Ice Actions Research Articles

Aspects of Designing the Ice-Resistant Drilling Platform to be Operated in the Taz Bay of the Gulf of Ob

Prospects for presence of gas production areas of the Taz Bay of the Gulf of Ob have been determined by the authors of the paper. Approaches to determining the optimal ice-resistant shape of the platform support base are considered. Scenarios of extreme ice impacts on structures operated in the Gulf of Ob have been determined. An assessment of global actions to occur during interaction of platforms with ice features was given, which directly affects the choice of the platform design and the way of securing it on the seabed. A calculation of global ice actions, determined by both domestic and foreign procedures, was performed. The obtained results of global ice actions, determined by use of several procedures prescribed by the main regulatory documents, were analyzed in various scenarios of the “platform/ice features” interaction. The prospects of further research were investigated.

Modelling of the Dynamically Positioned vessels and managed ice-field interaction dynamics using empirical-statistical techniques

Empirical models based on measurements are often preferred over high-fidelity numerical models when real-time predictions of broken ice-field actions on ships and floating offshore platforms are needed. This paper presents the methodologies and validations of a novel numerical model based on empirical-statistical techniques for predicting dynamically positioned (DP) vessels and ice-field interactions. The authors developed multiple regression models for predicting the time-averaged and average peaks of thruster forces and yawing moment of the DP vessel due to managed ice actions. Monte Carlo Simulations and other empirical techniques were then developed to predict the time series of the ice forces and moment on the DP vessel in managed ice scenarios. The authors then carried out model validations for several ice-field and vessel operating scenarios. The model showed reasonable accuracy in predicting the effects of several ice-field parameters on the forces and moments of two DP-controlled vessels. Subsequently, integration of the model into a DP-in-ice validation platform is offered, which can simulate and optimize the performance of a DP-controlled vessel in ice-infested water. Finally, the uniqueness and limitations of the models are discussed, and recommendations for future work are provided.

Numerical simulation of multi-tug towing of a gravity-based structure in broken sea ice

Towing is a critical mode of transportation for offshore platforms without self-propelled capability. In Arctic regions, the existence of ice floes might pose additional challenges to towing operations. It is essential to develop a reliable prediction of broken ice actions on towing systems in ice infested waters, especially for large scale floaters, which require assistance for navigation from multiple tugs in coordination. This paper proposes a numerical model for simulating towing process in broken ice, considering the coupled motions of multiple tugs, towed platform, associated with ice floes. The centroidal Voronoi tessellation (CVT) algorithm is adopted to generate a random broken ice field with polygon-shaped ice floes. Ice-ice and ice-structure contacts are simulated through discrete element method (DEM), which are verified by existing data of model tests. A catenary towline model is used to connect tugs and towed platform. A numerical case study of large volume gravity-based structure (GBS) towing is conducted to analyze the dynamics of towing system. General effects of different ice characteristics are discussed to provide insights on towing design and towing safety in broken ice.

Modelling of Dynamically Positioned vessels and managed ice-field interactions using multiple regression techniques

Empirical models based on full-scale or model scale measurements are often preferred over high-fidelity numerical models when real-time predictions for ice actions on ships and floating offshore platforms are needed. This paper presents multiple regression-based empirical models for predicting a Dynamically Position (DP) controlled vessel and managed ice-field interactions developed using a large dataset from ice basin tests of two distinct DP-controlled vessels. The authors carried out a detailed analysis of measurements from two physical modelling programs for developing a unique database. A multiple regression modelling procedure was developed to predict the time-averaged and average peaks of forces and moment of the DP-controlled vessels due to ice actions in multiple configurations of managed ice-field. The models were validated using model scale measurements for the predictions of time-averaged and average peak responses. The models were then used to evaluate the effects of each ice-field parameter on vessel forces and moment and interaction event parameters. The models presented in this paper showed reasonable accuracy in predicting the effects of several ice-field parameters on the forces and moments of two distinct DP-controlled vessels in a range of realistic operating conditions.

Seismic performance analysis of a wind turbine tower subjected to earthquake and ice actions.

Sea ice is one of the main loads acting on a wind turbine tower in areas prone to icing, and this threatens safe working life of the wind turbine tower. In our study, a simplified calculated model of ice, wind turbine tower, and water dynamic interaction under earthquake action was proposed, which could avoid to solve a large number of nonlinear equations. Then, the seismic behaviour of the wind turbine tower with and without the influence of sea ice was investigated, and we found that the influence of the greater mass of the sea ice on the seismic response of a wind turbine tower should be considered when the wind turbine tower is designed in an area with thick ice. With the influence of the most unfavourable ice mass, the deformation and energy dissipation capacity of the wind turbine tower are decreased, and the wall thickness or stiffening rib thickness should be increased to improve the seismic performance and ductility of the wind turbine tower; the shear force and bending moment increased significantly on the wind turbine tower, and the shear force changes at the bottom of the wind turbine tower and position of action of the sea ice: attention should be paid to the wind turbine tower design at these positions. Finally, we conducted the shaking table test, and verified the rationality of our proposed simplified model.

