Calibration of a Jackup Structural Analysis Procedure Using Field Measurements From a North Sea Jackup
Authors: Brekke J.N. , Campbell R.B. , Lamb W.C. , Murff J.D. 
Published: October 2008

During the winter of 1988-1989, a comprehensive structural measurement program was carried out on the Maersk Guardian jack-up rig while it was operating at a firm, sandy location in the southern North Sea (Silver Pit). The purpose of this program was to determine how well a structural analysis procedure models the actual behavior of a jack-up. In recent years, industry attention has been focused on the finding that differences in analysis procedures can lead to widely varying assessments of a rig's structural suitability for a given location. The results of this measurement program provide insight to help resolve these differences.

This paper focuses on how the measurements were applied to two stages of an analysis procedure:to calibrate a jack-up structural model; andto validate a procedure for calculating "static" wave load and dynamic response.

In the first stage, the measurements showed that the sandy soil affords significantly more structural resistance, or "spud can fixity", than is assumed in the typical "pinned" spud can model. A calibrated structural model was developed to incorporate direct measures of spud can fixity and leg/hull flexibility; this model was validated by the agreement of its natural period with that which was measured. In the second stage, the measurements confirmed the significance of dynamic response as predicted by rigorous analysis using the measured waves. Using the calibrated model, a time-domain analysis predicted static wave load and dynamic response at the natural period which are consistent with the measured response. Dynamic amplification factors (DAF's) quoted in this paper are similar to the DAF from a comparable rigorous analysis case presented in Reference 8. (The body of analysis cases in Reference 8 supports the use of a simplified, approximate DAF calculation method.)

Additional research is warranted to fully assess the significance of both spud can fixity and dynamic response. In separate studies, even conservative modelling of spud can fixity has provided up to 40 percent reduction of critical member stresses relative to the "pinned model", based on an assessment assuming a "fully-seated" spud can on dense sand. Further study is needed to investigate soil nonlinearities/degradation before the full potential benefits of spud can fixity are realized in general practice. In addition, further investigation of jack-up dynamics would improve confidence in predicted response under extreme wave conditions.

INTRODUCTION

Jack-up rigs are widely used as mobile offshore drilling units for the exploration and production of hydrocarbons. Prior to operating at a given location, structural assessments are performed, typically at varying levels of detail, to confirm a jack-up's "fitness-for-purpose". In recent years, industry attention has been focused on the finding that differences in jack-up structural analysis procedures can lead to widely varying assessments of fitness for purpose, particularly at deepwater locations.

In September of 1988, a structural measurement program was initiated on the Maersk Guardian rig at the Silver Pit location in the southern North Sea. The Silver Pit site has 230 feet of water and a firm sandy bottom, unlike a previous jack-up measurement site in the Gulf of Mexico where the soil was soft clay (Reference 1).