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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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

This paper was presented althe 9th Annual OTC in Housfon, Tex., May 2-5.1977. Tfiematerial is subjeclto correction by the ilutllor. Permission to copy is restricted to an'abstract of not more than 300 words. ABSTRACTIn offshore construction work related, to platforms, one of the difficult operations has been the connection of add-on sections to pin piles as they are driven into the sea floor. The reason for this difficulty has been the relative motion of the derrick barge and the fixed platform. This paper describes the design of and the application of a pile aligning system that makes this operation much easier to accomplish. It emphasizes the design of the system and its field operation conducted in the Gulf of Mexico. It descri bes the des i gn con,cepts, the actual hardware used to implement xhese concepts, and the successful operation of this hardware on the actual job site.

This paper describes the design and application of a pile aligning system that makes connecting add-on sections of piles in offshore platforms easier. Field operation of the system in the Gulf of Mexico is discussed. Design concepts, actual hardware used to implement these concepts, and successful operation of hardware on the job site also are discussed. Introduction Steel platforms are conventionally pinned to the ocean floor with large-diameter piling, which is driven through the legs of the platform and/or through guides around the base of the platform. When platforms are set in deep water, it is not feasible to handle the pipe in one section. It is necessary to add on additional sections as the pile meets and then is driven into the ocean floor. The platform itself is grounded to the sea bottom, whereas the derrick associated with a derrick barge for handling the add-on pile sections floats on the ocean surface. Consequently, there is relative motion between the section of piling to be added and the pile that already has been installed in the leg or guides around the platform. The amount of motion is, of course, a function of the wave action or sea state on the derrick barge. When the weather becomes a factor, it is common to wait for calm seas, even though the operation may cost as much as $50,000/day while waiting.The piling used today is either 48- or 54-in. diameter and some is more than 90 in. The add-on sections usually are joined to the pile by welding. Because welding must be performed in the field, it takes 3 to 12 hours or longer per connection, depending on wall thickness. Time is per connection, depending on wall thickness. Time is saved by using mechanical connectors that theoretically eliminate welding time and pay for themselves, even though they may never be recovered. On the other hand, experience to date has proven that with the motion, it is impossible to align the connectors and connect them successfully. Sometimes the operation takes longer than welding; therefore, these connectors are used reluctantlyWhen pile is driven through guides around the base of the platform (skirt piles), sometimes it must be cut off at the base of the platform to remove the upper section. Upper sections of pile are not necessary to improve jacket stiffness and, thus, considerable savings result. When the upper section of the driven pile must be removed, we need a disconnector at the appropriate level, which usually takes the form of the described mechanical connectors. During removal these upper pile sections must be held as well as disconnected. This study describes a way to speed up connecting time and make it more feasible to use mechanical connectors where possible. Design Solution The crux of the problem is the relative motion between the add-on pile and the pile already installed in the jacket. Eliminating this relative motion results in a satisfactory solution to the problem. Among secondary advantages of using mechanical connectors would be the ability to rotate the piling for connecting and disconnecting. Also, if the pile could be lowered or raised independently, several piles could be run simultaneously. This could result in piles could be run simultaneously. This could result in significant time savings for the over-all operation.The proposed solution to these problems is called the Pile Aligning System (PAS). PAS has four components: Pile Aligning System (PAS). PAS has four components:the aligning structure (Fig. 1),the self-powered hydraulic power supply (Fig. 2),the pickup elevator. andthe driving head (Fig. 3). JPT P. 1061