Riser and well head load monitoring

Abstract

A connected riser transmits loads acting on the rig and the riser, resulting from factors as current and waves, onto a subsea well at the seabed. This loading may induce fatigue damage to the well head (WH) and other subsea equipment. Riser monitoring systems (RMS) have been introduced in order to provide a more accurate calculation of this accumulated damage, as well as providing guidance for optimal rig position, so the operation envelope could be extended. This thesis looks into the possibility of calculating section forces and bending moment at two different locations, the lowermost end of the riser and at the top of the WH, by using different analytical algorithms. The input to the algorithms are assumed to be provided by a set of instrumentation, that measures forces and other important parameters at the lower tapered stress joint (LTSJ) and on top of the lower stack. The algorithms extrapolate loads from the measured positions to any location in the lower stack. The algorithms derived in this project are tested and compared with each other, for various combinations of wave loads, rig offset, current and boundary conditions for the well head. All the testing is done by using an FE analysis of the considered system to provide both input values to the algorithms, as well as reference values for comparison of the results. It is also tested how different configurations for placing the instrumentation at the LTSJ affect the analytical calculated results at the lowermost end of the LTSJ. The testing has showed that the analytically calculated results at the lowermost end of the riser and at top of the WH are in accordance with the results from FE analysis. It has also been found that the configuration for placement of instrumentation at the LTSJ strongly affect the accuracy of the algorithms' results.