Switchback transformations for 'Svenja' on Walney Extension work
“It was a ‘hot hop’ project for certain,” said Matthieu Moerman of SAL. In fact, the single ship handling both transition piece and monopile transport to the Walney III and IV Extension had to switch, fast, between not just one but two different kinds of extreme wind cargo.
More, it all took place on a ‘flexible schedule’ - with no break to allow for a refit.
The 659MW Walney Extension is situated 19km off the British coast in the Irish Sea and when it starts generating in 2019 it will be the biggest in the world. However, its scale required the carriage of 87 monopiles and the same number of transition pieces between manufacturers and installation base, Moerman told MJ.
As you'd imagine these components are equally large. At 33.6m the transition pieces were the highest ever made: additionally, concrete sections in the base resulted in the TPs' weight being put at 561 tonnes including contingency. However, this is dwarfed by the monopiles – at up to 72.3m long, these came in at around 1,100 tonnes apiece.
Despite the hefty nature of the cargos, the bottom line could only be stretched to hiring one vessel. Moerman explained: “The big challenges were the schedule and the budget – both were really under stress.”
The scope of work meant one of SAL’s largest ships, the 183-type Svenja, was chosen for the job partially because its pair of 1,000t cranes gave it the necessary capability. The rotation involved carrying 47 transition pieces from the Bladt terminal at Aalborg in Denmark, plus another 40 from the Redcar Terminal in Teesside where they were to be loaded from barges. Both of these pickup points meant navigating across the North Sea, before heading around the north of Scotland before moving south toward the drop off at the DONG (now Ørsted) base in Belfast.
The other element of the contract concerned carriage of the EEW SPC-built monopiles: these required loading in Rostock, Germany and once again meant a journey across the North Sea to the Irish terminal. This in itself held a few challenges, Pier 10 at Rostock was rather limited in size for Svenja’s 160.50m bulk although the authorities eventually agreed to the necessary exemptions if the vessel were to maintain DP mode at berth.
However, it wasn’t going to be easy; the nature of the schedule meant that a consignment of TPs would follow hard on the heels of a set of monopiles, and vice versa. As Moerman explained, “we didn’t often know which load batch was coming aboard next, where we were going, until we reached Belfast”.
Moreover, work couldn’t be started before November as the client’s installation scope included environmental restrictions, so the winter weather piled more pressure on. Despite this, SAL calculated the schedule could be met by sailing with an open hold, keeping to an 18kn or even 20kn transit if the six TPs onboard were carried upright using the company’s specifically developed grillages.
Other issues added to the complexity of the project. Moerman explained: “There were quite a number of challenges to overcome with the monopiles – they were to go into 87 different locations, that meant 87 different lengths, weights and even diameters.”
But there was another, potentially bigger, problem: “We are dealing with rising sizes – the market’s demanding higher megawatts and that means bigger structures, but the wall thickness itself isn’t growing to the same scale – in some cases, it’s even reducing,” explained Moerman. “As a result, the cargo is becoming more sensitive. You can imagine, carried in the horizontal position these 1100 ton monopiles can be like a boiled egg on a moving plate – susceptible to deforming if they’re left unsupported.”
All of which meant developing something that would allow for a fast, flexible switch over, but one that would also allow for the range of 60m to 72.6m lengths and diameters that could vary by up to a meter to be properly bolstered. The answer was a tailored set of seven movable twin saddles with heavy steel bases lined with 72 wooden insets 60cm to 80cm in thickness. “You can imagine it took a lot of time and effort just getting the curve of the wood supports to the optimal radius,” he explained.
The result was a relatively straightforward changeover; the vessel’s tween deck sat over the top of the empty TP grillages allowing two monopiles to rest down in the hold, while the others would be loaded up onto the main deck. It made quick work of the switch: after being given the signal, Svenja could be transformed from monopile to TP mode in just half a day.
Innovation didn’t end there, even the TP load and discharge gained a specialist tool. Moerman explained that a lifting mechanism, “like a big beam with two hydraulically driven steel pins on either end” would be craned to sit over the end of the structure, the flanges engaging with the piece’s external lifting points. This served both speed and safety as a green light showing when the TP was properly locked in.
In the end, Svenja made no less than 31 trips and carried a total volume of 550,287,16m3 in about 231 days.
Interestingly, Moerman admitted that while the project cargo crew could have been “slightly bored” with such a regular run as they are used to more diversity, the ship’s master got creative and motivated them to shave the time at berth while retaining a high level of safety.
The result, said Moerman, was impressive: “Of course you do get better at a repetitive task, but we managed to scale the loading and discharge time down by around 45% by the end of the project.”
By Stevie Knight
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