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Collaborating Authors
The Second International Offshore and Polar Engineering Conference
Influence Of Structural Anisotropy Of Sea Ice On Its Mechanical And Electrical Properties
Borodkin, V.A. (Arctic and Antarctic Research Institute) | Gavrilo, V.P. (Arctic and Antarctic Research Institute) | Kovalev, S.M. (Arctic and Antarctic Research Institute) | Lebedev, G.A. (Arctic and Antarctic Research Institute)
Numerous investigation show that polycrystalline ice formed at different hydrometeorological conditions has strongly marked difference in structure. These remarks were taken as basis for structural genetic classification of ice of natural reservoirs proposed by N.V. CHerepanov [1,2], who chose as main following indications:–Salinity composition of reservoir –Hydrometeorological regime of reservoirs before freezing and during ice growth –Perculiarities of crystal growth –Morphological parameters of crystals –Duration of ice existence and metamorphic transformation connected with it. Depending on ice formation conditions and water salinity ice of natural reservoirs is divided to four main groups:Ice, formed in fresh water of extremely weak salt reservoirs, water salinity S<‰ (index A) Ice, formed in weakly salt reservoirs S= 2…24, 7 ‰ (index B) Ice of sea reservoirs S > 24, 7‰ (index V) Ice, formed as a result of metamorphic change of long term existing ice cover (index G) Every group of ice, depending on formation conditions and structure character divides in separate types. So in sea ice (group V) ice types are marked out from V1, which is characterized by large fibrous crystals, having tens centimeters length to V9-infiltration ice, composed from small granular crystals. In sea reservoirs (S > 24, 7‰) with well developed vertical mixing ensuring stable homogeneous state of near ice water layer which temperature is equal to freezing temperature favorable conditions for crystal growth in direction of basal plane appear. Having no impediment for growth towards water most intensively grow crystals, which main optical axis (C-axis) in horizontal, promoting growth of crystals with perpendicular orientation of C-axis. Peculiar feature of such ice is the skeleton layer of lower surface, consisting of skeleton fibrous crystals.
- Europe (1.00)
- North America > United States (0.95)
INTRODUCTION The construction of artificial islands out of spray ice is at the same level of sophistication as was the construction of embankments out of earth sixty years ago. In 1933 Proctor published his landmark series of four papers on the design and construction of rolled-earth dams. These papers mark the start of engineering earth fill to meet strength, compressibility and permeability requirements. To-day the first attempts are being made to engineer spray ice to use as fill for offshore islands, also, as was the case with earth. to control strength, compressibility and, to a lesser degree, permeability. Six years ago when the first spray ice island, Mars Island, was being built to serve as an offshore platform for petroleum exploration, little was done to influence the way the material would perform. For all intents and purposes, the engineer simply accepted the "natural" properties of the material. One natural property of the granular, porous ice spray was high compressibility. At Mars Island in 1985, the island surface settled up to 0.7 or 0.8 metres, and the rig settled half this amount during its active (drilling) period (Shields, Domaschuk & Funegard, 1990). The extra metre of ice that had to be produced to offset the loss of freeboard due to settlement is estimated to have cost about a quarter of a million dollars. Fortunately, differential settlement was not a problem, and the blow-out preventer/conductor pipe/rig design clearances were adequate. Nevertheless, the question should be asked "Could mechanical compaction of the ice have reduced its compressibility to more tolerable limits with, perhaps, savings in overall cost?" Construction specifications for the most recent islands were written to ensure that the strength of the ice would meet or exceed the design value.
