Semisubmersible Rig Motion Studies Offshore of Alaska and Southern California

Wave and heave measurements were made from the Sedco 706 drilling in the Gulf of Alaska in the winter of 1976-77 and from the Odeco Ocean Prospector drilling offshore of Southern California in the first half of 1977. Wavestaff measurements from the moving semisubmersibles were corrected by adding the heave measured by accelerometers. This wave staff compensation was checked by spectral comparison with data from a Waverider buoy moored off of the Sedco 706 and was found to give excellent results. In addition to increasing the climatological data base for offshore California and Alaska, the measurements were useful for verification of rig motion model tests and computer programs. Prototype data during storm seas has rarely been available for this type of comparison, which is increasingly important since similar methods are being used in the design of tension leg structures. The simultaneous wave profile and heave measurements were analyzed by cross-spectral methods to give the amplitude and phase of the transfer function and the coherence between the signals. The coherences were very high in the energetic part of the spectrum, demonstrating that the heave may be adequately described by a linear transfer function. The measured transfer functions were correctly predicted by the model tests and computer programs.