Mensa Project: Subsea Manifold and Electrical Distribution Structure
Authors: Williams M.R. , Hale J.R. 
Published: October 2008

This paper presents the design, testing and installation of the subsea manifold and the Electrical Distribution Structure (EDS) for the Shell Offshore Inc. (SOI) Mensa subsea production system. The Mensa Project represents the world's deepest subsea manifold installation and the longest offset from a producing manifold to its host facility. Mensa's extreme water depth, high ambient pressures, low seabed temperatures and long offset posed a number of significant technical challenges to the project team. The innovative solutions developed have advanced the state-of-the-art for ultradeepwater subsea systems, equipment designs, and installation procedures. The Mensa manifold and EDS were subjected to an extensive systems integration test program during the latter half of 1996. The comprehensive SIT program generated a number of improvements in equipment designs, and rigging & handling procedures that proved valuable during offshore installation. The manifold and EDS installations were completed, with excellent results, in the 1st quarter of 1997, and the system began operating in July, less than 2 years from kick off of detailed design.

System Overview

The Mensa subsea manifold and EDS are located in Mississippi Canyon Block 685, approximately 140 miles southeast of New Orleans, in 5300 feet of water. The manifold is approximately 63 miles from its host platform in West Delta Block 143 and approximately 5 miles from the 3 Mensa wells in MC686. A pictorial representation of the complete Mensa subsea production system is provided in Figure 1. The Mensa satellite wells, which are all high pressure (9000 psi) gas wells, produce to the manifold via separate 6" flowlines (Fig. 2). The gas is commingled in the manifold and exported to the host platform via a single 12" flowline. The Mensa subsea production system employs no thermal insulation for hydrate prevention, but relies on glycol injection, supplied to the manifold from WD143 via a single 3" line. Low pressure (LP) and high pressure (HP) hydraulic control fluids and methanol are supplied to the manifold from WD143, via a 3-tube umbilical constructed of nominal 1" steel tubing. These fluids are distributed from the manifold to the satellite wells via individual 7-tube umbilicals, also constructed of nominal 1" steel tubing. The manifold provides the capability to tie in up to 4 subsea wells, using one 6" flowline and one 7x 1" hydraulic umbilical per well. Although hydraulic control fluids are routed through the Mensa subsea manifold, electrical controls are not. A main electrical umbilical delivers power and signals from WD143 to an EDS located adjacent to the Mensa manifold (Fig. 2). Separate electrical umbilicals connect the EDS to each subsea well. Both the subsea manifold and the EDS employ novel "Stab-And-Hinge-Over (S&HO)" connector technology (see description on pg. 4) that simplifies installation and connection of flowlines and umbilicals in deep water. The well flowlines, hydraulic umbilicals and electrical umbilicals were all installed as first-end S&HO/lay away connections. This direct S&HO connection approach provided several advantages in terms of reduced installation time, fewer leak paths, and less clutter adjacent to the manifold & EDS. Since the 12" export line was to be installed prior to the manifold, a second-end type connection was required, and an inverted U-type jumper was selected.