The prediction of extreme keel depths from sea ice profiles

The prediction of return periods of extremely deep pressure ridge keels is discussed, using as data a 1400 km submarine profile obtained by U.S.S. “Gurnard” in the Beaufort Sea. Three techniques of predicting return periods at a point are examined: the use of the negative exponential distribution; a depth crossing technique; and a probability plotting technique. The problem of predicting return periods along a line is then examined with reference to ice scouring across seabed pipeline routes. A technique which combines keel statistics and scour depth statistics is used to compute the pipeline burial depth necessary to avoid disturbance by ice for a specified period.     

Mapping sea ice overflood using remote sensing: Alaskan Beaufort Sea

The U.S. Department of Interior, Minerals Management Service (MMS), Alaska Outer Continental Shelf Region commissioned a study designed to map the extent of peak river overflooding onto the fast ice in the nearshore region of the Alaskan Beaufort Sea (Hearon et al., 2009). This phenomenon occurs annually during a brief period in the spring when the river stage increases rapidly as the snow pack melts, while the sea ice of the Delta front is still intact. River overflood constitutes a potential hazard to offshore oil and gas development, as it relates to facilities access, oil spill spreading, and the associated phenomenon of strudel drainage and potential seabed scouring. The overall goal of the study was to improve the knowledge of the spatial and temporal variability of overflooding and related pipeline and facility siting concerns. Historical overflood boundaries were mapped for the 13-year period from 1995 to 2007, using a combination of helicopter surveys and satellite imagery. Hazards associated with strudel scouring were assessed with strudel drain and scour databases developed for several industry projects.     

Iceberg grounding and scouring on the Labrador Continental Shelf

Icebergs drifting in seas of the eastern Canadian Continental Shelf present serious hazards to offshore drilling operations. Damage to offshore structures may be caused by direct collisions with a floating or gravity-based structure. Icebergs whose keels touch and plough through or scour the soft sediments of the seabed may crush and rupture seabed installations such as wellheads, anchoring/mooring systems, pipelines and telecommunication cables. Observations made at exploration wells on the Labrador Shelf from 1973 to 1981 are used to delineate areas of active iceberg scouring and to quantify the incidence of grounding and scouring icebergs. The term grounded is used to describe icebergs whose keels have contacted the seabed and which have thus been halted. Scouring icebergs are those whose keels have contacted the seafloor, but which continue to move forward. Observations of more than 1500 icebergs from twenty-two well sites have been analyzed and criteria for identifying grounded and scouring bergs have been established. Over fifty icebergs have been observed to ground and scour in eleven areas. Over the observation periods, the average grounding frequency for Makkovik and Saglek Banks were 3.3% (data collected in seven years) and 4.3% (data collected in six years), respectively. It appears likely, however, that these frequencies are an underestimate and that many more “possible” grounding and scouring icebergs will eventually be included in the data set. We show that many scouring icebergs can move over large (60 km) distances and are able to traverse significant (up to 45 m) ranges in bathymetry. We suggest that they may accomplish this through increases and decreases in draft by continual gradual rotation about a horizontal axis normal to the movement direction.     

New predictions of extreme keel depths and scour frequencies for the Beaufort Sea using ice thickness statistics

In 1983 Wadhams proposed a technique for calculating scour return periods and pipeline burial depths for scour avoidance for the coastal Beaufort Sea. The technique combined extreme keel depth predictions based on submarine data with the use of observed distributions of scour widths and incision depths, to calculate the extreme incision depth with a given return period for a pipeline of given length laid across the seabed. The Beaufort Sea submarine dataset used to illustrate the application of the technique was obtained in 1976 from a regime consisting largely of multi-year ice. In recent years first-year ice has come to dominate the southern Beaufort Sea, so we replaced the 1976 statistics with data from a winter 2007 survey of the same region by the submarine HMS “Tireless”. Holding other scour geometry parameters constant, we calculated new return periods and burial depths and found large changes which appear to be reflected in the results of recent resurveys of regions subject to ice scour. We reflect on the likelihood that these more moderate conditions will continue to prevail in the future.     

