An introduction to marine drilling

MARINE DRILLING Although land drilling for oil has been done for well over a century, it is only since the Second World War that marine drilling has really been in existence as an indust

THE AUTHOR

Malcolm Maclachlan was born in Dover, England in 1947 and first worked

at sea at the age of fourteen when he gained employment illegally on a

cross-channel ferry through a 'misunderstanding' with the ship's master After

training for three years on the cadet ship HMS 'Worcester' he sailed as

mid-shipman with the Blue Funnel Line and gained his second mate's certificate

at the age of 20 He served as a navigating officer with several deep sea and

coastal shipping companies, and after qualifying as a Master Mariner held

command of seven containerships whilst still in his early thirties He spent a

year lecturing in navigation and seamanship at Leith Nautical College before

joining the British drilling contractor Houlder Offshore Limited as a control

room operator/mate He served on the semi-submersible drilling rigs 'High

Seas Driller' and 'Kingsnorth UK' in the North Sea and aboard the

dynami-cally-positioned diving support vessel 'Orelia' in the Persian Gulf war zone

He has been writing and cartooning for many years and became a full-time

marine writer and illustrator in 1986, when he was a casualty of the

wide-spread cut-backs in the drilling industry caused by the slumping oil price He

has contributed many articles, short stories and drawings to various marine

journals and is currently writing a novel set on a North Sea rig Married with

a young family, he lives in Biggar, Scotland

CONTENTS

FOREWORD 7

PREFACE 8

ACKNOWLEDGEMENTS 9

Chapter 1: The Development of Marine Rotary Drilling 11 Hand-dug wells 11

Spring pole drilling 11

Cable tool drilling 13

Rotary drilling 18

Marine drilling 20

Chapter 2: Preparations for an Offshore Drilling Operation 25 The parties involved 25

The well owner 27

The operator 27

The drilling contractor 28

The drilling contract 30

Supply and service companies 31

Government departments 35

The costs of drilling offshore 36

Well types 42

Petroleum geology 45

The formation of hydrocarbons 46

Migration of hydrocarbons 46

Reservoirs 47

Anticlines 47

Fault traps 47

Stratigraphic traps 49

Unconformity traps 49

Exploration methods 49

Offshore surveying techniques 53

Magnetic surveys 53

Gravimetric surveys 53

Seismic surveys 53

Drilling rig site surveys 55

Chapter 3: Offshore Drilling Platform Types 57

Fixed drilling platforms 57

Fixed platforms with floating drilling tenders 59

Self-contained fixed platforms 59

Mobile drilling rigs 61

Submersibles 63

Self-elevating (jack-up) platforms 64

Semi-submersibles 71

Drill ships 81

Barge rigs 85

Chapter 4: The Offshore Rig and its Equipment 89 Basic rig components 89

The drill floor 89

The derrick 90

The drawworks 92

The blocks, hook &drilling line 94

The swivel, kelly &rotary hose 96

The rotary table , , 99

The drilling fluid circulation system 102

Drill string motion compensation 112

Downhole bumper subs 113

Surface drill string motion compensators 114

The power plant 120

Drilling equipment 123

API specifications 123

Drilling bits 123

Drill pipe 130

Drill collars 134

Stabilizers &reamers 136

The drill string &bottom hole assembly 139

Tubular handling tools 139

Other drilling tools 143

Sub-sea equipment 145

The temporary guide base 145

The permanent guide base 146

The wellhead/casing hanger system 146

Well control &the blow-out preventer stack 148

The marine riser 157

The riser tensioning system 162

Chapter 5: Drilling Operations 165 Running in the hole 166

Drilling ahead 171

Making a connection 172

Tripping 177

Running &cementing casing 180

Directional drilling 183

Drilling hazards 189

Stuck pipe 189

Fishing 190

Lost circulation 193

Kicks&blow-outs 194

Hydrogen sulphide 200

Weather &ice 200

Drilling operational sequence 202

Moving rig onto location &running anchors 203

Rigging up 203

Running the temporary guide base 204

Spudding in&drilling 36" hole 205

Running 30" casing &landing the permanent guide base 205

Cementing the 30" casing 207

Drilling 26" hole 209

Running &cementing 20" casing &running the 18314" wellhead 210 Running the 18 3 /4"BOP stack &the marine riser 210

Drilling 17112"hole 213

Logging 215

Running &cementing 13%" casing 215

Making a gyro survey 218

Drilling 12114"hole 219

Logging 219

Running &cementing 9518" casing 219

Displacing the hole to oil base mud 219

Drilling 8112"hole to total depth 219

Coring 220

Logging 220

Running &cementing the7" liner 222

Well testing 222

Well stimulation 225

Plugging &suspending or abandoning the well 227

Contingencies &weather 227

Chapter 6: Marine Operations 228 Basic rig stability 228

Displacement &the principle of flotation 228

The centre of gravity 230

The centre of buoyancy 230

Reserve buoyancy 230

The effect on the CG of adding, removing or shifting weights 230

The righting lever 230

The metacentre 234

Ballasting &free surfaces 234

Ballasting conditions 236

Rig structure &safety maintenance 237

Lifesaving &firefighting equipment 238

Work permits 239

Standby boats 240

Rig-moves 242

Navigation and pilotage 243

Towage 244

Approaching the location 245

Running anchors 247

Anchor types 250

Anchor patterns 251

Pre-tensioning 252

The moorings during drilling 253

Pulling anchors , 253

Dynamic positioning systems 255

Rig supplies 259

Helicopter operations 263

Chapter 7: Rig Personnel &Training 265 Semi-submersible rig personnel 265

Jack-up rig&drill ship personnel 279

Rig personnel training 280

6

FOREWORD

Over the years, I have seen many changes in the industry from the period of growth

in the 1970's to our present day recession, I have had the privilege to witness first hand some outstanding advances in techniques and technology and yet for me at least, drilling remains something of a mystery Infamous for its jargon, it is as com-plex as it is fascinating It is therefore a particular pleasure to discover, after so many years, a book which is both readable and comprehensive, and which succeeds in reve-aling to me, as I am sure it will do for all its readers, whatever their level of interest, something of the 'black art' of drilling.

It also gives me pleasure to think that much of the authors knowledge arises from his connection with Houlders First as a cadet on the "Worcester" established at Ingress Abbey on the Thames, my Grandfathers home and more recently during his time offshore with the company.

Houlder Marine Drilling is the offspring of the shipping company founded by my Grandfather in 1848 Interest in drilling arose in about 1973 as the result of a chance conversation with a Norwegian shipowner who introduced me to Bernard Larsen, another shipowner, who I believe to have originated the conceptual design for the famous H-3 which is illustrated on page 29.

Just as the author learned from Houlder, so Houlder in turn had learned from the established drilling industry which in turn had learned from the Moho project This was to discover by drilling, the composition of the core of the earth As much as pos-sible was to be drilled through water, and I think that the drillship owes more to the Moho technology than to the offshore shallows of Louisiana.

Houlder in turn claims to have originated the idea of transporting a semi-submersible drilling rig on the deck of another ship Page 39 shows the High Seas Driller being carried by this method I conceived the idea and made the pre-liminary calculations while waiting in the Boar-droom of the China National Oil Company in Beiching!

I think overall that Houlder has traditionally prospered by innovation and of the innovative approach used for the layout and content of this book is anything to go by, it should be an out-standing success.

This book was conceived during the author's own quest offshore for the

answers to a multitude of questions - questions that are inevitably asked by

any curious 'green hand' or 'boll weevil' in his first wondrous weeks aboard

a semi-submersible or jack-up rig

In a complex engineering environment such as an offshore drilling rig only

so much can be deduced from a silent observation of the strange and

mysteri-ous procedures, and invariably one must repeatedly seek explanations It is

not always easy, however, or diplomatic, to ask a driller to explain the

intricacies of his art when he is attempting to make a speedy 'round trip'

under pressure from a cost-conscious 'company man' Nor is a weary

roughneck or derrickman likely to want to spend an extra half hour in the

messroom after his 'tour' describing the arduous work he has just been doing

for a whole half day The drilling jig-saw puzzle can therefore take a

consid-erable time to piece together, and in retrospect I wish now that I had had the

benefit of a book on marine drilling to guide me through my first tentative

trip offshore

The simple aim of this book, then, is to explain to the new rig hand, to the

offshore job applicant, to those on the periphery of the offshore industry

who may never have the chance to go out to a rig and see for themselves, and

to the interested layman and student on the 'beach' the main operations of

this fascinating industry

It does not pretend to be a comprehensive or learned study of the oil

exp-loration business, nor does it masquerade as an instructional manual of

dril-ling technology; it is hoped merely to throw a little truthful light on some of

the operations of an industry which all too often is portrayed by the mass

media as being simply a matter of grim and grimy men heaving and slithering

on a wet deck as they struggle to latch massive wrenches onto a steel pipe

That is one interesting facet of marine drilling, but there are many more

which are rarely seen by those not fortunate enough to witness the workings

of an offshore rig

For any technical errors or omissions I can only apologise to the marine

drilling fraternity and ask for their forbearance

Biggar, ScotlandMarch 1987

ACKNOWLEDGEMENTS

Thanks are due to Adrian Rose, safety officer, and Ian Edwards, bargeoperator, both of Houlder Offshore, for their helpful clarifications; to JimLangley of Brown Brothers for information about motion compensators,and to Gavin Strachan, marketing manager of Atlantic Drilling, for his lucidexplanation of drilling contracts I must also acknowledge the generosity ofPhillips Petroleum and BP, who provided many illustrations, and the manycompanies who sent me research material Special thanks must also go to mywife Lesley for her encouragement and countless cups of coffee, and toDavid Gallimore of Dayton's Publishing who had the courage to back myidea form beginning to end

CHAPTER 1:

THE DEVELOPMENT OF MARINE ROTARY DRILLINGSince his earliest days, man has dug holes in the earth's crust in his search forwater, salt and other minerals Over the centuries his digging techniqueshave changed and become more efficient, culminating in the method todayknown as rotary drilling which is almost universally used in oil and gas explo-ration, both on land and offshore Rotary drilling with a land rig is a complexbusiness On an offshore rig it is even more involved, and at the same time

is made hazardous by the hostile elements An appreciation of the tive way in which oilmen have overcome their difficulties can be gained bylooking first at the earlier, simpler drilling methods which led to the develop-ment of rotary drilling

innova-HAND DUG WELLS

The age-old traditional method of digging a water well by hand was for oneman to pound a hole in the ground with a sharp implement like a big chisel

As the hole got deeper and the cuttings started accumulating at the bottom

of the hole, the digger had to load them into some sort of container which anassistant at the top of the hole then pulled out Digging had to be temporarilyhalted for this to be done and the well-digging operation was slow and tedi-ous

The walls of the hole had a tendency to cave in as it got deeper, so to vent this the well had to be lined with some material such as wood or brick

pre-as it progressed downwards These materials were the forerunners of what

is now known as casing

SPRING POLE DRILLING

Hand-digging was slow and dangerous for the digger, especially as he dugthrough a hydrocarbon-bearing zone and oil or gas started seeping into thehole A safer and more efficient mechanical method of digging was soughtand, according to ancient Chinese manuscripts, one was in use in China asearly as the 3rd Century AD These manuscripts described a method whichwas really only a logical development from hand-digging The Chinese 'dril-led' wells for brine using a percussion system in which a heavy, chisel-shapedbit suspended from a rope was jerked up and down by relays of men bounc-ing on a spring-board, thus progressively pounding the hole deeper

11

The Development of Marine Rotary Drilling

A v~riation of this method used in Europe and America in the eighteenth

century was called 'spring pole drilling' A large metal bit was suspended

from a flexible wooden pole by a long rope and allowed to drop to the

bot-tom of the well The bit chewed briefly into the formation, like a man

stab-bing with a chisel, before the springing pole bounced it back up As this

hap-pened the length of the rope would be extended a little by the 'driller'

con-trolling it at the surface, so that the bit struck with an unvarying force on each

successive blow and chewed in a little deeper on each downward stroke As

the bit was reciprocated up and down in this manner the rope twisted slightly

and varied the position of each blow on its descent, resulting in a roughly

cir-cular hole being dug

.

