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 shil3 structure design and the navigation securitv in ice-covered fields.

Uncertainty and joint probability of sea ice loads

Ice load is one of the most important factors for design of marine stuctures in cold sea area.The designcriteria of ice loads for marine structures consist of two parts: level ice thickness and crushing strength. Due to thestrong randomness of the affecting factors on ice thickness and crushing strength, such as ice temperature,air temperature, water temperature,salinity,wind field, numerical simulation method method can not produce satisfactory results. Thispaper proposes a method of uncertainty analysis and joint probabilistic prediction of level ice thickness and crushingstrength instead of traditional ice load criteria for marine structures.

Discrete Element Simulations of Ice Load and Mooring Force on Moored Structure in Level Ice

Moored structures are suitable for operations in ice-covered regions owing to their security and efficiency.This paper aims to present a new method for simulating the ice load and mooring force on the moored structure during ice-structure interaction with a spherical Discrete Element Method(DEM).In this method,the level ice and mooring lines consist of bonded sphere elements arranged in different patterns.The level ice model has been widely validated in simulation of the ice load of fixed structures.In the mooring line simulation,a string of spherical elements was jointed with the parallel bond model to simulate the chains or cable structure.The accuracy of the mooring line model was proved by comparing the numerical results with the nonlinear FEM results and model towing experiment results.The motion of the structure was calculated in the quaternion method,considering the ice load,mooring force,and hydrodynamic force.The hydrodynamic force comprised wave-making damping,current drag,and buoyancy force.Based on the proposed model,the interaction of a semi-submersible structure with level ice was simulated,and the effect of ice thickness on the ice load was analyzed.The numerical results show that the DEM method is suitable to simulate the ice load and mooring force on moored floating structures.

Dynamic ice loads on conical structures

Two series of model tests were performed to observe the dynamic ice loads on conical structures.The variable testing parameters include the water line diameter of the model cone and ice parameters.During small water line diameter tests,two-time breaking is found to be the typical failure of ice on steep conical structure,and also be controlled by other factors,such as ice speed and the cone angle.During big water line diameter tests,the ice sheet failed nonsimultaneously around the cone.Several independent zones of bending were found in the nonsimultaneous failure process of ice.With the increase of the ratio of D/h and the number of independent zones,the total ice force was found being gradually reduced.

Modal Parameters Identification of A Real Offshore Platform From the Response Excited by Natural Ice Loading

This paper investigates the possibility of utilizing response from natural ice loading for modal parameter identification of real offshore platforms.The test platform is the JZ20-2 MUQ jacket platform located in the Liaodong Bay,China.A field experiment is carried out in winter season,as the platform is excited by floating ices.The feasibility is demonstrated by the acceleration response of two different segments.By the SSI-data method,the modal frequencies and damping ratios of four structural modes can be successfully identified from both segments.The estimated information from both segments is almost identical,which demonstrates that the modal identification is trustworthy.Furthermore,by taking the Jacket platform as a benchmark,the numerical performance of five popular time-domain EMA methods is systematically compared from different viewpoints.The comparisons are categorized as:(1)stochastic methods versus deterministic methods;(2)high-order methods versus low-order methods;(3)data-driven versus covariance-driven stochastic subspace identification methods.

Fatigue Crack Propagation in Steel A131 Under Ice Loading of Crushing,Bending and Buckling

Three types of ice loading, which are most commonly present when ice acts on structures, are chosen and simulated for use of fatigue crack propagation tests on offshore structural steel A131. The three types of ice categorized in accordance with the failure modes when acting on structures called Crushing ice, bending ice, and buckling ice, respectively. This paper presents an experimental investigation on the fatigue crack propagation behavior of widely used high strength steel A131 for offshore jackets in the loading environment of ice crushing, bending, and buckling. The test results of fatigue crack propagation in steel A131 tinder these simulated ice loading at temperature 292K are presented and analyzed in detail in this paper, The amplitude root mean square stress intensity factor is optimized to be the fundamental parameter of fatigue crack propagation for all types of ice loading histories. The results are also compared with constant amplitude fatigue crack propagation conclusions as in wave load mode, and a joint investigation on the results from ice forces, ice-induced vibrations, and ice-induced fatigue crack propagation is conducted. Conclusions are drawn for reference in structural design and material selection for offshore structures in ice environments.

