Distribution and Variability of Icebergs in the Eastern Davis Strait 63 N to 68 N

Transcription

1 Greenland Survey Distribution and Variability of Icebergs in the Eastern Davis Strait 63 N to 68 N Bureau of Minerals and Petroleum Prepared by Håkon Gjessing Karlsen and Jørgen Bille-Hansen Greenland Survey, ASIAQ Keld Q. Hansen, Henrik Steen Andersen and Henriette Skourup Danish Meteorological Institute

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3 Greenland Survey Distribution and Variability of Icebergs in the Eastern Davis Strait 63 N to 68 N Bureau of Minerals and Petroleum Prepared by Håkon Gjessing Karlsen and Jørgen Bille-Hansen Greenland Survey, ASIAQ Keld Q. Hansen, Henrik Steen Andersen and Henriette Skourup Danish Meteorological Institute

4 Published by: Greenland Survey, ASIAQ Post Box 1003 DK-3900 Nuuk Tel: Fax: Printed in Denmark, 2001 By Book Partner, Nørhaven Digital ISSN ISBN

5 Preface In 2000, The Bureau of Minerals and Petroleum (BMP) commissioned The Greenland Survey to prepare a report compiling and interpreting all available information on the sea ice and especially the iceberg condition in that part of the eastern Davis Strait, which will be opened for licensing in year 2001 for hydrocarbon exploration. The Greenland Survey and the Danish Meteorological Institute has prepared the report as a joint effort. The report endeavour to give an overview of the seasonal and inter annual distribution of sea ice and icebergs in the licensing area. Main emphasis has been put on the sea ice and iceberg distribution in the existing licenses, the Fyllas Banke and the Sisimiut license. 3

6 Summary and recommendations The main objective of the study was to evaluate, characterize and compare the year 2000 iceberg observations with other data sources and historical data for the southern license area, Fyllas Banke, and the northern, the Sisimiut license area, respectively. Based on the available data, it was concluded that: The icebergs observed at Fyllas Banke, from early July until mid-september 2000 probably represent normal seasonal conditions rather than severe conditions. Evaluating the most representative dataset for the northern area, i.e. the last decade s records of icebergs, it is concluded, that the year 2000, does not deviate. Then, the year 2000 can be considered as a normal year concerning icebergs in the Sisimiut license area. It has been shown that the maximum distribution of multi-year sea ice occurs in June and the number of icebergs near Fyllas Banke will reach its maximum one or two months later. It is therefore reasonable to expect that the number of icebergs should be at its lowest from April to July. It has also been shown that the probability of occurrence of multi-year sea ice in the Fyllas Banke area is almost identical regardless if the exploration period is from April to June or July to September. David Strait first year sea ice occurs only occasionally in the Fyllas Banke area and is only present in April after severe winters. Consequently, to minimise the probability of icebergs it may be beneficial to use April to June as the exploration period. However, in case the exploration period must be prolonged into July or even August the probability of an increasing numbers of icebergs is quite high. In the northern area the West Ice affects the western margin of the Sisimiut license in the summer months, July September. However, the West Ice rarely has a distribution, which affects more than the margins of the area to be opened for licensing in year The number of icebergs varied only a little between spring-summer months and the early autumn months. Consequently, to minimise the probability of icebergs it may beneficial to use February to May as the exploration period. For future offshore operations is recommended that the iceberg environment south of the area of interest (Fyllas Banke licence) or north of the area of interest (Sisimiut license) is monitored continuously using satellite images several weeks before the actual drilling or exploration effort takes place to be able to forecast the iceberg density (number of icebergs on a certain area) and enable necessary actions to be taken well in advance. Direct evidence of the iceberg environment is limited and a baseline is therefore difficult to establish. RADARSAT satellite images provide detailed information about iceberg distribution and density. Consequently, it is recommended that additional available RADARSAT scenes should be acquired and analysed to gather more information about the iceberg environment variability and to provide better planning guidance. 4

7 Contents Preface...3 Summary and recommendations Introduction Icebergs and Ice Offshore West Greenland Main Oceanic Current Conditions Icebergs and Ice Offshore Southwest Greenland Icebergs and Ice Offshore Middle West Greenland Data Included in the Study Summary of the Year 2000 Season at Fyllas Banke Summary of Earlier Investigations of the Icebergs at the West Coast of Greenland Iceberg Distribution and Characteristics Seasonal Variation in Density Distribution Iceberg Mass Distribution Draft of Icebergs Drift Speed Analysis Analysis of Ice and Icebergs Drifting into the West Greenland Waters from South CFAR filtering of Radarsat SAR data Iceberg Data Based on Ice Chart Information Iceberg Data based on Ice Chart Information Analysis of Icebergs and Ice drifting into the West Greenland Waters from North and West Sea Ice Environment of the Eastern Davis Strait Analysis of Icebergs Observed at the Grand Banks of Newfoundland Statistics on the IIP Area 40 to 52 N and 39 W to 57 W Conclusions Ranking of the Year 2000 Season at Fyllas Banke Recommendations and Further Studies References

8 Qaanaaq Canada Baffin Island Baffin Bay West Navion Davis Strait Store Hellefiske Banke Fyllas Banke Disko Upernavik Aasiaat Sisimiut Maniitsoq Nuuk Greenland Ummannaq Ilulissat Ammassalik Ittoqqortoormiit Paamiut Nunarsuit 60 Cape Farewell Labrador Labrador Sea Greenland Iceland Norway UK Newfoundland Grand Banks of Newfoundland 40 USA Canada 50 Atlantic Ocean Figure 1. Sites mentioned in the report

