An Improved Lightweight Bottom-Founded Offshore Platform

Abstract

ABSTRACT This paper presents the design and analysis of an improved lightweight, bottom-founded, offshore platform for multi-purpose use in support of field development operations in water depths to 500 feet and possibly greater, and in severe as well as in mild marine environments. The concept, an innovation based on current tripod-type structures, utilizes a novel framing support system which results in an extremely efficient design with a broad range of applicability. The development of diverse applications for the concept and an assessment of its limits of applicability are discussed. This paper is of interest to the designer, the engineer, or others wh O are seeking an alternative, straightforward and cost effective approach to the design and installation of offshore platforms. The data presented is useful to those assessing the economic feasibility of an offshore field development. The paper discusses platform performance data, including member stresses, deflections, and natural periods of vibration, and offers criteria related to the environment, payload, and water depth for which the concept is best suited. Estimated platform weight and cost data versus water depth for various environments and development scenarios are presented and compared with data from more conventional platform concepts. It is demonstrated that the improved concept offers significant advantages over current designs. OBJECTIVE OF THE DESIGN This design concept was developed in response to the ever growing need for more cost effective solutions to the development of our offshore hydrocarbons reserves. The objective of the design was to develop an offshore structure that 1) is very lightweight, 2) has a relatively high deck load capacity, 3) is suitable for use in a large range of water depths, 4) is adaptable for the pre-drilling and completion of wells, 5) can be fabricated in a relatively short period of time, 6) can be installed with conventional lifting and/or drilling equipment, and 7) can be easily relocated for reuse at another offshore site where the water depth differs greatly from the first site. GENERAL DESCRIPTION The basic concept is a tripod structure as illustrated in Figures 1 to 3. The configuration shown in these figures is called a "Right Angle" configuration because it appears, when viewed from the side, as a 90° right angle. The right angle configuration offers greater access for the drilling of wells. In a "symmetrical" configuration the apex of the tripod is placed at the centroid of the triangular base. The symmetrical configuration is suitable for those applications that do not have wells. The principal component of the structure is the column. The column is designed as a separately installed element which is lifted or floated into place offshore in a single piece. It may contain risers, umbilicals, sumps, caissons, and other facilities as needed for offshore operations. The column is designed to support a cantilevered equipment deck.