ABSTRACT This paper reports an new data at land fast ice pressure measurements in the Beaufort Sea. The measurements were conducted at a grounded rubble field, 40 km North-East of Tuktayaktuk. The average ice movement rate against the rubble was approximately 0.15 m/day. Stresses appear to, be thermally induced and show semi-diurnal cycling that coincide with tidal cycles. The results are compared to, other available data of ice pressures that are thermal or correspond to, small movement rates. Statistics on ice force records are also, examined. INTRODUCTION Floating ice action on offshore structures is a function of the ice regime in which they are located. The ice regime depends on ice type, the environmental driving forces and ice movement rates. This paper is concerned with pressures of landfast ice that are thermally induced or correspond to relatively small movement rates. Such pressures develop against structures in shallow water depths within the landfast ice in the Beaufast Sea and the Canadian eastern Arctic. Wharfs built along ice covered waters would also experience such pressures. Field measurements of ice pressures that correspond to the regime considered here have been conducted at Adams Island which is located at the intersection of Lancaster Sound and Navy Board Inlet (Frederking et al. 1986 a, Frederking et al. 1986 b, and Sayed et al, 1988). Related measurements at Nanisivik wharf at Strathcona Sound were also reported by Frederking and Sayed (1986). Several other field programs were conducted in the Beaufort Sea at various artificial drilling islands. Sanderson (1988) and Blanchet (1990) gave overviews of these Beaufort Sea measurements, and listed some peak projects in the Beaufort Sea, including a comprehensive program at Tarsiut, were conducted at locations close to the edge of the landfast ice.
ABSTRACT For the last years several oil and gas fields were discovered on the North-Eastern Sakhalin offshore. Their construction is planned for the nearest future. From November till July water area of this region is covered by thick ice. Characteristics of such ice cover will determine design on selection of platform types and configurations. The results of the multi-year studies focused on fast and drift ice physico-mechanical properties are presented in this paper. The authors have worked out an ice sampling procedure, which help to avoid errors in determination of strength design values. Due to availability of the multi-year data for both cold and mild winters and due to the procedure mentioned above the design values for physicomechanical properties of sea ice in the region under discussion were obtained. INTRODUCTION The presence of a dynamic ice cover complicates development of facilities for the Arctic seas offshore construction. High cost and ecological responsibility of ice-resistant oil and gas producing structures result to strict safety requirements, which depend on reliability of design parameters. A large number of various approaches and methods for obtaining ice load design values account for complexity and insufficient investigation of the ice failure physical process during ice-structure interaction, peculiarities of structure design, unstable and conditional strength characteristics, variability of ice cover properties in various regions, relativelyinsufficient number of experiments, associated with in situ measurements of ice loads on offshore structures. In the case of same initial data the estimation results may vary greatly and complicate practical usage of investigation data (Vershinin, 1988). Multi-year experimental studies of the Sakhalin offshore ice regime were undertaken in order to meet requirements of design works on oil and gas field constructions (Fig.1).
- North America (0.70)
- Asia > Russia > Far Eastern Federal District > Sakhalin Oblast (0.55)
- Research Report > New Finding (0.34)
- Research Report > Experimental Study (0.34)
ABSTRACT The morphology and microstructure of icicles formed around the skirts of insulators energized under both direct and alternating voltages were analyzed in the present study. Wet ice accretion promoting the formation of icicles was grown at −12"C from 80 μm super-cooled droplets at a wind velocity of 4.2 ms. In general, the icicles grown under all three energized conditions are shorter than those formed without voltage. Bubble and crystal features show that icicles are very sensitive to heat exchange at the water/ice interface and they grow as hollow ice cores filled with liquid water at the tips. In the absence of applied voltage as well as in dc·, the core is circular, 5 mm in diameter, while in ac and dc+, where ionic wind velocity is stronger, they are deformed and shrink to a size of 4 mm and 3 mm respectively. The decrease in core diameter and increase in the ionic wind velocity at the tip of the icicle in dc+ and ac produce cooling effects. 1. INTRODUCTION In cold regions, ice accretions on power lines are the origin of a number of problems including a substantial reduction in the electrical insulation strength of insulators, (Kawai, 1970; Phan and Matsuo, 1983). Under atmospheric conditions favouring the formation of icicles, where the surfaces of ice deposits are covered by a water film, the spaces between the insulator units often become bridged by icicles. This considerably reduces the withstand voltages and causes insulator flashover under normal voltages and, consequently, line outages. The growth of icicles around the skirts of energized insulators is a complicated process and not only sensitive to atmospheric conditions but also to corona discharge intensity, affecting the heat exchange rate with the environment.