Deflection of a floating sea ice sheet induced by a moving load

A large number of time-deflection curves was obtained from a field experiment carried out on the sea ice sheet at Lake Saroma, Hokkaido, Japan, using a Skidoo snowmobile as a moving load with the test speed changed from 0 to 14.2 m/s. The thickness of the ice sheet was 0.17–0.18 m, the ice temperature was ?2.5 to ?4°C at the surface, and the water depth was 6.8 m beneath the ice sheet. The experimental results indicated the existence of a critical speed u_c and its value was estimated to be 5.8 m/s. At speed above it two ice waves were generated, one ahead of and the other behind the vehicle. A variation of wavelength with vehicle speed was expressed by a modified dispersion relation for the flexural ice wave in a floating ice sheet. The ice deflection pattern was classified into five stages. At u_c the depression around the vehicle was maximal in depth and minimal in width. The center of the depression began to lag behind the vehicle even at fairly low speeds below u_c.     

The structure and strength of first-year ice ridges in the Baltic Sea

Sea ice ridges are typical features in the Baltic Sea ice pack accounting for on average up to one-third of the total ice mass. They are difficult obstacles to winter navigation and cause large forces on ships and offshore structures. However, the internal structure and strength properties of ice ridges are quite poorly known. This paper presents the results of an experimental project on the structure and strength of first-year ice ridges in the Baltic Sea carried through during 1987–1989. Altogether, six freely floating ridges were investigated. Their total thickness ranges from 4 to 17 m. Valuable field data about the size and shape of ridges, consolidated and unconsolidated parts, block size and porosity have been obtained by drilling and sampling. Divers and underwater video cameras have been used for observing the ridge keel structure. The totally consolidated layer within the ridges was 1–2 times the thickness of the surrounding level ice. The average porosity was 29%. The strength of the keel part has been measured with a full scale loading test. The force required to shear the keel was determined as a function of the normal force, the settlement rate, and the porosity of the keel. The shear strength of the ridge keels was from 1.7 up to more than 4.0 kPa. Also small scale tests were conducted in an ice tank giving results in agreement with the full scale tests.     

An investigation of the failure envelope of granular/discontinuous-columnar sea ice

The three-dimensional failure envelope for granular/discontinuous-columnar sea ice has been investigated over a range of nominal strain rates from 9 × 10^?6 s^?1 to 4.4 × 10^?4 s^?1 by using confined compression tests. Both the applied load and side-confining load were measured. The tests were performed on samples cut from a solid block of ice in the Beaufort Sea which structurally was granular with some banding of discontinuous-columnar ice to a depth of 1.2 m. The results indicate the overall shape and size of the failure envelope of this ice, and show that although the shape is independent of loading rate, the size increases with loading rate. An analytical expression is derived which mathematically describes the failure envelope in three-dimensional space.     

State of the art of ice bearing capacity and ice construction

Ice is an effective and economical means of supporting loads for construction and resource extraction. The main requirement is to have continuous ice of sufficient thickness to support the intended loadings. Ice has been used to support heavy loads, both mobile and stationary and long-term loads such as oil and gas drilling rigs. It has been used to support the installation of offshore pipelines and related facilities. The paper describes the various uses of ice as a load-bearing medium and presents methods for determining required thicknesses.The bending of floating ice under applied load causes flexural stress to be imposed on the ice cross section. Because ice is weak in tension, the critical stress is the maximum tensile stress at the bottom of the ice directly under the load. The paper presents standard methods of calculating the maximum, extreme fibre stress for different types of loads and presents an allowable stress for safe use of the ice as a load-bearing medium. This allowable stress is also instrumental in ensuring that long term, creep deformation does not result in submergence of the ice surface. The paper presents a method of estimating long-term deflection and also presents a method of assessing the effects of dynamic or moving loads.Grounded ice roads require sufficient thickness to spread applied wheel loads and avoid overstressing of the tundra or seabed. A method of assessing the support provided by widely differing subgrade conditions is presented.Construction methods and techniques for ice roads and structures, including the best types of equipment to use, are presented, with photographs. Issues such as snow removal vs. flooding of thick snow are discussed. Durability of the road surface and surface repairs is outlined.During the construction phase of ice structures, the quality control (QC) tasks verify material quantity and material quality as required by the design. An outline of considerations for this task is presented. Failures and anecdotal information related to the design, construction and use of ice roads and structures are presented, with illustrative photographs.     

