Operation Of An Ocean Test Structure

Abstract

Abstract The Ocean Test Structure is a large scale engineering experiment designed to evaluate present wave force calculation procedures for fixed, space frame structures. This highly instrumented 6.1 × 12.2 × 36.6 m platform was installed in 20 m water depth in the Gulf of Mexico in November, 1976. Storm wave data, representing typical platform design conditions in reduced scale, were collected during two winter seasons and one hurricane season. The structure and its many sensors were calibrated during construction and checked after installation offshore. Data obtained include local wave forces on clean and barnacle-covered sensors, local wave kinematics, total base shear and overturning moment on the structure, forces on a simulated group of well conductors, and impact forces on a member above mean water level. The vast body of field data obtained during this measurement program are being used to evaluate present methods of wave force calculations. A present methods of wave force calculations. A digital data acquisition system was used, which allowed convenient computerized data processing. Special data analysis techniques were developed to handle the random vector time series responses produced by real ocean waves. Applications of the produced by real ocean waves. Applications of the data include evaluation of local force coefficients, total wave force, wave-induced velocity profiles and frequency response functions. When fully interpreted, these data will greatly improve our knowledge of wave forces on steel-jacket platforms. Introduction Present jacket-type structure wave force estimates are based on extensions of laboratory and field single pile measurements using simplified two-dimensional theories. Real ocean waves and the forces they produce on a three-dimensional structure may be substantially different than those used for design calculations. Actual ocean waves are of random shape and speed, and rarely exhibit uniformity along the crest. Waves which are assumed to exist for design purposes are uniformly crested, and produce two-dimensional velocity and acceleration produce two-dimensional velocity and acceleration fields. For typical steel-jacket platforms and typical design waves, we deal with waves having a length much greater than the diameter of a structural element. Total drag and inertia forces are calculated for each submerged differential element of the platform by the well-known Morison-O'Brien equation platform by the well-known Morison-O'Brien equation using the two-dimensional wave kinematics as input. Similar assumptions are made when calculating random force response in either the time domain or frequency domain. These latter results are used in long-term stress history predictions (fatigue analysis) and in dynamic analysis of deepwater structures. In order to gain more information on wave loading in real seas, Exxon initiated the Ocean Test Structure Program. Twelve companies have joined us in funding this program. Participation remains open to all interested parties. The test structure (Fig. 1) provided a replica of flow conditions encountered in design level seastates in reduced scale—about 1/3 to 1/6 the size of full-scale drilling and production platforms. Interpretation of the measurements will permit direct comparison between actual and predicted forces on the structure under conditions simulating full-scale wave loading for any continental shelf area. The validity of the experimental results is based on the concept of hydrodynamic scaling and on the high-quality, calibrated data acquisition system that was installed and maintained on the test structure. Figure 2 illustrates the overall timing of this Program. Installation of the Ocean Test Structure Program. Installation of the Ocean Test Structure (OTS) in 20-m of water in the Gulf of Mexico was completed in November 1976 and data recording began in early December 1976. The structure was located in South Timbalier, Block 67 at 28 degrees 49' 31.3" N, 90 degrees 23' 40.7" W. The total number of hours of storm seastate data was below our expectations during the 1976–77 winter due to the unusual climatic conditions in the Gulf of Mexico. While interpretation of those data began, arrangements were made to extend the Program another year. This provided the opportunity to measure large wave forces (relative to the scale of the test structure) when hurricanes passed nearby during the 1977 summer. P. 167