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    Nordic owners break the ice with new AHTS

    Vessel & ROV News // March 6, 2000
    Swedish shipowner Bylock & Nordsjofrakt (B&N) and Norwegian offshore shipping company Viking Supply Ships say they are about to take delivery of the first example of a trio of unique Anchor-Handling Tug-Supply (AHTS).

    The vessels can be justly described as unique because they are designed to spend part of the year engaged not in the supply ship/anchor handling role, but as icebreakers, working for the Swedish Maritime Administration.

    The trio of newbuildings is being built in Norway at the Kvaerner Leirvik shipyard. Though loosely based on the design of the Kvaerner Maritime (KMAR) 404, B&N felt that it needed larger, more capable vessels, with increased cargo carrying capacity.

    Apart from being somewhat larger, the KMAR 808 ICE design also differs significantly from its predecessor in a number of key respects, as B&N's Project Engineer Ulf Svard, explained.

    Intended mainly for operations in the North Sea in the roles usually assigned to an AHTS, the newbuildings - of which the first has recently been named Tor Viking - are built to Det Norske Veritas (DNV) 'Icebreaker Class 10' standard.

    Every winter, the Swedish Maritime Administration will have a three-month option on the ships to use the vessels as icebreakers along the Swedish coast. In milder, 'green' winters, the ships will remain in the North Sea.

    The icebreaking capability sets the KMAR 808 ICE design apart from any other AHTS vessels built to date. 'The challenge we faced was to design a ship that would meet the very demanding requirements of two quite different tasks, in two quite different operating environments', said Svard.

    The AHTS-related design of Tor Viking and her sisterships took account of experience gained both by Kvaerner Maritime, B&N and Norwegian partner Viking Supply Ships. But most of the icebreaking expertise comes from the Swedish Maritime Administration.

    "By combining Viking's offshore experience, the Swede's know-how in icebreaking and Kvaerner's experience in offshore designs we believe we have created a unique vessel", says Svard.

    The icebreaking capability manifests itself most obviously in the bow of the newbuildings, which is quite unlike that of a conventional AHTS. The hull underbody also derives largely from the requirement for icebreaking duties, says Svard.

    It has typical icebreaking lines, with a smooth, curved form for breaking through ice without getting jammed, a hullform which should enable the new vessels to break 1.0m of blue (clear) ice with a service speed of 3kt. In 0.7m of ice, the new trio should have a turning radius of less than 250m.

    Apart from the icebreaking lines, the hullform also incorporates seakeeping lines in order to reduce slamming and rolling, although the icebreaking features will mean that slamming will not be neutralised to quite the same extent that would be the case on a pure AHTS. Nonetheless, the Tor Viking and her sisterships will remain steady, stable platforms when engaged in the critical anchor-handling role.

    One other effect of the icebreaking lines has been to produce a hull with a little less buoyancy forward than would otherwise have been the case.

    Combined with the use of the somewhat heavier high strength steel required for icebreaking, this had had the effect of making the Tor Viking and her sisterships about 1,000 tonnes heavier, and reducing their deadweight by the same amount.

    Although B&N considered a diesel electric propulsion system for the Tor Viking and felt that such an arrangement could have many advantages on an AHTS, the company felt that adopting a conventional diesel mechanical arrangement would provide a higher bollard pull.

    The machinery arrangement selected for the newbuildings comprises a pair of MaK 8M32 medium speed diesel engines and a pair of MaK 6M32 engines in a father and son arrangement, driving a pair of specially designed four-bladed, 4m-diameter KaMeWa controllable pitch propellers through Renk reduction gears at about 145rpm.

    In addition to two shaft generators powered by the 6M32s (each of 2,500kW, 6,250kVA), the auxiliary engines and generators on board comprise a pair of 400kW diesel generators for use in harbour, and a 130kW, 165kVA emergency set.

    In order to reduce emissions and keep NOx emissions to a minimum, Selective Catalytic Reduction (SCR) equipment has also been fitted.

    Twin propellers will provide the Tor Viking and her sisterships with the high level of manoeuvrability they will require for both roles, and will allow the newbuilding to manoeuvre astern through ice by changing the thrust on the propellers - the rudderswill be locked in place to avoid damage.

    Another reason for choosing two somewhat smaller diameter propellers is that draught limitations preclude the use of a single, larger propeller.

    Icebreaking duties require that the propellers for the new class are somewhat heavier than would otherwise be the case, and, like the shaftlines, the Type 132XF5/4 propellers have been reinforced beyond the requirements of conventional AHTS.

    The controllable pitch propellers are mounted in nozzles - which is also advantageous with regard to bollard pull - but potentially disadvantageous with regard to ice clogging the propellers.

