Material Selection Process for Non-Metallic Flexible Buoyancy Bags Assembly in Decommissioning and Marine Salvage

Abstract

Offshore structure decommissioning and abandonment is a critical component in the life cycle of hydrocarbon production. In Malaysia, 11% of platforms and 8% of pipelines have been in service for more than 40 years, much exceeding their intended lifespan. Conventional methods of decommissioning offshore platform are inherently very costly with the mobilization and operation of HLV (Heavy Lift Vessel). This study developed an Offshore Flexible Buoyancy Tank (OFBT) as the buoyancy assembly to execute future small structure removal operation. This study applies the structured material selection process to determine suitable material as buoyancy assembly enclosure. According to the concept, material selection seeks to determine the lightest, pressure-sustaining, fracture-resistant and cost-effective buoyancy bags suitable for the application. This is a problem with 3 objectives: minimize weight, minimize cost, and withstand the internal pressure in above or submerged conditions. A range of material is tested based on the parameters, and it was determined that Two layers of PET membrane is proposed for OFBT with capacity not less than 380 kN/m. An additional layer is also required to protect the buoyancy bag from puncture or tear. Yard burst test testing is implemented to determine the accuracy of FEA model. A prototype OFBT is fabricated to consist of 3-layer flexible polymer sheet to create an air-containment enclosure. Overall, the FEA correlates well with the result from the burst test. Both burst tests indicate the failure to occur the flange area. The circumferential stiffeners were observed to fail first followed by Longitudinal stiffeners. Results from this work is integrated in the research towards the development of novel flexible offshore buoyancy tank assembly for the removal of jacket structure in decommissioning and abandonment suited to the application in Malaysia.