Numerical Simulations of the Ice Load of a Ship Navigating in Level Ice Using Peridynamics

In this study,a numerical method was developed based on peridynamics to determine the ice loads for a ship navigating in level ice.Convergence analysis of threedimensional ice specimen with tensile and compression loading are carried out first.The effects of ice thickness,sailing speed,and ice properties on the mean ice loads were also investigated.It is observed that the ice fragments resulting from the icebreaking process will interact with one another as well as with the water and ship hull.The ice fragments may rotate,collide,or slide along the ship hull,and these ice fragments will eventually drift away from the ship.The key characteristics of the icebreaking process can be obtained using the peridynamic model such as the dynamic generation of cracks in the ice sheet,propagation and accumulation of ice fragments,as well as collision,rotation,and sliding of the ice fragments along the ship hull.The simulation results obtained for the ice loads and icebreaking process were validated against those determined from the Lindqvist empirical formula and there is good agreement between the results.

Analysis and Research of Ice Loads Acting on Offshore Structures

Usually, the action of sea ice on offshore engineering structures is one of the controlling loads in cold waters engineering structure design. The reasonable selection of environmental condition and the physical mechanical properties of ice in the region are directly related to the structure design, operation and safety. In this paper, the sea ice force acting on the structure, the physical mechanical properties of ice and the selection of parameters in calculation are discussed. Some suggestions are proposed as to the calculation of various kinds of ice loads acting on the structure.

Interaction between sea ice/iceberg and ship structures: A review

For ship structural design and good maneuverability in an ice-covered sea, the local and global load of ice cover on ships should be well understood. This paper reviews the extensive work done on ice loads on ships, including： （a） Ice pressure and local load determination based on field and model tests; （b） Global ice loads on ships from full-scale field observations, model tests and numerical models under different ice conditions （level ice and pack ice） and ship operations （maneuvering and mooring）. Spe- cial attention is paid to the discrete element simulation of global ice loads on ships; and （c） Analytical solutions and numerical models of impact loads of icebergs on ships for polar navigation. Finally, research potential in these areas is discussed.

Time-and Frequency-Domain Analysis of Ice-Induced Displacement Responses Offshore Fixed Platforms

On the basis of ice- induced forced vibration model, ice- induced displacement responses of offshore fixed platforms are investigated in both time domain and frequency domain. The relationships of ice-induced displacement responses with ice breaking modes, ice acting directions and platform structures are analyzed and determined. The results lead to an important conclusion obtained for the first time that ice breaking frequency and the natural frequency of the first mode of the platform are the two main factors that dominate the degree of vibration. The present work provides a firm basis for both design and operation of fixed platforms against ice loading.

Efficient Approximate Method of Global Reliability Analysis for Offshore Platforms in the Ice Zone

Ice load is the dominative load in the design of offshore platforms in the ice zone, and the extreme ice load is the key factor that affects the safety of platforms. The present paper studies the statistical properties of the global resistance and the extreme responses of the jacket platforms in Bahai Bay, considering the randomness of ice load, dead load, steel elastic modulus, yield strength and structural member dimensions. Then, based on the above results, an efficient approximate method of the global reliability analysis for the offshore platforms is proposed, which converts the implicit nonlinear performance function in the conventional reliability analysis to linear explicit one. Finally, numerical examples of JZ20-2 MSW, JZ20-2NW and JZ20-2 MUQ offshore jacket platforms in the Bahai Bay demonstrate the satisfying efficiency, accuracy and applicability of the proposed method.

Ice Induced Low Temperature Fatigue Crack Propagation in Offshore Structural Steel A131

To investigate the low temperature fatigue crack propagation behavior of offshore structural steel A131 under random ice loading, three ice failure modes that are commonly present in the Bohai Gulf are simulated according to the vibration stress responses induced by real ice loading. The test data are processed by a universal software FCPUSL developed on the basis of the theory of fatigue crack propagation and statistics. The fundamental parameter controlling the fatigue crack propagation induced by random ice loading is determined to be the amplitude root mean square stress intensity factor K-arm. The test results are presented on the crack propagation diagram where the crack growth rate da/dN is described as the function of K-arm. It is evident that the ice failure modes have great influence on the fatigue crack propagation behavior of the steel in ice-induced vibration. However, some of the experimental phenomena and test results are hard to be physically explained at present. The work in this paper is an initial attempt to investigate the cause of collapse of offshore structures due to ice loading.