ABSTRACT A laboratory study of ice accretion from supercooled droplets on energized insulators was investigated. Wet ice which is associated with the highest probability of flashover, was produced at temperatures of −12°C and −7°C with water droplets of 50 and 80 μm and liquid water contents of 4.4 and 6.8 g·m. Wind velocity and the conductivity of freezing water played important roles in the uniformity and roughness of ice accreted respectively. It was also found that the mean power fed to the insulators by conduction current and corona discharges was about 40% of heat power liberated by the freezing process during wet ice accretion. 1. Introduction One of the serious problems with power transmission by overhead lines in northern countries and cold regions is atmospheric ice accretion due to freezing rain, in-cloud riming and wet snow combined with wind. The excessive accumulation of ice leads to mechanical damages, especially to the conductors and towers. Another problem with atmospheric ice accretion on power lines is the flashover phenomena occurring on insulators. Unlike the mechanical aspect mentioned above, the importance of problems caused by flashover phenomena on ice-covered insulators were not sufficiently considered in the past. Wet ice accretion and melting ice considerably change the dielectric properties of insulators and favour the occurrence of flashover thus the interruption of customer supplies. Due to the nature of these phenomena, failures by ice accretion often take a relatively long time to repair and cause considerable economic losses. The degree of loss in the dielectric properties of insulators depends essentially on the type and amount of ice accreted. To the best of our knowledge the systematic efforts to establish a relation between the type of ice and minimum flashover voltage of ice-covered insulators were undertaken by Phan and Matsuo (1983).
ABSTRACT In many countries, the design and reliability of power transmission lines are closely related to atmospheric icing overloads. Thus, accurate ice accretion modeling becomes increasingly important for the optimal design of new lines. At the present, existing icing models assume either a simple circular accretion shape or a complicated detailed profile. But field measurements of ice accretion on Cables have shown evidence of variable cable twisting during icing, which can be modeled by considering an eccentric ice load. By using a compromise between the circular or detailed icing shape cable twisting can be included in the simulation, and the accuracy of models can be improved. A model is presented to simulate numerically the dry growth of an ice accretion on a cable of a known torsional compliance. The ice mass and accretion size are calculated on a series of nodes along the cable. At each node, and for each time step, an elliptical ice accretion shape is assumed and the change in eccentricity and orientation is calculated knowing the icing intensity. From the mass, shape and orientation of the ice accretion, the torque due to gravity and the aerodynamic torsional moment are computed, and the resulting cable twisting is derived for static equilibrium. The aerodynamic moment is approximated by using a semi-empirical expression applicable to a two-dimensional elliptical shape. An average equivalent diameter and ice mass load per unit length are finally calculated as a function of time for a cable span. Results verify that twisting of a cable has a significant effect on the accretion shape and size. Eccentricity of the ice accretion shape is increased for a more rigid cable. Also, the total ice load for a cable span is more important for a smaller cable which is more flexible in torsion.
ABSTRACT This paper deals with the effects of a uniform and homogeneous ice cover on wind and tidally generated currents. The hydrodynamic equations which describe wind driven currents and tidally induced flow, are solved using an expansion in terms of vertical modes. The solutions at the sea surface are adapted to fast as well as drift ice cover of arbitrary thickness. At the sea bed, a no slip condition has been employed. The time evolution of the wind generated flow is examined with and without the ice cover. The magnitude and direction of the wind driven ice flow is considered for different ice thickness and eddy viscosity profiles. Tidal ellipses at different depths are computed both for the case of fast ice and for drift ice. It is demonstrated that the ellipse parameters as well as the wind currents are influenced by the ice cover. At the "critical latitude" the semidiurnal tidal frequency M2 coincides with the inertial frequency. The nature of the model solution at this latitude is examined as well. INTRODUCTION Over the past twenty years considerable efforts have been made by oil companies and others to gain insight into the environmental conditions that are important for offshore oil and gas activities, from the exploration phases to production. In some regions, such as the North Sea, understanding of the mechanisms that are important for the various phases, seems to be well established. With enhanced interest for offshore oil and gas exploration in cold climate regions, new challenges will appear, for example the presence of a permanent or seasonal ice cover which may be fast or moving with the winds and currents. The presence of ice will lead to load effects on constructions and may also hamper supply services and marine operations.
