Fifty-Two Cuts on a Tool That Did Not Exist Two Months Earlier

• 8 weeks Concept to mobilised, from scratch
• 52 Subsea cuts on the first campaign, no tool issues
• 5 Elements designed, built and tested as one package
• 500+ Cuts across all campaigns since

The problem

A caisson is a large-diameter tube hung off the side of a platform. Taking one off means cutting it below the waterline and getting the severed section back to the surface, and the cut is not the hard part. The hard part is everything around it: positioning a saw against a tube that was never designed to be interfaced with, holding it there while it cuts, and making certain the section cannot drop at the moment of severance.

Fugro came to us for the caissons on North Rankin A, on Woodside’s North West Shelf. The work was ROV-deployed, with no divers. Nothing in the rental fleet covered it.

What had to be built

Five things, scoped and delivered as one package rather than as five procurements:

• A bespoke diamond wire saw, sized to the caisson
• A coupon catching and ejecting system, so the cut section is captured at the moment of severance rather than released
• An ROV skid with a locking arrangement, so the tool stays where the ROV put it
• Bespoke drill and pin tools
• A basket-mounted deployment frame, so the spread goes down and comes back as one lift

Treating it as one package is what made the programme possible. Every element sat inside one scope we controlled, so a problem found on the test floor went back to the engineer who drew the part and was re-cut the same week rather than being argued between two suppliers. See the testing centre.

Eight weeks

Concept to a mobilised cutting spread took eight weeks. That figure is the whole argument for holding concept, design, fabrication, assembly and test in one scope, and it is not a number we could quote if the package had been split between vendors each managing their own interface.

Fifty-two cuts

The saw completed fifty-two subsea cuts on its first campaign without a tool issue. For something that had not existed two months earlier, that is the number worth quoting: not a first, just the absence of the thing that would have cost a fortnight of vessel time.

The package has since run further caisson campaigns on the same asset and on Goodwyn Alpha, where it took a bespoke coupon catcher. Saws in that family have now passed five hundred subsea cuts. Project references are available on request.

Decommissioning