Subsea wells are dynamically loaded during all drilling operations involving a drilling riser system which will cause fatigue loading of the subsea wellhead system and the upper structural components of a well. Estimation of loads and load resistance enables analysts to predict expected service life based on analytical models of well and riser systems. Analytical wellhead models may be a simplified representation of the reality but should still capture the essential physical behaviour of the well. Ensuring sound boundary conditions for the analytical model is important and subject to debate amongst analysts. This paper addresses the importance of the well cement as a lateral supporting boundary condition of the wellhead and surface casing in local modelling of a subsea wellhead system. A modified boundary condition approach is suggested that differs from the previously published way of modelling cement in a wellhead fatigue context. This modified boundary condition is applied both to a 2D FEM wellhead model and to a more complex 3D FEM wellhead model. Analysis results with the new boundary condition and the previously suggested boundary condition are compared to understand the effect this modified cement boundary condition has on the analytical results. The effective level of cement is varied to investigate the sensitivity of cement boundary condition modelling. Results indicate less conservative analytical results for the modified boundary condition both for 2D and 3D modelling. The modified cement boundary condition suggested herein is believed to be relevant for deepwater and high latitude subsea basins as it takes into account the hindered strength development of oil well cement cured close to seabed under the influence of low seawater temperatures. We advocate that installation of BOP before cement has properly cured will improve fatigue life of surface casing weld.
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