The Development of Marine Rotary Drilling

Large quantities of cuttings naturally collected at the bottom of the hole,and as this was almost as narrow as the bit, the cuttings had to be removed

by a self-shutting container, called a bailer, which was periodically lowered

to the bottom

When this method was eventually introduced to western countries ironbits were used, drilling holes of only 21f2to 4 inches (7 to 10 centimetres) indiameter Wells were seldom more than 240 feet (75 metres) deep, but there

is evidence of a brine well being drilled to just over a thousand feet (305metres) in the USA in the early 1840s To hold back the wall ofthe hole, thisAmerican well was 'cased' with lengths of wood shaped into half tubes andwrapped with twine The Chinese, on the other hand, cased their wells withhollow bamboo sticks, and are said to have drilled to depths of more than athousand feet by this method

Spring pole drilling was limited by the weight that the wooden pole couldrepeatedly lift without breaking, and of course this weight included that ofthe rope, which got progressively longer with the depth of the well If therope broke and fell to the bottom of the well with the bit or the bailerattached, the driller had to improvise a method of retrieving, or 'fishing', forthe 'fish', as the lost equipment was termed Various gadgets weredeveloped for this purpose and thus the forerunners of today's efficient fish-ing tools were devised

In the early part of the nineteenth century, whale oil was the most monly used fuel for lighting lamps in America and Europe The US, Britain,Norway and many other countries had large fleets of whalers, and othertypes of oil were not considered commercially important In America, 'rockoi!' was frequently found seeping into brine wells drilled near salt creeks,and at first it was regarded as no more than a nuisance However, the busywhaling fleets quickly depleted the stocks of whales and it was recognisedthat sooner or later another source of lamp fuel would be required Thevalue of the black oil that was often found seeping out of the ground in manyplaces was eventually realised, and this rock oil was henceforth harvested.However, it was to be some years before oil was specifically drilled for

com-CABLE TOOL DRILLING

The early nineteenth century saw the rapid mechanisation of many tries in the western world, and about 1850 a revolutionary new method ofdrilling called cable tool drilling was introduced It utilised a steam engine

indus-The Development of Marine Rotary Drilling

the hoist the drilling, bailing and fishing tools in and out of the well, and the

process of drilling a well thus became much faster and more efficient and was

almost completely mechanised, just as it is today

As the cable tool method gained wider favour, a wide range of ingenious

devices were developed to overcome problems encountered and make the

job of drilling easier and safer The now familiar drilling derrick, then made

of wood rather than steel, but tall enough to house lengths of drilling, bailing

or fishing t001s, was introduced Much heavier bits and more robust

equip-ment could now be used with the steam power, and wire instead of fibre rope

enabled deeper wells to be drilled Iron casing, replacing the old wooden

sheathing, could now be driven into the hole length by length as the well got

deeper, so as to retain the wall and to make it easier to extract any minerals

that were eventually found

.

The Development of Marine Rotary DrillingCable tool rigs were particularly useful for drilling medium-hard rocks,but the softer rocks encountered could not withstand the spudding, or jab-bing, action of the bit, and holes often caved in or allowed too much water

to seep into them for drilling to continue A new type of cylindrical steel ing was developed to replace the old iron type, and if caving occurred,lengths of it were screwed together and lowered into the hole to seal the wall

cas-of the hole When the casing reached the bottom, drilling then continuedusing a slightly smaller bit that could just run through it As each new prob-lem zone was encountered, so more casing was run inside the last 'string',and a correspondingly smaller bit was used to drill out the next section Inthose days oilmen were not blessed with the huge variety of tools that nowenable them to overcome nearly every kind of downhole problem, so if thecable tool well encountered too many hitches, and so many casing runs had

to made that the bit got too small to drill with, the hole would have to beabandoned, often before reaching its target depth The method was, there-fore, slow and inefficient, but to the early pioneers of the oil industry it rep-resented the pinnacle of drilling technology at that time

One of these pioneers, a retired railroad conductor named 'Colonel'Edwin L Drake, is now famed for supervising the drilling, between Juneand August 1859, of a cable tool well at Oil Creek near Titusville, westernPennsylvania, USA, that is considered to have founded the oil industry as wenow know it Drake did not actually own the well; a New York lawyer namedGeorge H Bissell owned the land it was drilled on, and his was the idea todrill specifically for oil, since the area had been noted for its surface seepages

of oil In any event, the Drake well, as it is now known, successfully located

an oil reservoir at a depth of only 691f2feet (21 metres), and oil flowed at arate of about 19 barrels a day, or 3,000 litres, which was phenomenal forthose days Bissell became a founder of the Pennsylvania Rock Oil Com-pany, and further successes with cable tool drilled wells made that state theleading oil producing area for the next fifty years Its lead stimulated thegrowth of the drilling industry in other areas, notably in Canada, Poland andCalifornia From the oilfields of these areas drillers took their knowledge ofthe newly successful cable tool drilling techniques all over the world, and theworldwide drilling industry thus began

By the turn of the twentieth century, after fifty years of continuous use anddevelopment, it was possible to drill wells by the cable tool method down toabout 3000 feet (1000 metres) in favourable conditions The heydey of themethod, however, was the period from 1900 until the great economic depre-ssion of the 1930s, and many of the best improvements came during thistime Steel derricks replaced the old timber structures, and bits becamelarger and tougher The most important development, however, was the

15

The Development of Marine Rotary Drilling

introduction of the cementing of casing This was first done in 1903 whensome liquid cement was dumped from a bailer into a well and a string of steelcasing was lowered into it A few days later, when the cement had set, thehard cement inside the casing was drilled out The cement in the annulus bet-ween the casing and the wall of the hole was found to have sealed off a zone

of water in the formation, and it had also anchored the casing to the wall ofthe hole

This success was further improved on when in 1910 a procedure was duced for pumping a measured amount of liquid cement down a hole A vol-ume of cement was held between two wooden plugs, spaced one above theother, and was pumped down the inside ofthe casing, round its bottom end,and up the narrow annulus outside, where it was left to set hard Althoughall the cement was expelled from the 'shoe' at the bottom of the casing, theplugs were retained inside it, but these were drilled out when drillingresumed with a smaller bit Again it was found that the cement in the annulusbetween the casing and the hole had set hard, and henceforth in this mannereach section of casing was run into the hole and cemented to surface, thusovercoming many of the formation difficulties previously encountered Bas-ically, the same procedure is still used today in 'cement jobs', and one of itsseveral virtues is that it allows wells to be carefully planned in advance fromstart to finish in fields where the formation types are known

intro-By 1918 the world's deepest well, drilled by the cable tool method, was7,386 feet (2,251 metres) deep, and the technology of cable tools nearly atits zenith It was possible to 'spud', or start drilling, one of these wells with

a 24-inch (61 em) diameter bit and, if necessary , cement up to seven separatestrings of casing, from 20-inch (51 em) diameter down to 5-inch (12.7 em), asproblems in the well were encountered However, the rotary system, inwhich the drill bit was rotated under power on the end of a steel tube instead

of being reciprocated on the end of a wire, was also being developed, and bythen had become the preferred method for drilling holes deeper than 4,000feet (1,219 metres) It was recognised that the great disadvantage of thecable tool method was that there was no means by which a drilling fluid could

be circulated so that cuttings could continuously be brought to the surface,and so that the wall of the hole, together with any oil, water or gas in it, could

be held back

During the 1920s a type of combination rig was developed which couldemploy both cable tool and rotary methods at different stages of the wellwhen it was thought profitable to do so, but the rotary type of rig was by thistime rapidly gaining favour Up to 1930 there were still far more active cabletool rigs than rotary rigs, and that was the situation obtaining when the pre-

The Development of Marine Rotary Urilling

war depression drastically reduced drilling activity all over the world Bythat time the deepest well yet drilled by the rotary method was just over10,000 feet (3,050 metres) deep

When drilling activity resumed after the depression the industry hadbecome leaner, fitter and more competitive, and deeper wells were required

to find and exploit new oil reserves to meet the world's revived thirst forenergy Cable tools, which could rarely drill more than 60 feet (18 metres) aday, had virtually reached the limits of their technology, but rotary rigs,which could, in favourable conditions, drill 2,000 feet (609 metres) in eighthours, seemed to offer unlimited scope for improvement As a result, after

1930 cable tools never recovered their previous popularity, and they weregradually superseded by the rotary system The deepest cable tool well evercompleted was the Kesselring No 1 NYS Nat Gas Corp., completed in

1953 at 11,145 feet and taking over 21f2years to drill Today the cable toolmethod is rarely used

ROTARY DRILLING

Rotary drilling evolved from the carpenter's method of boring a hole inwood with a rotating tuol which itself flushed out the cuttings The derrick,initially introduced for cable tool drilling, was now employed as a simplecrane from which a hook, suspended from a block and tackle, was moved up

or down by a wire running over a hoisting drum called a drawworks The bitwas secured to the bottom of a string of steel pipes, and was rotated by powertransmitted by an engine on the surface A rotary drilling machine waspatented in 1845 but the system was first used in Texas in the early 1890s in

an attempt to solve the soft rock problem that had plagued cable tools there

Up until the end of the 19th century, cable tools were still favoured for mostdrilling situations

In January, 1901 Anthony Lucas's well at Spindletop, near Beaumont,

Texas, was completed with a rotary rig after several attempts to complete itwith cable tools failed due to running quicksand The quicksand was easilyheld back with casing when the rotary equipment was brought in and the wellfinally blew out, producing 84,000 barrels of crude oil a day from a depth of

a little over 1000 feet The 'gusher' blew all the drill pipe out of the 60-foothigh derrick and shot more than 200 feet into the air above it, provingbeyond doubt the value of rotary rigs

On the strength of its Spindletop success rotary drilling soon held sway inthe US Gulf coastal areas and was competing strongly with the cable toolmethod elsewhere Although the early equipment was radically different tothat used today, the concept of rotary drilling is fundamentally unchanged

