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Challenges of Dredging in the Arctic and Other Deep Ocean Locations

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Challenges of Dredging in the Arctic and Other Deep Ocean Locations Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" CHALLENGES OF DREDGING IN THE ARCTIC AND OTHER DEEP OCEAN LOCATIONS R. E. Randall1 and C. K. Jin2 ABSTRACT Dredging in the Arctic Ocean is challenging due to ice cover, permafrost, iceberg scour, whaling season, ice gouging, and remote location. In the deep ocean 1000 m (3280 ft), dredging is a technique for recovering minerals from the deep ocean waters where water depth is a major challenge for the pumping system. Another application for dredging in the deep ocean and beneath ice covered waters is for recovering petroleum reserves. This paper reviews the recent literature of current systems used for deep ocean mining and the need for developing oil and gas development beneath ice covered water. Conceptual ideas are discussed for overcoming the dredging challenges that include the use of remotely operated vehicles, trenchers, hopper dredges and self-propelled cutter suction dredges. Keywords: Dredging, trenching, arctic, ocean mining, deep water. INTRODUCTION The demand for essential industrial resources, such as oil and gas, has been increasing, which accelerates the decrease in such resources on land and in coastal regions. Accordingly, expanding the exploration to harsh environmental regions, especially the Arctic and deep oceans, turns out to be a promising solution since they are far less developed. For example, it is reported that the area north of the Arctic Circle comprises 13% and 30% of the undiscovered oil and gas in the world, respectively (Gautier et al. 2009). As one of the most important applications in a severe ocean environment, dredging has been utilized to excavate material and minerals, or even improve the environment (Bray et al. 1996). Various innovative concepts for dredging have been developed, including the usage of remotely operated vehicles, trenchers, trailer suction hopper dredges (TSHDs), and self-propelled cutter suction dredges (CSDs). The major difficulties of developing reserves in Arctic Oceans are to protect pipelines from ice and sensitive climate, which can be accomplished with trenching techniques. Considering that repair costs for downtime occupy up to 75 % of the capital expenditure (CAPEX) related to pipeline facilities (Vershinin et al., 2008), it is essential for the pipeline to be protected from possible hazards. In particular, the damage of subsea pipelines from ice gouging, strudel scour, and upheaval buckling should be well managed (Abdalla 2008; Jukes et al. 2011). The primary way is to bury pipeline below seafloor through trenching, where the water depth and trench depth are the main factors. Currently, conventional excavation, hydraulic dredging, ploughing, jetting, and mechanical trenching have been used as the typical trenching methods in Arctic areas (Paulin et al. 2014a). In addition, unique concepts have also been suggested, such as the arctic subsea bucket ladder trencher (Vaartjes et al. 2012). Dredging in the deep ocean is technically and environmentally challenging. Dredging in deep water calls for the improvement in current equipment, which leads to an increase in capital expenditures (CAPEX). Besides, the maintenance of dredging equipment working under such conditions can result in increased operating expenditures (OPEX) and environmental disruption (Stuifbergen 2012; Vershinin et al. 2008). Furthermore, environmental aspects could not be ignored as well. For example, building islands and developing oceans or lands from dredging may destroy natural habitats and prohibit restoring the environment or recreating habitat. Therefore, both eco- friendly equipment and safe operations are required (Bray, 2008). Actually, dredging is an essential technology in deep water for oil and gas production as well as for ocean mining. Firstly, dredging has been used in the offshore oil and gas industry to prepare for the development of petroleum 1 Professor and Director, Center for Dredging Studies/Haynes Coastal Engineering Laboratory, Texas A&M University, College Station, Texas 77843-3136, USA, T: 979-845-4568, Email: [email protected] 2 Ph.D Candidate, Texas A&M University, College Station, Texas 77843-3136, USA, T: 979-204-3454, Email: [email protected] 345 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" reservoirs and the installation of subsea components such as subsea manifolds and separators. Since rugged seafloor topography can have a negative influence on the installation of these components, dredging is often performed to make the seafloor more flat (Stuifbergen, 2012). Secondly, dredging in the deep sea for mining is also used