December 21, 2021
The company has incorporated the features intoa new gangway, Gannet, for next-generation requirements such as the service operation vessels (SOVs) being sought for offshore windfarms on the Dogger Bankin the North Sea.
In an exclusive interview with OWJ, Van Aalst Group chief executive Wijnand van Aalst said the Gannet 3Dmotion-compensated offshore access system was developed in response to evolving requirements for walk-to-work systems such as those being sought by Equinor and its partners for the massive offshore windfarms they plan to build on DoggerBank. However, it is equally well suited to emerging requirements elsewhere, such as in Taiwan.
Equinor has issued a tender for a new class ofSOV for its Dogger Bank projects, a tender that is particularly demanding for a number of reasons. The first is that the access systems installed on the vessels will need to be able to connect and transfer personnel and equipment in a significant wave height of 3.5 m Hs, compared to the industry norm of Hs2.5m. The second is that the SOVs on which the motion-compensated gangways will be installed will be somewhat smaller than those built to date in the NorthSea.
“The Gannet is based on our experience to date with a fleet of Safeway access systems but takes the concept even further,” Mr van Aalst told OWJ. “It will meet the Dogger Bank requirements and other demanding requirements for SOVs that are emerging in other markets such as Taiwan.
“Equinor is looking for very compact vessels for Dogger Bank,” he explained “They want to do transfers in 3.5 m Hs with a high landing point from what will be very compact ships. That will be a challenge, as are the wind and wave conditions on the Dogger Bank, which are great for wind turbines but challenging for vessel operations.”
SOVs built to date for operation in the North Sea are in the order of 80-90 m in length. Those for Dogger Bank will be 70-80m long. Smaller vessels will be less expensive to build and operate but, given that a vessel’s stability usually increases in relation to size, opting for smaller ships will make motion-compensated access harder to achieve without the adoption of walk-to-work systems that are more capable than existing equipment.
“That’s where the features of the Safeway access system and of the Gannet will come into play,” said Mr van Aalst.“Equinor wants a very high level of workability. We believe that the Gannet can provide that.”
Mr van Aalst’s confidence is based on some of the unique features of Safeway’s motion-compensated gangway. These include its well-known ‘hover mode’ and its roll compensation capability.
Hover mode means that the gangway does not physically push against the landing point on a turbine or offshore structure.“That means that you can ‘land’ the gangway with the vessel in any position,”Mr van Aalst explained. “The freedom of landing point that you get with hover mode is a big advantage because it means that you can select the best heading for the vessel towards the waves, whatever the conditions, and maximise workability.
“In the longer term, we also see hover mode as particularly well-suited to the floating wind market, where you are conducting transfers from a vessel, which is moving, to a turbine, which is also moving.
“Essentially, what hover mode does is it helps you compensate for having to land on a moving target. Whether you are transferring to a bottom-fixed turbine or a floater, it also enables the vessel to place the gangway further over the target structure, which eliminates the gap that you sometimes get with a gangway that is trying to connect directly to a specific landing point.”
“Some windfarm operators, such as Ørsted, have expressed concern to us about the gap. They see it as a potential safety issue,” he said.
Roll compensation has become an increasingly important capability as the height above sea level at which windfarm technicians need tobe transferred has grown. Any vessel, however well designed, will experience some rolling motion, but its effect on a gangway when extended increases as the height at which transfers take place grows.
Much existing equipment was designed with a transfer height of20-21 m in mind but as wind turbines have grown and more windfarms are built in areas with significant variation in conditions and in water depth, so gangways need to be able to safely transfer personnel at 28-30 m or more above sealevel. At that kind of height, even a small amount of vessel roll can make a lot of difference.
To date, gangways have compensated for this with a high telescoping speed, but Mr van Aalst said this can lead to a certain amount of resistance from personnel being transferred. “A lot depends on what thetelescoping speed is, but when it is high the personnel on a gangway can experience being moved towards a fixed object at a speed that they can find uncomfortable.
“The sensation of being 30 m above sea level and of being transported towards a metal structure at speeds of up to 15 km/hr could make users a little nervous or hesitant,” said Mr van Aalst. “In contrast, with the roll compensation capability in Gannet and our other gangways, we can reduce telescoping speed significantly, remaining at a telescoping speed of up to 7km/hr. Our aim is to make crossing a gangway as uneventful as possible, combining reduced motions with a fully enclosed, protected walkway, so the experience is as comfortable as walking onto an aircraft.”
Apart from enhanced safety and greater confidence in the use of a gangway, Mr van Aalst said that, combined with hover mode, this also reduces the time it takes to transfer personnel and their equipment. “We believe that compared to a system without roll compensation, you can complete transfers 25-30% more quickly,” he claimed. That translates into a significant time-saving and cost reduction.
Like some of the other gangways from Safeway, the Gannet will also be able to transfer cargo using the hook on the tip of the gangway. “Being able to switch from people mode to lifting mode in 20 seconds also significantly enhances workability,” Mr van Aalst explained. “Being able to transition from one mode to another means that you can change the mode of operation of the gangway while it is slewing back to the vessel. It gives you a high-performance, motion-compensated lifting capability with a small, agile hook the load on which can easily be man-handled and set down quickly and precisely.”
Safeway’s gangways are currently capable of lifting 1,000 kg, but the Gannet will provide an option for lifting 2,000 kg, a level of capability that is also understood to have been specified by Equinor.“Typically, a gangway might lift generator sets or batteries on to a turbine,”said Mr van Aalst. “The extra 2,000-kg lifting capacity of the Gannet will mean that loads can be consolidated and the number of lifts required to deliver equipment will be reduced, which will speed up the process and reduce time between turbines.”
Turbine OEMS such as Siemens Gamesa Renewable Energy are known to have tight targets for transfers from turbine to turbine. “They like to see personnel transferred to a turbine and two transfers of equipment, with the vessel moving on to the next turbine, in 30 minutes,” Mr van Aalst explained.“ A lot obviously also depends on how well the vessel is handled and on key personnel such as the ship’s dynamic positioning operator, but the ability of agangway to work like this obviously counts for a lot and it is very important to the speed at which a job can be completed.”
In addition to personnel and cargo transfer, the Gannet will also be able to carry trolleys of the type that many offshore operators like to use to take equipment and tools across to a turbine. By combining trolley operation with cargo lifts, Mr van Aalst said Safeway is confident that the Gannet will be popular in the demanding and fast-growing SOV market worldwide.
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