Ridge Keel Research Articles

A field study of grounded ice features and associated seabed gouging in the Canadian Beaufort Sea

During the late summer of 1983, the southern Canadian Beaufort Sea was invaded by vast areas of thick sea ice features called hummock fields. Thousands of these features drifted into shallow water (10–27m water depth), became grounded and remained in place throughout the following winter. During the grounding event, it was assumed that the deeper ice ridge keels would have gouged the seabed. In April 1984, eight features with gouging potential were visited and investigated from the ice surface. Conditions allowed for measurement by sonar of the geometry of seabed gouges at two locations, named M1 and M2, as well as the geometry of the ice keels that caused the gouges. Volume and mass of the ice features were estimated from stereo air photography gathered earlier in the winter, from which elevation profiles were obtained. On-site elevation profiles confirmed elevations. The hummock field at site M1, estimated at 277,000tonnes, produced a gouge 2m in depth and a front mound about 3m high ahead of its keel. This feature had also been uplifted 2m at its leading edge. The deepest gouge was 3.5m, caused by hummock field M2, with an estimated mass of 1.16milliontonnes, which fragmented during the grounding process. A seabed gouging survey conducted in August 1984, in open water conditions, confirmed gouge depth at M2. Data on seabed soils were obtained at the M2 site one year later, in September 1985, as part of a geotechnical field survey. These cases of seabed gouging by massive sea ice features are unique in that they were found and documented with the ice features that made them still in place. All gouge, ice and soils data relevant to these two events are summarized herein, as a full-scale data set intended to increase understanding of seabed gouging and allow validation of models.

A critical review of Arctic pack ice driving forces: New sources of data

This paper provides the details of ice loading events that can be used to further the understanding of pack ice driving forces in the Beaufort Sea. Several methods have been reviewed and employed including in situ stress measurements, loads on the Molikpaq offshore caisson, shoreline pile-up events, pile-ups and rubble fields on offshore shoals and relic berms, analysis of shear walls on offshore rubble fields, and analysis of deep ridge keels. Over 50 different events are identified with 33 suitable for a pack ice analysis. The data are considered in terms of both the ISO 19906 (2010) Arctic Structures Standard and pack ice pressures that can be exerted across various widths. A new approach is proposed, in which the calculated and measured values from past pack ice pressure events are used to predict limit force.

Estimating operability of ships in ridged ice fields

A method for estimating ship's resistance caused by sea ice ridge keels is revised and used as a part of a method for predicting performance of ships in ridged ice conditions. The resistance method is based on a continuum plasticity model of ridge rubble and is simple to compute. The performance prediction method combines deterministic simulations of ship motion with probabilistic modelling of ridged ice fields. Performance estimates given by the model are distribution of attainable mean speeds for given ice conditions and probability of the ship being able to operate independently.A comprehensive sensitivity analysis was performed to gain insight into the model and identify possible problematic parameters. The sensitivity analysis covered both the ice conditions and modelling assumptions.Two data-sets were used to test the simulation method. One set included the depth profile of sea ice, machinery data and the speed of a ship operating in ridged ice. The resistance method was able to predict the mean speed over 3km well. The second data-set consisted of a history of ship's speeds and positions from AIS data and ice conditions estimated by a numerical ice model HELMI, developed in the Finnish Meteorological Institute. Observed mean speeds were mostly well within the distributions of mean speeds simulated by the transit simulation model. Predictions of independent operation were also promising.

