Wellhead fatigue analysis considering the effect of wind-sea and swell waves by using Univariate Dimension Reduction Method

Abstract

In recent years, the observation of dynamic motions in wellhead structures during drilling operations turned wellhead fatigue into a great concern for the oil industry. As stated by DNV in its Recommended Practice E104 (DNVGL, 2018), the fatigue damage evaluation demands an accurate representation of environmental loads acting on the whole drilling structure, including rig, drilling riser and BOP/LMRP. In terms of computational costs, this representation can make the fatigue analysis very expensive. A high number of loading cases may be needed to take into account the statistical independence between environmental parameters, mainly when wind-sea and swell waves are taken into account. To solve this issue, this paper proposes the use of the Univariate Dimension Reduction Method (UDRM) to compute the fatigue damage in wellhead structures considering the effects of both types of waves. The main advantage of this procedure is that it replaces the solution of the four-fold integral associated to the problem by the solution of four one-dimensional integrals, which drastically reduces the number of numerical simulations required in the fatigue assessment. The method is applied to two study cases considering the environmental conditions of the Brazilian coast. In the first, the drilling unit is a semi-submersible platform, while in the second case a ship-based drilling unit is considered. The obtained results indicate a substantial reduction in computational costs without loss of accuracy of UDRM when compared to a Monte Carlo Simulation-based approach.