Kaj Riska

NUMERICAL SIMULATION OF SHIP TURNING IN LEVEL ICE

Ice loads represent the dominant load for ice-going ships, and it is important to estimate both global and local ice loads on ship hulls. The global ice load governs the ship’s overall performance in ice, and it is an integrated effect of local ice loads over the hull area. Information on the distributions of local ice loads around the hull can be used for more effective design of ice-going ships both in terms of overall operation and from the structural point of view. The present thesis focuses on a numerical model for simulating ice–hull interaction and ship maneuvers in level ice. This model is partly based on the empirical data, by which the observed phenomena of continuous icebreaking can be reproduced. In the simulation of a full-scale icebreaking run, the interdependence between the ice load and the ship’s motion is considered, and the three degree-of-freedom (DOF) rigid body equations of surge, sway and yaw are solved by numerical integration. The thickness and strength properties of the ice encountered by the ship are assumed to be constant or predefined based on the statistical data. Accordingly the global and local ice loads on ship hulls can be obtained in a deterministic or probabilistic way. The convergence tests are carried out to make sure that this numerical method can give a convergent solution of both global and local ice loads. The computation time is also examined with the purpose of determining a balance between the computation time and the convergence. The influences of the assumptions and simplifications made in this numerical model are analyzed by changing different parameters and comparing with an empirical method and the measured data. The simulation results are discussed through three case studies, in which the global ice load effects on ship’s performance, the probabilistic and spatial variations of local ice loads around the hull and the short-term distribution of maximum ice loads on a frame are respectively analyzed and compared with field measurements conducted in the Baltic Sea. The ship’s performance in level ice is usually described by the speed that the ship can attain in the ice of a certain thickness. A case study with an icebreaker, Tor Viking II, is carried out by using the simulation program. In this study, the ice encountered by the ship is assumed to be uniform. The ship’s motion is obtained by solving the equations of motion in which the thrust and the global ice load both are identified. The speed that the ship can attain is then simulated in the ice of different thicknesses. The simulation results agree well with the full-scale measurements. The turning circle diameter which is a measure of the ship’s maneuverability in ice is also investigated by using the simulation program. Herein the ship is assumed to turn freely with a given rudder angle, and the simulated turning circles are comparable to the full-scale ice trials. It is also found that the ship’s performance can be considerably affected by the geometry of simulated icebreaking patterns. If the shoulder crushing takes place (i.e. the ice is continuously crushed by hull shoulder without bending failure) both the forward speed and the turning rate of the ship will be significantly slowed down. This phenomenon is also observed in full-scale trials, but it is difficult to learn about its effect on ship’s performance as the actual ice conditions in-service usually are uncontrollable. It is expected that numerical simulations can supplement full-scale tests in providing more details about the continuous icebreaking processes and the global ice load effects on ship’s performance. The local ice loading process has a clear stochastic nature due to variations in the ice conditions and in the icebreaking processes of ships. A case study with an icebreaking tanker, MT Uikku, is carried out by using the simulation program. In this study, the thickness and strength properties of the ice encountered by the ship are assumed to be constant or randomly generated using the Monte Carlo method. It is found that the variation of simulated ice loading process on a frame is noteworthy even if the ice properties are fixed. If the statistical variation of the ice conditions is considered, then the distributions of simulated load peaks are found to be comparable to the measured statistical distributions. In this case study, the spatial distribution of local ice loads around the hull is also investigated. The simulation results agree with previous experimental studies, that the turning operation may develop a high load level on the aft shoulder area. Ice conditions and ship operations in ice vary in the short term from voyage to voyage and in the long term from winter to winter. Long-term ice load measurements conducted in the Baltic Sea consist mainly of 12-hour load maxima which are gathered during the normal operation of the ship over several years. A case study with a chemical tanker, MS Kemira, is carried out by using the simulation program. In this study, the statistical data on the strength properties of Baltic Sea ice are applied and the thickness of the ice encountered by the ship are classified referring to the full-scale measurements onboard MS Kemira. The 12-hour maximum ice loads on a frame are evaluated by fitting a Gumbel I asymptotic extreme value distribution to the simulated 10-min load maxima in a certain ice condition. It can be expected that if a reasonable variance of the ice thickness is defined, the simulation results can be used for a preliminary estimation of the maximum ice loads within a 12 hours’ voyage in level ice. By applying the different ice thicknesses in simulations, the probable correlation between the simulated load maxima and the ice thickness is analyzed. A potential way to evaluate the long-term ice load statistics based on short-term simulations is then introduced. Up to now the main source of knowledge about ice load statistics has been field measurement. While field measurements will continue to be important, numerical methods can provide useful information, since they can be easily used to study the effect of different parameters. As far as we know the present numerical model is the first one to deal with multiple subjects including the ice–hull interaction, the overall performance of icebreaking ships and the statistics of local ice loads around the hull. It is hoped that further studies on this numerical model can supplement the field and laboratory measurements in establishing a design basis for the ice-going ships, especially for ships navigating in first-year ice conditions

