construcyion of marine and offshore structures

First published in 1986, this second edition of Construction of Marine and Offshore Structures has been updated and augmented.. Construction operations have been alreadycarried out in 15

To Martelle and Ellen,

whose love and understanding sustained

the efforts of preparing this book

Introduction

0.1 General '" 1

0.2 Geography ·3

0.3 Ecological Environment , 3

0.4 Legal Jurisdiction 4

0.5 Offshore Construction Relationships and Sequences 5

0.6 Typical Marine Structures and Contracts 7

0.7 Interaction of Design and Construction 8

1 Physical Environmental Aspects of Marine and Offshore Construction 1.0 General 15

1.1 Distances and Depths 15

1.2 Hydrostatic Pressure and Buoyancy 16

1.3 Temperature , 18

1.4 Seawater and Sea-Air Interface Chemistry, Marine Organisms 19

1.5 Currents , 20

1.6 Waves and Swells 24

1.7 Winds and Storms 30

1.8 Tides and Storm Surges 34

1.9 Rain, Snow, Fog, Whiteout, 'and Spray; Atmospheric Icing, Lightning 35

1.10 Sea Ice and Icebergs 36

1.11 Seismicity, Seaquakes, and Tsunamis 41

1.12 Floods 42

2 Geotechnical Aspects: Seafloor and Marine Soils 2.1 General 43

2.2 Dense Sands , 45

2.3 Calcareous Sands 45

2.4 Boulders on and near the Seafloor Surface; Glacial TilL 46

2.5 Overconsolidated Silts 47

2.6 Sub-Sea Permafrost and Clathrates 47

2.7 Weak Arctic Silts and Clays 47

2.8 Ice Scour and Pingos : 48

2.9 Methane Gas 48

2.10 Muds and Clays 48

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2.11 Coral and Similar Biogenic Soils; Cemented Soils 50

2.12 Unconsolidated Sands 51

2.13 Underwater Sand Dunes ("Megadunes") 53

2.14 Rock Outcrops 53

2.15 Cobbles 54

2.16 Deep Gravel Deposits 54

2.17 Seafloor Oozes 54

2.18 Seafloor Instability and Slumping; Turbidity Currents 55

2.19 Concluding Remarks Concerning Seafloor 56

3 Ecological and Societal Impacts of Marine Construction 3.1 General 57

3.2 Oil and Petroleum Products 57

3.3 Toxic Chemicals.m •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 58 3.4 Contaminated Soils 58

3.5 Construction Wastes 58

3.6 Turbidity 59

3.7 Sediment Transport, Scour, and Erosion 59

3.8 Air Pollution 60

3.9 Marine Life: Mammals and Birds, Fish, and Other Biota 60

3.10 Aquifers 60

3.11 Noise.mm ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 61 3.12 Highway, Rail, Barge, and Air Traffic 61

3.13 Protection of Existing Structures 62

3.14 Vibration 63

3.15 Safety of the Public and Other Vessels 63

4 Materials and Fabrication for Offshore Structures 4.1 Steel Structures for the Offshore Environment 65

4.2 Structural Concrete 74

4.3 Hybrid Steel-Concrete Structures 87

4.4 Plastics and Synthetic Materials, Composites 88

4.5 Titanium 91

4.6 Rock, Sand, and Asphaltic-Bituminous Materials 91

5 Marine and Offshore Construction Equipment 5.0 General 93

5.1 Basic Motions in a Seaway 94

5.2 Buoyancy, Draft, and Freeboard 96

5.3 Stability 98

5.4 Damage Control 99

5.5 Barges 102

5.6 Crane Barges 105

5.7 Offshore Derrick Barges (Fully Revolving) 108

5.8 Catamaran Barges 112

5.9 Semisubmersible Barges 112

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5.10 Jack-Up Construction Barges 115

5.11 Launch Barges 119

5.12 Offshore Dredges 120

5.13 Pipe- Laying Barges 124

5.14 Supply Boats 127

5.15 Anchor-Handling Boats 128

5.16 Towboats 128

5.17 Drilling Vessel 130

5.18 Crew Boats 130

5.19 Floating Concrete Plant 130

6 Marine Operations 6.1 Towing 133

6.2 Moorings and Anchors 140

6.3 Handling Heavy Loads at Sea 150

6.4 Personnel Transfer at Sea 156

6.5 Underwater Intervention, Diving, Underwater Work Systems, ROVs, and Manipulators 160

6.6 Underwater Concreting and Grouting 168

6.7 Offshore Surveying, Navigation, and Sea-Floor Surveys 179

6.8 Temporary Buoyancy Augmentation 184

7 Seafloor Modifications and Improvements 7.1 General 187

7.2 Controls for Grade and Position: Determination of Existing Conditions 188

7.3 Seafloor Dredging and Obstruction Removal 189

7.4 Dredging and Removal of Hard Material and Rock 196

7.5 Placement of Underwater Fills 200

7.6 Consolidation and Strengthening of Weak Soils 204

7.7 Prevention of Liquefaction 206

7.8 Scour Protection '" 206

7.9 Concluding Remarks 210

8 Installation of Piles in Marine and Offshore Structures 8.1 General '" 213 8.2 Fabrication of Tubular Steel Piles 216

8.3 Transportation of Piling 217

8.4 Installing Piles 219

8.5 Methods of Increasing Penetration 239

8.6 Insert Piles 242

8.7 Anchoring into Rock or Hardpan 242

8.8 Damaged Piles 243

8.9 Pre-Stressed Concrete Piles for Marine Structures 243

8.10 Handling and Positioning of Piles 245

8.11 Drilled and Grouted Piles 247

8.12 Belled Footings 251

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8.13 Other Installation Methods and Practices 254

8.14 Improving the Capacity of Piles 254

9 Harbor, River, and Estuary Structures 9.1 General 257

9.2 Harbor Structures 257

9.3 River Structures 266

9.4 Piers for Overwater Bridges 274

9.5 Submerged Prefabricated Tunnels (Tubes) 296

9.6 Storm Surge Barriers 302

9.7 Flow Control Structures 307

10 Coastal Structures 10.1 General 315

10.2 Ocean Outfalls and Intakes 315

10.3 Breakwaters 321

10.4 Offshore Terminals 328

11 Offshore Platforms: Steel Jackets and Pin Piles ILl General 343

11.2 Fabrication of Steel Jackets 343

11.3 Load-Out, Tie-Down, and Transport 345

11.4 Removal ofJacket from Transport Barge; Lifting; Launching 353

11.5 Up-ending of Jacket 360

11.6 Installation on the Seafloor 363

11.7 Pile and Conductor Installation 366

11.8 Deck Installation 369

11.9 Examples 371

12 Concrete Offshore Platforms: Gravity-Base Structures 12.1 General 387

12.2 Construction Stages 389

12.3 Enhancing Caisson-Foundation Interaction 427

12.4 Sub- Base Construction 431

12.5 Platform Removal 432

13 Other Applications of Offshore Construction Technology 13.1 General 433

13.2 Hybrid Concrete-Steel Platforms 433

13.3 Single-Point Moorings 434

13.4 Articulated Columns 437

13.5 Seafloor Templates 444

13.6 Underwater Oil Storage Vessels 450

13.7 Cable Arrays, Moored Buoys, and Seafloor Deployment 453

13.8 Ocean Thermal Energy Conversion 454

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14 Moored Floating Structures

14.1 General 457

14.2 Fabrication of Concrete Floating Structures 459

14.3 Launching 462

14.4 Use of Concrete Barges for Cryogenic Service: FPSOs for LPG and LNG 463

14.5 Steel Structures for Permanently Floating Service 463

14.6 Mating Afloat 465

15 Installation of Submarine Pipelines 15.1 General 467

15.2 Conventional S-Lay Barge 470

15.3 Bottom- Pull Method, Coastal Pipelines 486

15.4 Reel Barge 493

15.5 Surface Float 494

15.6 Controlled Underwater Flotation {Controlled Subsurface Float) 495

15.7 Controlled Above-Bottom Pull 495

15.8 J-Tube Method from Platform: Single- and Double-Pull 496

15.9 J-Lay from Barge 497

15.10 S-Curve with Collapsible Floats 497

15.11 Bundled Pipes 498

15.12 Directional Drilling 498

15.13 Laying Under Ice 490

15.14 Protection of Pipelines: Burial and Covering with Rock 490

15.15 Support of Pipelines 506

16 Plastic and Composite Pipelines, Cables 16.1 Submarine Pipelines of Composite Materials and Plastics 507

16.2 Cable Laying 508

17 Topside Installation 17.1 General 511

17.2 Module Erection 511

17.3 Hook- Up 513

17.4 Giant Modules and Transfer of Complete Deck by Heavy Lift 515

17.5 Float -Over Deck Structures 515

17.6 Integrated Deck 517

18 Underwater Repairs 18.1 General 519

18.2 Repairs to Steel Jacket-Type Structures 520

18.3 Repairs to Steel Piling 523

18.4 Repairs to Concrete Offshore Structures , 523

18.5 Repairs to Foundations 525

18.6 Fire Damage 526

18.7 Pipeline Repairs 527

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19 Strengthening Existing Structures

