Ivan Metrikin

Image Processing for the Analysis of an Evolving Broken-Ice Field in Model Testing

Dynamic positioning (DP) experiments in model ice were carried out in the ice tank at the Hamburg Ship Model Basin (HSVA) in the summer of 2011. In these experiments the behavior of two different ships in a broken-ice field were studied. One of the main parameters characterizing a broken-ice field is the ice concentration, defined as the fraction of the total water area covered by ice. In this paper, image processing techniques are applied to derive the ice concentration in the model basin. Several points in time are analyzed in order to describe the evolution of the ice field. The applied techniques include methods for identifying individual ice floes and calculating the ice concentration in the vicinity of the model ship. Ice floe boundaries are then obtained, and the ice floe size distribution and shape factor may further be extracted from the images. The image processing methods applied in this work are object extraction and edge detection algorithms, which are further customized to ice identification. The obtained results can be used for relating the ice field characteristics to the model test results, such as the vessel’s displacements and the corresponding ice forces.Copyright

Dynamic Positioning in Ice: Comparison of Control Laws in Open Water and Ice

Dynamic Positioning (DP) systems are intensely used in a large range of ship operations nowadays. The growing interest of Arctic exploration and exploitation may introduce a new application area for those systems. The very few full scale DP operations in the Arctic have demonstrated the need for improvements in DP systems for ice-covered waters. External forces due to the ice environment are very different from open water forces and especially the dynamic component of the loads is much higher. This paper firstly reviews DP in open water and spotlights the needs for adaptations to ice-covered regions. The architecture of a controller answering ice requirements is then presented. This system has been successfully tested at the large ice tank of the Hamburg Ship Model Basin (HSVA) in 2012 within the European R&D project DYPIC [1]. The designs of open water and ice control laws are then compared in two simulation frameworks. The first framework involves only the current, wind and waves, while the second framework deals with the ice conditions. The numerical ice simulator, utilized in this paper, is a novel high-fidelity modelling tool developed by the Norwegian University of Science and Technology (NTNU).© 2013 ASME

Numerical Simulation of a Floater in a Broken-Ice Field: Part II — Comparative Study of Physics Engines

Numerical simulation of a floater in ice-infested waters can be performed using a physics engine. This software can dynamically detect contacts and calculate the contact forces in a three-dimensional space among various irregularly shaped bodies, e.g. the floater and the ice floes. Previously, various physics engines were successfully applied to simulate floaters in ice. However, limited attention was paid to the criteria for selecting a particular engine for the simulation of a floater in broken-ice conditions.In this paper, four publicly available physics engines (AgX Multiphysics, Open Dynamics Engine, PhysX and Vortex) are compared in terms of integration performance and contact detection accuracy. These two aspects are assumed to be the most important for simulating a floater in broken ice. Furthermore, the access to code, documentation quality and the level of technical support are evaluated and discussed. The main conclusion is that each physics engine has its own strength and weaknesses and none of the engines is perfect. These strength and weaknesses are revealed and discussed in the paper.Copyright

Capability Plots of Dynamic Positioning in Ice

Dynamic positioning (DP) capability plots in open water are essential tools both for ship design and operational risk assessment. Currently the capability plots are widely used by the whole ship sector. Furthermore, the calculations and representations of the capability plots have been standardized by the International Marine Contractors Association (IMCA). In a capability plot, the wind, current and wave loads are taken into account and the plots are given in polar coordinates [1]. Recent research and ice basin experiments have demonstrated the feasibility of DP operations in ice-covered waters [2]. However, the design of a similar analysis tool as the capability plot is complicated by both a large range of ice parameters and the lack of understanding of the ship/ice interaction physics at low relative velocities [3]. In this paper, the influence of several ice parameters is studied in order to identify the most important variables for DP in ice. The study comprises both ice basin experiments and numerical simulations. The ice basin test results are extracted from the trials carried out at the large ice tank of Hamburg Ship Model Basin (HSVA) within the scope of the European research and development project DYPIC [2]. The numerical modelling is based on a novel high-fidelity simulation approach which is discussed in the paper. Finally, “ice capability plots” are drawn under certain hypothesis and assumptions.Copyright

Numerical Simulation of a Floater in a Broken-Ice Field: Part I — Model Description

