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Pile Design and Construction Practice, Fifth edition

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Piling for marine structures 417<br />

relatively shallow <strong>and</strong> sheltered waters. Whereas in berthing structures the dominant forces<br />

are those caused by the berthing of ships, the offshore platform is served only by small<br />

vessels <strong>and</strong> the environmental forces resulting from waves, winds, <strong>and</strong> currents have a<br />

dominating influence on design. In very deep water, the environmental forces can account<br />

for three-quarters of the total load on a main supporting member.<br />

The economics in the design <strong>and</strong> construction of offshore platforms for petroleum <strong>and</strong> gas<br />

production <strong>and</strong> wind farms are viewed from a st<strong>and</strong>point very different from that applied to<br />

jetty design. In the case of jetties <strong>and</strong> wind turbine installations the main requirements are<br />

low capital cost, ease of maintenance, <strong>and</strong> a long life. The time required for construction is<br />

not usually a critical factor in the design of shear-shore structures. However, only a limited life<br />

is required from oil <strong>and</strong> gas production platforms but assurance of stability in the most severe<br />

exposure conditions is of vital importance, <strong>and</strong> rapidity of installation at sea is essential.<br />

This is because of the limited periods during which the state of the sea will permit the<br />

operation of large floating cranes <strong>and</strong> other constructional plant.<br />

The principal activity for the offshore construction industry in the 1970s <strong>and</strong> the 1980s<br />

was in the fabrication <strong>and</strong> installation of platforms for oil <strong>and</strong> gas production <strong>and</strong> laying<br />

sea-bed pipelines. The platforms constructed in the UK <strong>and</strong> Western Europe were mainly in<br />

the relatively shallow waters of the North Sea where multi-pile foundations of the type<br />

shown in Figure 8.15 were an economical form of construction. Gravity base platforms<br />

where the foundation consisted of a large caisson floated into place <strong>and</strong> sunk on to the<br />

prepared sea bed were also constructed.<br />

Figure 8.15 Fabrication of platform for Forties Field (North Sea) of British Petroleum Co., showing<br />

guides for clusters of piles around each leg.

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