Sea-ice ploughmarks in the eastern Laptev Sea, East Siberian Arctic shelf

One of the major observations during marine interdisciplinary expeditions in 2011 and 2012 on the RV Akademik M.A. Lavrentyev and RV Viktor Buinitskii was the occurrence of a large number of ice scours in the East Siberian Arctic shelf (Lobkovsky et al. 2013). The scours vary in size, depth, spatial arrangement and age of generation. Maximum water depth for sea-ice ploughing of the shallow continental shelf depends on sea-ice ridge keel depth (Ogorodov 2011). Linear and curvilinear scours on the floor of the eastern Laptev Sea shelf at water depths ≤40 m are interpreted here as ploughmarks generated by sea-ice pressure ridges (Fig. 1). Ice ploughmarks in the Laptev Sea were first discovered in 1970 during the East Siberian Arctic shelf transect by US Navy nuclear submarine USS Queenfish (McLaren 2008). Fig. 1. Sea-ice ploughmarks on the eastern Laptev Sea continental shelf. ( a ) General bathymetric map of the study area from digitized navigation charts (Nikiforov et al. 2004). ( b ) Location of study area (red box; map from IBCAO v. 3.0). ( c , e , f , i ) Side-scan sonar sonographs of sea-ice …

THE STATISTICAL MODEL OF PRESSURE RIDGE MORPHOMETRY ON THE NORTHEAST SHELF OF SAKHALIN ISLAND

The work presents characteristics on geometry and inner structure of ice ridges investigated at offshore the northeast coast of SakhalinIsland. A formula was obtained which allows one to calculate the ice ridge keel depth by the height of its sail. Plots of the probability distribution density for ice ridge characteristics are given. A model of morphometry of a mean statistical ice ridge was constructed, and its mass is determined. Factors influencing the hydrostatic ice ridge equilibrium are considered.

Thermodynamic consolidation of ice ridge keels in water at varying freezing points

The permeability of sea ice ridge keels to surrounding seawater was investigated using data from field measurements in the Barents Sea. Two models that describe the consolidation of ice rubble due to the penetration of low salinity water at freezing point inside the rubble were formulated and analyzed when the water salinity decreases with time. The sea ice enthalpy depends on the temperature and salinity with considerable increase towards the 0°C. It is shown that this dependence significantly influences the amount of newly formed ice around the rubble. The presence of brine pockets in the sea ice explains such dependence. When sea ice is heated brine solution should adjust its salinity to stay in thermodynamic equilibrium with pure ice. Reduction of salinity in the closed pocket is possible due to melting of ice on the wall of the pocket. This latent heat of fusion defines so high specific heat capacity of sea ice and as a result allows extracting enough coldness from sea ice rubble to reach high values of its consolidation. It was shown that in both models a key role in consolidation is played by the process of interchange between microporosity of sea ice and macroporosity of rubble field.

Effect of vertical inhomogeneity of the ridge keel filling on its freezing rate

Field data on ice ridges in the south-eastern and eastern parts of the Barents Sea collected by specialists of the Arctic and Antarctic Research Institute (AARI) in 2003–2007 has been used in attempt to understand the vertical inhomogeneity of the ridge keels filling. New ridges originated from one-year sea ice (with no consolidated layer or or less than 5–10 cm thick) have been analyzed. Reduction of original observations under certain additional assumptions has allowed formulating the vertical distribution of the ridge keel fill factor. This approximation has been applied in the one-dimensional thermodynamic ride model developed in AARI to simulate the ice ridges behavior in the Central Arctic and Arctic seas. The modeling of ridge freezing rates for different geographical sites has shown the more intensive freezing of ridge keels in initial stage than expected from previous modeling runs. The new results are consistent with field observations on consolidated layer thickness in ridges. Model calculations for a longer period show no big difference in results obtained with or without the approximation proposed.