19.1 General 531

19.2 Strengthening of Offshore Platforms and Terminals, Members, or Assemblies 531

19.3 Increasing Capacity of Existing Piles for Axial Loads 532

19.4 Increasing Lateral Capacity of Piles and Structures in Interaction with Seafloor Soils 537

19.5 Seismic Retrofit 539

20 Removal and Salvage 20.1 General 541

20.2 Piled Structures (Terminals, Trestles, Shallow-Water Platforms) 542

20.3 Offshore Drilling and Production Platforms (Jackets with Piles) 543

20.4 Gravity- Base Platforms 544

20.5 New Developments in Salvage Techniques 546

21 Constructibility 21.1 General 547

21.2 Construction Stages 548

21.3 Principles of Construction 552

21.4 Facilities and Methods for Fabrication and Launching 552

21.5 Assembly and Jointing Afloat 556

21.6 Material Selection and Procedures 557

21.7 Construction Procedures 558

21.8 Access '" 563 21.9 Tolerances 564

21.10 Survey Control 565

21.11 Quality Control and Assurance 566

21.12 Safety 567

21.13 Control of Construction: Feedback and Modification 568

21.14 Contingency Planning 568

21.15 Manuals 569

21.16 On -Site Instruction Sheets 571

21.17 Risk and Reliability Evaluation 571

22 Construction in the Deep Sea 22.1 General 575

22.2 Considerations and Phenomena for Deep-Sea Operations 576

22.3 Techniques for Deep-Sea Construction 576

22.4 Properties of Materials for the Deep Sea 577

22.5 Platforms in the Deep Sea, Compliant Structures 581

22.6 Tension -Leg Platforms 587

22.7 SPARS 59I 22.8 Deep-Water Moorings 591

22.9 Construction Operations on the Deep Seafloor 594

22.10 Deep-Water Pipe Laying 598

22.11 Deep-Water Bridge Piers 600 x

23 Arctic Marine Structures

23.1 General 607

23.2 Sea Ice and Icebergs 608

23.3 Atmospheric Conditions 612

23.4 Arctic Seafloor and Geotechnics 613

23.5 Oceanographic 615

23.6 Ecological Considerations 615

23.7 Logistics and Operations 616

23.8 Earthwork in the Arctic Offshore 618

23.9 Ice Structures 621

23.10 Steel and Concrete Structures for the Arctic 623

23.11 Deployment of Structures in the Arctic 629

23.12 Installation at Site 631

23.13 Ice Condition Surveys and Ice Management 640

23.14 Durability 641

23.15 Constructibility 643

23.16 Pipeline Installation 644

23.17 Current Arctic Developments 644

Epilogue 647

References 649

Index 651

xi

Ben C Gerwick, Jr., was born in 1919, the son of a renowned marine contractor and engineer Following

graduation from the University of Caliifornia at Berkeley, he served at sea for five years with the U S.Navy during World War II He concluded his service as Commanding Officer of the US.S Scania. Hethen returned to work in his family's construction company, working his way up from field and officeengineer, to vice president, to president of the firm in 1952 During this period, the company was involved

in the construction of wharves, outfalls, and the foundations for overwater bridges on the West and EastCoasts of the US., as well as offshore terminals in Cook Inlet, Alaska In 1959, the company mergedwith J H Pomeroy, and extended their combined operations to Kuwait, Saudi Arabia, Iran, Singapore,Korea, Australia, and Central and South America

In 1967, the firms became part of Santa Fe, International Mr Gerwick was in charge of their offshoreconstruction projects in Bass Strait, Australia, and in the North Sea In 1971, Mr Gerwick accepted aposition at the University of California as Professor of Civil and Ocean Engineering

Mr Gerwick is currently Chairman of Ben C Gerwick, Inc., Marine Consultants, of San Fransisco

He has served as consultant on numerous marine and offshore projects in the US., the North Sea, theArctic, Canada, India, Bangladesh, Japan, China, Norway, Denmark, and the Arabian-Persian Gulf

Mr Gerwick is a member of the U S National Academy of Engineering, a Fellow and HonoraryMember of ASCE, AC1, and PCI, and an Honorary Member of several European Engineering andTechnological Associations He has served as Chairman of the U S Marine Board and as President ofthe International Federation of Prestressed Concrete

In addition to Construction of Prestressed Concrete Structures, Mr Gerwick is also the author of over

200 technical papers and articles

xiii

The objective of this book is to assemble in one volume the state-of-the-art practice in the construction

of marine and offshore projects First published in 1986, this second edition of Construction of Marine

and Offshore Structures has been updated and augmented The developments of the past decade have

been truly phenomenal, culminating in the current capability to carryon construction operations inwater depths of 1,500 m and more Of even greater importance to the construction industry, the devel-opment of heavy-lift equipment, remote-sensing devices, remote-operating vehicles (ROVs), the GlobalPositioning System (GPS), and underwater technology, originally for the purpose of offshore construc-tion, is now being transferred to coastal, harbor, and river construction These developments have madeinshore marine construction more efficient and economical, even in unstable soils and adverse environ-ments

This text is designed to serve as a guide and reference for practicing engineers, both designers andcontractors, for, in the marine environment, each discipline is dependent on and intertwined with theother It is also intended as a text for graduate engineering students interested in this highly challengingfield of endeavor

Until now, this information has been available in scattered fashion among hundreds of technical papers,engineering journals, and conference proceedings While there have been several excellent books pub-lished on the design of marine and offshore structures, they have touched only superficially on construc-tion technology and procedures

Construction of Marine and Offshore Structures represents the culmination of the author's involvement

of over 50 years in the field of marine and offshore construction He has not only extracted and compiledrelevant information from published sources, but has augmented these with in-house reports madeavailable by individuals and industry, as well as by direct experience - the problems and the successes.The text has been illustrated by numerous illustrations and photographs, many of which have beengraciously made available by the firms and individuals involved in marine construction To them, manythanks are extended

xv

The oceans are the dominant features of Earth, comprising more than two thirds of its surface, stabilizingits temperature so that life as we know it can exist, providing the water vapor which later falls as rain onthe continental "islands;' the original source oflife and the ultimate collector or sink of all surficial matter,including waste Oceans have been both a barrier and a conduit over which people and goods have movedwith relative ease, spreading culture while garnering Earth's remote resources.

Yet the ocean is fiercely inhospitable, making us dependent on land bases for support Storm waveshave destroyed even the largest vessels, as well as the puny attempts of humans to protect the coastlinefrom the oceans' attack The northernmost ocean, the Arctic, is almost completely covered with perpetualsea ice, while the southern, the Antarctic, carries with it huge tabular icebergs that stretch beyond thehorizon

Opportunity and challenge, safety and terror, wealth and destruction: these are the paradoxes of theseas

Since before recorded history, oceans have been used for transport, for food, for conquest, and forwaste disposal The Phoenicians sailed as far as Norway to the North and Capetown to the South, perhapseven on to South America; the Polynesians crossed the Pacific to where they sighted the great wall of theAndes, which to them marked the "end of the world;' and to Japan and Indonesia; and navigators fromKerala reached Africa and Indonesia, completing early man's circumnavigation of the globe Much latercame the Arabian sailors whose sea empire extended from West Africa to the Philippines; the Vikings,who sailed to Venice and Canada; and eventually the Western European navigators of the Age of Explo-ration, who challenged the utmost corners of the globe, including both the Arctic and the Antarctic.Today, more than 30,000 ships ply the trade routes of the world

Mahan's brilliant insights in The Influence of Sea Power on the History of the World (1890) demonstrated

the decisive role that has been played by the navies of nations who strove either to dominate the world

or repel the challenger As Mahan points out, it was the Greek sea victory at Artemis that blunted theexpansion of the Persians; the Roman domination of the Mediterranean that forced Hannibal to hisaudacious but futile march through Spain and across the Alps to try to break Rome's stranglehold onCarthaginian trade; Drake's defeat of the Spanish Armada and Nelson's victory at Trafalgar that eventuallyled to Britain's worldwide empire; and the temporary repulse of the British fleet by the French navy whichenabled Washington to force surrender of the food- and munitions-starved Cornwallis Similarly, it wasthe U.S Navy's destruction of the Japanese fleet that led to victory in the Pacific in World War II.The oceans have been regarded until recently as an inexhaustible source of food; fishermen need only

be clever enough to trap the fish which roam along its coastlines and its great internal rivers where thecold water, rich in nutrients, intermixes with the warm Fishermen have learned to survive the stormwaves, hurricane winds, dense fogs, and "black" ice that have destroyed their less-able predecessors

