SECTION JACK UP OFFSHORE OPERATIONS COURSE

THE JACK UP - OFFSHORE OPERATIONS COURSE - PIDC

SECTION 1 THE JACK UP

CHAPTER 2 ENVIRONMENTAL LOADS ON THE JU 24

CHAPTER 4 THE JACK UP IN ELEVATED POSITION - PART 1 50

CHAPTER 5 THE JACK UP IN ELEVATED POSITION PART 2 59

CHAPTER 1 GENERAL ON JACK-UPS

1.1 INTRODUCTION

Jack up drilling rigs represent about 60% of the worldwide Mobile Offshore DrillingUnits (MODU's) fleet Transocean Sedco Forex has more than 50 JU’s, which representabout 30% of the fleet within the company and 15% of the worldwide fleet These figuresshow the importance of the JU design for the offshore drilling industry

Compared to other type of drilling rigs, the JU is rather special since it involves specificproblems such as leg penetration, punch through and moving with the legs fully raised

The purpose of this section is to introduce the basic concepts, which have involved thestructural and naval architectural aspects of jack ups The recommendations in the MarineOperations Manual are a result of the analysis, which is rig specific

Historically the JU was built to operate in mild environments up to 250ft of water depth.The modern largest JU's are built to operate world wide with at present up to maximumwater depth up to 450ft

1.2 ADVANTAGES AND DISADVANTAGES OF A JACK-UP

In comparison to semi-submersibles, a jack-up has some definite advantages:

a) Lower construction costs

b) Less personnel required to run the rig

c) Because of (a) and (b) lower day rates

d) The possibility to work over a fixed platform

e) It is cheaper for the operator to use a jack-up:

 Less power full boats to move the rig

 No mooring system required -no lost time to run anchors

 Less maintenance costs

 Surface BOP without sub sea system

 Simple well head assembly

f) Less down time:

 No wait on weather due to motions

 Drilling equipment can be handled faster and easier

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However, the jack-up’s have some disadvantages:

a) Limited water depth The maximum water depth for the largest JU is 450ft.b) Depends on bottom condition The bottom soil conditions may cause a punch

through or deep leg penetration

c) In case of a blow-out the rig can not move off location

d) More fragile Many incidents and damages during moving and because of a

punch through Statistics have shown that over 75% of the incidents occurunder tow or during jack-up/jack-down operations

e) Safe operations require strict procedures

1.3 TYPES OF JACK-UP's

There are two types of jack-up's (Fig 1.1):

1) The independent leg type

2) The mat supported type

3) For both types of rigs the derrick structure can be fixed over a slot in the hull

or mounted on a skid to allow the derrick to slide out (cantilever type) over a

platform (Fig 1.2)

Fig 1.1 Types of JU

SSlot

Typical Extension: 45ft (typ.)

Lateral extension: 12ft (typ.)Note: The hook-load/setback depends

on cantilever and substructureposition

Fig.1.2: Slot type versus Cantilever JU.

1.4 THE INDEPENDENT LEG JU

Most JU's have three legs but some JU's have four legs The leg structure can be thelattice type trusses construction of the K type It is called K type because the tubularsform a horizontal letter K For additional strength, some of the harsh environment jackups have the X-type trusses

The spud cans on the bottom of each leg are generally polygonal shaped structuresdesigned with a heavy point to provide support on the hard seabed as well as to ease thepenetration in soft soil The pressure on the seabed by the spud can is between 25t/m² and

35t/m² (Fig:1.3)

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The independent leg jack-ups are designed for a wide range of bottom conditionsincluding an uneven slanted seabed The leg utilization becomes restrictive when thepenetration becomes excessive The design penetration for most rigs is about 30 ft.However, some of the TSF rigs have worked with a penetration of 150ft.

