Calibration of Jackup Leg Foundation Model Using Full-Scale Structural Measurements
Authors: Brekke J.N. , Murff J.D. , Lamb W.C. 
Published: October 2008

The fitness of jack-up rigs to operate in varying water depths and weather conditions is determined by "site-specific" structural assessments. These assessments are conducted by a wide range of companies which includes oil companies, rig owners, rig designers, classification societies, warranty companies, and regulators. The results of a particular assessment can differ significantly, depending on the assumptions and modeling details used in the structural analysis. Key aspects of the analysis include the leg foundation model, the calculation of wave load, and dynamic amplification effects.

This paper describes how structural measurements during a storm on an operating jack-up rig were used to calibrate a leg foundation model. The measurements show that, for significant leg penetration in soft clay, the foundation affords considerably more structural resistance, or "fixity", than is assumed in the typical, "pinned spudcan" model. When applied to an assessment, the revised leg foundation model leads to a 20 percent reduction in the maximum leg moment and an additional 20 percent reduction in the structure's natural period which further reduces the load.

INTRODUCTION

Jack-up rigs are self-elevating mobile drilling units which are used extensively offshore to drill both exploratory and production wells. Unlike most offshore structures, a jack-up typically operates at a given site for only a few months before moving. The unit is emplaced by floating it over a location and lowering its legs until the spud cans contact the seabed. Following proof-testing of the foundation (pre-loading), the hull is elevated to provide a stable platform from which drilling operations can be conducted.

In an effort to resolve some of the uncertainties in the analytical idealization of jackups, a field measurement program was undertaken. Measurements were made during February-May, 1988 on the KEYES 301 jack-up (rig type L-780 Mod II) while it was operating in the Gulf of Mexico. Structural response and metocean data were collected during a storm in which the largest waves measured 25 feet from crest to trough. The data were processed to determine 1) reaction loads at two elevations on one of the three legs, 2) motion of the hull, and 3) metocean conditions. The measured loads and motions were then compared with analytical predictions based on the measured metocean conditions.

This paper focuses on the calibration of a leg foundation model with the measured structural response data. The text includes 1) a summary of the measurements, with emphasis on leg foundation fixity, 2) a description of the leg foundation model and its basis, 3) comparison of measured and predicted foundation response, and 4) a projection of the results from the measured loading to extreme loading used in assessments.

STRUCTURAL MEASUREMENTS

Measurement Plan

The goal of this measurement program was to resolve uncertainties that have significant impact on jack-up assessments. In keeping with this goal, the program was tailored to measure leg foundation fixity and wave-induced dynamic amplification.