Improved platform design

Abstract

Exxon has built a 3.5 million-dollar yardstick, the Ocean Test Structure, to measure wave forces on a three-dimensional structure in the ocean. Installation of the Ocean Test Structure in South Timbalier Block 67 was completed in November 1976 and data recording was begun in early December. Thus far, data collection has gone smoothly; however, the total number of hours of storm sea state data is below expectations due to the unusual climatic conditions in the Gulf of Mexico during the past winter. We are presently evaluating plans to extend the program for another year. Present space-frame structure wave force estimates are based on extensions of single pile measurements using simplified two-dimensional theories. Real ocean waves and the forces they produce on a structure may be substantially lower than those used for design calculations. Actual ocean waves are of random shape and speed and rarely, if ever, exhibit uniformity along the crest. Waves which are assumed to exist for design purposes are uniformly crested and produce two-dimensional velocity and force fields. The objectives of the Ocean Test Structure are simply to 1) evaluate present design force calculation procedures and 2) refine those procedures. Based on the data, industry may be able to eliminate costly overconservatism where it exists. The greatest value of the Ocean Test Structure data will be the direct comparison of real forces with those computed by present design practice. The validity of the experimental resuits is based on the concept of hydrodynamic similitude (scaling) and on the high-quality calibrated data acquisition system that has been installed and maintained. Individual large rare waves in storm sea states will produce external loading conditions on the Ocean Test Structure that are reduced-scale replicas of design level waves on space-frame structures three to six times larger. The total base shear and overturning moment caused by these waves will be carefully analyzed and compared with calculated forces and moments for the measured waves striking the structure. Other sensors in the structure will permit a detailed study of forces on the simulated group of conductor pipes, forces on nonvertical members, impact forces on members above mean water level, and the effects of marine growth on local forces. Spectrum analyses of the input conditions and resulting forces will treat the structure as a black box which transforms wave action into structural loads. These data will permit us to evaluate methods employed by structural engineers for dynamic analysis and long-term stress history predictions. About five man-years of effort by several leading consultants will be devoted to data interpretation. The massive amount of data being collected will be evaluated in detail in order to draw meaningful engineering conclusions from the experiment.