18

The Development of Marine Rotary Drilling

One of the most significant developments was the introduction of the rolling

cutter bit, first made and used by Howard Hughes in 1909 There have been

many improvements made since then to Hughes' original design, but his

basic concept remains the same today Modern metallurgy and components

now ensure that bits last many times longer than in those days, and there are

now variations on the basic design to suit all types of formation Bit

develop-ment has recently been aimed at making the bit match the characteristics of

the rock it is drilling, but many bits are designed to cope with a range of

var-ious rock types

Most other items of rotary drilling equipment have also undergone great

changes since the pioneering days of the method and vast amounts of money

have been spent by the oil companies on developing new techniques and

ideas to improve the efficiency of drilling Drilling fluids have been

formu-lated for use in every well condition, and means have been evolved for

alter-ing the fluid's chemical and physical properties as necessary duralter-ing the

circu-lation process Fluid circulating systems now have greater capacities and can

be more precisely controlled and powerfully pumped Metallurgical

research has discovered ways of combatting corrosion in drilling tubulars

and in withstanding the stresses that are imposed from great depths,

temper-atures and pressures Downhole equipment has been developed to meet

every conceivable need, permitting complex tasks to be performed

thousands of feet down a narrow borehole

A tool inside a borehole can only move in three fundamental directions:

up, down and round On the face of it, therefore, drilling holes in the earth's

crust might appear to be a simple job that has been made easy with modern

equipment On the contrary, it remains a highly complex operation

demand-ing grit, determination and much technical expertise and a great deal of

money, and nowhere is this more the case than offshore

MARINE DRILLING

Although land drilling for oil has been done for well over a century, it is only

since the Second World War that marine drilling has really been in existence

as an industry in its own right, and many of the practices now used offshore

evolved only in the last twenty years

The first 'offshore' wells, however, were a shallow well drilled over the

water from a pier at Santa Barbara in southern California in 1897, and a well

drilled in 1911 in Caddo Lake, Louisiana, where a steam-powered rotary rig

was erected on a wooden, bottom-supported platform

.

The Development of Marine Rotary Drilling

In the 1930s techniques were introduced for drilling in the swamps ofLouisiana, USA, in Lake Erie in Canada, and in the large-scale develop-ments then taking place in Lake Maracaibo, in Venezuela The first floating

ria was a simple barge used in 1933 to drill in the bayous of southernLouisiana All subsequent 'marine' drilling, which was at first confined to.wamps and lakes, was carried out using ordinary land drilling equipmentmounted on simple, flat-topped, flat-bottomed barges

As the US oil industry rapidly expanded following the end of the SecondWorld War, so it set its sights on drilling in the seas off the US Gulf states,necessitating the development of special craft capable of supporting drillingequipment in a sometimes hostile environment The first seagoing 'mobileoffshore drilling unit' of any kind was a submersible platform that drilled in

a water depth of only 20 feet in 1948, while the first 'floater' was a coverted

USArmy wartime transport barge that had a rig fitted on an overside tilever Many other 'submersibles' were built for use in the US Gulf area,these having decks supported by tubular, bottle-shaped columns fittedabove pontoons which could be ballasted like those of a modem semi-sub-mersible These platforms were, in fact, the direct descendants of the mod-

can-em 'scan-emi', although some of thcan-em bore little rescan-emblance to today's duty, harsh environment' units

heavy-The next development after submersibles came in the form of ing barges that had tall legs on which the drilling platform could be jacked up

self-elevat-to sit well clear of the water, the feet of the legs resting in or on the sea bed.These craft, of which the first was built in 1954, evolved from floating docksthat were used by the US army during the War The first crude 'jack-up'units were used initially in the shallower parts of the Gulf of Mexico and theArabian Gulf, but the designs grew steadily more sophisticated with theincreasing demands of the industry for oil from deeper water, eventuallyproducing units with legs 300 feet high However, this was still not highenough

After the early makeshift barges and submersibles, converted ships wereused to support drilling rigs, but as exploration moved out into deeper water,

so fixed platforms were developed that could be towed into position andsunk so that they rested squarely on the drilling location At first these, likejack-up units, were only used in relatively calm and shallow waters near thecoast, but larger and stronger platforms were later built for the deeper andless tranquil waters further offshore

In 1955 the first well was drilled by a vessel on which a rotary rig wasmounted and by 1957 a further milestone was reached when a well was dril-

led by a drill ship in 100 feet of water The equipment used on these sions was the most advanced then available, but it would look primitivealongside the sophisticated offshore drilling units operating today The shipsand barges then used could only operate in a maximum depth of 600 feet,while submersibles were restricted to 90 feet By 1960 about 70 barges andconverted ships were being used for offshore drilling, much of this workgoing on off California

occa-The 1960s saw the introduction of the first semi-submersibles, which couldeither sit on the bottom like a submersible or float like a drill ship About 30

of these units were built during the decade, of many different types Thenumber of jack-up rigs quadrupled at the same time, and the new designsstarted to utilise the canted legs that are now a common feature of manyunits

Purpose-built ocean-going drillships with large storage capacities for fuel,drilling fluids and other supplies were also developed for use in remote areasfar from supply bases They were built with their own propulsion so that theycould move themselves between locations and dispense with the need fortugs In the 1960s drillships were first fitted with one of the most importantdevelopments in deep-water drilling technology: dynamic positioning (DP)systems These systems enabled drillships, and later, semi-submersibles, tomaintain position with the aid of computer controlled thrusters thatresponded to the commands of position monitoring inputs and dispensedwith the need for anchors However, the systems were, and still are, expen-sive and the vessels using them relatively few

The upward surge in the price of crude oil and the availability of offshoreconcessions and exploration licences from many countries prompted a fran-tic spate of rig-building in the 1970s, when an average of 30 drilling unitswere built each year - more than at any time before or since This brought theworld's offshore rig fleet up to over 500 units, and with this increase camemany improvements in equipment and operating techniques, including theuse of surface motion compensators

Having reached a level of about 750 units of which many are surplus torequirements, the 1980s have seen a fall in the number of drilling units built,along with a fall in the price of oil Offshore technology has, however, keptmoving ahead Jack-ups with legs 600 feet long are now able to drill in 450feet of water, while dynamically-positioned semi-submersibles can nowoperate in 10,000 feet depths Offshore drilling is going on in nearly everymaritime area of the world, and seems likely to continue and expand as theoil price again rises Even in the Arctic wastes drilling is carried out from

23

The Development of Marine Rotary Drlllln,

man-made islands - the latest type of submersible - whose huge caissons are

floated into position, ballasted and sunk, and fortified against the powerful

natural forces within the ice cap Sea-bed cores have been recovered by a

drill ship from a depth of 23,000 feet and experimental well drilling has been

conducted in water 13,000 feet, although no commercial oil drilling has yet

been carried out in such depths However, the technology is available to

exploit oil reserves in the deep oceans well away from continental shelves; it

only requires a stronger demand for oil at a commercially viable price to

stimulate the drawing board ideas into action

or the North Sea One region may be more attractive than another, larly if strong incentives are offered from the host country

particu-After acquiring permission from the host government to carry out theexploratory work, which may take months or even years, large amounts ofmoney and time are spent in carefully examining the sub-sea geology of thechosen area for likely hydrocarbon-bearing locations When a promisingarea has been identified by surveys and an available drilling site selected, theoil company must then go through the machinery of applying for the neces-sary permission from the controlling government authorities to go aheadwith the project This alone can take months or, with some governments,even years to finalise Governments have to be sure that the project will bebeneficial to their own interests as well as those of the investors, and thesafety of the personnel involved and the marine environment has to be regu-lated by binding agreements with the oil company involved

With an offshore well often costing between £10 million and £20 million todrill, and far more likelihood of it being a 'dry hole' than a commercialbonanza, the viability of the well programme has to be very carefully consi-dered before large amounts of investors' money are ploughed into the pro-ject If the chances of success are calculated to be worth the risks involved,finance then has to be raised and partnerships sometimes entered into tospread the enormous costs involved (Not every offshore well is drilled by ahuge multi-national like Shell or Mobil) Then contracts for the supply of adrilling rig and its crew, and for every conceivable item of equipment or ser-vice that will be required during the programme have to be sought Once therig has been hired and drilling has begun at a cost to the oil company ofperhaps £60,000 a day, time is so costly and precious that not a day can bewasted on waiting for something that should have been ordered monthsbefore With so much at stake, the drilling rig's operation is naturally con-trolled extremely rigidly by the oil companies, which largely accounts for thefeverish and tense atmosphere often felt onboard

Preparations for an Offshore Drilling Operation

THE WELL OWNER

Theoretically, virtually anyone can own an oil or gas well, and private viduals who invest money in insurance companies, pension funds, banks andother institutions may be unaware that they might in factha part-owner ofone or more wells However, in most cases the majority owner of an offshorewell is almost always an oil company, either one of the big 'majors' who haveinterests in virtually every sector of the oil production, refining and market-ing industry, or one of the smaller 'independent' oil companies which arepurely concerned with production and sale of oil or gas

indi-The independents, which number in their thousands in the US but arerelatively few in Europe, are primarily concerned with land drilling,although they have made significant contributions in the development ofoffshore areas such as the North Sea About 80% of all the land wells in the

US are financed by independents, the remainder being paid for by themajors These, which include household names such as Shell, Exxon (orEsso), BP, Mobil and Texaco, concern themselves mainly with offshore andoverseas operations as far as drilling is concerned, although some majors arehighly active in land drilling as well

In many cases offshore, ownership of a well is not by a single company but

by a consortium of investors which may include one or more oil companies

as well as virtually any type of firm or institution with money to invest in theproject A typical consortium might be composed of:

Power & Light company 'D' 4.50%

Consortia often acquire rights to drill in several areas, and some may stay

in existence for a number of years, while others split up after a while withvery little drilling activity to their credit

THE OPERA TOR

If the well is owned outright by one company, then that company is known

as 'the operator' , since it would normally operate the well during its tion phase when the exploratory drilling has been completed In the casewhere a consortium exists to finance the well, one of the participating com-panies - usually the company with the largest equity ownership - will be

produc-"',.,

Preparations foranOffshore Drlmnl Operation

designated the operator by mutual agreement The operator acquires therights to drill on a location and holds the various licences required, but at thesame time he shoulders the huge responsibility for the safety of the hundreds

of personnel who will invariably be involved, as well as for the protection ofthe environment The operator is, therefore, answerable to the governmentbody which regulates drilling activity in the host country concerned Most ofthe operators in countries such as the US and Britain have formed associa-tions through which their own interests can be represented in dealings withgovernments UKOOA, the British operators' association, has about 40members, most of whom are British subsidiaries of American oil companies

THE DRILLING CONTRACTOR

In the first half of the twentieth century it was common for oil companies toown their own rigs and to drill their own wells, but nowadays most prefer, forreasons of cost-effectiveness, that specialist drilling contractors do this workfor them, both on land and offshore Drilling contractors have the necessarymen and skills, and a vast store of drilling experience, and between themthey maintain a large fleet of many different types of rig that any operator,from a giant major to the smallest independent, can call on to tackle anytype of drilling job Even the largest oil company, on the other hand, isunlikely to have a wide range of rig types in its own fleet

