Wind turbine towers are being planned in ice covered regions subject to pressure ridges (e.g. the Great Lakes). Conical collars are often employed to reduce ice loads from level ice and their associated dynamics. For level ice, downward breaking cones have some advantages. It is not clear if this is the case for pressure ridges. This paper presents an improved method for ridge loads on wind turbines with downward breaking cones and makes comparisons with upward breaking cones.
First year pressure ridges can be formidable ice features and usually control design ice loads in the sub-Arctic. Important components of a ridge creating ice loads are the consolidated layer at the surface (which is considered as solid ice) and the ridge keel below consisting of ice rubble, but much thicker. The load due to the consolidated layer is usually derived as if it is thick level ice. On a cone, methods for level ice assume it can be idealized as a plate on an elastic foundation (the water) and equations have been developed for upward and downward breaking cones. But for a ridge on a downward cone, to break the consolidated layer downwards requires it to be pushed into the keel rubble below. This will have a different foundation modulus than water buoyancy. A method is developed to account for this difference. The method uses an iterative approach to determine the point of breaking of the consolidated layer (and associated load) accounting for the ridge geometry, keel rubble shear strength, the flexural strength of the consolidated layer and the buoyancy forces. The keel loads on the vertical shaft below the conical collar are based on the method currently in ISO 19906 (2010) but modified to add the effect of additional rubble in the keel from breaking the consolidated layer downwards.
In examples, it is shown that the breaking force can be about twice that of breaking the consolidated layer without the keel present. This might be seen as a disadvantage for downward breaking cones vs upward breaking. However, it is also shown that the clearing forces on an upward cone are higher; which tends to balance out the lower breaking force. Example loads are given on typical wind turbine bases due to typical ridges. Upward and downward breaking configurations are compared.
The paper provides new methods for ice loads due to ridges acting on wind turbine structures not currently covered by existing methods.
Numerous field studies show that mature first-year (FY) ridges consist of a solid ice consolidated layer (CL) below which is a keel of ice rubble. The CL is generally thicker than the ambient level ice. With the decrease in multi-year ice in the Arctic, FY ridges may become the controlling ice feature at many Arctic locations. In sub-Arctic regions ridges are smaller but they also usually control design ice loads. For vertical structures, ISO 19906 (Standard on Arctic Offshore Structures) suggests calculating the load due to crushing of the CL and adding this to the load to fail the keel. In ISO a formula is provided for the load to fail the keel on a vertical-face which assumes that the keel material has Mohr-Coulomb properties. No specific algorithms are given in ISO for FY ridge loads on sloping structures. The work described in this paper addresses this gap.
The study investigated three bounding methods and compared them with the measured loads due to FY ridges on Confederation Bridge (which has piers with upward breaking cones). The three bounding methods are: Model A; which assumes the CL breaks in bending and rides up as level ice and the keel load is calculated assuming a "dead wedge" is created on the slope which converts the slope into a vertical face against which the keel fails. This model can make use of the methods in ISO 19906 for calculating these two components and can be considered to be the implied "current ISO approach". Model B assumes that the CL layer fails in bending as "level ice" on an elastic foundation and rides up the slope with the accompaniment of additional ice rubble scooped-up from the sail and keel of the ridge. Model C assumes that the FY ridge can be idealized into an equivalent "solid ice" beam using composite beam theory. Then the beam on elastic foundation method, as used for solid ridges, is used to estimate breaking and clearing loads.
These various approaches are reviewed and the derived loads are compared to failure modes and measured loads from Confederation Bridge for selected events involving FY ridges. Based on these comparisons a hybrid of Models B and C is recommended and the paper gives the details of how to apply this method.
When used for example structures, the new model gives loads which are 40 – 50% lower than the current approach implied in ISO 19906. The method can be adapted to downward sloping structures.
Marine vessels and offshore structures functioning in Earth's frigid zones require ice management to continue their routine operations. Icebreakers are the most influential vessel in assisting marine operations in Polar Regions. The present study is set to analyze the clearance area of level ice using Azimuthing propeller jet in bollard condition, by means of full-scale and model scale experiments. Moreover channel widening and heeling test is performed to analyze the escorting ability of an icebreaker with only using propeller jets. Scope of the current investigation can be incorporated in designing new icebreakers and maintaining desired channel width based on propeller jets effect.
