Offshore Shipping Online

A publication for the offshore shipping industry published by Clarkson Research

  • Offshore Intelligence Monthly
  • Menu

    More details on new generation heavy lift ships

    Vessel & ROV News // May 8, 2000
    Offshore Heavy Transport in Norway has provided more details about the new, large semi-submersible heavy lift ships it has recently taken delivery of.

    The first of what it describes as a new generation semi-submersible heavy lift vessels, named Black Marlin, has now been in service for a couple of months; the second vessel, Blue Marlin, built to the same design by the same yard, CSBC in Khaoshiung, Taiwan, was delivered in early April.

    Designed to meet the requirements of the offshore industry to transport new types of larger, heavier, taller rigs which are designed for operation in deep water, the first of the new vessels begun work by loading a drilling rig, Deepwater Nautilus, in Korea and has now delivered the rig to its owners, R&B Falcon.

    Utilising the Black Marlin to transport the Deepwater Nautilus was a job that took full advantage of the large lifting capacity of the new vessel.

    On completion of manoeuvring trials - which confirmed the ship's extremely good turning circle and favourable speed predictions - Black Marlin was fitted with four specially designed sponsons in order to transport the rig to the Galveston, and the rig was subsequently loaded in just 12 hours.

    Having loaded the rig from the quayside, Black Marlin left the port in order to discharge the rig so that it could be re-loaded and seafastened before departing for the US on 17 February en route to the US via Singapore and Cape of Good Hope and across the Atlantic.

    All of the forces acting on the heavy lift ship during the voyage to the US were analysed in detail prior to departure using Finite Element Modelling (FEM) techniques.

    Offshore Heavy Transport says their size makes the Black Marlin and Blue Marlin especially suitable for transporting the largest semi-submersible rigs, jack-ups, TLPs and SPAR buoys built to date.

    According to the owners, the Marlin class will be capable of carrying semi-submersible rigs of up to 30,000 metric tonnes. Their large, free and open deck area makes the ships very suitable for skidding or rolling large cargoes onboard, and the ballasting system is especially designed for such operations.

    In keeping with conventional practice, floating cargoes are loaded by the float-on method. This is done by partly submerging the ship, until only the forward deck and stability towers aft remain above the surface.

    The cargo is then floated over the cargo deck, which is now deep (up to 10m) under water. When the cargo is in the correct position for loading, the heavy lift ships pump out water and de-ballast, leaving the cargo on deck.

    Non-floating cargoes can be loaded directly from a quay onto the deck of the ship by skidding or sliding the cargo over the quayside and onto the deck. Since the two stability towers on the Black Marlin and Blue Marlin can be removed, this can be done over the side of the ship or over the stern.

    Because the new generation of semi-submersible rigs and TLPs will produce very concentrated loads on the deck of the new heavy lift ships, they have also been built with extra strong decks.

    As the company explains, because the new generation of spars and TLPs has a comparatively small 'footprint' compared with conventional rigs they generate larger shear and bending stresses when loaded on deck.

    To withstand the forces from such cargoes, the decks were designed for an average, evenly distributed pressure of 27.5 tonnes/m2, with more to spare in certain areas of the deck.

    When designing the Marlin class ships, Offshore Heavy Transport knew it wanted a design in which the ballast tanks were arranged in three 'layers', a configuration which would use more steel and be more expensive to build, but would offer many advantages.

    Arranging the ballast tanks in three layers or levels triples the number of tanks available, and provides a very high level of flexibility in ballasting the ship to make it stable, and counteract the bending stresses produced by the huge cargoes carried.

    Smaller tanks are easier to control for complex 'skid on' operations, having a smaller free surface and providing better compensation for weight transfer and tidal compensation.

    Another important consideration is that double deck and double bottom tanks provide a large element of safety in case the hull is accidentally punctured during loading or unloading, and the risk that the ships might be de-stabilised by bottom damage or rupturing the hull is greatly reduced.

    As Dagfinn Thorsen, Senior Vice President, Chartering & Operations at Offshore Heavy Transport explains, the ability to manipulate the stability of the ships with such a high level of precision is important both for smaller cargoes and large ones.

    When lighter cargoes are being carried, it is important to be able to reduce rolling motions in heavy seas - so when lighter cargoes are being transported, the double deck tanks are filled to make the ship 'top heavy'. When larger, heavier cargoes are being transported, extra stability can be gained by filling the tanks in the ships' double bottoms, thus lowering the centre of gravity.