9 1 Introduction This report gives a short review of the summer season sea ice and iceberg conditions in West Greenland waters. Special emphasis is given to the icebergs 1 in the area offshore West Greenland to be opened for licensing in 2001, Figure 2. Icebergs can cause problems for drilling operations, especially if they are big in size and high in frequency. The environmental consequences of a collision between a drilling rig and an iceberg could be serious in an Artic environment. A special effort has been made to determine the amount of icebergs at the Fyllas Banke (64 N,54 W). Fyllas Banke is particularly interesting because icebergs there annoyed a drilling operation performed by Statoil. The observations took place during an offshore drilling operation, from early July until mid- September It would be desirable to know if the number, frequency, and distribution of icebergs observed in the summer of 2000 were normal or extreme when compared to available observations. The objectives of the study is to: evaluate and classify the iceberg observations from the 2000-season, compare these data with earlier data and hereby estimate the severity of the season. In there has been rather detailed studies in the block system area, in connection with exploitation operations (DHI/GTO, 1979c). These studies were conducted mainly during the months of June through October and included investigations on the distribution and amount of icebergs, their mass and draft. The distribution of icebergs in the area 63 N to 68 N and east of 58 W is influenced by the West Greenland Current going northwards and the Baffin Island current going southwards and the interaction between them. Therefore, investigations reporting on icebergs and sea ice in the north and south going currents have been the primary data sources. Investigations directly related to the target area have only been performed in relative short periods of time, but they are included in this study whenever possible Baffin Island The Sisimiut licence Border with Canada West Navion Aasiaat Sisimiut The Fyllas Banke licence Maniitsoq Nuuk Ilulissat Greenland An iceberg is defined as an ice block that reaches a minimum of 5 meters above sea level, is longer than 15 meters and is broken away from a glacier. Figure 2. Map of the investigated area. Block system and existing licenses offshore West Greenland between 63 N and 68 N. 7

10 In addition to icebergs, sea ice occurs in the following main types: Storis, mainly multi year drift ice and West Ice, mainly first year drift ice. The sea ice is normally present in the Davis Strait from November to mid-summer. The waters off Southwest Greenland, south of Nuuk, are normally free of sea ice, but can be covered with sea ice during late winter for short periods of time. During spring and early summer months, multi-year sea ice can drift into the area. 1.1 Icebergs and Ice Offshore West Greenland Icebergs occur everywhere in the West Greenland waters. They are produced from a large number of glaciers along the coast of Greenland. Occasionally, many small icebergs and bergy bits are calved in the Southwest Greenland fjords, but normally these ice masses melt quickly, and they rarely affect ocean areas further offshore. The glaciers which produce the most and largest icebergs are located in the Ittoqqortoormiit area, Figure 1, on the east coast, and in the Disko Bay on the west coast and north of this bay (Valeur et al., 1997). In the process of calving from the front of a glacier, an infinite variety of icebergs, bergy bits and growlers are produced. Calving can occur throughout the year. The length of an iceberg is defined as the longest horizontal extent of the iceberg above the waterline, while the width is defined as the shortest horizontal extent. By height and draft is meant the maximum vertical extent from the waterline (Bernitt et al., 1998). Icebergs are described according to their size and the following size classification is normally used, Table 1 (Valeur et al., 1997). Table 1. Size classification of iceblocks and icebergs. Type Height [m] Length [m] Growlers < 1 < 5 Bergy bit < 15 Small iceberg Medium iceberg Large iceberg Very large iceberg > 75 > Main Oceanic Current Conditions The general circulation pattern of the ocean currents, Figure 3, is important since the iceberg drift patterns mainly respond to the currents. Figure 3. General ocean surface current circulation in the Greenland water (Nazareth & Steensboe, 1998) The main currents in the Baffin Bay and northern Davis Strait are fairly simple. There is a relative warm north-flowing current along the Greenland coast and a cold south-flowing current along the Baffin Island continuing down to the Labrador coast. However, transitory eddies influence this basic flow pattern and cause westward flowing branches of the Greenland current, which have significant impact on the numbers of icebergs and their residence time (Nazareth & Steensboe, 1998). The dominant current in the eastern part of the Davis Strait is the north going West Greenland Current. The surface layer (0-50 m) of this current is dominated by cold water from the East Greenland Current, while the underlying layer consists of warm water from the Irminger Current - a branch of the North Atlantic Current (Mosbech et al., 1996). 8

