Today’s cable-lay vessels must carry more cable, install it more quickly and do so in more challenging conditions
Increased cable-carrying capacity is a standout feature among many of the new cable-lay vessels currently entering service or at the design stage. This additional capacity translates into higher transit speeds, less time in transit between ports to load more cable, and fewer cable joints. Combined with an ability to lay cable and undertake jointing operations in adverse conditions, and to do so accurately and quickly, this next generation of craft should prove far more efficient installation machines.
Still, there is a feeling among industry insiders that even these capabilities may not be ample to manage the challenges of cable installation for floating turbines and interconnectors in very deep water. For this task, new equipment will be required.
Discussing the changing requirements of these vessels, NKT head of high-voltage solutions Andreas Berthou said additions to its fleet, such as NKT Victoria, were designed with long, deepwater interconnector cables and long-range offshore windfarms firmly in mind.
Mr Berthou noted that the increasing cable capacity of new vessels has been driven by the larger diameter of high-voltage cables and longer cable routes. “Vessels have got larger as a result,” he explained, noting that larger vessels also make for more stable work platforms, helping them to cope with adverse weather conditions.
Mr Berthou explained that NKT Victoria could continue working in 3-4 m waves, conditions that would defeat older, less capable vessels. He pointed out that unless the vessel is stable, with integrated cable-lay gear, the accelerations induced by wave action on both the vessel and cable can easily lead to damage. A stable vessel not only overcomes these obstacles, but is also able to install cable on the seabed at very high levels of accuracy.
NKT Victoria can lay cable with a level of accuracy of +/- 25-30cm, thanks to onboard software that monitors the cable as it is laid on the seabed. Mr Berthou noted that some earlier generation vessels – considered sophisticated in their time – are only capable of levels of accuracy measured in metres.
The vessel can carry 9,000 tonnes of cable on two turntables. Mr Berthou said this capacity allows it to complete even the largest work scope in a couple of campaigns.
NKT Victoria is capable of simultaneous dual HVDC and fibre optic cable-laying and deepwater HVAC installation, using a high capacity tensioner system. To enable complete cable-lay capabilities ranging from deep water to the shore, NKT Victoria is designed to be beachable in a fully laden condition. The vessel is also fitted with a six-point mooring system. For work in ultra-deep water, the deck has been prepared to accommodate a vertical lay tower to enable sufficient high-tension hold-back capabilities.
The ship’s versatility is further strengthened by a fully integrated navigation and survey system that will enable NKT Victoria to operate in high sea states and conditions that would rule out many other cable-lay units. The offshore market’s stringent safety requirements are met throughout the installation process thanks to sophisticated roll reduction technology that mitigates the effects of challenging sea conditions. Fire and flooding containment systems protect essential systems, ensuring ongoing operations are not compromised.
NKT Victoria is an innovative vessel is a number of other respects, not least in its ability to use shore power when in port, thus reducing emissions from the vessel. It also has ABB’s OnBoard DC Grid, which reduces fuel consumption and batteries that can store power. The Onboard DC Grid system will increase the efficiency of the vessel by allowing the ship’s engines to work at variable speed, in combination with energy storage for peak shaving and enhanced dynamic performance, optimising energy consumption and reducing engine maintenance.
Energy storage is also used for back-up regarding shore connection during cable loading, allowing the ship to be emission free during the process. This means that fuel consumption and emissions from the vessel are significantly lower than on other vessels, a feature that Mr Berthou believes is particularly important in the environmentally-conscious renewable energy industry. NKT Victoria’s overall fuel consumption is approximately half that of less sophisticated units, he said.
Such innovation requires that the cable installation equipment onboard a vessel be an integral part of the overall design. Gavin Rippe business development director at MAATS Tech – which supplied the carousel and associated equipment for NKT Victoria – explained how his company is devising new ways of achieving this.
“Fuel consumption and emissions are significantly lower than on other vessels, a feature that is particularly important in the environmentally-conscious renewable energy industry”
Carousel in a carousel
Alongside its work on NKT Victoria, the company is contracted to do likewise for Nexans’ newbuild, Aurora, which is being built in Norway at Ulstein Verft, based on a Skipsteknisk design. MAATS Tech will supply three deck tensioners and another in the gooseneck for the system, along with a capstan hold back, lay wheels and integrated control system.
The carousel that MAATS Tech is designing for Nexans’ new unit will be capable of carrying 10,000 tonnes of cable, in an innovative ‘carousel in a carousel’ arrangement.
Mr Rippe explained that the concentric concept would enable a ship such as Aurora to carry more cable without making the vessel excessively large and costly: “The deck of a vessel needs a significant amount of space aft of the carousels, around 50 m, to accommodate tensioners and jointing equipment. Installing two turntables on the deck in order to give you the kind of capacity that Aurora will have would not be feasible. You can install one on deck and one below deck, but it is not as operationally efficient a solution as the ‘carousel in a carousel’ approach.”
Aurora's 'carousel in a carousel' will allow it to carry more cable without significantly increasing the vessel's size or cost
The concentric ‘carousel in a carousel’ concept on Aurora will be able to process two cables simultaneously, or alternatively it can store a single length weighing 10,000 tonnes. The vessel will have two firing lines.
Like NKT Victoria, Aurora will be a dynamic positioning class 3 (DP3) cable-lay vessel. The ST-297 design from Skipsteknisk will be outfitted for power cable-laying, including bundle laying, cable jointing and repair and for cable protection and trenching. The vessel is designed for installation of HVDC and HVAC cable systems, even in severe weather conditions, and for deep sea and nearshore cable-laying. As is also the case with NKT Victoria, the design has been developed for operation in rough weather and features high levels of manoeuvrability and stationkeeping. The vessel will be 149.9 m long with a deadweight of 17,000 tonnes and able to accommodate 90 people.
Aurora is due to be delivered in Q2 2021. In addition to power cables for export lines for offshore wind and deepwater interconnector projects, it will be capable of carrying and deploying fibre optic cables and will operate a seabed trenching machine.
Another of the ‘big beasts’ of the cable-lay market, Prysmian, also has a sophisticated new vessel that will be constructed at Norwegian yard, Vard.
The vessel will be capable of installing cables in water depths exceeding 2,000 m – an important attribute in the interconnector market – and will undertake a range of operations using a variety of burial systems, including heavy duty ploughs. The vessel, which will be designed to have a reduced environmental footprint, will have a length of 172 m and a beam of 34 m and accommodation for 120 people. Construction is expected to commence by the end of the year.
Prysmian has described the vessel as “a strategic asset” that will bolster the company’s ability to undertake turnkey projects and deliver end-to-end engineering, procurement, construction and installation projects. It will also strengthen the group’s interconnection and offshore wind project execution capabilities, replacing the now elderly cable-lay vessel Giulio Verne.Like NKT Victoria and Aurora, it will have a much larger cable-loading capacity compared to earlier units and like its competitors, this will enable it to install cables with fewer joints. It will also be able to carry out cable laying and burial operations simultaneously, thus shortening the time required to undertake large-scale projects such as export cables and interconnectors.