Mensa Project: Subsea Tree System
Authors: da Costa M. , Hartley H.J. 
Published: October 2008

This paper describes the design, product development and offshore installation of the subsea trees for the Shell Offshore Inc. (SOI) Mensa Project. The Mensa Project represents the world's deepest subsea tree at the time of installation and the longest offset from the host platform. The extreme water depth, high flow rate and erosion resistance requirements resulted in a number of significant technical advancements in the subsea tree design, installation equipment and installation techniques.

Introduction

The Mensa development plan utilizes three satellite wells in 5,300 ft water depth, completed with 10,000 psi working pressure, guidelineless, diverless subsea trees producing to a subsea manifold located five miles from the trees. A single main flowline carries the commingled produced fluids from the manifold to a shallow water platform located 63 miles from the manifold. The subsea tree design applied existing guidelineless technology extended to deeper water. A split tree was designed to resist wear and enable retrieval of components subject to erosion wear while leaving the master valves in place. In the first quarter of 1997, the first tree was subjected to the factory acceptance and system integration testing program, which simulated the offshore installation. The first tree installation was completed in July 1997 and the second in September 1997. The third tree is scheduled for installation in May 1998.

Subsea Equipment Overview

The Mensa wells are located in Mississippi Canyon Block687, approximately 140 miles southeast of New Orleans, in 5,300 ft of water. The total offset distance from the subsea trees to the host platform is 68 miles. Three wells were completed using guidelineless diverless subsea trees. Each tree is connected to a subsea manifold via a 6" flowline and a steel tube hydraulic umbilical. The hydraulic umbilical incorporates 7 lines for control of the tree functions and chemical injection. The 6" flowline and hydraulic umbilical were installed using a first end stab & hinge over connection at the manifold and the second end laid down near the tree using terminationsleds with vertical hubs. The distance between the trees and the manifold is approximately 5 miles. The flowline and umbilical connections between the tree and the termination sleds were accomplished using inverted U shaped jumpers, approximately 60 ft long. All stab & hinge over and inverted "U" jumper connections feature ROV operated hydraulic connectors. The subsea manifold accommodates the tie-ins for the three 6" production flowlines and hydraulic umbilical to the subsea trees. The manifold production header connects to the 12" gas export line. The hydraulic umbilical and glycol supply lines also connect to the manifold. A sensor module is installed and there are provisions for connection of a hydrate remediation umbilical. The production control system for the subsea trees is electro-hydraulic. A common electrical umbilical connects the Master Control Station (MCS) on the production platform to the Electrical Distribution Structure (EDS) near the manifold. Separate electrical umbilical link the EDS to each tree control pod and to the manifold. The distance between the manifold and the production platform is approximately 63 miles. A schematic of the subsea production system arrangement is shown in Figure 1.