This paper presents a novel concept for simulating the ice-floater interaction process. The concept is based on a mathematical model which emphasizes the station-keeping scenario, i.e. when the relative velocity between the floater and the ice is comparatively small. This means that the model is geared towards such applications as dynamic positioning in ice and ice management.The concept is based on coupling the rigid multibody simulations with the Finite Element Method (FEM) simulations. The rigid multibody simulation is implemented through a physics engine which is used to model the dynamic behaviour of rigid bodies which undergo large translational and rotational displacements (the floater and the ice floes). The FEM is used to simulate the material behaviour of the ice and the fluid, i.e. the ice breaking and the hydrodynamics of the ice floes. Within this framework, the physics engine is responsible for dynamically detecting the contacts between the objects in the calculation domain, and the FEM software is responsible for calculating the contact forces. The concept is applicable for simulations in a three-dimensional space (3D).The model described in this paper is divided into two main parts: the mathematical ice model and the mathematical floater model. The mathematical ice model allows modelling both intact level ice and discontinuous ice within a single framework. However, the primary focus of this paper is placed on modelling the broken ice conditions. A floater is modelled as a rigid body with 6 degrees of freedom, i.e. no deformations of the floater’s hull are allowed. Nevertheless, the hydrodynamics of the floater and the ice is considered within the outlined model.The presented approach allows implementing realistic, high fidelity 3D simulations of the ice-fluid-structure interaction process.Copyright

Experimental and Numerical Investigation of Dynamic Positioning in Level Ice

Offshore operations in ice-covered waters are drawing considerable interest from both the public and private sectors. Such operations may require vessels to keep position during various activities, such as lifting, installation, crew change, evacuation, and possibly drilling. In deep waters, mooring solutions become uneconomical and, therefore, dynamic positioning (DP) systems are attractive. However, global loads from drifting sea ice can be challenging for stationkeeping operations of DP vessels. To address this challenge, the current paper investigates DP in level ice conditions using experimental and numerical approaches. The experimental part describes a set of ice model tests which were performed at the large ice tank of the Hamburg Ship Model Basin (HSVA) in the summer and autumn of 2012. Experimental design, instrumentation, methods, and results are presented and discussed. The numerical part presents a novel model for simulating DP operations in level ice, which treats both the vessel and the ice floes as separate independent bodies with six degrees-of-freedom. The fracture of level ice is calculated on-the-fly based on numerical solution of the ice material failure equations, i.e., the breaking patterns of the ice are not precalculated. The numerical model is connected to a DP controller and the two systems interchange data dynamically and work in a closed-loop. The structures of the models, as well as the physical and mathematical assumptions, are discussed in the paper. Finally, several ice basin experiments are reproduced in the numerical simulator, and the results of the physical and numerical tests are compared and discussed.

Post-simulations of Ice Basin Tests of a Moored Structure in Broken Ice - Challenges and Solutions

Interaction between a moored structure and drifting broken ice is a complex process. To document the expected structure response, ice basin tests of the interaction are common practice. The outcomes of ice basin tests need to be carefully analyzed before extrapolation to expected full-scale target responses. The preferred strategy is to use numerical simulations to correct the measurements. The numerical model needs to be qualified by successful post-simulations of the achieved ice basin interactions. Post-simulations of interactions between drifting broken ice and a moored floating structure are of high complexity. The response of both the structure and the ice field needs to be replicated. This requires a good modeling of the ice field properties that matter (such as the floe size distributions and concentrations) and the boundary conditions affecting the interactions (such as the effect of the ice basin walls). Statoil’s SIBIS numerical model is used to post-simulate ice basin tests of the moored Cat-I drillship. The present paper discusses the challenges with such post-simulations and presents the philosophy chosen for achieving successful postsimulations. Background There is a limited experience with design and operation of moored structures in ice infested waters. Per today, the screening, feasibility or detailed design phases of such concept rely greatly on ice basin tests. This is inline with ISO 19906 (2010) normative requirements which states: “Appropriately scaled physical models and mathematical models may also be used to determine the response of structures to ice actions, in combination with current, wind and wave actions”. The outcome of ice basin tests has to be interpreted and corrected to be exploited in the design process. Different correction methods can be applied, and the use of empirical formulations is a common practice (see e.g. Tatinclaux, 1988). The interaction between a moored structure and drifting ice is a complex process, as changes in the action will affect the structure response and vice-versa. It can be challenging to only use empirical formulations to correct the measurements due to the complex interdependency between the ice action and the structure response. The preferred strategy to correct ice basin measurements is thus to combine ice basin tests with numerical modeling (see also Jensen et al., 2011, Bonnemaire et al., 2014): 1. Simulate numerically the ice basin tests, under achieved conditions, 2. Compare measurements and simulation outcome and qualify the numerical model for the considered interactions, 3. Use the qualified numerical model to simulate the response of the structure to the relevant ice interactions, under target conditions. This methodology results in a correction of the ice basin outcome for the effect of all deviations in the achieved conditions under testing. In addition, the numerical model is qualified and can be used further for simulating additional similar interactions. This procedure applied to moored floating structures in drifting ice is presented and discussed in for instance Jensen et al. (2011) and Bonnemaire et al. (2014). These studies focused on the interaction with intact level ice and ridges. The present paper discusses challenges and solutions in the application of the procedure for the interaction between a moored structure and drifting broken ice. Focus is put in particular on item 1 and 2, the post-simulation of ice basin tests. For an example on item 3 see Metrikin et al., 2015.