Medium-scale experiment in consolidation of an artificial sea ice ridge in Van Mijenfjorden, Svalbard

This study characterizes a consolidation of undeformed level ice and ice ridges. Field investigations were performed in the Van Mijenfjorden, Svalbard for 66 days between February and May of 2017. The thickness and properties of the level ice that was used to make the ridge were measured, and thermistor-strings were installed in the ridge and the neighboring level ice. The ridge was visited four times for drilling and sampling. During our field experiment, the level ice (LI) grew from 50 to 99 cm, the consolidated layer (CL) grew up to 120 cm, and the ridge initial macroporosity was about 0.36. The experimental results provided enough information for accurate growth prediction and validation of ridge consolidation models.Two analytical resistive models and two-dimensional discretized numerical models are presented. All models need general met-ocean conditions and general ice physical properties. The ridge model includes the effect of the inhomogeneous top and bottom surfaces of the consolidated layer. The models were validated against the field measurements, and the further details of the analytical models were validated against the numerical model.The analytical resistive ridge model with convective atmospheric flux captures the relevant phenomena well and could be used for prediction of the consolidated layer thickness in probabilistic analysis of ice actions on structures. The model including the radiative terms predicted heat fluxes in level ice and ridge better than the convective model but required more input data. Vertical temperature profiles through the consolidated layer and further into respectively a void and an ice block may result in significantly different estimations of the consolidated layer thickness. The difference between fresh and saline ice growth is becoming significant only during the warming phase due to significant change of sea ice microporosity.

Glacial ice impacts: Part II: Damage assessment and ice-structure interactions in accidental limit states (ALS)

Floating glacial ice features of various sizes pose a substantial threat to the structural integrity of offshore structures in ice-prone regions. Relatively small ice feathers, e.g., bergy bits and growlers, are a major concern because they are more difficult to detect by marine radars and perform concurrent ice management operations, especially in extreme sea states.This two-part companion paper aims to assess potential loads of accidental ice actions that are exerted on an offshore semi-submersible platform and the consequences of these actions. Paper I investigates the probability distributions of glacial ice impact velocities and the associated impact heights under different sea states. Given the critical impact energy identified from Paper I, Paper II investigates ice crushing and structural damage during glacial ice impacts in accidental limit states (ALS) and ice-structure interactions. Three different approaches, i.e., the integrated approach, weakly coupled approach and fully coupled approach, are employed. The methods are based on different coupling strategies between ice and the structure and are used to identify the critical local ice sharpness for stiffened panels of the platform column, which causes maximum structural damage given an energy demand. The accuracy of the simulation results, calculation efficiency and scope of application using the three approaches are compared and discussed.With the identified critical sharpness of the ice model, numerical simulations are performed to impact various locations of the platform column. Apart from scenarios with head-on ice impacts, several oblique impact scenarios are simulated, where the glacial ice feature slides along the contact plane with different initial indentations of the panel. The results are discussed with respect to the resistance and energy absorption of the ice and structure and ice-structure interactions.

Research of R/V “Akademik Tryoshnikov” hull structures response to ice actions during the first stage of the expedition “Transarktika-2019”

At present, oil-producing offshore structures, as well as transport vessels transporting oil products extracted in the seas of the Russian Arctic are equipped with ice load monitoring systems (ILMS) to prevent environmental disasters. The ice-resistant self-propelled platform (IRSPP) that is under construction now, according to the design should be equipped with this system. First of all this system is the main system for ensuring the platform’s safety in ice conditions, and secondly makes the platform’s hull a unique tool for solving a wide range of tasks to study the effects of ice on any construction.The main goal of the research during the expedition “Transarktika-2019” was to obtain the necessary data for the development of an ice load monitoring system of the constructing IRSPP “North Pole” and testing the prototype of the ILMS at long vessel’s drift in ice.The measurements of stresses in the hull structures of the R/V “Akademik Tryoshnikov” were carried out during impacts on ice ridges and during ice compressions.The standard ship ice load monitoring system (SILMS) of the R/V “Akademik Tryoshnikov” and strain gauges additionally installed on the frames and a shell plating in the middle part of the hull were used to perform the measurements.The analysis of the obtained data showed that the maximal loads on the hull occurred during the forcing of ice ridges but the level of maximum stresses was not a danger to the hull.Compressions during the drift did not have a strong effect on the ship’s hull. The data obtained made it possible to identify a number of features for the operation of ILMS in similar conditions.Based on the results of the expedition research, recommendations for the design of the architecture of the ILMS for IRSPP were issued. The results of further analysis of the obtained materials will be used in the development of data processing algorithms for ILMS for IRSPP, as well as for the development of the prospective programs of scientific research of deformation, fracture and other processes of various scales that occur in drifting ice during the future drifts of the IRSPP “North Pole”.The authors have no competing interests.

