A Comparison of Full-Scale Measurements and Numerical Riser Response Simulation

Data from full scale measurements have been collected for a marine drilling riser connected to a moored semi-submersible rig which is operating at a site with a water depth of 325 m. The flex joint angles at the bottom of the riser just above the lower marine riser package are recorded and applied for the purpose of estimating the accumulated fatigue damage by means of rainflow cycle counting and Miner-Palmgren summation. A numerical simulation model is also established. Both unidirectional wave excitations as well as directional spreading are considered, where the latter generally implies a reduced response level. Different main heading directions of the waves are analyzed. These are 0, 30, 60 and 90 degrees relative to the rig. The effect of applying a linear versus a non-linear stiffness model for the lower flex joint is also investigated. The frequency content of the measured full scale motion versus the simulated motion is studied in some detail. Observed differences are discussed.