Mensa Project: Subsea Control System
Authors: MacNeill J.C. , Murthy R.R. 
Published: October 2008

Abstract
This paper reflects on the development of a subsea production control system for the Mensa Project. It will present the technical challenges encountered and the solutions implemented during development, and will identify the management of interfaces critical to the successful installation of the subsea control system and umbilicals.

Introduction
In the Gulf of Mexico, in 5300 feet of water, situated in Mississippi Canyon Block 687, lies a gas field named Mensa. Mensa is a subsea development incorporating three trees connected by flowlines and control umbilicals first to a subsea manifold, 5 miles distant, and then 63 miles to Shell Offshore Inc. (SOI) platform located in West Delta 143. The 68 mile offset is the world's longest and represents an important technological advancement in linking very remote deepwater fields to existing production infrastructure within the Gulf of Mexico shelf.

Control System Overview
Mensa utilizes a multiplex electrohydraulic production control system (Fig. 1). The control system's design is based upon comprehensive system operability reviews convened in the early phases of the project, as well as throughout its various execution phases. These reviews were extremely beneficial, resulting in specifications that clearly defined the operating constraints for both platform and subsea control equipment. Platform Equipment. Platform-mounted equipment consists of a production hydraulic power unit (PHPU) and a dual redundant computer Master Control Station (MCS) including an uninterruptible power supply (UPS) for the control and monitoring of the subsea equipment. The MCS is the primary control center for the production control system. From the MCS, the operator can monitor the status of the MCS, subsea communication, subsea electrical distribution, an individual well, the Hydraulic Power Unit (HPU), the chemical injection pumps, or the control pod on a video display terminal. The operator can issue commands to the Surface Controlled Subsurface Safety Valve (SCSSV) and tree valves and to some HPU functions from the keyboard provided with the system. The MCS is located in the control room of the platform. The HPU provides regulated hydraulic pressure for use in the control system. It provides regulated high pressure (HP) at 7,000 psi, regulated low pressure (LP) at 4,000 psi, and a spare supply that can be used either for HP or LP. These pressures are directed to the subsea control pod via the umbilical. Three electric motor driven pumps provide the hydraulic power. One is used for the HP system pressure. A second pump is used for the LP pressure. The third pump acts as back-up pump for the LP and HP pumps. A fourth pump circulates the reservoir fluid. Filters clean the hydraulic fluid and regulators regulate the pump output to the desired pressures. Pressure relief valves protect circuit components from excessive pressure. Two piston accumulators store the HP hydraulic supply and four bladder accumulators store the LP supply. Electrical pressure transducers provide automatic control for the pumps and provide pressure information to the MCS. Subsea Equipment. The Electrical Distribution Structure (EDS) provides for the subsea termination of the main electrical umbilical from the WD 143 platform.