Mensa Project: System Design and Operation
Authors: Lang P.P. , Rainey R.M. , Bankston C.H. 
Published: October 2008

This paper reviews the system engineering issues related to development of the Mensa Project subsea .system: from conceptual through detailed design, and the philosophies and strategies planned for operation of the system.

Overall System Design

Design Basis.

The Mensa gas is a dry accumulation with nearly 100% methane at a reservoir temperature and pressure of 1760 F and 10,040 psi, respectively. The wellhead shut-in tubing pressure is 8970 psi and the hydrate formation temperature is 80° F.

System Description.

The Mensa subsea development system consists of three subsea gas wells that are connected by 5 miles of 6-inch flowlines to a manifold in about 5300 ft of water. From there, the gas is transported through a 63-mile 12-inch flowline to a processing facility at the West Delta 143-A platform. The wells are continuously choked at the wellhead to vary the production rate and reduce the flowing pressure into the flowline system. It is anticipated that the wells will be continuously choked for several years until the reservoir pressure declines. After subsea choking is no longer required, the system production will be a function of flowline pressure losses. Based on the maximum well shut-in tubing pressure, the 6-inch flowlines have a Maximum Allowable Operating Pressure (MAOP) of 9000 psi. The 12-inch flowline has a variable MAOP along its length. The MAOP is 6000 psi at the manifold and 4000 psi at the platform. For hydrate inhibition and gas dehydration, Triethylene Glycol (TEG) is injected at each tree. The dehydration process is accomplished in the 6-inch flowlines, thereby eliminating the need for surface dehydration. Glycol is transported to the trees via a combination of a common 3 1/2-inch supply line between the platform and the manifold and four I-inch tubes between the manifold and each tree. Transition of the glycol between the 3 1/2-inch supply line and the I-inch tubes occurs at the manifold. The subsea system is monitored and controlled with, a multiplexed electrohydraulic control system. Control IS relayed back to the WD143-A platform, allowing the operator to control and monitor the changing characteristics of the wells. In addition, the Master Control Station (MCS) automatically shuts-in the wells if conditions occur that are outside the normal operating range.

Tree to Manifold Distance.

The wells are arranged in a "spread" cluster with the manifold located 5 miles away (Fig. 1). The distance between the wells and the manifold was selected to ensure that, with maximum flow of 150 MMSCFD, the gas within each flowline would be cooled to seabed temperature, prior to entering the manifold. As the gas is cooled to seabed temperature, the water condenses out and IS inhibited with glycol in the 6-inch flowlines. If the inhibition system ever failed, a hydrate plug therefore would most likely form within the 6-inch flowlines. Should this occur, immediate well shut-in is not essential since the flowlines are rated for full shut-in pressure. Also, each flowline can be individually isolated from the entire flowline system and depressurized to ambient (atmospheric) using a surface deployed intervention umbilical.