In the 2019 version of DNVGL-RP-C203 Fatigue Design of Offshore Steel Structures, significantly improved methods have been added on how to establish M-N curves representing the fatigue resistance of preloaded connectors subject to cyclic bending. The M-N curve parameters are typically provided by the manufacturer and used by operators and drilling contractors for calculating the wellhead fatigue life for planned drilling operations.
DNVGL-RP-C203 provides specific advice on how to establish design M-N curves based on analysis, and the augmentation by possible testing, where testing may grant more favorable M-N curves and thus extended fatigue life for any given case.
The paper provides background and introduction to the improved analysis methodology and relevant S-N curves for high-strength steels for wellhead systems, given in the 2019 version of the DNVGL-RP-C203. It includes a worked example in order to demonstrate the detailed use of the method, applied on a Baker Hughes preloaded BOP connector, connected to a 27” wellhead mandrel. This example describes the finite element model set up, FE model mesh refinement in hot-spots, the application of cyclic loads, extraction of hot-spot cyclic stresses, and the establishment of the M-N curve for the connector.