1.5 THE MAT SUPPORTED JU

Mat supported JU's are designed specifically for very soft bottom conditions The legs ofmat supported JU are round steel columns connected at the bottom to an A-shapedstructure

The large bottom contact area with the seabed provides for a much lower bearing pressurebetween and 2 t/m² and 3 t/m² This type of rig is a logical choice for soft soil conditions.Because the legs cannot be adjusted for a sloping bottom the mat supported units aredesigned to work with a seabed slope of up to 1.5 only

Typical soil pressure:

Independent leg JU: 25-35t/m²Mat type JU: 2-3t/m²

Fig: 1.3 Soil pressure independent leg versus mat supported JU

1.6 TYPE OF JACKING SYSTEMS (Fig.1.4)

The jacking systems for most the independent leg units are of the electrically poweredrack and pinion type Some designs use an electro-hydraulic system to power the rack andpinions For many mat supported units a hydraulic jacking system is used to operatehydraulic jacks and associates yoke pins to fit in pin holes along the column shaped legs

Fig: 1.4 Pictures of jacking house and jacking systems.

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Jack -house and

Jacking

Examples of jacking systems gear trains

Hydraulic jacking system

Joke pin Fixed pin

8

Sea bed

From a structural behavior standpoint, the jacking systems for the independent leg unitscan be classified into two types:

1) The floating jacking system

2) The fixed jacking system

The environmental forces wind wave and current try to overturn the rig These forces

cause bending moments, shear forces and axial forces in the legs.(Fig 1.5) Depending on

the type of jacking system the reaction forces are resisted with a horizontal or verticalcouple at the upper and lower guides

LEG INTERNAL EFFORTS

1.6 THE FLOATING JACKING SYSTEM (Fig 1.6)

Environmental forces as well as gravity forces create bending moments on the JU legs Inthe floating system the leg bending moment is resisted by a horizontal couple of forces

acting at the lower and upper guides (Fig 1.6 and Fig 1.7) This creates a large shear

force on the leg between the guides

Consequently the floating system JU needs heavy leg sections The result is that themoment is mainly resisted by a couple of horizontal forces acting at the upper and lowerguide

Fig 1.6 The floating jacking system

The floating jacking system diagramEffort at Leg-Hull connection

Can't jack up with full pre-load Truss members are

heavy solicited

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Fig 1.7 The floating jacking system -horizontal forces at upper and lower guide.

Typical examples of JU with the floating jacking system are JU's such as the Trident 6,Trident 8 Modec 300 and 400 design

Not al floating jacking system have the rubber shock absorbers installed For example onthe MLT-116C design (like Trident 2, Trident 4,Ron Tappmeyer, D.R.Steward) whereeven though the jacking system is welded fixed to the hull, the structural behavior is of afloating system Because in this design the stiffness of the pinion is small in comparison

to that of the jack house and guiding structure As a result the moment is mainly resisted

by a couple of horizontal forces acting at the upper and lower guides

1.7 THE FIXED JACKING SYSTEM (Fig.1.8)

In this system the stiffness of the jacking unit is high Therefore the leg bending moment

is mainly resisted by a couple of vertical forces acting at the pinions Strong and heavybuild pinions support the vertical reactions As the leg bending moment does not create alarge shear force in the leg between the guides, the leg section can be lighter than with thefixed jacking system To show the difference; the leg sections of the Trident IX (fixedsystem) is 5.7t/m compared to the 7.1t/m on the Trident II (floating system)

The rack-chock system is another example of the fixed system As show in Fig 1.9 the

The bending moment created in the leg after the engagement of the rack-shock is resistedentirely by the later The advantage of this system is an increase in holding capacity Thedisadvantage is that the system is slow.