1

2 Construction of Marine and Offshore Structures

The coasts are the boundary between the continents and the oceans This interface is constantlyundergoing change; uplifting as an active plate subducts under the continental margin, eroding underthe constant pounding of the surf, or accreting from sediment discharge, while periodically dischargingimmense flows of sand or mud that scour out great submarine canyons

The great rivers of the world drain the continents, providing fresh water for human and animalconsumption, for agriculture and industry Their flow, however, is not uniform but characterized byperiods of low water offset by raging floods which often devastate the adjacent lands yet leave behindthe fertile silts and clays These rivers also provide the easiest and most economical roadways into theinterior of the continents; their navigation has been the mainstream of commerce throughout history.Rivers empty into harbors, around which man has built his great cities The harbors are ports of refugefor ships from the storms of the oceans, and they are the locale of transshipment from oceangoing vessels

to land and river transport As cities have grown, clustered mainly at the junctures where great riversmeet the ocean, so have the problems of waste disposal grown, whether it be sewage, the effluent fromindustrial processes, the runoff of waste oil from urban lands and of nutrients from farmlands, or thewarm water discharges from power plants The sea has been a compliant receiver, quick to disperse anddilute all but the most toxic wastes The oils have been consumed by bacteria, and most of the excessminerals precipitated to the seafloor

The above describes the state of the ocean until recent times, the latter half of the 20th century Nowsuddenly humankind has burst out with explosive force, increasing both population and human activities

at an exponential rate In the forefront of this revolutionary expansion, this "big bang" of culturalspreading, has been the technological eXploitation of the oceans

In the field of transport, we see new ship types and modes, from containerization to catamarans, hovercraft,and very-large crude oil carriers (VLCCs), while kilometer-long "trains" of barges ply inland rivers madenavigable by locks and dams In fishing, we see electronic search, sea ranching, and the beginning of exploi-tation of Antarctic krill, that tiny shrimp whose numbers render it the most abundant source of protein onEarth Even though we live in the Space Age, it remains the seas in which military might dominates, for thenuclear-powered submarine with its awesome destructive power lurks almost undetectable in the oceandepths or underneath sea ice cover.Waste discharges continue, but now there is a global awareness of theneed for at least primary treatment and mechanical dispersal to avoid over-concentrations along the vulner-able coasts

The thermal attributes of the ocean, as a source of cooling water, a sink for warm water, and even apotential source of energy, have long been recognized Although ocean thermal energy conversion (OTEC)projects are not currently economically viable, their technological feasibility has been demonstrated Inthe long term, it will probably be the unlimited source of cooling water of the oceans combined withtheir capacity to accept discharges that will lead to seaborne industrial processing plants on a large scale

It has only been in the latter half of the 20th century that full recognition has been given to the oceansand their sediments as a major source of mineral wealth, both hard minerals and petroleum Offshoreoil and gas now supply almost one third of the world's energy needs: in fact, it has been stated by theU.S Geological Survey that the offshore sedimentary basins within the U.S Economic Zone hold forththe greatest potential for major new discoveries

An immense amount of publicity has been given to the manganese nodules which cover large areas

of tropical and subtropical seafloors More recently, the scientific world has been excited by the discovery

of the thermal vents from seafloor rifts, with their strange new forms of life and their apparently richdeposits of polysulfide minerals Extraction of soluble minerals from the sea has been carried out sinceprehistoric times: salt (sodium chloride) and, in modem times, magnesium and bromine

Coastal sediments are also rich deposits of precious minerals such as gold, tin, and probably chromiumand platinum Seabed mining of such unsophisticated minerals as sand and gravel is of major importance

in Japan, many European countries, and the Arctic However, because of the tremendous economicimportance of offshore oil and gas and the concentrated development of technology for their exploitation,much of the recent marine construction practice has been devoted to the installation of facilities to servethe needs of the petroleum industry, and hence this will serve as focal subject matter of this book

Introduction 3

The marine environment, however, includes far more than the deep oceans: it includes the coasts,against which the sea beats with relentless fury It includes the harbors and their terminals for thetransshipment of containers, bulk cargo, and liquids, primarily petroleum It includes the deep rivers,and their locks and dams which keep them navigable even in seasons of low water, while providing floodprotection when needed It includes the overwater bridges which boldly span not only rivers and harborsbut today challenge the straits of the ocean margins

The great ocean basins, long thought to be relatively simple, with stable slopes and flat seafloors, haveturned out to be exceedingly complex and dynamic The study of plate tectonics has revealed theunderlying mechanism by which seafloors spread and sediments are eventually subducted The seafloor

is marked by deep canyons and steep escarpments Great volcanic mountains rise above the ocean floorfar higher than Everest rises above its base Seamounts which have not yet reached the surface or whichhave been eroded or submerged below it now sport crowns of coral

Among these oceans float the continents, whose margins extend well beneath the sea That whichessentially extends the continents out under the sea is known as the continental shelf, an area rich insediments washed off the continents and eroded from the shore to be deposited here in relativelyconcentrated zones At the outer edge of the continents is the continental slope, dipping down to theabyssal plain (whose surface is often far from planar) Sediments accumulated on the shelves periodicallyflow down the slopes, as turbidity currents, to form giant fans at their base

The shelves and slopes are then inherently unstable, geologically speaking, at least in their surficialdeposits Many of the most striking geological features have occurred during episodic events As opposed

to gradual continuous erosion and deposition, these episodic events include gigantic submarine landslidesand turbidity current flows Similarly, coasts and rivers are periodically altered drastically by episodicevents

A basic property of the oceans, affecting all human activities thereon, is their vastness, their "illimitableexpanse" which necessitates long-distance transport of all materials, structures, equipment, and person-nel There are no easy geographic reference points, no stable support for adjoining activity or storing ofsupplies This problem of logistics dominates all considerations of construction activities and integratesconstruction with the transport functions upon which it so heavily depends This same concern forlogistical support occurs to a lesser degree on all marine projects

Interest in the life of the sea has expanded in recent years beyond the fascination of Moby Dick and thelocating of desirable schools of fish to a deep concern for all living creatures, especially those of the sea,which share our common source of life and which have evolved along parallel lines to relatively highlevels of intelligence As with all newly emerging concerns, there have perhaps been excesses of zeal, butthe underlying recognition that the life of the sea must be protected from wholesale depredation hasbecome a basic ethical tenet of our society

Thus, construction activities in the ocean, especially those in the coastal zones, must take cognizance

of ecological and environmental constraints, whether these be limitations on noise generated in the watercolumn by dredging, which may affect the navigational and communications abilities of marine mam-mals, or pollution by persistent chemical discharges Paradoxically, the aesthetically and legally unfor-givable presence of a sheen of oil on the water may also be the least environmentally harmful, due to itsrapid biodegradation into edible protein Massive oil spills, however, cause extensive damage to the biota

of the coasts, but fortunately the damage is not permanent Perhaps the worst effects occur in the wetlandswhich adjoin the estuaries

The seas have long presented a curious contrast between freedom and politically imposed controls As aresult, a patchwork set of arrangements has emerged The recent Law of the Sea conferences were anattempt to establish a more logical and politically viable legal basis for the rapidly expanding development

of the oceans

Freedom of navigation, except in a narrow zone close to the shores, and freedom of innocent passagethrough straits have long been established and enforced by the world's great sea powers More recently,freedom of scientific research has been promulgated, only to founder on the narrow distinctions amongresearch, exploration for mineral resources, exploitation of pharmacological compounds, and militaryintelligence

While the Law of the Sea Treaty has been signed by several nations, it has been rejected by the UnitedStates because of inclusion of the concept of a supranational seabed authority to be granted jurisdictionand concomitant expropriation-like rights over seabed mineral resources and the technology used toproduce them Despite the current lack of treaty ratification, however, most of the treaty's other provisionsare coming into being as common law, through voluntary observance and through unilateral proclama-tions of clauses similar to those of the treaty, such as the establishment of 200-mile-wide economic zones.The most immediate results pertain to control of fishing in these vastly expanded national jurisdictionsand the right to produce offshore oil and gas For example, the United States has by one stroke increased

by almost 25% the area of the globe over which it asserts jurisdiction

Political jurisdiction in the Arctic remains confused, with some of the nations bordering the Arcticasserting the sector theory, that is, the extension of a meridian from their northernmost land boundarydirectly to the North Pole The five nations bordering the Arctic Ocean are Greenland (which belongs

to Denmark), Canada, the United States (in Alaska), Norway, and Russia, the sector of the last extendingalmost halfway around the globe Russia claims the shallow waters of the Barents, Kara, and East SiberianSeas as territorial seas Canada similarly claims that the channels between the Arctic islands are territorialwaterways, whereas the United States asserts that they are international straits with free right of passage.The Antarctic seas, south of the 60th parallel, remain an anomaly under a regional authority set upunder U.N provisions The Antarctic Treaty Organization, originally formed to prohibit military use ofAntarctica and to foster exchange of scientific information, has more recently been expanded by theestablishment of a "Living Resources" regime, primarily to control the exploitation of krill in the cold,upwelling waters around the continent Whales and seals are currently protected under a parallel Treaty

on Marine Mammals Similarly, a "Mineral Resources" regime constrains development of potentialpetroleum resources such as the vast submarine sediments of the Ross, Weddell, and Bellingshausen Seas.With more practicable approach to rights and obligations, the protection of the operator, and the sharing

of gain, this could eventually set a pattern for revision of the objectionable provisions of the Law of theSea Treaty