Truncorotalia crassaformis from its type locality: Comparison with Caribbean plankton and Pliocene relatives

Abstract Truncorotalia crassaformis has been identified in Pliocene–Holocene assemblages globally but there has been little analysis of specimens from its type locality at Lomita Quarry, California. This has led to confusion about some diagnostic criteria, particularly the presence of a peripheral keel. To better understand variation specimens are studied from the type locality (Pleistocene, c. 400–600 ka), supplemented by material from a plankton trap in Cariaco Basin and from ODP 925, Ceara Rise (Pliocene, c. 4.3 Ma). The damaged holotype has a weak topographic ridge (keel) at the periphery of early chambers of the outer whorl. Several well-preserved specimens have a keel on all chambers of the whorl. Encrustation obscures the periphery on some and masks shell shape. Several outliers in a morphometric analysis of axial shape have distinctive discoidal outlines but ventroconical (cone-like) forms are common. Lomita Marl was deposited on a sheltered, shallow shelf in Chron 1. Foraminifera reworked from the unconformably underlying Repetto Siltstone are present. Specimens resembling the holotype are very rare and often damaged. Morphological disparity is high. It is unlikely that an autochthonous population was sampled. The weak peripheral keel present on some living specimens from Cariaco Basin is built incrementally by a thin featureless calcitic veneer deposited between the morphogenesis of each chamber. The process progressively obscures pores in the primary wall. Its earliest stages have been misidentified as a poreless zone. Ventroconical form is weak in the Ceara Rise Pliocene specimens and is distinguishable from the Cariaco sample. There is only a veneer at the periphery. Although the study does not provide a population-based diagnosis of T. crassaformis it indicates that the name should not be applied to early Pliocene forms.

Loads from Ice Ridge Keels – Analytical vs. Numerical

Action of ice ridge keel on vertical structures is studied by means of analytical and numerical methods. The analytical method involves keel load calculations using Russian and European recommendations provided in SP 38.13330.2012 and ISO/FDIS 19906 respectively. The numerical method involves finite element calculation. It is demonstrated that numerical methods can provide an important insight into ridges keels behavior. The numerical and analytical results obtained using Russian recommendations for vertical structures are in a good agreement.

Ice ridge keel geometry and shape derived from one year of upward looking sonar data in the Fram Strait

Ice ridge keel geometry was studied by analyzing one year of upward looking sonar data collected in the Transpolar drift stream at 79°N, 6.5°W in 2008/2009. Ridges were identified using the Rayleigh criterion with a threshold value of 2.5m and a minimum draft of 5m. The keel shape was studied after the identification of ridges from temporal data. On average ridge keels were symmetric both with respect to the centroid of the keel and the keel crest location. By quantifying the ratio between observed keel area and the keel area of an assumed triangular keel shape (often assumed for first year ridges) we observed that in 79% of the cases the ridge cross sectional area would be underestimated by a triangular keel shape. Because keel loads on ships and structures increase with keel draft and keel area it is important that an assumed keel shape maintains the observed keel area. Thus we suggest that a better generalization of the shape of first year ridges is a trapezoidal keel shape rather than triangular. Based on the observations the mean trapezoidal keel, representing both first year ridges and old ridges, has a keel bottom-width which on average is 17% of the keel width. For the deepest keels (>15m) the mean keel bottom width was 12% of the keel width. The mean keel draft was 7.3m and the deepest ridge was 25m. The temporal data was converted to spatial data based on an ice drift speed estimate which assumed free drift. From the spatial data we found that the mean keel width was 28m and the mean keel cross sectional area was 164m2.

Effects of a Shallow Pycnocline and Surface Meltwater on Sea Ice–Ocean Drag and Turbulent Heat Flux

Abstract Comprehensive boundary layer measurements from a drift station on first-year ice in the late summer of 2012 in the Nansen basin, when stable stratification in the upper ocean extended all the way to the surface, are analyzed. Observed quadratic ice–ocean drag coefficients, based on measurements of wind stress, are roughly 3.6 × 10−3, consistent with neutral-stability Rossby similarity scaling. The turning angles of 32°–39° between surface velocity and stress are larger than Rossby similarity predicts and obey a different scaling. This can be explained by the shallow pycnocline forcing the Ekman transport into a thin layer and modeled roughly employing a simple first-order correction to Rossby similarity. Turbulent shear stress in the ice–ocean boundary layer is on average 3 times smaller than the estimate based on wind stress, possibly because internal wave drag was significant. This lowers vertical scalar fluxes by 38% compared to a scenario where turbulent stress accounts for the total drag. The authors measure an average upward ocean–ice heat flux of 10 W m−2, which is 50% smaller than predicted by a bulk heat flux parameterization. This reduction is attributed to additional sources of heat and freshwater that alter the ice–ocean interface salt balance. This study shows that a commonly used bulk heat flux parameterization is a special case of a simple downgradient parameterization allowing for a modified interface salt budget. For similar wind forcing, observed ice–ocean fluxes of heat and salt were 40%–100% larger when the ice-relative current approached from a nearby pressure ridge keel than otherwise.