Some drilling contractors are active solely in marine drilling, while othersown both land rigs and offshore units, which might include fixed platformrigs as well as floaters and jack-ups Some marine drilling companies ownjust one elderly barge rig in the US Gulf, while others own large fleets ofmodern jack-ups, semi-submersibles and dynamically-positioned drillshipsthat are scattered from the Canadian Arctic to the jungle creeks of WestAfrica

The drilling contractor might own the units he operates, or he might age some or all of them on behalf of other owners such as finance houses orshipping companies Some marine drilling contractors have their roots in theshipping industry and diversified their activities into drilling when their trad-itional cargo fleets dwindled in the mid-seventies Many others havegraduated to the offshore sector from long-established land drillingbackgrounds

man-In most cases the drilling contractor has no equity interest in the well but

Preparations for an Offshore Drilling Operation

The marine drilling contractor might directly employ the entire crew of

the offshore drilling unit, comprising the drilling, marine, engineering and

catering departments, or he might sub-contract some of the manning out to

a crewing agency where this is to his financial advantage When the price of

oil is low, reflected in a slump in drilling activity and the mass 'stacking' or

laying-up of rigs, contractors need to have the flexibility in their crewing

arrangements to accommodate the lower hire rates that working rigs are able

to command The entire catering department, for example, is in many cases

nowadays composed of agency personnel who are unlikely to-be so well paid

as the directly-employed staff of the drilling contractor

THE DRILLING CONTRACT

When an operator plans to drill an offshore well, he seeks tenders for the job

from a number of selected marine drilling contractors Three or four months

before the well is to be 'spudded' , or begun, a telex is sent to the contractors

outlining the well programme and its requirements in terms of the rig and

drilling equipment On the basis of the replies from any interested

contrac-tors, the operator then sends out the bid documents along with very detailed

specifications of the type and capabilities of the rig required and the

equip-ment to be used for the programme The specification contains numerous

detailed stipulations on every matter concerning the rig and its operation, all

of which the contractor must be able to meet

The drilling contractor is thef1 able to assess his costs were he to take on

the work, and depending on market conditions he makes a bid for the

con-tract, quoting a price that he thinks he can command in the prevailing

indus-try climate When the market is in his favour, this might far exceed his

break-even level, but more often than not, at the time of writing, it will mean

operating at a loss In early 1987 the break-even point for a typical modern

semi-submersible's operating costs (which exclude bank loan repayments

and depreciation) was in the region of$18,000 a day, while many rigs of this

type were commanding day-rates of only$12,000, or even less However, as

the price of oil rises, so rig day-rates generally rise, and semi-submersibles

have in the past obtained as much as$95,000 a day when the oil price has

been high A rough guide used by some marine drilling contractors is that

they need approximately 10% of the capital cost of the rig to adequately

cover all their costs Thus, when a rig has been bought for$60 million, about

$60,000 a day is needed for profitability

The various contractors in contention for the job make their bids for the

contract and the operator evaluates all the bids on their individual economic

merit, considering factors such as past performance of a contractor, his

abil-ity and integrabil-ity, his safety record and the present location of his rig and the

time needed to re-Iocate it

30

Preparations for an Offshore Drilling OperationWhen a rig is being taken over by one operator after the expiry of a drillingcontract with another, the new operator's responsibility normally com-mences at the time of 'racking', or securing onboard, of the last anchor pul-led in at the previous well site, immediately before the transit to the newlocation It may not be in the best interests of the operator, therefore, to hire

a rig that is in every other respect suitable, if it is lying at an uneconomic tance from the planned drilling location At the same time, a contractor whohas the right type of rig lying very near to the new location, and who claims

dis-he can complete tdis-he well faster than all tdis-he otdis-her bidders, might not get tdis-hecontract if his safety record is less than commendable

The document of hire of the rig is called the 'drilling contract' This forms the same function as a ship's charter party, laying down each party'sresponsibilities, but it is much more detailed, running to as much as ahundred pages or more It stipulates, amongst other things, the rates that thecontractor will receive for each type of operation during the well prog-ramme There will be a top rate for normal drilling operations, a slightlylower 'standby rate' for periods when drilling has to be suspended whilstwaiting on equipment, a lower-still 'repair rate' for periods of downtimewhen the contractor's own machinery has failed, a 'force majeure rate' forsituations out of the contractor's control, such as a strike in a supply base,and a 'moving rate' for periods in transit between two wells of the same wellprogramme The hope of both parties, naturally, is that the drilling bit will

per-be on the bottom of the hole for as much time as possible

SUPPLY AND SERVICE COMPANIES

In the course of any offshore drilling operation a large number of companies,apart from the drilling contractor, are invariably called upon to perform cer-tain specialised jobs and provide special equipment of one sort or another.The operator may directly contract these supply and service companies, just

as he contracts the drilling contractor, or else the drilling contractor tracts them, but in any event, the final cost will ultimately be borne by theoperator Running casing, cementing, mud logging, diving, fishing (ordebris retrieval), inspection, and directional drilling are all typical serviceswhich are commonly put out to tender, while items such as drilling fluid,cement, fuel and water are amongst the essential supplies which must bereceived regularly on demand from supply bases

sub-con-There are numerous firms in the oil industry specialising in the ture of individual items of oilfield equipment or the provision of specialisthelp, and their products and services are described in four large volumes ofstandard reference catalogues which run to nearly 8000 pages in all and arefound in the toolpusher's office on all rigs Drill pipe, for example, can be

manufac-31

Preparations for an Offshore Drilling Operation

supplied by more than forty manufacturers, while blow-out preventers aremade by over thirty firms Whenever a problem arises with the equipmentsuprlied by one of these firms, their technicians - 'service hands' as they arecalled - are brought out to the rig to effect speedy repairs Consequentlythere may be many personnel on an offshore drilling unit who are onlyaboard for a few days and who work for a variety of different firms

The operator also has to have craft available to get all this equipment, andthe men who will use it, out to the drilling unit In most areas of the worldthis means chartering supply boats for the heavier and bulkier cargoes, andhelicopters for the men and the lighter, or more urgent, small items Insmooth-water areas, however, crews often travel by fast launch since theseare cheaper to hire than helicopters Two or three supply boats, not neces-sarily all owned by the same company, might be chartered for the duration

of the well programme, while others might be 'spot chartered' as required on

a single voyage basis Normally one helicopter company is contracted to vide a regular flight schedule out to the unit, with additional flights beingpaid for as required when extra personnel or freight have to be transported

pro-In addition to hiring supply boats, the operator has to provide a safetyboat in some parts of the world, to stand by close to the rig whenever it ismanned in case of an emergency requiring its evacuation The standby boatmay never actually be used in earnest, but it is an unavoidable expense whichmust be borne nevertheless as a condition of holding a licence to explore

Re-Iocating many mobile rigs from a previous location will involve usinganchor-handling vessels These are often dual-purpose ships that maybecome the rig's supply boats once their anchor-laying work has finished.Because of their more arduous duties, for which they need much greaterpower, anchor-handlers normally command higher rates than supply boats,and different rates again will be earned by boats in different powercategories In a particular week in early 1987, for example, an anchor-hand-ler of 12,000+ brake horse power could earn £2,850 a day in the North Sea,while a vessel of 8,000-10,000 bhp could earn £2,650 Large supply boatscommanded around £2,500, while smaller vessels on supply runs could com-mand a maximum of £2,400 Charter rates fluctuate wildly from week toweek according to demand and availability of vessels, and rates muchhigher, as well as much lower, might prevail according to the dictates of themarket As far as the well operator is concerned, he usually wants his boats

to be reliable as well as cheap, and he would rather have an expensive boatthat can stay 'on location' alongside a rig with essential supplies in badweather than a cheap but unreliable boat that delays the drilling programme

Preparations for an Offshore Drilling Operation

Before the operator can begin any offshore exploration, he must usuallyobtain permission in the form of a licence from the government in whosewaters the proposed well will be By a United Nations convention, maritimenations have sovereignty over large offshore areas for the purposes of thedevelopment of oil and gas resources, and almost every sea in the world isdivided by the median lines of all the bordering countries The North Sea,for example, is shared between Norway, Denmark, West Germany, Hol-land, Belgium, France and the United Kingdom, each country administering

a sea area roughly in proportion to the length of its own bordering coastline.For the purpose of administration of oilfield activity and the award oflicences, offshore waters in most maritime areas of the world are divided into

numbered blocks In the British sector of the North Sea these are defined by

lines of latitude at ten-minute intervals and lines of longitude at minute intervals, making thirty blocks in each one-degree by one-degreesquare on a chart On the British continental shelf all oil and gas explorationand production activity is regulated by the UK government's Department ofEnergy, and from time to time a number of blocks will be put up for auction

twelve-by the DEn in the hope that oil companies will make bids for the explorationand production rights on them The DEn grants two kinds oflicence, one forexploration, which in this context means geophysical surveying and bottom-sampling, and the other for production, which includes exploratory drilling.Licences are only awarded after a thorough examination of an applicantoperator's proposed drilling programme and his ability to carry it out safely,

as well as his survey work on the area, so that the government will be fied that he will operate safely and in the best interests of the nation and itsresources

satis-Preparations for an Offshore Drilling Operation

Once the operator has been granted his licence to explore and has awarded his drilling, supply and service contracts, drilling can theoretically begin However, there are usually many other official regulations to be satis- fied before he can legally bring his rig on location While the Department of Energy is primarily concerned with the regulation of offshore exploration and production activity, including the operation of fixed platforms, the safe operation of British floating vessels on the oilfields remains the province of the Department of Transport The DoT is reponsible for the registry and regulation of all British ships as far as their safe manning and operation is concerned, and as such it has an interest in all British-flag mobile offshore drilling units, wherever they are operating It is also the examining body for British seafarers' certificates of competency and it decides the minimum manning scale for individual rigs, so that whatever the unit's operational mode, whether drilling or in transit, and whether on a long ocean passage or

a short shift of location on the same oilfield, the vessel will always be safely manned with a properly qualified crew This only applies to marine crew; the DoT does not make regulations governing the carriage or certification of any other personnel onboard The American body with broadly similar official powers to the DoT is the US Coast Guard, which enforces US marine regu- lations on American-flag rigs working in US and overseas waters Mobile offshore drilling units are, therefore, treated by government departments like ships in some respects, but like fixed oil installations in others, and the interests of the different government bodies involved sometimes overlap.

THE COSTS OF DRILLING OFFSHORE

Drilling an offshore well can cost ten times as much as drilling a land well, and an operator's expenses might well run to $100,000 a day for 100 days or more. In sea areas such as the North Sea or the Canadian Arctic costs are raised due to two main factors One is the harsh operating environment, which necessitates rigs and equipment which are more robust and therefore more expensive than those needed on land and in less hostile sea areas such

as the Arabian Gulf and Lake Maracaibo The other is the longer time required for drilling the well, due partly to the harsh conditions and partly to the need to use special additional equipment Minimising the time spent from 'spudding' to completion of the well is usually the most important fac- tor to any operator, and oil companies are normally prepared to pay what- ever is necessary to obtain the right kind of reliable equipment and services

to get the job done expeditiously.