Propeller jets can be used to break level ice, when the ship is stationary or moving, where the amount and capacity of breaking or clearing the ice are based on the thrust of the propeller, angle between the propeller jet axis and free surface, and thickness of the ice as well as propeller running time. This paper presents a comparison between full-scale experiments data (carry out in the Gulf of Bothnia, March 2017) and model scale trials performed in Aker Arctic testing facility on the level ice sheet. These experiments were based on image data from external camera and propeller flow parameters, where the area, as well as coordinate calculation, were within 3% of the accuracy from the acquired images. Full-scale ice thicknesses utilized in the experiments were selected and confirmed from surveillance videos. Model-scale images were corrected using Hugin software while ImageJ was used to calculate ice clearance parameters.
Propeller thrust and area analysis show 10-22 % of the variation in the results of the model and full-scale experiments for 16 mm thick model ice. 16 mm thick model ice results are much closer to full-scale trials than 25 mm thick model ice. Test results at 30° and 90° pod angles could be extrapolated to design a prototype vessel.
Channel widening shows worthy outcome, with the use of Azipods at a speed of 8 kn channel width of 36 m can be attain by positioning the stern Azipods at 30° puller configuration. Changing the pod inclination by 30% will increase the channel width to 20%. In the widening of new level ice channel, 30° pod angle is the most suitable.
This paper aims to numerically simulate the loading process when a moored ship is intruded by an ice ridge. Ice force caused by ice keel is calculated based on suggestions from ISO while the ice force due to consolidated layer is taken as level ice and simulated with circumferential crack method. The equation of motion is solved at each time step. A case study is given to show main features during the moored ship and ice ridge interaction. The result shows that the present numerical simulation is promising to be used in the design for moored structures in ice ridge.
In the Arctic, there exist many different types of features such as pure level ice, brash ice, ice rubble and ridges, ridge fields and icebergs, all with different structural and mechanical properties and behavior. For ships and offshore structures, first year ice ridge is a key consideration due to the extreme ice loads acting on the structures. It is crucial to determine the design load levels for offshore structures in ice-infested waters, can also bring a threat to shipping and navigation activities.
Typically, an ice ridge is formed when ice sheets are compressed against each other due to environmental factors, such as wind, current in the sea, thermal expansion etc. From geometry aspect of ice ridge, it is composed of three parts: sail, consolidated layer and keel. The above water part, called the sail, has pores filled with air and snow. The underwater part, called the keel, has pores filled with water and air pockets can exist. The ridge keel is further separated into an upper refrozen layer called the consolidated layer and a lower unconsolidated part. The consolidated layer grows through the ridge lifetime as a function of the surrounding meteorological and oceanographic conditions, air and water temperature, snow depth and the velocity of the wind, and surrounding currents are of principal importance. There was a wide variation in the shapes of the first-year sea ice ridges (Timco & Burden, 1997).
By developing general constitutive laws for ice ridge, Heinonen (2004) and Serré (2011) used finite element software to simulate the ice ridge load. At present, moored ships are often used to oil exploration and exploitation in ice-infested waters. For example, starting in the mid-1970s to the late 1980s, Dome Petroleum deployed floating drill-ships named Canmar during the summer months. In some water, the ice ridge action should be taken into consideration. A sketch of the moored ship in ice ridge is shown in Figure 1.
This paper present a proposal of a numerical model for 2D simulation when an icebreaker is advancing into ice-covered waters. The ship-ice contacts, ice failure, and ice floe motion are modeled. The numerical simulation calculates the repeatable ice breaking and removal. Numerical modeling demonstrates ice management with a race track. The distribution of ice floes, open channels, and time history of ice force caused by the icebreaker’s ice management are obtained numerically. Example calculations demonstrate that the proposed numerical model can be useful to identify an efficient way of ship handling in ice-covered water.
Ice management is necessary to reduce ice loads acting on tankers and drilling vessels operating in Arctic regions. An icebreaker works upstream of the vessels to create a continuous channel and to reduce floe size to manageable levels. Ice management requires a sufficiently large channel width and small floe size in drifting sea ice. Efficient ice management has to estimate the ice channel managed by the icebreaker, and support proper planning of vessel operations and deployed configuration.