Numerical Simulation of Turret Moored Ship Performance in Variable Ice Drift Based on Heading Control

The frequent change in ice drift direction poses a significant challenge for turret moored ship in ice. Variability in ice drift is mainly caused by the winds and currents. To solve this problem, a new method with numerical simulation based on heading control is applied to reduce the risk of operation of The Arctic Tandem Offloading Terminal(ATOT),which includes an offloading icebreaker(OIB) moored to a submerged turret and a shuttle tanker moored at the stern of the OIB in this paper. An icebreaking tanker, MT Uikku, was modeled in a simulation program. Then the level ice load on the tanker was calculated with different ice thicknesses and drift speeds, after which a heading controller assisted with mooring system is used to simulate the horizontal motion of the tanker under the ice action.

Influence of Ice Size Parameter Variation on Hydrodynamic Performance of Podded Propulsor

During ice-breaking navigation, a massive amount of crushed ice blocks with different sizes is accumulated under the hull of an ice-going ship. This ice slides into the flow field in the forward side of the podded propulsor, affecting the surrounding flow field and aggravating the non-uniformity of the propeller wake. A pulsating load is formed on the propeller, which affects the hydrodynamic performance of the podded propulsor. To study the changes in the propeller hydrodynamic performance during the ice podded propulsor interaction, the overlapping grid technique is used to simulate the unsteady hydrodynamic performance of the podded propulsor at different propeller rotation angles and different ice block sizes. Hence, the hydrodynamic blade behavior during propeller rotation under the interaction between the ice and podded propulsor is discussed. The unsteady propeller loads and surrounding flow fields obtained for ice blocks with different sizes interacting with the podded propulsor are analyzed in detail. The variation in the hydrodynamic performance during the circular motion of a propeller and the influence of ice size variation on the propeller thrust and torque are determined. The calculation results have certain reference significance for experiment-based research, theoretical calculations and numerical simulation concerning ice podded propulsor interaction.

Failure of Semi-infinite Beams of Variable Thickness and Curvature on Elastic Foundation under Contact Loading

In early winter it is usual, in cold regions, that ice features approach offshore structures, like offshore platforms, impacting them, in a slow process of constant deformation build up. Interaction follows, in many cases, up to the point where ice-failure caused by bending fracture takes place. This supposes very large contact forces that the structure has to resist. Therefore, quantification of these efforts is of vital importance to the structural design of platforms. In several designs, these platforms are constructed with inclined walls so as to cause ice to fail in a flex-compression mode. In such a case the ice feature is analyzed as a beam constituted of a linear elastic material in brittle state with constant ice thickness. The simplification renders the problem solvable in a close form. However, this hypothesis goes against field observations. Marine currents action, wind and the sequence of contacts among features lead to thickness variations. Here this factor is addressed in the construction of a model, for harmonic forms of variation of thickness profile, and the accompanying curvature variations, whose solution determines field variables used to address the failure question. Due to the deformation dependency of the loading, a numerical scheme for the two-point boundary value problem in the semi-infinite space is developed. Failure pressures are computed based on a Rankine locus of failure. Variations of the order of 20% in the failure loads, as compared to the uniform beam model, are observed.

