Deepwater-Structure-Installation Challenges Offshore Australia

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 26720, “Structure Deepwater-Installation Challenges—Northwest Shelf, Australia,” by Trent Broadway, SapuraAcergy, and Vincent Tachoires, Subsea 7, prepared for the 2016 Offshore Technology Conference Asia, Kuala Lumpur, 22–25 March. The paper has not been peer reviewed. Copyright 2016 Offshore Technology Conference. Reproduced by permission. This paper provides an insight into the challenges encountered and overcome during installation of 20 subsea structures, some close to 1000 t in weight and in water depths of up to 1350 m, for the Gorgon project offshore Western Australia. Introduction The Gorgon project development comprises the Gorgon and Jansz-Io gas fields located off Australia’s northwest coast, in water depths of 200 and 1350 m, respectively. Ten structures were installed in 20 separate lifts, using the multipurpose heavy-lift and pipe-laying vessel Sapura 3000. Tight angular tolerances and low landing speeds were required to meet the structures’ design criteria. All structures were loaded onto cargo barges and towed to site for installation. Liftoff and deployment through the splash zone were performed with the Sapura 3000 main crane. Depending on the size of the lift and the installation water depth, the structures were either deployed directly to the seabed using the crane (in the Gorgon field only) or transferred to a deepwater lowering system (DLS). Structure foundations were either skirted mud-mat types (deep water, soft-seabed locations) or suction piles, and were all fitted with fixed guideposts to assist with module-landing operations. Installation Analysis Subsea-structure foundations and modules are typically designed with landing-speed limitations; exceeding these might cause overstress within the structures. On the Gorgon project, the landing speeds were limited to less than 0.3 m/s. Initial structure-landing analysis showed that the prescribed landing velocities would be difficult to achieve without further heave compensation because the sea-state limits would be too low to be realistically achievable given the prevailing weather conditions at the site. Because the main crane of the Sapura 3000 is not fitted with a heave-compensation system, a passive heave compensator (PHC) was introduced in the lift-rigging model to achieve better installation sea states. Each of the 20 Gorgon and Jansz-Io structures required different PHC stiffness and damping because of their individual properties such as mass, rigging properties, perforated areas, and associated drag effects. Lift-off analysis from the cargo barge was performed for the sea states identified by the governing installation step (landout). To achieve sea-state limitations, different PHC settings (stiffness and damping) were required for structural liftoff.