Sören Ehlers

Simulating the collision response of ship side structures : A failure criteria benchmark study

Simulations of the collision response of three different ship side structures are performed using the finite element method. The aim is to obtain a force-penetration curve based on the inner mechanics without considering outer dynamics. Respective structures were previously tested in large scale under the right angle ship-ship collision with a bulbous bow and these experiments were used to validate the simulations. The analysis focus is to determine the influence on the collision results from different failure criteria including the mesh size sensitivity. This paper applies the Germanischer Lloyd (GL) criterion based on thru thickness plastic strain, the criterion based on studies of Peschmann and the Rice-Tracey and Crockcroft-Latham (RTCL) criterion. The comparison of results attained here with the experimental results brings an insight into the sensitivity of the failure criteria. The final conclusion of this work is the performance of the studied failure criteria and further guidelines for collision or grounding simulations.

A procedure to optimize ship side structures for crashworthiness

This paper presents a procedure to optimize a conceptual ship side structure from a crashworthiness point of view. As an example, this procedure is presented for a chemical or product tanker. A particle swarm optimization algorithm is used for the procedure. The classification society compliance of the conceptual design is checked through service loads that are applied to the ships hull girder according to Det Norske Veritas. The collision simulations to assess the crashworthiness are carried out with the non-linear finite element solver LS-DYNA. An element length-dependent constant-strain failure criterion is chosen to simulate rupture. A numerical simulation of a stiffened plate serves to validate the material relation until failure. This material relation and failure definition makes possible an accurate prediction of the structural energy absorbed until inner plate rupture. The procedure is adjustable and applicable to other ship types and scenarios.

Decision Support Framework for Exploiting Northern Sea Route Transport Opportunities

Abstract This paper presents a decision-support model identifying the most viable ice class for a liner vessel transiting along the Northern Sea Route. As input, this model requires parameters, some of which are uncertain. These include the time-dependent length of the Northern Sea Route sailing season and corresponding roundtrip times, the additional capital expenditure and operational expenditure for ice class capabilities for the vessel, as well as fuel price. Furthermore, the sensitivity of the model is discussed on the perspective ice extent, respectively the ice class allowed to enter the Northern Sea Route and possible delays, on the basis of current trend predictions.

Iceberg Shape Sensitivity in Ship Impact Assessment in View of Existing Material Models

Large natural resources in the Arctic region will in the coming years require significant shipping activity within and through the Arctic region. When operating in Arctic open water, there is a significant risk of high-energy encounters with smaller ice masses like bergy bits and growlers. Consequently, there is a need to assess the structural response to high energy encounters in ice-infested waters. Experimental data of high energy ice impact are scarce, and numerical models could be used as a tool to provide insight into the possible physical processes and to their structural implications. This paper focuses on impact with small icebergs and bergy bits.In order to rely on the numerical results, it is necessary to have a good understanding of the physical parameters describing the iceberg interaction. Icebergs are in general inhomogeneous with properties dependent among other on temperature, grain size, strain rate, shape and imperfections. Ice crushing is a complicated process involving fracture, melting, high confinement and high pressures. This necessitates significant simplifications in the material modeling. For engineering purposes a representative load model is applied rather than a physically correct ice material model.The local shape dependency of iceberg interaction is investigated by existing representative load material models. For blunt objects and moderate deformations the models agree well, and show a similar range of energy vs. hull deformation. For sharper objects the material models disagree quite strongly. The material model from Liu et.al (2011) crush the ice easily, whereas the models from Gagnon (2007) and Gagnon (2011) both penetrate the hull. From a physical perspective, a sharp ice edge should crush initially until sufficient force is mobilized to deform the vessel hull. Which ice features that will crush or penetrate is important to know in order to efficiently design against iceberg impact.Further work is needed to assess the energy dissipation in ice during crushing, especially for sharp features. This will enable the material models to be calibrated towards an energy criterion, and yield more coherent results. At the moment it is difficult to conclude if any of the ice models behave in a physically acceptable manner based on the structural deformation. Consequently, it is premature to conclude in a design situation as to which local ice shapes are important to design against.Copyright

Optimization-based material parameter identification for the numerical simulation of sea ice in four-point bending

The bending strength of the sea ice cover is crucial in the design of ice-going vessels. Therefore, a series of in situ four-point bending tests have been performed earlier, which are now utilized to present a particle swarm optimization–based procedure to identify the material parameters needed for numerical simulations. The resulting numerical simulations are found to comply well with the experimental results in terms of force, failure time, and displacement. Therefore, it can be concluded that the presented optimization-based material parameter identification procedure can also be used in the future if similar material parameters are to be found.

