An advanced technology for structural crashworthiness analysis of a ship colliding with an ice-ridge: Numerical modelling and experiments

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Highlights

  • Nonlinear structural problems associated with a ship colliding with an ice-ridge are studied.

  • Effects of buckling, collapse, crushing, plasticity and fracture together with environmental and operational factors, such as the loading speed (strain rate), temperature and salinity are taken into account.

  • An advanced technology for numerical computations of structural crashworthiness in the event of a ship colliding with an ice-ridge is developed.

  • The nonlinear finite element method is used for modelling the problem, in which the ship structures are modelled by plate-shell finite elements and the ice-ridge structures are modelled by solid elements together with the KOSORI ice material models.

  • Two sets of experiments are performed to validate the numerical computations.

  • It is concluded that the developed technology is very useful for computing the structural crashworthiness of a ship when colliding with an ice-ridge.

Abstract

The structural engineering problem associated with a ship colliding with an ice-ridge involves highly nonlinear mechanisms including buckling, collapse, crushing, plasticity and fracture together with environmental and operational factors, such as the loading speed (strain rate), temperature and salinity. The objective of this paper is to develop an advanced technology for numerical computations of structural crashworthiness in the event of a ship colliding with an ice-ridge. The nonlinear finite element method is used for modelling the problem, in which the ship structures are modelled by plate-shell finite elements and the ice-ridge structures are modelled by solid elements together with the KOSORI ice material models. Two sets of experiments are performed to validate the numerical computations. In the first set of experiments, ice is dropped on a steel plate from a height of 2 m, and in the second, a steel solid (rigid) body is dropped on a steel plate under the same conditions. The results of the two experiments are compared to determine the differences between the ice responses and solid (rigid) body responses on the steel plate. It is concluded that the developed technology is very useful for computing the structural crashworthiness of a ship when colliding with an ice-ridge. Details of the test results are documented.

Introduction

In the Arctic region, ships and offshore structures are increasingly being exposed to the impact loads arising from collisions with ice ridges and floating in ice infested waters [1]. Thus, predicting the ice loads has become a necessary parameter in the structural design of ships and other marine structures. The ice loads are an ice ridge-marine structure collision event can be characterised by the external and internal mechanics, as shown in Fig. 1 [2].

The internal mechanics relate to the energy absorbed by the colliding bodies in terms of the damage they incur. In some studies, the ice is assumed to act as a rigid material in a collision [3], and thus, all of the collision energy is absorbed by the steel structure. In reality, however, steel is much stronger than ice and the collision energy is shared by both colliding bodies [4] This approximation is referred to as the shared energy method and is indicated in Fig. 2. The material models of both the steel and ice materials should be characterised in numerical computations because the collision impact energy is dissipated in the deformation of the colliding bodies.

Ince et al. [5] developed the so-called KOSORI ice model, which includes a constitutive equation and a fracture model of the ice structural mechanics. This comprehensive model incorporates the effects of influencing parameters, such as the strain rate, temperature and salinity, based on experimental results.

The motivation of this paper comes from the need to validate the model. In this paper, the ice strength and fracture behaviour in the KOSORI ice model are combined with the traditional metal strength model to implement as a user defined material model (UMAT) in a nonlinear finite element program to solve the ice-steel interaction problem. An experiment is conducted on the interaction between a steel plate and ice to validate the developed methods. In addition, a steel body which is stiffer than plate, drop test is performed in similar conditions to the ice drop test to show the difference between considering ice as rigid and as deformable.

Section snippets

Experiments

There is a lack of experimental evidence on the impact between ice and steel, with only a few numerical and experimental studies [6], [7], [8], [9] in the literature. Two main approaches are used to model the impact between ice and steel. First, all of the energy is absorbed by the steel and second, the energy is absorbed by both the ice and the steel structure. The experiments presented in this paper were performed to understand the interaction between ice and steel under impact loading

Numerical model

Numerical computations are the easiest and most economical way to understand material responses without conducting experiments. For numerical simulations first step is meshing process. Computer programs run on the mesh model of the design. Therefore, high quality mesh is required to take good results from computer simulations. In the literature some studies are focused mesh qualities and mesh size [10]. It is not the target of this thesis, but simply will explain the mesh technic and mesh size.

Comparison and discussion

In this section, the steel body drop test and ice drop test results are compared and the KOSORI ice model is compared with the ice drop experiment.

Conclusions

The objective of this paper was to develop an advanced technology for computing the structural crashworthiness of a ship colliding with an ice-ridge. Some impact tests were conducted to verify the ice and steel interaction simulation. The responses of ice under the rigid assumption by using stiffed steel object and real ice responses on steel plate were compared with the experimental results and the results were found to be quite different.

The results confirmed the need for a well-defined ice

Acknowledgements

This study was undertaken at the Lloyd's Register Foundation Research Centre of Excellence at Pusan National University, Busan, Korea. Lloyd's Register Foundation (LRF), a UK registered charity and sole shareholder of Lloyd's Register Group Ltd, invests in science, engineering and technology for public benefit, worldwide.

References (18)

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