The overall costs of any exploratory drilling venture can be grouped under three main headings: initial costs, equipment costs and operating costs Ini- tial costs cover the preparatory work necessary before any drilling starts, including seismic surveying, the purchase of a licence and the annual licence rental fee.

Preparations for an Offshore Drilling Operation

Seismic surveying is carried out by specialist companies who operate

spe-cial seismic survey vessels and sophisticated equipment and employ highly

trained seismologists and geologists Although it is quicker and cheaper to

carry out a geophysical survey over the sea than on land, it is still very

expen-sive, and oil companies often club together to pay for surveys of large areas

before going their separate ways to investigate smaller concessions at their

own individual expense The survey company is us,ually hired on an area

basis, for example to investigate a particular block, and payment is made on

the basis of the number of 'line miles' of seismic shot The price the survey

company charges reflects not only its own costs but also the importance of its

findings to the oil company in accurately determining the position of possible

oil- or gas-bearing geological structures, thereby saving the operator

expen-sive but wasted drilling time

As already explained, most offshore operators hire the services of

specialist contractors to carry out nearly every function onboard a drilling

unit including the actual drilling operation itself, and their own equipment

costs are therefore low A few majors such as Shell and BP, however, still

own their own rigs, unlike most oil companies which hire drilling

contrac-tors' units and crews on a time basis

As the quest for richer oil reserves pushes out the frontiers of exploration

into more northerly and deeper waters, so offshore rigs are becoming more

robust and sophisticated However, these improvements are reflected in the

enormous cost of a new rig The latest semi-submersibles, which can drill

holes 25,000 feet deep in all but the severest weather in water more than

2,000 feet deep, cost more than $110 million to build, even before the drilling

tools are put onboard Their capital cost can be split into three main

compo-nents: the basic hull, the drilling equipment and the ancillary equipment

such as the power plant, auxiliaries and accommodation The basic hull

gen-erally makes up half of the total cost, with the drilling package amounting to

and third and the ancillary equipment the remainder However, these

prop-ortions vary with the degree of sophistication of the unit and the unit's type

With such an enormous investment required, rigs are normally only ordered

from building yards on the basis of secure work contracts for a period of

months or years ahead

Whether it is hired from a contractor or owned outright, the largest single

cost to the operator is invariably that of the drilling rig He normally starts

paying for its hire as soon as it has completed its previous contract with

another operator, and the cost of relocating it is one of the major factors

con-sidered in its selection Towage costs, if these are involved, vary with the

length and the difficulty of the tow, but are usually high In this respect, an

38

Preparations for an Offshore Drilling Operation

advantage of drill ships over nearly all jack-ups and most semi-submersibles

is that they travel under their own power, often at considerable speed, andthere are no towage costs involved For newly-completed rigs built in distantyards, carriage aboard special semi-submersible heavy lift ships, usuallyoperated by specialist Norwegian or Dutch heavy-lift shipping companies, issometimes chosen in preference to prolonged, difficult and risky oceantowage

Preparations for an Offshore Drilling Operation

The hire costs of mobile offshore drilling units vary considerably with thetype, sophistication and capabilities ofthe individual unit, with the overrid-ing factor being the current price of crude oil, which determines demand to

a large extent Because of the worldwide slump in exploratory drilling ity caused by the low oil price, day-rates were generally far lower in early

activ-1987 than rates obtaining only a year earlier for similar rigs Day-rates forsemi-submersibles peaked in the period 1980-82 with sums of over $90,000being the norm for some types, but by the end of 1986, a rig ofthis type could

be hired for less than $10,000 a day, and numerous units of all types werestacked all over the world, keeping hire rates at a ruinous low for many dril-ling contractors There were calls for the scrapping of older rigs to relieve theover-tonnaging that had dogged the rig market since the mid-seventiesbuilding boom, and this has been put into effect, with the fleet's size of about

750 units worldwide beginning to show a net loss

The overall cost of hire will naturally depend on the time the drilling tractor takes to drill the well, and this can depend on many factors, not least

con-of which, in the North Sea and other hostile areas, is the weather Wells inthe more placid southern half of the North Sea may only take about 65 days

to drill, while 100 or more days might be needed for a well of the same depthfurther north, where time spent 'waiting on weather' may account for a con-siderable part of the total cost But the weather is only one of the hazardswhich may extend the drilling period Problems in the hole, such as lost andirretrievable equipment necessitating a deviated hole being drilled round theproblem zone are by no means uncommon, and a planned three-month wellcan easily end up taking five months or more to actually complete

The other operating costs during drilling as far as the operator is cerned are mainly the fees charged by the specialist firms for the servicesthey provide during the drilling operation These expenses are for supplyboats, anchor-handlers, helicopters, materials such as pipe, casing, mud andchemicals, fuel and water, and well and mud logging services

con-Mud logging and well logging is invariably carried out by specialist panies using their own personnel and equipment While mud logging can becontinuously carried out whilst drilling, well logging sometimes requires thehole to be emptied of equipment like drill pipe, and payment is consequentlycharged on a pounds-per-hole basis

com-Almost none of the oil companies use their own supply boats, which, inline with the fall in rig hire rates, can, (in early 1987,) be chartered for sub-stantially lower rates than in previous years The same applies to helicopters,which in the North Sea, the Arctic and many other parts of the world are

Preparations for an Offshore Drilling OperatJon

indispensable They are normally hired for a monthly fee, plus an additional

hourly rate when the machines are actually in use, on top of which the oil

company pays for the fuel used on its flights

Drilling rig crew wages are normally paid by the rig owner or manager,

and an operator will only be responsible for the wages of its resident

super-visor and a small number of experts which may include a drilling engineer,

a geologist and a materials co-ordinator For the drilling contractor,

how-ever, the wage bill is a major cost, since a semi-submersible in drilling mode

is normally crewed by between fifty and sixty personnel, excluding any

operator's staff or 'service hands' The driliing contractor also pays for his

crew's food, their insurance and their travel to and from the base heliport,

although the crew are transported out to the rig in the helicopters paid for by

the oil company

North Sea drilling rig wages at one time reflected the input of expertise

contributed by American oilmen in the development of offshore drilling

skills and techniques, but in recent times the American element has largely

diminished to the point where only a few supervisory staff remain The

wages of North Sea rig crews have fallen behind those of their American

counterparts, and have been restrained somewhat by the slumping oil price,

but by shore standards they are still high To keep this in perspective,

how-ever, it must be remembered that rig crews often do arduous and

unsatisfy-ing work in difficult conditions for twelve hours out of every twenty-four, for

fourteen days at a time On the rigs that they spend half their working lives

on their only comforts are good food, clean accommodation, freshly

laun-dered working clothes for every shift and a daily movie show, and many

shoreside workers would find an isolated offshore existence in these sort of

conditions far from ideal But virtually every company involved in the

dril-ling business appreciates this, and is usually prepared to reward its offshore

employees accordingly

WELL TYPES

To most of the personnel on an offshore drilling rig the well being drilled is

simply 'the hole' whatever its purpose in the operator's scheme, but every

well can be labelled according to its function Most of the names likely to be

met with are listed below

Wildcat Well: An exploratory well drilled in an unproven area, remote from

any existing producing well In some areas only about one wildcat in ten is

productive, but even the dry holes yield valuable information from core

sam-ples about the geological structures in the area, and on pressures and

temp-eratures to be expected in other wells drilled The drilling of a wildcat might

42

Preparations for an Offshore Drilling Operation

be in complete isolation from other rigs, or there might be other units'wildcatting' in the same area Only about one wildcat in forty worldwideyields petroleum, although in the North Sea the figure is one in four and inthe US it is one in nine However, the yield might still not be 'commercial'.Exploration or Exploratory Well: A well drilled in a search for a new reser-voir of hydrocarbons It might be a wildcat well, or a well drilled on an exist-ing field to seek a new productive formation

Discovery Well: An exploration well that produces evidence of oil or gas incommercial quantities Wells that produce 'shows' of uncommercial hyd-rocarbons are labelled 'dry holes' or 'dusters'

Oil Well: A well that produces hydrocarbons in a liquid state from an ground reservoir

under-Gas Well: A well that produces hydrocarbons in a gaseous state from anunderground reservoir Sometimes a well that was hoped to be an oil wellyields gas instead If the gas is in commercial quantities it might be sold whenequipment is mstalled for exploiting it

Dry Hole: A well in which no commercially significant evidence of bons is found Dry holes are usually plugged and abandoned They aresometimes called 'dusters'

hydrocar-Appraisal Well: A well drilled following the drilling of a discovery well inorder to determine the extent of the reservoir or field Several appraisalwells may be drilled, each close to a discovery well, in order to map out theoutline of a new field In this case they are called 'step-out wells' or 'deline-ation wells'

Development Well: A well drilled after a discovery well, and usually afterseveral appraisal wells, to commercially exploit an oil or gas field Mostdevelopment wells are drilled from fixed platforms built once the field hasbeen appraised They may be 'vertical wells', but are very often 'deviatedwells'

Step-Out Well: A well drilled close to a discovery well but in an unprovenarea, so that the boundaries of the producing formation can be determined.Depending on the phase of the operator's programme, the step-out wellmight be further classified as a development well, an appraisal well or adelineation well

43

Preparations for an Offshore Drilling Operation

Infill Well: One of a number of wells drilled to fill in between establishedproducing wells on a block or concession They reduce the spacing betweenwells in order to increase production from the reservoir

Stripper Well: A well in its later stages of production, when its yield is ing For US pricing and taxation purposes a stripper well is defined as a wellproducing 10 barrels per day or less The term is more usually heard onshorethan offshore, since an offshore well producing as little as this would not becommercially viable

reduc-Satellite Well: A well in a field that is being tapped by a production form's own wells, but which was drilled independently, and usually verti-cally, by a mobile rig and later tied in to the platform by a sub-sea productionpipeline

plat-Re-Entry Well: A well that is re-entered following earlier plugging for somereason Wells are usually plugged and abandoned if found dry, or pluggedand suspended if 'commercial' but not required to be completed at the time

of drilling They might be re-entered and production equipment installedwhen the price of oil is more favourable

Vertical Well: A well that is drilled without intentional deviation from thevertical, except perhaps to sidetrack an obstacle

Deviated Well or Directional Well: A well drilled at an angle from the cal, so that a reservoir is tapped at some distance horizontally from the sur-face location Most wells drilled from fixed platforms are deviated wells

Many types of crude oil are found in the ground in different parts of theworld, but all are natural products of the earth, complex mixtures of chemi-cal compounds called 'hydrocarbons' and 'non-hydrocarbons' In this

natural state crude oil is properly known as 'petroleum'