Closer to the continental shores, within those areas of national jurisdiction, is a 12-mile zone underfull control of the adjoining nation In the United States this is further subdivided by a 3-mile zone underthe jurisdiction of the adjoining state This latter is administered under the provisions of the CoastalZone Management Act, which takes into account the onshore impacts of offshore activities as well as thedirect activities themselves

The environmental impact statements, carried out under the laws governing these several zonessurrounding the coasts, have resulted in a series of agreements concerning specific projects, includingtheir construction These constraints may affect the procedures, methods, and sequence of construction.They carry the force of law Of particular interest to construction are those constraints which relate todredging and dredge disposal Discharge of oil in harbors, and disposal or capping of contaminatedsediments, is a major concern On our rivers and estuaries, migrations of fish may restrict the times whenoperations maybe carried out, whereas in harbors, the prohibited zones and periods may relate to breedingand nesting times

Many offshore projects relate to the development of an ocean resource, especially oil and gas Therefore,the sequence and relationships of the parties involved will help constructors in planning their operations.For offshore oil and gas development, leases for specific offshore areas are granted by the sovereignnation having jurisdiction to a petroleum company, with provision for payments, royalties, taxes, andconduct of operations These latter may involve specific agreements regarding alliances with other con-tractors, training and employment of nationals during the construction period, use of local contractors,fabricators, and suppliers, purchase of local materials, areas in which work will be carried out, andresearch and educational activities to be supported Many of these act as constraints upon the constructioncontractor, who may be required to associate with a local partner or a national enterprise Constructorsmay be subject to restrictions regarding the number of foreign workers, including skilled workers andsupervision, that they may employ Often they are required to build new construction equipment in thatcountry or, if they bring in outside equipment, to do so under bond assuring its subsequent export.Federal U.S laws - for example, the Jones Act - prohibit the use of foreign-registered dredges withinU.S waters and restrict the use of foreign towboats.

Following lease arrangements, the oil company will carry out extensive geophysical investigations,including seismic surveys At this time it may also get shallow core borings, bathymetric data, andenvironmental information Exploratory drilling is usually carried out by floating vessels, drill ships andsemisubmersibles being the most used in deep offshore areas and jack-ups in more limited depths Inthe Arctic and in shallow waters, exploratory drilling may be carried out from bottom-founded mobilestructures These vessels are collectively called "mobile offshore drilling vessels."

The petroleum company, having carried out geophysical exploration and exploratory drilling, mayconfirm a discovery It now carries out delineation drilling to determine reservoir characteristics It alsointensifies planning for offshore structures and development, carrying out feasibility studies and prelim-inary engineering to select a concept and contractors

Most often, these studies are carried out by one or more engineering contractors or integratedengineering-construction firms However, some of these may be carried out by in-house teams of thepetroleum company with the aid of consultants At the same time, the company proceeds to obtain more-accurate, site-specific geotechnical and environmental data for use in design Arrangements must bemade for shore-based facilities Environmental impact statements must be prepared Financing arrange-ments are made

During this entire period, the oil company operator will often have put together a consortium ofcompanies to participate in the project These may include the national oil company and from 1 to 20other petroleum companies Usually, but not always, the operator has the largest percentage, exceptperhaps for the national oil company The position of operator is much sought after by petroleumcompanies, as it confers control of the project, usually with substantial fees to cover management andoverhead costs It also enables the company to develop advanced in-house engineering expertise andmanagement capabilities

With the project approved, the operator now lets contracts for the offshore platform In many casesthese will be broken up into the following segments:

Design of the substructure

Design of the deck

Fabrication of substructure

Procurement of process equipment

Fabrication of deck and provision for installation of equipment

Installation of platform

Offshore hookup

Production drilling

6 Construction of Marine and Offshore Structures

Several of these may be combined in logical groups and awarded to one contractor Increasingly, many

of the above functions are being combined in design and construct contracts or "alliances."

Pipelines are usually divided into segments such as the following:

• Design of submarine pipeline

• Procurement of pipe

• Coating of pipe

• Installation and trenching of pipeline

To oversee and manage such a complex series of contracts, the operating oil company may set up its ownmanagement team or may engage a construction manager In the latter case it may integrate its staff intothe construction manager's activities

Most nations that have major offshore oil activities in their economic zones have established regulatoryagencies to control and supervise their development These governmental agencies are typically assignedresponsibility for ensuring safety during development and operation with respect to the following:

• Prevention of pollution

• Prevention of loss or waste of the resource

• Prevention of injury and death to personnel working on or in conjunction with the developmentNational agencies (e.g., the Minerals Management Service of the U.S government) have establishedrules for the conduct of offshore minerals development, including especially offshore oil and gas Theserules provide for a review of design, fabrication, and installation by an independent, third-party "veri-fication agent." The constructor is intimately involved in the fabrication and installation phases and musttherefore not only submit properly prepared and documented plans for approval, but must carry out thework in compliance with these documents and sound construction practice Provision must also be madefor quality assurance during construction This mayor may not be part of the verification process but,

of course, must be integrated with it

Following completion of the platform, development drilling is carried out from the platform In manycases, production will start after a few wells are completed, with concurrent drilling continuing After allwells are operating, workovers must be carried out from time to time in order to ensure the continuingproductivity of the wells Water and gas injection may be instituted to enhance well productivity Subseasatellite wells may be tied into the primary platform On the platform, the gas and oil must be separated.Produced water and sand must be separated out, with the water requiring treatment prior to dischargeinto the sea In some cases, oil is stored at the offshore platform, usually by the saltwater displacementsystem Discharge ballast water must have the hydrocarbons removed by separators to the concentrationpermitted by regulation

Most shipment of oil and of gas is by pipeline to a shore terminal Some gas is used on board to powerthe platform operation Flaring of gas from the platform is usually limited by regulation to the initialperiod of operations and to emergencies Oil may also be shipped by tanker In this case, a submarinepipeline from the platform runs to a loading buoy for direct transfer by means of a swivel head to thetanker

During the operating life of the platform, maintenance must be carried out, repairs performed, andmodifications made While these are usually relatively small contracts in magnitude, they frequently areequally demanding in terms of technical skill and specialized equipment

When the field has reached the end of its economic lifetime, usually after 20 to 30 years, regulations

of most countries provide that all facilities be removed to several meters below the mudline The wellsare capped and cemented off, and the topside equipment is removed Piles and well casing are cut off.The platform is now dismantled and removed

For other types of marine construction, whether it be in river harbors or coasts, an EnvironmentalImpact Statement is required and a permit must be obtained by the operator The conditions attached

to this permit are binding upon the constructor and have the force of law Similarly, the OccupationalSafety and Health Act requires full compliance by the constructor Often marine projects have special

The range of offshore and marine structures is very great A few of the more important types are illustrated

in Figures 0.6.1 to 0.6.10

Most structures in harbors, on coasts, in estuaries, and deep rivers are carried out by public bodies.The dominant current practices in the United States are to select an engineer to prepare plans andspecifications or, alternatively, to perform this work with an in-house engineering staff Then, separateconstruction contracts are awarded, usually on the basis of competitive bidding A growing trend world-wide is a single contract for design and construction In the case of very large projects, such as majoroverwater bridges, the contract may include financing Both BOT (build, own, and transfer) or BOOT(build, own, operate, and transfer) contracts are employed These contracts include the engineeringdesign to meet the basic criteria established by the agency Constructibility is thus a major consideration

by the contending constructors

This book is directed to the physical operations in the marine environment, their conception, planning,preparation, and execution Such operations, subsumed in the general category "construction;' obviouslyinvolve many design-related activities, including engineering of a degree of sophistication appropriate tothe activity Once completed, the structure must perform satisfactorily under service conditions whilesafely enduring extreme environmental events and credible accidents The structure must not sufferprogressive collapse as a result of such extreme events as earthquake, iceberg impact, extreme storm, oreven ship collision It must withstand the repeated loads typical of the marine environment: an offshoreplatform, for example, may experience 2 x 108cycles of wave loading during its design service life Harborstructures and bridges must withstand earthquakes; river structures must also withstand floods anderosive scour.

