Gemini Pipelines
Authors: Ritter J.E. , Doiron D.D. , Beck J.E. 
Published: October 2008

This paper describes the design characteristics, contracting philosophy and management, installation planning and project execution associated with the installation of the Gemini flowlines. Additionally this paper addresses unique design features associated with deepwater subsea tie-back systems and focuses on the installation planning, contingency planning and risk management. The dual 12" Gemini flowlines are 27 miles in length and installed to a maximum water depth of 3,400 feet. The shallow end of the flowlines are connected to conventional risers at a host platform in 340 feet of water. The deep end of each flowline was terminated at depth utilizing a Pipeline End Termination skid (PLET).

The Gemini flowlines, PLET's and risers were installed under budget and without delaying or interfering with other installation or drilling activities associated with the project. The success of the Gemini flowline installations enforce the philosophy that good contracting strategy, proper installation planning and good risk management will produce desired results.

Introduction

The Gemini Subsea Development is a deepwater subsea tieback system developed to transport natural gas / condensate directly from three wells located in Mississippi Canyon Block 292 (MC 292) to an existing host platform in Viosca Knoll Block 900 (VK 900). The host platform located in the VK 900 block is the VK 900 "A" platform.

The Gemini Subsea Development is a joint venture between Texaco (60%) and Chevron (40%) located in MC 292 in the Gulf of Mexico. The development consists of three subsea wells flowing into a subsea manifold. The subsea manifold is tied back to the VK 900 "A" platform via two 12-inch diameter, 27-mile flowlines. A four-inch diameter electro-hydraulic umbilical follows the route of the flowlines and is utilized to control subsea manifold and well functions from the host platform in VK 900 (Figure 1).

Front-End Engineering and Design (FEED)

In mid 1997 the Gemini Project Team initiated FEED process for the Gemini Subsea Development. The Gemini Project Team elected to hire various engineering firms with specialized expertise in lieu of obtaining the services of one engineering firm. The FEED process included development of the chosen subsea tie-back option, level two schedule development, development of a contracting strategy and Approval For Expenditure (AFE) development.

Since the flowline installation was deemed a critical path item on the overall project schedule, one goal of the flowline FEED process was to develop a plan to expedite the procurement of flowline materials and develop a contracting plan to reduce the flowline installation cycle time.

Route Survey

Available topography information was gathered and analyzed to develop an acceptable flowline route between MC 292 and the host platform in VK 900. Particularly in deepwater, the goal is to avoid environmentally sensitive areas and subsea features which could induce pipeline spans. Because of the lack of good quality bathymetry data in the area of the proposed route, the Gemini Project Team elected to perform the survey in two phases scheduled several months apart.