The theory of random processes has been shown to be an effective means of describing real ocean waves. This paper illustrates the application of the theory to the prediction of drilling barge motions due to waves; measured data from a drilling barge in the open ocean are presented to verify the results.
The theoretical model provides a means of computing the power spectra of drilling vessel motions from the directional power spectrum of the ocean waves and the wave response functions of the vessel. (The wave response functions are obtained by determining the motions of the vessel in simple, sinusoidal waves of all frequencies and directions). Statistical proper ties of vessel motions, such as the rms motion, the "significant" amplitude, or the expected largest motion over the duration of a storm may then be computed from the power spectra of the motions and a probability law. The theoretical model is adequate for many practical engineering problems and can be feasibly applied within the present state-of-the-art.
An example of results from the theoretical model is presented using a sample of data obtained from a drilling barge during operations References and illustrations at end of paper by Standard Oil Company of California offshore Oregon in the summer of 1965. The data was analyzed by computing auto spectra and cross spectra from the measured time series of vessel motions and waves, and by estimating the directional wave spectrum with a least squares fit to the measured wave spectra. Power spectra of vessel motions were then computed from the random vibration model using the estimated directional wave spectrum and wave response functions; the results are compared with the vessel motion spectra obtained from the measured data. Average and extreme values from the measured data are compared with corresponding values predicted from the theoretical probability law. Results from the comparisons support the validity of the theoretical model as a practical engineering tool.
Floating drilling vessels are presently in operation on continental margins throughout the world in the search for petroleum reserves. Experience has shown that drilling vessel motions caused by particular wave conditions can significantly impair or halt drilling operation and that the capability for operating in a particular wave environment varies considerably among drilling vessels currently available. Information from various sources has served, in the past, as a basis for selection of a drilling vessel and drilling equipment for an operation in a particular area; results from model test data, theoretical studies, and actual experience sometimes gained at considerable expense, have all served as a basis for selection.
Requirements are continuing to arise for conducting floating drilling operations in new areas and in more severe wave and weather environments than have been previously experienced. As these requirements increase, the need for more accurate means to predict vessel motions in particular wave environments increases, and as more complete oceanographic data becomes available, the use of more complete theoretical models for calculating vessel motions becomes practical.
