Moonpool operations on Havila Subsea - improvement study

Abstract

Increasing challenges with regards to remote fields, reservoir conditions and deep water, forces the petroleum industry to adopt new technology. A large part of this technology comes in the form of subsea equipment. Larger and heavier subsea modules are manufactured and installed in order to meet the field and reservoir conditions. Inspection, maintenance and repair (IMR) operations presents a key element in a subsea field life cycle. Lifting through moonpool is preferred when performing such an operation. A critical factor which has been identified for lifts through moonpool, is when equipment is to be docked onto the cursor frame. During the docking, relative motion between vessel and equipment can lead to large impulse loads. The structural strength of the cursor frame is seen as a limiting factor and a risk element. This is the main reason for Subsea 7 and Statoil to initiate this thesis. This thesis addresses the module handling system on Havila Subsea. Emphasize has been made on structural challenges of the cursor frame and the possibility of improvement. Both manual capacity calculations and Staad.Pro analysis has been conducted for both existing and alternative cursor frame. The loads have been applied as static loads acting on the tip of the prongs, this is to simulate a worst case scenario where the funnels are just docket at the prongs and the vessel experiences a large pitch or roll motion. The alternative cursor frame has been modeled with new and flexible prongs. The flexible prongs can deflect 5 degrees in any direction. Analysis shows that the existing cursor frame has a high structural capacity and the prongs are the cursor frame weakest members. The prongs have been proven to have a capacity of 11.8 Te per prong. Effectively this gives a total static cursor frame capacity of 23.6 Te. The flexible prongs have been shown to be beneficial with regards to impulse loads and fatigue. However, for a final recommendation, detailed dynamic analysis and full scale tests are recommended. Even though structural challenges have been emphasized in this thesis, operational aspects have been regarded and concluding remarks been made.