Ice Drift Direction Research Articles

Experimental study into the effect of wind-ice misalignment on the development of ice-induced vibrations of offshore wind turbines

The effect of misalignment between wind- and ice loading direction on the development of ice-induced vibrations of offshore wind turbines has been investigated experimentally. In the experiments a hybrid test setup was used to study the structural response to combined loading from physical model ice and numerically applied wind. The motivation for this study was the high uncertainty in the design of offshore wind turbine support structures in cold regions, caused by scarcity of full-scale and model-scale data on ice-structure interaction. Test results revealed that misaligned scenarios result in the development of sustained ice-induced vibrations in the ice load direction. The test results also showed that ice-induced vibrations can develop up to higher ice drift speeds for misaligned scenarios than for aligned scenarios. Both observations are considered to be related to low total damping in the ice drift direction for a misaligned scenario. Further comparison between a 90°-misaligned operational and an aligned idling scenario revealed that wind-induced structural displacements perpendicular to the ice drift direction do not cause the ice to fail. On the contrary, it was shown that the ice constrains the wind-induced motion for low relative velocities between ice and structure. For high relative velocities, wind-induced displacements approach those in open water as the ice fails in rapid succession at the sides of the structure during crushing. The analysis of a misaligned scenario with a smaller misalignment angle revealed that vibrations occur perpendicular to the ice drift direction and are characterized by relatively low amplitude and high frequency. The ice, being in contact with the structure, neither prevented those vibrations nor failed.

Ice rose diagrams for probabilistic characterization of the ice drift behavior in the Beaufort Sea

The purpose of the present paper is to investigate the viability of the so-called ice rose diagram, which is a graphic tool to provide a succinct view of the ice drift information such as speed and direction of ice drift events in terms of their relative distribution at a local observation station to support the dynamic ice-structure interaction in cold regions. The ice drift data is collected by using local subsurface measurements based on acoustic Doppler current profilers (ADCP) in the Beaufort Sea during 2006–2017. Probabilistic analyses are performed in order to characterize the time-series data from these ADCP measurements. Both the ice drift speed and direction are treated as random variables for which analytical probability density functions are fitted. It is found that sea ice in the Beaufort Sea tends to drift at a lower speed during the winter season (i.e. the growth season) than during the summer season (i.e. the melting season). The Weibull distribution provides the most appropriate fit to the data of ice drift speeds during the growth season. For the melting season, a mixed distribution provides the most appropriate fit for the drift speed. Regarding directionality of the ice drift, a mixed Von Mises distribution is applied in order to represent its statistical properties. In this work, the relationship between ice drift speed and wind speed is also studied. It is found that the magnitude of the ice drift speed is approximately 2.5% of the wind speed during the winter season.

Physical model testing I for investigating the effects of managed ice-field characteristics on a dynamic positioning vessel

This paper presents details of an extensive physical model testing program for evaluating the performance of a dynamic positioning (DP) controlled vessel in different realistic managed ice environments. It describes the model vessel, the preparation of the ice-field, the procedure of the model tests, the methodologies of data analysis, and highlights of the measurements. A simplified generic drillship model at 1:40 scale was tested in multiple managed ice configurations. A total of 372 test runs were carried out in 17 ice sheets to evaluate the thruster forces and motion responses of the vessel in various ice conditions. The physical and mechanical characteristics of the ice-field were modelled by controlling ice concentration, thickness, floe size, flexural strength, and the ice drift speed and direction. The ice concentration ranged from a light condition (7/10th) to a heavy condition (9/10th+) with multiple ice floe sizes ranging between 25 m to 100 m. Also, multiple ice thicknesses ranging between 0.60 m to 2.0 m and multiple ice drift speeds (0.2 knots, 0.5 knots, and 1.2 knots) with various moderate to extreme ice drift angles were investigated. The thruster forces on the model vessel along the surge and sway directions and the yawing moment were measured. Besides, the vessel model's 6-DOF motions and accelerations were recorded. Multiple cameras were used to capture the global and local ice-structure interactions placed in different above-water locations. All reported values presented in the paper are in full-scale unless otherwise specified.The time series of thruster forces and yawing moments are presented along with the vessel's positioning capabilities in various managed ice conditions. Results and discussions on the effects of various managed ice-field parameters on the time-averaged thruster forces and moment are offered. Generally, the analysis reveals that each of the ice parameters under investigation had a moderate to strong effect on global thruster force in the surge direction. The measurements and the videos showed complex and highly stochastic ice-vessel-basin wall interactions and different ice failure modes. Overall, the DP system used in the testing demonstrated the vessel's capabilities in maintaining the station for the majority of test head-on test cases and the oblique ice tests not exceeding 10°. The program generated a large dataset and video recordings to evaluate and improve understanding of the dynamic ice-vessel interactions and provided sufficient information for developing novel ice force models for real-time applications.

