Repair of the Macaroni 10 x 6 in. Pipe-in-Pipe Flowline
Authors: Gilchrist Robert 
Published: October 2008

The Macaroni Flowline system was the first J-lay installed pipe-in-pipe insulated flowline in the Gulf of Mexico. The flowline initiated at a subsea manifold in 3700 feet of water and terminated with a steel catenary riser (SCR) at the Auger Tension Leg Platform (TLP) in 2980 feet of water. To limit the consequence of loss of casing pipe integrity each flowline was divided into ten bulkhead-isolated compartments each some 6000-ft long. During construction of the second of two parallel flowlines the laybarge A&R cable failed with 3800-ft of pipe suspended in 3400-ft of water. The pipe fell to the seafloor. All of the pipe came to rest aft of the lay vessel in a pile of large loops. The barge was rigged to recover pipe alongside and away from the J-lay tower. The damaged section was severed using an ROV deployed shaped charge cutter and abrasive wheel. The end of the damaged line was recovered to the surface using a drop-on recovery clamp. The pipe was incrementally lifted and abrasive cut into 165-foot long lengths for onshore recycling. Upon removal of all damaged pipe the top of the suspended span was transferred to the J-lay tower pedestal and construction resumed.

INTRODUCTION

The Macaroni flowline system was designed and installed with a view to providing an extremely reliable facility to transport produced fluids from a group of subsea wells in Garden Banks Block 602 to the Auger tension leg platform in Garden Banks Block 426. The subsea manifold and the Auger TLP are 62,000-ft apart. Unique features of the system are:An insulated pipe-in-pipe system was installed for flow assurance. The insulation minimizes heat loss and maintains the flowing commodity stream above the hydrate formation temperature with some time allowance for shut-in.Dual flowlines were installed to permit pigging and commodity displacement in the event of an extended shut-in.Because the insulated annulus is at atmospheric pressure, the flowline pressure design could not take credit for external hydrostatic pressure like typical deepwater pipelines. The casing is designed for collapse.Each insulated flowline was divided into ten compartments separated by steel bulkheads. This is was intended to limit the consequence of loss of casing integrity.The flowline system was installed by J-lay to facilitate pipelay start-up (a connection sled was pulled in and pipelay initiated beneath an operating drill rig), minimize top tensions, minimize installation bending strain and facilitate SCR fabrication and hand-over to the Auger TLP.

The fabrication and construction of the system entailed numerous new activities:Design of forged bulkheads.Design of sliding pipe-in-pipe assembly method, including precise dimensioning to permit operation of automatic welding machines and automatic ultrasonic testing scanners.Onshore insulation of double joints with polyurethane foam.Onshore fabrication of pipe-in-pipe J-lay quad joints. The J-lay barge was designed for 165-ft long quad joints.Upgrading of the J-lay system for pipe-in-pipe and an increased top tension design load of 775 kips for this and future projects.Upgrade A&R capability to 490 kips.Design of repair and recovery contingency equipment for pipe-in-pipe.