Automatic Welding and Ultrasonic Inspection for J-Lay of the MARS Tension Leg Platform Export Pipelines

Abstract

Abstract The paper describes the use of automatic welding and ultrasonic weld inspection for the deepwater J-Lay of the 18-inch oil and 14-inch gas export pipelines from the MARS Tension Leg Platform (TLP) in the Gulf of Mexico. While automatic welding has seen frequent use on offshore pipelines, its use on deepwater J-Lay was a first, and a significant development effort was undertaken to ensure success during offshore pipelay. To maximize the probability of detecting significant weld defects in these deepwater pipelines, especially those defects with significant through-wall height, Shell specified use of automated Ultrasonic Inspection (UT). This type of inspection can be used to quickly and accurately locate and sue the through-wall dimensions of a defect. These features, in combination with the high toughness of the automatic welds, made it possible to use the Alternative (i.e. Fitness for Purpose) Acceptance Criteria of the Appendix to API 1104, instead of the workmanship criteria of Section 6 of API 1104. As such, a good balance was achieved between, on the one hand minimizing the overall repair rate by not having to repair indications that were not harmful to the weld, an4 on the other hand, maximizing the probability of finding and suing significant weld defects. This first time use of automated UT and application of fitness for purpose weld acceptance criteria on an offshore pipeline project in the Gulf of Mexico goes a long way towards validating the approach as a suitable and cost effective inspection method. Introduction The MARS Tension Leg Platform (TLP), a 72%28% joint venture between Shell Offshore, Inc. and BP Exploration, is located in Garden Banks Block 864 in 2940 ft water depth. The two export pipelines, a 14-inch gas pipeline, and an 18- inch oil pipeline were to be installed by J-Lay, using the same J-Lay equipment and lay vessel (the dynamically positioned Derrick Barge DB 50) as used previously to install the 12- inch export pipelines to Shell's Auger TLP'. Dunng J-Lay 165-ft-long pre-fabricated quadruple joints are brought onboard in large storage bins. The quadruple joint is upended in the J-Lay tower and welded in a single station. While on the Auger Project acceptable cycle times were achieved using manual welding (SMAW), the larger pipe diameter and wall thickness on the MARS project warranted an investigation of using automatic gas metal arc welding (GMAW) to obtain much faster joint cycle times than could be achieved using SMAW. The pipelay contractor J Ray Mc Dermott and Shell had negotiated a risk sharing agreement, where both would pay for the incremental cost of automatic welding and UT, but also would share savings resulting from a faster lay rate.