VESSEL REVIEW | Altera – Large cable layer with shallow-water capability

Netherlands-based N-Sea Group has taken delivery of Altera, its newest vessel built for cable installation and repair.

Built by Neptune Marine, the vessel combines a 4,500-tonne cable capacity with a shallow-draught hull intended for projects involving interconnector, export or inter-array cables.

The Dutch-flagged vessel has an LOA of 99.9 metres (328 feet), a moulded beam of 28 metres (92 feet), and a depth of 7.5 metres (25 feet). The draught range is between 3.3 and five metres (11 and 16 feet). The draught increases to 7.4 metres (24 feet) with the retractable thrusters deployed.

The vessel’s gross tonnage is 7,460 while the deadweight is 7,400.

Diesel-electric propulsion coupled with suitability for year-round operation

N-Sea said the combination of DP2 capability, a six-point mooring system, and beaching capability is intended to support year-round operations in both very shallow and deep waters. The mooring system comprises six units rated at 60 tonnes on the first layer.

Power is provided by a diesel-electric arrangement with a total output of 8,780 kW. Veth Propulsion supplied two VL-1550 1,920kW L-drives, two VL-1250i 1,425kW integrated retractable thrusters, and two VT-700 590kW tunnel thrusters. The L-drives and retractable units have permanent magnet motors.

The propulsion enables safe navigation even in a 25-knot wind, a 2.5-knot surface current. and a two-metre significant wave height. Published class particulars include unrestricted navigation, DYNAPOS AM/AT R, strengthened-bottom and in-water-survey provisions.

The cable is carried in a 26-metre (85-foot) carousel. The vessel’s main deck has an area of 1,800 square metres (19,000 square feet) and a load rating of 15 tonnes per square metre. The classification particulars also list a heavy-cargo main-deck notation of 150 kN per square metre.

Installation spread configured for a range of cable and work types

N-Sea said it designed and procured the deck spread using lessons learned from previous offshore wind and interconnector cable installation and repair projects.

Subsea Cable Assets provided Altera with a turnkey cable lay spread. Its scope included the carousel, the loading tower, the tensioning equipment, cable routing hardware, and related control systems.

MacArtney meanwhile supplied a vertical tensioner rated at three tonnes for the loading tower, a 15-tonne main-deck tensioner with breakback functionality, quadrant handling, the carousel drive system, and cable lay controls. There are cable spread options for 3.3-metre and five-metre minimum bend radii, together with infield and export modes.

The offshore knuckle boom crane is positioned aft on the starboard side and is rated at 25 tonnes at a reach of 25 metres, or 15 tonnes at 37 metres. A separate auxiliary crane forward on the port side is arranged to reach the carousel.

Ample amenities for extended-duration deployments

Altera is methanol-prepared with tank capacities of 1,400 cubic metres (310,000 gallons) for methanol and 600 cubic metres (130,000 gallons) for diesel.

The vessel has an operating endurance of between 40 and 60 days while a shore connection will provide electricity for the onboard systems while berthed.

Air-conditioned accommodation is provided for up to 99 people in 58 cabins, 17 of which are single cabins. Facilities include separate male and female welfare areas, four client offices, a control room, and crew recreation spaces.

Safety equipment is specified for the full 99-person complement, and the class particulars include COMFNOISE 3.

The vessel can be optionally fitted with removable boat landings on both sides within crane reach and a survey moonpool.

The latter is intended for USBL equipment, DGPS reference systems, and a multi-beam echosounder.

Neptune Marine built Altera’s hull at its partner yard in Poland and constructed the accommodation block at Hardinxveld-Giessendam in the Netherlands.

N-Sea’s original programme named Altera’s first planned campaign as laying inter-array cables at the 1.1GW Inch Cape offshore wind farm in Scotland.