Effectively utilizing existing Gulf of Mexico shelf infrastructure will allow predictable fast tracking of new deepwater subsea developments. In the case of the Gemini subsea development, this meant upgrading an over twenty-year-old oil handling host platform to accommodate up to 240 MM ft3/day of gas and 4,320 bbl/day of liquids. The Viosca Knoll 900 "A" host platform was modified to include approximately 18,000 ft2 of additional deck space with an operating weight of more than 1,700 tons. The upgrade was executed in a simultaneous operations mode with limited amount of host platform production downtime. Topside facility challenges are highlighted including equipment added, equipment placement issues, installation methods and interfaces between new and existing producing equipment.

While many fixed water production and drilling platforms are aging and nearing abandonment, deepwater subsea completions are becoming more common place in the Gulf of Mexico. Properly evaluating this aging infrastructure can lead to profitable deepwater developments with short cycle time, few startup problems and high production availability.


In August 1997, front end engineering and design was initiated for the Gemini subsea development. An integrated joint Texaco (60% working interest) and Chevron (40% working interest) project team was established. The development consisted of drilling and producing Allison gas reserves subsea with a tie back to a shallow water host facility. The following October, the jointly owned Chevron (75%) / Texaco (25%) Viosca Knoll 900 "A" platform was selected as host platform. Front end loading for topsides commenced late October 1997, with board approval by both companies granted in January 1998.

The Gemini project team was charged with drilling and completing wells in 3,393 ft water depth and producing back to the host facility located in 340-ft water depth via two 27 mile flowlines. First production was targeted for July 1, 1999.

Topside facilities were soon identified as critical path. Topside scope of work included providing gas processing facilities to accommodate new production and interfacing with existing host platform liquids handling equipment. To allow for future expansion, Gemini topside facilities were designed for 240 MM ft3/day of gas and 4,320 bbl/day of liquids. The Gemini upgrade added 18,000 ft2 of new deck area to the existing 28,000 ft2, a 64% increase. First production was brought online one month ahead of schedule on June 2, 1999.

Topside Challenges

The topside project team was faced with several challenges:

  • Upgrading host platform to accommodate over 1,700 tons (operating weight) of new structure and equipment

  • Integrating new and old production and safety systems

  • Fast track project execution

  • Integrating Texaco and Chevron operations philosophies. Texaco was charged with managing project execution, while Chevron would operate the host platform

Host Platform Description

Viosca Knoll 900 "A", an eight-pile 24-slot drilling and production platform, was installed by Gulf Oil Company in 1976 and is located approximately 90 miles southeast of New Orleans, Louisiana in 340 ft of water.

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