Geologists and other scientists have differing theories on the precise gins of petroleum, but there is no doubt that it occurs mainly in what aretermed 'marine sedimentary rocks', especially in sandstones, dolomites andlimestones Like coal, petroleum is a fossil fuel, and since it is an organic sub-stance, organic matter must have been present during the process by which

ori-it came to accumulate in the reservoirs ori-it is today found in This process maynever be completely understood, but it is thought to have been a combina-tion of a chemical and bacterial action that took place millions of years ago

Preparations for an Offshore Drilling Operation

THE FORMATION OF HYDROCARBONS

In prehistoric times the earth was covered by dense vegetation and by seas

that teemed with minute living organisms, or plankton As quantities of

plants and animals died, layers of dead and decaying organic matter built up

on the ground and on the seabed Meanwhile, sand, gravel and earth was

being eroded from the land by the weathering action of wind, water and ice

The resulting silt flowed down the rivers to merge with the decaying

plankton, forming sedimentary deposits on the flood plains of the lower

river reaches, in the delta estuaries and on the sea bed of the waters beyond

the coastline

These deposits continued to build up over the ages, each layer in turn

becoming buried by another Sealed deep inside them, starved of the oxygen

that was necessary for the slow decaying process to continue, were the dead

marine organisms The lowest layers of sediment gradually became

com-pressed and heated by the enormous layers above, and eventually they

became so compacted that they were tranformed into soft, permeable,

sedimentary rocks such as limestone, chalk, sandstone and shale Under the

combined effects of pressure, heat, bacterial action and age the organic

deposits in these rocks, their oxidation halted, were eventually tranformed

into the substance that we now know as petroleum, and as more and more

weight was added above, so the petroleum was forced to seep into adjacent

rocks

MIGRATION

The newly-formed oil was trapped in the minute pores of the coarse-grained,

permeable, sedimentary rock, forming a large 'reservoir' in the same way

that a sponge can hold a reservoir of water But since the oil was lighter than

water it was able to move, or 'migrate', within the rock and displace any

water ahead of it Its natural tendency was to migrate upwards through the

pores of any permeable rocks, although it moved perhaps only a matter of a

few centimetres each year Eventually, however, it either reached the

sur-face of the earth and seeped out of the ground, or it arrived underneath a

layer of hard, fine-grained, impermeable rock through which it could not

pass In many cases it was possible for the petroleum to continue migrating

horizontally below this 'caprock', but if the geological structures around the

caprock were such that it could not move any further, an accumulation

formed underground, rather like air trapped under a cup immersed in water

This trapping, millions of years ago, of large volumes of hydrocarbons

thus made possible today's oil and gas drilling industry which searches for

new energy accumulations for humanity The science of petroleum geology

is devoted to discovering where these accumulations are likely to exist

46

Preparations for an Offshore Drilling Operation

RESERVOIRSPetroleum geologists classify hydrocarbon accumulations in three types,termed 'anticlinal traps', 'fault traps' and 'stratigraphic traps' By far themajority of known offshore reservoirs occur in either anticlines or faults,which are known collectively as 'structural traps' Examples of structuraltraps are 'domes', 'faults', 'folds' and 'unconformities' In domes and foldsthe petroleum is trapped at the top of the structure, while in faults andunconformities it is trapped when an impermeable layer lies next to a perme-able layer containing the petroleum

ANTICLINESThe most common form of trap is the 'anticline', which is a formation with

an upwardly folded convex structure, looking like a hummock ofsandwiched rocks lying deep underground In an anticline a coarse-grained,porous, permeable reservoir rock is capped by a fine-grained, relativelyimpermeable formation such as clay, shale, marl or salt that has folded up toform a dome or inverted bowl, leaving a cavity below the summit in which oil

or gas can accumulate Inside the trap, oil will lie on top of any water sent, and any gas will lie over the oil

pre-FAULT TRAPSFault traps are commonly found where there are sideways displacements ofgreat sections of subsurface rocks Millions of years ago great stresses causedsections of the earth's crust to crack, and the two split, or 'faulted', faceswould be forced to slide across each other so that the formations lying in eachface were no longer lined up with each other

Occasionally it happened that a permeable layer moved so that it nowfaced and was sealed by an impermeable layer which was at a shallowerdepth A 'fault' trap was thus formed, any petroleum accumulating in itbeing prevented from escaping by the barrier of the impermeable layer

The existence of a fault trap does not guarantee the existence of oil in thetrap, however Sometimes further movements of the earth's crust allowedpreviously trapped oil to leak out, and in other cases oil never reached thetrap This partly explains why so many 'dry holes' are drilled after a fault traphas been accurately located by geologists

Large petroleum accumulations in fault traps have been found in theNiger Delta, the US Gulf coast and in SE Asia, and many of the reservoirs

in the North Sea are in this type of trap

47

Preparations for an Offshore Drilling Operation

Stratigraphic traps are seams of oil trapped inside envelopes of impermeablesealing rock such as impervious clay or shale which prevent further migra-tion of the oil The shape of the trap is not caused by the structure of the sur-rounding rock formations but only by the sedimentary process that left avoid in the rock while the surrounding sediments became hard Stratigraphictraps are harder to detect than faults or anticlines, and many of the dis-coveries in them too date have been accidental However, as geophyisicalsurvey techniques become more sensitive and accurate, many more willprobably be found all over the world

Unconformities are usually regarded as stratigraphic traps but they arereally structural features In an unconformity trap the reservoir rock isslanted up towards and cut off by a fine-grained, impervious rock that liesacross its end, resulting in a wedge-shaped reservoir Several importantNorth Sea oilfields, as well as the North Slope of Alaska are over unconfor-mities, but they are relatively uncommon compared with other trap types Insome cases the fine-grained rock sealing the wedge-shaped reservoir mayactually be the source of the petroleum, migration in this instance havingbeen downwards rather than upwards

The objectives of the oil companies in their search for hydrocarbons,whether on land or at sea, are to locate potentially oil- or gas-bearing struc-tures such as the anticlines, faults and stratigraphic traps described above, totest them for hydrocarbons, and then exploit any reservoirs of oil or gasfound Unfortunately, whether or not oil or gas actually exists in a particulararea cannot be determined by interpreting geological maps or even by read-ings of the most sensitive instruments on the surface, but if the right kind ofpotentially oil-bearing structure can be identified, a chance of finding oil orgas in it may exist The proof of the matter can only be established then byexploratory drilling

In the early days ot'land exploration for oil, before the advent of marinedrilling, petroleum geologists were not equipped with sophisticated survey-ing devices like those used today, and they had to rely to a great extent on'field maps' to assist in the location of the right types of formation Fieldmapping, in which rocks were collected from all parts of the area underinvestigation, helped to establish the ages and chronological sequence ofsedimentary rocks and to determine the position of possible oil-bearingstructures

Preparations for an OtTshore Drilling Operation

Nowadays, although petroleum geologists have the benefit of highlysophisticated survey equipment, field mapping is still important in the searchfor oil At sea, cores are taken by special core-drilling vessels from the seabed in the area under investigation, and are sent to a laboratory where theyare examined, first by palaentologists for fossils, and then by sedimen-tologists to determine their nature and that of the basin where they accumu-lated Geochemists will also test the samples to determine the degree towhich the organic matter inside them has been changed by the effects ofpressure, heat and age The findings of these three specialists are togetherused to reconstruct the history of the sedimentary basin under investigation

To enable particular formations to be dated, which is essential to thesearch for petroleum, geologists have had to devise a 'geological time scale'against which rocks and their fossils could be compared This time scale cov-ers the periods within the great 'epochs' of the earth's history that petroleumgeologists are really interested in - the 600 million years or so from the 'prim-ary' or 'Palaeozoic' epoch that followed the formation of the first life forms,through the 'secondary' or 'Mesozoic' epoch that began about 220 millionyears ago, to the relatively modern 'tertiary' and 'quaternary' ('Cenozoic')epochs

Within each epoch, (sometimes called 'ages' or 'eras') are 'systems' whenrocks of different types were laid down Within the Mesozoic epoch, forexample, the Triassic, then the Jurassic, and finally the Cretaceous rock sys-tems were laid down These are further subdivided into the Upper andLower Cretaceous, Upper, Middle and Lower Jurassic, and so on, each sub-division often being associated with a particular geographical area where therock type is found

Oil and gas might be discovered in the rocks of almost any of the epochsand systems, with the exception of the most modern formations of theQuaternary epoch However, in a particular basin it is more likely to befound in some than others In the North Sea, for example, numerous fieldshave been discovered in Jurassic rocks, while only one has been developed

in the Carboniferous rocks of the Palaeozoic epoch and none have beenfound in the Cretacious

As an exploratory well gets deeper, many different formations of varioustypes of material are encountered, such as claystone, shale, sand and limes-tone, each of a distinctive colour and sometimes mixed with other materials.These are the rocks of the different epochs and systems which have been laiddown in their chronological order, so that the oldest formations are at thedeepest depths and the youngest are at the sea bed To aid identification by

51

Preparations for an OtTshore Drilling Operation

the geologists on the rig and in the shore laboratories, each layer is given adifferent name which is often associated with the drilling locality, so that alayer of pink and white coloured limestone, grading to marl and encountered

by the drill bit in a depth of between 8800 and 9100 feet in the Central NorthSea might be recognised as the 'Ekofisk' formation, while the chalky whitelimestone with grey marl interbeds found just beneath it might be detected

as the 'Tor' formation

Whilst on land some idea of the types of underground formations can begained from the study of the surface geology, it is obviously not easy toobtain rock samples from deep below the sea-bed, and other means of iden-tifying potentially petroleum-bearing structures have had to be developed.Geophysical surveying applies the principles of physics to the study of geol-ogy, and this is the means by which nearly all offshore drilling locations arefound Three geophysical survey methods are used to examine formationsbelow the sea bed: magentic, gravimetric and seismic One or more of thesemethods is invariably employed to make a detailed survey of any petroleumprospecting area before any drilling commences

MAGNETIC SURVEYS

A magnetic survey measures the effect of the magnetic properties of rocks

on the earth's magnetic field, and is carried out by an aircraft carrying aninstrument called a 'magnetometer' Certain types of rock that contain ironare magnetic, whereas oil-bearing rocks are usually non-magnetic, and var-iations in the magnetometer readings are used to give an image of the under-ground layers This method does not provide very detailed information but

it is useful for forming an overall picture of a prospecting area It was oftenused to good effect in the early days of North Sea exploration

Gravity, or gravimetric, surveys measure the effect of variations in thedensity of different rocks on the earth's gravitational field Subsurface rocksexert different degrees of 'pull' on the gravitational force at the surface, and'gravimeters' are used to measure these variations However, this methodagain provides no detailed information, and is of limited value offshore

SEISMIC SURVEYS

Seismic surveying provides the petroleum geologist with accurate detailsabout depths and extents of layers of sedimentary rock, and is by far the mostwidely used survey method offshore The method is based on the principlethat different rock types respond with different absorption and reflectioncharacteristics to shock waves which are produced by an energy source