ABSTRACT During the International Geophysical Year, and for several years thereafter, supply ships escorted by ice breaker moored to the sea ice off McMurdo Station, Antarctica, and discharged cargo onto sleds. In the mid-sixties, Winter Quarters Bay was cleared of ice and ships began to dock directly along the shoreline at McMurdo. Elliott Quay, a major dock structure built to arrest the resulting progressive erosion, was destroyed by a freak storm in March 1972. Since then, a succession of man-made ice wharves has been built to handle ship traffic. In January 1991, the newest ice wharf broke into several fragments as a vessel was moored alongside. This paper describes the events preceding the mishap and presents the findings of the task force. INTRODUCTION McMurdo Station [77°51" S, 166° 4O"E] is the largest multipurpose research and logistics center in Antarctica and serves as both air and sea hub for the United States Antarctic Program. The station proper is located on bare volcanic rock on the southern reaches of the Hut Point Peninsula, an eleven-mile extension of Ross Island that marks the farthest south land mass accessible by ships traveling in the Ross Sea. Figures I and 2, respectively, present a map of the south polar region and expanded view of the McMurdo Sound study area. Structures at McMurdo Station include science laboratories, dormitories, repair facilities, administrative buildings, stores, clubs, warehouses, a firehouse, powerplant, water distillation plant and communications center that are linked together by an above-ground utility system of water, sewer, telephone and power lines. In addition to the island station and its harbor, located in adjoining Winter Quarters Bay, there are outlying air-support facilities operated seasonally on the sea-ice of McMurdo Sound and on the deep snow blanketing the McMurdo Ice Shelf.
- Antarctica > Antarctica (0.81)
- North America > United States > Alaska (0.28)
- Transportation > Marine (1.00)
- Transportation > Freight & Logistics Services (0.69)
- Health, Safety, Environment & Sustainability (0.68)
- Management (0.46)
ABSTRACT The shoreline of the mainland and barrier islands of the U.S. Beaufort Sea from Bullen Point to Brownlow Point were studied to determine the long-term (1908–1982) and short-term (July-September 1982) rates of change from historical records, charts and air photos, and from 67 survey stations at nearly evenly spaced intervals along the area. The short-term rates were determined from profile line measurements taken at the beginning of the open water season (mid-July) and again in early September. Several of the measurement site were revisited in 1983 and 1988 and these data were added to the data set. Island migration rates of 24m per year have been documented over the "long term (1908–1950), and much higher rates have been observed over shorter intervals. The historical records also show the bluff erosion rate on Flaxman Island to he about 5m per year (long term) and is sporadic from season to season. The data collected during the Summer 1982 surveys indicate erosion rates as high as 7.5m on Alaska Island between July and September. This was due primarily to a minor storm which occurred in late July. As expected, greatest changes were recorded near the ends of the barrier islands, and least changes were observed on the mainland shore that lies in the lee of the barrier islands. Survey markers were specially designed and placed along the shoreline at the 67 locations. Several markers have been over-washed by storm wave processes, however, we believe that few have been lost, thereby allowing this survey data base to be extended into the future. INTRODUCTION Studies of coastal changes in the Pt. Thomson region of Northern Alaska on the southern shore of the U.S. Beaufort Sea (Figure 1) have been conducted to establish an environmental baseline for the Point Thomson area.
- Africa > Middle East > Egypt > Gulf of Suez > Gulf of Suez Basin > July Field (0.99)
- North America > United States > Utah > Island Field (0.98)
- North America > United States > Gulf of Mexico > Central GOM > West Gulf Coast Tertiary Basin > Green Canyon > Block 627 > Hopkins Field (0.93)
- North America > Canada > Alberta > North Star Field > Gulf North Star 10-22-90-23 Well (0.89)