Station-keeping in Ice

The challenge

The Arctic Ocean is the smallest and shallowest of the Earth’s five major oceans. Almost completely surrounded by Eurasia and North America, it is partly covered by sea ice throughout the year (and almost completely in winter). It exists in a polar climate: winter features continuous darkness (polar night), profound cold (to-51°C) but stable, clear weather conditions; the brief (typically 3 months) summer features continuous daylight (known as the midnight sun), cool temperatures (to +10 °C) damp, foggy weather, and weak cyclones with rain or snow.

The Arctic Ocean’s surface temperature is fairly constant, near the freezing point of seawater. Sea ice forms when the upper 100–150 meters of ocean water cools to the −1.8°C. Sea ice floats on the ocean’s surface and is the dominant surface type of ice throughout the year in the Arctic Basin. It is relatively thin, generally less than 4 metres (13 feet), with thicker ridges. At its maximum extent, in March, Arctic sea ice covers about 15 million km² (5.8 million sq mi).

Winds and ocean currents cause the sea ice to move. Generally speaking, these motions carry sea ice from the Russian side of the Arctic Ocean into the Atlantic Ocean through the area east of Greenland, while they cause the ice on the North American side to rotate clockwise, sometimes for many years. The Arctic basin is also variously frequented by vast ice floes, pack ice, drifting ice, icebergs and ice islands. This environment makes year-round exploration and production impossible, and reliable operational windows for drill ships and floating platforms extremely difficult to predict or achieve. The challenge was to extend the operating season for drill ships and floating platforms by creating systems to enable the vessels to safely remain on station in the presence of ice, thus reducing downtime and improving the efficiency of Arctic operations.

Existing solutions

Station-keeping systems must take into consideration local ice conditions, water depth and ice management support. In many Arctic regions of interest for hydrocarbon development, shallow water depths necessitate small vessel offsets, favouring designs that use moorings to maintain location. For developments in deeper water, larger offsets are permissible and dynamic positioning (DP) solutions may be a viable alternative. Experience gained by industry from past Arctic operations provides very valuable insights into station-keeping issues.

Disconnection systems currently available (for example, that used on the Terra Nova FPSO offshore Newfoundland), are designed to allow for emergency disconnection of the vessel to avoid possible iceberg impact. Lessons learned from the design and operation of these systems can be leveraged to develop turrets for heavier ice conditions, where new disconnection systems are required, to enable frequent operational disconnection and reconnection of a vessel under high ice loads.

The response of moored vessels in managed ice is an important area of research. Lessons learned from drilling operations onboard the Kulluk, which operated in the Beaufort Sea in the 1980s, are an important source of information.

For DP vessels, accurate prediction of the environmental loads acting on a vessel operating in dynamic ice conditions is a significant challenge. During the ACEX coring expedition in 2004, the Vidar Viking utilized dynamic positioning to keep station in broken ice, with ice management support from the Sovetsky Soyuz and the Oden. This expedition serves as an important source of information regarding the performance of DP vessels in managed ice, and serves to illustrate how multi-vessel ice management strategies can be effectively used to significantly reduce station-keeping loads in dynamic pack-ice conditions.

Research topics

To help reduce downtime and improve vessel station-keeping in ice, CARD proposed to model ice loads in the design of moored and dynamically positioned (DP) systems, selecting a small number of particular locations for study, and to implement the results in order to determine mooring loads and study DP systems, both selected in consultation with the Industry Advisory Committee. Moored vessels operating in ice will need to be able to disconnect under high ice loads, and floating systems must be evaluated in terms of feasibility and cost. Detailed understanding of ice conditions, ice mechanics and loading, ice management and the response of floating systems are necessary inputs in the development of station-keeping solutions for ice environments.


Scale-Model Testing of Floating Systems

Fundamental questions remain regarding the scaling of results from model ice basins to predict full-scale behaviour. For issues such as the prediction of loads, ice clearing, and ice accumulation for vessels operating in ice, research is needed to evaluate and improve methodology and scaling of results for scale-models of floating structures. Scale-model testing of ice-induced loads on mooring systems and dynamically positioned floating systems have been identified as areas for further research.

Modelling of Loads on Moored Floating Platforms

Modelling ice-induced loads on mooring systems for different loading scenarios, ice features, types of ice, metocean conditions, vessel designs, anchor systems, and mooring configurations is needed. To aid in the assessment of mooring loads used in design of systems for selected development concepts and locations, probabilistic models must be developed. Research is also needed to improve understanding of issues associated with the disconnection of turrets under high ice loads, reconnection of mooring in ice conditions, or interactions of individual mooring lines with ice features.

Modelling of Loads on Dynamically Position Platforms

The response of DP systems to loads from managed and unmanaged ice is complex and at present is not well understood. To improve understanding of such systems, models of ice-induced loads and dynamic positioning system response for different loading scenarios, ice features, types of ice, metocean conditions, vessel designs and DP system configurations are needed. The development of a probabilistic framework for studying and evaluating DP system response has been identified as an area for further research.

Concept Evaluation and Downtime Reduction Tools

To aid in the evaluation of different floating system concepts for a range of environmental conditions and design configurations, new tools were developed. This includes decision-support tools to assess expected downtime for different design concepts, which helps assess the required number and types of support vessels, and the optimal platform concept to reduce operational downtime.