Production Techniques From Tension Leg Platforms

Abstract

ABSTRACT Production techniques and designs have been developed to take full advantage of the unique dynamic characteristics of a Tension Leg Platform (TLP). The application of the information presented in this paper could be used to economically develop deep water reservoirs considered to be marginal if conventional fixed platforms were used. Data obtained in this development program indicates that the production risers can remain connected under all environmental conditions that the TLP would experience. Included are graphs, specialized hardware designs, sketches of special operating techniques and TLP performance data. INTRODUCTION For many years, the petroleum industry has developed offshore production from fixed platforms. Initially these platforms were installed in shallow water (<100'). Over the years, fixed platforms have been used in ever increasing water depth and have established a history of proven performance. In 1977, Shell Oil Company began installation of the Cognac platform offshore Louisiana in 1025 feet of water. Deep water fixed platforms are expensive. Only large and prolific reservoirs can be considered for development using a fixed platform in deep water. Development of comparatively small reservoirs or those located in water depths beyond the economic limit of a fixed platform require alternate production concepts. One alternative that has been considered is a completely submerged seafloor production station. In 1972 the Shell-Lockheed production system was installed on a well in the Gulf of Mexico at a water depth of 375 feet. A second alternative which takes advantage of readily available semi-submersible drilling platforms and much proven equipment and techniques is the floating platform production system. The first such installation is Sedco-Hamilton's Floating Production Facility (FPF) on the Argyll Field in the North Sea. The system has been operating for approximately three years. A sub sea manifold receives prediction from four satellite sub sea wells where individual production risers transmit the flow up to the platform. After processing in onboard separators, the product is transferred through a ten (10") inch central riser back to the seafloor manifold. A Single Point Mooring (SPM) system and tankers are used to deliver the crude to shore facilities. The production and transfer risers are supported from the platform by riser tensioners which accommodate a maximum rig heave of ten (10') feet. Prudent operation of such a conventionally moored facility requires that the wells be shut-in, and that the risers be purged of oil and retrieved well in advance of weather which would impede such operations. The result is downtime and a less cost-effective program. Also, when production is resumed, flow rates are often lower. The Tension Leg Platform production system offers potential for improved production efficiency because the platform does not heave, roll, or pitch in response to wave motion. The absence of heave not only simplifies the riser design and tensioning, but also permits the risers to remain connected during all weather conditions.