Moran et al. (2006) reported ice management operations in the Arctic Coring Expedition (ACEX) conducted during August-September 2004 by the Integrated Ocean Drilling Project. In ACEX, two icebreakers worked upstream of the drill ship, which was able to stay at the location in heavy ice conditions about 250 km from the North Pole. He concluded that success was achieved through the efforts of the ice management, comprising individuals with extensive experience with Arctic icebreaking, ice prediction, and weather forecasting. Hamilton et al. (2011a, 2011b) developed a numerical simulator and quantified the ice management performance using real ice condition data collected in the Canadian Beaufort Sea. They studied effects of ice management strategies quantitatively, with examination of parameters of the icebreaker’s icebreaking pattern, speed of the ice, channel width, and ice floe size in the managed ice channel. Results show that the simulation provides variable insights into ice management fleet composition and fleet deployment. Full scale sea ice management trials in waters northeast of Greenland were conducted by the Oden Arctic Technology Research Cruise (OATRC) during the summers of 2012 and 2013 (Lubbad et al., 2013; Scibilia et al., 2014). Farid et al. (2014) investigated the sea ice breaking patterns of several short-term ice management activities during a research cruise in OATRC 2013, later proposing a preliminary analysis. They demonstrated that the maximum floe size resulting from the numerical simulations was roughly equivalent to that of an actual ship trial. Lu et al. (2015, 2016) examined ice floe fracture phenomena during the icebreaker’ s ice management. They proposed analytical and theoretical models of in-plane and out-plane ice failure for implementation into a numerical simulator of ship-ice interaction (e.g., Lubbad and Loset, 2011). The numerical simulation, as explained above, can determine an efficient ice management strategy. However, simulations have not led to efficient planning of ice management because of complexity of ice breaking and removal during ice managements.
An improved numerical method for calculation of icebreaking force is proposed in this paper. First, the dynamic bending failure criterion is introduced considering the loading rate, which is of great significance to the icebreaking process. The parametric expression of the dimensionless coefficient (λt) related to velocity is determined by dimensional analysis. The approximate numerical expression of λt is obtained at last. Second, the secondary fracture is introduced. The secondary fracture has an enormous influence on the period of ice loads. A time-domain analysis curve of icebreaking force is obtained based on the tests carried out by Tianjin University. The approximate expression of the icebreaking force in the whole fracture process is determined at last. The direct sailing and the turning motions of the Swedish icebreaker, Tor Viking II, are simulated in level ice. The numerical simulation results are compared with full-scale trial data and semi-empirical formulas to validate its rationality.
In recent years, global warming is melting the ice sheet in the Arctic Ocean increasingly. The complete opening of the Arctic Route is just around the corner. This newly opened route will inevitably exert a significant influence on the international ocean transportation pattern. The icebreaker is a service ship used to break ice, open channels and guide ships in the ice region. In view of the risk of working environment and the specificity of missions, the research on icebreaking mechanism, motion trajectory and ice loads of the icebreaker appears to be particularly important, which is directly related to its safety and efficiency.
At present, the study of the icebreaker has been widely concerned by scholars around the world, and the computing method for icebreaking force in level ice is becoming a research hotspot. Wang (2001) used a new ice loads model to simulate the icebreaking process of conical structure in level ice. The model simplified the icebreaking process as three continuum processes of crushing, bending and rubble formation. A model for establishing the time history of ice loads during the ice-hull interaction was developed based on a geometric grid method. Liu, Lau and Williams (2006) separated ice forces into three independent components. The breaking, buoyancy and clearing forces, representing individual processes identified during the ship-ice interaction, were calculated separately and summed as the total ice forces. Su, Riska and Moan (2011) introduced a numerical model to investigate both global and local ice loads on the ship hull. This model was partly based on empirical data. The interdependence between ice loads and ship motions was considered, as well as the variations in the thickness and strength properties of ice. Tan, Riska and Moan (2013) concluded that the dependency of ice resistance on hull speed was observed to be linear as was predicted by Lindqvist's semi-empirical formulas. Liu, Xue, Lu and Cheng (2018) calculated ice loads and simulate the ship-ice interaction process by using the peridynamics method.