Water content estimates of a first-year sea-ice pressure ridge keel from surface-nuclear magnetic resonance tomography

AbstractThe porosity of a sea-ice pressure ridge keel is an important but poorly known variable, relevant for determining the mass budget and evolution of the Arctic sea-ice cover. Determination of keel porosity from drillholes is time-intensive and only yields limited information because of their limited lateral extent. Since the porosity within a keel equals its liquid water content, surface-nuclear magnetic resonance (surface-NMR) methods can be used to estimate porosity within such features. Surface-NMR tomography measurements were made in April 2011 using seven surface coil positions across a first-year pressure ridge on landfast sea ice near Barrow, Alaska, USA. The inversion results indicate water contents of 30 ± 7% and 40 ±10% in the ridge’s shallow and deep parts, respectively. These values are much higher than those obtained from drillholes, which are ∼10% and ∼27%, respectively. In contrast to drilling, surface-NMR tomography yields average porosity values for the entire subsurface volume. However, the inversion process is sensitive to the electrical conductivity distribution; uncertain conductivity estimates limit the reliability of the inverted water contents. Nevertheless, the results suggest that ridge porosities obtained from invasive measurements such as drilling may lead to substantially overestimated sea-ice volume.

On modeling cohesive ridge keel punch through tests with a combined finite-discrete element method

This paper introduces a technique for modeling partly consolidated ice rubble using a two-dimensional combined finite-discrete element method and an application of the technique on ice rubble punch through experiments. In the technique, each ice block within the rubble, the contact forces between the blocks, the block deformation, and the rubble freeze bonds are modelled. Simulations with various freeze bond strengths and block to block friction coefficients were performed. As a main simulation result, the close relationship between rubble deformation patterns and load records is demonstrated in detail. It is shown that the buoyant load component due to the rubble becoming detached from the surrounding rubble field and displaced during an experiment is of crucial importance when interpreting punch through experiment results. The consequences of simulation results on ice rubble material modeling are discussed.

Regulation of extreme ice loads acting on hulls of azimuth propulsion systems for ice ships

Vessels with an azimuth propulsion system (APS), including double-acting tankers (DATs) designed for autonomous ice navigation in astern-running mode, which makes it possible to improve their icebreaking capability and operation performance in open water by means of bow shape optimisation, are considered to be promising ship types for hydrocarbon and ore material transportation along Arctic Passage routes. This paper covers the regulation issues of the extreme ice loads on the APS pods of conventional ships and DATs. It was traditional for Russian practice to use a hydrodynamic model (HDM) of solid body/ice impact; however the latest full-scale and laboratory experiments carried out in Russia, as well as abroad, show a higher degree of ice load localisation (or peaks) as compared with HDM predictions. For the regulation of the ice loads on the DAT azimuth propulsion system, a modified hydrodynamic model (MHDM) is used, which provides both satisfactory agreement with the experimental data and a good succession of the HDM's approaches and empiric coefficients. This paper suggests an approximate formula for the ice force determination, and this formula is based on MHDM approaches. The paper covers the methods of regulating the calculated parameters for the APS ice impact under ahead-running, astern-running and in circulation mode. It also considers the possibility for creating an efficient APS protection from ice impact under astern-running, and takes into account the double-acting tanker APS specifics. The paper suggests an engineering calculation model to determine the load on DAT APS when the vessel goes through an ice ridge under astern-running mode. It examines the case of the ridge keel interaction not only with the blades of the pulling propeller, but with the APS pod as well. The paper also considers the circumstance that the areas of the ridge keel adjacent to the zone destroyed in the course of interaction with the propeller blades are rather hollow.

