With the help of a dynamic positioning (DP) system, a vessel can automatically maintain position using its propellers and thrusters. This kind of system works well in open water, but in ice conditions, performance varies.
As a recent report from Petroleum Research in Canada highlighted, for the time being, there is much uncertainty involved in defining ice loads in terms of how the load builds during interaction with a platform or a vessel. Hopefully, research into this area will allow for a better understanding of the peak loads to which a station-keeping system is subjected during an interaction with different ice features.
A station-keeping system is an integral component of an ice-management plan. Limitations of dynamic positioning/station-keeping systems determine the extent of ice management required to stay on location during operations. For projects occurring in ice regions within a pre-existing operational window, research and development relating to improvements to station-keeping systems could undoubtedly reduce the ice-management requirement. This, in turn, would lead to large reductions in operational costs.
“The development of enhanced station-keeping systems without reduction in the level of external ice management has potential to extend the operating window for petroleum operations in harsh Arctic conditions,” the report said, noting that advances in ice management could help alleviate station-keeping requirements in terms of reducing design loads. An integrated design of the ice-management and station-keeping systems would result in an optimised solution.
The only alternative to using DP in icy conditions is to use a mooring system – essentially a station-keeping system that uses mooring lines and anchors to hold an installation at the desired location. A mooring system arrangement is dependent on the anticipated loading and hence is individually selected for each project. In general, redundancy is built into the mooring system design to prevent disconnection in the event of a mooring line failure. Mooring systems are limited by an upper bound on water depth above which they become uneconomical. They are also limited by the geophysical properties of the seabed since they cannot be used in certain adverse conditions.
DP systems have been used in Arctic conditions, for instance, during the Sakhalin project as well as the Arctic Coring Expedition (ACEX). The Sakhalin project was subject to less intensive ice conditions than the ACEX project, which occurred in the Beaufort Sea, near the North Pole. This is because the Sakhalin region only has first-year ice, while the Beaufort Sea region has multiyear ice and ridges. An example of a moored system is the Kulluk drilling platform that operated in the Beaufort Sea. The Kulluk’s mooring system consisted of 12 mooring lines deployed at equal angle intervals around the structure and was designed to withstand the loads from unmanaged pack ice. The Terra Nova and Sea Rose floating production, storage and offloading (FPSO) units are also examples of moored platforms, which operate on the Grand Banks, off the coast of Newfoundland.
In contrast, DP relies on the use of a combination of sensors, computer programs, propellers and thrusters to hold an offshore structure on location. The information gathered by the sensors (motion sensors, wind sensors, ice sensors and position reference sensors) is sent to the computer program, which calculates the required thrust magnitude and direction required to maintain position. The calculations conducted by the DP programme are fed to the propulsion system (propellers and thrusters) to direct thrust output. DP operations can also be performed manually but are generally limited to short durations.
Defining ice loads in terms of how the load builds during interaction with a platform involves much uncertainty. Research into this area would allow for a better understanding of the peak loads to which a station-keeping system is subjected during an interaction with different ice features. An enhanced knowledge of ice load development would lead to a comprehensive design of a station-keeping system capable of withstanding ice loads that correspond to the region of operation.
Most existing ice load predictions are based on models for fixed structures. An improved understanding of the effects of compliant systems on load development is required for accurate representation of non-fixed structures. A new generation of ice load models could be integrated into a DP system to enhance operational performance. A DP system with ice load prediction capability could prompt for engine ramp-up in preparation to exert the required thrust to maintain station.
An enhanced knowledge of ice load development would lead to a comprehensive design of a station-keeping system capable of withstanding ice loads that correspond to the region of operation. Most existing ice load predictions are based on models for fixed structures – an improved understanding of the effects load development is required for accurate representation of non-fixed structures. A new generation of ice load models could be integrated into a DP system to enhance operational performance.