11 Mixing occurs, especially in the shallow areas off the Continental Shelf. Tidal generated currents are superimposed on the general current circulation (DHI/GTO, 1979a) Icebergs and Ice Offshore Southwest Greenland The icebergs observed near Fyllas Banke originates from the East Greenland glacial outlets. Here, thousands of large icebergs are calved every year. When not grounded or stationary in fast ice (firmly fixed ice) near the shore, the icebergs drift southwards with the East Greenland Current which most of the year also contain large amounts of multi-year sea ice from the Arctic Ocean. This sea ice regime is called Storis. Many icebergs drift outside the sea ice edge and melt relatively quickly, depending on the water temperature, the wave/swell action, and the size of the icebergs. While drifting within the sea ice edge in the cold East Greenland Current, the deterioration of the icebergs is significantly slower since sea ice protects the icebergs. Under normal conditions, sea ice occurs in the Cape Farewell area from late December until early August. Peak season is in spring and early summer. From Cape Farewell, the West Greenland Current carries most of these sea ice/icebergs northwards. Some years the sea ice drifts into the Fyllas Banke area. Figure 4. Bathymetry of southern Baffin Bay, Davis Strait and northern Labrador Sea, after (Nazareth & Steensboe, 1998). The West Greenland Current, the tide, meteorological forcing and hydrographic phenomena generate the general current conditions along the west coast of Greenland. The current is strongly affected by local bathymetric conditions, Figure 4. The drift tends to be parallel to the isobaths, especially in relatively shallow areas (Bernitt et al., 1998; DHI/GTO, 1979c). Due to the different melting rates of the multiyear ice and icebergs, the iceberg density in sea ice-free waters off Southwest Greenland is to a certain extent controlled by occurrence and distribution of multi-year ice 1-2 months earlier (Valeur et al., 1997). Therefore, one may expect the maximum iceberg density off Southwest Greenland to occur in early and mid-summer, decreasing to a minimum in the fall and early winter. Large variations in the number and size of the distribution of icebergs rounding Cape Farewell are expected due to the variability of the currents, the large distance to major sources, the varying amounts of protecting sea ice, and of course, the extreme weather conditions. An important factor controlling the iceberg environment offshore Southwest Greenland is the input of icebergs to the East Greenland 9

12 Current at high latitudes during summer. Frequently, the movements of icebergs are controlled by the occurrence and drift of sea ice. If the fast ice in fjords with major iceberg sources does not melt during summer or if the East Greenland sea ice does not drift offshore, a decrease in the amount of icebergs at lower latitudes near Cape Farewell may be the result. This event will probably reduce the input of icebergs to the East Greenland Current. growlers, which never affect offshore areas, Figure 5. The east Greenland icebergs drift southwards along the coast to Cape Farewell and then northwestwards, controlled by the West Greenland Current. Occasionally, under the effect of wind or in the absence of a welldeveloped Irminger Current, icebergs may continue south past Cape Farewell reaching as far as miles to the south or southwest. However, the majority goes northwards under the effects of the relatively warm West Greenland Current, disintegrating rapidly, seldom drifting north of latitude 65 N. Some icebergs drift westwards across the southern Davis Strait to the coasts of Labrador and Baffin Island where they join the main stream drifting southwards. Some large icebergs may survive several months if the water temperature remains cold enough or if the wave action on the icebergs is low. But even in this low-melt situation, deterioration is significant. Icebergs from the major sources in northern Davis Strait and Baffin Bay will normally not be present near Fyllas Banke in the sea ice-free season, due to the flow patterns of the currents in the southeastern part of Davis Strait. Generally, the number of offshore icebergs decreases significantly when drifting north in the West Greenland Current. In other words, a large majority of the icebergs present near Cape Farewell melt here and will never reach as far north as Fyllas Banke. Local glacial outlets normally produce bergy bits or Figure 5. Large-scale iceberg drift pattern in the Davis Strait. No significant occurrence of iceberg calving 61 N to N (Nazareth & Steensboe, 1998) Icebergs and Ice Offshore Middle West Greenland The highest iceberg density and the largest icebergs occur in the Sisimiut licence area. The iceberg drift patterns, Figure 5, seem mainly to respond to the current, Figure 3. In consistently high wind situations, the effect of the winds become significant, particularly if the surrounding area is free of sea ice, and wind-generated surface water currents may develop (Nazareth & Steensboe, 1998). The tidal currents primarily influence the smallscale iceberg movements (Bernitt et al., 1998). 10

13 Many of the icebergs produced in the Disko Bay will leave the bay. The icebergs leaving the bay south of Disko will partly drift towards north along the coast of Disko Island and partly drift towards the southwest and approach the concession areas from the north (DHI/GTO, 1979c). As a result, most of the observed icebergs in the northern parts of the Davis Strait (north of 66 N) are estimated to originate from glaciers in the Disko Bay and eastern Baffin Bay. The north going icebergs from the Disko Bay and the icebergs calving further north will probably drift along with the general largescale anticlockwise current circulation in the Baffin Bay. Most of these icebergs leave the Baffin Bay along the east coast of Canada and enter the Labrador Current southwest of Greenland (DHI/GTO, 1979c). Anyway most icebergs disintegrate near the location they are produced. Most of the icebergs drifting southwards from Baffin Bay in the western part of Davis Strait occur within km of the shore of Baffin Island. It is well known that the largescale iceberg drift is more or less related to the presence and drift of sea ice. Northerly winds and a southwards sea ice drift dominate the Davis Strait in most of the winter. Ice that is formed along Baffin Island and carried by currents into central Baffin Bay and the Davis Strait and is named West Ice. Sometimes the West Ice advances to a very easterly position. It may be expected that a significant amount of icebergs, which are present kilometres west of the Disko Bay and the Uummannaq Fjord, drift southwards to the central/eastern parts of the southern Davis Strait. The Fyllas Banke is included in severe winters. This process seems to occur and is indirectly proved in the observations from the pre-season survey conducted by International Ice Patrol (IIP) in January and February, Figure 6. Late in the sea ice free period, icebergs occur infrequently in the central and eastern Davis Strait between 63 N and 67 N (Valeur et al., 1997) Baffin Island Canada Labrador Figure 6. February average , of icebergs in the Davis Strait (International Ice Patrol, 1978) February average of icebergs , West Navion Greenland