In this paper, a nonlocal meshfree method, peridynamic theory is utilized to simulate the process of the interaction between level ice and a wide inclined structure. Since in peridynamic theory, the integration in the equation of motion is applied to calculate the force acting on the particles of the body instead of traditional differentiation, it has a huge advantage when handling spontaneously forming discontinuous issues such as the propagation of cracks. During the process of the interaction, with high initial velocity of the level ice, the ice behaves as a linear elastic material with a brittle mode of failure. To testify the accuracy of implementing peridynamic method on the ice-structure interaction, the result of the simulation in terms of ice force has been compared with the analytical formula, showing high agreement. Finally, for further study, several parameters that influence the fracture radius of the level ice had been discussed in detail.
Currently, with the acceleration of polar ice melting resulting from the global warming, the opening of the Arctic Passage becomes possible. Therefore, the polar engineering and technology which contributes to icebreaking and the exploration of polar resource appeal lot scientists to study. To analyze the interactions between ice and marine structures, numbers of experiments and analytical solutions have been proposed in the past days. Based on the beam bending theory, Croasdale (1980) introduced a basic two-dimensional model for ice action on a slope structure, in which the ice is depicted as an elastic beam. This simple 2D analysis for ice breaking and ride-up on a sloping structure has been improved to be a full 3D analysis model by Croasdale and Cammaert (1994). To testify the applicability of Frderking-Timco theory, Li and Riska (2001) performed the experiments of the level ice interacted with 45°, 60°, 75° inclined walls under shallow water. With development of computational technology in recent years, the numerical simulation plays a vital role in the analysis of ice-structure interactions, due to its accuracy and low-cost advantages. A two dimensional discrete element method was implemented by Paavilainen et al. 2006 to study the ice pile-up process against an inclined plate. In this model, the contact forces between particles were calculated with an elastic-viscous-plastic material model combined with an incremental Mohr-Coulomb tangential force model. Compared with the experiment, it appeared that in the simulations the rubble piles were more loosely packed, resulting from the edge crushing. The ice forces obtained from the simulation were consistent with the results from analytical methods. Based on the cohesive zone theory, the cohesive element-based approach was utilized by Lu et al. (2012) to simulate the ice-sloping interactions. A random ice field and bulk energy dissipation considerations were introduced to alleviate the mesh dependency issue.
While operating on the Arctic route, ships may face various issues. The ice environment, such as level ice, pre-sawn, pack ice, ice ridge and brash ice, is one of the sources of those issues. Prediction of ship resistance in brash ice is very important for safe operation. There are three ways to estimate the ice resistance: using a mathematical model, numerical simulation, and using empirical formula. In this paper, empirical formulas are used. The main aim of the study is to develop a computer program (I-RES) for prediction of attainable speed in brash ice and for ice resistance estimation. To achieve this goal, first, the brash ice environmental characteristics were analyzed. The results of I-RES were evaluated by comparing with the model test results of brash ice. The accuracy of I-RES calculations was found to be around 5%.
As global warming reduces Arctic sea ice, Russia's Arctic resource development is taking place in earnest. In recent years, Russia has successfully built up Yamal LNG vessels. Interest in the Arctic route has been increasing as a result of the use of the Arctic Sea as a means of shipping and transportation, which saves time and money compared to the existing Suez Canal. Ships operating on the Arctic route are exposed to various ice conditions such as collision with ice and friction. For this reason, it is important to determine the engine power at the initial stage of the ship design, because the ship operating at the Arctic route has a larger hull resistance. For this purpose, research is being conducted in various ways including analytical methods and model tests. The Arctic sea routes have various types of sea ice such as brash ice, which is formed by overlapping small ice, level ice which is frozen flat in a large area, pack ice where ice pieces of different sizes float, ice ridge which is formed by overlapping ice and flat ice. Since ice resistance has a very different value depending on the type of ice, it is important to establish a method for estimating the ice resistance accordingly. The method of estimating the ice resistance includes a mathematical model, a method using a simulation, and use of empirical formulas. The method of using a mathematical model and the method using simulation has an advantage that relatively accurate results can be obtained and the result analysis is also easy. However, these methods are time-consuming to define the shape and characteristics of ships and ice. Therefore, in this study, an empirical formula that can estimate ice resistance in a short time (Kim et al., 2015) was used to estimate ice resistance. The purpose of this study is to expand the application range of ice resistance and to adopt the safe speed estimation program for the brash ice, which was first developed for the level ice. The process of determination of the attainable speed from the estimated ice resistance and calculated engine power was summarized. Engine power was determined from the characteristic curve derived from the relationship between the engine and the propeller. To verify the accuracy and validity of the results of previous studies, we compared the model test results in level ice, pre-sawn, pack ice environment with the I-RES program results. The ice resistance estimation of brash ice developed ice resistance estimation algorithm through the environmental characteristics analysis. Also, the velocity estimation algorithm of brash ice using empirical equation is verified by comparing with the model test. The I-RES program, which has been supplemented with the proven algorithm, can be used to determine the maximum engine power of a ship working at the Arctic route.
