Mensa Project: Electrical Umbilica
Authors: Marosko Ronald J. 
Published: October 2008

Abstract
This paper reviews the definition, manufacture, testing and installation of the electrical umbilical used on the Shell Offshore Inc. (SOI) Mensa Project. It covers the cable definition as part of the subsea control system in light of its extreme length of 63 miles (100 km) and depth of operation, 5300 feet (1.6 km). It describes how the power distribution and signal attenuation for the multi-well system were handled. Some of the problems encountered in manufacturing the umbilical in one contiguous length, transportation of the total length, and installation are likewise discussed.

Preliminary System Design
The approach to this project was a relatively standard one designing a cable that would provide acceptable attenuation for the signal and acceptable power transfer for the power while keeping size to a minimum. It was decided that signal and power would be kept separate to allow the use of three phase power which keeps the power conductors as small as possible. Overall size of the umbilical was very important due to the desire to have the umbilical in one continuous length for installation purposes. The physical sizes of the carousels on vessels expected to be used for installation were limited. The signal cable was designed by configuring a quad of conductors of various sizes with polyethylene insulation to define the range of possible configurations. The main requirements were attenuation and characteristic impedance that were defined by the standard configuration of the control electronics. The quad was sized to achieve the desired characteristic impedance, and then the attenuation was calculated based on the basic R, L and C parameters. Alternate calculations were made with the capacitance doubled to allow for water flooding of the quad. An acceptable configuration was achieved using 6 mm2 conductors. The three-phase power configuration was based on using the same conductor sizes in a triad configuration. For the expected maximum load, a supply voltage of 720 volts was selected. Power factor correction inductors at both ends of the umbilical were planned due to the high expected capacitance of the power triad. Final selection of the values was deferred until the umbilical was manufactured. Resistance for a single conductor in the umbilical is approximately 300 ohms.

System Configuration
The Mensa subsea control system comprises an interfield umbilical approximately 63 miles in length from West Delta 143 (WD 143) to the manifold and repeater module and then three 5-mile intrafield umbilicals from the repeater module to the three wells. Redundant three-phase power and signal is connected to each of the wells (See Fig 1 for Umbilical cross section). Power transfer from the surface to the repeater module is at a nominal 720 volts. There it is stepped down to a nominal 240 volts, three phase for the three wells. Power factor correction inductors are included in the repeater module as well as the surface master control unit. Redundant signal using a communications quad is supplied to the repeater module where it is amplified before being passed to the wells. The repeater module also operates as a data acquisition unit for pressures and temperatures on the manifold. Communication is half duplex, 1200 bits per second (FSK), based on Bell 202 frequencies.