While broad references to the design of the structure cannot be avoided in a book on construction,the two aspects being inseparable, no attempt will be made to include detailed descriptions of analyticalprocedures for determining dynamic response, fatigue, soil consolidation, and so forth; instead, these

will be briefly categorized and identified at the beginning of each section, leaving a description ofappropriate analytical matters to other texts and technical papers, many of which are cited in theReferences at the end of this book Indeed, the technical literature in the field is extremely rich, consisting

of the journals of professional engineering societies and the proceedings of conferences and symposiasuch as the Offshore Technology Conferences, International Coastal Conferences, and Bridge Confer-ences Many of these sources are listed in the References

Consideration of the many demands of construction and their interaction with design, regulatoryrequirements, the environment, logistics, economics, schedule, risk, and reliability have led to the devel-

opment of the concept of constructibility: a new term describing a process which has been evolving over

many years Constructibility denotes a process that has input to every phase of an offshore project, fromconception to maintenance, repair, and eventual removal It requires consideration of all the applicableprovisions described in this book Because of the growing importance of this concept, a special chapter(Chapter 21) has been devoted to the methodology

Marine engineering practice is in the process of adopting the universal 51 units for measurement, butremnants of the English system and the older metric system (which employs kg (f) and centimeters) stillare found, sometimes all mixed in the same set of documents Thus yield strength of structural steel may

be expressed in MPa, N/mm2 (both 51), kg/mm2 (Japanese metric), or psi (U.S.) In general, the systemmost commonly employed will be given first, followed by the equivalent value in the other system withinparentheses

Offshore construction, with its many current opportunities and its tremendous demands and lenges, is emerging as one of the most exciting fields of engineering practice, one which will test human-kind's ability to rise to new heights of skill and courage

chal-Come, my friends

'Tis not too late to seek a newer world.

Push off, and sitting well in order, smite

The sounding furrows; for my purpose holds

To sail beyond the sunset and the baths

Of all the western stars, until I die.

Alfred, Lord Tennyson, "Ulysses"

The oceans present a unique set of environmental conditions which dominate the methods, equipment,support, and procedures to be employed in construction offshore This same unique environment also,

of course, dominates the design Many books have addressed the extreme environmental events andadverse exposures as they affect design Unfortunately, relatively little attention has been given in pub-lished texts to the environment's influence on construction Since the design of offshore structures isbased to a substantial degree upon the ability to construct, there is an obvious need to understand andadapt to environmental aspects as they affect construction These considerations are even more dominant

in many coastal projects where breaking waves and high surf make normal construction practices sible To a lesser extent, they have an important role in harbor and river construction

impos-In this chapter, the principal environmental factors will be examined individually As will be repeatedlyemphasized elsewhere in this book, a typical construction project will be subjected to many of theseconcurrently, and it will be necessary to consider their interaction with each other and with the con-struction activity

As noted in the Introduction, most marine and offshore construction takes place at substantial distancesfrom shore and even from other structures, often being out of sight over the horizon Thus, constructionactivities must be essentially self-supporting, able to be manned and operated with a minimum depen-dency on a shore-based infrastructure

Distance has a major impact upon the methods used for determining position and the practicalaccuracies obtainable The curvature of the Earth and the local deviations in sea level need to beconsidered Distance affects communication It necessitates arrangement of facilities to deliver fuel andspare parts and to transport personnel Distance requires that supervisory personnel at the site be capable

of interpreting and integrating all the many considerations of making appropriate decisions Distancealso produces psychological effects People involved in offshore construction must be able to worktogether in harmony and to endure long hours under often miserable conditions

Offshore regions extend from the coast to the deep ocean Construction operations have been alreadycarried out in 1500 m water depth, exploratory oil drilling operations in 6000 m, and offshore mining

15

16 Construction of Marine and Offshore Structures

tests in similar water depths The average depth of the ocean is 4000 m, the maximum over 10,000 m,deeper than Everest rises above sea level

The ocean depths, even those in which work is currently carried out, are inhospitable and essentiallydark, and thus require special equipment, tools, and procedures for location, control, operations, andcommunication Amazing technological developments have arisen to meet these demands: the worksubmersible, remote-operated vehicles (ROVs), fiber optics, acoustic imaging, and special gases for diveroperations While some of these advances have extended human's capabilities in the deep sea, it isimportant to recognize the limitations which depth still places on construction operations

The external pressure of seawater acting on a structure and all of its elements follows the simple hydrauliclaw that pressure is proportional to depth:

where h=depth, v =density of seawater, and P=unit pressure This can be very roughly expressed inthe SI system as 10 kN per square meter per meter of depth More accurately, for seawater, the density

is 1026 kg/m3•

Hydrostatic pressure acts uniformly in all directions: downward, sideways, and up The pressure is, ofcourse, influenced by wave action: directly below the crest, the hydrostatic pressure is determined by theelevation of the crest and is therefore greater than that directly below the trough This effect diminisheswith depth, with differences due to moderate waves becoming negligible at 100 m and those due to stormwaves becoming negligible at 200 m

Hydrostatic pressure is also transmitted through channels below structures and channels (pores) inthe soil The difference in pressure causes flow Flow is impeded by friction The distribution of hydrostaticpressure in the pores of soils under wave action is thus determined by the water depth, wavelength, waveheight, and friction within the pores or channels The effects from wave action usually disappear at 3 to

4 m in depth of soil

Hydrostatic pressure is linked with the concept of buoyancy Archimedes' principle is that a floatingobject displaces a weight of water equal to its own weight From another viewpoint, it can be seen thatthe body sinks into the fluid, in this case, seawater, until its weight is balanced by the upward hydrostaticpressure In the case of a submerged object, its net weight in water (preponderance) can also be thought

of as the air weight less either the displaced weight of water or the difference in hydrostatic pressuresacting upon it See Figure 1.2.1

Hydrostatic pressure not only exerts a collapsing force on structures in total, but also tends to compressthe materials themselves This latter can be significant at great depths, and even at shallower depths formaterials of low modulus, for example, plastic foam Confined liquids or gases, including air, also aredecreased in volume and increased in density when subjected to hydrostatic pressure This decreases thevolume and buoyancy while increasing the density

Hydrostatic pressure acts as a driving force to force water through permeable materials, membranes,cracks, and holes In the cases of cracks and very small holes, flow is impeded by frictional forces At thesame time, capillary forces may augment the hydrostatic force, and raise the water level above the ambient.Hydrostatic pressure acts in all directions Thus, on a large-diameter jacket leg, which has a temporaryclosure, it will produce both transverse circumferential compression and longitudinal compression Thecombined stresses may lead to buckling

It is important for the construction engineer to remember that full external hydrostatic pressure can

be exerted in even a relatively small hole, for example, an open prestressing duct or duct left by removal

of a slip-form climbing rod Hydrostatic pressure acting on gases or other fluids will transmit its pressure

at the interface to the other substance Thus, where an air cushion is utilized to provide increased buoyancy

to a structure, the pressure at the interface will be the hydrostatic pressure of the seawater

The density of seawater increases slightly with depth This can be important in determining net weight

of objects at great depths The density of seawater also varies with temperature, salinity, and the presence

of suspended solids such as silts See Chapter 22, "Construction in the Deep Sea;' in which the effectsare quantified

Special care must be taken during inshore or near-shore operations, where buoyancy, freeboard, andunderkeel clearance are critical, and where large masses of fresh water may be encountered, with their

18 Construction of Marine and Offshore Structures

lowered density and consequent effect on draft An example of such suddenly occurring reduction ofbuoyancy is the annual release of the lake behind St George Glacier in Cook Inlet, or a flood on theOrinoco River, whose effects may extend almost to Trinidad A more static situation exists north ofBahrain in the Arabian Gulf, where fresh water emerges from seafloor aquifers

The surface temperature in the seas varies widely from a low of -2°C (28°F) to a high of 32°C (90°F) The higher temperatures decrease rapidly with depth, reaching a steady-state value of about 2°C (35°F)

at a depth of 1000 m (3280 ft) However, water and soil temperatures at 250 m depth on Australia's

Northwest Shelf exceed 30°e.

Temperatures of individual masses and strata of seawater are generally distinct, with abrupt changesacross the thermal boundaries This enables ready identification of global currents; for example, a rise

in temperature of as much as 2°C may occur when entering the Gulf Stream.

While horizontal differentiation (on the surface) has long been known, vertical differentiation andupwelling have recently been determined as major phenomena in the circulation of the sea Rather definiteboundaries separate zones of slightly different temperature, chemistry, and density These zones will haverecognizably different acoustic and light transmission properties, and the boundaries may give reflectionsfrom sonic transmissions

Temperature affects the growth of marine organisms, both directly and by its effect on the amount ofdissolved oxygen in the water Marine organisms are very sensitive to sudden changes in the temperature:

a sudden rise or fall produces a severe shock that either inhibits growth or kills Cold water containsmore dissolved oxygen than warm water

Air temperatures show much greater variation In the tropics, day air temperatures may reach 40°e.

