Wellhead axial movements in subsea wells with partially cemented surface casings

Abstract

In subsea wells, the surface casing string should be fully cemented but, eventually, this is not possible due to the long length of the string and/or the soil characteristics. As the surface and the structural casings are the foundations of any subsea well, a partially cemented surface casing may allow significant wellhead movements, which affect the structural response of the well. This article, therefore, addresses the prediction of wellhead axial movements considering subsea wells where the surface casings are partially cemented. These movements are assessed with a multi-string model composed of nonlinear springs and bars that represent the casing strings. Nonlinear hysteretic t-z curves represent the soil and a developed finite element (FE), which combines these curves and the stiffness of the structural casing string, represents this string. A typical subsea well is analyzed assuming different soil-structural casing conditions and uncemented lengths. These analyses evidenced the significant impact of the uncemented length on the wellhead movements and on the stresses in the casing strings, especially if the structural casing load capacity is overcome. Comparisons with results from an analytical and a FE model developed in a commercial software showed good agreement and highlight the need to adequately modeling the structural casing-soil interaction.