    To provide the maximum possible thrust, the propellers are mounted in nozzles. The Tor Viking will require immense thrust when going through ice and to provide the bollard pull necessary for the North Sea.

    In fact, the Tor Viking and her sisterships will have a bollard pull of approximately 200 tonnes, which only a very few AHTS vessels in the world can equal. The machinery installed will provide the Tor Viking with a bollard pull of a minimum of 200 tonnes continuous forward and a minimum of 120 tonnes continuous astern.

    Housing the propellers in nozzles afford the propellers some protection against the effects of ice, but could have caused problems with clogging. However, tests carried out prior to construction have confirmed that it should not be a serious problem.

    Using controllable pitch propellers will enable the Tor Viking to go from full ahead to full astern in just six seconds, a manoeuvre, which adopted in response to clogging. The pitch of the propellers can also be reduced very quickly, which will be veryhandy when entering ice from open sea.

    When this happens, the propellers have to transition very quickly from working in water to working in an ice/snow slurry, which could easily overload the main engines, but reducing the pitch of the propellers can alleviate the problem.

    The main machinery is designed to ensure that torque on the propeller and propeller shaft is consistently maintained while breaking through ice. Each of the main engines therefore has exceptionally large flywheels - of some 13 tonne each - fitted betweenthe main engines and the gearboxes to maintain the torque on the propellers as they work that much harder in the slurry and ice.

    Using the rotating mass of the flywheels to this end also protects the main engines from overloading.

    Although controllable pitch propellers are well known for the high level of manoeuvrability they provide, changing over from full ahead to full astern in such a short space of time required some lateral thinking on the part of propeller manufacturer KaMeWa.

    To meet the demand, the hydraulics department at KaMeWa designed an innovative system based on four electric motors per propeller system, each driving a triple pump in order to handle the enormous total oil flow per propeller (about 1,000 litres of oil every minute).

    The manoeuvring equipment on board the newbuildings is completed by two spade rudders with two sets of 45 degree steering gear, a pair of 1,200bhp tunnel thrusters, one forward and one aft, and a 1,200bhp retractable, azimuthing thruster forward, all ofwhich are being supplied by Brunvoll AS in Norway.

    Although intended exclusively for use in the AHTS role and not during icebreaking duties, the thrusters have been modified and strengthened by Brunvoll to cope with the static loads encountered in ice.

    Other interesting features of the design include particular attention to reducing noise levels on board, and a diesel overflow tank system and alarm.

    The design is also capable of accepting a demountable helicopter deck for a 2.4 tonnes helicopter, if required for regular day or nighttime operation, mainly during icebreaking operations. Otherwise the helicopter deck will be stored ashore unless specifically required by the charterer. A helicopter refuelling installation is also fitted.

    Designed with a deck area of 602m2 (40.20m x 15.00m) and deck loading capacity of about 1,350 tonnes the deck strength is 5 tonnes/m2 and 10 tonnes/m2 aft of frame 23. The cargo rail height is 3,000mm, and the height of the bulwark 1,200mm, increasing to1,500mm in the last 10m of deck. The deck equipment comprises a couple of Brattvaag CM6315, 15 tonne capstans, a pair of BrattvaagAKM6315, 15 tonne hydraulic tugger winches, a 250 tonne Smit bracket, two hydraulic cranes and a single telescopic crane, ahydraulic anchor windlass and six searchlights atop the wheelhouse, each operated via a joystick.

    In terms of its role as an supply vessel, the Tor Viking will be fitted with four cylindrical dry bulk cargo tanks for cement and other materials totalling 282m3, and a total of six cylindrical tanks for liquids, plus fresh water tanks, and tanks for diesel oil and brine.

    The towing and anchor handling equipment comprises a single BSL440WX/SL440WX waterfall-type towing/anchor handling from Brattvaag, and a declutchable towing/anchor drum with dividing flange.

    Two free surface passive stabilising tanks are fitted, one at tween-deck level between the main deck and one to be positioned a D-deck level, and the tank on D-deck may also be used for pre-heated fresh water supply to the mud tank washing system.

    During icebreaking, two of the wing tanks connected with a crossover line will also be utilised as heeling tanks, and will be capable of creating a 10 degree list in 20 seconds.

    Tor Viking will be owned jointly by B& N and Viking Supply Ships through B&N Viking Icebreaking & Offshore AS. Viking Supply Ships will act as Commercial Manager for the AHTS, and B&N will act as Technical Managers.

    Tor Viking will be followed into service by numbers 02 and 03 in the series in the third quarter of this year and the first quarter of 2001. Ship 02 will bear the name Balder Viking, but a name for the third ship has yet to be released.

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