Mensa Project: Flowlines
Authors: Gilchrist R.T. , Kluwen F.A. 
Published: October 2008

This paper describes engineering designs, installation particulars and learning points from development of the Mensa pipeline transportation system. The information presented will be of interest to engineers involved in design, construction or repair of offshore pipelines and subsea flowlines. The Mensa 12" × 63 miles interfield flowline was S-Iaid to a depth of 5300 feet. The second end was terminated at depth using a Pipeline End Manifold (pLEM). The PLEM was fitted with vertical connection hubs and a horizontal jumper was installed between the PLEM and the Mensa manifold. The flowline maximum allowable operating pressure (MAOP) varies with location and has been calculated considering maximum possible flow rates, pressure relief facilities and hydrostatic pressures. Damage during construction was repaired using shaped-charge cutting devices, ROY-operated lift frames, ROY-operated pipe recovery tools and ROY-operated pipe repair tools at 5000 feet. Seven miles of pipe from depths between 5300 feet and 4700 feet was recovered up the stinger by 'reverse lay' and later reinstalled. Three 6" × 5-miles long intrafield flowlines were initiated using stab and hinge tools and terminated with vertical hub PLEMs adjacent to subsea wells. The stab & hinge tools were deployed down an S-lay vessel stinger. The PLEMs were welded to the flowlines on the surface and the entire assembly was lowered into place. During raising/lowering sequences of pipe ends, with and without PLEMs, rotations in excess of 5000 were observed. End cuts were made using a long baseline acoustic positioning system for reference. These repeatedly yielded actual positions within one meter of target. Each intrafield line was fitted with 15 lift frames at 500 ft intervals starting at the subsea wells. These were placed using a coordinated procedure involving lowering by cable and near-bottom ROY guidance. The purpose of these frames is to lift the pipe into the seaway to facilitate cooling of the produced gas.

Introduction

The Mensa flowline system is designed to transport produced gas from three subsea wells in Mississippi Canyon 687 (MC 687) through a subsea manifold in Mississippi Canyon 685 (MC 685) and on to the West Delta 143A Platform (WD 143A). In addition to the produced gas flowline system there is a flowline to convey glycol from WD143A to the production manifold. The 3" glycol line, electrical umbilicals, and hydraulic umbilicals generally follow the flowline routes. This paper covers design and construction of the production flowlines. The flowline system is comprised of two distinct subsystems:The interfield sub-system is a 12.75" OD main flowline between the subsea manifold in MC 685 and the West Delta 143 "A" (WD 143A) Platform.The intrafield sub-system is three each 6.625" OD gas flowlines running from the A-I, A-2 and A-3 well sites in MC 687 to the subsea manifold in MC 685

12" Interfield Flowline Route.

The interfield route was designed to avoid two government-defined explosive dump areas, a rock outcrop area, and to cross surface faults and changes in seafloor elevation in the least severe manner (Figs. 1 & 2). Depth ranged from -369 feet at WD 143A Platform to -5,304 feet at the subsea manifold site, MC 685.