Mensa Project: Completion Riser and Workover Control System
Authors: Alford B.J. , Ellis C.M. , Smith J.D. , Going W.S. 
Published: October 2008

Abstract
This paper presents an overview of the development of a Completion Riser and Workover Control System for use by Shell Offshore Inc. (SOI) in the Mensa Project located in 5,300 feet of water in the Gulf of Mexico. The system was developed as an upgrade to a system previously used for SOI's Popeye and Mars Projects in water depths of 2,040 feet and 3,150 feet, respectively. Major changes to the existing system were the development of a multiplexed electro-hydraulic control system, use of a monobore running string to land the tubing hanger running assembly and tubing string, construction of a modular BOP Spanner Joint, and new hydraulic and electrical umbilical systems for use in water depths up to 6,000 feet. Early resolution of system interfacing and start of manufacturing work were essential to meet the well completion schedule. Use of this Completion I Workover (CIW) System at world record water depths has expanded the envelope for which proven and practical technology can be used in a subsea field development.

Introduction
The C/W System for Mensa Project was a logical extension of the system initially built for the installation of the Popeye and Mars subsea wells. All subsea equipment for the two earlier projects was designed for water depths up to 6,000 feet. However, the original control umbilicals were limited to 4,000 feet. While application of the original piloted hydraulic control system at 6,000 feet was feasible, the preferred system for deeper water was an electro-hydraulic multiplexed (mux) control system. With a mux control system, the reduced umbilical size makes it more manageable and the response time for hydraulically actuated components is drastically reduced. One guiding philosophy from the outset of the project was that any required new equipment would be constructed as stand-alone units without any re-configuring of existing equipment. The goal was to allow the creation of the required new equipment to proceed on a development path independent of the field utilization schedule for the existing equipment. This concept was rigidly enforced as design alternatives were evaluated which would require modification of the existing equipment to incorporate the features of the new control system. By requiring that the new control system act as an overlay to the existing piloted hydraulic system, the ability to return to the former operating mode was preserved. Changes to the subsea hardware were driven by the performance on prior projects and new challenges of working at the greater water depth. Figures 1 and 2 identify the major components of the CIW System for the tubing hanger installation mode and tree installation mode, respectively. Experience gained working with the original BOP Spanner Joint prompted a redesign to simplify assembly and increase reliability of the tool. Evaluation of the limitations of the completion rig's hook motion compensation system led to the adoption of a lighter running string to land the completion string.

Riser System Mechanical Components
Apart from general refurbishment activities following the Mars subsea well completion, there were no initial plans to modify any of the mechanical components of the riser system.