14 2 Data Included in the Study The data sets analyzed in the present study could be grouped in: 1). Data sets on the occurrence of sea ice and icebergs covering the Fyllas Banke and the Sisimiut license area and 2). Data sets with parameters of relevance to the area. The following data sources were found to cover the Fyllas Banke and Sisimiut license area: Radarsat ScanSAR Narrow scenes plus filter products for Fyllas Banke, early July-mid-September Infrequent ScanSAR Wide scenes, spring and early summer 2000, 1999 and Iceberg observations recorded onboard West Navion during the drilling season Data/ice charts from infrequent ice reconnaissance flights for the period Charts from reports, showing the iceberg density, mass and draft distribution (DHI/GTO, 1979b). 6. Probability charts on the occurrence of sea ice in July, August and September. Due to limited relevant data covering Fyllas Banke and the Sisimiut licence area (listed above), indirect but more frequent data for the areas were analyzed. Data related to the Fyllas Banke license area: Radarsat ScanSAR Wide scenes for the western Cape Farewell area, April - September data from about 17 reconnaissance flights in June-July Radarsat ScanSAR Wide scenes for the western Cape Farewell area, April - September 1999 plus data from 13 reconnaissance flights in June-July Radarsat ScanSAR Wide scenes for the western Cape Farewell area, April - September weekly high resolution ice charts for the Cape Farewell area based on aerial data or satellite data for the period Data related to the Sisimiut license area: 1. Indirect information and data coverage, latitude and longitude of observed icebergs, size and class (40 to 52 N and 39 W to 57 W). These data, from The International Ice Patrol (IIP) Iceberg Sightings Database, were obtained from the National Snow and Ice Data Center (NSICC), University of Colorado at Boulder. In the present study, it is assumed that the iceberg input to the East Greenland Current is constant on an annual basis. Also the sizes and classes of the icebergs are only known very roughly. The utilization, analysis and interpretation are described in Chapter 4 to 6. 12

15 3 Summary of the Year 2000 Season at Fyllas Banke More than 200 icebergs, bergy bits and growlers were registered near West Navion throughout the drilling season (Bullock et al., 2001), Figure 7. The precise coordinates of the drill site were N and W. Based on the Statoil observations, more than 50% were characterized as bergy bits or growlers, which easily may be towed or deflected from the exploitation site. Due to current eddies or wind changes, the icebergs entered the area from various directions, but most icebergs entered the area from southerly to northeasterly directions. Figure 7. The sites (cross) where the icebergs, growlers and berg bits were detected around West Navion (bold cross), in the summer months July, August and September The circle around West Navion has a radius of 12 nautical miles (1 nautical mile = 1852 m) (Data source: AMEC). However, it is theoretically possible that some icebergs have been in and out of the area twice (out of the radar observation limit for a shorter or longer period). The ocean currents are significantly influenced by the local bathymetry and more intense near the edge of the continental shelf. The 1000 m isobaths is east west oriented in the vicinity of Fyllas Banke, which explains why some of the icebergs entered the area from easterly directions, rather than southerly directions, although the main drift would be in parallel to the overall sea bed contour, where isobaths are running in the south-north direction, Figure 4. Icebergs were monitored continuously throughout the drilling operation year 2000 by the ice watch team onboard the drilling vessel, West Navion. Especially during the first month of operation, a relatively large number (larger than expected) of icebergs were observed near West Navion and deflection activities were found necessary and often carried out by supply vessels Jul Aug Sep Number of targets/icebergs observed Date 10 UTC 21 UTC Figure 8. Summary of West Navion iceberg observations (Source: AMEC). It is remarkable that several icebergs, sometimes 8-10 were observed simultaneously near West Navion (in a radius of about 15 nautical miles) through the first weeks of the drilling period in mid-july, Figure 8. Also periods up to several days with no or very few iceberg occurred frequently. Overall there is a significant decline in the number of icebergs observed from July to September, probably representing the normal seasonal variability of the iceberg environment of Southwest Greenland. 13

16 Based on ice chart information, the distribution of multi-year sea ice (of Arctic Ocean origin) peaked 29 June 2000 near 63 N after a couple of weeks with significant amounts of sea ice (mixed with glacial ice) near and south of 62 N, Figure 9. Sea ice was observed between 62 N - 63 N from the first week of June until the first week of July. Multi-year ice was present near or west of 48 W from early March until mid-july (close to normal conditions). Based on satellite information, it was evident that in April and May a significant number of icebergs had passed Nunarsuit (Figure 1) and were drifting northwards in the West Greenland Current, see example below from 15 May 2000, Figure 10. Figure 9. Typical ice chart for the Southwest Greenland waters, summer The ice chart indicates that multi-year ice and many icebergs were present west of Paamiut (Source: DMI). The "Egg symbols" refer in coded format to the hatched areas. The uppermost group in an "egg" describes the total sea ice concentration (in tenths of the surface covered with ice), partial concentrations (left blank here), icetype (third group) and floe sizes (fourth group), combined with additional symbols for bergy water and few/many growlers/icebergs. In this case the icetype is multi-year ice arranged in floes less than 100 m in diameter. For a complete description of the "Egg code", see Figure 10. Radarsat ScanSAR Wide image over the Southwest Greenland waters (60 5 N 62 5 N) from 15 May 2000, 21:00 UTC. Overlay is the target/iceberg detection filter product, CFAR. The Storis is present to the south. The coastline is marked in green (Source: Canadian Space Agency, RSI and DMI). 14

