CALM Buoy Re-lifing

Location: Stag Field, Western Australia


The ONA team undertook an asset management challenge to ‘relife’ a CALM buoy and mooring system. The CALM buoy system was reaching the end of its design life and would fall out of class requirements. Our brief was to assess the condition of the CALM buoy system, from the topside components and hawser down to the mooring chain and anchors, and then develop an asset management strategy to satisfy Class requirements and enable the buoy to continue its working life.


The scope began with our team completing a re-lifing plan to ensure the client agreed with the direction of the re-lifing study. This roadmap set out the steps required for the CALM buoy to continue to meet Class requirements.

From there, we conducted a mooring load and capacity review of the system, including developing a hydrodynamic model of the CALM buoy system. This review and model identified the decrease in allowable return period of extreme events.

Furthermore, we oversaw the subsea inspections, which involved both diving and ROV surveys of the buoy, mooring chains and anchors. These findings, as well as the mooring review results, were collated into Class submissions.

Outcomes and benefits

Our team managed the engineering, oversaw the inspection scopes, liaised with Class and developed the Class submissions as a single point of contact on the project. The analysis and review support we conducted de-rated the designed extreme event, reducing the ongoing risk of the system.

The asset management program we developed ensures the CALM buoy system continues to stay under Class, and also gives the client a thorough understanding of what is required going forward, with a view to reducing risk and planning operational expenditure.

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