Efficiency Improvements for Subsea Operations in Deepwater Angola

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 119651, "Improvements in Efficiency for Subsea Operations in Deepwater Angola," by Jason Zook, SPE, ExxonMobil Development Company, and Arran Keith, Swift Technical Group, originally prepared for the 2009 SPE/ IADC Drilling Conference and Exhibition, Amsterdam, 17–19 March. The paper has not been peer reviewed. Esso Exploration Angola Limited (EEAL) has realized substantial improvements in the efficiency of subsea operations in the deepwater environment of Angola Block 15 (AB15). EEAL has increased its efforts to improve efficiencies for subsea operations, specifically during simultaneous operations. Improvements in efficiency and schedule certainty were realized as a result of dedicating a work vessel primarily to subsea operations. The full-length paper presents case histories of various operations and discusses the effect on efficiency of a dedicated work vessel. Introduction EEAL along with coventurers is developing several fields in AB15, 145 km west of Soyo, Angola, and approximately 386 km northwest of Luanda. Water depths range from 700 to 1435 m. Exploration rights were awarded in 1994, and drilling began shortly thereafter. AB15 currently is producing from five floating production, storage, and offloading (FPSO) facilities. The Kizomba C development project, which supplies FPSOs Mondo and Saxi-Batuque, is under way, and a Kizomba Satellites development project is in the planning stages. To bring these developments on line in the timeliest manner possible, as many as six rigs have operated simultaneously on the different fields within AB15. Two of these rigs are tension-leg platforms, and the remaining rigs were mobile offshore drilling units (MODUs) working on a drilling schedule of more than 100 subsea wells distributed amid 18 subsea drill centers. This high amount of activity within the block created the potential for large schedule conflicts as a result of installation-vessel and MODU simultaneous operations. In response to these potential conflicts, EEAL evaluated operations to identify which nondrilling tasks could be separated from critical-path MODU operations. Removing these tasks from the MODU schedule increased schedule flexibility and decreased cost. Suction-Embedded-Plate-Anchor (SEPA) Installation Early in the project planning for the AB15 development, drill centers were planned in close proximity to one another to enable the most cost-effective subsea architecture. The decision was made that AB15 would use one or possibly more conventionally moored MODUs for its subsea development wells. To achieve such a subsea layout and its related cost savings, the anchor pat tern of a conventionally moored MODU must become more compact. This more-compact anchor pattern required the use of taut-leg-mooring (TLM) systems to allow a MODU to moor safely over drill centers set relatively close to each other.