For local ice loads, and based on numerical simulation, this paper presents a method for calculating fatigue damage to a ship's structure as it navigates in ridged ice fields. A semi-empirical method is introduced to develop the numerical model of ship-ice interaction in level ice and consolidated layer in ice ridges. Rankine's plasticity model is applied to calculate ice loads in ridge keels. A Weibull model is useful to describe ice load peaks. The structural fatigue stress is found using structural beam theory. According to the ice thickness distribution and a proper S-N curve, fatigue damage can be estimated based on the Palmgren-Miner rule. An example of fatigue damage calculation is presented. The calculated fatigue damage in ridged ice is much greater than that in level ice because of the ice ridge effects.
Fatigue damage can be important for ships operating in the harsh environment of ice-covered waters. Damage can entail oil leakage or even catastrophic failure, threatening overall structural safety. Nevertheless, research into fatigue damage caused by ice action has not been developed well compared with wave action. To date, most studies of fatigue damage caused by ice-induced loads have been conducted using field measurements. Zhang and Bridges (2011) introduced deterministic fatigue assessment using the Ship Right FDA ICE Procedure, as proposed by Lloyd's Register, to assess fatigue damage to a ship's structure induced by ice loads. Suyuthi et al. (2013) derived closed form expressions of fatigue damage for several statistical models of stress amplitude. However, the field measurements are usually quite limited and incomplete. For that reason, it is difficult to evaluate fatigue damage correctly and to provide guidance for the design of new structural components or new ship routes. Compared to field measurements, the ice conditions and ship hull can be easily varied in a numerical simulation, which is useful to complement the lack of ice load data in some regions, or to predict the fatigue life for new structures when only ice condition data are needed. The numerical method seems promising to evaluate fatigue damage.
Shi, Wei (Dalian University of Technology) | Tan, Xiang (Nanyang Technological University) | Zhou, Li (Jiangsu University of Science and Technology) | Ning, Dezhi (Dalian University of Technology) | Karimirad, Madjid (Queen's University)
The ice loading process has a clear stochastic nature due to variations in the ice conditions and in the ice-structure interaction processes of offshore wind turbine. In this paper, a numerical method was applied to simulate a monopile fixed-bottom and a spar-type floating wind turbine in either uniform or randomly varying ice conditions, where the thickness of the ice encountered by the spar were assumed to be constant or randomly generated. A theoretical distribution of the ice thickness based on the existing measurements reported in various literatures was formulated to investigate the response characteristics of the monopile wind turbine and spar wind turbine in such ice conditions. The effect of the coupling between the ice-induced and aerodynamic loads and responses for both operational and parked conditions of the rotor was studied. Moreover, the dynamic response of wind turbine in randomly varying ice was compared and verified with that of the wind turbine in constant ice.
So far, more than 80% of the energy all over the world comes from fossil fuels. Excessive and improper use of fossil fuels has caused climate change and threatened human security and development. The Paris Agreement, which entered into force on 4 November, 2016, is a major step forward in the fight against global warming. Due to severe smog, forty Chinese cities reel under heavy air pollution. Air pollution becomes one of the key words in China in 2016 (PTI, 2016). Renewable energies play an important role for reducing greenhouse gas emissions, and thus in mitigating climate change. Offshore wind energy is recognized as one of the world's fastest growing renewable energy resources. By the end of 2015, totally 12,107 MW of offshore wind energy was installed around the world according to Global Wind Energy Council (GWEC) report (Fried, 2016). In Europe, 3230 turbines are now installed and grid-connected, making a cumulative total of 11,027 MW (Ho, 2016). However, governments outside of Europe have set ambitious targets for offshore wind and development is starting to take off in China, Japan, South Korea and the US. The 1.2 GW of capacity installed in Asia as of the end of 2015 was located China and mainly in Japan.