Model tests of ice forces on fixed and oscillating cones

A series of model tests has been carried out on downward breaking conical structures using low salinity model ice. Comparison with information in the literature showed that a downward breaking cone experiences lower ice forces than an upward breaking cone. Fixed cones of angles of 15, 30, 45 and 60 deg were tested in 50-mm-thick ice of 60 kPa flexural strength at velocities ranging from 0.01 to 0.5 m/s. The effects of thickness and flexural strength variations were also investigated. Horizontal forces were observed to increase with increasing velocity. Ice thickness variation was observed to have a stronger influence on the vertical forces than on the horizontal forces. Vertical and horizontal forces showed a decrease with decreasing flexural strength.Two test series were run with the 45 deg conical structure arranged so that it oscillated. Horizontal force reductions of up to two thirds were observed.     

Experiments on level ice loading on an icebreaking tanker with different ice drift angles

A series of tests was performed with a laboratory-scale model ship to simulate the effects of ice load parameters on an icebreaking tanker. A model of the icebreaking tanker Uikku was mounted on a rigid carriage and towed through an unbroken ice sheet in the ice tank of the Marine Technology Group at Aalto University. Two ice sheets and 11 different experimental configurations were used. The carriage speed, heading angle of the model ship, and ice thickness were varied, and the forces, accelerations, ice cusp sizes, carriage positions, and ice pile dimensions under the intact ice sheets were measured.This paper includes results for the measurements of ice rubble loads against the model hull in the horizontal plane. Phenomena such as ice failure modes and ice rubble accumulation on the upstream side of the hull beneath the ice sheet were observed in some tests. The icebreaking lengths and dimensions of ice rubble were analyzed for some tests. The effects of towing speed, heading angle under the intact ice sheet in front of the hull, and the accompanying ice loads on the formation and build-up of ice rubble were analyzed. In addition, the evolution of ice rubble geometry, in cross sections and the horizontal plane, was investigated. There was good agreement over several orders of magnitude between the measured and calculated values of the lateral ice forces. These results are relevant to the modeling of ice loading on hulls and the design of moored or dynamic positioned structures for operation in ice-covered waters. Some parameters obtained from these tests can be used as input for future numerical simulations.     

Experimental studies on ice shells in Asahikawa

An ice shell is thin, and its structural material is snow-ice. It may be an efficient form of instant shelter for snowy and cold regions. This paper describes two field studies on ice domes carried out in Asahikawa. The first one is an investigation on the creep collapse of a 5-m span model under a concentrated load acting on a circular area at the apex. Normal displacements and temperatures were measured up to the collapse. Experimental collapse time was examined, introducing the classical creep buckling value of a completely spherical shell under uniform external pressure. The second study deals with both the construction technique and the creep test of a 10-m span model. The construction technique consists of: (1) inflating a membrane bag covered with rope, (2) spraying it with snow and water, (3) solidifying the snow-ice sherbet on it, and (4) removing the bag and rope for reuse. Subsequently, a creep test was carried out under snow load, and its structural behaviour up to the collapse was examined. Based on the results of these studies, the production of 20–30-m span ice shells may be practicable.     

Fracture in the compression of columnar gained ice

Failure behavior of fine-grained columnar ice is investigated under compression. A special experimental set-up is used to achieve a uniform stress (strain) state in the specimen. The strength is then determined at several constant temperatures and strain rates.Three failure mechanisms have been identified: cleavage fracture, plastic flow, and linkage of grain boundary cracks. The failure mechanism is believed to be a material property. The failure mode, however, is dependent on the specimen geometry and the loading condition. Thorough investigation is made on the failure modes of ice specimens (4 × 8 × 12 cm blocks).A major conclusion is that ice is an excellent material to be used in the investigation of the failure of brittle polycrystalline materials.     