Assessment of the Feasibility of the Arctic Sea Transportation by Using Ship Ice Transit Simulation

Arctic sea transportation has drawn a lot of attention in the recent years. The possibility of using a shorter route between Europe and Asia interests many actors in the shipping industry. Benefits from the shorter route may, at first, seem attractive. However, there might be factors affecting the feasibility of the route that are not obvious at first. The estimated transit speed along the Arctic route is not necessarily reached due to ice and other prevailing conditions. Simplified methods can underestimate the actual transit time, so the use of advanced methods is advisable. A thorough assessment of ship performance along the selected route can reveal factors that affect the feasibility of trans-Arctic shipping.This paper presents how the ship transit along an Arctic route can be simulated and how the ship performance can be assessed based on the simulation results. In this paper comparative results of ship performance in different ice conditions are shown and the benefits and challenges of the ship ice transit simulations are discussed. The effect of the input ice conditions to the ship performance estimates and how the ice information from various sources can be used for the simulations are discussed.Ship performance in the Arctic transit is tested using a probabilistic model called COSSARC for ship performance simulation in ice and open water. The tool can be used for assessing the economic feasibility of ship designs and transport concepts. One of the main benefits of the ship ice transit simulations compared to the simpler methods is that ice ridges are described in the realization of ice conditions statistically and the ship performance in ridges is modeled. A ship might get stuck in ridges and might be forced to wait for assistance from an icebreaker, or a double acting ship might travel slower than anticipated through ridge fields. This increases the transit time significantly, which is not necessarily revealed by simpler methods. The main outcome from the ship ice transit simulations described in this paper, is a more realistic estimate of transit time for a given route. This can be used as input for economic or other assessments. It is possible to estimate the probability of getting stuck in ice ridges from the ship ice transit simulation results, and thus the need for icebreaker assistance can be assessed. The probabilistic simulation of ship performance can be done in the design phase of the ship to assess various design concepts or while selecting what kind of ship or fleet of ships is the most suitable for the given transport task.Copyright

Numerical Simulation of Moored Ship in Level Ice

The dynamic ice forces on a moored icebreaking tanker induced by drifting level ice were simulated with a two dimensional numerical model. Based on a heading controller which aimed to keep the hull head towards the drifting ice, ice resistance on ship was mainly estimated when taking the relative motion between the hull and ice into account. The mooring force and responses of the moored vessel were also looked into through parameter sensitivity studies with different ice thicknesses and ice drift speeds.Copyright

