A Z-Bow for Zenith

BMT Nigel Gee Z-Bow design ‘Njord Zenith’ on site
BMT Nigel Gee Z-Bow design ‘Njord Zenith’ on site
Njord Offshore’s new support vessel, ‘Zenith’, designed by BMT Nigel Gee
Njord Offshore’s new support vessel, ‘Zenith’, designed by BMT Nigel Gee

The hull form under Njord Offshore’s new support vessel, 'Zenith', is an interesting response to recent evolutionary pressures within the windfarm industry: it’s looking at transporting more cargo, more fuel – and, since those long-awaited regulatory changes, more than a dozen technicians at a time.

However, there are still some constraints said Jago Lawless of designer BMT Nigel Gee: “The new support boats have for the moment mostly settled on the 26m long, 24-person design, as this means that they can operate as offshore support but still fulfil the criteria for the MCA workboat code which requires a maximum 24m load line.”

Further, although everyone now wants to raise the number of ‘industrial personnel’ onboard, that’s not the only criteria for efficient wind turbine maintenance. “It is all well and good taking 24 people out to site, but all that effort is wasted if you can’t get them off the boat to work,” pointed out Lawless.

He explained that the new Z-Bow is incorporated into the already highly efficient BMT hull. So, like the others in the designer’s portfolio, it still has a very fine entry but has a longer waterline length for efficiency and speed. However, some careful ‘tuning’ of volumes was necessary as the vessel is rather different from its predecessors.

To start with, the design moves the resiliently mounted superstructure aft and dispenses almost entirely with the rear deck in order to create a big, 92m2 area forward. Making the most of it is a Palfinger PK23500 crane with capacity for 2.7 tonnes at an 8.5m outreach.

Up on the bridge, full-length floor to ceiling windows give the helm a clear, uninterrupted view of operations of the enlarged foredeck: “There’s nothing in front of the helmsman, all controls have been pushed away to each side of the centreline steering position,” said Lawless. Downstairs, the resiliently-mounted superstructure also has room for 24 technicians in an unbroken gallery. Aft of this is a separate wet-room and lockers, plus comfort facilities and so on, while below there are sleeping quarters for ten.

However, as a result of moving the superstructure back “we had to play with balance” said Lawless. He explained that a certain amount of volume in the hull was necessary as this vessel is designed to carry up to 52,000l of fuel and 50dwt of cargo “the largest capacity of all the BMT Nigel Gee designs” he explained, and probably one of the biggest on the market for this size vessel. On deck, a variety of fastening points maximises flexibility by allowing five different positions for 10ft standard containers or two positions for 20ft containers.

This, in turn, meant more attention has been paid to the dynamics during transfers. In fact, the hull’s shape has been designed to avoid issues resulting from any extra buoyancy: “While giving extra waterline length, the ‘Z’ bow coupled with BMT’s finer forward sections results in reduced vertical forces and therefore, less reaction from wave motion when pushing onto the turbine,” said Lawless and added that there’s lower vertical movement in head seas and raised efficiency during transit.

However, Lawless is keen to underline that it’s not all down to a single ‘magic bullet’ and added: “Given the complexity of these boats... there’s no one-stop solution.” He explained the new design’s efficiency comes from refinements suggested by owner Njord Offshore (which already has 16 vessels of BMT design), resulting in a fast 26kn service speed.

Volvo Penta's Inboard Performance System (IPS) takes care of the propulsion and drive from prop to engine feet: a real blessing for the Cheoy Lee Shipyard, responsible for building Zenith and its twin. A pair of D13 engines sit behind a quad IPS 900 configuration, each yielding 700hp output: moreover, as each of these four quads are completely independent propulsion units, they can still operate even if one fails. “So if there’s some kind of engine problem, Njord Zenith could still provide a service – albeit at a slightly reduced speed – but it won’t lose operational days and in this game, that’s a massive advantage,” said Lawless.

Lastly, like many of BMT’s recent windfarm service vessels, Zenith and its sister will have the company’s ‘Active Fender’ system to reduce the impact loads.

Visit BMT Nigel Gee at Seawork International 2018 on PG75.

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