given that such materials near shore have a tendency to be exhausted. For example, sea sand acquired from the deep ocean turns out to be a promising and feasible substitute for sand on land or coastal areas (van Duursen and Winkelman 2011a; van Duursen and Winkelman 2011b). The objective of this paper is to review current challenges of dredging and dredging technologies that can be applied to the Arctic and deep ocean locations. Innovative techniques that include the usage of trenchers, trailer suction hopper dredges, self-propelled cutter suction dredges, and remotely operated vehicles are discussed. TRENCHING IN ARCTIC REGIONS Challenges for Arctic Subsea Pipeline and Trenching Arctic subsea pipelines are exposed to a harsh environment mainly because of the presence of ice. Possible environmental loads that pipelines may experience are the result of ice gouging, strudel scour, thaw settlement, and upheaval buckling (Paulin et al. 2014b). The formation of each environmental load and the corresponding influences are explained. Moreover, even though trenching can be applied to eliminate the loads, it also shows potential difficulties related to a regional location, operation, maintenance, expense, and environmental issues, which is also discussed. Ice gouging is formed by either icebergs or sea ice ridges. The iceberg comes from the breaking off of a glacier, and the ice ridge is formed by wind and current forces that act on ice movements to pile up sea ice (Barrette, 2011). When the depth of an ice keel is greater than the water depth, gouges can be developed as a result of lower parts of an ice keel contacting the seafloor. Ice gouging can exert substantial loads of 10 to 100MN on the seafloor (Kenny et al. 2007; Palmer and Tjiawi 2009), which can result in the pipeline damage. Since changing the design of pipeline to guard against the loads is impractical in high loading conditions, trenching and pipeline burial is often used to protect subsea pipelines. It is reported that the trench depth of roughly 6m is sufficient to protect pipelines (Vaartjes et al. 2012). Strudel scour is generated by bottomfast ice. The bottomfast ice sheet is normally formed in near-shore arctic areas during the freezing season. When river flows meet the bottomfast ice sheet during melting season, special flows passing through holes in the bottomfast ice sheet are created. If the speed of the seawater flow is high enough, it leads to scour of seabed sediments with high hydrodynamic loads acting on subsea pipelines (Reimnitz 1974; Paulin et al. 2014a). The strudel scour tends to take place in offshore area close to river deltas in water depths of 2 - 9m (Leidersdorf et al. 1996). Thaw settlement is another significant challenge for arctic subsea pipelines. Since permafrost is not uniformly distributed in space and time, there is uneven temperature distribution. When production of hydrocarbons is conducted in permafrost, the temperature of surrounding area of the pipeline increases. These local rises in temperature cause permafrost thawing, creating local thaw bulbs (Abdalla et al 2008). In this condition, pipelines experience significant overstress and bending strain (Lanan and Ennis, 2001). As a result, trenching is required to remove the permafrost and replace the area with stable materials to guard against a thaw (Paulin et al. 2014a). Upheaval buckling of a pipeline takes place when the temperature and pressure of an operating pipeline are higher than that in the installation period. Since pipeline movement is constrained because of surrounding soil, the axial compressible load is inevitable. This condition leads to the longitudinal expansion of the pipeline and making the pipeline move upwards. This phenomenon referred to as upheaval buckling. In this case, the pipeline may be exposed to seawater; thus, insufficient trenching can lead to additional damage by the ice gouging. Even though it is not a unique phenomenon in arctic regions, the temperature difference in arctic areas merits more emphasis than that in other ocean conditions (Abdalla et al. 2008; Paulin et al. 2014b). Pipeline trenching is one of the optimum methods to prevent the ice gouging loads on pipelines in Arctic area. However, it also has several difficulties because of the severe Arctic environmental conditions. The areas are remote from land and generally covered by ice even during summer. In addition, highly developed low light cameras may 346 Proceedings of Western Dredging Association and Texas A&M University Center for Dredging Studies' "Dredging Summit and Expo 2015" be needed to conduct trenching due to darkness even during daytime. Climate conditions also lead to challenges with respect to operation and maintenance of trenchers, and the utilization of high performance trenchers.
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