Commissioning, Startup, and Early Operation of the Gemini Development
Authors: Holbrook J.A. , Golson L.A. 
Published: October 2008

Operability throughout field life is an important element of any field development. The Gemini project team set system uptime and reliability as key project goals. To achieve these objectives, the team included strong offshore operational involvement throughout the development.

The Gemini subsea development is a fast track, deepwater project. In the first seven months of operation, the development has achieved a high system availability percentage, exceeding project objectives. Commissioning and start up occurred with few problems. These successes resulted from cooperative project execution and the ability to quickly overcome the few challenges arising during this period.

Introduction

First production from the Gemini subsea development began on June 2, 1999, approximately sixteen months from project approval in January 1998 by the board of directors of both partners. In the first seven months of operation, the surface systems and subsea hardware achieved a high degree of reliability and uptime performance. A major contributor to this success was the effective integration of offshore operations personnel into all aspects of the project, from design, equipment selection and installation planning through commissioning and startup.

The Gemini field is located in the Gulf of Mexico, Mississippi Canyon Block 292, in a water depth of approximately 3,400 ft. The project is a joint development between Texaco (60%) as operator of MC292 Development and Chevron (40%). The field consists of the Allison gas reservoir and several deeper potential zones. The partners selected the Allison reservoir for a subsea development as an early production system. Gemini production flows to the host platform in Viosca Knoll Block 900, located 27 miles northwest of Gemini in 340 ft. of water. VK 900 is jointly owned by Chevron (75%) as operator and Texaco (25%).

The subsea development consists of three producing wells connected via flexible jumpers to a four slot cluster manifold, (Fig. 1). The manifold and well functions are controlled from VK 900 using a multiplexed electro hydraulic control system. The manifold has two headers connected via a pigging loop with an actuated pigging/isolation valve. Each well is connected to both headers via actuated manifold isolation valves, providing flexibility to direct a well into either flowline. Tree mounted subsea chokes control flow rate from individual wells into the manifold. Each manifold header is connected to a 12-inch flowline via rigid steel pipe jumpers.

Dual 12-inch uninsulated flowlines transport produced fluids to the host platform. The flowlines are rated for shut-in tubing pressure and sized to accommodate future production from the surrounding Gemini development area. Reservoir performance modeling predicts limited water production throughout field life. Therefore, continuous methanol injection was selected to control hydrate formation in the subsea system.

The host platform has been upgraded with gas processing and metering equipment, subsea control systems and chemical injection capability to service the Gemini wells. In addition, liquid handling capacity at the host was increased to optimize the use of existing equipment and platform deck space. The upgrade was made possible by the addition of two large production modules.