Fig.1.8 The fixed jacking system - vertical loads at the pinions

The fixed jacking system diagramEffort at Leg-Hull connection

AXIAL FORCES

MOMENTS

(Can jack up with full pre-load) Truss members are not

heavely loaded

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LEG

12

The Rack Shock System

 Main advantage

- increase in holding capacity

 The STDL-780 Mod II system can hold 3500t per chord vertical loads

 Main disadvantage -slow

Fig 1.9 The Rack Shock system 1.8 SUMMARY FLOATING AND FIXED JACKING SYSTEM (Fig 1.10)

THE FLOATING SYSTEM THE FIXED SYSTEM

Heavy legs (7.1t/m) Light legs (4.1t/m)

Lot of drag forces Less drag forces

Normal jacking system Very strong jacking system

Not capable to jack with full preload Capable to jack with full preload

Low sensitive to Rack Phase Difference Sensitive to RPD

EXAMPLE LEG COMPARISON

FLOATING SYSTEM

TRIDENT 9 WITH FIXED SYSTEM

Fig 1.11 Rectangular leg structure

Having the strong jacking system the fixed system offers many advantages

The large normal and emergency lifting capacity gives a rig the capability to jack up anddown with almost full preload Preloading is done to simulate the maximum leg load atthe location to ensure that one of the legs will not punch trough the soil layers in a stormcondition

The strong jacking system improves the rig safety and can save time in cases of large tidedifferences or quick changing weather conditions during preloading In addition, in case

of uneven leg penetration or punch through during preloading, the large jacking capacitygives the operator a better way to respond to the problem and this enhances safety

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K joint

Chord section

Bracing diameter

Horizontal

brace

Leg chord with chord rack Jack

house

14

Because of the lighter leg weight with the fixed system the rig may be dry towed with thelegs full length of legs Time and money are saved by not cutting some leg bays

Compared to the floating system the fixed jacking system is heavier and more expensive.However, these disadvantages are offset by the weight reduction of the legs and the otheradvantages as mentioned

1.9 SOME EXAMPLES OF JACKING SYSTEM CAPICITIES

The pinion capacity is based on three conditions:

1) Normal lifting capacity

2) Emergency lifting capacity

3) Holding capacity

The pinion capacity is rig specific

As an example of the pinion capacity of the Modec 300 design is:

Rating: 220st normal lifting

250st emergency lifting - gravity loads only350st maximum safe holding without damage This is also the preload limit.500st maximum holding under criteria storm conditions This is considered asurvival condition and some localized damage may be incurred requiringrepair before the next operation

Jacking speed under 220st load per unit:

Approximate 1 ft per minute raising hull

1.10 REDUNDANCY IN JACKING SYSTEM

Because the pinion capacity is rig specific the operations manual should be consulted incase one of the pinions can not be used because of a pinion failure or maintenance It isadvisable to consult the TSF engineering department and/or the manufacturer

The loads will be distributed among the remaining working pinions In the example of

Fig 1.12 Example of pinion redundancy-load distribution One pinion damaged

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0 200 400 600 800 1000

Intact Damaged

Pinion 3L

Pinion 1R

Pinion 2R

Pinion 3R

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1.11 THE MAIN CHARACTERISTICS OF A JACK UP

Jack-ups are designed for different water depths and for different environmental criteria.The characteristics of jack-ups vary from, one to another Following are the main

characteristics controlling the capacity or future enhancement of a jack-up Detaileddiscussion of these characteristics will be presented in other sections

1) OPERATING TEMPERATURE: Depended on its design temperature, a jack

up might not be able to work in cold areas such as the North Sea, because ofthe steel properties

2) SURVIVAL CONDITIONS: This includes the design criteria of the rig and

indicates the maximum strength of the rig The survival criteria are waterdepth, wind, wave, current, and deck load capacity, penetration and air gap

4) PRELOAD CAPACITY: It allows simulating the maximum leg reaction on

the spudcan and indicates the possibility of increasing the preloading in case

of suspicious soil

5) JACKING CAPACITY: This indicates the level of gravity load, as well as

the environmental loads to which the jack up can be upgraded to, in case offixed jacking type

1.12 INCIDENTS WITH JACK UP'S

Most serious incidents with heavy structural damage and total loss of the JU occur duringrelocation such as:

 Re-enter in old foot prints

To prevent these incidents strict procedures must be followed This course explains in the

to following chapters the theory and practice to allow the person in charge of the JU's tounderstand the importance of the operations procedures for the JU The MOM of each rig

A punch through is a rapid uncontrolled leg penetration caused by failure of the

underlying soil structure (Fig 1.13)

SAND CLAY

Fig 1.13 Punch through condition Fig 1.14 shows the damage to a small platform caused by a punch through

Fig 1.14 Damage caused by punch through

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Fig 1 15 and 1.16, show examples of damage by pulling a leg with high RPD, old foot

print re-entry pulling legs and a fatigue crack

Fig 1.15 Leg chord deformation by pulling with high RPD

1.13 THE TRANSOCEAN SEDCOFOREX JU DESIGNS AND FLEET

The Transocean SedcoForex fleet of over 50 JU is made up of various designs of whichthe most important are listed below:

1) The Marathon LeTourneau (MLT) 116C Water depth 300' Like the D.R.

Steward, Randolph Yost, Don Tappmeyer, and Trident 2 and 4

MLT 116C 2) The Marathon LeTourneau 53 SC Water depth 300' FGMCClintock and

C.E.Northon

MLT 53 SC

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3) The Marathon LeTourneau 150 44 C Water depth 150' RBF 150 and RBF154.

4) The Friede and Goldman L-780 Mod II Water depth 300' George H.Galloway,

Roger W Mowell, J.T .Angel, and Harvey H.Ward

Friede and Goldman L780 ModII

5) The Sonat Offshore Orion and Sonat Offshore Drilling Water depth resp 300'

and 170' or 250ft Interocean III, Transocean Comet, Transocean Jupiter, and

Transocean Mercury

Sonat Offshore Drilling design (Transocean Jupiter)

6) The Bethlehem JU 100/150/200MC and 250MS Class Water depth between

100 and 250 depending on design Many RBF Series like RBF 110, 205 etc.

Bethlehem JU 250MS aand BMC 150IC

7) The Baker Marine Corp (BMC) 150/200/250 Waterdepth 150'to 250'

depending on design Many RBF Series like RBF151, 191 atc.

8) The Baker Marine Corp.BMC 300 Water depth 300' Trident 12 and 14.

9) The MODEC 300C and 400C Water depth 300' and 400 Trident 6, 8, 9, 15,

16, and 17

MODEC 400C-35 (Trident 9) and MODEC 300-C-35 (Trident 8)

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10) Various single rig designs such as CFEM 300'WD (Shelf Explorer 300'),

Levingston Class 011 150' WD (RBF155) and the CS MODV 400' WD (Trident 20)

The CFEM T-2601-C (Transocean Nordic) and CS MODV (Trident 20)

CHAPTER 2 ENVIRONMENTAL LOADS ON THE JU

2.1 INTRODUCTION

While elevated or afloat a jack-up is subjected to environmental loading namely wind,waves, and current, in addition to gravity loads Various mathematical formulae havebeen developed to compute the wave forces Summary of present methodology will bepresented in this section

The objective is to introduce some of the fundamental concepts as well as technical wordsfrequently used in the industry In later sections examples of strength calculations will beexplained

2.2 ENVIRONMENTAL LOADS

Generally environmental loads include wind, wave, and current As a design basis it isconservative to assume that the maximum wind speed, wave and current actsimultaneously in the same direction For some locations such as California, Japan, and

Alaska one may have to include loads caused by earth quakes and/or ice Fig 2.1 shows

the different types of environmental loads on a JU

WIND

WAVES CURRENT

EARTH

QUAKES Fig 2.1 Environmental loads

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2.3 ENVIRONMENTAL DATA

The environmental data depends upon the geographic location, the water depth, the time

of the year, and the amount of available data For instance, the environmental data fromthe southern North Sea is different from those from the northern part The environmentalforces in the summer are considerable lower than those for the winter