Preparations for an OtTshore Drilling Operation

released either just below the ground surface or just below sea level The

'echoes' sent back by the rock formations are recorded on the surface by a

seismograph (an instrument which measures vibrations) and give a more

detailed picture of the subsurface formations than the other geophysical

sur-veying methods

An offshore seismic survey is made by a special seismic survey vessel

tow-ing devices capable of generattow-ing the necessary shock waves and, behind

this, a recording cable possibly two or three miles in length The energy

source is usually an array of many different sized compressed-air guns that

look rather like artillery shells, each of which produces a powerful burst of

acoustic or sound energy in a different frequency band to its neighbours

These 'sleeve exploders' are strung out on a cable called a 'streamer' which

is several hundred feet long and towed at a depth of several feet below the

surface If the guns are all fired at the same instant they produce an

extremely strong sonic pressure wave for a very short duration

The pressure wave travels down through the water and into the underlying

sediments and rock structures thousands of feet below the sea bed Some of

the wave energy is reflected directly back to the cable from the rock layers,

while some is refracted as it enters or leaves a layer before returning to the

surface

Preparations for an OtTshore Drilling Operation

The recording cable is towed just below the surface and is fitted withnumerous sensitive hydrophones which detect the returning pressure waves.Transducers convert the pressure energy into electrical impulses and theseare transmitted along wires inside the towed cable to recording instruments

on the ship

The instruments basically measure the time taken for each energy pulse toreturn, and they relate this to the distance from the energy source to thereceiving hydrophone The voltages received from the numerous channels inthe cable are then processed by a computer which converts them to data thatcan be written on magnetic tape

When the sea survey is complete the magnetic tapes are sent to a sing centre ashore which contains mainframe computers that can efficientlyhandle large amounts of data, although some vessels are now able to carryout processing onboard, enabling interesting 'lines' to be re-shot withoutdelay After pro.;essing the tapes, a seismogram - a cross-sectional viewthrough the earth below the line of the shoot - is produced, and from thisinterpreters build up contour maps of the structures under the sea bed.These maps are used, like field maps, to pinpoint the positions of potentialoil-bearing structures

proces-It is usual nowadays to survey an area and process the data in three sions 3-D surveying is much more expensive than conventional 2-D survey-ing, but the cost of the survey is normally a relatively small part of the overallexploration budget of the oil company However, ships are not normallyable to carry the powerful mainframe computer equipment necessary to pro-cess 3-D data, and 3-D processing usually has to be done ashore

dimen-Oil companies often club together to pay for seismic surveys of a large areathat all of them might be interested in, and trading of seismic informationgoes on to their mutual benefit But even when the geological structurerevealed by seismic or other survey methods looks promising, there is still nocertainty that oil is present Only exploratory drilling can prove whether ornot the geological traps located by the surveys actually contain oil The timenow approaches when the operator will have to move his rig in and startexploratory drilling

THE DRILLING RIG SITE SURVEY

Apart from surveys of large areas to detect the presence of structures thatmight bear hydrocarbons, an offshore operator also commissions a survey ofthe precise location where he intends a drilling rig to operate This is also car-ried out by a seismic survey vessel, and covers an area of perhaps 30 square

55

Preparations for an Offshore Drilling Operation

kilometres to encompass the scope of sea bed in which the rig's anchors will

be laid and the hole will be 'spudded', or begun The drilling rig site survey

reveals information on bathymetry (water depths), sea-bed features such as

obstructions, the geology of the sea-bed, anchoring conditions and any

prob-lems foreseen for drilling operations, such as the presence of shallow gas

pockets This information helps the operator decide how and where exactly

his rig should be positioned to drill down to the target formation

CHAPTER 3:

OFFSHORE DRILLING PLATFORM TYPES

When an operator requires a well drilled, the rig he selects must be capable

of doing the job efficiently and safely, but at the same time it must be of atype suitable for the nature of the operation It would be economic folly, forexample, to build a massive fixed platform over the proposed location of anexploration well where there was a ninety percent chance that no hydrocar-bons would be found in commercial quantities, so in this case a mobile rigwould normally be hired do the exploratory work and a platform might bebuilt on the location later if the well proved commercially productive Ingeneral, therefore, exploration wells are drilled by 'floaters', which includesemi-submersibles, drill ships and barges, or by self-elevating 'jack-up' rigs,while development wells, drilled to exploit a field already discovered, aremostly drilled from fixed platforms

Whether a floater or a jack-up is used for the exploratory drilling dependsmainly on the water depth Jack-up rigs can normally be hired more cheaplythan floaters, but if the water is not shallow enough for jack-ups to stand in,they obviously cannot be used The choice would then lie between a semi-submersible or a drill ship (or barge in some areas), and many factors would

be taken into account before the operator made his decision These wouldinclude the water depth at the proposed location, the prevailing weather andsea conditions there, the amount of deck load intended to be carried aboardthe rig, and the logistic problems of keeping the unit adequately suppliedfrom a shore base

Even when a particular type of craft has been decided upon, there stillremains the question of the capabilities of individual units, since each rig of

a particular class may have been modified at stages in its life to fulfil differentfunctions for different operators, and unlike classes of ships, no two rigs of

a class are usually exactly alike Every detail of a rig's layout and equipment

is therefore closely examined before an operator awards a drilling contract

Generally, offshore drilling platforms can be grouped under three maincategories: fixed platforms with floating drilling tenders, self-containedfixed platforms, and mobile units The mobile units are the 'rigs' that do vir-tually all the exploratory drilling for the oil and gas industry Fixed platformsare basically production units, but because most have facilities to drilldevelopment wells they are described briefly below

Offshore Drilling Platform Types

FIXED PLATFORMS WITH FLOATING DRILLING TENDER

In this type, which is seen mainly in the Middle East, the US Gulf, Nigeriaand Lake Maracaibo, a small platform is constructed on piles driven into thesea bed over a discovered reservoir, and a drilling derrick and drawworks aremounted on the platform The drilling tender, which is usually a barge, ismoored close alongside the platform and carries the power supply, mudpumps, mud pits, pipe racks, storage space for miscellaneous equipment,and accommodation for the platform crew A helicopter platform is gener-ally also fitted on the tender A 'catwalk' or 'bridge' allows the crew to crossbetween the two units, while power cables and hoses for fuel, water and dril-ling fluids are connected from the tender to the platform's drill floor.After the drilling phase of the development, the platform usually hasremote-controlled production and pumping equipment installed, and thetender departs, to be used again at another platform

At one time this was the most common type of offshore drilling platform,but there are relatively few remaining nowadays They are obviously unsuit-able for the more hostile sea areas and would normally only be designed tooperate in sheltered waters A great deal of time would otherwise be wasted

in the tender disconnecting and 'standing off' to ride out bad weather, cially if it is not self-propelled and requires towing, which most do They arenot, therefore, seen in areas such as the North Sea

espe-SELF -CONT AINED FIXED PLATFORMS

These are massive platforms erected on the sea-bed, and are generally built

of either steel or concrete There are many designs, derived from two basictypes: tubular steel structures built on a shore construction site, from wherethey are floated out to the location and pinned to the sea-bed by piles, andreinforced concrete gravity structures of such enormous mass that, oncetowed to the location and positioned vertically on the sea bed, they can standfreely

The largest platform of the concrete gravity type reaches nearly 800 feetabove the sea bed and weighs more than 600,000 tons in air Like other grav-ity platforms it was built, with the exception of its steel topside modules, in

a drydock, and towed to its location in an upright position and sunk Mostgravity platforms have a ring of cylindrical concrete tanks surrounding theirbase, which serve as storage for oil awaiting transportation

The tubular steel fixed platforms may be as tall, or even taller than theconcrete type, but they are naturally much lighter They are normally con-structed on their sides in a dock and floated to the location either on a barge

59

Offshore Drilling Platform Types

from which they are tipped to sink vertically, or using steel 'buoyancy tles' attached to their legs Once on location, the buoyancy bottles are care- fully flooded so that the steel 'jacket' (as the base structure is called), tips and sinks vertically Piling is then driven into the sea bed around the legs to pin them Including the length of the piles, some platforms of this type are more than 1100 feet tall to the peak of the drilling derricks.

bot-Drilling from production platforms is mostly 'directional drilling' scribed in Chapter 5) in which the hole is deviated at a slanting angle after being made vertical for a certain distance This enables the extremities of some fields to be exploited from one central platform, from which up to sixty directional wells might be drilled (One platform in the US Gulf has 96 dril- ling slots.) Horizontal distances from a platform of about two miles can be reached using this technique, which would otherwise necessitate the re-Ioca- tion of a mobile rig many times to drill each well vertically.

(de-The largest fixed platforms are able to accommodate two drilling rigs as well as full production facilities and accommodation for several hundred personnel The power generated on board these enormous structures is often enough to supply a small city Electric power for the drilling equipment may

be provided by conventional diesel plant, but it is frequently generated by gas turbines utilising gas diverted from the production equipment The dril- ling and well control equipment and most of the drilling procedures are simi- lar in most respects to those used on any jack-up rig, and the drill crews are often hired from a drilling contractor who may also operate mobile units Marine personnel involvement on fixed platforms is usually limited to sea traffic direction and co-ordination and the maintenance of lifesaving equip- ment, although many installation managers are former shipmasters.

As each well is completed on the platform it is then brought 'on stream', provided sub-sea pipelines have been installed to transport the oil or gas to shore, or tanker-loading facilities or storage facilities exist at the location The drilling rig, meanwhile, can be hydraulically jacked over several feet to another 'drilling slot' to drill the next well in the development programme.

In many instances on platforms the rig or rigs have completed their functions and are removed, or else retained onboard for 'workover' (well mainte- nance) purposes or for use in a later stage of the development programme.

MOBILE DRILLING RIGS

The category of mobile rigs consists of two general types: those which rest on the sea-bed during drilling and those which float The group of floaters includes semi-submersibles, drill ships and barges, while the bottom-sup- ported rigs are jack-ups and submersibles.

