Calibration of a Dynamic Analysis Procedure Using Measurements from a North Sea Jack-up in a Severe Storm

Abstract

ABSTRACT Field measurements taken from the Maersk Guardian jackup operating in 75m of water in the North Sea are used to calibrate dynamic analysis procedures. These procedures include wave load modeling, damping, and, most importantly, foundation fixity. The measurements demonstrate the existence of foundation fixity during a large storm event. However, the measurements also indicate some nonlinearity occurring during the storm. This paper describes the measurement program, along with a summary of the data recovered. It continues with a comparison of these measurements to time-domain analytical results. These time-domain analyses are based primarily on procedures specified in the Recommended Practice for Site Specific Assessment of Mobile Jackup Units developed as a joint-industry project. Time-domain analysis was used to study various nonlinear foundation mode/s. The load-shedding model was found to directionally produce the best fit to the measurements. /t is concluded that neglecting foundation fixity may be conservative in a dynamic analysis of a jack-up. INTRODUCTION During the winter of 1990-1991, structural response data were collected during a field measurement program on the Meersk Guardian jackup. At the time, the rig was operated by Phillips Petroleum Notway in 75m of water at the 2/4W platform in the Ekofisk complex in the central North Sea. The purpose of the program was to improve confidence in dynamic analysis procedures used for jackups. A key uncertainty is the influence of the foundation behavior on the jackup dynamic response. Instrumentation was designed to provide data on hull acceleration, water surface elevation, wind speed and bow leg forces. From these data, it was possible to extract information about natural periods, damping, and foundation fixity. The dynamic behavior of a jackup can be important in site specific assessments that determine the overall suitability of a jackup for a given location. Dynamics can be important because jackups are relatively more flexible than, for example, fixed platforms. This increased flexibility will produce a proportionally greater dynamic loading as the jackup moves in response to wave forces. Concurrent with this measurement program, a series of scale model tests of jackup spud can foundations has been conducted at Cambridge University (References 2,3,4). These tests provide the basis for a detailed calibration of an analytical foundation fixity model. The field measurements described here provide an opportunity for overall verification of the general foundation model behavior developed from these model tests. Other measurement programs have provided useful information on jackup behavior (References 1,5,6,12). The program reported here is an extension of an earlier measurement program using the same Maswsk Guardian jackup (Reference 1). The key feature of this program is a particularly severe storm, which occurred on December 12, 1990, which produced a measured significant wave height of 11.7 m. This same storm event is reported in Reference 5. This program provides confidence in applying dynamic analysis procedures to site specific assessments, which typically include extreme storm conditions. The dynamic analysis procedures used herein are based primarily on the Recommended Practice for Site Specific Assessment of Mobile Jackup Units developed as a joint industry project (Reference 7).