Patterns of sea ice drift and polar bear (Ursus maritimus) movement in Hudson Bay

Sea ice habitats are highly dynamic, and ice drift may affect the energy expenditure of travelling animals. Several studies in the high Arctic have reported increased ice drift speeds, and consequently, polar bears Ursus maritimus in these areas expended more energy on counter-ice movement for station-keeping. However, little is known about the spatiotemporal dynamics of ice drift in Hudson Bay (HB) and its implications for the declining Western Hudson Bay (WH) polar bear subpopulation. Using sea ice drift data from 1987-2015 and polar bear satellite telemetry location data from 2004-2015, we examined trends in drift speeds in HB, polar bear movement relative to drift, and assessed annual and individual variation. In contrast to other areas of the Arctic, we did not find an increase in ice drift speed over the period examined. However, variability in ice drift speed increased over time, which suggests reduced habitat predictability. Polar bear movement direction was not strongly counter to ice drift in any month, and ice drift speed and direction had little effect on bear movement rates and, thus, energy expenditure. On an annual scale, we found individuals varied in their exposure and response to ice drift, which may contribute to variability in body condition. However, the lack of a long-term increase in ice drift speed suggests this is unlikely to be the main factor affecting the body condition decline observed in the WH subpopulation. Our results contrast findings in other subpopulations and demonstrate the need for subpopulation-specific research and risk evaluation.

Modelling of the passive turning of a turret moored vessel in conditions of compact ice

The concept of the offshore oil and gas field development using floating ship-shaped platforms, frequently named floating production units (FPU further), with turret mooring is widely used in the world, including regions with harsh environment. The direct transfer of this concept to the Arctic seas is not possible, mostly due to the difficulties of maintaining a weathervaning, or passive turning regime in heavy ice conditions. The main danger relates to the fact that the expected FPU rotation under the action of high ice loads can be accompanied by a translational displacement of the FPU away from the mooring point. As a result, the mooring system may reach an overloaded state until the FPU turns to a favourable position relative to the ice drift direction. In the paper, we are focused on the investigation of a mathematical model of the passive FPU turning on a spot under the assumption that the ice cover is described by a rigid-plastic continuum. The study is performed both analytically and by numerical simulations. A number of specific FPU motion patterns are analytically derived from the model in quasi-static approximation in the form of successive limit states of the system FPU – ice continuum. Some results of the corresponding numerical simulation are presented that confirm the existence of similar solutions in the full dynamic setting of the problem. A partial parametric analysis of the problem is also performed.

Near-field ice management tactics – Simulation and field testing

Robust near-field ice management tactics have been developed for use in Arctic floating drilling. The tactics provide high confidence that a station-keeping drilling rig can be kept within the managed ice channel throughout periods of complex changes in ice drift direction and speed, and thus effectively mitigate the potentially high economic cost of an emergency disconnection. Kinematic simulations were used to design the systematic arched racetrack tactics which were implemented and tested during the Oden Arctic Technology Research Cruise 2015 (OATRC2015) conducted in the pack ice north of Svalbard at about 82°N latitude and 16°30′E longitude. The long duration field tests, conducted over 10 days using two icebreakers, show the virtual drilling rig was maintained within the managed ice channel in complex ice drift conditions that included multiple ice drift loops, cusps, and reversals. Moreover, the field observations, including satellite and helicopter imagery, confirm the fundamental simulation methods. The field results are extended through simulations to assess the impact of icebreaker fleet configuration and capability on outlet ice conditions (floe size and brash content) and the performance of the tactics against long-term ice drift records (over 1200 days with over 18,000 km of drift). Results demonstrate the importance of systematically executing the tactics and confirm that the geostationary drilling rig is kept within the managed ice channel.