Performance Simulation of a Dual-Direction Ship in Level Ice

This thesis is a summary of studies that were carried out as part of candidacy for aPhD degree. The purpose of these studies was to evaluate some factors in shipdesign that are intended for navigating in ice using numerical simulations. A semiempiricalnumerical procedure was developed by combining mathematical modelsthat describe the various elements of the continuous-mode icebreaking process inlevel ice. The numerical procedure was calibrated and validated using full- andmodel-scale measurements. The validated numerical model was in turn used toinvestigate and clarify issues that have not been previously considered.An icebreaker typically breaks ice by its power, its weight and a strengthened bowwith low stem angle. The continuous icebreaking process involves heave and pitchmotions that may not be negligible. The numerical procedure was formulated toaccount for all of the possible combinations of motions for six degrees of freedom(DOFs). The effects of the motion(s) for certain DOF(s) were investigated bycomparing simulations in which the relevant motion(s) were first constrained andthen relieved.In the continuous-mode icebreaking process, a ship interacts with an icebreakingpattern consisting of a sequence of individual icebreaking events. The interactionsamong the key characteristics of the icebreaking process, i.e., the icebreakingpattern, ship motions, and ice resistance, were studied using the numericalprocedure in which the ship motions and excitation forces were solved for in thetime domain and the ice edge geometry was simultaneously updated.Observations at various test scales have shown that the crushing pressure arisingfrom the ice–hull interaction depends on the contact area involved. A parametricstudy was carried out on the numerical procedure to investigate the effect of thecontact pressure on icebreaking.The loading rates associated with the ship’s forward speed have been anticipatedto play an important role in determining the bending failure loads, in view of thedynamic water flow underneath the ship and the inertia of the ice. The dynamicbending behavior of ice could also explain the speed dependence of the icebreakingresistance component. A dynamic bending failure criterion for ice was derived,incorporated into the numerical procedure and then validated using full-scale data.The results obtained using the dynamic and static bending failure criteria werecompared to each other.In addition, the effect of the propeller flow on the hull resistance for ships runningpropeller first in level ice was investigated by applying the information obtainedfrom model tests to the numerical procedure. The thrust deduction in ice wasdiscussed.

Non Parametric Probabilistic Approach of Ice Load Peaks on Ship Hulls

Due to its complex phenomenon, ice-induced load process could not be physically possible to be modeled by a specific well established probabilistic model. Therefore, a non parametric approximate probabilistic approach should be performed based on the available data. This paper describes the procedure and also the extrapolation to get the short term and long term extreme values. Comparison with the classical approach, where the initial distribution of ice-induced load is assumed to be one of the well established probabilistic models, was made. The comparisons discussed in this paper were explored by using the same available data of full scale measurement on board a coastal guard vessel KV Svalbard during the winter 2007. There was a tendency that the non parametric approach produces more conservative results.Copyright

Towards mission-based structural design for arctic regions

ABSTRACT Structures used in Arctic regions must comply with ice-induced loading without failure, asset loss, or loss of life. Current structural design regulations, particularly for ships, rely primarily on the experience gained from first-year ice, e.g. in the Baltic Sea. Therefore, this paper presents first, the most relevant design rules for both ships and offshore structures, and second, first principle-based pressure and occurrence determination methods to obtain corresponding scantlings. To demonstrate these first principle-based methods, the design ice load is assessed for a transit operation along the Northern Sea Route. In conclusion, this paper seeks to motivate a more mission-based design methodology, in addition to the present design methods, using the presented first principle-based methods. The paper is based on the ISSC 2015 V.6 Arctic Technology committee report.

Variation of the Short Term Extreme Ice Loads Along a Ship Hull

This paper focuses on the short term probabilistic analysis of ice loads acting on a ship hull. The ice load data was obtained from full scale measurement onboard the Norwegian coast guard vessel KV Svalbard during the winter of 2007. The available data corresponds to discrete peak amplitude time histories of estimated ice impact loads as well as corresponding measurements of ice thickness in addition to ship speed and course. There were several number of sensors installed along the hull, either on the port side and starboard side of the bow part. The present paper focuses on the variation of the predicted extreme ice loads acting on the ship hull for a short time duration. The short term prediction of ice loads as an integral part of an Ice Loads Monitoring (ILM) system is very important in relation to the tactical navigation plan. An inexpensive ILM system would requires less number of sensors mounted on the hull. By addressing the variation of the extremes along the hull, it will be possible to make decisions regarding the minimum number of sensors and their location without loosing the accuracy of the predicted extremes. Three different approaches for predicting the short term extremes are considered, i.e. the classical extreme value distribution approach, the time window approach, and the up-crossing rate approach. In general, all the approaches involve the following two steps: (i) establishment of the estimated distribution model, (ii) calculation of the expected largest extreme ice impact load for an extrapolated duration. Comparison of the results obtained by the three different approaches is made, and some limitations of the various approaches are discussed.Copyright