Collision consequence estimation model for chemical tankers

Chemical tanker traffic constitutes a hazard for the environment; large chemical spills caused by, for example, ship–tanker collisions can have catastrophic effects in similar manner to oil spills. For this reason, it is important to have a generalized spill model for chemical tankers, so that the spill size for a given geographical area can be assessed. The spill volume can be used as an indicator of risk in the given area. Furthermore, such a model can be used to analyze the possible effects of potential risk-mitigating measures such as requiring wider double hulls for chemical tankers. In this article, a spill model is proposed, which for a given collision scenario can model the penetration, spill probability, and size caused by a ship striking a chemical tanker. This is done using finite element method, MATLAB, and statistical metamodels with ship particulars extracted mainly from automatic identification system data from the Gulf of Finland.

Ice Model Tests in Context of the Investment Value of an Offshore Vessel

Offshore activities and shipping in Arctic regions increased significantly in the past decade due to fossil resources. These areas hold about 15% of the worlds oil and gas. Exploration or transportation in such harsh Arctic environments possesses additional risks for the crew, the material and the environment. Hence, ships need to be able to handle low temperatures and ice impacts. Ice class certificates issued by classification societies reflect the ships level of ice capability. They are further required to be admitted to ice covered waters or particular regions in seasons with a certain probability of ice occurrence. In most cases, offshore operations are not continued in ice and ships need to transit through ice after abandoning a site upon ice arrival. However, the daily costs of such specialized vessels are high with up to 0.5M

An Assessment Procedure of the Crashworthiness of an LNG Tanker Side Structure

day that are not reimbursed in downtimes or transit. Therefore, in Northern Arctic regions a higher ice class can significantly enhance the ships workability and therewith its economic value.The lower Polar ice classes, respectively Baltic ice classes, can only be determined analytically with empirically validated formulae for common cargo ships. Other ship types and ships with low L/B ratios are typically required to prove their ice capability through ice model tests. Nevertheless, ice model tests determine only the ice class of the propulsion system, whereas the ice class of the hull structure is determined by calculations. Furthermore, ice model tests are typically conducted towards the end of the design phase where eventual modifications are expensive and potentially threaten the construction schedule of the vessel. Often steel and equipment have already been procured and the manufacturing has begun. This paper presents an iterative procedure of ice model testing and design updates in order to enhance the performance of a particular offshore vessel to meet the requirements for a particular ice class. Thereby, it will be shown how an increase in investment costs for the design changes is compensated by the increasing value of the ships capabilities due to the higher ice class. Furthermore, the drop in value of the ship for the next lower ice class will be indicated as well as the economic consequences should the ship fail to reach the targeted ice class.Copyright

A Multi-Objective Optimisation-Based Structural Design Procedure for the Concept Stage – A Chemical Product Tanker Case Study

Abstract In this paper a procedure is presented for the conceptual design of a crashworthy side structure of a liquefied natural gas (LNG) tanker. The crashworthiness of an LNG tanker is assessed taking into account the deformation restrictions of the containment system. A particle swarm algorithm is used for the structure optimisation. The classification society compliance of the conceptual design is checked for a characteristic service loading condition. Collision simulations are carried out with a non-linear finite element solver in order to assess crashworthiness. An element length-dependent constant strain failure criterion is used to model possible rupture. The resulting optimised conceptual side structure is compared with the initial rules-based concept and with a minimum weight concept, demonstrating the influence of the structural concepts and the containment system restrictions on the crashworthiness.

A procedure to Assess the Damage of a Grounded Ship: A Full-Scale Validation Case Study

Abstract The paper presents a structural design procedure based on multi-objective optimisation and using a decisionsupport algorithm to identify the competitive optimum. The procedure is applied to a chemical product tanker. In addition to the rule compliance under service loading, optimisation is performed with respect to the ship weight, production cost or service life. Appropriate assumptions and simplifications are proposed for not yet known structural solutions or loading conditions. Objectives of the stakeholders are identified in interviews. The paper studies the influence of various objectives on the resulting structural alternatives.