The storm conditions include a set of environmental data such as wind speed, wave heightand period, current, velocity, tide and storm surges These values are given by the client

or the insurance surveyor or independently obtained from private consultants Generally,values obtained from one source will not agree with those obtained from another In mostcases, the ones given by the client are most severe The discrepancy is usual Depending

on location, the observed weather data bank can be very limited and subjected to differentinterpretations

The normally accepted recurrence (return) period for a JU is a design storm of 50 years.The design storm for a semi-submersible is 100 years because in general the semi-submersible remains longer on location The statistical computations are based on existingdata The probability of a 50-year storm to happen in a specific year is 2%, however thestorm can happen at any time of the year and may even happen more than once For otherapplications the value of 50 may be deviated For example for a platform production unit,the 100-year storm may be required

2.4 THE WIND LOAD

A) The Wind Speed

The total wind load on the JU structure depends on the wind speed Generally, oneassumes that the wind is steady, i.e not variable with times In reality, this is far fromtrue The wind speed does vary from none moment to another Therefore, it is important

to agree on and define what wind speed is

Weather stations compute the wind speed obtained from various locations recorded byweather stations Wind records are broken into time intervals The standard time intervalsused for the analysis are three seconds, one minute, and one hour

 THREE SECOND GUST is defined as the maximum value of the three

-second interval average wind speed This represents the instantaneous windspeed Generally it is used for the individual design criteria for structuralmembers on the open deck area and not for the entire rig

 ONE MINUTE WIND is the maximum value of the average wind speed in one

minute intervals (Fig 2.2) This wind speed is applied with the maximum wave

in the design storm criteria This wind speed is generally used for most of the rigstructure

 ONE HOUR WIND is the maximum value of the average wind speed in one

Fig: 2.2 Wind speed graph and wind averages.

The one-minute wind is less than the three second gust The ratio of one-minute wind tothe three-second gust is 0.85 The one minute wind is larger than the one hour wind Theratio between the two values is 1.16

B) The Wind Force.

The wind force F (Fig 2.3) is obtained by wind tunnel test, or by existing formula such as

one given be the ABS Rules based on the Code for the Construction and Equipment ofMobile Offshore Units (MODU) 1991

minute One

V

m in area Projected A

2.4) (Fig.

MODU ABS

from

t coefficien Shape

C

2.4) (Fig.

MODU ABS

from

t coefficien Height

C

kg in e Windforc F

V A C C 0.0623 F

2

s h

2 s

Fig 2.3 Wind force formula

Fig 2.4 Example of height and shape coefficients from ABS

HEIGHT COEFFICIENT

SHAPE COEFFICIENT

An example of the calculation of wind forces acting on various parts of a JU is shown in

(Fig 2.5 and 2.6) It should be noted that the height coefficient Ch and the shapecoefficient Cs in the wind formulae may vary from one Classification Society to another

Fig 2.5 Example of wind force calculations for some of the sections

t

WIND DIRECTION 33.3T

17.5t

28

SECTION AREA (M² ) CS CH F

(Ton)

H(m)

23.0016.50

17.50-2.30

The minimum wind velocity for Unrestricted offshore service for all normal drilling and

transit conditions is not to be less than 70knt All units in unrestricted offshore service are

to have the capacity to withstand a severe storm condition wherein a steady wind of notless than 100knt is assumed Units, which, due to intended limited service, are not

designed to meet the above criteria, may be considered for Restricted service,

classification with a minimum steady wind not to be less than 50knt

In some parts of the world such as India, China, the Gulf of Mexico, with hurricaneconditions, the one-minute wind speed can range from 110knt to 130knt

In the ABS formula as shown, the wind force is proportional to the square of the windvelocity This means that a 10% increase of wind speed will increase the wind force by21%

Furthermore, in a case of a JU, the major part of the wind force acts on the upper hull, theliving quarters and structure above the main deck, this is at a higher level than where thewave forces act Thus, generally the wind is more critical than wave This is generallytrue except for the case of an exceptional high wave and current.

