Dynamic Ice Forces Research Articles

Investigating the Ice-Induced Fatigue Damage of Offshore Structures by Field Observations

The oil and natural gas resources of the Bohai Sea are mainly marginal oil fields, and there are currently a large number of structures that are approaching or have reached the end of their service life called aging structures. The interactions between ice and offshore structures could lead to significant ice-induced vibration. Ice-induced vibrations may evoke fatigue damage in tubular joints, which results in severe dangers for offshore platforms in the Bohai Sea. Dynamic ice force models and sea ice fatigue environmental parameters have not been well developed in the current design codes. It is not accurate to evaluate the fatigue damage of structures only with numerical simulation. In this paper, a faster method for evaluating fatigue damage on aging structures is proposed. Firstly, the approximate linear relationship between the fatigue hot spot stress and the vibration response of the deck structure has been discovered using dynamic analysis for jacket platform of Bohai Sea. Then, the procedure of the fatigue damage evaluation of the aging structures in the Bohai Sea is established. Finally, the numerical simulation of the structure is carried out considering ice thickness, ice velocity, ice direction, and action height of sea ice. Fatigue damage is calculated by the fatigue hot spot stress and the Palmgren–Miner rule. The measured data on the platform in the Bohai Sea are selected to obtain the fatigue hot spot stress using a mathematical model. The fatigue damage results considering the actual ice conditions of a jacket structure in the Bohai Sea are compared using the proposed method and finite element analysis. The comparison of the results verifies the rationality of the method proposed in this work.

Numerical study of an ice-offshore wind turbine structure interaction with the pile-soil interaction under stochastic wind loads

In order to better understand the ice-induced collision/vibration mechanism of offshore wind turbines (OWTs) with ice-breaking cones, the Tsai-Wu yield criterion of ice for bending failure is coupled to the nonlinear collision simulation tool ANSYS/LS-DYNA in this paper, and a numerical approach is adopted to predict dynamic ice force. Through comparisons of ice force between simulations and physical tests, the accuracy of dynamic ice force simulated by the proposed approach is verified. Meanwhile, to consider the pile-soil interaction (PSI) effect, the Euler-Bernoulli Beam theory and the Timoshenko Beam theory are used, respectively, and the dynamic characteristics of the OWT are compared and analyzed. The discrepancies between the calculation results of the two methods and the influence of PSI on OWTs under combined wind-ice conditions are discovered. Further, the fully coupled interaction model between ice and flexible OWT with PSI under wind and ice loadings is established. Based on the full three-dimensional (3D) interaction model, the influence of the ice velocities, ice thicknesses, and cone angles on the dynamic ice force and structural response is investigated. The area damage rate and fatigue damage rate of the monopile foundation are proposed to further carry out damage and fatigue analyses under wind and ice loads. Higher fatigue damage is associated with smaller area damage rate. In addition, the ice thickness has a more significant effect on OWT damage and fatigue compared with the ice velocity.

Study on the Dynamic Ice Load of Offshore Wind Turbines with Installed Ice-Breaking Cones in Cold Regions

The dynamic ice load of conical offshore wind turbines (OWTs) in cold regions is unclear. The ice force period is the key parameter used to establish an ice force model for conical structures. To obtain ice load data, a field monitoring system was installed on an OWT in a cold region in China. Based on the monitoring data, a new formula for calculating the ice force period of conical structures was established. By comparing the period calculated with this formula and the measured ice force period, it was found that the calculated data generally agreed with the measured data. Then, a random dynamic ice force model for conical OWTs can be established. Based on this ice force model, the ice-induced vibration of an OWT was analyzed with the ANSYS finite element software. The results are in good agreement with the measured data obtained from the OWT in the time and frequency domains. Therefore, the random dynamic ice force model established in this paper can be used to evaluate the ice resistance performance of conical OWTs in cold regions.

