Wave Forces: Data Analysis and Engineering Calculation Method

Abstract

This method for calculating the force of ocean waves on offshore drilling structures utilizes a mathematical model for the flow field of highly nonlinear waves; the method is considered applicable to a broad range of wave conditions commonly encountered in offshore design. Introduction An extensive body of knowledge is developing in the area of ocean wave forces on cylinders. Excellent summaries appear in Refs. 1 and 2; important contributions of mathematical models used in wave-force calculations are described in Refs. 3 through 5; and wave-force measurement installations are described in Refs. 6 and 7. From theoretical and experimental work has come a variety of wave-force calculation methods for use in engineering; one such example is presented in Ref. 8. Most published methods are presented in Ref. 8. Most published methods are based on the analysis of relatively few waves of low to moderate height and mathematical models that involve major simplifications in the representation of real ocean waves. There has been a need, therefore, to improve the mathematical model and to use information from larger waves. A mathematical model was developed and proved to be a substantial improvement over the available models. Using this model and data from large-amplitude waves measured in the Gulf of Mexico, Standard Oil Co. of California developed the method reported here for use in design of offshore structures. Selection of a Mathematical Model The mathematical model used in the wave-force calculation method was picked to be compatible with specification of design wave conditions in terms of a single wave; the calculated forces are used in a space-frame loading program to calculate the loads on a complete structure. Although this deterministic, single-wave approach appears to conflict with the fact that the sea is non-deterministic and can be described only statistically, for the following reasons the deterministic approach was chosen.Natural periods of structures considered when the method was developed were much shorter than the period range of significant wave energy; hence, wave loads could be treated as static loads.Nonlinear wave theories could be used for calculating the single wave, whereas time series representations used in the statistical models were not developed to the point that nonlinear wave characteristics could be modeled.Statistical characteristics of a sea that are of importance to the static load case could be approximated in selection of design wave heights and calculation of wave forces. Considerable progress is being made on calculation methods using time-series description of waves and forces, and such methods will be used when longer period structures, requiring dynamic analysis, are studied. JPT P. 368