Subsea Tree Fatigue Mitigation Solutions For Shallow Water Drilling

The offshore drilling industry is advancing technologies to extend deep water drilling technologies and attain feasibility of operations at deeper depths and higher pressures. However, shallow water operations themselves pose a certain unique set of challenges that need to be addressed with customized and innovative solutions. While shallow water poses certain benefits and conveniences to the operations, like ease of retrieval and better access to wells, there are significant challenges in terms of operational down time caused by limited operability and poor drilling riser and subsea hardware fatigue performance. Shallow water operations do not have the advantage of deep water drilling where the motions and loads imparted to the subsea blowout preventer (BOP) are relatively decoupled and damped out by hydrodynamic damping from the significant length of the water column. Thus, the vessel motions and wave hydrodynamic loads imparted on the riser are transferred to the wellhead and subsea hardware. In this paper the fatigue challenges encountered for drilling wells in 530 ft water depth from a sixth generation moored semi-submersible rig are explored. The fatigue loading is critical for the subsea tree connector which is characterized by a high stress amplification factor (SAF). Multiple riser space-out solutions were evaluated including fairings, helically-grooved buoyancy, joints with rope, and modifications to the telescopic joint each of which will be presented in the paper along with combination of different damping parameters to optimize the fatigue performance. The paper will present the subsea tree connector fatigue performance for different riser space-out options and make recommendations for future operations with similar conditions. Other challenges encountered in fatigue evaluation will be discussed. This will highlight the current assumptions and unknowns in data that can form the subject of evaluation for a future joint industry study.