Discrete element analysis of ice loads on ships and structures

In this paper, the versatility of discrete element method (DEM) in modelling ice-related problems is discussed and further demonstrated using the results from the DEM works conducted by the National Research Council's Institute for Ocean Technology (NRC-IOT) using a commercial code DECICE. These works include a wide range of ice-structure and ice-ship interaction problems of current interest, i.e. ice loads on conical structures, jamming of floes at bridge piers, modelling of the mechanical behaviour of ice rubble, pack ice stability and associated forces on offshore structures, rubble loads exerted on an inclined retaining wall, ridge keel resistance during seabed scouring, dynamic response of a moored conical drill-ship in ice and ship manoeuvring performance in ice. Representative simulations for each case are presented including load, motion and/or interaction process, whichever is appropriate. The simulations from DECICE were compared with experimental data and found satisfactory in terms of accuracy and real-time simulation capability. The accuracy is important for design and engineering of marine structures and ships, whereas the real-time simulation capability allows it to be used in marine simulators for personnel training and marine operation assessment. The performance of DECICE is also addressed and improved via the implementation of a new contact detection sequence and parallel considerations.

A parameterization of the ice‐ocean drag coefficient

A parameterization of the ice‐ocean drag coefficient (Cw) was developed through partitioning the oceanic drag force into three components: (1) form drag on the floe edge, (2) form drag on the ridge keel, and (3) skin friction on the ice bottom. Through these quantities, Cw was expressed as a function of observable sea ice geometric parameters. Sensitivity studies were carried out to investigate the influence of varying sea ice conditions on Cw. The results revealed that Cw increases first and then decreases with increasing ice concentration (A), similar to the observations of the air‐ice drag coefficient, and which is mainly attributed to the nonmonotonic variation of the form drag on the floe edge with ice concentration. Moreover, the form drag on the floe edge is always the dominant component, having a proportion of more than 60% in sea ice with a large aspect ratio (draft/length ≥ 1/100), indicating the necessity of including this term in sea ice dynamic models, particularly for the marginal ice zone (MIZ). The form drag on the ridge keel becomes dominant only when the ridging intensity is extremely high (depth/spacing ≥ 1/20). Additionally, a large value of Cw cannot be caused only by the inclusion of form drag terms but also by large skin friction over rough ice bottoms. Finally, for typical situations in the MIZ with moderate ridging intensity, the parameterization will underestimate Cw by approximately 30% for a rough ice bottom and by over 80% for a smooth ice bottom if no form drags are considered.

Numerical modelling of ice ridge keel action on subsea structures

Abstract The present paper describes a numerical simulation of the keel–structure interaction where the keel geometry, ice thickness and interaction speed present a scaling ratio of 1:20. The rubble is represented by a Drucker–Prager material with cohesive softening and no dilatation. The partial differential equations are solved with the non-linear Eulerian finite element method of Abaqus Explicit V6.8.2. The results are compared to physical experiments conducted in the ice basin of the Hamburg ship model basin (HSVA). Two identical cubic subsea structures were impacted into the unconsolidated keel portion of two ice ridges with different thermal properties. The results indicate that only slight dilatation occurs, and the models are able to estimate the rubble action and deformed shape. A progressive failure of the rubble occurs. The rubble action is influenced more by the friction angle than it is in punch tests, due to higher confinement at the failure surface.

