Analysis and Improvement of Calculation Method of Riser Top Tension in Deepwater Drilling

Abstract Top tension plays an important role in stability of riser system. Either excessive or insufficient value of it tends to cause negative effects on risers' proper function and then pose a threat on drilling operation. But there is no unified calculation method about the setting value of riser top tension. Therefore, the purpose of this paper is to establish a calculation method of riser top tension that both meets the requirements of theory and engineering. The improved method is based on the theory of effective axial force in tubing mechanics. It considers the requirement of risers stability and over-pull force to lower marine riser package (LMRP) in field drilling operation. Take three deepwater wells in South China Sea as examples. Utilizing finite element analysis software ANSYS to model the riser, subsea wellhead and conductor systems respectively, then exerting the calculated top tension and environment load, the analysis results can be used to check the accuracy of the proposed calculation method. After analyzing the related methods proposed by American Petroleum Institute[1] (API) and Institute of French Petroleum[2] (IFP), it is found that both of them have their limitations. The former one only concentrates on stability but ignores the over-pull force required by LMRP in field operation. The latter one takes the requirement of overpull into consideration though, it regards this force as effective tension rather than true tension. According to the related theories of tubing mechanics, the effects of true axial force and effective axial force on the riser mechanical behavior is analyzed. The results show that, the bottom tensile force to LMRP is determined by true axial force, and the risers' stability depends on effective axial force. Through the force analysis of risers which simultaneously bear the inside and outside pressure, the correspondent mechanical model was established, and a new calculation method of top tension was derived, which modifies the deficiencies of algorithms proposed by API and IFP. The accuracy of this method is verified by the values if field operating and numerical simulation of ANSYS. The novelty of this method is to propose a calculation method of riser top tension that could improve the deficiencies of API and IFP and meet the requirement of stability and bottom over-pull force to LMRP in field operation.