The Stabilised Platform (STAPLA) is a variable draught semi-submersible vessel Which is capable of supporting both drilling (up to 1000m of water depth) and production. The vessel is able to change draught from 50m to 10m enabling it to be brought into a conventional dock for inspection, repairs and refitting for re-use on another marginal field if required. The vessel provides the maximum flexibility to suit the differing requirements of different fields. STAPLA has been designed to withstand the most severe environmental conditions which are expected in the North Sea and offers improved motion response characteristics over conventional designs because of its deep operating draught. STAPLA was conceived as an alternative to a fixed platform for marginal fields where exploitation by a fixed platform may be uneconomic. This paper discusses the special problems associated with the design of a variable draught semi-submersible.
The STAPLA vessel is a concept developed for marginal field exploitation which may occur in the Northern North Sea over the next 5 to 10 years. Forecasts of UK development suggest that 80% of future production will be from fields of under 100 million barrels recoverable reserves. The development of marginal fields will also be significant worldwide. The design philosophy of STAPLA is based on reducing capital and operational costs, by devising a versatile, easy to redeploy, variable draught semisubmersible. The vessel is column stabilized, but capable of changing its draught from 50m to 10m by use of a conventional Jacking mechanism. By dropping the lower hull to a 50m operating draught, the vessel achieves very good motion response characteristics Which are normally achieved by purpose-built large draught vessels. On the other hand, by raising the lower hull to a 10m draught, it is possible to perform dry dock inspection and maintenance.