In semi-enclosed areas such as the Arabian-Persian Gulf and the Arabian Sea, air temperatures may evenreach SO°e Humidity is extremely high in such areas, resulting in rapid evaporation, which can produce

a "salt fog" in the mornings, causing saline condensation to form on the surfaces of structures

The other extreme is the Arctic, where air temperatures over the ice may reach -40°C to -SO°e When the wind blows, air friction usually raises the temperature 10 to 20°e However, the combination of low

temperature and wind produces "wind chill," which severely affects ability of people to work Wind maysimilarly remove heat from materials (e.g., weldments or concrete surfaces) far more rapidly than whenthe air is merely cold but still

Air temperature in the temperate zones varies between these extremes The ocean's thermal capacity,however, tends to moderate air temperatures from the extremes that occur over land The rate of soundtransmission varies with temperature The temperature of the surrounding seawater has an importanteffect on the behavior of material, since it may be below the transition temperature for many steels,leading to brittle failure under impact Properties of many other materials, such as concrete, improveslightly at these lower temperatures Chemical reactions take place more slowly at lower temperatures:this combined with the decrease in oxygen content with depth reduces greatly the rate of corrosion forfully submerged structures

Temperature also has a major effect on the density (pressure) of enclosed fluids and gases which may

be used to provide buoyancy and pressurization during construction The steady temperature of theseawater will constantly tend to bring the enclosed fluid to the same temperature Where this enclosedfluid, such as oil, is subject to transient phenomena, density and thermal gradients will be set up in it.The atmosphere immediately above seawater is greatly modified by the water temperature Neverthe-less, it can be substantially below freezing, as for example in the sub-Arctic, or substantially above thewater temperature, as in areas off Peru, where cold water contrasts with warm air This produces a thermalgradient and thermal strains in structures which pierce the water plane These above-water structuresmay also be directly heated by the sun Thus there may be a significant expansion of the deck of a barge

or pontoon, leading to overall bending of the hull, with high shears in the sides and longitudinal bulkheads.Conversely,at night, the radiation cooling may lower the air temperature well below that of day

Physical Environmental Aspects of Marine and Offshore Construction 19

Where the structure contains heated products (e.g., hot oil) or extremely cold products (such asliquefied natural gas, LNG), the thermal strains may be severe and require special attention, particularly

at points of rigidity such as structural intersections and corners These thermal strains are discussed morefully in Chapter 4

The dominant chemical characteristic of seawater is, of course, its dissolved salts, which typically stitutes 35 parts per thousand (3.5%) by weight The principal ions are sodium, magnesium, chloride,and sulfate These ions are of importance to the construction of structures in the ocean in many ways.Chloride (Cl-) acts to reduce the protective oxidized coatings which form on steel and thus acceleratescorrosion

con-Magnesium (Mg2+)will gradually replace the calcium in various chemical constituents of hardenedconcrete Magnesium salts are soft and tend to high permeability Sulfates(SOl-) attack concrete, espe-cially in fresh water They affect both the cement paste and the aggregates, causing expansion anddisintegration Fortunately, the other constitutents of seawater tend to inhibit sulfate attack

Oxygen is present in the air immediately adjacent to the seawater-air interface and is also present inthe water in the form of entrapped air bubbles and dissolved oxygen Oxygen plays an essential role inthe corrosion of steel in the sea environment, whether the steel is exposed, coated, or encased in concrete.Carbon dioxide (C02) and hydrogen sulfide (H2S) are also dissolved in seawater in varying degreesdepending on location and temperature They lower the pH of seawater In addition, H2S may causehydrogen embrittlement of steel

Entrapped bubbles of water vapor, as in foam, may collapse suddenly, leading to cavitation which pitsand erodes the surface of concrete structures This phenomenon occurs when the surface of a structure

is exposed to high-velocity local flow, as with surf, or over a spillway

Silt and clay are suspended in water, usually in colloidal form, as the result of river runoff and also asthe result of bottom erosion and scour due to current and waves Colloidal silt in fresh water will dropout of suspension upon encountering seawater: this, as well as reduced velocity, accounts for the formation

of deltas The zone or band where such deposition takes place is often very narrow, resulting in adisproportionate deposition and buildup in this zone Fine sand, silts, and clays, and even gravel mayalso be carried along with strong currents or wave action to be deposited as soon as the velocity dropsbelow critical for that particular particle size and density This results in horizontal stratification ofdeposits The colloidal and suspended silts render vision and optics difficult due to their turbidity, whichscatters light rays Thus in many harbors, rivers, and estuaries, diver and submersible observations arelimited when using normal light spectra

Moving silt, sand, and gravel may erode surfaces, removing coatings and paint as well as the protectivefilm of rust from steel, exposing fresh surfaces to corrosion

Marine organisms have a number of adverse effects upon sea structures The first is the increase ofdrag due to the obstruction of the free flow of water past the surface of the structure This is the "fouling"

of ship bottoms Mussels may clog intakes to power plants Eels have entered circulating water systemsand then have grown and plugged the system Barnacles and algae increase the diameter of steel piles.Fouling increases the size of the member and, more importantly, increases the surface roughness Because

of this latter, the drag coefficient, CD>used in Morrison's equation is often increased by10to20%

Fortunately, most marine organisms have a specific gravity only slightly greater than that of the seawateritself; thus they do not add an appreciable mass Fortunately, also they also tend to be fragile, and areoften torn or broken off by storms Barnacles and sea urchins secrete an acid which pits and erodes steel.Sea urchins are partially active near the sand line and can attack the steel piling and jacket legs.Mollusks secreting acids bore into rocks and soft concrete Very aggressive mollusks exist in theArabian-Persian Gulf These bore holes into the hard limestone aggregate of high-strength concrete: they

20 Construction of Marine and Offshore Structures

also can eat through bitumastic coatings on steel piles Marine organisms have occurred at depths up to

60 m, but are concentrated near the surface where sunlight penetrates

Of particular importance to the constructor is the attack of marine organisms on timbers Teredoenter into wood through a relatively small hole, eating out the heart, while Limnoria attack the surface

of the wood, generally in the tidal range The action of teredo may be very rapid, especially in fast-flowingclean seawater Untreated timber piles have been eaten off within a period of 3 months!

Fish bite, attacking fiber mooring lines, is of increasing concern for deep-sea operations Sharksapparently exercise their teeth on the lines, causing them to fray, which then attracts smaller fish Fishbite is especially severe in the first month or two of exposure, apparently due to the curiosity of thesharks Fish bite attacks occur in depths up to 1000 m in sub-Arctic waters and probably twice that depth

in tropical waters

Marine organisms play a major role in the soil formation on the seafloor and in disturbing andreworking the surficial soils Walruses apparently plow up large areas of sub-Arctic seafloors in search ofmollusks, leading to turbidity and erosion Algae and slime can form very rapidly on the surfaces ofstones and riprap, preventing the subsequent bond with grout and concrete Mussels, especially zebramussels can rapidly build up clusters on substrates of stone and steel In the case of the anchorage caissonfor the Great Belt suspension bridge, a cluster of mussels built up in the short interval between finalscreeding and the placement of the caisson, preventing it from proper seating

Marine growth is influenced by temperature, oxygen content, pH, salinity, current, turbidity, and light.While the majority of growth takes place in the upper 20 m, significant growth has occasionally beenfound at three times that depth Algae inhibitors such as copper sulfate and covering to cut off sunlighthave been employed to protect enclosed areas temporarily during construction

Anaerobic sulfur-based bacteria are often trapped in the ancient sediments of the oil reservoir Uponrelease to the saltwater, they convert to sulfates, and upon subsequent contact with air they producesulfides (H2S) These bacteria and the sulfides they produce, with the dramatic scientific name of Theo-

bacillus concretivorous, attack weak and permeable concrete as well as causing pitting corrosion in steel.

