ICE EVENT ON AN ARTIFICIAL ISLAND IN THE CASPIAN SEA

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 2002 International Association of Hydraulic Engineering and Research ICE EVENT ON AN ARTIFICIAL ISLAND IN THE CASPIAN SEA D.C. Mayne 1, E.M. Lemee 1 and M. Metge 2 ABSTRACT An artificial island constructed in the Kashagan East area of the North Caspian Sea was subjected to ice movement from 18:00 January 30 to 02:45 January 31. During this period there were three distinct ice movements occurred causing significant ice pile-up on, and away from the island slopes. An analysis of the ice movements and environmental conditions during the event are examined and observations of rubble formation, ride-up, and rafted ice are presented. INTRODUCTION The Kashagan field is the site of a major hydrocarbons discovery that is currently undergoing an appraisal-drilling programme with production scheduled to begin in An artificial island (Figure 1) was constructed in the summer of 2001 as part of the development to serve as an exploration and production installation. The island design was chosen due to the shallow water of the North Caspian Sea. The average water depth in the North Caspian is 5 metres and at the location of this ice event it is nominally 4 metres. The island has a sea floor footprint of 225 by 135 metres with the freeboard rising 4 metres above mean sea level. It has three sloping sides and one vertical side (SE) which serves as the quay for the island. Of the three sloping sides, one is a 1:4 slope (NE) and the remaining two are 1:2 slope. The ice event described here Figure 1: Kashagan East Artificial Island occurred on the two faces with the 1:2 slope. 1 Department of Civil Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada 2 AgipKCO, The Hague, Netherlands

2 EVENT SUMMARY The ice event of 30 January 2002 occurred in three distinct movements that took place between 18:00 and 02:30 local time. The ice began to move at approximately 18:00 local time and encroached the SW face of the island past the ice protection blocks. The second ice movement started at 19:20 and lasted 25 minutes. During the second movement, ice encroached past the ice protection blocks on the SW face and engulfed the flare boom structure on the W corner of the island. Rubble was also forming on the NW face due to a slight shift in the direction the ice was moving. The third ice movement began at 00:45 on 31 Jan and lasted just over 2 hours. In the final ice movement, there was little to no ice encroachment onto the island surface. The ice continued to pile up to an estimated height of 15 m but this occurred at a distance from the island such that there was no danger to personnel on ice management strip. The water level surged a total of 38 cm from 04:00, 30 Jan to 04:30, 31 Jan and is believed to be one of the triggers of this ice movement. Although a considerable amount of ice piled up around the island there were no injuries and no equipment or island installations were damaged. The extent of the ice rubble can be seen in Figure 1. FIRST ICE MOVEMENT The first ice movement began at approximately 18:00 local time and was reported to last approximately three minutes and estimated to have travelled 50 m. The wind was 12 kts from 202 gusting to 20 kts and the water level had surged to ORL 0.34 m, 13 cm since 06:00 (ORL 0.47 m) that morning and 5 cm since 15:00. (ORL 0.39) The ice came into contact with the south west face of the island and created two distinct failure zones: the western portion of the face had an intact ice sheet (Figure 2) with the edge reaching the top of the island slope and the centre and eastern portion of the slope had ice rubble pile-up shown in Figure 3. The largest concentration of ice rubble was in the centre of the island face. The ice had moved past the ice protection blocks and stretched approximately 3 m onto the ice management strip. The ice thickness was 40 cm and the ice rubble reached a maximum height of approximately 3 m Edge of Intact Ice Sheet Figure 2: Intact ice sheet at island edge Figure 3: Ice rubble encroachment on island surface

3 above the island surface. Ice clearing actions were initiated using the loader and excavator but were stopped due to the risk of further ice movement. The generators and generator fuel tank were deemed to be at risk and efforts were initiated to place ice protection blocks in front of the structures to prevent any damage. The placement of the blocks was only partially completed due to the commencement of the second ice movement. SECOND ICE MOVEMENT The second ice movement began at 19:20 and lasted 25 minutes with and estimated 200 m of ice travel. Ice was initially observed moving approximately 20 m from the SE face of the island before any movement began on the slopes. The wind speed was 14 kts from 218 gusting to 25 kts. During this movement the wind shifted from 218 to 243 and increase to 28 kts gusting to 39 kts by the time the ice stopped moving. When the ice began to move, all non-essential personnel were evacuated from the area and all work to place additional ice protection blocks was stopped due to potential for the existing intact ice sheet to continue to ride up onto the island. Approximately 2 minutes after the ice movements away from the island were observed, the ice began to move onto the island slopes at the west corner as well as the SW and NW faces. Ice rubble began to form on the west corner and pile up around the flare boom structure. By the end of the ice movement, the flare boom structure was covered by ice Figure 4 Figure 4: Ice rubble accumulation around flare boom structure rubble as shown in Figure 4. A brief survey of the rubble pile on the SW face of the island by the Ice Advisor revealed that the ice sheet had been bent close to vertical on the outside edge of the pile-up. The rubble had reached an estimated height of 4 m above island surface and ice thickness was later measured in the cm range. The water level was measured at 21:00 at ORL 0.27 m and was found to have increased 7 cm since 18:00. A small wave was observed travelling towards the island in the ice sheet just before the ice started to move on the island slopes, it is not know if this was due to a change in the water level. THIRD ICE MOVEMENT The third and final ice movement began at approximately 00:25 on 31 Jan and lasted until 02:30. The ice travel was measured from a satellite image and is approximately 2.25 km total from the three movements. The wind was 24 kts from 253 gusting to 33 kts. During the event, the wind was kts and shifted between 248 and 262,