Operational modal analysis on a lighthouse structure subjected to ice actions

The sea ice interaction with a structure may cause system changes and affect the feasibility of common approaches to track modal parameters. In this paper, a covariance-driven stochastic subspace identification method is used to identify the natural frequencies from 190 time series of ice actions against a lighthouse structure. The results are sorted into groups defined by the observed ice conditions and governing ice failure mechanisms during the ice-structure interaction. The identified natural frequencies vary substantially within each individual group and between the groups. Recordings with flexural failures and a north-south ice-drift direction consistently rendered the same identified frequencies, whereas crushing seemed to create large amounts of variability in the identified frequencies. The results show the need for more high-fidelity data to assess whether modern system identification methods can be used to monitor system changes and support decision-making for operations of structures in ice-infested waters, such as offshore wind structures.

Seasonal and interannual variability of the Arctic sea ice: A comparison between AO-FVCOM and observations

A high-resolution (up to 2 km), unstructured-grid, fully ice-sea coupled Arctic Ocean Finite-Volume Community Ocean Model (AO-FVCOM) was used to simulate the sea ice in the Arctic over the period 1978–2014. The spatial-varying horizontal model resolution was designed to better resolve both topographic and baroclinic dynamics scales over the Arctic slope and narrow straits. The model-simulated sea ice was in good agreement with available observed sea ice extent, concentration, drift velocity and thickness, not only in seasonal and interannual variability but also in spatial distribution. Compared with six other Arctic Ocean models (ECCO2, GSFC, INMOM, ORCA, NAME, and UW), the AO-FVCOM-simulated ice thickness showed a higher mean correlation coefficient of ∼0.63 and a smaller residual with observations. Model-produced ice drift speed and direction errors varied with wind speed: the speed and direction errors increased and decreased as the wind speed increased, respectively. Efforts were made to examine the influences of parameterizations of air-ice external and ice-water interfacial stresses on the model-produced bias. The ice drift direction was more sensitive to air-ice drag coefficients and turning angles than the ice drift speed. Increasing or decreasing either 10% in water-ice drag coefficient or 10° in water-ice turning angle did not show a significant influence on the ice drift velocity simulation results although the sea ice drift speed was more sensitive to these two parameters than the sea ice drift direction. Using the COARE 4.0-derived parameterization of air-water drag coefficient for wind stress did not significantly influence the ice drift velocity simulation.

Impact of surface wind biases on the Antarctic sea ice concentration budget in climate models

We derive the terms in the Antarctic sea ice concentration budget from the output of three models, and compare them to observations of the same terms. Those models include two climate models from the 5th Coupled Model Intercomparison Project (CMIP5) and one ocean–sea ice coupled model with prescribed atmospheric forcing. Sea ice drift and wind fields from those models, in average over April-October 1992-2005, all exhibit large differences with the available observational or reanalysis datasets. However, the discrepancies between the two distinct ice drift products or the two wind reanalyses used here are sometimes even greater than those differences. Two major findings stand out from the analysis. Firstly, large biases in sea ice drift speed and direction in exterior sectors of the sea ice covered region tend to be systematic and consistent with those in winds. This suggests that sea ice errors in these areas are most likely wind-driven, so as errors in the simulated ice motion vectors. The systematic nature of these biases is less prominent in interior sectors, nearer the coast, where sea ice is mechanically constrained and its motion in response to the wind forcing more depending on the model rheology. Second, the intimate relationship between winds, sea ice drift and the sea ice concentration budget gives insight on ways to categorize models with regard to errors in their ice dynamics. In exterior regions, models with seemingly too weak winds and slow ice drift consistently yield a lack of ice velocity divergence and hence a wrong wintertime sea ice growth rate. In interior sectors, too slow ice drift, presumably originating from issues in the physical representation of sea ice dynamics as much as from errors in surface winds, leads to wrong timing of the late winter ice retreat. Those results illustrate that the applied methodology provides a valuable tool for prioritizing model improvements based on the ice concentration budget–ice drift biases–wind biases relationship prevailing in the simulation of Antarctic sea ice over the last decades.