Experimental study on surface wave propagating through a grease–pancake ice mixture

A laboratory experiment was carried out in two wave tanks in a cold room. This experiment was designed to better understand the dispersion relation and amplitude attenuation for surface waves propagating through ice covers consisting of a grease and pancake ice mixture. Each test started from a calm condition to avoid reflection effects. The portion of the wave signals before the arrival of reflections was used to measure the wave number and the attenuation rate. The comparison between experimental results and the two-layer viscous model (Keller, 1998) indicates that modeling a grease–pancake ice cover as a viscous fluid is not sufficient to describe the observed dispersion relation and amplitude attenuation. Extended models including other mechanisms may be necessary to better describe wave–ice interactions for a grease–pancake mixture.     

Maximum ice force on the Molikpaq during the April 12, 1986 event

On April 12, 1986, the Molikpaq, a caisson-type offshore drilling structure, experienced a series of loading events when second-year and multi-year ice moved against the structure. The highest loads that the Molikpaq experienced during the 1985–86 season were during this day. Extensive ice thickness measurements had been taken in the ice around the Molikpaq prior to April 12. Thickness of up to 6 m were measured and 8 to 12 m estimated for a multi-year hummock. MEDOF panels on the outer face of the caisson and strain gauges on inner bulkheads of the structure were the primary instrumentation used to calculate forces on the faces. Detailed analysis of strain gauge and extensometer data indicated that the maximum global force on the Molikpaq was no more than 420 MN and most likely was 380 MN. This is in contrast to higher and lower values quoted in the literature for the April 12th event. Other loading events during the day when multiple strain gauges and MEDOF panels were being recorded produced global loads of the order 250 MN. Global ice pressures for the 8 to 12 m thick multi-year hummock crushing on the 59 m long east face were not greater than 0.8 MPa.     

Acoustic emissions from polycrystalline ice

The acoustic emission response from fine-grained polycrystalline ice subjected to constant compressive loads was examined. A number of tests were conducted with the nominal stress ranging from 0.8 to 3.67 MPa at a temperature of ?5°C. The acoustic emission response was recorded and the data are presented with respect to time and strain. The source of acoustic emissions in ice is considered in terms of the formation of both microfractures and visible fractures that develop without catastrophic failure of the ice. A model to describe the acoustic emission response is developed.     

Mechanical properties of atmospheric ice

Mechanical properties of atmospheric ice obtained in a wind tunnel are measured. The ice is grown from supercooled droplets on a rotating aluminium cylinder of 31.5-mm diameter and 6.5-μm rugosity. Compressive strength is measured at two speeds of deformation (0.76 and 26 mm/min) for glaze and rime samples, as a function of air temperature for different atmospheric conditions (0.4 and 0.8 g/m³ liquid-water contents, 20 and 40 μm mean volume droplet diameters, and 4, 8, 15, and 20 m/s air velocities). These values of compressive strength are compared to the adhesive strength on aluminium, measured at a 26 mm/min speed of deformation. The ratio of compressive to adhesive strength has a maximum value of 135 for hard rime accreted at −10°C, with a wind velocity of 15 m/s, a liquid-water content of 0.8 g/m³ and a mean droplet diameter of 40 μm. The maximum compressive strength measured for the lower speed of deformation is 17395 kPa and 10745 kPa for the higher speed of deformation. The maximum adhesive strength measured is 181 kPa. On the other hand, compressive strengths measured at deformation speeds varying from 0.015 to 288 mm/min show that atmospheric ice has a ductile-brittle behaviour approaching that reported for snow ice and fine-grained lake ice.     

