Deep Draft Semi Submersible

High Pressure and High Temperature wells together with harsh environmental conditions puts a tough challenge on the flowline and riser design. The flexible riser which has been the favorite choice for many field developments with subsea wells have several technical limitations. Large gas fields such as Ormen Lange require large bore export risers. Steel risers are the preferred solutions with respect to durability and cost. Unfortunately, it is difficult for steel risers to accommodate the vessel motions of a semi submersible or ship. Heave restricted vessels such as TLP or SPAR are used together with toptensioned risers and surface trees. Based on the limitations of flexible risers Hydro is pursuing extensive research on riser and floater technology. As part of this research program a study on the effect on riser design caused by reducing vessel motions. The main goal is to introduce steel catenary risers (SCR) on semi submersible platforms. The typical draft of a semi submersible production platform is 20–25 m. By reengineering the design of such a platform and increasing the draft from 21 m to 40 m the vessel motions were reduced significantly. Hence, an opening for novel riser solutions was made. ULS and FLS analyses for 10″ and 30″ SCR in 300 and 1000 m water depth were carried out. The key result is that the 10Prime; riser satisfies both the ULS and FLS requirement for both vessel drafts and water depth and the 30Prime; riser satisfies the both the ULS and FLS requirement for the 40 m vessel draft at 1000 m water depth.