Mechanical properties of model ice ridge keels

Abstract Rubble ice presents pressure dependent yield strength and its behaviour can be described by mathematical models based on several mechanical parameters. They are investigated for HSVA model rubble ice through the analysis of three different tests: the oedometer test, the pile test and the punch test. This last test is analysed with the non-linear Eulerian finite element method. The tests were performed on 4 ice ridges with two different submersion times. A 0.5 to 1.2 kPa model scale Mohr–Coulomb cohesion (0.6 to 1.5 kPa Drucker–Prager cohesion), depending on the ridge history, was used in the simulations of the model scale punch tests. The friction angle is estimated between 30 and 45° (40 and 50° Drucker–Prager friction angle). The upper value was used in the punch test simulations. A 0.9 MPa Young modulus was derived and the hydrostatic compressive yield curve was determined. The numerical model is able to estimate the rubble action during the entire penetration of the punch test in the keel and it is shown that a cohesive softening occurs in the rubble. In order to reproduce the experimental load time series for the short submersion time ridges it was necessary to use a vertical distribution of the cohesion representing the vertical distribution of the freeze-bond strength. A sensitivity analysis of the punch test shows that the keel depth and the ice density are the main parameters governing the keel frictional resistance. A precise determination of these parameters is therefore crucial for a correct determination of the rubble mechanical properties from the numerical simulation of experimental punch tests. The punch test is not appropriate for the determination of the friction angle due to the low confinement pressure at the failure plane. The numerical analysis of the punch test allows the estimation of different assumptions used in analytical models for the rubble failure: the cohesion averaging is an under-conservative approximation, and the non-simultaneity of the cohesive and shear resistance maximum values can be considered in the peak load estimation by the computation of their quadratic mean. The comparison with full scale values shows a reasonably good scaling of the cohesion for the model ice ridges with a long submersion time.

Ice ridge keel characteristics and distribution in the Northumberland Strait

Ice ridges impacts are a major design consideration for offshore structures in the Arctic. Consequently, field programs on the frequency and characteristics of ridges can provide valuable empirical information for the design of future offshore structures. The Confederation Bridge ice force monitoring program monitors ice ridge keels through sonar instrumentation at the bridge. A total of 3199 keel cross-sections were identified during the 2007 and 2008 ice seasons using a new processing method. Of the 3199 keels identified, 137 keels caused loads over 1 MN. The shape of each keel was visually identified and classified as one of four shapes: triangular, trapezoidal, w-shaped, and multiple peak keels. Triangular and trapezoidal keels made up more than 60% of the keels that cause loads over 1 MN. Four of the five largest loads had keels that were trapezoidal. For the 3199 keel cross-sections, the depth, width, leading and trailing keel angles, bottom width and area were identified and distributions of these keel properties were developed. The distribution was then compared to the design keel depth distribution from the Confederation Bridge. The observed 2007 and 2008 keel depth distributions was slightly lower than the design distribution likely due to the significant number of multiple peak and w-shaped keels. Additionally, the maximum keel depth suggested by the observed distribution was shallower than the maximum predicted keel depth.

Preparatory Tests with an Explorer Class Autonomous Underwater Vehicle for Missions under Sea Ice

The autonomous underwater vehicle (AUV) operations described here are preparatory missions to enable operation of an AUV under sea ice in polar regions. The proposed polar projects include operations under land fast ice in the Arctic through an ice moon pool and a planned project to assess sea ice mass balance and habitat assessment in the Southern Ocean in East Antarctica. This paper focuses on the preparatory missions, done in open water, and the planned Southern Ocean project. The plan is to use an autonomous underwater vehicle (AUV) for the under ice component of measurements. The ultimate goals are to quantify the size and shape of ridge keel structures and their contribution to the sea ice mass balance over a study region; to understand the processes that link sea ice with the distribution of ice algae and krill; to provide the necessary field measurements, over sufficiently large areas, for the calibration/validation of satellite and aircraft-based measurements of the sea ice and snow cover thickness; and to provide baseline measurements of sea ice thickness for future climate monitoring.