Deformation of the Arctic Ocean ice cover after the 2007 record minimum in summer ice extent

We examine the deformation of the Arctic Ocean sea ice cover after the record minimum in summer extent in 2007. The period spans ~ 2.5 months between September 15 and December 1. Ice drift and deformation inside the ice edge, within a domain that initially covers ~ 0.76 × 10⁶ km² of the western Arctic, are derived from high-resolution RADARSAT imagery from the Alaska Satellite Facility. Poleward of 80°N, we find a net convergence of more than 14% over the period. This large convergence is associated with the strength, location, and persistence of the Beaufort high-pressure pattern that led to prevailing on-shore winds north of Ellesmere Island and Greenland. This can be contrasted to the nearly 25% divergence of the ice cover, accompanied by a large regional vorticity of − 0.93 (or a clockwise rotation of ~ 53°) south of 80°N. The same atmospheric pattern produced openings as the ice cover drifts southwest towards the unconstrained ice-free part of the southern Beaufort and Chukchi Seas. These sustained strain rates, especially convergence, impacts the area and thickness distribution of the sea ice cover in the Arctic Basin. If unaccounted for, this deformation-induced decrease in ice coverage (in this region with predominantly multiyear ice) could be incorrectly ascribed to ice export with a concurrent decrease in Arctic sea ice volume, when in fact the ice volume is conserved but with a local redistribution in thickness.     

Scale effects on the in-situ tensile strength and fracture of ice. Part II: First-year sea ice at Resolute, N.W.T.

A set of lab- to structural-scale 0.5 < L < 80 m) in-situ full thickness (1.8 m) fracture tests were conducted on first-year sea ice at Resolute, N.W.T. using self-similar (plan view) edge-cracked square plates. With a size range of 1:160, the data is used, via size effect analyses, to evaluate the influence of scale effects on the fracture behavior of sea ice over the range 10-1 m (laboratory) to 100 m and to predict the scale effect on tensile strength up to ≈1000 m. Details of this large-scale sea ice fracture test program are presented in this paper. The experimental results are presented as well as the fracture modeling of the data. The influence of scale on the ice strength and fracture toughness is dramatic. The applicability of various size effect laws are investigated and criteria for LEFM test sizes are presented. For the thick first-year sea ice tested, the size-independent fracture toughness is of order 250 kPa , not the 115 kPa that is commonly used. The number of grains spanned by the associated test piece is 200, much larger than the number 15 typically quoted for regular tension-compression testing. The size-independent fracture energy is 15 J/m2, while the requisite LEFM test size for the edge-cracked square plate geometry (for loading durations of less than 600 s and an average grain size of 1.5 cm), is 3 m square. Size effect analyses of sub-ranges of the data show that unless the specimen sizes tested are themselves sufficiently large, the true nature of the scale effect is not revealed, which was a concern raised by Leicester 25 years ago. In the case of the fracture tests reported in this paper, based on the lab-scale and field-scale strength data measured between 0.1 and 3 m and using Bažant's size effect law, it is possible to accurately predict the tensile strengths for all of the remaining tests, up to and including 80 m. This revised version was published online in July 2006 with corrections to the Cover Date.     

Test method for measuring resistance of industrial floor overlays to impact loading

Durability of industrial floors depends to a great extent on the quality of floor surface, which must be highly resistant to mechanical loads, chemical attack and other types of influence. In order to choose the correct overlay material, it is necessary to define relevant characteristics and use reliable test methods. In this work, the new method is shown developed specifically to test the resistance of industrial floor overlays to impact loading. Compared to other standard test methods, the method suggested here has been adapted to the demands of industrial environments with respect to impact load size, specimen dimensions and shape, as well as boundary conditions. Laboratory tests have been carried out on specimens 40 × 40 × 10 cm and 100 × 100 × 10 cm, and a floor structure with layout dimensions 2.6 × 3.5 m, using different types of overlays.A theoretical model of industrial floor overlay under impact loading has been formulated with the help of the analysis package COSMOS/M, using various parameters such as specimen size and overlay characteristics for the sake of detail evaluation of their particular influence.