Gap Onshore Construction: Welding and NDE Challenges

MURPHY Sabah Oil Co. Ltd. has developed the Kikeh Field located offshore Malaysia in the South China Sea in a water depth of 1325m. This field development is based on a Floating Production Storage and Offloading unit (FPSO) and a Spar Dry Tree Unit (DTU). Fluids are transported in fluid transfer lines (FTL) using SBM’s newly developed and patented Gravity Actuated Pipe (GAP) system. Two key areas in the construction of the Kikeh GAP Fluid Transfer Line were Welding and Non-Destructive Examination (NDE). The welding of flowlines was performed using the Gas Tungsten Arc Welding (GTAW) and Shielded Metal Arc Welding (SMAW) processes. The welding of the carrier pipe utilized the double-sided Submerged Arc Welding (SAW) process for tubular prefabrication and the manual/automated GTAW processes for girth welding in the field. Welding process selection for the carrier pipe was driven by the need for high weld quality in order to provide optimum fatigue performance. Extensive welding procedure development was carried out for the manual/automated GTAW techniques in order to determine optimum welding parameters as this constituted the most critical welding activity. Project specific welding procedure qualification was undertaken for all field welding activities. An Engineering Criticality Assessment (ECA) was performed to determine maximum allowable defect sizes for the welded joints. This ECA established that the welded joints had a low defect tolerance, which consequently dictated the necessity for an NDE technique providing a high level of flaw detection and accurate flaw sizing. Automated Ultrasonic Testing (AUT), using a Phased Array (PA) system based upon the ‘Pulse Echo Method’, enhanced with mapping images and ‘Time of Flight Diffraction’ (TOFD), was therefore nominated as the volumetric inspection method for the carrier pipe. The AUT system capabilities were verified by qualification, prior to mobilization to the field, to determine the system Probability of Detection values (POD) and imperfection height sizing accuracy as well as assessing the AUT operator(s) performance.