Mensa Project: Hydraulic Umbilicals
Authors: Williams V.T. , Wolfson S.L. , Thompson J.M. , Lecomte H. 
Published: October 2008

The Mensa Project is a remote satellite subsea production system located in the Gulf of Mexico, Mississippi Canyon 687, in approximately 5300 ft. of water. The development of this field required a production control umbilical for hydraulic and chemical injection service to the subsea wells. A steel tube umbilical design utilizing relatively large diameter, carbon steel tubing was selected for this project. Design verification testing was conducted and the umbilical was successfully manufactured and installed. The viability of utilizing continuous carbon steel tubing for subsea hydraulic umbilicals as well as the fabrication and installation of large steel tube umbilicals in ultra-deepwater has been demonstrated on the Mensa project.

Introduction

The development of the Mensa Field required that production control umbilicals be installed from the host platform in West Delta 143, some 68 miles away, to the subsea wells. Services required to produce the wells include hydraulics for valve control, Triethylene Glycol (TEG) injection, methanol injection and well annulus vent. The main or interfield umbilical, which consists of 3 tubes for methanol/annulus vent service, low-pressure hydraulics and high-pressure hydraulics, connects the host platform to the manifold in Mississippi Canyon Block 685. TEG is supplied to the manifold via a conventional 3.5 inch flowline. Three 7-tube intrafield umbilicals connect the manifold to each of the three wells in order to provide the above mentioned services. Reference Figure 1 for field layout. The challenges for this umbilical were significant. The very long offset of the wells required that the tubes be relatively large diameter, i.e. 1 inch ID, to obtain the necessary flowrates for chemical injection and hydraulic fluid. Also, the well pressures required that the tubes in the umbilical have an operating pressure of 10,000 psi. Finally the ultra-deepwater presented unique installation challenges. This paper addresses the selection and development of a new product, SeaCAT?carbon steel tubing, for use in the umbilical. The design verification testing, manufacture, and installation of the umbilicals are also discussed.

Selection

The combination of large diameter, high pressure and ultradeep water for Mensa required the selection of a steel tube umbilical. Early in the conceptual design phase, it became evident that carbon steel tubing would offer significant cost savings for this application. However, to use carbon steel tubing required that several technical challenges be met, primarily external corrosion protection in the ultra-deep subsea environment and internal cleanliness. While the usage of continuous carbon steel (low alloy) tubing has been the subject of study for some time, super duplex stainless steel has been the primary material choice for steel tube umbilicals. The few carbon steel umbilicals that have been manufactured have, for the most part, relied on conventional coating, relatively short length, and cathodic protection at the ends. Due to the extreme length of the Mensa umbilicals, protection from the ends was not possible; thus, a distributed cathodic protection system was required for the carbon steel tube option. Internal cleanliness was also a concern with carbon steel. Continuous carbon steel coiled tubing must be cleaned after manufacture to remove mill scale and other potential contaminates to the control fluids.