Efficient discrete element simulation of managed ice actions on moored floating platforms

The current design standard lacks the detailed approach to estimate managed ice actions, which in turn forces the industry to rely on ice basin model tests and calibrated numerical simulations. To address the need for a reliable assessment of managed ice actions on large-scale floaters, this paper proposes an efficient two-dimensional discrete element simulation with polygonal elements, which integrates smoothly with a mooring system modeling and a hydrodynamic force representation. The study aims at establishing a range of numerical parameters suitable for managed ice, as well as the boundaries within which the two-dimensional assumption remains acceptable. The study includes validations against the empirical data on isolated ice impacts and the towing model test conducted in the Hamburg Ship Model Basin (HSVA), for which the discrete element simulations demonstrate reasonable agreements. Using this validated tool, this paper conducts a case study using a hypothetical floater based on the Kulluk, a heavily studied conical drilling platform with years of deployment in the Arctic. The study examines the effect of different ice and mooring characteristics on the simulated load, which provides insights on the design of floating platforms and its mooring systems for managed ice conditions.

Stochastic subspace identification of modal parameters during ice-structure interaction.

Identifying the modal parameters of structures located in ice-infested waters may be challenging due to the interaction between the ice and structure. In this study, both simulated data from a state-of-the-art ice-structure interaction model and measured data of ice-structure interaction were both used in conjunction with a covariance-driven stochastic subspace identification method to identify the modal parameters and their corresponding variances. The variances can be used to assign confidence to the identified eigenfrequencies, and effectively eliminate the eigenfrequencies with large variances. This enables a comparison between the identified eigenfrequencies for different ice conditions. Simulated data were used to assess the accuracy of the identified modal parameters during ice-structure interactions, and they were further used to guide the choice of parameters for the subspace identification when applied to measured data. The measured data consisted of 150 recordings of ice actions against the Norströmsgrund lighthouse in the Northern Baltic Sea. The results were sorted into groups defined by the observed ice conditions and governing ice failure mechanisms during the ice-structure interaction. The identified eigenfrequencies varied within each individual group and between the groups. Based on identified modal parameters, we suggested which eigenmodes play an active role in the interaction processes at the ice-structure interface and discussed the possible sources of errors. This article is part of the theme issue 'Environmental loading of heritage structures'.

Large inelastic deformation resistance of stiffened panels subjected to lateral loading

This paper presents a simplified formulation for the assessment of large deformation resistance of stiffened panels subjected to lateral loading. The method is based on rigid plastic material assumptions and the use of yield functions formulated in terms of stress resultants. The method considers the flexibility of the panel ends with respect to inward motion, while the rotational boundary conditions are free or clamped. Concentrated and distributed loads are considered, as is patch loading. The resistance-deformation curves predicted by the proposed method are compared with results from experiments and using LS-DYNA, and good agreement is obtained for panels that are not dominated by shear failure and tripping or local buckling of stiffeners at the early stage of deformation. The formulation provides a useful tool for quick estimates of panels subjected to abnormal or accidental static and transient lateral loads such as ship collisions, dropped objects, explosions, slamming, hydrostatic pressure and ice actions.

Limit state design and methodologies in ice class rules for ships and standards for Arctic offshore structures

Sea ice poses hazards to ships and offshore structures which create challenges for designers. Designers have to take into account the ice actions resulting from the various ice conditions, interactions and operational scenarios which may occur during the lifetime of structures. In this paper the methodologies used in ship and offshore structure standards, such as the Finnish Swedish Ice Class Rules, International Association of Classification Societies Polar Class rules and ISO 19906, are considered with respect to Limit State Design principles and design air temperature definitions. The paper presents an overview of the design approach used in these standards and especially the criteria for the structural limit used. Case study examples based on ship measurements in the Baltic, as well as design air temperature data from the Russian Arctic, are used to highlight the different approaches and their implementation. The analysis shows clearly that explicit design ice action and structural limit are required for the rational basis in the future development of Limit State definitions in standards accounting for ice actions.

