This chapter describes the factors influencing the design of semisubmersible floating production systems, exemplified by a design produced by the authors' company.
The oil price crisis of 1973 stimulated oil and gas exploration and production in Northern European waters The initial field developments were a significant extrapolation of existing offshore technology, and many of these early developments suffered cost and programme overruns due to technical problems, cost underestimation and inflation. However, the oil price continued to rise and the great majority of the early installations were profitable despite their high costs. With the experience now available and relatively low levels of inflation, it is possible to build fixed oil production installations with a high confidence that they will be completed to programme and within budget. The declining oil price and the fact that most of the yet undeveloped fields have small or difficult reservoirs or are in very deep water means that, at the present time, very few offshore oil fields in North-West Europe can be economically developed using conventional fixed structures One solution would be to take no action on existing but unexploited discoveries but, generally, exploitation is preferred if an economical solution can be found. Floating production systems can provide a solution which makes the production from these marginal reservoirs economical There have, so far, been relatively few floating production systems in North-West European waters and the semisubmersibles have all been conversions of drilling rigs or an adaptation of an existing drilling rig design. As the requirements for floating production systems become more severe, purpose-designed vessels will become the only economical field development solution. This chapter is intended to highlight the main problems in producing such a design, and it is hoped that this will lead to better understanding of the rather different philosophy required for a floating system compared with a fixed structure.
Floating production systems are being used in many different parts of the world The designs can vary dramatically according to the local environmental conditions. This chapter addresses a solution for environments such as the seas off northern Norway. The environment is generally cold and windy with a high incidence of rough seas. Survival waves exceed 30 m and water depths range to more than 500 m.
The development of floating equipment for drilling or production has traditionally been carried out by engineering companies who supply to oil companies or service contractors. The engineering companies normally subcontract the actual building of the vessel to suitable shipyards or fabrication yards.