Calculation method of riser top tension in deep water drilling

Abstract

Through mechanical analysis, a calculation method which meets the requirements of both the riser stability and the bottom over-pull margin was deduced. It was verified by an example of ultra deepwater well in South China Sea, and compared with two other common methods. Based on the related theories of pipe string mechanics, the effects of true axial force and effective axial force on the riser mechanical behavior were analyzed. The results show that, the bottom tensile force to Lower Marine Riser Package is determined by true axial force, and the riser's stability depends on effective axial force. Through the force analysis of riser which simultaneously bears the internal and external pressures, the correspondent mechanical model was established, and a new calculation method of top tension was derived, which remedies the deficiencies of algorithms proposed by American Petroleum Institute and Institute of French Petroleum. Field application shows that, the value of top tension calculated by this method is basically the same as the value set by field operation, and moreover, it is more accurate than the value obtained by the algorithms of American Petroleum Institute and Institute of French Petroleum. This method can be used to guide the design and operation of conductor. Key words: deepwater drilling, riser, top tension