Simulating transverse icebreaking process considering both crushing and bending failures

The present numerical model simulates ice actions on marine structures in the vicinity of the waterline caused by breaking of intact ice. Intact ice mainly fail in bending or/and crushing mode on the interaction surface between ice and the hull. In this paper, a new method has been proposed to simulate non-simultaneous crushing failure in time domain based on previous research on simulations of bending failure. The simulated results are also compared with model test results. It shows that the simulated ice loads are in good agreement with the experimental results in terms of mean value, standard deviation, maximum and extreme force distributions, though there are some deviations between predicted and measured results for certain cases.

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.

Time-dependent spatial distribution of thermal stresses in the ice cover of a small reservoir

Static ice loads (ice actions) are a key design parameter for dams in cold climates. However, their theoretical description is still elusive, introducing uncertainty in design and hindering development of remediation measures. We present and analyze measurements of stresses due to thermal loads in a small reservoir in northern Norway. Several weeks of observations, including both cold and warm spells, were well-described by a simple equation that accounts for thermal expansion and temperature-dependent creep. One model parameter was found to depend systematically on the location of measurements within the reservoir. Biaxial stress measurements showed that the stress field was not homogeneous. Results suggest that the stress field in reservoirs should be predictable from first principles with numerical methods and point toward a promising, simple parameterization.

Sea Ice Problems in Bohai Bay Oil and Gas Exploitation

ABSTRACT Zhang, D.; Xu, N.; Yue, Q., and Liu, D., 2015. Sea ice problems in Bohai Bay oil and gas exploitation. Sea ice infests the Bohai Bay every winter. Normally, ice condition is slight but sometimes heavy ice conditions may occur. It has been difficult for offshore designers to determine the conditions and the ice loads. Two jackets were pushed over by ice and others were designed too conservative as ice load were overestimated. In recent years some of the platforms have met severe ice induced vibrations, which has caused harmful accidents. In this paper, sea ice problems in oil and gas exploitation of Bohai Bay are briefly introduced. The dynamic behavior of ice resistant jacket structures and the potential failure modes that arise from cyclic ice actions are described.

Influence of pitch motion on level ice actions

The effect of pitch motion on the forces applied to conical structures by ice has been analysed by means of model tests and numerical analyses. The model test setup allowed a direct comparison between a cone in fixed condition and compliant condition in which it was able to rotate only in pitch. A numerical analysis was performed to demonstrate results and conclusions from the model tests.It was determined that the structural response (pitch motion) affected the loading process and the ice forces. The model test demonstrated that compared to the fixed cone, the compliant cone exhibited a reduction in the measured ice forces caused by drifting level ice. The numerical analyses supported the findings of the model test campaign. Several loading conditions were investigated by changing the ice drift velocities. Both the structural response and the loading process of the compliant cone changed under different ice drift velocities due to different dynamic response behaviours. It was concluded that the pitch rotation significantly influenced the ice forces acting on the cone, illustrating the importance of including structural response in the design of Arctic structures when calculating ice actions.

Dynamical Processes in the Drifting Ice in the Arctic Ocean

Information about physical and mechanical processes in the ice based on instrumental observations on the drifting and land fast ice is presented. Methods of measurement of the parameters of ice oscillations and waves in Russian expeditions in the Arctic were used. Sea ice cover reacts to oscillations, surface and internal waves in atmosphere and in ocean. Force of interaction of sea ice generates large-scale effects of self-excited oscillations. Natural dynamical processes in the drifting ice floes (system ice-ice) reflect many forms of deformation and failure of the ice, observed at the interaction in the system ice-structure (lock-in or self- excited vibration). Especially it relates to the effects discussed in ISO 19906 2010 as a lock-in or self-excited vibration. The mechanics of interaction of a sea ice with icebergs also can be considered as analogue of interaction in the system ice-structure. Sea ice cover is a dynamical dissipative system of the geophysical level in which physical-mechanical processes of different scale and intensity occur simultaneously. Study of physical-mechanical properties of sea ice cover is based on consideration of ice as a material, and construction as a geophysical discrete medium which is under impact of atmosphere and ocean (1). Periodic horizontal shears on significant space of ice are registries; they are related to the class of self-excited oscillations. The mechanical processes in dynamic system ice-ice are very similar to processes in system ice-structure. Intermittent crushing of the contacts of ice floes, with regimes of resonance events, chaotic vibrations and impacts in ISO 19906 2010 (2) are revealed It follows a necessity of a complex investigation of the sea ice for solution of weather and climate tasks and ice technology interconnected with ice impact to offshore engineering structures. Scientific and applied problems of the sea ice are interconnected and are solved by experimental and theoretical methods. A structure of the sea ice cover is analyzed by using theories of elasticity, viscosity and plasticity. Thus general strategy of investigation was realized at natural experiments and monitoring of the mechanical state of land fast and drifting ice in the Arctic. As a rule, except for traditional measurements, two main tasks were set: — investigation of physical-mechanical processes of deformation and failure of ice to obtain data for development of forecast method of ice state and its failure in advance of several hours; — consideration of possibility in the future use of obtained data for solution of tasks of ice bearing capacity and evaluation of ice actions onto shores, bottom and structures of the Arctic shelf. The technology of monitoring ice state is based on interconnected tasks of ice mechanics, morphometry and geophysical processes, which occur under action of external forces from atmosphere and water. The inverse tasks allow using mechanical changes in ice and giving estimate of forces, which cause these changes. It makes a possibility of prediction of time and place of extreme ice phenomena, which is especially important during exploitation of the offshore structures and navigation in ice. Such approach allows to integrate applied and scientific tasks: evaluation of ice strength of local and meso scale, short-term forecast of sea ice compression, evaluation of ice loads on structures.

