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Due to increased functional and pressure capacity requirements of subsea BOPs, there is a steady increase of weight and height. A down-side is that the loading of subsea wellheads due to marine riser loads is also increasing. This becomes particularly critical when deep-water rigs with large BOP stacks are used for drilling in the relatively shallow water in the Norwegian Continental Shelf. These loads may be characterized by their maximum single load and by their cyclic load variation and number of load cycles. Several wellhead system designs incorporate a rigid lockdown mechanism, where the wellhead housing is vertically pre-loaded down onto an internal reaction shoulder in the conductor housing. The pre-load and force reaction pre-stress the components that make up the lockdown mechanism. This pre-stress in particular may limit the bending capacity of the wellhead.
The purpose of this study is to evaluate a lateral reaction force couple for the load transfer between the wellhead and conductor housing, instead of a traditional vertical pre-load, per the request of an operator experiencing extremely high fatigue loading in the subsea wellheads. The aim is to show favorable fatigue performance with a lateral based reaction system. Three new wellhead concept designs were developed, analyzed, and evaluated following the practices of DNVGL-RP-C203 and using generic load parameters for soft-clay and loose-sand soil types. M-N curves of all hotspots were plotted and compared against the M-N curve of a perfect 36" × 2" WT girth weld with C1 quality (considered the best fatigue performance a wellhead system can achieve for this particular conductor size) found in NORSOK U-001, Appendix B. The fatigue resistance was presented as an M-N curve.
The new concept designs were refined through a series of iterations around the areas of high stresses until the worst hotspot in the system was isolated around the 36" × 2" WT conductor housing weld. The new concept designs were each evaluated and a single one was selected to continue with the fatigue optimization work. For the selected concept design, additional iterations were analyzed with and without pre-load between the wellhead housing and the conductor housing. The results are presented in the form of M-N curves for individual hotspots and for the entire wellhead system. Based on detailed FE analysis, the first iteration of all three conceptual designs showed promising results when compared to the 36" × 2" WT benchmark. For the non-preloaded conceptual design selected, analysis showed a possible solution, both with respect to fatigue resistance and structural capacity. The new conceptual wellhead design consists of only four major components. Although the overall size of the system has increased slightly compared to conventional systems, a preliminary commercial review shows reduction/simplification in manufacturing processes involved.