Foundation Performance of Mat-Supported Jackup Rigs in Soft Clays

Abstract

Summary Mat-supported jackup rigs often experience mat penetrations approaching the thickness of the mat in soft soil penetrations approaching the thickness of the mat in soft soil areas. Data obtained with an electronic bottom sensor show that actual mat penetrations differ from divers' observations because of a soil mound that forms near the mat's edge. The paper compares the mat penetration data with various bearing capacity procedures and strength data to help assess which procedure gives the most accurate prediction. The paper describes geologic features and soil properties that may influence the foundation performance of these rigs. The paper concludes by performance of these rigs. The paper concludes by recommending types of geophysical and geotechnical studies to be performed to evaluate the expected foundation performance of mat-supported rigs more thoroughly. performance of mat-supported rigs more thoroughly. Introduction About 60% of the offshore oil and gas exploration is being conducted with a fleet of more than 350 mobile jackup 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 soils are very soft underconsolidated clays. In these cases, an accurate measurement of the mat penetration and an assessment of the resulting foundation 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. showed that foundation performance of mat-supported jackup rigs was safe and performance of mat-supported jackup 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 do, however, indicate that vertical and lateral movements have occurred in very weak soils during four severe huricanes. The purpose of this paper is to assess the geotechnical and geological factors that may influence the foundation performance of mat-supported jackup rigs operating in areas with very soft clay soils. The paper then presents a series of field measurements made at two sites in the West Delta Area to determine soil strength characteristics and mat penetration at various stages during rig placement and later, after Hurricane Allen (July 1980). These measurements show:classical bearing capacity equations underpredict actual mat penetrations of a jackup rig,a soil mound forms adjacent to the edge of the mat, resulting in divers making misleading observations of actual mat penetration below the seafloor, andthe method of placement can greatly influence mat penetration. Later sections describe the type of soil-strength data that should be used with bearing capacity equations to allow more accurate predictions of mat penetration in very soft deltaic clay predictions of mat penetration in very soft deltaic clay soils. JPT P. 2958