Benguela-Belize Compliant Piled Tower: Tower Design

Abstract The compliant tower is a proven bottom-founded platform which has been used successfully for three developments in the Gulf of Mexico. The compliant tower offers a number of advantages as it provides a stable, conventional, cost effective base for offshore drilling and production operations using dry trees, and fixed or steel catenary risers. Construction of compliant towers is readily achievable with existing and proven fabrication facilities and offshore construction equipment. The Benguela-Belize tower represents the first compliant tower application outside the Gulf of Mexico in an environment substantially different from Gulf of Mexico environments. The Benguela-Belize development scheme required a concept which could effectively serve as a major hub facility, with large wellcounts, large numbers of risers, and a record setting operating payload of 40,000 mt. The advantages offered by the tower were exploited effectively to satisfy the challenges presented by the Benguela-Belize development. This paper describes the design of the compliant tower for the Benguela-Belize development, which presents a number of challenges not experienced in Gulf of Mexico developments. Reviews of the compliant tower concept are presented, along with Gulf of Mexico experience, and comparisons with the tower configuration and global response in West African environmental conditions. Descriptions of the Benguela- Belize project requirements are given, along with design criteria and discussions of differences with Gulf of Mexico criteria and the impact on the tower design. Design and analyses procedures, and system characteristic responses and the impact on wear and fatigue are discussed. The unique design features, such as the largest piles ever installed offshore, the active leveling system required to ensure verticality constraints are achieved, and field connections for the offshore mating of tower sections, will be summarized and presented. The technology summarized in this paper can be used effectively in future offshore projects worldwide. Introduction Chevron Corporation's Angolan subsidiary, Cabinda Gulf Oil Co. Ltd. (CABGOC), successfully completed the installation of the world's first compliant tower outside the Gulf of Mexico in May of 2005. Planning for this milestone event began with discovery and appraisal drilling at the Benguela and Belize sites in the late 1990s and the year 2000. Following concept evaluations, a design competition, and the selection of the compliant tower as the central hub facility for development of the Benguela and Belize reservoirs, the preliminary design of the tower, more specifically the Compliant Piled Tower (CPT), was awarded to Mustang Engineering in 2001. CABGOC awarded the EPCI contract to Daewoo Shipbuilding and Marine Engineering in February of 2003, with commercial first oil planned for the last quarter of 2005. Additional information related to project development is contained in two accompanying papers by Kim, et al, and McNeilly, et al. The development schedule resulted in a design and build schedule, including installation offshore West Africa, which can only be described as aggressive. Design and build schedules for such deepwater structures typically can extend over 30 months, and more. The Benguela-Belize schedule resulted in a design and build schedule of 26 months, which presented the design and fabrication teams with significant challenges. ‘Fast-track’ scheduling between the EPCI contractor and fabrication subcontractors, Kiewit Offshore and Gulf Marine Fabricators, and the CPT design team was identified early in the project as an essential element for success.