17 4 Summary of Earlier Investigations of the Icebergs at the West Coast of Greenland The most detailed information about the distribution of icebergs was compiled from 1975 to 1978 (DHI/GTO, 1979c). In addition to the distribution of icebergs, the mass and draft are the most important iceberg parameters in connection with drilling operations. The mass sets the limits for towing operations, while the maximum draft determines the safe level for installations on seabed (Bernitt et al., 1998). 4.1 Iceberg Distribution and Characteristics Mean iceberg density along the southwest coast of Greenland was studied, using a radar mounted on a survey ship. The radar reached 12 nautical miles from the ship. The survey ship was crossing along the coast repeatedly. The observations took place in the months May through October The mean iceberg density is presented in a grid with mesh size 1 x The mean value calculated for each grid cell is representative for the density of icebergs in that cell, Figure 11. The minimum iceberg densities were found between 65 N and 66 N. From this relative minimum, the iceberg density increased both towards north and south, reaching mean densities higher than 6 icebergs in areas south of latitude 64 N and north of 68 N (DHI/GTO, 1979b). Although the West Navion site was not covered in the investigations, neighbour areas were. The neighbour grid cells mean counts ranged from zero to nine, Figure 11. Mean values can mask a big variation, e.g. one observation with many counts and the rest with low counts. However, in the area with minimum densities, N, counts greater than 10 were never reported in the investigation (DHI/GTO, 1979b). The relatively few counts made by The International Ice Patrol and Canadian Ice Service in the same area support the findings (Valeur et al., 1997) Baffin Island Ilulissat Aasiaat Sisimiut Mean density in number of icebergs within 12 nautical miles West Navion Greenland 13 8 Figure 11. Mean iceberg density, May through October (DHI/GTO, 1979b). 4.2 Seasonal Variation in Density Distribution In the investigation, the seasonal variation in the number of icebergs was investigated using the same grid as above and 7 6 Maniitsoq Nuuk

18 comparing the mean count from May to July with the counts from August to October. The densities varied very little between the two periods north of 65 N. The late summer counts had a tendency to be lower south of latitude 65 N. The distribution of icebergs was not uniform in the grid in the early summer period, May to July. Going from north to south, the highest mean densities were found farthest from the coast between latitude 67 N - 68 N, by contrast the highest densities were found nearest to the coast south of latitude 66 N (DHI/GTO, 1979b). 4.3 Iceberg Mass Distribution An analysis of the mass distribution versus latitude and longitude was also included in the study. The largest icebergs were most frequently found south of latitude 64 N and north of latitude 66 N, Figure 12. South of latitude 64 N, the mean mass varied between 1.4 and 4.1 million ton with maximum estimated mass of 8 million ton. Between latitude 64 N and 66 N, mean masses were between 0.3 and 0.7 million ton, the maximum mass was 2.8 million ton Aasiaat Ilulissat Aasiaat Ilulissat Greenland Greenland Baffin Island Sisimiut Baffin Island Sisimiut Maniitsoq Maniitsoq Nuuk Nuuk Mean iceberg masses in million ton West Navion Maximum iceberg masses in million ton West Navion Figure 12. Distribution of mean and maximum iceberg masses, May through October In the grid are the mesh size 1 x 1 (DHI/GTO, 1979c). 16

19 North of latitude 66 N, the mean iceberg mass was larger, as a consequence of the nearby calving in the Disko Bay. The biggest icebergs were found at the Sisimiut licence area and in the Disko Bay. At the Sisimiut licence area the mean masses were estimated to be near 2 million ton with maximum estimates of 15 million ton for individual icebergs. In the Disko Bay, mean masses were estimated to be in the range of 5-11 million ton and individual maximum estimates on 32 million ton (DHI/GTO, 1979c). 4.4 Draft of Icebergs The distribution of iceberg draft in the grid net nearly followed the mass distribution, Figure 13. The mean draft south of latitude 64 N was m with a maximum draft of 138 m. Between latitude N, a minimum was found with mean drafts of m and maximum estimated draft of 125 m. North of 66 N, the mean draft was generally between 80 and 125 m and the maximum estimated draft was 187 m. The overall draft of icebergs in the observation area has been estimated to be less than 230 m (DHI/GTO, 1979c). The increasing drafts towards the north indicate that a significant part of the icebergs enter the area from the north, probably from Disko Bay (DHI/GTO, 1979b) Ilulissat 126 Ilulissat Aasiaat Aasiaat Greenland Greenland Baffin Island Sisimiut Baffin Island Sisimiut Maniitsoq Maniitsoq Nuuk Nuuk West Navion West Navion Mean iceberg drafts in meters Maximum iceberg drafts in meters Figure 13. Iceberg draft distributions in selected sectors on the southwest coast of Greenland, with mean and maximum draft, May through October In the grid are the mesh size 1 x 1 (DHI/GTO, 1979c). 17

