Calibration of Jackup Leg Foundation Model Using Full-Scale Structural Measurements

by J.N. Brekke, Exxon Production Research Co.,
J.D. Murff, Exxon Production Research Co.,
R.B. Campbell, Exxon Production Research Co.,
W.C. Lamb, Exxon Production Research Co.

Part of: Offshore Technology in Civil Engineering: Hall of Fame Papers: Volume 12

Abstract: 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.

Copyright holder: Offshore Technology Conference, Copyright 1989

Subject Headings: Structural models | Foundations | Dynamic loads | Jacking | Calibration | Structural analysis | Storms


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