More than intervention for the UT777
Although Island Offshore want to keep the UT777 design’s ‘competitive advantage’ under wraps for as long as possible, Yrjar Garshol of Rolls-Royce gives Stevie Knight a sneak preview.
“It’s one of the first really new innovations in the field that we’ve seen for many years” he says, as the UT777 goes much further than its predecessor, the Island Wellserver, proposing a shift in well operations by offering a vessel that can stay in place long enough to take on the top hole drilling – a move that holds the potential of lowering the costs of opening up a well considerably.
Island Offshore are convinced this new ship is a game changer, and it has a very different raison d'être to its predecessors. “It’s not a vessel that has its hull criteria set by speed, the main factor is that it can continue operating under North Sea weather conditions,” he says. It helps that it’s a big vessel, almost 170m long 28m wide: “Overall it’s a much more stable platform than even most well intervention vessels, and it can continue to operate even in sea state 7, or 14m high waves,” says Mr Garshol.
It also has an effect on crewing, as it is envisaged being onsite for long periods. “In the Osgaard field helicopter operability is typically under 30%, but the new vessel puts this figure up to allowing helicopter transfers to take place 70% of the time - and there’s no need to stop the vessel working to land,” explains Mr Garshol.
If it’s not about speed getting out to site but sticking there, how does it achieve this? “The most obvious point is that it has a contoured stern as well as bow: this means it can weather waves from all angles. The hydrodynamic acceleration has been a particular focus: the whole shape of the vessel allows the ship to move through the waves, not over them.” He adds that getting the design right, “took a combination of experience, advanced CFD and a lot of North Sea weather data from the Meteorological Institute”.
Station keeping is obviously a very big concern: its DP3 capability even in a worst single failure scenario can still deal with 7m wave heights.
While not designed for speed, it is capable of 11kt transits alongside its station keeping ability, so a pair of Rolls-Royce ULE 255 DPN FP electrically-driven azimuthing retractable thrusters outputting 2,200kW of power each are backed up by a pair of TT2800-SS DPN CP tunnel thrusters of 2,000kW each. Behind these are four 720rpm Bergen B32:40L9ACD gensets creating 4190kw each and two B32:40V12ACD gensets yielding 5,587kW each.
Most importantly the design has been taken as a whole in a literal sense. Top hole drilling, when compared to traditional well intervention, is a much heavier job; it requires a taller tower and also means putting hundreds of tonnes of equipment and drilling appliances through the moonpool in order to be able to place them 1km underwater.
So Rolls-Royce has sought some novels answers. Firstly rather than being a ‘bolt-on’, the UT777’s 28m handling tower is integrated right into the structure: “This way the stress is distributed across the deck and hull, rather than a normal tower which just brings all the forces down into its footings,” says Mr Garshol.
This approach has also been applied to the moonpools too. The main pool is a huge 9m by 9m in order to take on the biggest construction items but forward of this are two smaller pools for the ROVs set side by side. The reason for this is that the equipment has to work together but not get in each other’s way despite conditions, after all the ROV systems are designed to be launched or recovered in 6.5m waves and heave compensated up to 8.0m. Mr Garshol points out: “You don’t want the possibility of equipment clashes or umbilicals getting tangled with each other so this design keeps them close but still separate.”
However, this kind of extensive moonpool layout results in additional considerations for integrity and hull strength: although the beam of the ship is large, “Again, it was a challenge we met by spreading the loads more evenly; we took them away from being concentrated in the keel at the bottom and redistributed them around the sides of the hull.”
Lastly, while the vessel is designed for global use it is specifically designed for cold areas, benefiting not just from a covered, heated topside but also from a partially covered stern, too. It has a DNV 1B Iceclass and winterisation, so with the right support it could even take on operation in the high north. “It’s a vessel for the future,” concludes Mr Garshol.
By Stevie Knight
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