Offshore Drilling Platform Types

SUBMERSIBLES

When drilling began in the swamps and marshes bordering the US Gulfstates and in the creeks of West Africa, the waterlogged land presentedproblems for the teams sent ahead of the rigs to prepare the drill sites andtheir access ways In many cases, transportation by water was the only ans-wer, and dredgers sometimes had to be brought in to dig shallow access can-als

Meanwhile the drilling rig, accommodation and storage facilities weremounted on a wide, flat-bottomed, shallow-draught barge which was towedonto the location and ballasted down so that it sat on the bottom, effectivelybecoming a 'submersible' The well was drilled, and the barge was then de-ballasted to float it out There was a narrow slit in the barge hull runningfrom the centre of one end to a position just beyond the mid-point of thedeck, immediately below the derrick This not only permitted drillingthrough the hull but also allowed safe clearance during the tow-out over thewellhead equipment which was left protruding from the swamp bed Not allthese 'swamp barges' were designed to be submersible, many being able todrill whilst afloat in shallow water These are still used in some parts of theworld

From 1948 onwards a larger type of drilling platform evolved from thesubmersible 'swamp barge' in which the hulls of the vessel were ballastedand allowed to submerge and rest on the bottom, while the drilling andaccommodation deck remained well clear of the water on tall pillars Thehulls were either ship-shaped or were a framework of large-diameter hollowpiping, while the deck, which became known as the 'Texas deck', was sup-ported by posts or stanchions In later versions buoyant columns, called'milk bottles' because oftheir shape, were fitted to provide the necessary sta-bility when submerging or refloating, and today's semi-submersibles aremostly derivatives of this later design

Many submersibles were built, but few now remain, mostly in the Gulf ofMexico area and Nigeria They are not usually self-propelled and they aretightly restricted in the depths that they can drill in, which is the main reasonwhy they have been superseded by other types of rig One unit is able to drill

in 100 feet of water, but the largest examples were able to drill with their'Texas decks' 25 feet clear of the sea and their pontoons resting on the bot-tom in 175 feet of water

Even at such depths there are still considerable wave forces, and ing' , or the washing away of the sea bed by underwater currents, sometimesoccurs around the hulls of submersibles that could eventually upset the plat-

'scour-63

Offshore Drilling Platform Types

form if not controlled Divers are therefore used to reinforce the sea-bedalongside the pontoons with sand bags Scouring is also a problem for thetype of unit that has largely replaced the submersible, namely the 'jack-up'rig

SELF-ELEVATING (JACK-UP) PLATFORMS

'Jack-ups' comprise about half of all the mobile rigs in the world, and areused for shallow-water drilling They are self-contained platforms resembl-ing a flat bottomed barge hull with three, four or more vertical legs fittingthrough openings on the outer hull edges These legs have 'teeth' notchedinto them and can be raised or lowered by a jacking mechanism on the deckthat usually employs a hydraulic or electric rack and pinion arrangement

In its drilling mode the barge hull is raised on its legs well out of the waterand serves as the drilling, storage and living platform It is thus firm and sta-ble and experiences none of the motions due to the sea that affect floaters.After the well has been completed the 'drilling package' , which includesthe drill floor and the derrick, is 'skidded' clear of the wellhead, and thebarge hull is jacked down the legs until it floats freely Jacking continues,with the legs now being supported by the floating hull rather than the hullbeing supported by the legs The legs are jacked up at least until their bottomends are sufficiently clear of the sea bed to permit safe clearance during therig-move to the next location

In its 'transit condition' the legs of a jack-up rig can usually be seen ing high above and around the drilling derrick, giving an impression of insta-bility There have been several accidents during jack-up transits, but the vastmajority of rig-moves are carried out in complete safety

tower-On arrival at the next location the legs are jacked down until they touchthe sea-bed The jacking continues after a test period called 'pre-loading',the legs penetrating the sea-bed for some distance while the barge hullbegins to climb up the legs When the hull is high enough to be clear of thehighest waves expected at the location, the legs are locked and remain in thisposition until the well has been completed

Most jack-ups have vertical legs, but several designs incorporate legswhich slant outwards at the bottom to obtain a wider standing 'spread' andbetter stability The legs are all independently jacked, and their position can

be adjusted so that the barge stands horizontal on a sloping sea bed The legs

of some units are fitted with a large, flat steel frame at their lower ends, led a 'mat' This affords better stability on some bottom soil types andreduces the danger of capsize due to scouring

cal-Offshore Drilling Platform Types

Jack-up rigs are normally stable in their drilling mode, although therehave 'been some instances of rigs collapsing and sinking when a leg has eithersunk into the sea bed or its steel structure has failed They are, however,designed to withstand strong wind and wave pressures and are usually only

at risk from the sea during the critical hours of the jacking phase when thehull is either just leaving or just returning to the floating position, and duringbad-weather transits when their low 'freeboard' allows water to comeaboard the hull easily

Early types were dogged by the excessive penetration by their legs of softsea beds, but this has largely been eradicated in recent types by the fitting oflarge feet called 'spud cans' or 'spud tanks' at the bottom of the legs How-ever, as the legs of modern rigs get taller the problem of structural bendingstresses in them is becoming more difficult to resolve For this reason somejack-ups' legs are shortened by dismantling during long transits when there

is likely to be much rolling or pitching motion

The deepest water normally operated in by jack-up rigs is about 400 feet,which means having legs almost 600 feet tall to enable the hull to be jackedclear, Most ofthe 300-plus jack-up rigs worldwide, however, do not operate

in water more than 350 deep The vast majority of these are not fitted withpropulsion and have to be towed or carried on special ships between loca-tions

The following particulars are for a typical modern jack-up unit that candrill in water of 300 feet depth This is a 'cantilever' type in which, for dril-ling, a strong platform that supports the entire drill floor and derrick (thedrilling package) is moved outboard on tracks until it is projecting over theafter end of the hull An alternative to this design is for the drilling to be donethrough a drilling slot in the barge hull The drilling equipment and its useare described in more detail in Chapter 4 and 5

Distance from centre of fwd leg to centre line of

Maximum distance of rotary centreline from

66

Offshore Drilling Platform Types

JACKING SYSTEM: electro-mechanical rack and pinion type, total jackingcapacity 9000 tonnes

POWER SUPPLY: 3 x 12 cylinder supercharged diesel engines driving 3 x600valternators

DERRICK: Height 44.8m (147ft); base 11m x 11m (36ft x 36ft); load ity 1.39 million lbs (620 tons)

capac-DRAWWORKS: 2000 hp

BOP STACKS: 1 x 13-5/8" 5,000 psi stack (annular)

1 x 13-5/8" 10,000 psi stack (single ram)

1 x 13-5/8" 10,000 psi stack (double Upreventer)

1 x 21-1/4" 2,000 psi (annular)

1 x 21-1/4" 2,000 psi (single ram)

1 x 10,000 psi choke manifold

MUD PUMPS: 2 x 1,600 hp

ACCOMMODATION: 38 x 2-men cabins

2 x I-man cabins

1 x 3-man hospital

Offshore Drilling Platform Types

This ri~ is roughly triangular in hull shape, with the apex of the triangle forming the forward end and the cantilever beams projecting over the after end The accommodation block stands on the hull just aft of the forward leg, and aft of that are the pipe racks The drilling package, which is capable of 'slant drilling' (see Chapter 5) is further aft on the cantilever beams which extend to the middle of the deck It can be skidded completely off the rig and onto a platform jacket so that slant drilling can be carried out from the cor- ners of the jacket, but before transits it is skidded to the centre of the main deck.

Beneath the main deck, inside the hull, are large spaces for generating machinery, electrical equipment, mud tanks and mud pumps, and below this level, in an 'inner bottom', are numerous tanks for 'pre-load water', 'drill water', potable (drinking) water, fuel oil, and drilling fluid or 'mud' There are numerous designs of jack-up in service but most follow this general pat- tern.

SEMI-SUBMERSIBLES

There are several claimants to the distinction of being the designer of the first semi-submersible, but to whoever it belongs the marine drilling industry owes a great deal Because it can operate in deep or relatively shallow water

it is probably the most versatile of all drilling platforms, and for that reason

it has become almost a hallmark of the marine drilling industry 'Semis' have drilled thousands of exploratory wells in virtually every sea area of the world and are used in deep and shallow waters from the Canadian Arctic to the South China Sea.

Offshore Drilling Platform Types

A French design for a semi-submersible warship was patented in 1937, and

in 1943 a British design was patented for a salvage vessel with twin hulls and five pairs of circular columns, rather on the lines of today's semi-submersible rigs But it was the limitation of the jack-up rig to shallower waters that led

to the development in the early 1960s of a type of mobile drilling rig that could be used in deep water or, alternatively, resting on the bottom in shal- low water Basically the industry required a type of unit that, whilst having the ability to float and drill in the depths attainable by drill ships and barges, would have better stability qualities than the conventional floating vessels The column-stabilized semi-submersible, a logical development from the column-stabilized submersible drilling platform, fulfilled the industry's needs.

The foremost problem in drilling in deep water from floaters is heave, the vertical 'up and down' motion of a vessel as it rides the waves One way of minimising heave is to keep the 'waterplane area' of the hull to a minimum, and this was achieved in semi-submersibles by retaining the pillar-shaped buoyant deck supports used in submersibles and by having counterbalancing pontoons at the bottom of the columns in which ballast could be kept.

Many designs of semi have been produced in the last twenty or so years, and most are broadly similar in concept although perhaps widely differing in structure The more modern designs feature a roughly rectangular working deck with two, three or four vertical circular-sectioned columns fitted beneath the deck at each side, terminating in underwater pontoon hulls con- taining large tanks for ballast, fuel and fresh water The columns and pon- toons provide the buoyancy to keep the vessel afloat, and some of the tanks

in them can be ballasted to submerge the vessel to a sufficient depth to maximise stability and minimise movement in reaction to wave forces, thereby providing an extremely stable platform for drilling from In addition

to providing stability, the columns also support the deck, but they are usually assisted in this function by large struts or 'braces' which cross diagonally in fore-and-aft and athwartship directions Other braces cross between the col- umns to provide complete structural integrity.

Semis are usually moored on a location by a system of multiple anchors with chain, wire, or chain-and-wire combination cables, but a few are fitted with 'dynamic positioning' equipment which dispenses with the need for anchors altogether The DP units are more expensive both to build and oper- ate, but are sought after for drilling in deep water far beyond the reach of a jack-up rig and a conventionally moored semi.

Offshore Drilling Platform Types

Semi-submersible design has been the subject of much intense ing effort over the past two decades and the latest vessels are extremelysophisticated compared with older units Conventionally anchored semisoften drill in 2000 or more feet of water, but modern DP units can work infive times this depth without moving off station more than a few yards.Some semi-submersible drilling rigs have been converted to accommoda-tion units, while others have become floating production systems, beingused initially to drill production wells where water depths were too great forthe safe erection of a fixed platform and then to provide the production

engineer-Offshore Drilling Platform Types

The largest examples of this type displace in the region of 50,000 tons fully

loaded, but most semis are about half that size The following particulars

give some idea of the size and capabilities of a typical modern, heavy-duty,

column-stabilised semi-submersible drilling rig The drilling equipment and

its use are described in more detail in Chapters 4 and 5.

POWER SUPPLY: 4 turbocharged diesels each providing 2714 bhp, driving

4 x 600v alternators Power is converted to DC by an SCR system, powering

drilling equipment motors Propulsion is provided by 2 x 2000 bhp motors in

each pontoon.

MEAVE COMPENSATION: Motion compensators have 600,000 lbs

capac-ity with a 20ft stroke There are 6 riser tensioners, each with 80,000 lbs

capacity and a 50ft stroke Total tensioning capacity is 800,000 lbs.

Guideline tensioners with 16,000 lbs capacity and a 40ft stroke are also

fit-ted.

DRILLING EQUIPMENT: The derrick is 48.8m (160ft) high, with a base

measuring 12.2m x 12.2m (40ft x 40ft) Load capacity is 1.4 million lbs The

crown block is rated at 650 ton capacity and has 7 x 60" sheaves.

The travelling block, hook and swivel are each rated for a 650 ton capacity.

There is a 3000 hp drawworks driven by 3 x 940hp DC motors.

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Offshore Drilling Platform Types

Many semi-submersible designs have been tried; this one is shaped like a cross and has five toons.

pon-79