Abstract
Hyperbaric welding habitats provide a dry environment where divers can perform subsea welded repairs to structures or pipelines. The Ninian Southern Platform jacket structure in the northern North Sea has suffered a history of weld defects and fatigue-induced brace failures since it was installed, a number of which have required extensive hyperbaric weld repair work.
Previous repair projects have typically been long and expensive as they have relied on a purpose built welding habitat which requires a long lead time and significant up front cost for each repair. The need to have in place a repair readiness strategy for the Ninian Southern Platform jacket, to minimise the time and cost involved in carrying out repairs to critical welds led to the development of a modular system which can allow access to a large number of different weld locations.
This paper describes the concept, design, building and testing of a unique Modular Habitat system to fit 80% of the brace welds on the Ninian Southern Platform. The design has kept diver safety as the over-arching driver, but with flexibility, speed and quality of repair as key objectives of the project.
Evolution of the design to suit the maximum number of weld configurations is explained, through detailed design to the importance of sealing onto the different brace configurations. Key to this achievement was a collaborative approach that involved CNRI, Subsea 7 and Atkins; input from diver experience of working in bespoke jacket repair habitats was invaluable. As well as developing an optimised enclosure small enough to be located in confined areas but large enough for practical working, the opportunity was taken to include state-of-the-art features in the Habitat’s integral diving system, making it as far as possible a safe and comfortable place in which to work. The design was refined by the use of full scale mock-ups, and proven by wet-testing of the completed Habitat Modules.
The Modular Habitat system plays a key role in the overall life extension strategy for the Ninian Southern Platform, ensuring that the costs, risks and production downtime associated with future subsea repairs are minimised.
