Semisubmersibles and FPSOs are the two most common floating production solutions and they compete in many field developments. The first production semisubmersible in 1975 preceded the prototype FPSO by two years. Both concepts were more or less on a par for the next two decades, until the dramatic surge in popularity of the FPSO in the late 1990s. However, the continued adoption of semisubmersibles indicates that they retain advantages over FPSOs in certain situations.
Semisubmersibles have seen application in most petroleum provinces around the world outside Australasia. Traditionally semisubmersibles were converted drilling rigs used for EPS and short field life developments. Many are located in only moderate water depths. However, recent semisubmersibles have been adopted for large production facilities. They have also been used in ultra-deepwater, and several semi-submersibles have held deepwater production records. Many of these developments are off Brazil and Norway. These changes in focus have required new-build semisubmersibles with novel hull forms in comparison with traditional drilling units.
The early popularity of semisubmersibles over FPSOs is related to their advantages in harsh environments. With the development and advancement of internal turrets, FPSOs have recently seen acceptance in severe North Sea conditions. Today, a principal driver for selection between semi-submersibles and FPSOs is the relative importance of workover capability and infield storage. Other key factors include the opportunity for gas export, the number of risers required and the availability of vessels for conversion.
The paper traces the evolution of the production semisubmersible and gives a comprehensive database of applications to date. A comparison of the relative merits of semisubmersibles and FPSOs for different production scenarios is also included.
Introduction
The forerunner of the semisubmersible was the submersible. A submersible barge is floated to location and then ballasted down to sit on the seafloor prior to operation. As the deck must remain above water, submersibles are suitable only for shallow water. The first submersible for open water use was constructed in 1948 and the last was built in 1963 for a water depth of 53m.
The first semisubmersible appeared in 1962. Semisubmersibles were built initially for drilling operations; however, they are also used for other temporary functions including accommodation and installation. During the 1970s, when much research was undertaken to develop alternatives to fixed platforms for deepwater application, conversion of a semisubmersible to production was seen as an attractive field development option. Semisubmersibles have low motions in operation, can support a large number of risers and are able to combine production with drilling and workover services. They are also suitable for a range of water depths and environments.
The first application of a semisubmersible for the more permanent function of oil production was in 1975. Argyll was also the first offshore oil production in the UK. At this time semisubmersibles, rather than drillships, were preferred in the North Sea due to their superior operating characteristics. Hence, a semi-submersible production vessel was a natural choice over a monohull.
Further semisubmersibles have entered production at a relatively steady rate until the present time, where there are around 33 units in operation. Production semisubmersibles have evolved from simple converted drilling rigs used as early production systems (EPS) or for marginal oil fields in moderate water depths, to recent new-build units for ultra-deepwater and large oil and gas accumulations. These and other details of production semisubmersible applications to date are listed in the Appendix and discussed in the Sections below.
