Model Test of Wave Forces On A Structurally Dense Jacket Platform
Authors: Gu G.Z. , Parsley M.A. , Berek E.P. , Calve J.J. , Johnson R.C. , Petruska D.J. 
Published: October 2008

Introduction

In the Gulf of Mexico, there are a significant number of jacket platform. built in the 1950's and 60's which are still in operation. Typically, these platforms have a large number of closely spaced legs and densely arranged bracing members (Figure 1). Since most of these platforms are beyond their design lives but the reservoirs are still producing, their safely, serviceability and fitness-for-purpose must be re-assessed in order to continue producing from them.

During Mobil's in-house re-qualification ion effort, it was found that the predictions by structural analysis programs (such as SACS and KARMA) were inconsistent with the platform inspection results. The programs predicted a large number of joint can failures during design storm (such as hurricane Andrew), but underwater inspections indicated only few failures had actually occurred. It was apparent that the procedure used for the assessment was conservative--either the wave loads the platforms experienced during the hurricanes were overestimated and/or the structural resistances were un - dercsti mated. This paper addresses the wave load issue.

Heideman and Weaver (1992) and Petrauskas, Heideman and Berek (1993) describe the procedure outlined in API RP-2A 20th Edition for calculating wave/current forces on space framed structures. The procedure assumes that the structure is transparent to the waves and that members do not shield other members from wave effects, if the spacing between major memtxrs is greater than four member diameters. The procedure has been calibrated extensively against data from a number of field and model tests, and is considered fairly accurate for modem jacket structures which normally have few legs and sparsely spaced bracing members. However, for aging platforms in the Gulf of Mexico built in the 50's and 60's which normally have 20, 40 or even more legs and densely spaced bracing members, not enough field or laboratory data, were available for similar calibrations. It was felt that the wave force recipe in API RP-2A 20th Edition might over predict wave/current forces for structural y dense plat - forms because it neglects wave shielding which can be significant. To calibrate the force algorithms typically used in structural analysis programs, a model test of a typical aging jacket platform was conducted in the wave basin at Chicago Bridge & Iron in 1994.

Description of The Model Test

The platform modeled has 40 legs and a large number of bracing members. The model scale was 1:20 and the largest member diameter in the model was about 1 inch (20 inches in prototype). The water depth was 42 ft. The model was tested under design wave and current conditions. The 100 year extreme hurricane wave height and current speed were among the cases tested.

The full jacket of 40 legs was modeled as two identical 20-leg modules which were individually instrumented with load cells at the base. For each 20-leg module, the two orthogonal components of base shear and overturning moment were measured. This allowed the testing of a single 20-leg module separately from the complete 40-leg model, and also enabled the simultaneous measurement of forces on individual 20-leg modules in full model tests.