    The Marlin class is equipped with four high capacity ballast pumps, which enable the new vessels to load and unloaded very quickly and efficiently, a facility which will be particularly useful when weather limits the time available for loading and unloading.

    According to Thorsen, Black Marlin and Blue Marlin have six sets of six ballast tanks, of which three sets are arranged longitudinally, divided by two longitudinal bulkheads.

    Uniquely, for ships of this size, the Black Marlin and Blue Marlin also have ballast tanks in the double bottom and double deck tanks, thus providing still more flexibility for difficult cargoes - large or small - and the ability to tune GMt to any desired level.

    Altogether, there are 54 large ballast tanks in the main cargo area, and another 14 tanks arranged fore and aft, thus providing a total of 68 tanks, making it easier to find suitable ballast conditions for awkward, difficult cargoes.

    In addition to the ballast pumps, a total of 36 ballast tanks can be free-flooded, thus reducing the critical contact period, a feature which the heavy lift specialist says will again reduce the time required for ballasting down during mating operations.

    With all four ballast pumps in operation and flooding of tanks, the ship can alter its draught by 1.0m in just 20 minutes, says the company. When the cargo deck is submerged the same change in draught can be accomplished in just five minutes.

    When lighter cargoes are being carried, it is important to be able to reduce rolling motions in heavy seas - so when lighter cargoes are being transported, the double deck tanks are filled to make the ship 'top heavy'. When larger, heavier cargoes are being transported, extra stability can be gained by filling the tanks in the ships' double bottoms, thus lowering the centre of gravity.

    The Marlin class ships are fitted with a main engine capable of more than 17,000bhp continuous output, sufficient to provide an estimated speed of 14.5kt on design draft and an estimated average speed of 13.5kt when loaded. Their maximum speed whilst intransit and unloaded is around 15.5kt.

    Offshore Heavy Transport says the hullform and size of the newbuildings is such that they should experience little loss of speed due to high seas, and their fuel capacity is sufficient to sail half way around the world without refuelling. A typical voyage, from Rotterdam to Rio de Janeiro, with a large drilling rig onboard, will take just 16 days.

    "The ships are powered by a proven, reliable, slow speed diesel engine combined with a large-diameter controllable pitch propeller", explained Thorsen. "We felt that the highest level of reliability would be ensured by adopting a direct drive arrangementwith no gearboxes".

    A Becker rudder considerably increases steering reliability under slow speed and extreme weather, and a large bow thruster has been installed in the bow in order to further enhance manoeuvrability. Apart from the two anchors forward, the newbuildings arealso fitted with a heavy duty Bruce anchor aft for improved holding power.

    The ships are equipped with five electrically driven winches for cargo positioning, four of which have double drums, and both ships have been designed in such a way that water and power can be delivered to cargoes as required. Up to 2,000kW of 44 V can be delivered from the heavy lift ships to their cargoes.

    As Thorsen explained, compared to most heavy lift ships, the Marlin class has an unconventional hullform which is rather deep compared to a conventional barge shaped hull, and the ships' lines are those of a deepsea ship rather than a barge.

    Together, these features, and the ability to use the sophisticated ballasting system to produce a low GM, ensure that Black Marlin and Blue Marlin will have very gentle motions in heavy seas. Roll angles should be very moderate, says Thorsen, and with excellent motion characteristics, the cargoes the ships carry will be subject to reduced motions.

    Another key feature is that a very high freeboard is achieved in the design, and maintained even with heavy cargoes loaded onboard the ship.

    A large freeboard will provide cargoes carried by the Marlin class with a high level of protection heavy seas, a fact that is of particular relevance to large, overhanging and more vulnerable cargoes.

    Main particulars

    Length overall: 217.50m
    Length between perpendiculars: 206.57m
    Breadth (moulded): 42.003m
    Depth (moulded): 13.304m
    Summer draught: 10.079m
    Deadweight: 57,021mt
    Gross/Net: 37,938
    Maximum deck load: 27.5 mt/m2
    Free deck length: 178.20/157.20m
    Free deck area: in excess of 7,216m2
    Main engine output: 12,640 kW
    Bow thruster: 2,000 kW
    Speed: 14.0kt
    Range: 25,000nm
    Accommodation: 50-55

    More articles from this category

    More news