GPU-Based DEM Simulations of Global Ice Resistance on Ship Hull During Navigation in Level Ice

The ice resistance on a ship hull affects the safety of the hull structure and the ship maneuvering performance in ice-covered regions. In this paper, the discrete element method (DEM) is adopted to simulate the interaction between level ice and ship hull. The level ice is modeled with 3D bonded spherical elements considering the buoyancy and drag force of the water. The parallel bonding approach and the de-bonding criterion are adopted to model the freezing and breakage of level ice. The ship hull is constructed with rigid triangle elements. To improve computational efficiency, the GPU-based parallel computational algorithm was developed for the DEM simulations. During the interaction between the ship hull and level ice, the ice cover is broken into small blocks when the inter-particle stress approaches the bonding strength. The global ice resistance on the hull is calculated through the contacts between ice elements and hull elements during the navigation process. The influences of the ice thickness and navigation speed on the dynamic ice force are analyzed considering the breakage mechanism of ice cover. The Lindqvist and Riska formulas for the determination of ice resistance on ship hull are employed to validate the DEM simulation. The comparison of results of DEM, Lindqvist, and Riska formula show that the DEM result is between those the Lindqvist formula and Riska formula. Therefore the proposed DEM is an effective approach to determine the ice resistance on the ship hull. This work can be aided in the hull structure design and the navigation operation in ice-covered fields.

Dynamic ice force estimation on a conical structure by discrete element method

This paper aims to numerically estimate the dynamic ice load on a conical structure. The Discrete Element Method (DEM) is employed to model the level ice as the assembly of numerous spherical particles. To mimic the realistic fracture mechanism of ice, the parallel bonding method is introduced. Cases with four different ice drifting velocities are considered in time domain. For validation, the statistics of time-varying ice forces and their frequencies obtained by numerical simulations are extensively compared against the physical model-test results. Ice properties are directly adopted from the targeted experimental test set up. The additional parameters for DEM simulations are systematically determined by a numerical three-point bending test. The findings reveal that the numerical simulation estimates the dynamic ice force in a reasonably acceptable range and its results agree well with experimental data.

Risk assessment of sea ice disasters on fixed jacket platforms in Liaodong Bay

Abstract. Sea ice disasters seriously threaten the structural safety of oil platforms in the Bohai Sea. Therefore, it is necessary to carry out a risk assessment of sea ice disasters on oil platforms in the Bohai Sea. In this study, a risk assessment of sea ice disasters on fixed jacket platforms in Liaodong Bay, in the Bohai Sea, was performed in five steps. Firstly, the formation mechanisms of sea ice disasters were analyzed and the sources and modes of sea ice risks were summarized. Secondly, according to the calculation formulas of extreme ice force, dynamic ice force, and accumulated force, several ice indices such as thickness, motion, strength, period, and concentration were proposed as the hazard indices, and corresponding values were then assigned to the proposed indices based on ice conditions in the Bohai Sea. Thirdly, based on four structural failure modes – structural overturning by extreme ice force (Mode 1), structural fracture failure caused by dynamic ice force (Mode 2), the damage of facilities caused by dynamic ice force (Mode 3), and structural function failure caused by accumulated ice (Mode 4) – the structural vulnerability index, overturning index, dynamic index, ice-induced vibration index, and function index were proposed, and corresponding values were assigned to the structural vulnerability index of fixed jacket platforms in Liaodong Bay. Fourthly, the weight of each risk index was determined according to previously recorded sea ice disasters and accidents, and the sea ice risk was then calculated with the weighted synthetic index method. Finally, with the above index system and risk assessment methods, the risk assessment of sea ice disasters on 10 jacket platforms in three sea areas in Liaodong Bay was carried out. The analysis results showed that efficient sea ice prevention strategies could largely mitigate the sea ice-induced vibration-related risks of jacket platforms in Liaodong Bay. If steady-state vibration occurs (usually in front of the vertical legged structure) or the structural fundamental frequency is high, the structural vulnerability is significantly increased and the calculated risk levels are high. The sea ice risk assessment method can be applied in the design, operation, and management of other engineering structures in sea ice areas.

Classification criterion of narrow/wide ice-resistant conical structures based on direct measurements

Ice-induced structural vibration generally decreases with an increase in structural width at the waterline. Definitions of wide/narrow ice-resistant conical structures, according to ice-induced vibration, are directly related to structure width, sea ice parameters, and clearing modes of broken ice. This paper proposes three clearing modes for broken ice against conical structures: complete clearing, temporary ice pile up, and ice pile up. In this paper, sea ice clearing modes and the formation requirements of dynamic ice force are analyzed to explore criteria determining wide/narrow ice-resistant conical structures. According to the direct measurement data of typical prototype structures, primary quantitative criterion of the ratio of a cone width at waterline (D) to sea ice thickness (h) is proposed. If the ratio is less than 30 (narrow conical structure), broken ice is completely cleared and a dynamic ice force is produced; however, if the ratio is larger than 50 (wide conical structure), the front stacking of broken ice or dynamic ice force will not occur.