4 gusting between 32 and 38 kts. The water level was ORL 0.19 m, up 8 cm from the previous movement. During the event, the water level rose another 10 cm to ORL 0.09 m and peaked at 04:30 at ORL 0.08 m. Figure 5: Ice ride-up locations, 30 and 60 cm ice The ice began by piling up away from the island with piles reaching an estimated 15 m above sea level. Some of the peaks of these piles appear to be level with the helideck but this needs to be verified with a detailed survey. The ice rubble was surveyed visually in the days following the event and the ice thickness was found to be cm. The 80 cm ice was rafted in layers of two or three and can be found in several areas of the rubble. Ride up locations can be seen in the rubble as shown in Figure 5, the ice thickness is 30 and 60 cm. RAFTED ICE AREA Prior to the ice event, the NW face of the island had a small amount of ice rubble on the slope as well as some ridging approximately 50 m from the edge of the island. This ice was accumulated during the Figure 6: Rafted ice area before and after ice event

5 ice movement in late December. Between the island and the ridge, the area was previously thought to be thicker than surrounding areas to due rafting. This section of ice survived the ice event intact and was later measured in several locations with thickness ranging from metres. The before and after can be seen in Figure 6. SATELLITE IMAGE A portion of the RADARSAT satellite image acquired 01 Feb is shown here in Figure 7. From the image, the distance the ice travelled can be inferred by the length of the open water lead. In this case it was measured to be approximately 2.25 km. The direction of movement can also be inferred by the direction of the lead, in this case the ice was moving on a heading of 78, which is consistent with the wind direction. There are two stamukha in the area that have also experienced the ice movement. The image shows an open water lead shorter than the one created by the island, indicating that Figure 7: Radarsat image showing lead due to ice movement they may have been moved by the ice. The distance that the stamukha were moved has not yet been determined but they have been noted as possible locations for ice scour studies. Several other stamukha in the area were identified on the 01 Feb image and investigated for ice scours after the ice cover permitted. WEATHER SUMMARY The summary of the wind direction, speed and gusts is shown in Figure 8 with the rel- Figure 8: Weather conditions during the ice event

6 ative water level rise overlaid. Figure 9 shows the actual water level curve for the period surrounding the ice event. Figure 9: Water level during the ice event ICE CHARACTERISTICS The ice present in the rubble piles surrounding the island is between 30 and 80 cm thick. The ice that is cm thick is mostly a single layer whereas the cm ice is a result of rafting in two or three layers. It is possible that thicker ice or more rafting layers are present but it has not been observed as yet. During the event, ice sheets were observed to have ridden up the entire face of the rubble with some still intact. Figure 10 shows 80 cm ice rafted in three layers. ICE PROTECTION BLOCK PERFORMANCE Ice Protections Blocks (IPBs) were designed as additional protection from ice encroachment. These blocks are 1.5 m cubes with one camfered side. There were approximately ten ice protection blocks that came into direct contact with ice and ice rubble during this event. Of these, only one is was moved by the ice as shown in Figure 11. This particular block did not slide, it Figure 10: 80 cm rafted ice in three layers was rotated forward by the ice. There were four IPBs completely covered by the ice movement, of which, none were found to have moved. but several are still buried by ice rubble. Figure 11 shows the ice protection block that was moved. In several areas the ice protection blocks held back the ice rubble and prevented its encroachment onto

7 the island surface. CONCLUSIONS The Island performed as designed during a significant ice movement with ice rubble forming on the slopes and building away from them as the event progressed. The thicker rafted ice areas triggered failure of the oncoming ice sheet away from the island slope. The flare booms structure was deemed to be at risk from future ice movements and the island has since been modified in order to better protect these structures from a Figure 11: Ice protection block moved by the ice. similar ice movement. A similar ice movement on the quayside would have rendered is unsuable until the IBSVs were able to clear away the ice rubble. As a result of this, rock berms have been placed around the island to prevent ice accumulations in areas critical to the operation. As the design of an artificial island is now proven in the North Caspian, these structures continue to be the choice for operations.