The sections to follow will show that the Design Criteria for a JU includes wind speed,wave height, current velocity, water depth, air gap, and leg penetration It is possible toreduce any of these parameters such as wind speed in order to increase the others such aswave and current

The wind force can be 300t to 400t or more The example calculation of a MODEC 400C

(Trident 9) shows that in 400ft of water depth the 100knt wind force is 411t (Fig 2.6).

This 411t wind force contributes for 75% of the total over turning force The other 25% isfor a 40ft wave However, for the Harsh Environment JU design, wind only accounts for25% of the total force The MLT 116C (Trident 2 and4) the wind force is 365t and for theMODEC 300C (Trident 6) 324t

2.5 WAVE LOADS

The so-called Morison equation is used to compute the wave forces on the JU legs withlong cylindrical or prismatic members The Morison equation states that wave forces

acting on a submerged body are composed of two components (Fig 2.7):

 The inertia force caused by the acceleration of the water particles (Fi)

Fig 2.7 The Morison Formulae

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body the around particles

water of

Velocity V

body the of area projected A

cylinder.

a for 0.7 to 0.5 from ranges

t coefficien Drag

C

body.

the around cles

waterparti of

on accelerati α

body.

the of Volume V

seawater.

of mass specific ρ

cylinder.

a for 1.8

water.

of mass displaced the

on based

t coefficien Inertia

C

) ρA(V 0.5C F

and

Vα ρ C F

W

D w

M

2 W D D

w M i

Amplitu de

CRES T

L =wave length

H =Wave height Double amplitude Direction of propagation

To simplify the calculations one can compute an equivalent diameter and the associatedinertia coefficient (CM) and drag coefficient for the entire leg, taking onto account alltubulars, the exposed rack, as well as other parts on the legs Sometimes certain amount ofmarine growth needs to be accounted for to include the larger diameter and correspondinghigher drag coefficient

2.6 WAVE KINEMATICS

Each regular wave is defined by the wave height i.e the distance between the wave crest and the wave trough and the wave period i.e the time between two consecutive wave crests (Fig 2.8.and Fig 2.9)

Fig 2.8 Wave characteristics

Fig 2.9 JU and wave motion

The study of the motion of waves is quite complicated Various wave theories exist topredict the action of the wave particles for a given wave Each wave theory has itslimitations The application depends upon deep, intermediate, and shallow water Deepwater is when the water depth is more than half of the wavelength Some basic wave

formulae are show in Fig 2.10.

Fig 2.10 Wave formulae

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amplitude double

for period wave

whereT frequency

wave T

2 ω

frequency.

wave angular

height wave H

length wave l

where 1.56T

l

crest wave the

at velocity maximum

Vmax where ω 2

H Vmax and

level water still

at on accelerati maximum

αmax where ω

2

H αmax

2 2

Following are some of the wave theories frequently used in the offshore industry.

Example: a wave with a 10sec period has a length of 156m

For water particles below still water level, the water particles motion propagete by a

decaying factor as shown in Fig 2.11 and Fig 2.12 Theoretically, the wave disturbance

is negligible at a distance of half wavelength below SWL

Fig 2.11 Water particles orbit and decaying factor and shallow water effect.

Linear wave theory is easy to use but for shallow water or in case of breaking waves the

Decaying to zero 0.5

wavelength.

Fig 2.12 Water particle path

B)

STOKES FIFTH RIDER WAVE: This is the expansion of the linear wave toinclude non-linear terms As the water depth decreases, the percentage of steepness of a

wave increases Fig.2.13 shows the change in profile of a standing wave The Stoke Fifth

Rider Wave is used mostly for JU’s because it is applicable for intermediate and deepwater

Fig.2.13 Change of wave profiles from deep to shallow water depth.