Ice force identification on the Norströmsgrund lighthouse

Ice forces on bottom-founded structures can be measured by load panels or identified from response measurements. This paper presents a comparison between the measured and identified dynamic ice forces acting on the Norströmsgrund lighthouse. The dynamic ice forces are identified from the measured responses using a recently developed joint input-state estimation algorithm in conjunction with a reduced-order finite element model. A convincing agreement between the measured and identified forces was found. The response predictive ability of the algorithm is further used to estimate the response of the structure at unmeasured locations including at the ice-action point.

Experimental study of dynamic conical ice force

Dynamic ice force is one of the key factors in the design and operation of narrow offshore structures in ice-covered areas. In the paper, a series of physical model tests were carried out to simulate field ice conditions on the oil jacket structures with an ice breaking cone in the Bohai Sea. The model tests involved two geometric scale factors (3 and 6), 3 structural models, and 3 ice sheets. In the total 17 groups of tests, the controlled parameters of ice velocity, ice friction coefficient, and the shape of the ice-resistant structure at the waterline were different. The time history of ice forces, accelerations of the model, and the ice–cone interaction process were recorded synchronously. Ice failure modes and global ice loads obtained from the model tests were compared with the data measured on the JZ20-2MUQ oil jacket platform. Similar to the results in the field measurements, two ice failure modes were observed on the cones in the lab test. The plate failure mode was observed on the inverted cone. For the upright cone, when the cone diameter was increased, the ice failure mode was transformed from the wedge failure mode into the plate failure mode and the ice load on unit area was decreased.

Study on a Simplified Calculation Method for Seismic Response Analysis of Bridge Pier in Icy Water

Under earthquakes, bridges in icy water are subjected to both hydrodynamic pressure and dynamic ice force, and yet effective calculation methods for seismic analysis are not available. In this article, based on the test and numerical study on hydrodynamic pressure and dynamic ice force, a simplified calculation method for seismic response analysis of bridge piers in icy water was established to analyse the influence of free ice and consolidated ice on seismic responses of the pier. Comparison was carried out between the FEM analysis and shake table test, and the results showed that the simplified calculated method is efficient and accurate.

Field test system for investigating dynamic ice forces on jacket structures and platform safety guarantee in the Bohai Sea

In order to investigate the dynamic ice forces acting on compliant jacket platforms and to guarantee the platform safety in the Bohai Sea, an extensive measuring system is planned and installed on several prototype structures, which consists of: (1) specially designed load panels to detect dynamic ice forces of high fluctuating frequencies; (2) suitable sensors to measure structure topside's acceleration or displacement; (3) high-definition video cameras to record videos of ice, which are employed to observe the ice–structure interaction phenomenon and to calculate ice thickness/velocity; and (4) meteorological station to record corresponding information like air temperature, wind speed and direction etc. All the signals from different devices are concurrently processed and stored into a central computer. The measuring system has provided fruitful information for the research of dynamic ice force. Meanwhile the structural acceleration response prediction model and the platform safety guarantee system in the Bohai Sea were established based on the measuring system. Some typical events and results are shown in the paper.

Dynamic Ice Force Analysis on a Conical Structure Based on Direct Observation and Measurement

In order to study dynamic ice force induced by ice-structure interaction, we adopted the most reliable method to directly measure ice force on full-scale structure. This paper mainly demonstrates the qualitative description on the basic model for dynamic ice forces based on direct measurement on the jackets with ice-breaking cone in the Bohai Sea. Temporal variations of ice force are recorded by the ice load panels, and corresponding ice failure processes on conical structures are recorded by video camera. It is found that, when an ice sheet acts on the upward narrow cone, bending failure occurs and broken ice pieces are completely cleared up by the side of the cone. The basic form of dynamic ice force in time domain is a series of impulse signals with minimum load of zero.

Discrete element modeling of ice loads on ship hulls in broken ice fields

Ice loads on a ship hull affect the safety of the hull structure and the ship maneuvering performance in ice-covered regions. A discrete element method (DEM) is used to simulate the interaction between drifting ice floes and a moving ship. The pancake ice floes are modelled with three-dimensional (3-D) dilated disk elements considering the buoyancy, drag force and additional mass induced by the current. The ship hull is modelled with 3D disks with overlaps. Ice loads on the ship hull are determined through the contact detection between ice floe element and ship hull element and the contact force calculation. The influences of different ice conditions (current velocities and directions, ice thicknesses, concentrations and ice floe sizes) and ship speeds are also examined on the dynamic ice force. The simulated results are compared qualitatively well with the existing field data and other numerical results. This work can be helpful in the ship structure design and the navigation security in ice-covered fields.

