Chapter 14
Waves and offshore
engineering
14.1 Introduction
A large number of offshore structures of various types and sizes have been
built to extract fossil fuels from wells at considerable depths below seabed.
Offshore engineering is a multi-disciplinary activity with inputs from civil,
mechanical, marine and chemical engineering and geology.
Offshore structures may be either fixed or floating and they support
platforms which in turn support various components such as drilling der-
ricks, processors, flare stacks, radio masts, accommodation and a helideck.
Steel or concrete or a combination of both is used for the construction.
The majority of structures are fixed jacket platforms that are tubular steel
members welded together to form a three-dimensional truss and held in
place by means of piles driven through the jacket legs into the sea floor.
Another common fixed structure is one that has a massive concrete base
that rests on the seabed and the stability of this installation is ensured by
its weight. Once the structure is in position, the base can be used as a
storage tank for crude oil. A floating structure on the other hand is held in
position by either cables or vertical tethers anchored to the sea bed or it
may simply be a floating rig connected to a wellhead located at the sea bed
and kept in position by thrusters. Chakrabarti (1987) describes the differ-
ent types of offshore platforms and Mathur (1995) gives an overview of
the various offshore activities concerning the production of the oil and gas.
Oil is usually transported from the offshore platform by means of
tankers or submarine pipelines. These pipelines are usually but not always
buried in the seabed but they will be exposed to waves and currents during
pipe laying and in deep waters. The common geometrical shape of the
various structural elements forming an offshore platform and obviously
of submarine pipelines is cylindrical. They can be subjected to severe
environmental forces due to wind, currents and waves.
Under unfavourable conditions flow-induced vibrations could occur