Wind ro-ro - with a twist
The Twisties solution, showing base and adaptors to allow blades to sit down. Image, DNV GL
The wind power industry is getting closer and closer to its goal of producing cost-competitive electricity: “In fact it is predicted that it will reach parity with fossil fuels by 2025,” explained Chris Garrett of DNV GL – Energy “something which only comes with scale.” However, it also stands to highlight the importance of enhancing present deployment practices.
Mr. Garrett explained: “The next generation of wind turbines are likely to be around the 10 MW or 12 MW size.... manufacturers are gearing-up for 11 m diameter monopiles, these may weigh something like 2,000 ton with 100 m blades, absolutely dwarfing anything we have got going today – they are real giants.”
However, the trend has pushed up ship sizes, which have carried costs along with them.
The previous generation of Wind Turbine Installation Vessels (WTIVs) were used to erect a number of the 3.6 MW turbines “but over the last few years we have seen WTIVs like the NG9000C design, at around 130 m with a 40 m beam this is, he pointed out, more than double the size of the previous generation. Further, as these bigger jack-ups cost between €100,000 and €200,000 per day “the standard day rate is somewhere between one and two Euros a second.”
However, according to Mr. Garrett, this simply is not the right paradigm. “The costs are so high, what you really want to do is to stop these big boys from cycling into and out of port – there is absolutely no point in using them as cargo carriers, they are far too expensive for that, especially with the new wind farms like Hornsea being so far offshore that even at 12 knots you would get little change out of a day’s sailing, every cycle.”
“So, what you need is to have WTIVs sitting out in the wind farm, and feeder vessels supplying them. You need to make sure they never stand still.”
The problem, he admitted, is that “many before have tried and failed to get a workable solution... because when it comes to big wind farm components, vessel to vessel offshore lifts are an absolute nightmare.” Further, the wind blades especially fall under very strict regulations, “and often we have lift a limitation of something between 9 and 10 m/s wind speeds... this is a wind farm after all and this leaves you sitting around doing nothing half the time during winter when the wind picks up.” So while specialist equipment has widened the weather window a little, this has not supported a real alternative.
However, the potential cost advantages of a feeder operation are growing with the scale of the turbines and distance to shore, so despite the issues associated with vessel to vessel moves, DNV GL – Energy and Mr. Garrett in particular believed an engineering solution was well worth pursuing.
Therefore for the last couple of years he and the DNV GL – Energy team have been working on a ‘Fast Feeder Vessel’ or FFV concept which will allow an entire set of turbine components to be unitised and rolled on multiaxle Self Propelled Modular Transporters (SPMTs) from the quay to feeder and feeder to installation vessel in one piece. “The beauty of these SPMTs is that they can manoeuvre in any direction,” he added, “making shuffling around much faster.”
Because the two jack-ups – the FFV and the WTIV - will have their legs down, they will be stable at close quarters, explained Mr. Garrett. Therefore the connection between them will simply be a retractable bridge of a design already used by heavy duty roro cargo vessels and barges.
At the heart of the concept is a frame nicknamed the ‘DNV GL Twistie.’ “This is a modular frame high enough for an SPMT to travel underneath, which fits together like Lego, to be expandable to virtually any configuration,” he added.
Adaptors allow it to seat virtually any size or shape component and a compatible seven-flanged ‘Turbine Cassette’ allows it to take on a number of wind turbine blades and towers in a vertical position. Hinged units allow blades to be loaded horizontally then raised for storage, transport and offshore ro-ro transfer, and finally lowered again, ready for installation.
Obviously, it needs robust fixings underneath but Mr. Garrett’s approach “is not to go reinventing the wheel.” Instead of making a frame to fit a non-standard deck structure for every job, the Twisties are tailored around 10’ ISO container-spaced twistlocks which lend them their name. The staple of the container industry, these can individually take 25 ton of force in any direction; because there is a set of four at every ‘node’ point, (down-rated to 50 ton in combination) each single Twistie will have between 16 and 32 pinning it down. “It will allow rapid sea-fastening just like a rack of containers,” he explained.
Further, given the standardised approach, the deck of the vessel can be adapted once-and-for-all so the deck fastenings won’t need individual time-consuming modifications each time the jack-up is mobilised.
The drop in costs does not just come from keeping the big ships working, it might also turn back the clock. Mr. Garrett told MJ; “the idea attacks the evolutionary driver that has been pushing vessels’ growth.... potentially smaller but still capable mid-range jack-ups can be used as they won’t need the carrying capacity. The feeders can keep them supplied, without the WTIV’s crane stopping operation.”
Further, whilst custom-designed FFVs may yield best value, initially they “would not need to be anything too fancy” said Mr. Garrett, so a jack-up retrofitted with the retractable bridge and twistlocks could prove the concept. Given the relatively simplicity, it may be breathtakingly fast to implement: the industry could even see the first ‘Fast Feeder’ operation by the end of 2018 and “it may start appearing in tenders even earlier than that” concluded Mr. Garrett.
Lastly, it is an idea “with legs” in more ways than one. Mr. Garrett explained the concept could be used for jacket foundations as well as monopiles and there is good reason other project cargo components may consider using the Twistie solution.
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