Petroleum Research currently has two joint industry projects underway to advance industry’s knowledge of the magnitude and time-dependent behaviour of global ice loads on both moored and DP floating vessels (and vessel responses) in ice. Phase 1 of the Ice Loads on Floating Structures joint industry project (JIP) was recently completed by a consortium led by the Harsh Environment Technology Center at classification society ABS. Phase 2 plans were reviewed late in 2014. Potential work includes full-scale field data collection of ice loads on and responses of moored floating structures in managed and unmanaged pack ice conditions and related assessment of numerical and physical model tests and load predictions.
The second project is a multiyear project to develop enhanced DP operations in ice environments, executed by the Marine Institute’s Centre for Marine Simulation, the National Research Council of Canada and Kongsberg, started in 2014. The estimated total cost of this project is C$8.6 million (US$7.0 million), including major contributions from the governments of Canada and Newfoundland and Labrador. The objective is to enhance DP control algorithms to respond to ice loads. The scope includes an extensive series of model tests in ice in support of algorithm development and deployment of the software into a simulation environment to assist in training, operational assessment, risk analysis and equipment design.
On the other side of the Atlantic, Aker Arctic in Finland is also developing an enhanced DP system for ice use, which is ready for pilot studies at the company’s model test basin. The development work is being carried out in co-operation with Navis Engineering Oy, which specialises in DP equipment.
“We have been working on the new version for a year now,” Riku Kiili, a project manager at Aker Arctic, explained. “Our target was to develop better algorithms for the system and to improve the performance in ice. Another goal was to build the required equipment for a functional DP system for our ice test basin. We wanted to improve our model testing possibilities and also to further develop the DP system by studying how the improved system works in ice.”
Mr Kiili said the equipment for the model test basin is now ready for use. The models will be equipped with specially developed thrusters, tailored systems for positioning, various sensors and a connection link to the computerised DP system.
“We are using a Qualisys camera system that provides detailed information about the position of the model and its movements,” said Veikko Immonen, the person at Aker Arctic responsible for the development of the technology. “The DP computer is now connected to the camera system. We can also make more accurate measurements of reaction speed, movements, turns, speed, torque and thrust.”
“Most specialised vessels have DP systems, but they don’t work well in ice as the system easily gets confused,” said Mr Kiili. “In open water, the wind, wave and current forces are relatively constant and do not change quickly. Ice, on the other hand, can change rapidly and can impart significant forces, particularly when you have big ice ridges. Ice is so powerful that a conventional DP system and a ship’s propulsion system cannot react quickly enough.”
“An additional challenge is the temperatures in ice-prone regions, which means that the equipment has to be winterised so that it does not freeze. Positioning can become a further difficulty, as GPS signals are not always accurate in the far North, and other position reference systems can be affected by fog and snowfall. Another challenge is to prevent ice reaching the propellers and interfering with their operation.
“In more severe ice conditions, the DP system needs information about the surrounding ice field and a method to forecast incoming forces. If the ice loads could be forecast in advance, the ship’s machinery and propulsion would have enough time to react,” Mr Kiili explained.
“There is still a lot of development work to be done, but now that we have the DP system in place for model testing, we can use it for customer projects to help find solutions for challenging operations.”
DCN in France has also been working on DP in ice, looking at issues such as the effects of ice loads; ice pressure of the hull of a vessel; bending, breaking, splitting and crushing effects of ice; ice ingestion into thruster intakes; the weak coverage and reduced accuracy of positioning sensors in Arctic and ice-prone regions; and safety conditions associated with the use of DP in ice.
Like Petroleum Research and Aker Arctic, it recognises the need for an enhanced form of DP that would be capable of handling the high amplitude, extremely variable loads ice exerts on the hull of a ship. Working closely with Sirehna, classification society Bureau Veritas and ice physics experts CNRS-LGGE; it has received funding for the work from Total, GDF Suez E&P International and Saipem. The aim of the multiyear project was to conduct first-of-a-kind full-scale trials of a DP system in ice on a vessel of around 100m in length. The project team was hoping to conduct trials at sea in 2016, but the current status of the project is not known. OSJ
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