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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.

Understanding the response of the drilling riser in ultra-deep water is critical for effective riser management and successful drilling operations. Due to the relative shortage of industry experience in ultra-deep water, analysis conducted prior to an operation is a key supplement to judgement and common sense. Damage or loss of the riser or the BOP leads to high costs for riser replacement and rig downtime. The riser can be subjected to various loading configurations during a drilling operation. Storms and/or high current conditions can be encountered while the riser is in the following configurations: deployment; connected and tensioned; subjected to vessel drift-off; subjected to recoil after an emergency disconnect; and suspended in a hang-off condition. The unique aspects of each of these riser loading configurations in ultra-deep water warrant careful attention by riser analysts to provide operations personnel with well-founded guidance and an idea of what to expect in the field. This paper provides an overview of the key elements of drilling riser analysis in ultra-deep waters. Analysis results are presented on the topics listed above, along with operational applications for several ultra-deep water drill ships. Shortcomings and uncertainties in riser analysis are discussed. The results show that analysis of key drilling riser configurations is critical for understanding riser response during operations and effective riser management. Introduction In the past decade, the oil industry has made substantial investments in floating drilling rigs, leases, geological and geophysical work for water depths of 6000 to 10,000 feet. Global Marine, Inc. has been active in new construction and modification of rigs including three drillships. One of these drillships is the Glomar Explorer, which was leased by Global Marine, Inc. from the U. S. Government. The modifications on the Glomar Explorer were completed in 1998, allowing the vessel to drill in water depths up to 7800 feet. Global Marine, Inc. also owns and operates two new-built rigs, the Glomar C. R. Luigs, which is equipped to drill in 9000 feet of water and the Glomar Jack Ryan, which is equipped to drill in 8000 feet of water. The Glomar C. R. Luigs is the subject of analysis to be discussed in this paper. In drilling an ultra-deep water well, key elements of the riser design and operation warrant special attention to ensure the integrity of the riser system. These include:Riser Hang-offDrift-OffRiser Tensioning / Wear AvoidanceVortex-Induced VibrationsRiser CollapseRiser Loading on the BOP, Wellhead, and CasingRiser Recoil The first portion of this paper provides a discussion of analysis procedures and sample results for the first two topics listed above, riser hang-off and drift-off. The sample results for these two topics are based on Glomar C. R. Luigs operations in 8850 feet of water in the Gulf of Mexico. Following this, a brief discussion is given for each of the other five items listed, focusing on key topics of interest for ultra-deep water.

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: 1) to calibrate a jack-up structural model; and 2) to 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 model-References and illustrations at end of paper.ling 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.

In planning for a deepwater well, running/retrieval of a drilling riser in advance of rapidly developing seas was identified as a critical operation. The harsh seastates required careful attention to the well-specific operating criteria, (WSOC), or the metocean conditions that limit specific operations. Certain conditions would warrant the operation being shut down so that the riser could be pulled or run before conditions became excessive. In deep water, a running/retrieval operation can take several days and the consequences of being shut down part way can be severe. Moderately high current profiles can cause the riser to bind in the diverter housing, preventing further running or retrieval. If binding occurs, a dynamically-positioned vessel can carry out “drift running” operations to increase its operability. However, if conditions prohibit further running/retrieval, the riser may need to be hung off “hard” (with no compensation) at the drill floor or “soft” on the tensioners and/or drill string compensator. In such a case, in addition to large top riser angles, vessel heave motion can cause tension variation, depending on the hang-off conditions and the length of the deployed riser. This paper discusses how riser analysis was used to support the operational understanding of the riser running/retrieval process for this deepwater well. The results were used to assist in the planning and decision-making involved in drilling this well.