Model Tests With a Compliant Cylindrical Structure to Investigate Ice-Induced Vibrations

A compliant cylindrical structure has been built and tested in a series of model tests in ice in the Large Ice Model Basin at HSVA. The structure's stiffness in ice plane is higher in ice drift direction than crosswise, enabling the model to vibrate in different geometrical oscillation patterns. In total, four ice sheets have been used to perform tests in different ice thickness, covering a wide range of ice drift velocities between 0.005 and 0.15 m/s in model scale. Several events of ice-induced vibrations were observed throughout the test campaign. Oscillations are found to reach different types of beginning steady states, depending on ice drift velocity and ice thickness. Dynamic amplification of structural response in ice plane as well as ratio of static and dynamic forces is highly dependent on the type of vibration. While the dynamic amplification is highest when the ice load's frequency equals the first natural frequency of the structure, the highest dynamic forces occur when the crushing frequency is an integer fraction of the natural frequency. The paper describes the design of the test setup, instrumentation and calibration, performance and analysis of conducted tests, and general findings.

Virtual radar ice buoys – a method for measuring fine-scale sea ice drift

Abstract. Here we present an algorithm for continuous ice drift estimation based on coastal and ship radar data. The ice drift is estimated for automatically selected ice targets in the images. These targets are here called virtual buoys (VBs) and are tracked based on an optical flow method. To maintain continuous ice drift tracking new VBs are added after a given number of VBs have been lost; i.e. they can not be tracked reliably any more. Here we also apply the algorithm to data of three test cases to demonstrate its capabilities and properties. Two of these cases use coastal radar data and one ship radar data. Ice drift velocity and direction information derived from the VB motion are computed and compared to the prevailing ice and weather conditions. Also a quantity measuring the local divergence or convergence is computed for some VBs to demonstrate the capability to estimate derived kinematic sea ice parameters from VB location time series. The results produced by the algorithm can be used as an input for estimation of the dynamic properties of sea the ice field, such as ice divergence or convergence, shear, vorticity, and total deformation.

Heading control for turret-moored vessel in level ice based on Kalman filter with thrust allocation

This paper mainly focuses on the heading control of a position-moored vessel operating under level ice regime. A dynamic ice simulator interconnecting the vessel motions with the ice dynamics is used for the design of the heading control system. The strategy is to ensure that the vessel is kept at an appropriate position within the safe limits. Using a heading controller based on a Kalman filter, the desired control force is computed to counteract the environmental disturbances. A thrust allocation method is developed to go with the heading controller. To keep the ice forces to a reasonable level, the moored vessel should be aligned with the ice drift direction and small angles up to 15° in changes on heading against the ice flow could be possible. Therefore, heading control of a moored vessel exposed to level ice with drift angle 0° and 15° is simulated since the dynamic positioning system needs to resist ice yaw moments and lateral ice forces on the hull. The simulation indicates that the proposed control strategy performs satisfactorily for a moored vessel in level ice.

Numerical and physical simulations of moored tanker behaviour

The behaviour of a tanker moored to a point terminal under a change of ice drift direction has been investigated. One end of the mooring line is fastened to the point terminal and the other one to the mooring device on the vessel's bow. The tanker's motion in drifting broken ice is described by equations of motion, and its behaviour is determined by the balance of the environmental forces (ice, aerodynamic and hydrodynamic) acting on the vessel, as well as of the tighting force of the mooring line, of thrusters’ forces and of inertial forces. For the ice loads determination, the discrete element method is applied: the broken ice is represented by a set of floating disks interacting with each other as viscous-elastic bodies. The results of numerical calculations have been compared with experimental data of analogous model tests performed in an ice tank. It is shown that the developed numerical model may be used for theoretical analysis of behaviour of a moored tanker in drifting ice.

