Numerical Simulations of a Tanker Collision with a Bergy Bit Incorporating Hydrodynamics, a Validated Ice Model and Damage to the Vessel

Cold Regions Science and Technology 81 (2012) 26–35 Contents lists available at SciVerse ScienceDirect Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions Numerical simulations of a tanker collision with a bergy bit incorporating hydrodynamics, a validated ice model and damage to the vessel R.E. Gagnon ⁎, J. Wang Ocean, Coastal and River Engineering, National Research Council of Canada, St. John's, NL, Canada A1B 3T5 article info abstract Article history: Numerical simulations of a collision between a loaded tanker and a bergy bit have been conducted using LS- Received 8 March 2012 Dyna™ software. The simulations incorporated hydrodynamics, via LS-Dyna's ALE formulation, and a validat- Accepted 16 April 2012 ed crushable foam ice model. The major portion of the vessel was treated as a rigid body and a section of the hull, located on the starboard side of the forward bow where the ice contact occurred, was modeled as typical Keywords: ship grillage that could deform and sustain damage as a result of the collision. Strategies for dealing with the Bergy bit collision highly varying mesh densities needed for the simulations are discussed as well as load and pressure distribu- Numerical simulation Ship damage tion on the grillage throughout the course of the collision. Realistic movement of the bergy bit due to the vessel's bow wave prior to contact with the ice was observed and the damage to the grillage resembled published results from actual grillage damage tests in the lab. A load measurement from the lab tests com- pared reasonably well with a rough estimate from the simulation. The collision eventually ruptured the hull in a ripping fashion resembling documented incidents of vessel impacts with ice masses. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. 1. Introduction ship was also conducted (Gagnon et al., 2008). In parallel with the ex- perimental aspects of the project the numerical simulation work has Marine transportation off the East Coast of Canada is seriously af- been progressing. The data acquired in the Tow Tank proximity fected by the presence of glacial ice masses. The same can be said for tests, growler impact tests and the field study are invaluable assets Arctic regions and areas off the west coast of Greenland where there for the validation of the numerical model. The computational work are new hydrocarbon prospects. This is a problem for tanker transport has involved simulations of the CCGS Icebreaker Terry Fox colliding of crude oil from current and future offshore production facilities, and with a bergy bit (Gagnon and Derradji-Aouat, 2006), simulations of for the transport of supplies to northern communities such as Chur- the growler impact lab tests (Gagnon, 2007) and the development chill Manitoba and minerals from Labrador. Of most concern for sup- of a numerical ice model that incorporates spalling behavior ply ships and tankers are bergy bits (house-sized glacial ice masses), (Gagnon, 2011). which are difficult to detect using marine radar in rough sea states Recently simulation results were obtained for a loaded tanker col- and when imbedded in sea ice flows. Should these make contact liding with a substantial bergy bit. The model tanker had actual ship with a ship's hull, the impact forces will depend on the masses of grillage on a portion of its bow where the collision occurred that the vessel and ice, the hydrodynamics of the interaction, the ship could sustain damage during the collision. This is the first simulation structure, the shape of the ice mass and its local crushing properties. of a vessel colliding with a glacial ice mass that includes the hydrody- NRC Ocean, Coastal and River Engineering has been studying var- namics of the interaction, a validated ice model and damage to the ious aspects of the problem for several years, with the overall objec- vessel associated with the collision. The purpose of this paper is to tive of creating a validated numerical model of ship/bergy bit show the techniques that were utilized to deal with the various chal- collisions. The work has involved extensive physical model testing lenging aspects of simulations such as this and to show that reason- of a tanker transiting in proximity to bergy bits in NRC's Tow Tank able results were obtained for a realistic collision scenario. Using (Gagnon, 2004a), impact experiments in NRC's Ice Tank using real readily available hardware and commercial software it is obvious growlers (Gagnon, 2004b) and strength and crushing experiments that these same methods can be applied to a wide variety of ship- on iceberg ice and lab-grown ice (e.g. Gagnon, 2004c; Jones et al., ice collision cases. 