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A new vessel for the Arctic
28 February 2011
A new kind of vessel is being specially designed to tolerate the tough, frigid conditions in the Arctic to allow for the repair and maintenance of sub sea installations
In addition to having good operating characteristics in the open sea, the 120-meter intervention vessel has been designed to operate in and make its way through the ice. Another design consideration has been to provide good working conditions for those on board. Given the harsh environment of the Arctic, the vessel has been planned with superstructures to be built in areas where people work - including attempts to get a large crane on the work deck.
The bottom of the ship contains an open well of 7.7m2 that allows the onloading and offloading of modules from sub sea oil and gas installations. In addition, there are additional wells farther back in the ship that will allow the operation of remote-controlled underwater vehicles (ROV - Remotely Operated Vehicles). During transit from the home harbour to the work area at sea, these wells can be closed off to reduce the ship’s drag during transit.
Resistance, propulsion and fuel
SINTEF researchers have conducted tests on the resistance, propulsion and sea keeping in towing tank and the ocean basin at MARINTEK to develop an optimal design. Project chief Tor Einar Berg and his colleague Bjørn Ola Berge have led the tests, which have used a scaled-down model.
The researchers have also used calculations and numerical simulations to come up with a vessel with optimum properties for working in the Norwegian sector of the Barents Sea. The design parameters are based on the tasks that the ship should be able to execute – including a consideration of the criteria and thresholds that will allow these tasks to be performed. An important factor has been able to reduce fuel consumption and emissions through the design of the boat.
In addition to SINTEF, the design company STX OSV, Statoil, NTNU, Aker Arctic Technology in Finland and VTT (a Finnish equivalent of SINTEF) are partners in the three-year KMB project, which is being partially funded by the Research Council of Norway.
Adapted to the waves and motion
“The boat is specifically designed for work on the Olga field, east of Svalbard. We’re also looking at possibly using it on the Goliath field and the Halten Bank,” says Tor Einar Berg.
The Barents Sea is characterized by very different weather and sea conditions than are found in other seas around Norway. The waves there are both lower and shorter, and the weather changes more rapidly - especially in connection with polar lows. There is also seasonal ice in the north-eastern part of the sea. It was thus very important to find as large an a operating window as possible in terms of the sea conditions.
“If the movement of the boat is too great, for example, the workers cannot deploy equipment and ongoing operations have to be interrupted. By adapting the dimensions of the boat to the wave conditions, we can expand its operational limits,” said Berge.
Reduced fuel consumption
Since there can be long distances between the ship’s land base and the areas where the seafloor installations are located, the design and layout of the vessel are also important in determining fuel consumption and the amount of greenhouse gases and particulate matter emitted during its operation. The vessel has been designed to have a small environmental "footprint". The choice of fuel for the ship, whether diesel or natural gas, also affects emissions. In particular, the use of marine diesel results in dark particulate emissions in the ship’s exhaust. These fine particles fall on the ice, and because of their colour, absorb sunlight and thus melt ice – which is not a desirable in the Arctic.
An icebreaker in the stern
For oil companies, one of the most important features of a vessel of this nature is that it be able to operate when there is ice in the Barents Sea. Currently all equipment maintenance has to be scheduled for the ice-free season, but since problems can arise at any time, companies must also be able to work when there is ice. If a production system has to be shut down because of equipment failure and it takes months to get the system up and running again, the financial losses can be quite high.
The proposed ship has been designed so that it can be turned and operated with the stern facing forwards to push through the ice. The propeller is turned so that the powerful devices to which the propeller is attached – the so-called “head boxes” – can cut through the ice like knives. This allows the ship to operate as an icebreaker.
“These units are designed to withstand ice and ice loads, but our job is to find a design that prevents them from becoming too cumbersome, which would slow the ship,” says Berge, who added that the five-metre test model of the boat had been sent to Finland in February to study how it behaves in the ice.
Design requirements for offshore vessels
The two researchers have thought carefully about the rating requirements and the demands from oil companies in terms of operating limits. The use of wave height as a the main criterion will not contribute to the development of specially designed vessels that can operate in northern oil fields. -
“There should be much more focus on the ship's response, and the characteristics of the ship’s movement should also play an important role. That’s how to design ships that are specifically intended for areas such as the Arctic,” say Berg and Berge.