20 4.5 Drift Speed Analysis Iceberg drift velocities were calculated in the investigation by using data sampled by three drill rigs, Mobil, Chevron and Arco respectively, Table 2. At the Arco and the Mobil positions, the one-hour mean iceberg drift speed was about 20 cm/s, while it was about 30 cm/s at the Chevron site. The drift direction was depending upon the mass of the icebergs. Icebergs with masses > ton tended to drift northwards, whereas smaller icebergs predominantly drifted towards southeast (DHI/GTO, 1979c). It appears, that the highest one-hour iceberg drift speed occurred at the Chevron position, Table 2. Less than 0.1 % of the icebergs observed at the Arco and the Mobil rig have velocities higher than 77 cm/s, but 98 cm/s at the Chevron rig. Table 2. One-hour mean iceberg drift velocities (in cm/s and in km/hr in brackets) at the three drill rig positions (DHI/GTO, 1979c). Exceedance frequency Mobil N W Chevron N W Arco N W 50% 20 (0.7) 30 (1.1) 20 (0.7) 10% 36 (1.3) 55 (2.0) 42 (1.5) 1% 58 (2.1) 80 (2.9) 61 (2.2) 0.1% 77 (2.8) 98 (3.5) 77 (2.8) 0.01% 94 (3.4) 116 (4.2) 91 (3.3) Number of observations At positions for the Mobil and the Chevron rigs, the great majority of one-hour iceberg drift velocities were towards northern directions. At the Arco drill rig, located at the northwestern part of the Store Hellefiske Banke, the frequency of south going drift velocities was almost equal to the frequency of north going drift velocities (DHI/GTO, 1979c). 18

21 5 Analysis of Ice and Icebergs Drifting into the West Greenland Waters from South 5.1 CFAR filtering of Radarsat SAR data The problem of detecting icebergs in satellite SAR data mainly consists of identifying them against the background sea clutter and, equally important, reducing false alarms. It has been found that the normalized second moment of the probability distribution, the Power-to-Mean-Ratio (PMR), is very useful for the detection of possible icebergs against the background sea clutter. To reduce the number of spurious targets, iceberg detection based on the Constant False Alarm Rate (CFAR) method, has recently been implemented. Figure 14. In this grid definition, the number of icebergs/targets using the CFAR method was counted in each sector. The grey circle and the tiny frame at the drilling location outlines areas investigated in details in the present study, see also Figure 15 and 16. In the DMI (Danish Meteorological Institute) version the statistics of the background are described by the gamma distribution (limiting case of the k distribution). The false alarm probability is the probability that the backscattered signal, above threshold intensity, is misjudged as an iceberg, acts as a threshold, and is used to remove false targets. This approximation results in significant reduction in computation time, which is an important issue in an operational environment (Gill et al., 2000). The PMR method is not used in these investigations, because the CFAR method is found more accurate for iceberg detection. 63 Radarsat imagery available at the Danish Meteorological Institute from 1997, 1999 and 2000 covering the Southwest Greenland waters were reprocessed for the spring and summer months using the CFAR method. The accuracy and the limitations of the methods for detecting icebergs have not been evaluated thoroughly; however, a relatively large number of Radarsat scenes were reprocessed and analyzed to reduce the influence of real false icebergs, i.e. ships, which are impossible to exclude. In addition, most images covered the area in the far range part of the images. Here the image quality is normally very high. Therefore, the results shown in Table 3 and Table 4 for the Southwest Greenland waters are believed to give good estimates of the seasonal and inter annual variability of the number of icebergs. A geographical grid (½ Lat. x 1 Long) was defined and used in the registration and the statistical analysis of the data sets, Figure 14. The results are shown in Table 3. 19

22 20 Table 3. Number of targets pr ½ Lat. x 1 Long extracted using the CFAR method. I indicates the presence of multi-year sea ice (N show that the number of targets not registered). The analysis (number of targets) is not corrected for the presence of West Navion, supply vessels, and other ships. The waters near Fyllas Banke and 'West Navion' is highlighted green (Data source: DMI).

23 The results of the analysis of Radarsat ScanSAR Imagery for the years 1997, 1999 and 2000 using the CFAR method are shown as averages, in Table 4. Interpretation of results and conclusions based on only three years of data always have to be carefully carried out to avoid misleading results. The geographical coverage of data for 1997 and 1999 is also limited, however, there are some important characteristics: 1) The number of targets generally decreases when the distance to the Greenland shore increases. 2) The number of targets in the northern part of the study area (near Fyllas Banke) is generally smaller than in the southern part. 3) Significant seasonal variability is also identified. In general, the number of icebergs peak in July and relatively few targets were detected in the spring months, April and May. 4) The 2000 season is not significantly different from year 1999 and The number of targets was also counted near the West Navion position, in 1) a 0.25 Lat. x 0.5 Long. grid outlined by N and W, and 2) within 10 nautical miles around West Navion, Figure 14. Table 3 (the two columns to the far right) compare satellite observations with observations from West Navion. If the satellite data is corrected for known vessel positions, there is generally a good agreement between the data in these two columns, but not a 1:1 correlation. Not all vessel positions are available and false targets may appear, which are impossible to exclude in the data set. Table 4. Average number of targets in ½ Lat. x 1 Long boxes using the CFAR method, averaged over the four sectors 61 N - 62 N, 62 N - 63 N, 63 N - 64 N and 64 N - 65 N. White colour in the tables indicates no data available (Data source: DMI). 21