A panel method for modelling level ice actions on moored ships. Part 2: Simulations

A semi-empirical method for calculating local ice forces on moored ships was described in Part 1 and is implemented for a real hull in this article. The method splits the ice actions into actions in the vicinity of the waterline caused by breaking of intact ice and rotation of broken ice floes, and actions below the waterline caused by ice–hull friction. The hull was discretized into flat panels and ice forces were calculated for each panel and summed to obtain the total ice action. The resolution of the discretization was studied and stable results were obtained with relatively few panels. The model was compared with model test data and satisfactory agreement was found for the mean mooring forces. Surge oscillations were larger in the simulations than in the model tests, probably due to underestimated ice-induced damping forces in the simulations. The dynamic surge response of the ship was investigated by varying the ice drift speed and the surge natural period. The coefficient of variation for the surge motions was largest for speeds around 0.10–0.15 m/s for all surge natural periods. The importance of parameterizing the ice forces in terms of the penetration of the ship into the ice is highlighted, as this strongly affects the dynamic response of moored ships in level ice. The model can be applied in studies of dynamic response of moored ships in level ice with constant drift direction and it can be extended to variable ice drift direction.

Arctic Offshore Loading Downtime Due to Variability in Ice Drift Direction

Four arctic offshore loading concepts are selected, loading from the corner of a platform, loading in the wake of a loading tower, Submerged Turret Loading (STL) and Single Anchor Loading (SAL). The influence of variations in the ice drift direction on the performance of these concepts is discussed and critical drift events are determined. Ice drift measurements from eight ARGOS/GPS buoys deployed in the Pechora Sea in winters 1995 and 1998 are analysed to estimate downtime rates of these loading systems due to ice drift heading changes. Depending on the location in the Pechora Sea and the chosen concept, downtime rates range from 6 to 72%. A discussion on how these rates will vary with different assumptions, different ice conditions or different ice management is given. Finally the loading concepts are compared through a qualitative risk analysis.

A numerical study of the western Cosmonaut polynya in a coupled ocean–sea ice model

Employing results from a 0.4°, 40‐level fully global, coupled ocean–sea ice model, we investigated the role of physical processes emanating from atmosphere, ocean, and ice in the initiation, maintenance, and termination of a sensible heat polynya with a focus on the western Cosmonaut polynya that occurred during May–July 1999. The Cosmonaut polynya first appeared in early May 1999 in the form of an ice‐free embayment, transformed into an enclosed polynya on 5–9 July, and disappeared by late July, when the ice from the surrounding regions began to encircle the embayment. Except for the differences in ice concentrations, the time of appearance, size, and shape of the Cosmonaut polynya simulated by the model are in approximate agreement with the Special Sensor Microwave/Imager (SSM/I) observations. Between May and July 1999 the Cosmonaut Sea experienced two synoptic storms, both lasting ∼5 days. Followed by the passage of the first storm on 12–19 June, there was a remarkable growth in the size of the embayment by 21 × 103 km2. Associated with this, the sea surface temperature (SST) rose by 0.15°C, the upward heat flux jumped from 5 to 94 W m−2, and a net freshwater flux into the ocean increased by 2 cm d−1. By running the model simulation with a 20% wind speed increase, it is demonstrated that the twofold increase in SST and upward heat flux increased the embayment area by 15 × 103 km2 and decreased the ice concentration by approximately 10% from the control run. A similar, but somewhat weaker wind event that took place on 30 June to 10 July had less influence on the embayment area although the upward heat flux (65 W m−2) was comparable to the first event. By examining the vertical displacement of the −1.6°C isotherm depth prior to, during, and after these two storms, we demonstrate that the impetus provided by these storms was able to raise the −1.6°C isotherm depth by 30 m through wind‐driven mixing, making sufficient oceanic heat input from beneath the mixed layer available to prevent freezing and/or delay ice formation while ice in the adjacent regions continued to grow. A sudden shift in the ice drift direction from southwest to northeast (3 July) followed by the second storm, accompanied by large air‐sea temperature differences, caused the enclosure of the embayment, subsequent formation of the polynya, and its termination.