Solar Cooling Alternatives for Residential Houses

The energy consumption rate of non-OECD(non-Organisation for Economic Co-operation and Development)countries rises about 2.3 percent per year as compared to the OECD countries which is 0.6 percent.If developing countries use energy efficient technology and integrate renewable energy systems in the new building their carbon dioxide emission rate reduces by 25 to 44 percent.However,even now,renewable energy integrated buildings are hardly considered while constructing them.This paper focuses on the study of solar cooling system options for residential house of Bahir Dar city,in Ethiopia.To meet the demand of housing in the city,different types of apartments and villa houses are under construction.For the analysis case study was made focusing on two types of residential houses,condominium apartment and Impact Real Estate Villa house.Simulation results of IDA ICE software show that the average operative temperatures and cooling loads for condominium apartment and Real-estate Vila are 31.8℃ and 30.7℃,5.53 kW and 5.73 kW respectively.Most of the residences are not satisfied at this operating temperature.There are different types of solar cooling systems.Solar sorption cooling systems are commonly used which can also be classified into absorption,adsorption and desiccant cooling systems.Solar adsorption cooling systems are easy to manufacture locally as compared to solar absorption cooling systems.They do not have moving parts.Some of the working medium pairs used in adsorption cooling system are:activated carbon/ammonia,silica gel/water,zeolite/water.Adsorption chillier with silica gel/water as a working pair was selected since it can operate at regeneration/desorption temperature as low as 45℃ coming from flat plate collectors.At 75℃ regeneration temperature,the system delivers 9℃ chilled water temperature.From cooling load simulation result direct solar irradiation is the highest source of cooling load for both houses.This gives an opportunity for passive solar cooling technology.

Numerical Simulation of Multiple Collisions Between Ice Ridge and Polar Ship

Ships navigating in ice-covered regions will inevitably collide with ice ridges.Compared to other ice bodies,ice ridges exhibit more complicated mechanical behaviors due to the scale and structure characteristics.In this paper,nonlinear finite element method is used to investigate the interaction between a polar ship and an ice ridge.The ice ridge is modelled as elastic-plastic material based on Drucker-Prager yield function,with the consideration of the influence of cohesion,friction angle and material hardening.The material model is developed in LS-DYNA and solved using semi-implicit mapping algorithm.The stress distribution of ice ridge and ship,and the ice load history are evaluated through the simulation of multiple collisions.In addition,parametric analysis is performed to investigate the influence of ridge thickness and impact velocity on the ice load and energy absorption.

An Integrated Approach to Fatigue Life Prediction of Whole System for Offshore Platforms

The failure of one or even more components usually does riot lead to the collapse of the whole structure. Most of the analysis of fatigue is centered on only a single component which the researchers are interested in or Much attention should be paid to. However, the collapse of a structure is the result of failure of a series of components in a specific order or path. This paper proposes an integrated approach to fatigue life prediction of whole structural system for offshore platforms, mainly describing the basic principles and prediction method. A method is presented for determining the failure path of the whole structure system and calculating the fatigue life in the determined failure path, The corresponding final collapse criteria for the whole structure system are discussed, A simple method of equivalent fatigue stress range calculation and a mathematical model of structural component fatigue life estimation in consideration of sea wave and sea ice loads are provided. As an application of the proposed approach, a fixed production platform Bohai No. 8 is chosen for the predication of fatigue life of the whole structure system by means of the software OSFAC developed based on the present methods.

基于态基近场动力学的多重失效弹性模型及其在船-冰碰撞中的应用

Due to complex mesoscopic and the distinct macroscopic evolution characteristics of ice,especially for its brittle-to-ductile transition in dynamic response,it is still a challenging task to build an accurate ice constitutive model to predict ice loads during ship-ice collision.To address this,we incorporate the conventional multi-yield-surface plasticity model with the state-based peridynamics to simulate the stress and crack formation of ice under impact.Additionally,we take into account of the effects of inhomogeneous temperature distribution,strain rate,and pressure sensitivity.By doing so,we can successfully predict material failure of isotropic freshwater ice,iceberg ice,and columnar saline ice.Particularly,the proposed ice constitutive model is validated through several benchmark tests,and proved its applicability to model ice fragmentation under impacts,including drop tower tests and ballistic problems.Our results show that the proposed approach provides good computational performance to simulate ship-ice collision.

多桩锥体海洋平台结构冰荷载遮蔽效应离散元分析

在海冰与多桩锥体海洋平台结构的相互作用中,平台结构总冰力在海冰流向和桩腿方位的影响下呈现出显著的遮蔽效应。采用具有粘结-破碎效应的离散元方法,基于GPU并行的高性能计算,对不同冰向下锥体海洋平台结构的冰荷载进行了数值分析,确定了不同冰向下平台结构各桩腿的冰力衰减系数并分析了总冰力的遮蔽效应。最后,对自由边界影响下多桩平台结构冰荷载遮蔽效应的产生机理进行了讨论。本文工作可为多桩腿平台结构的冰荷载特性、冰激结构振动以及冰区结构设计提供参考。