24 The iceberg information from each ice chart was extracted manually, tabulated and analyzed by use of simple statistical methods. The area of interest was divided into five sectors, Figure 15. For every chart each of the sectors were characterized by a score (0 to 6), ranking from excellent to worst ice environment for a drilling operation, Table 5. The main areas of interest for sea floor activity are sectors A and B. Interpretation of the results was primarily based on the following simple assumptions: Figure 15. Definition of sectors used in the ice chart study. 5.2 Iceberg Data Based on Ice Chart Information Data and statistics on the presence of icebergs near Fyllas Banke are sparse. The icebergs observed here in summer are only a fraction of the icebergs that passed the Cape Farewell area 1-3 months earlier. For navigation safety reasons the ice conditions in the waters near Cape Farewell are mapped operationally 1-3 times a week using aircraft or in the recent years by using high resolution SAR satellite images. The ice charts do not contain specific data on individual icebergs, but indicate whether there are many, few or no icebergs. The mapped areas as well as the contents of information vary from chart to chart. Direct information on the amount of icebergs north of 62 N is infrequent. 1) Few/no icebergs in sector 1, 2 and 3 lead to very few/no icebergs in sector A and B 1-3 months later. 2) Many icebergs in sector 1 and 2 lead probably to many icebergs in sector A and B 1-3 months later. 3) Many icebergs in sector 3 lead not necessarily to many icebergs in sector A and B 1-3 months later. Only data recorded in the summer season, April - September, were found relevant to be included in the investigations. Sea ice/iceberg records for or 26 seasons were studied. Because the number of available data and the observation dates varied from year to year, the observations were regarded as ensemble measurements. Table 5. Severity index. The ranking used for each descriptive wording for iceberg severity. Index Description value - No data in this particular area 0 Open water, no icebergs 1 Bergy water (icebergs occur only occasionally) 2 Few icebergs (icebergs occur infrequently) 3 Few/many icebergs (many bergs in some areas) 4 Many icebergs 5 Numerous icebergs 6 Multi-year ice and icebergs The error using this method decreases, when the number of observations increases. Ensemble averages were calculated daily 22

25 over a period of 11 days (5 days before and 5 days after a given reference date). A summary of the ice chart study and the calculations are shown in the tables 7-9 for sector A, B and 1. The legends in these tables are shown in Table 5 and Table 6. Sector A represents the offshore area near the drilling location in year Sea ice rarely occurred in that area and was therefore only occasionally mapped. Basically, the evaluated data cannot be regarded as representative for a long time period. Anyway Table 7 shows that more severe ice conditions than in the 2000 season, have been observed earlier in sector A. Table 6. Legend for the tables and parameters in Table 7-9. N_obs Number of observations available for the reference date in the period Number of observations without sufficient data coverage for the reference date in the period (%) Percentage of the ice observations included Category 0 (Open water, No icebergs) 1 (%) Percentage of the ice observations included Category 1 (Bergy water) 2 (%) Percentage of the ice observations included Category 2 (Few icebergs) 3 (%) Percentage of the ice observations included Category 3 (Few/many icebergs) 4 (%) Percentage of the ice observations included Category 4 (Many icebergs) 5 (%) Percentage of the ice observations included Category 5 (Numerous icebergs) 6 (%) Percentage of the ice observations included Category 6 (Multi-year sea ice and icebergs) Ave. Average value of the ice observations in the Categories 1-6 for this reference date frac25 25% fraction of the ice observations in the Categories 1-6 for this reference date median Median of the ice observations in the Categories 1-6 for this reference date frac75 75% fraction of the ice observations in the Categories 1-6 me2000 Median of the ice observations in the Categories 1-6 for this reference date for 2000 only me median (negative values indicate that median for year 2000 observations were lower than the median for ) Table 7. Summary of the results ( ) and statistics in sector A, for legend see Table 5 and Table 6. 23

26 Table 8. Summary of the results ( ) and statistics in sector B, for legend see Table 5 and Table 6. Table 9. Summary of the results ( ) and statistics for sector 1, for legend see Table 5 and Table 6. 24

27 Sector B and sector 1 are located respectively east and south of Fyllas Banke. Generally these areas have the worst ice conditions in June and July. In sector 1 almost all iceberg observations have severity index 1, 2 or 3 in August and September, but in the year 2000, the waters were generally characterized by index 2, which means that some earlier years had more icebergs than in 2000 and other years less icebergs. Due to the physics in the area, it is reasonable to assume that the pattern would be the same between 63 N - 65 N, but the result of deterioration will normally lead to fewer icebergs. This is to some extent confirmed, Table 7 and Table 8, taking into account the low number of observations in sector A and B. 5.3 Iceberg Data based on Ice Chart Information Multi-year sea ice information for the Southwest Greenland waters was extracted from all available navigation ice charts (~100 high resolution icecharts pr. year). Due to the large amount of data and the nature of multiyear sea ice in the area, the waters were divided into three sectors: Multi-year ice observed west of 48 W Multi-year ice observed 62 N - 63 N Multi-year ice observed north of 63 N Every month in the Storis season (December August) was divided into weekly intervals and the presence (but not the amount) of multi-year ice was registered for each 7-8 days period for each of the years , Table This data set does not include iceberg information and is there significantly different from the data set for the period analyzed in section 5.2. The deterioration rate from common to occasional of the meridional multi-year ice distribution is identified when the tables are compared. Additionally, a simple multi-year ice presence index was calculated, as monthly means, for three time periods, July - August, April -June and April - August. The range of the index is between 0 (no ice at all in the period) and 1 (ice throughout the whole period), Table 14. From time to time the multi-year sea ice drifts north of 63 N. In order to increase the resolution of the data in the Fyllas Banke area, the area between 63 N and 65 N was split into 18 subsections, Figure 16, and the occurrence of multi-year sea ice was studied in each of these subsections. The results of this high-resolution study are only shown for Subsection 10, which is identical with the area, where the drilling in year 2000 was performed, Table 13. Multi-year ice is a rare phenomenon here and when it enters Subsection 10, it is nearly always in June or July. The ice observations in year 2000 showed, that the sea ice extended up to 63 N (near the shore). This is close to normal conditions and in most of the years studied; the multi-year ice did not drift north of this latitude. It is, however, also noted, that there are several examples of severe ice conditions in the Fylla Banke area. Then, sea ice and probably many icebergs can occur near the Fyllas Banke area until mid-august, although rarely. Figure 16. The occurrence of multi-year ice between N was investigated in a grid net with 18 subsections (½ Lat. x 1 Long). 25

