A semi submersible production platform with steel catenary risers and taut leg mooring system has been developed for deepwater and harsh environmental conditions. Design and analysis iterations have been performed to establish a system that is cost optimal when considering platform, mooring and risers in total. Extensive parametric studies have been performed to fully explore the behavior of the total system and to optimize the solution established.
The generic concept developed is a platform with a topsides of approximately 25.000t and with Steel Catenary Risers for gas export of up to 24 inch. Various measures to improve the riser fatigue has been evaluated and analyzed. It has been concluded that the use of a deep draft semi in combination with weight coating on a limited length of the risers is an effective measure to achieve the required integrity of hull, mooring and risers.
The feasibility of the concept has been demonstrated through extensive coupled analysis and a model test where mooring and riser lines are truncated and full depth simulated. Further, post test analysis of a coupled semi, riser and mooring system has been performed, due allowance made for the model findings. Through this process a design and analysis process has been developed and verified.
Steel Catenary Risers (SCR) have seen their merits in Gulf of Mexico, mainly in combination with TLP's and Spar platforms. This type of riser in combination with semi submersible platforms is also being introduced in Gulf of Mexico and offshore Brazil.
For harsh environmental conditions, e.g. offshore Norway, the semi submersible platforms with flexible risers have during the last decades proved their excellence in use for development of oil and gas fields in medium to deep water. However, as the developments in these areas now extend to even larger water depths, the introduction of SCR's is becoming a desired option.
A semi submersible platform with a draft of 40m, and with a 24 inch steel catenary gas export riser has been developed for these harsh environmental conditions. The integrated design and analysis approach for the total system (semi, riser and mooring) has been adopted and the feasibility demonstrated through model testing of a truncated system.
The work has been executed in a DEMO 2000 Joint Industry Project with Statoil, Hydro, Shell, BP and Marintek. In addition DNV has been in an advisory position to the JIP. Included are key issues to enhance the confidence in this concept to a level where the technology will be ready for development projects to come. The work has concentrated on a 1500m water depth case for a gas development offshoreNorway. The technology applied can also be transferred to other similar conditions.