Multiple Well Caisson In Mudslide Area

ABSTRACT An unusual design was used in the Gulf of Mexico to develop a small oil reserve in 63-ft of water in ar area of unstable soil conditions. Front end project planning emphasized minimal monetary exposure by staging expenditures and deve1oping specialized procedures to accomplish installation using the upper 1imits of avai1ab1e construction equipment. The result was the installation of a 5 well 8' × 12' diameter satellite production caisson installed adjacent to a discovery well and designed to resist loads induced by a mudslide and a 100-year storm. INTRODUCTION The Main Pass (MP) 74-B caisson project originated in the first quarter of 1984 in response to the need to develop a small oil prospect in an. offshore mudslide-prone area. The prospect is located twenty-five miles east of Venice, Louisiana in Louisiana State Waters and is in the active delta area of the Mississippi River. As in most of the Mississippi de1taregion, sediments in this area have been deposited in large volumes over short periods of time. This results in very soft under consolidated surface soi1s that may be subject to mudslide or downslope mass-movements. Therefore, design of platforms for this area must consider loads induced by soil movement in addition to the typical environmental and functional loads. The existance of Exxon's nearby (3 miles to the north) mp 72-A platform precluded the need for extensive facilities on the MP 74-B structure, and the number of wells required for development was relatively small. Although these functional requirements were minimal, the major challenge of this project was to minimize up-front investments and cost exposure until the results of the prospect confirmation well (B-1) were known. To meet the challenge, a stair step project and expenditure schedule was formulated requiring design and fabrication of the structure to take place while drilling the B-1 well. The resulting satellite structure was a mudslide resistant caisson protecting five internal conductors and supporting one external conductor. A 35'x 35' lower deck was designed to support test facilities and a 50' × 60' upper deck serves as a combination heliport and concentric workover rig platform. An 8-in diameter pipeline was designed to carry two phase production across several mudflow features to the "A" platform. The location of the satellite structure was chosen to minimize water depth and to avoid active mudflow gullies. The MP 74-B project required eight months of planning and engineering and an additional six months for fabrication and installation. Production start up began 14 months after project initiation. DESIGN BASIS Functional Requirements The design basis for the prospect was a mudslide resistant satellite structure with a maximum of six wells. Major design considerations established by the development plan included.