Fatigue damage of ship hulls due to local ice-induced stresses

Fatigue damage in a ship hull could lead to leakage of ballast or cargo, water intrusion, degradation of hull strength, or even worse, i.e. catastrophic structural failure. For ships navigating in ice-covered waters, fatigue damage due to ice actions can be of importance. The present paper proposes a systematic procedure for fatigue damage assessment in relation to ships navigating in ice-covered waters. Closed form expressions for the fatigue damage based on the Palmgren–Miner’s rule are derived for several different statistical models of the stress amplitudes, i.e. the exponential, the Weibull’s, and the three-parameter exponential distributions. Additional expressions are also provided when stress concentration factor and/or a lower threshold of the stress amplitude are taken into account. The proposed procedure for fatigue damage assessment reflects the variation in ice conditions, vessel’s speed, and operational modes. Some practical concerns related to application of the proposed procedure are also discussed. A particular example of fatigue damage calculation is presented, where data obtained from full scale measurement performed on board of the vessel KV Svalbard in the winter 2007 are utilized.

Performance, Operability, and Risk Assessment of the Shtokman Floating Platform in Ice

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 22100, ’Approach to Performance, Operability, and Risk Assessment of the Shtokman Floating Platform in Ice,’ by Pavel Liferov, Guillaume Le Marechal, Marc-Marie Albertini, Michel Metge, and Erik ter Brake, Shtokman Development A.G., prepared for the 2011 Arctic Technology Conference, Houston, 7-9 February 2011. The paper has not been peer reviewed. Copyright 2011 Offshore Technology Conference. Reproduced by permission. The Shtokman gas/condensate field (SGCF) is 610 km from Murmansk, Russia, in the Barents Sea. The water depth is approximately 340 m. The offshore facilities of the SGCF Phase-1 development will include an ice-resistant ship-shaped disconnectable-turret-moored floating platform (FP). Significant sea-ice invasions occur at Shtokman approximately 3 out of every 10 years. Icebergs also may occur in the SGCF area. Ice and iceberg management is planned to support the FP operations. The performance, operability, and risk of the FP in waters where occasional invasion of sea and glacial ice is anticipated were assessed. Introduction The main challenge was to achieve an appropriate reliability level at acceptable costs, while minimizing operational downtime. The main ice-related challenges identified in the prefront-end engineering and design (FEED) and updated during the FEED included FP loss of station keeping because of actions from sea ice, FP loss of station keeping because of actions from icebergs, and underhull ice interaction in the riser-connection area. It was decided early on to establish challenging but realistic design targets for the main systems (e.g., hull, mooring, and disconnection system) and, in parallel, work on design of operational measures, including assessment of efficiency and reliability. Further, quantitative operability and risk assessments of the entire system were performed to evaluate potential optimization needs. Ice-Related Design and Operating Philosophy The FP was designed as an ice-resistant disconnectable production unit, able to withstand independently almost all ice and iceberg actions expected to occur at Shtokman. The following conditions are the design limits, and acceptable response criteria under these conditions and interaction scenarios must be satisfied. Ultimate-limit-state (ULS) mooring safety factor under: Head-on interaction with unmanaged 100-year-return-period (YRP) ice ridge (with corresponding companion environmental actions) Ice vaning in unmanaged 100-YRP level ice (with corresponding companion environmental actions) Interaction with 10,000-YRP iceberg in open water (for demonstration purpose, not for use in operations) 5000-t horizontal load (as practically achievable value, on the basis of results from initial studies)