Moonraker: more than 30,000 tonnes of lift
The UK is aiming to enter the offshore decom market with a splash: Moonraker, currently in design in Singapore, will be capable of lifting over 30,000 tonnes at a time.
The design for this innovative, six-column semi-submersible has sprung from a group of oil and gas industry veterans which founded Offshore Decommissioning Services (ODS) in 2017. It’s an ambitious engineering feat, and for ODS’ Morgan Howes-Roberts it promises to put a British presence on the map: “Something we missed out on during the early days of North Sea construction,” he explains.
Usefully, a semisub has a distinct advantage over typical installation ships: while it doesn’t have particularly fast transit pace, making an adequate 4 to 6kts, it makes up for this in stability. “The water acts as a hydraulic damper so Moonraker can work in harsher conditions than standard designs: for example, it can pick up a platform in 2m to 3m significant wave heights,” says Howes-Roberts.
However, it’s still a rather different beast from most others of its kind. Its 114m width is divided by a 64m-wide, 90m long slot: this enables a neat trick - the resulting U-shape receiving area wraps snugly around the platform’s underside while still allowing for a 3,500m2 laydown area on the deck.
Twelve lifting beams, six on each side, have been designed to slide out across the gap: each has an active heave compensated head “that can remain static with respect to the topsides” he explains, adding that given the very limited required outreach, the lifting beams which nominally have a capability of 2,500 tonnes, “will in reality, be able to handle as much as 2,700t each”.
The mechanisms that deploy the beams, along with the beams themselves, have been integrated into the structure of the vessel itself, adding strength and bringing down the overall weight.
To get this up close and personal, the vessel carries out the final approach using its dynamic positioning capability: this is achieved by eight 4.5MW fixed pitch, fully azimuthing variable speed thrusters, a pair on each pontoon extremity, powered by eight, 8.85MW dual fuel four-stroke gensets. It’s underpinned by a plethora of positioning devices and four separate engine rooms to give it DP3 redundancy.
However, to understand why Moonraker is reliant on no less than 71MW’s of installed power, it’s necessary to dig into the details of the lift operation itself.
On reaching the working arena, the vessel ballasts down to a point where the beams will slip under the platform “with roughly a 2m margin”, says Howes-Roberts: this usually requires bringing the vessel down from its transit draft of around 11m by another 18m to 23m as most topsides have a 20m + air gap the former is more prevalent. However, in some cases, Moonraker will have to ballast to its lowest point to pick up a platform that might only have 14m of clear space beneath.
Therefore, the vessel relies on “an extensive, sophisticated ballast solution” says Howes-Roberts: eight 2,500m3p/h pumps divided across four pump rooms supply the tanks seated in the columns with a combined 71,000m3 capacity.
Once in position around the topside, the vessel is then deballasted so that between 80% and 90% of the platform weight is then resting on the lifting heads at the end of the beams: sensors help balance the load between them.
Then comes a particularly demanding manoeuvre: the platform’s legs are simultaneously severed using diamond wire cutters and the powerful lifting rams rapidly elevate the topside.
Why the need for speed? Despite the extreme weight, there’s a time limit, explains Howes-Roberts: “Basically, we want to make sure we avoid contact between the sheared legs and the remaining jacket, so we need to gain clearance inside one wave period.” He adds: “In the North Sea that usually means 9 to 12 seconds, so we want to achieve a 2m to 3m lift in that time.”
Therefore the water pumps also have an essential role in granting an extra safety margin: while a single beam failure wouldn’t be too problematic, he points out “in the event of a major catastrophe, we’d have to rely entirely on the lift from deballasting”.
Lastly, the process can be reversed to offload onto a shuttle barge or bring the topside into the decommissioning port itself.
Moonraker need not stop at the topside. “We can lift around 8,000 tons of jacket,” he says, accomplishing a 35m or, with an A-frame extension, a 60m vertical pick up, using the beams’ 1,000t capacity linear winches. Drum winches, also incorporated into the beams, can be used to recover pipe from the seabed. For this the four onboard ROVs cut the pipeline into approximately 100m sections; after lifting the pipes are neatly slotted into storage cassettes attached to the inner surfaces of the ‘U’, making for a quick operation: “Depending on pipe diameter, up to 8Km can be recovered per day,” says Howes-Roberts.
Of course, there’s other equipment supporting operations: 1,000 tonne and 300 tonne electrohydraulic cranes are on hand to transfer loads to laydown areas or move items around the deck, a helideck and 42M AHC gangway can be utilised for personnel transfer. Since around 70 crew will be needed in rotation, there’s accommodation for 134 people in double/single cabins.
It will also need a support team back at base: a up to 50 in the engineering office, and maybe more again to cover the logistics.
This isn’t an inexpensive ship, but it’s got scale on its side. Having said that, money has still been kept in mind. It’s not, after all, aiming for decommissioning the heaviest platforms as that would necessitate adding an extra 50% to its overall capacity; “Instead we are aiming at the 90%+ that fall between 16,000 to 30,000 tonnes,” says Howes-Roberts.
This may well give Moonraker more than a thin edge over the competition: “We aim to reduce current decommissioning costs by as much as 35%.”
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