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Heavy constructor can install larger components with ease

Wed 06 Sep 2017

Heavy constructor can install larger components with ease
Seven Arctic was designed for worldwide operations, is adapted for both tropical and winter environments, and capable of operations in water depths up to 3,000m

Subsea 7 believes the latest addition to its fleet of pipelay/construction ships is sufficiently capable to do the work of several conventional vessels

 

Seven Arctic, Subsea 7’s new vessel, was designed to address the growing operational challenges for subsea construction in ultra-deep water and hostile environments. It is the first vessel of its kind in Subsea 7’s already highly specified fleet and combines an adaptable tiltable lay system that is suitable for use in different water depths with an immensely capable crane that has several modes of operation that can be selected according to the scope of work. Its versatility provides clients with more options than older vessels, such that the company believes it can undertake work that would have required two or three older less-capable units.

“This is a vessel that will be equally at home operating in the Gulf of Mexico, North Sea or west coast of Africa,” said Subsea 7. The preliminary design of the vessel was conducted jointly with Wärtsilä Ship Design, and detailed design and construction was completed by Hyundai Heavy Industries in Korea. It is equipped with a 600-tonne Huisman vertical pipelay system, a 7,000-tonne underdeck basket for storage of flexible pipe/umbilical and a newly designed, 1,000-tonne rope-luffing knuckleboom crane also designed by Huisman in the Netherlands.

The crane is an active heave compensated unit and was developed to provide a particularly high level of flexibility and capability for a range of different operational requirements. To ensure maximum efficiency, it can work in a number of different modes: 1,000-tonne triple-fall mode; 660-tonne dual-fall deepsea mode; 660-tonne dual-fall standard mode; 660-tonne dual-fall high-lift mode; and 330-tonne single-fall mode. For deepsea mode, the rope-luffing knuckleboom design, together with the rope capture system, allows the two wire rope falls to be widely separated at the surface, while the wide twin sheave block provides separation and anti-twisting movement at depth. This combination of features helps to eliminate the risk of cabling while maintaining a manageable wire size for more standard operations.

Speaking to OSJ at the time that the vessel was first ordered, Dr Stuart N Smith, VP asset development at Subsea 7, said the main benefit of the new crane is to maintain knuckleboom functionality for offshore construction activity, while not suffering weight penalty and the associated impact on ship stability when operating conventional knuckleboom crane designs.

Looking at other similar vessels in Subsea 7’s fleet, he said that Seven Seas and Skandi Acergy and vessels of that era typically had 400-tonne cranes, so the new crane is a significant increase. “We’re responding to our clients’ needs for something bigger,” he said.

Seven Arctic’s rope-luffing knuckleboom crane is a development of the Huisman pedestal-mounted offshore crane using an innovative (for offshore applications) knuckling system on the main boom, which is actuated using wire ropes rather than hydraulic cylinders. The benefit of this is to maintain knuckleboom functionality, which is essential for offshore construction activity, whilst not suffering the weight penalty (with impact on ship stability) of conventional knuckleboom crane designs.

The crane’s large lift capacity is matched by significant workability advantages. With a 58 m radius, it can move equipment from every corner of the deck of the vessel, thereby reducing the need for deploying skidding systems and allowing the vessel to maximise its operational time.

The vessel’s 600-tonne top tension tiltable lay system is suitable for laying flexible flowlines and risers, umbilicals and power cables via a twin tensioner and double A&R system. The four-track twin tensioners are retractable to facilitate passing large pipe-end fittings and other structures. The lay system also has the capability to retrieve rigid products from the seabed to the hang-off system and install pipeline end terminations and other structures. Product can be loaded into the underdeck basket through one of two loading hatches. The aft deck of the vessel is suitably strengthened to accommodate an optional 3,000-tonne capacity deck carousel or a multi-reel drive system. A basket for storage of flexible pipe/umbilical – developed jointly with MAATS – is used for storing and transporting multiple flexible pipes or the longest and heaviest umbilicals and power cables, meeting the needs of the trend to underwater processing or for wind energy projects. Dr Smith said most comparable vessels had a 5,000-tonne underdeck loading capacity or less. Having 7,000-tonne loading capacity will enable Subsea 7 to keep the deck of the vessel free of obstructions and enhance operability. A large deckload capacity of 4,500-tonnes at 5 m, combined with a large strengthened deck area, will facilitate load-out of large, heavy equipment.

Dr Smith explained that Seven Arctic will, in Subsea 7’s view, bring an important change in subsea construction capability, particularly in ultra-deepwater fields, which pose increasingly technical challenges.

“This vessel will offer clients the opportunity to install larger, heavier infrastructure components more quickly,” he explained. “A very specific advantage is that, with our principal crane, we have chosen to pursue a knuckleboom solution rather than a rigid boom, which additionally will give significant safety benefits by minimising load swing while offering important advantages in workability.”

He highlighted the fact that, for example, spool pieces are getting longer and more difficult to handle and that umbilicals and cables are getting longer and heavier. “Our clients need us to be able to carry out heavier lifts for items such as manifolds and the growing amount of subsea processing equipment,” he told OSJ. Another important advantage of the new crane is that it will have a higher lifting point, enabling it to lift lengthier, heavier loads.

Many operations that Seven Arctic will undertake will not need to make use of the full capacity of the crane, which is where its ability to work in single-fall mode over the entire deck, lifting up to 300-tonnes more quickly and with more easily handled hooks, comes in.

In addition to the new crane, the 160 m by 32 m-breadth vessel has two additional cranes in order to ensure efficient deck operations and to allow a number of lines simultaneously to the seabed for complex or simultaneous operations. One of the cranes and the bulwarks are removable for long overhanging items. Both the main crane and auxiliary cranes will have a subsea heave compensated capability.

Twin purpose-built work-class remotely operated vehicles (ROVs) and handling systems in a dedicated hangar area will be installed on the new vessel, which will have a Norwegian specification helideck.

As one would expect of a vessel named Seven Arctic, the new unit will be ice strengthened and winterised to extend the operating season in the far north. Intended for global operations, the vessel will also have air conditioning for operation in tropical regions.

The vessel will provide accommodation for 132 people, have a total deck area of around 2,600 m², have dynamic positioning to DP3 class and will be Special Purpose Ship (SPS) Code-compliant. The thruster configuration, in conjunction with the main propulsion shafts and controllable pitch propellers, affords the vessel a positionkeeping capability able to withstand harsh operating conditions. Although it has DP3 class notation, it can also work in DP2 configurations, allowing different environmental limits depending on the requirements of the operation being undertaken.

“Combined, the capabilities of the new vessel will also give us the ability to undertake projects in a single mobilisation,” said Dr Smith, “and the lifting capacity of the crane and its hoisting speed will be a major asset. Overall, the vessel will reduce costs and timescales for projects, and in many cases, our clients will be able to avoid the expense of dedicated heavy-lift vessels,” he concluded. 

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