Foundation Performance Of Mat-Supported Jack-Up Rigs In Soft Clays

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Abstract
Even though mat-supported mobile jack-up rigs are designed to develop low seafloor bearing pressures, the observed mat penetration in soft soil areas may approach the thickness of the mat. An accurate measurement of penetration is often critical since designers caution against using the rig at locations where the top of the mat penetrates below the seafloor. Geological features and geotechnical properties of very weak soils in areas such as the Mississippi River Delta also greatly influence foundation perforJllance. This paper describes how ge010gic features and geotechnical properties can influence the foundation performance of jack-up rigs and includes field performance measurements at two sites during rig placement and later, after Hurricane Allen. The field measurements indicate the method of placement can greatly influence mat penetration. An electronic bottom sensor proved very useful inetermining the actual mat penetration since diver's observations produced misleading indications of mat penetration below the seafloor due to a mound of soil that forms near the mat edge. The observed performance data are used to develop an improved method to more accurately predict mat penetration in very soft deltaic clays than possible with classical bearing capacity theory and undisturbed shear strengths of cohesive soils. The paper concludes by recommending that integrated geotechnical and geophysical studies be performed to develop a better understanding of geologic and geotechnical factors that can influence foundation performance of a mat-supported jack-up rig. Introduction About 60 percent of the offshore oil and gas exploration is being conducted with a fleet of over 250 mobile jack-up drilling rigs. Although there are many different mobile rig designs, the rigs can be divided into two broad categories according to their foundation type:individual footings, ormat-supported. Many of the mats are A-shaped. Mat-supported rigs have a much larger bearing area and develop lower bearing pressures than rigs with independent footings. The lower bearing pressures enable mat-supported rigs to operate in areas covered by very soft clay soils with only a few feet of mat penetration below the seafloor. Observed mat penetrations, however, can approach the mat thickness in active delta areas around the world, such as the Mississippi River where the soi1s are very soft, underconso1idated c1ays. In these cases, an accurate measurement of the mat penetration and an assessment of the resulting foundation stability is important since designers caution against using the rigs at locations where the mat top penetrates below the seafloor. Hirst et al (1976) showed that foundation performance of mat-supported jack-up rigs was safe and acceptable as indicated by a record totaling 176 rig years without loss of a rig due to wind, wave, or current activities during drilling. Their performance data does, however, indicate that vertical and lateral movements have occurred in very weak soils during four severe hurricanes.

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