The offshore wind industry and interconnector markets are driving demand for new vessels and cable-lay equipment that can carry more cable, install it more quickly, in more challenging conditions and reduce the number of joints required
New cable-lay vessels that entered service recently and those on order have a number of features in common that will make them more efficient installers of export cables for offshore windfarms and interconnector projects, of which there are a growing number.
Among the main features are significantly greater cable-carrying capacity – which means less time in transit to and from ports to load more cable and fewer joints in cables – and higher transit speeds, for when it is required. Others include the ability to continue laying cable and undertaking jointing operations in adverse conditions and do so more accurately and quickly.
In the longer term, experts suggest, new types of equipment will be required onboard to handle the challenge of cable installation for floating turbines and interconnectors in very deep water.
Speaking exclusively to OWJ in early August, NKT’s head of high-voltage solutions, Andreas Berthou, said additions to its fleet such as NKT Victoria, which recently completed 12 months in operation, were designed with offshore windfarms that are further from the shore and long, deepwater interconnector cables firmly in mind.
He explained that the diameter of high-voltage cables has also increased. Together with longer cable routes, this has meant that the cable capacity of vessels such as NKT Victoria has needed to increase compared with earlier generation vessels. “Vessels have got larger as a result,” he explained, “which also addresses another important issue, the ability to keep working in adverse weather conditions.” Larger vessels make for more stable work platforms.
Mr Berthou explained that NKT Victoria could continue working in 3-4 m waves, conditions that would defeat older, less capable vessels that might only be able to continue laying cable in 2 m waves. Unless you have a very well-designed, stable vessel with integrated cable lay gear, the accelerations induced by wave action on a vessel, and hence on a cable, can all too easily damage it. The stability of the vessel and its ability to continue to work in challenging conditions also enhances its ability to install cable on the seabed with a very high level of accuracy.
Combined with software programmes on the vessel that monitor cable as it is laid and touches down on the seabed, NKT Victoria can lay cable with a level of accuracy of +/- 25-30cm. Some earlier generation vessels – considered sophisticated in their time – are only capable of levels of accuracy of several metres, Mr Berthou said.
Mr Berthou told OWJ that the cable installation equipment onboard a vessel is an integral part of the overall design, a sentiment MAATS Tech’s business development director Gavin Rippe agreed with.
MAATS Tech supplied the carousel and associated equipment for NKT Victoria and is contracted to do likewise for Nexans’ newbuild, Aurora, which is being built in Norway at Ulstein Verft based on a Skipsteknisk design. The company 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..
NKT Victoria can carry 9,000 tonnes of cable on two turntables, a feature Mr Berthou said ensures that the vessel can complete even the largest work scope in a couple of campaigns, but 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 told OWJ that the concentric ‘carousel in a carousel’ concept would enable a ship such as Aurora to carry more cable without making the vessel excessive large and costly. “Were you to have to install two carousels on deck, a vessel would have to be much longer and larger,” Mr Rippe told OWJ.
“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 wouldn’t be feasible. You can install one on deck and one below deck, but it’s not as operationally efficient a solution as the carousel in a carousel approach.”
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.
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 the deepwater to the shore NKT Victoria is designed to be beachable in fully laden condition. The vessel is also fitted with a six-point mooring system. For work in ultra-deepwater, 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 prevent most cable-lay units for operating. 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, optimizing the energy consumption and reducing engine maintenance.
Energy storage is also used for back-up for shore connection during cable loading, allowing the ship to be emission free during cable loading. 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.
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 deepsea and nearshore cable-laying. As is also the case with NKT Victoria, the design has been developed for operation in rough weather and has high levels of manoeuvrability and stationkeeping. The vessel will be 149.9 m long with a deadweight of 17,000 tonnes and 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, repair and joint cables will operate a seabed trenching machine.
Another of the ‘big beasts’ of the cable lay market, Prysmian, also has a sophisticated new vessel under construction. Having first announced plans to build a new vessel in March 2018, Prysmian awarded the contract to build the ship to another Norwegian yard, Vard.
The vessel will be capable of installing cables in water depths exceeding 2,000 m – a feature that will be important 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 get under way by end 2018.
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 strengthening 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 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.
Deeper water and floating offshore wind present new challenges
Widely seen as the next generation of offshore wind projects, floating offshore windfarms will present cable lay vessels and equipment on them with some challenges to overcome, Mr Rippe believes, as will the installation of interconnectors in ever-deeper water.
In a presentation at Global Offshore Wind 2018, he explained that cables connected to floating turbines would probably need buoyancy modules attached to them. This will make installing such cables complex and increases the risk of kinking. Buoyancy modules will be required to maintain cables in position such that they are not subject to excessive fatigue loads.
Techniques exist that have been used in the offshore oil and gas industry to install cables in similar applications, but they require the use of a lay tower unlike those found on a conventional cable-lay vessel (although as highlighted above, Victoria can be fitted with one if required).
In response to this challenge, MAATS Tech has developed a module launcher for conventional cable lay vessels. It consists of a chute around a nominal 6 m minimum bend radius (MBR) and moving pads that transport the cable/module assembly from horizontal to vertical position. Where modules occur on the cable, a ‘pocket’ in the moving pad line will accommodate the profile. The pads are profiled in such a way that the MBR is not compromised. A further expansion of the launcher is a module management system just inboard of the equipment, which ensures safe and efficient handling of modules on deck.
Cable lay operations usually take place with a top tension of up to 10 tonnes, for which a simple fixed chute is adequate. However, As Mr Rippe explained, in deeper water, such as are found in interconnector installation projects, dynamic top tension becomes significant and cable could be damaged if deployed over standard chutes.
In the oil and gas industry a vertical lay tower is used to overcome this challenge, a solution that would drive up the cost of an interconnector project. MAATS Tech’s solution is a variation of the active chute for module launching described above. It has a series of PU lined pads which do not present point loads to the cable and form a continuous moving belt which the cable rests in. The pads are U-shaped around the cable, which further helps to maintain cable shape during deployment. The chute is motorised to neutralise cable friction and can accommodate top tensions of up to 150 tonnes and an 8 m radius. The chute works with a MAATS Tech horizontal four-track tensioner on the deck.