28 Table 10. The occurrence of multi-year sea ice (M) west of longitude 48 W. West Ice not included (Data source: DMI). Table 12. The occurrence of multi-year sea ice (M) north of 63 N. West Ice not included. (Data source: DMI). Table 11. The occurrence of multi-year sea ice (M) 62 N - 63 N. West Ice not included (Data source: DMI). Table 13. The occurrence of multi-year sea ice (M). West Ice not included The results of the study for subsection 10 (equivalent to the drilling area in year 2000) for (Data source: DMI). 26

29 Table 14. Multi-year ice presence index (definition, see text) categorized into 5 colour groups. 27

30 6 Analysis of Icebergs and Ice drifting into the West Greenland Waters from North and West The icebergs observed in the northern parts of Davis Strait (north of Sisimiut) and at the Grand Banks of Newfoundland originate from the glaciers in Disko Bay and eastern Baffin Bay. The aim of this section is to investigate a possible connection between the amounts of icebergs in the two areas. For inter-annual and seasonal variations in the number of icebergs passing south of 48 N of eastern North America, an empirical relationship between the numbers of icebergs and the distribution of sea ice has been derived. The distribution of the Labrador spring ice influences the number of icebergs present in different years and was either determined by or closely correlated to the area covered by midwinter Davis Strait ice (Marko et al., 1994). 6.1 Sea Ice Environment of the Eastern Davis Strait In the summer season, the currents in the Davis Straits are the dominant factor influencing the distribution of sea ice, since new ice does not form in the summer season. It is evident that the probability of sea ice decreases through the summer, Figure 17. The West Ice is formed along Baffin Island and is carried by currents into central Baffin Bay and the Davis Strait. The extent of the West Ice is at its minimum in October, and it normally reaches its maximum distribution in April. Figure 17. The probability (%) of the occurrence of sea ice, 2. of July, 6. of August and 3. of September. The probability lines in NW refer to West Ice (Nazareth & Steensboe, 1998). 28

31 The West Ice affects the western margin of the Sisimiut licence area in the summer months, July - September. However, West ice rarely drifts into the area covered by the Sisimiut licence. Charts showing the average distribution of the sea ice through the years are available (Nazareth & Steensboe, 1998). 6.2 Analysis of Icebergs Observed at the Grand Banks of Newfoundland The International Ice Patrols (IIP) observations of icebergs are a main source for icebergs originating from the Disko Bay and eastern Baffin Bay. The Ice Patrol s main mission is to map the southern limits of the area of iceberg danger. The iceberg observations cover the years from 1960 to The area monitored by IIP covers the area from 40 to 52 N latitudes and 39 to 57 W longitudes. The data do not provide the exact number of icebergs south of 48 N, but the number of observed/estimated icebergs. The Ice Patrol uses the number of icebergs drifting south of latitude 48 N as a measure of the severity of the iceberg season. In order to improve the IIP database, new sampling methods have been employed over the years. In 1983 and during the first years of using Side Looking Airborne Radar (SLAR), there were difficulties in discriminating between vessels and iceberg targets. Concurrent with the exploration of the Hibernia oil field in 1985, an increase in the estimate of icebergs crossing 48 N was seen for all four years the exploration lasted. In 1991, reports from ships accounted for over 50% of the icebergs observed. The number of flights used in iceberg reconnaissance increased in 1991 resulting in an increase in the number of iceberg counts. Then, an improvement in the method of sampling data, may affect comparisons over time Statistics on the IIP Area 40 to 52 N and 39 W to 57 W If it were possible to show the dependence between the number of icebergs in relation to latitude in the IIP data set, it would in turn be possible to use the IIP data south of 48 N as an indicator for the number of icebergs in the Davis Strait. No icebergs Latitude Figure 18. The number of icebergs in relation to latitude, from Cape Dyer 67 N to Newfoundland 48 N, through the years The prediction and confidence limits correspond to the red and green bounds on the graph. The equation of the fitted model is: Number of icebergs = ( )/latitude P < , r² = 98% Then, there is a statistically significant relationship between the number of icebergs and latitude at the 99.9% confidence level. The model explains 98% of the variability in number of icebergs, r 2 statistics. The strong regression of the number of iceberg with latitude, Figure 18, indicates, that the yearly amount of icebergs south of 48 N can be considered an indicator of the severity of the iceberg season in the West Greenland waters north of 66 N. The close relationships between iceberg density in the Davis Strait in the winter and south of 48 N in the spring are physically explicable. Net southerly drift rates of approximately 15 km day -1 in areas south of 67 N are typical. Ice and icebergs at 65 N in Davis Strait should cross 48 N in late May, 29