Current measurements in support of fixed platform design and construction

Abstract

Storm-driven currents can be an important part of the design hydrodynamic flow field for fixed platforms. In addition, the currents which exist during platform construction can significantly affect the installation. Planning for the installation of a large platform off the Mississippi Delta was facilitated by climatological current data collected with electromagnetic and Aanderaa meters, which were supported from a semisubmersible drilling rig and from a subsurface mooring. In addition, Cyclesondes and electromagnetic current meters were used to provide real time current data during the installation of the base section of the structure. Once a platform is in place, it provides an excellent site for the study of near-surface, storm-driven currents and waves. The fast response time of electromagnetic current meters makes them seem ideally suited for this application, and their effectiveness has been demonstrated through five years of experience at three sites in the Gulf of Mexico. Early problems with reliability of the meters during long-term, unattended operation have now been mostly eliminated. During tropical storm Delia, surface currents over 2 m/sec were measured. The electromagnetic current meters also provide information on the kinematics of storm waves. Comparison of the measured particle velocities with wave theories shows that the directional spread of the wave energy is important. The measured particle velocity spectra agree with the predictions of linear theory to within a few percent over the energetic frequency range, increasing confidence in the current measurements.