Platform Hermosa as a Wave Force Transducer

Abstract

ABSTRACT Chevron's Platform Hermosa, located offshore California in 602 ft of water has been instrumented in order to extract global wave forces for correlation with measured waves and currents. Finite element studies showed that global forces on the platform could be calculated from measurements of axial leg strains near the platform base. Field calibrations to determine the relationship between applied force and measured strain were performed using an ocean-going tug. The wave load measurement approach, the field calibration procedure, the analysis of the calibration data, and comparisons of the calibration results with finite element predictions are prcscntcd. Discrepancies between prediction and measurement in these comparisons emphasize the importance of field calibrations when relying on strain measurements for extraction of wave forces. INTRODUCTION Wave tank model test results and available fullscale wave force data are not entirely satisfactory for evaluating wave force calculation procedures for drilling and production platforms. Data from wave tank tests suffer from scale effects. Field data either lack field calibration or are from structures with simple geometries. For example, data from Wave Projects I and II 1 are difficult to apply to full structures because only local forces on individual vertical cylinders were measured. The Conoco Test Structure wave forces2 and Platform Magnus3 were inferred from measured strains in structural members for a full platform, but lacked a field calibration. Conclusions regarding the adequacy of wave force calculation procedures drawn from uncalibrated field data are questionable. The Exxon 0TS5 was notable in that the structure was calibrated in the field; however it was a l/3-scale model of a structure with a relatively simple geometry. The Hermosa wave force project was designed to address shortcomings of existing data by obtaining full-scale wave and current force data from a field-calibrated platform. Platform Hermosa in the Point Arguello field, offshore California (see Fig. 1), was designed to allow convenient deployment of instruments to underwater locations. A square access tube, running from mudline to the well deck, is welded to each of its corner legs. The availability of these tubes provided motivation to find a way to use the platform response to measure applied overturning moment due to wave and current forces. Theoretical studies and computer simulation showed that such measurements were possible by measuring the leg strains near the base. An important ingredient of the method is a calibration accomplished by applying known loads to the instrumented structure while recording the strain and motion response. Some instrumentation was already in place on the platform. This was augmented with strain sensors and additional accelerometers. The calibration was performed using a 5000 hp tugboat to apply the known loads. PLATFORM INSTRUMENTATION A sketch of Platform Hermosa showing the location of many of the installed sensors is shown in Figure 1. Diagonally opposite pairs of triaxial accelerometers are situated at four elevations, including one not shown at the well deck (cl. +51 ft).