Mackerel/Tuna Platform Design And Installation

ABSTRACT An improved conventional template design has been developed to minimize construction weather sensitivity and reduce offshore installation time. Major innovations include incorporating deck legs and support structures to the (+)55 ft elevation (integral jacket cap) as part of the launched template, carrying maximum amounts of piling with the template during launch, and the use of an on-template construction module to speed pile installation and reduce dependence on weather-sensitive derrick barge operations. An effective technique for installing drilled and grouted piles was developed and proven in the field. The first platform of this design was installed in the Mackerel field in 304 ft of water in the Bass Strait, Australia. A second platform, which was designed concurrently with the Mackerel platform, is currently being installed in 195 ft of water at the nearby Tuna field. Construction is in progress on Tuna. Construction savings over conventional design as a result of innovative design improvements are estimated to be about seven weeks derrick barge time per platform. INTRODUCTION In an effort to reduce offshore construction costs in the Bass Strait (see Figure 1), Esso Australia Ltd. participated with Exxon Production Research Company in a pre-design screening study of alternative platform concepts. Prior to that time, during the period 1967-1971 Esso Australia Ltd.(EAL) as operator for the Esso-Broken Hill Proprietary Ltd. (BHP) joint venture installed a total of five oil and gas platforms and a network of some 135 miles of submarine pipelines in the Bass Strait. Development was done primarily by adapting existing Gulf of Mexico practices to the conditions of the Bass Strait. Hostile weather conditions and subsequently determined unusual foundation characteristics encountered during initial development work indicated there was a large incentive to modify conventional Gulf of Mexico design and construction practices to decrease vulnerability to these adverse conditions. After considering several concepts (ranging from novel self-erecting designs that did not require derrick barge services to conventional self buoyant tower designs later utilized in the North Sea), the improved conventional template described in this paper was selected, and detailed design followed for two 18 well platforms (named "Mackerel" and "Tuna"). Mackerel facilities are designed for 100M platform B/D, whereas Tuna is designed for 39M platform B/D with 14MMscf/D of gas requiring compression. Subsequent gas service after well recompilation could be at rates up to 100MMscf/D. As the Mackerel and Tuna platforms are conceptually similar, the focus of this paper is on Mackerel, the first and larger of the two. DESIGN GOALS Initial construction experience in Bass Strait indicated that subsequent installations would benefit by an early design start and an orderly development of design. This would allow special emphasis to be placed on a thorough appraisal of lifetime platform service requirements and design-influenced offshore construction details. These two factors were considered the key elements in reducing construction costs.