ABSTRACT: The fate and erosion history of abandoned artificial islands in the Beaufort Sea

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Abstract

Over 30 artificial islands have been constructed in the shallow water region of the Canadian Beaufort Sea since the early 1970's. These structures were used as stable drilling platforms, but then abandoned and left to the natural elements of wind, waves and ice to erode. This study was initiated to help assess the degree of hazard these sites may pose to navigable waters in the Beaufort Sea region. The fate and erosion history of four abandoned artificial islands; Arnak L-30, Issungnak 0-61, Itiyok I-27 and Kaubvik I-43 are examined in this report. The erosional morphology of these submerged features can be generalized as having roughly concentric to oblong dimensions with the steepest slopes occurring on the landward side. This southeast section of the islands is also where the shoalest water depths occurred for all four sites which ranged from 3.0 m for Arnak to 3.7 m for Itoyok. This morphology results from erosion of the (higher energy)seaward slopes with the entrained sediment deposited on the (lower energy) lee slope. The seabed foundation that the islands are all built on is comprised of Unit A clays and silt. By contrast, the islands themselves are constructed of fine to coarse sand and gravel. The presence of sand on the island crest was confirmed by side scan sonar data, however, the interpretation of gravel was constrained by the possibility of construction debris in the area. Sediment transport bedforms were found at all four sites which indicate that entrainment and transport of the island sediments is occurring, at least during major storms. ... An important aspect of this study was the identification of possible debris features left on the island's surface after abandonment. Such materials might include; dredge pipes, drill pipe, drilling mud barrels, sand bags, filter cloth, chain link fence, cable netting and soil anchors. Although the operators were required to clean theisland's surface upon abandonment, there is some evidence to suggest that these operations were inadequate. The exact identity of each target type is difficult to confirm based on the acoustic, side scan sonar data alone. ... Analysis of the 1989 survey data and review of the historic literature allow us to summarize our interpretation regarding erosion rates of artificial islands in the Beaufort Sea as follows; 1) The rate and depth to which islands will ultimately erode is controlled by water depth and the frequency of extreme storm wave conditions. 2) Those islands in deeper water become submerged within 2 or 3 years, whereas those in very shallow water (2 m or less) do not erode as rapidly to the water line. 3) The vertical erosion rate, following submergence is initially high, but decreases with time. The islands appear to approach an equilibrium elevation between -3 and -5 m, within 5 to 15 years of abandonment. This elevation depends on ambientwater depth, with islands in depths greater than 20 m, decreasing to an elevation of -5 m. 4) The islands all show an increase in diameter with time in at least one dimension, that usually indicates sediment transport towards the southeast. The islands therefore appear to migrate landward at rates of up to 10-20 m/year. There is no evidence in the present data for a stabilization of this lateral motion, although a slow down might be expected as the island elevation becomes stabilized. 5) Once the equilibrium elevation has been attained, the islands will probably not be significantly lowered by natural processes over the next 100 years, except by the occurrence of extreme storms. Such events have return periods greater than 10 years. It is unlikely that any island would be eroded to much below -5 m within a time period of several decades. [The following information is provided for each artificial island on the folded map sheets: Oil Company Operator,construction start date, construction end date, spud date, rig release date, date abandoned, water depth (m), type of island structure, construction method, aggregate source site, volume of aggregate (mł), island dimensions (surface), surface height MSL (m), slope below MSL and beach/berm protection.]

Bibliography of Canadian Geomorphology