2003). A field study involving bergy bit impacts with an instrumented 2. Overview of the planned program of simulations ⁎ Corresponding author. Below we present the techniques, configuration details and results E-mail address: [email protected] (R.E. Gagnon). from numerical simulations for one particular scenario. This is part of 0165-232X/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2012.04.006 R.E. Gagnon, J. Wang / Cold Regions Science and Technology 81 (2012) 26–35 27 Fig. 1. Side view of the meshed tanker half-ship. The small dark rectangle at the bow is the grillage segment. Only the front half of the vessel is necessary for the simulation. Table 2 Water box details. Dimensions 344 m×144 m×40 m Density 1000 kg/m3 Fluid computational method ALE (Arbitrary Lagrangian–Eulerian) Element type *SECTION_SOLID (brick) Typical element dimension 2 m Material properties *MAT_NULL Number of elements 247680 What must be conducted in parallel with simulations is a program of validation. To the extent possible, simulation results have to be validated with results from various carefully designed lab and field studies. No single organization that presently exists could undertake all aspects of that work so we anticipate that in the coming years various organizations will, possibly to an even greater extent than in the past, conduct lab and field studies to provide crucial validation data for numerical simulations. For its part, in addition to the activities men- tioned above, NRC has worked with collaborators on various field Fig. 2. Perspective view of the meshed tanker half-ship (top) showing the location of the meshed grillage segment (bottom) on the starboard bow. The differing element sizes between the vessel and the grillage segment are evident. The rectangular attach- ment frame is at the periphery of the grillage segment. a broad plan for simulation studies that could be used as a tool for ship designers and for those who draft operational strategies, and for the general improvement of codes and rules. In the next few years simulations for a variety of vessel and grillage types will be conducted where the influences of ship speed and impact trajectory, and ice mass size, shape and orientation will be investigated. Table 1 Ship details. Dimensions 145 m half length; 48 m beam; 23.9 m depth; 18.9 m draft Vessel type Tanker with bulbous bow Displacement ~ 170,000 ton (full length ship, fully loaded) Velocity 5.14 m/s (10 knots) after 10 s start-up Initial ship-ice 96 m (from bulbous bow to ice knob) separation Element type *SECTION_SHELL Fig. 3. View of the ice knob (bottom) and its location on the bergy bit ice mass (top). Typical element 2m The differing element sizes between the ice knob and ice mass are evident. The bottom dimension layer of elements at the base of the ice knob is treated as a rigid body that is connected Material properties *MAT_RIGID to its companion rigid body, i.e. the ice mass. The rest of the elements of the ice knob Number of elements 3432 are deformable with crushable foam properties during the collision. 28 R.E. Gagnon, J. Wang / Cold Regions Science and Technology 81 (2012) 26–35 Table 3 Table 4 Ice mass details. Air box details. Dimensions: lower portion 48 m×28 m×4 m Dimensions 344 m×144 m×20 m Dimensions: upper portion 40 m×20 m×12 m Density 1.1845 kg/m3 Density and mass 870 kg/m3; 13000 ton Fluid computational method ALE (Arbitrary Lagrangian–Eulerian) Element type *SECTION_SOLID (brick) Element type *SECTION_SOLID (brick) Typical element dimension 2 m Typical element dimension 2 m Material properties *MAT_RIGID Material properties *MAT_NULL Number of elements 1872 Number of elements 123840 experiments aimed at understanding ice crushing behavior. It also participates in MUN's (Memorial University of Newfoundland) STePS2 (Sustainable Technology for Polar Ships and Structures) Pro- Table 5 ject where large-scale ice crushing and impact experiments, which Grillage segment details. incorporate real ship grillage, are planned for the near future in Dimensions 6 m length×3 m height MUN's Structures Lab. Additionally, NRC has for some years been pre- Plate thickness 10 mm paring and testing components of a novel impact panel in order to Frame thickness and height 8 mm×200 mm fi conduct another major eld study involving an instrumented vessel Frame T-top thickness 10 mm×75 mm colliding with bergy bits. and width Frame spacing 350 mm 3. Hardware, software and ice model Stringer thickness and height 8 mm×250 mm Stringer spacing 2 m Density 7850 kg/m3 The simulations were run on a HP Z800 Workstation that has two Young's modulus 200 GPa Intel Xeon Processors X5570 QC, where each has 4 CPU's running at Plastic tangent modulus 1.0 GPa 2.93 GHz.

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