Offshore construction gets heavy treatment

March 20, 2014

Subsea 7’s new heavy construction vessel, under construction in South Korea, is set to make a splash when it joins the company’s fleet in 2016. The company gave Elaine Maslin a look at the new vessel.

As operators build larger and more complex subsea equipment in deeper, and more remote waters, contractors and vessel operators are striving to stay ahead, including Subsea 7.

The company has launched a new heavy construction vessel, the Seven Arctic.

The 160m-long, 32m-wide, vessel is due to be delivered in early 2016, from Hyundai Heavy Industries’ (HHI) yard in South Korea.

Its main equipment includes a newdesign, 900-tonne, Huisman rope-luffing, knuckle-boom crane, and a 7000-tonne MAATS underdeck basket, for storage of flexible pipe or umbilicals.

Vice President, Technology and Asset Development Stuart Smith says the vessel’s scope is partly driven by an increasing focus on subsea processing, particularly in areas like Norway, where Statoil is deploying the industry-leading Åsgard subsea compression project.

Contained within projects like Åsgard are multiple modules, weighing up to 300-tonne a piece, and requiring installation, replacement, and, eventually, removal. Industry predictions suggest the weight of modules will reach up to 450- tonne, says Smith.

Norwegian energy analysts Rystad predict the subsea processing market will grow from US$500 million a year today, to $8 billion by 2020.

Developments are also moving into deeper and deeper waters, with subsea infrastructure growing in size to meet the demands placed on it by the higher pressures. “We see subsea lifts getting heavier and deeper,” Smith says.

“Spool pieces are getting longer and more difficult to handle. Umbilicals and cables are getting longer and heavier, and project load-outs are getting heavier, with more equipment.

“Heavier lifts are required for manifolds, and subsea processing plant, caissons, and suction piles. We wanted a versatile vessel to carry out all these tasks.”

Key features of the new vessel are a new active heave compensation subsea construction crane, developed by Huisman, with Subsea 7, and the vessel’s capacity—it could load out with 5000- tonne in its under-deck carousel and 4500-tonne up to 5m high on deck.

Knuckle boom crane

“We spent a lot of time on the crane, looking at the weights that are going to be lowered down to 3000m water depth, having a large radius, and higher lowering speeds for greater efficiency,” Smith says.

The crane employs a knuckling system on the main boom, which is actuated using wire ropes, or “rope luffing,” rather than hydraulic cylinders. Smith says it also allows for a higher lifting point for lifting tall, heavy loads.

The crane is primarily designed for subsea construction in single and double fall configuration, but will have triple for capability, with some limitations.

In single fall mode, the crane’s radius can cover the entire deck, with up to 300- tonne load, in double fall mode, the crane will be able to install a 580-tonne module to 2000m water depth, covering a 29m radius and lower to 3000m, due to having 6000m of hoist wire.

In double fall mode, the crane also has a hook system and wire handling system to ensure twisting is prevented. In triple fall, the main hoist capacities is up to 900- tonne, with 21m radius.

The main hoist can lift 600-tonne out to 30m radius, in harbor, and the same subsea, but at a slightly lesser radius, subject to the physical size of the load.

Hoisting speed is 14-18m/min in single fall, and 20-40m/min in twin fall mode. Wire diameter has been kept to 109mm to avoid the heating and subsequent degradation problems associated with heave compensation on larger diameter wires, Smith says.

Deck and marine equipment

The vessel has two additional cranes, for efficient deck operations, and complex or simultaneous operations. One of the additional cranes, and the bulwarks, are removable for long overhanging items.

“Some additional simple features include extra deck strengthening, for additional capacity, and the 7000-tonne under-deck umbilical and flexibles carousel,” Smith says. “Most comparative vessels have 5000-tonne, or less, capacity. To help install these umbilicals and flexibles, stored onboard, we have a 325-tonne vertical lay system.”

Preliminary vessel design was conducted jointly by Subsea 7 and Wärtsilä, with detailed design and construction to be completed by HHI.

The vessel is DP3, can transit at 15knots and operator in up to 4.5m significant wave height. It is special Purpose Ship code compliant, has a Norwegian specification helicopter-deck, and accommodation for 132 people.

It has a 2600sq m deck area and 4500-tonne deck load capability at up to 5m above deck, combined with a large strengthened deck area for load outs of large heavy equipment.

It also features an ice-strengthened hull and winterization features, including de-icing equipment on deck, to extend the operating season in the far North, and air conditioning for operation in tropical regions.

The vessel has two shaft-driven propellers, with diesel electric drive, and a total installed power capacity of 25MW, split between two engine rooms.

The new vessel will increase the capacity in Subsea 7’s fleet. Its existing heavy construction vessels mostly have up to 400-tonne capacity cranes, Smith says. “This vessel gives us either 600-tonne capacity, a 50% increase, or 900-tonne. It has also increased the hoisting speed compared to other vessels in the fleet,” he says.

“It also has very good loading capacities. It can cope with 5000-tonne under deck with 4500-tonne up to 5m above deck.”