Even more serious, the hydrogen sulfide which is formed is deadly poisonous and may be odorless Hence,entry to compartments previously filled with stored oil must be preceded by thorough purging not only

of hydrocarbons, but also of any hydrogen sulfide These anaerobic bacteria may also react with each

other to produce methane and hydrogen Theobacillus bacteria in a seawater canal in the Arabian Gulf

have attacked polysulfide sealants, turning them into a spongy mass

Currents, even when small in magnitude, have a significant effect on construction operations Theyobviously have an influence on the movement of vessels and floating structures and on moorings Theychange the characteristics of waves They exert horizontal pressures against structural surfaces and, due

to the Bernoulli effect, develop uplift or down drag forces on horizontal surfaces Currents create eddypatterns around structures, which may lead to scour and erosion of the soils Currents may cause vortexshedding on piles, tethers, and piping

Even before the start of construction, currents may have created scour channels and areas of deposition,thus creating surficial discontinuities at the construction site The vertical profile of currents is conven-tionally shown as decreasing with depth as a parabolic function Recent studies in the ocean and onactual deepwater projects indicate, however, that in many cases, the steady-state current velocities justabove the seafloor are almost as high as those nearer the surface There are substantial currents in thedeep sea, just above the seafloor

There are several different types of currents: oceanic circulation, geostrophic, tidal, wind-driven, anddensity currents, as well as currents due to river discharge Currents in a river vary laterally and withdepth The highest river currents usually occur near the outer edge of a bend River currents are alsoaugmented locally around the head of jetties and groins Some of these may be superimposed upon eachother, often in different directions See Figure 1.5.1

The worldwide ocean circulatory system produces such currents as the Gulf Stream, with a defined "channel" and direction and velocity of flow Other major current systems exist but are often morediffuse, having a general trend but without the characteristics of a river Thus the prevailing southeasterlytrending current along the California and Oregon coasts givesan overall southward movement to sedimentarymaterials from river outflows These major currents may occasionally,often periodically, spin off eddies andbranches; the lateral boundaries of the current are thus quite variable Strong currents may thus occur manymiles from the normal path of a current such as the Gulf Stream Within local coastal configurations, a branch

relativelywell-of the main current may sweep in toward shore or even eddy back close to shore

Recent research has indicated that many of these current "streams" are fed by upwelling or downwardmovements of the waters and that there are substantial vertical components These will become ofimportance as structures are planned and built in deeper waters and will require that accurate measure-ments be taken at all depths, for both vertical and horizontal components of the current

Another major source of currents is tidal.changes The stronger tidal currents are usually in proximity

to shore but may extend a considerable distance offshore where they are channeled by subsurface reefs

or bathymetry While they generally follow the tidal cycle, they frequently lag it by V2 to 1 hour; thus a

tidal current may continue flooding on the surface for a short period after the tide has started to fall.Actually tidal currents are often stratified vertically, so that the lower waters may be flowing in whilethe upper waters are flowing out This is particularly noticeable where tidal currents are combined withriver currents or where relatively fresh water of lower density overlies heavier saltwater This stratificationand directional opposition also occurs at the entrance to major bodies of water, e.g., at the Strait ofGibraltar where evaporation from the Mediterranean produces a net inflow

Since tidal currents are generally changing four times a day, it follows that their velocity and directionare constantly changing Since the velocity head or pressure acting on a structure varies as the square ofthis current velocity, it can have a major effect on the mooring and control of structures during criticalphases of installation The current velocities are also superimposed on the orbital particle velocities ofthe waves, with the pressure and hence forces being proportional to the square of the vectorial addition.While in regular harbor channels the tidal currents may move in and out along a single path, at mostoffshore sites the shoreline and subsurface configurations cause the directions to alter significantly,perhaps even rotate, during the tidal cycle Ebb currents may be directed not only 180°but often 150°, 120°, or even90° from flood currents, and this variance itself may change periodically Tidal currents

may reach speeds of 7 knots (3.6 m/s) and more.

River currents, especially those of great rivers with large discharges, such as the Orinoco, extend farout to sea Because the density of the water is less, and perhaps because of silt content, the masses ofwater tend to persist without mixing for a long period; thus substantial surface currents may reach to

22 Construction of Marine and Offshore Structures

considerable distances from shore River currents may, as indicated earlier, combine with tidal currents

to produce much higher velocities on ebb and reduced velocities on flood

Wind persisting for a long period of time causes a movement of the surface water that is particularlypronounced adjacent to shallow coasts This may augment, modify, or reverse coastal currents due toother causes

Deep-water waves create oscillatory currents on the seafloor, so that there is little net translation ofsoil particles due to waves alone When, however, a wave current is superimposed upon a steady-statecurrent, the sediment transport is noticeably increased, since its magnitude varies as the cube of theinstantaneous current velocity The vertical pressure differentials from the waves lift the soil particles,which are then transported by the current

Adjacent to the shore, the translational movement of the waves produces definite currents, with waterflowing in on top and out either underneath or in channels Thus a typical pattern of the sea will be tobuild up an offshore bar, over which the waves move shoreward and break on the beach This piles excesswater on the beach, which may move laterally, then run out to the sea The outflowing current cutschannels in the offshore bar The seaward-flowing current becomes the infamous "undertow." Theselateral and seaward-flowing currents may be a hazard or may be taken advantage of to keep a dredgedchannel clear through the surf zone

In the deeper ocean, currents are generated by internal waves, by geostrophic forces, and by deeplypromulgated eddies from major ocean streams such as the Gulf Stream It appears that currents ofmagnitudes up to 0.5 knots exist on the continental shelf and slope and that currents up to 2.6 knots

Strong currents can cause vortex shedding on risers and piles, and vibration of wire lines and pipelines.Vortex shedding can result in scour in shallow water It can also result in cyclic dynamic oscillations ofcables, tethers, moorings, and vertical tubulars, such as piling, which can lead to fatigue Vortices occurabove a critical velocity, typically 2 to 3 knots These vortices spin off in a regular pattern, creatingalternating zones of low pressure Vortices and whirlpools can form at the edge of obstructions to riverflow, e.g., around the end of groins or at the edge of an underwater sand wave, leading to severe local scour

As mentioned earlier, water moving over a submerged surface or under the base of a structure produces

a vertical pressure (uplift or downdrag) in accordance with Bernoulli's theorem This can cause significantconstructional problems, of which the following examples may be given:

1 A large concrete tank being submerged in the Bay of Biscay had its compartments accurately sizedfor filling to create a known preponderance for controlled sinking, without free surface Whenfilled and submerged a few meters, the waves moving over the top had their oscillatory motionchanged to a translatory current, thus creating an uplift force This has been called the "beacheffect." The tank would sink no further When, as an emergency measure, additional ballast wasadded to cause sinking to continue, then at a depth of some 30 m, the current effect was reducedand the uplift force diminished The tank was now too heavy and plunged rapidly See Figure 1.5.2

2 A caisson being submerged to the seafloor behaves normally until close to the bottom, when thecurrent is trapped beneath the base and its velocity increases This "pulls" the caisson down, while

at the same time tending to scour a previously prepared gravel base In loose sediments, such asthe Mississippi River,the loose sand "mudline" may drop almost as fast as the caisson is submerged,unless antiscour mattresses are placed beforehand

3 A pipeline set on the seafloor is subjected to a strong current which erodes the sand backfill fromaround it The pipeline is now subject to uplift (from the increased current flowing over it) and raisesoff the bottom The current now can flow underneath; this pulls the pipeline back to the seafloor,where the process can be repeated Eventually the pipeline may fail in fatigue See Figure 1.5.3

4 The placement of a structure such as a cofferdam in a river leads to accelerated currents aroundthe leading corners, and the formation of a deep scour hole either at the corners or some distancedownstream where a vortex has formed These have reached depths of 10 m below the adjoiningbottom and have resulted in general instability

Installation of a box caisson pier in the 0resund crossing between Denmark and Sweden led tosignificant erosion along and under one corner of the base due to currents induced by a storm Currentsproduce both scour and deposition It is important to note that eddies formed at the upstream anddownstream corners of structures, such as those of a rectangular caisson, produce deep holes, whereasdeposition may occur at the frontal and rear faces.

Scour is extremely difficult to predict Model studies indicate tendencies and critical locations but areusually not quantitatively accurate because of the inability to model the viscosity of water, the grain sizeand density, and the effect of pore pressures However, models can be effectively utilized to predict howthe currents will be modified around a particular structure

Currents have a significant effect on the wave profile A following current will lengthen the apparentwavelength and flatten the wave out, so that its slopes are much less steep Conversely, an opposingcurrent will shorten the wavelength, increasing the height and steepness Thus at an ocean site affected

by strong tidal currents, the same incident waves will have quite different effects on the constructionoperations, depending on the phases of the tidal cycle See Figure 1.5.4

Currents have a serious effect on towing speed and time, a following current increasing the effectivespeed, an opposing current decreasing it Translated into time, the decrease in time for a tow of a givendistance is only marginally improved by a following current, whereas an opposing current may signifi-cantly increase the time required

By way of example, assume that a towboat can tow a barge 120 miles at 6 knots in still water, thusrequiring 20 hours With a following current of 2 knots, the trip will take only 120/(6+2) or 15 hours,

a saving of 5 hours or 25% With an opposing current of 2 knots, the trip will require 120/(6 - 2) or 30hours, an increase of 10 hours or 50%

Waves are perhaps the most obvious environmental concern for operations offshore They cause a floatingstructure or vessel to respond in six degrees of freedom: heave, pitch, roll, sway, surge, and yaw Theyconstitute the primary cause of downtime and reduced operating efficiency The forces exerted by wavesare usually the dominant design criterion affecting fixed structures See Figure 1.6.1

Waves are primarily caused by the action of wind on water, which through friction transmits energyfrom the wind into wave energy Waves that are still under the action of the wind are called "waves,"whereas when these same waves have been transmitted beyond the wind-affected zone by distance ortime, they are called "swells."