C)

STREAM FUNCTION WAVE: This wave theory is good for deep, intermediate,and shallow water depth The calculation requires an interactive process and takes a lot oftime

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DEEP

SHALLO W

VERY SHALLOW

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The following graphs show the effect of the wave period (Fig 2.14) and water depth (Fig

2.15) on the wave force for the Trident 9 The wave forces produce an Overturning Moment (OTM) The graphs show the difference in wave force (OTM) between the

AIRY and STOKE FIFTH wave calculations

Fig 2.14 Wave force (OTM) versus period for Trident 9

2.7 DESCRIPTION OF SEA STATE

Typically, the Design Criteria for a JU or the storm conditions specified by the operator is

a wave of a certain height and period This description of sea state refers to a determinedwave height and profile In reality, the sea state almost never consists of a single waveprofile The sea state is actually built up of a combination of various wave and swell

profiles (Fig 2.15).) The result is a spectral or in-deterministic sea state.

Although the spectral sea state is mainly applied to semi submersibles and vessels and not

to the JU, a short explanation is necessary to introduce the concept

Fig 2.15 Design (single profile) sea state versus spectral sea state

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The spectral sea state describes the sea as a summation of many waves of variousfrequencies and heights centering around a dominating wave, or in other words the energy

in the sea spread over a range of frequencies rather than concentrated in one wave Thedescription of spectral sea state is:

A) Significant wave height H s , which is the average of the highest one-third

waves

B) Zero (up) crossing period, which is the average period of the spectrum.

C) The maximum wave height H m , computed from the significant wave height

Where: H m =1.86 H s

Without going into the details of spectrums used for wave forces it is enough to know thatthe two most used spectrums are the JONSWAP and PIERSON –MOSKOWITZ Thedifference in these two types is the spread of the energy in the sea

2.8 CURRENT LOADS

A)

FORMULATIONS : As in the case of wind, the current is assumed as a constantflow with time The profile will vary with the depth Two types of currents exist:

 The tidal current

 The wind generated current

The values obtained from clients, weather service companies or marine surveyors includeboth types Current only generate drag force The combined formula for the wave drag

forces and current drag forces is (Fig 2.16):

Fig 2.16 Combined wave and current drag formula

The above formula shows that the velocity part is the square of the sum of the velocityforces of wave and current (vw+ vc )² This is larger than the sum of vw ² + vc ² In otherwords, the effect of the current is much more than one would expect Consider as anexample the Trident 9 Design Criteria of a 40ft wave at 12sec with zero current and with a

velocity current

v and particles water

of velocity v

body.

the of area projected A

water.

sea of mass specific ρ

cylinder.

for 0.7 to 0.5 from

t coefficien drag

C

) v ρA(v 0.5C F

c w

D

2 c w D D

Fig 2.17 Example of effect of current on the total wave force (OTM) for Trident 9

Usually the current values are specified at sea bottom and at still water level In mostcases, the current profile varies linearly from seabed to still water level Sometimes it is

specified at intermediate depth like the DNV current profile Fig.2.18 shows the linear

profile versus API and the DNV profile The total final result of the combinedwave/current force depends on the applied profile

Linear and API current profile DNV Current profile

Fig 2.18 Examples of current profiles.

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CURRENT AT WAVE CREST: The wave force occurs when the crest of the wavepasses through the JU leg The question is the amount of current velocity to apply for theparticular instant It makes a difference apply the value of vc with or without the presence

of the wave There are two schools of thought on this subject:

To be on the conservative side some operators require the constant maximum current withthe highest vc factor Fig 2.19 displays graphically the difference in vc factors for eachtheory

Fig 2.19 Vc Factors

CHAPTER 3 BASICS OF SOILS AND MECHANICS

3.1 INTRODUCTION

The statistics reveal that in addition to transit, soil is a major factor causing mishap anddamage to JU’s Soil can have a harmful effect to the rig in case of a punch through,scouring, and too much penetration However, soil can enhance the rig operationalcapability as well as providing partial fixity at the spud can The practical aspects of soil

in operations will be discussed in a later section This chapter covers the basics of the soilmechanics

Solid Liquid

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