Extreme weather conditions (low polar pressure, wind and wave) Polar lows Polar lows

Besides the photos about the white view due to covered by the ice and snow, another condition in most people’s mind about the Arctic is related to the heavy snowfall, strong winds, high waves and poor visibility, all these factors might hamper offshore operations such as navigation of vessel, loading and offloading of material, flight of helicopter and some other routine operations. In fact, all the physical conditions mentioned above are mostly caused by the Polar Lows, or in other words, the Polar Lows could make the conditions severe and more challengeable. The Polar Lows has been discussed in the chapter of introduction, but it would be discussed further due to its disaster results when combined with other extreme weather conditions, and it is one of the most distinctive features of the Arctic differentiating with other regions.

Gudmestad, O. T. and Karunakaran, D. (2008) have stated that Polar Lows are small, rather intense low pressure systems in the Arctic. Essentially they are rare special cases of strong troughs, which could limit offshore operation to some extent. DNV (2008) proposed that Polar Lows are examples of small scale features because common low pressure systems with the diameter are of thousand kilometers or more, much bigger than the Polar Lows with the diameter of only a few hundreds kilometers. When an air stream northward flows over the warm sea surface, Polar Lows might be induced. That is why the Polar Lows occurs more frequently in the north Barents Sea than in the south. Figure 2.23 shows one typical photo taken by a satellite over the north of Scandinavia.

. Figure 2.23 Satellite picture of a polar low over the north of Scandinavia

(Copyright: University of Dundee).

Ref. Helmholtz-

For oil and gas operations in the Arctic, Polar Lows during the period from September to the early summer of the second year have been concerned because they would influence normal offshore operations to high degree, which is risky and negative in economics for a rig with the day rate of several millions US dollars.

The best solution for the Polar Lows and other extreme weather is to predict them precisely, which is another challenge, because not enough data are available in the far north part of the Arctic, this condition is same for the reliable models. Figure 2.24 shows the example of stations that report into the global observing system for weather forecasts in the North Sea and the Barents Sea. So lots of work about data collection in the Barents Sea are still necessary, including establishment of more weather observation stations.

Figure 2.24 Example of stations that report into the global observing system for weather forecasts. Left: North Sea; right: Barents Sea.

Courtesy to US National Climatic Data Center

Hamilton, 2004 and Kristiansen et al., (2011) have concluded the main characteristics of the Polar Lows:

· It is usually formed in cold air outbreaks winter time;

· normally developed rapidly;

· in most cases in the forms of gale or storm force winds, seldom hurricane;

· heavy snow showers, icing, changing wind direction from time to time;

· they might last for 6 hours to 1-2 days

· their diameters are in the scopes from 100 km to 500 km

• Might be more frequent due to global change

The main challenge for the offshore operation in the Arctic is the combination of the Polar Lows with other tough physical environmental factors such as high waves and strong winds, because the former could make the latter factors worse. Due to its small scales, the Polar Lows can be generated without any indicators and change frequently, but the destroy

results they would make to the local area could be destructive. Besides, they are not easy to be forecasted. To avoid this effect, Polar Lows’ power must be considered when the offshore facilities are to be designed for operation in the Arctic, and sufficient safety factors need to be put in the design principles. When the decision makers trade off on the design principles, over design is preferred to risk from accidents due to offshore facilities’

insufficient capability to withstand the complicated destroy from the Polar Lows.

Precise forecasting has been researched by lots of oil companies and institutes, two main jobs have been concentrated by them: data collection and prediction model optimization.

The first job requires sound cooperation of different countries around the Arctic, only after they share their data selflessly with other partners, could the data base be enriched and more reliable. For the second job, there is no other choices except the prediction models would be optimized through repeated contrasts between the forecasting and actual condition, and then improve the models.

DNV (2008) has reported some successful cases for forecasting of the Polar Lows by the use of prediction model from the Norwegian Meteorological Institute. Figure 2.25 illustrates the forecasting of a Polar Low in the Barents Sea.

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Fig 2.25 Forecast of a polar low in the Barents Sea with the Norwegian Meteorological Institute’s mesoscale weather prediction model (horizontal resolution 4 km).

Courtesy to the Norwegian Meteorological Institute

Wind and Waves

Sea state and tidal currents are representative features of physical environment in the Arctic for determining design criterion. In NORSOK N-003, DNV (2008) has compared 100 years’ data of sea state and tidal currents in the North Sea. Figure 2.26 demonstrates that even in the same sea, there are significant varieties in both of the conditions. The north part of the Arctic is similar to the Barents Sea.

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Fig. 2.26 100 years sea state (left) and tidal current (right) on the Norwegian shelf (NORSOK N-003)

Courtesy to DNV (2008)

There are quite a few shortcomings of Meteorological data in the Barents Sea, which has been concluded by DNV (2008)

• Only short records are available, rare cases for 5 years or more;

• No sound spatial coverage;

• Most of the measured temperature data from open sea are not reliable;

• Among the poor records, most records for the open ocean (buoy) are data for the late 1980’ies or early 1990’ies, poor coverage the last15 years, which we are interested in.

Based on the records available, DNV (2008) has depicted one location distribution for the rough positions of wind and waves records on the Norwegian shelf that are 5 years and longer, and regular Meteorological stations, as shown in Figure 2.27.

Fig. 2.27 Rough positions of wind and waves records on the Norwegian shelf that are 5 years and longer.

Courtesy to DNV (2008)

Wilcken (2012) has cited a typical example about offshore operation influenced by the Polar Lows in the Barents Sea. One pipeline was planned to be laid in Snøhvit oilfield from October to November of 2004 by the reel lay vessel CSO Apache. This job was designed to two trips: the estimated offshore work time are 12 days for the first trip and 18 days for the second trip respectively. As per the capability of CSO Apache, the acceptable operational wave height was 2.5m to 3.0m and at least 2-3 days of acceptable weather was required for the start of installation work. Final result is: the first trip took 12 days and then take 30 days for waiting for the acceptable weather, but after 30 days of waiting, the work had to be abandoned and could not be completed due to the bad weather; the remaining work has to be postponed to the next year, which is caused by the Polar lows in November 2004.