RENEWABLES Ӏ OFFSHORE WIND
Smooth operator: change of plan ensures No tide delays at Nigg The conventional approach would have
The port of Nigg is a marshalling facility for Seagreen Offshore Wind Farm. Seagreen will be the world’s deepest fi xed-bottom offshore wind farm. It lies 27km from the coast, where winds are stronger and more reliable. It is due to become operational this year and will be capable of powering two-thirds of Scotland’s population. The 114 wind turbine foundation jackets at
Seagreen stand 95m tall and are robust enough to support the new 10MW variants of Vestas turbine. They weigh 2,200 tonnes apiece. They arrive by sea and are loaded-in and loaded-out onto their transport barges at Nigg. Nigg is tidal with water levels rising and
falling by more than four metres every six hours. This extreme tidal range is diffi cult to compensate for using ballasting; you cannot keep the barges level with the quay. So load-in or load-out of large jackets is delayed when waters are too high or low; that leaves the installation vessel waiting, miles offshore, with no work to do. And the installation vessel is the project’s most critical asset. Working years in advance, Mammoet
proposed a solution to speed up the work. The need was to fi nd a methodology that
would keep the installation vessel as productive as possible. In other words, it needed always to have a jacket to hand, on a barge at the offshore site, ready and waiting to be set on the seabed. The team looked beyond the jacket handling scope to determine how all scopes could be optimised to protect this critical path of the installation campaign.
been to mobilise a fl eet of SPMTs, then use these to complete all manoeuvres to place the jackets onto barges. It is the obvious solution and was, in fact, the initial plan. But Mammoet realised this would impose limitations on how the work can be performed as it needs the barge to be level with the quay. On days with high tidal range the operational window would thus be closed. Also, if SPMTs need to drive on and
off the barge then deck space needs to be permanently cleared. This limits the area available for stowing the jackets, grillage beams and sea fastenings. Larger and stronger barges can overcome these limitations, but this reduces the size of the fl eet that can be used for the project, causing supply chain issues. Since 114 jackets required load-in and load- out ineffi ciencies would quickly add up. So Mammoet suggested a different
approach: a combination of SPMT transporters and the company’s PTC210-DS ring crane. This approach removed the impact of tidal shifts on the project schedule. The fi ve- hour operational time that would be expected for an SPMT load-out of this nature typically allowed only a single manoeuvre per tide. Crane lifting has no such limits as the barge deck no longer needs to be level with the quay. The ring crane also enabled smooth parallel operations, as it could load-out jackets to a waiting vessel while SPMTs offl oaded others. Outside the delivery windows, jackets could be brought to the
quayside, within lifting range of the crane, well in advance of a vessel arriving from offshore, cutting down on crane waiting time and so maximising the output of the installation vessel.
MAXIMISING OUTPUT Where it takes around one shift to load each jacket using SPMTs – even if conditions are good – the PTC210-DS has been loading-out two jackets in a single shift, providing there are barges ready for it. This has increased the feeder vessel cycle time and reduced downtime for the installation vessel at sea. Was the decision to use a large ring crane a simple one? “Not at all,” says Mammoet UK technical sales manager Oliver Smith. “As ever, when an organisation is deciding on a project of this scale, cost-effi ciency is always an important factor. We were involved with this project years in advance, well before some of the full details came to light. It was only further down the line that we needed to reassess the situation and move away from SPMT-only operations. “This change in plan wasn’t small either.
We knew that our customer would, in turn, need to secure buy-in from their customer that the proposed solution would deliver results. We integrated engineering teams to demonstrate the fl exibility of the solution and work through the risk, while in parallel we were working with our customer on the commercial calculations, proving that the numbers did indeed add up.” The fi rst jackets were delivered to Nigg
Energy Park in July 2021. They were loaded- in using SPMTs, then offl oaded and stored in their pre-positioned blocking arrangement on-site. In September that year the jackets were lifted from this arrangement and transferred to the quay edge, again by SPTM, and from there were loaded onto the feeder barges using the ring crane. They were then towed in a 36-hour journey to the installation vessel and set up 27km from the coast. Smith says everything proceeded as planned and the feedback from the client has been positive: “We knew this method was safe, fl exible and more cost-effective than using SPMTs. Now we can share it with the wider industry so it can serve as a template for how to get offshore wind infrastructure in the water faster and more cost-effi ciently; especially where tidal variances are high.”
32 CRANES TODAY
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