Kristjan Tabri

Numerical and Experimental Investigation on the Collision Resistance of the X-core Structure

This paper analyses the collision resistance of the X-core structure. The analysis includes a detailed investigation of the non-linear plate and laser weld material behaviour using optical, full-field strain measurements. The resulting material relationships are implemented into the finite element model. Furthermore, the finite element model includes the influence of the ship motions to accurately predict the collision resistance. The verification of the numerical results is done by a comparison of the experimental and numerical force versus penetration curves and by a comparison of the deformed geometries. The latter is achieved through a digitised three-dimensional model of the post-experimental X-core structure. As a result, the accuracy of the collision simulations is presented and discussed.

Influence of coupling in the prediction of ship collision damage

This paper studies the influence of coupling between the ship motions and the structural resistance in predicting ship collision damage. Several collision scenarios are simulated using coupled and decoupled approaches. A coupled approach implies a time domain simulation, in which a precise description of the whole collision process together with the full time histories of the motions and forces involved is achieved. In this approach, the ships’ motions are evaluated in parallel to the structural deformations, that is, the coupling between the external dynamic and internal mechanics is preserved. A decoupled approach is based on the conservation of momentum and allows fast estimation of deformation energy without providing exact ship motions. This method is based on the ship masses and velocities, and there is no coupling with the structural behaviour. The extent of deformation is defined by applying some calculation method, such as a finite element method for example, to evaluate the penetration depth along some prescribed path required to absorb this energy. The comparison of the outcomes of two methods reveals that while the deformation energy is predicted with a reasonable accuracy using both methods, the difference in penetration path is significant. The decoupled approach precisely predicts the penetration in symmetric collision, but in non-symmetric collisions under oblique angles, the results differ from those of the coupled solution and also from the experimental measurements. The forward velocity of the struck ship does not significantly influence the precision of the decoupled approach.

Ship collision simulations using different fracture criteria and mesh size

There is a wide range of fracture criteria available in the literature to simulate the ductile fracture in large structures. Almost all criteria depend in some form on the mesh size and some criteria also account the effect of the stress state on the fracture ductility. Furthermore, a material model employed could considerably influence the analysis results. Therefore, in this study, four different fracture criteria, three different mesh densities and two different material models are used to simulate ship collision with a rigid bulb. Thereby, plastic dissipation energy, force-displacement curves and structural failure mechanism is compared between different fracture criteria. Advantages and disadvantages of each criterion are discussed.Copyright

Interaction of ice force in ship-ship collision

The maritime transportation system in the Northern Baltic Sea (NBS) is complex and operates under varying environmental conditions. The most challenging conditions relate to the presence of ice-cover, which for the NBS e.g. for the Gulf of Finland or Bay of Bothnia, can remain up to several months. The number of maritime accidents in these two areas is the highest during winter season, which can involve accidents like groundings, collisions, damages due to the ice etc. The paper presents the model to predict the ice interaction in ship-ship collision dynamics. The ice forces are included in the time-domain simulation model and also in a simplified model based on the momentum conservation. The simplified model is proposed for a rapid estimation of increase in deformation energy due to the presence of ice forces. The analysis shows that the ice influence depends strongly on the ship mass, ice thickness and collision speed. The maximum increase in the deformation energy in the studied collision scenarios was up to 19%.Copyright

Collision Consequence Assessment of ROPAX Vessels Operating in the Baltic Sea

A Ship collision accident represents a daily threat for vessels operating in dense traffic zones. The collision consequences may include loss of life or severe injuries if passengers are on board. The latter would be the case for ROPAX vessels, which are fairly dominant in the Baltic Sea connecting various member states. Furthermore, their routes tend to be in cross-traffic with the cargo vessels travelling through the full extent of the Baltic Sea. Therefore, it is of utmost importance to be able to assess the collision consequences for ROPAX vessels operating in the Baltic Sea with sufficient accuracy. This will result in an overview of possible damage scenarios for the actual traffic situation at a given location. As an example location the dense cross traffic between Helsinki and Tallinn will be analyzed and discussed.The analysis procedure combines three steps: (1) determination of possible accidental scenarios based on traffic statistics; (2) assessment of the structural resistance of the colliding ship and (3) the evaluation of selected accidental scenarios using a time-efficient semi-analytical approach. The level of structural resistance of the chosen ships is assessed in a quasi-static manner using finite element method. This information is the basis for the calibration of a semi-analytical collision simulation model used to simulate large number of the accidental scenarios typical to the selected location. The presented results will be limited to the initial choice between vessels and dimensions, respectively masses, but the procedure can easily be extended to cover a vast amount of colliding vessels. However, the actual collision risk can be obtained using the presented results if the traffic along the vessels route is known.Copyright

An online platform for rapid oil outflow assessment from grounded tankers for pollution response

The risk of oil spills is an ongoing societal concern. Whereas several decision support systems exist for predicting the fate and drift of spilled oil, there is a lack of accurate models for assessing the amount of oil spilled and its temporal evolution. In order to close this gap, this paper presents an online platform for the fast assessment of tanker grounding accidents in terms of structural damage and time-dependent amount of spilled cargo oil. The simulation platform consists of the definition of accidental scenarios; the assessment of the grounding damage and the prediction of the time-dependent oil spill size. The performance of this integrated online simulation environment is exemplified through illustrative case studies representing two plausible accidental grounding scenarios in the Gulf of Finland: one resulting in oil spill of about 50 t, while in the other the inner hull remained intact and no spill occurred.