Analogue Simulation of Dynamic Interaction of Sea Ice and a Pier Column Structure

The process of interaction of sea ice and a pier column structure was studied by using analogue simulation method in the paper, and the dynamic ice force acting on the pier column structure was achieved. Combining with theory analysis, it modified the dynamic ice force model suggested by Croteau. The results showed that the loading stiffness suggested by Croteau is too big to use in the dynamic ice force model and the Withalm formula and simulated results are more suitable. It suggested the calculation formulas of relative displacement in ice force unloading stage and the continuous ice force in ice force continuous stage.

Dynamic ice forces of slender vertical structures due to ice crushing

Abstract The dynamic ice forces and structure vibrations generated by crushing failure of ice sheet were investigated by full-scale tests conducted on a cylindrical compliant monopod platform in Bohai Bay, China. The load panels recorded three ice force modes distinguished by ice speed, which cause structure quasi-static, steady-state and random vibrations respectively. Based on the tests data, the physical process of dynamic ice forces in crushing failure was probed, by analysis of mechanical behaviour of ice under compressive loading related to relative loading speed of ice-structure interaction. It is proved that the three ice force modes takes place in loading speeds which make ice fail in ductile, ductile–brittle transition and brittle range respectively. In addition, the key problem from the structure design point of view involved in each ice force mode is discussed.

A Spectral Model for Forces Due to Ice Crushing

This paper presents a model of dynamic ice forces on vertical offshore structures. The model concerns a loading scenario where a competent ice sheet is drifting and crushing against the structure. Full scale data obtained on two offshore structures were used in the derivation of a method that applies both to narrow and wide structures. A large amount of events with directly measured local forces was used to derive formulas for spectral density functions of the local ice forces. A nondimensional formula that was derived for the autospectral density is independent of ice thickness. Coherence functions were used to define cross-spectral density functions of the local ice forces. The two kind of spectral density functions were used to obtain the spectral density of the total ice force. The method takes into account both the spatial and time correlation between the local forces. Accordingly, the model provides a tool to consider the nonsimultaneous characteristics of the local ice pressures while assessing the total ice force.

Three-dimensional non-linear finite element model for the Molikpaq, Gulf's mobile caisson

The paper describes the development and testing of a comprehensive finite element model of a mobile arctic caisson which has been used for offshore drilling in the Canadian Arctic, and which has been exposed to significant dynamic ice forces. The specific structure, the Molikpaq, is owned by Gulf Canada. The structure consists of an eight-sided annular steel caisson, whose overall width is 110m. The caisson is set down on a berm such that the effective water depth is approximately 19m. The central core is then filled with sand to provide the overall stability and to increase the resistance to lateral loads. The system is designed for year-round drilling, and therefore must be capable of resisting loads from impacts from thick multi-year ice floes. The model described in this paper consists of a finite element representation of the caisson which is developed using macro-elements to model the complex structural steel arrangement of bulkheads and stiffened steel plates in the caisson. These macro-elements are then combined with a brick element representation of the core, and various alternative models for the berm. The static and dynamic behaviour of the model are tested, and the results compared to the behaviour of the prototype.

Ice interaction with offshore structures

Oil platforms and other offshore structures being built in the arctic regions must be able to withstand icebergs, ice islands, and pack ice. This reference explain the effect ice has on offshore structures and demonstrates design and construction methods that allow such structures to survive in harsh, ice-ridden environments. It analyzes the characteristics of sea ice as well as dynamic ice forces on structures. Techniques for ice modeling and field testing facilitate the design and construction of sturdy, offshore constructions. Computer programs included.

Dynamic tests and analysis of a massive pier subjected to ice forces

This paper describes the field tests and analysis of a massive bridge pier that is subjected to the action of dynamic ice forces. The field results indicate that the pier has a natural frequency of 8.9 Hz and a damping ratio of 0.19 in the fundamental mode of vibration. By using these dynamic characteristics, the response of the pier to dynamic ice forces was analyzed. This analysis confirms that the pier is capable of responding in full to short duration peak ice forces.

Erratum: Dynamic ice forces on piers and piles. An assessment of design guidelines in the light of recent research

The first part of the paper reviews and assesses various analytical approaches and design formulas used to estimate forces due to the impact of moving ice on piles, piers, and towers. The applicati...