relatively shallow and sheltered waters. Whereas in berthing structures the dominant forces
are those caused by the berthing of ships, the offshore platform is served only by small
vessels and the environmental forces resulting from waves, winds, and currents have a
dominating influence on design. In very deep water, the environmental forces can account
for three-quarters of the total load on a main supporting member.
The economics in the design and construction of offshore platforms for petroleum and gas
production and wind farms are viewed from a standpoint very different from that applied to
jetty design. In the case of jetties and wind turbine installations the main requirements are
low capital cost, ease of maintenance, and a long life. The time required for construction is
not usually a critical factor in the design of shear-shore structures. However, only a limited life
is required from oil and gas production platforms but assurance of stability in the most severe
exposure conditions is of vital importance, and rapidity of installation at sea is essential.
This is because of the limited periods during which the state of the sea will permit the
operation of large floating cranes and other constructional plant.
The principal activity for the offshore construction industry in the 1970s and the 1980s
was in the fabrication and installation of platforms for oil and gas production and laying
sea-bed pipelines. The platforms constructed in the UK and Western Europe were mainly in
the relatively shallow waters of the North Sea where multi-pile foundations of the type
shown in Figure 8.15 were an economical form of construction. Gravity base platforms
where the foundation consisted of a large caisson floated into place and sunk on to the
prepared sea bed were also constructed.
Piling for marine structures 417Figure 8.15Fabrication of platform for Forties Field (North Sea) of British Petroleum Co., showing
guides for clusters of piles around each leg.