Water waves can also be generated by other phenomena, such as high currents, landslides, explosions,and earthquakes Those associated with earthquakes (e.g., tsunamis) will be dealt with in Section 1.11

A wave is a traveling disturbance of the sea surface The disturbance travels, but the water particles withinthe wave move in a nearly closed elliptical orbit, with little net forward motion

Wave and swell conditions can be predicted from a knowledge of the over-ocean winds Routineforecasts are now available for a number of offshore operating areas They are provided by governmentalservices such as the U.S Naval Fleet Numerical Weather Control at Monterey, California Many privatecompanies now offer similar services These forecasts are generally based on a very coarse grid, whichunfortunately may miss local storms such as extratropical cyclones

The height of a wave is governed by the wind speed, duration, and fetch (the distance that the windblows over open water) The wave height for a fully developed storm can be roughly calculated by theformula:

Deep-water wave forecasting curves can be prepared as a guide See Figure 1.6.2 These values aremodified slightly by temperature; for example, if the air is lOOCcolder than the sea, the waves will be20% higher, due to the greater density and hence energy in the wind This can be significant in the sub-Arctic and Arctic.

Some interesting ratios can be deduced from Figure 1.6.2:

1 A lO-fold increase in fetch increases the wave height 2.5 times

2 A fivefold increase in wind velocity increases the wave height 13 times

3 The minimum-duration curves indicate the duration which the wind must blow in order for thewaves to reach their maximum height The stronger the wind, the less time required to reach fulldevelopment of the waves

The total energy in a wave is proportional to the square of the wave height While wave height isobviously an important parameter, wave period may be of equal concern to the constructor Figure 1.6.2gives the typical period associated with a fully developed wave in deep water Long-period waves havegreat energy When the length of a moored vessel is less than one half the wavelength, it will see greatlyincreased dynamic surge forces

Waves vary markedly within a site even at the same time Therefore, they are generally characterized

by their significant height and significant period The significant height of a wave is the average of thehighest one third of the waves It has been found from experience that this is what an experienced marinerwill report as being the height of the waves in a storm If the duration of the strong wind is limited to

Physical Environmental Aspects of Marine and Offshore Construction 27

less than the minimum duration, then the wave height will be proportional to the square root of theduration A sudden squall will not be able to kick up much of a sea

The majority of waves are generated by cyclonic storms which rotate counterclockwise in the northernhemisphere and clockwise in the southern hemisphere The storm itself moves rather slowly, as comparedwith the waves themselves Thus the waves travel out ahead of the generating area Waves within the

generating area are termed seas, those which move out ahead are termed swells Swells can reach for

hundreds and even thousands of miles The area embraced by the cyclone can be divided into fourquadrants The "dangerous quadrant" is the one in which the storm's forward movement adds to theorbital wind velocity

The Antarctic continent is completely surrounded by open water It is an area of intense cyclonicactivity Storms travel all the way around the continent, sending out swells that reach to the equator andbeyond The west coast of Mrica, from southwest Africa to Nigeria and the Ivory Coast, and the westcoast of Tasmania are notorious for the long swells that arrive from Antarctica The long, high-energyswells that arrive at the coast of southern California in May are generated by tropical hurricanes in theSouth Pacific

The swells eventually decay Energy is lost due to internal friction and friction with the air The period (high-frequency) waves are filtered out first, so that it is the longest of the long-period swellswhich reach farthest Swells tend to be more regular, each similar to the other, than waves Whereas wavestypically have significant periods of 5 to 15 seconds, swells may develop periods as great as 20 to 30 s ormore The energy in swells is proportional to their length Thus even relatively low swells can cause severeforces on moored vessels and structures

shorter-Deep-sea waves tend to travel in groups, with a series of higher waves followed by a series of lowerwaves The velocity of the group of waves is about half the velocity of the individual waves This, ofcourse, gives the opportunity to wait until a period of successive low waves arrives before carrying outcertain critical construction operations of short duration, for example, the setting of a load on a platformdeck or the stabbing of a pile Such periods of low waves may last for several minutes

The average wave height is about 0.63 Hs' Only 10% of the waves are higher than Hs One wave in

1000 is 1.86 times higher than H,;this is often considered to be the "maximum" wave, but more recentstudies show that the value may be closer to 2 Wave height H is the vertical distance from trough to

crest, and T is the period, the elapsed time between the passage of two crests past a point Wavelength

L is the horizontal distance between two crests Velocity V,often termed celerity C, is the speed ofpropagation of the wave Rough rule-of-thumb relationships exist between several of these factors

In the SI and metric systems:

28 Construction of Marine and Offshore Structures

As noted in Section 1.5, currents have a significant effect upon wavelength, steepness, and height Afollowing current increases the length and decreases the height, whereas an opposing current decreasesthe wavelength and increases the height, thus significantly increasing the wave steepness (SeeFigure 1.5.4.) Note that the influence of an opposing current is much more pronounced than that of afollowing current Note also that the wave period remains constant When seas or swells meet a strongcurrent at an angle, very confused seas result, with the wave crests becoming shorter, steeper, and sharperand thus hazardous for offshore operations

Seas are often a combination of local wind waves from one direction and swells from another Wavesfrom a storm at the site may be superimposed upon the swells running out ahead of a second storm that

is still hundreds of miles away The result will be confused seas with occasional pyramidal waves andtroughs

Waves are not "long-crested"; rather the length of the crest is limited The crest length of wind wavesaverages 1.5 to 2.0 times the wavelength The crest length of swells averages three to four times thewavelength These crests are not all oriented parallel to one another but have a directional spread Windwaves have more spread than swells From a practical, operational point of view, the majority of swells

tend to be oriented within ±15°, whereas wind waves may have a ±25° spread.

When waves in deep water reach a steepness greater than 1 in 13, they break When these breakingwaves impact against the side of a vessel or structure, they exert a very high local force, which in extreme

cases may reach 30 tons/m 2 (0.3 MPa), or 40 psi The areas subjected to such intense forces are limited,and the impact itself is of very short duration; however, these wave impact forces are similar to theslamming forces on the bow of a ship and thus may control the local design of floating constructionequipment

Data on wave climates for the various oceans are published by a number of governmental organizations.The U.S National Oceanic and Atmospheric Administration (NOAA) publishes very complete sets ofweather condition tables entitled "Summaries of Synoptic Meteorological Observations" (SSMOs), based

on data compiled from ship observations and ocean data buoys The published tables tend to timate the wave heights and period in the Pacific; recent data for the Pacific indicates that there issignificant wave energy in longer periods (e.g., 20 to 22 s) during severe storms The swells from suchstorms may affect operations even at a distance of many hundreds of miles

underes-The "persistence" of wave environmental conditions is of great importance to construction operations.Persistence is an indication of the number of successive days oflow sea states one may expect to experience

at a given site and season To the offshore constructor, persistence is quite a different thing than apercentage chart of sea states exceeding various heights

For example, assume that the limiting sea state for a particular piece of construction equipment is21/2m The percentage exceedance chart may show that seas greater than 21/2m occur 20% of the month

in question This could consist of two storms of 3-day duration each, interspersed between two 12-dayperiods of calm Such a wave climate would allow efficient construction operations Alternatively, this20% exceedance could consist of 10 hours of high waves every other day, as typically occurs in the BassStraits between Australia and Tasmania Such a wave climate is essentially unworkable with conventionalmarine equipment

Typical persistence charts are shown in Figures 1.6.3 and 1.6.4 Further discussion on persistence isfound in Section 1.7 Wave height-wave period relationships are shown in Figure 1.6.5

As swells and waves approach the land or shoal areas, the bottom friction causes them to slow down;thus the wave front will refract around toward normal with the shore This is why waves almost alwaysbreak onshore even though the winds may be blowing parallel to it Multiple refractions can createconfused seas and make it difficult to orient a construction barge or vessel for optimum operationalefficiency At some locations, two refraction patterns will superimpose, increasing the wave height andsteepness

Submerged natural shoals and artificial berms increase the wave height and focus the wave energytoward the center Waves running around a small island, natural or artificial, not only refract to convergetheir energy on the central portion but run around the island to meet in the rear in a series of pyramidal

peaks and troughs Such amplification of waves and the resultant confusion of the sea surface may makenormal construction operations almost impossible Running along or around the vertical face of a caisson,waves will progressively build up in an effect known as "mach stem" and spill over onto the island, butwithout radial impact Both phenomena combined to cause overtopping and difficulty in operations atthe Tarsiut Offshore Drilling Island in the Canadian Beaufort Sea.

Waves approaching a shore having a deep inlet or trench through the surf zone will refract away fromthe inlet leaving it relatively calm, while increasing the wave energy breaking in the shallow water oneither side As waves and swells move from deep water into shallow water, their characteristics changedramatically Only their period remains essentially the same The wavelength shortens and the heightincreases This, of course, leads to steepening of the wave, until it eventually breaks