SPH - Pacific ESI

Examples of Ship Motion and Wash Predictions by Smoothed Particle Hydrodynamics (SPH) Bruce Cartwright (1), Paul Groenenboom (2), Damian McGuckin (1) Pacific Engineering Systems International Pty Ltd, Artarmon, NSW, Australia Engineering Systems International BV, Krimpen aan den Ijssel, The Netherlands ( Emails: [email protected], [email protected], [email protected] respectively ) Abstract Examples of challenging scenarios for the analysis of ship performance in waves are discussed. The software tool that allows these scenarios to be analysed in a conceptually clean and coupled numerical problem, employs smoothed particle hydrodynamic techniques (SPH) integrated into an existing explicit finite element structural analysis code. Introduction The novel use of smoothed particle hydrodynamics (SPH) embedded in a finite element (FE) program, is proving to be an effective tool for the performance evaluation of commercial and naval vessels in limit-state operating conditions such as rogue waves [1], breaking waves, or large swells. This short technical note demonstrates the non-steady motion of several types of vessels in varying sea conditions using the industrially proven, commercial software code PAM-SHOCK that includes such SPH features. The conventional approach to the computational assessment of hydrodynamic loads on ships is based predominantly on the numerical solution of a set of equations for fluid flow within a continuous domain [7]. Usually, this includes simplifying assumptions such as the flow being irrotational, or the solution being based on diffraction theory. Most solutions methods have serious limitations with respect to the shape of the free surface and often rely heavily on empirical formulae. For reasonably smooth waves and/or simple hull shapes, such solutions may be adequate. For the common phenomena of breaking waves and green water, or the extreme case of a rogue wave, the validity of such assumptions is doubtful. The validity of these assumptions becomes even more doubtful when applied to novel hull shapes, for which limited performance data is available. Summarising, the currently available empirical tools are often not suitable for extreme events or novel hulls as they do not allow the accurate consideration of physical scenarios such as slamming or violent wave impact, where the effect of the entrapped air or local rough water may be relevant. A common approach to severe waves or novel hull forms would be a de-coupled approach where a theoretical hydrodynamic analysis predicts pressures on a rigid hull, and then these pressures are transferred to a finite element code to calculate the structural response for the vessel. In some situations, this approach may be flawed. For severe waves, the deformation of the vessel can influence the wave loadings. For example when the duration of a wave load on the ship structure is small compared to the period of vibration of the ship structure, the influence of structural deformation on the hydrodynamic phenomena, or hydro-elasticity, may no longer be neglected. For that reason, the numerical prediction of the structural Copyright Pacific ESI 2004

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response of ships to extreme waves demands a fully coupled solution in which the fluidstructure interaction effects are included. The SPH method was introduced some 30 years ago for astrophysical simulations [3]. This mesh free method was found to be quite attractive for a number of other challenging numerical simulation problems. These include hyper-velocity impact of space debris and fluid flows [5] involving free surfaces or interfaces [8]. The coupling of SPH within a finite element code for structural dynamics facilitates the simulation of fluid-structure interaction. This approach has been validated for a range of applications such as sloshing in fuel tanks, impact on water surfaces [6], bird strike on aircraft [11], the opening of a heart valve, the collapse of the wall of a dam and wave effects on offshore structures [2]. The SPH technique has been shown to be particularly well suited to non-steady events such as freak waves, broaching, and wave-slamming [9]. Scale effects common to model ship testing are non-existent with the SPH technique as the ship model is 100% full scale, and speeds are at real speeds. Approximations endemic in empirical relationships are removed because the SPH analysis makes no assumptions on ship behaviour, or wave formation – the results are those of physical of interactions. An SPH approach also has none of the drawbacks of physical prototyping because extreme or unusual wave conditions can be handled just as easily as calm ones. With no requirement for the construction of an actual vessel, there are the usual virtual prototyping benefits such as significant savings in cost and dramatic reductions in lead times. In this new application, the SPH particles display the characteristics of large waves on the ocean surface. An FE model of a ship was then allowed to interact with the SPH waves. The resulting simulations demonstrated very realistic vessel motions such as buoyancy, dynamic lift, heave, pitch and roll. The software can display the movement of the vessel to allow the prediction of tie-down strengths for cargo, sea keeping, and even ride-quality and seasickness tendencies for occupants. Of course, raw structural issues such as the hog, sag and twisting of the hull itself are also easily visualized if elastic structural ship models are used. In addition to vessel movements, the prediction of forces acting between the water and the vessel allow wave loadings to be observed. Although a non-deformable ship model has been used in the cases noted here, subsequent work will use an elastic ship structure to develop hull loads and the overall ship response due to the SPH wave motions. The use of SPH techniques allows the virtual try-out of full-sized ships in full-sized ocean conditions, providing an extremely useful, predictive tool to a naval design team during both early and subsequent stages of the design. Virtual try-out is particularly useful when a wealth of experience is missing, as would be the case during the development of new generation hull forms, or where the cost of construction of physical prototypes and associated experiments are impractical for one reason or another. The following examples demonstrate a small set of scenarios that have been modelled to date with the SPH technique. They do not go into detail about the underlying SPH and FE theory that is quite well known for many years now. They also do not look at the modelling issues involved in the SPH representation of waves [10], boundary conditions or the detailed validation and verification work that is currently underway to ensure reliability and robustness in this type of analysis. For some of these aspects, the references to this document could be consulted, although this list is by no means a complete bibliography on the topic. Copyright Pacific ESI 2004



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INCAT 91 metre Wave Piercing Catamaran This is an aluminium passenger and vehicle ferry, launched in 1997 by INCAT Tasmania. Vessels of this form have recently received praise as military logistics support vessels for the US Navy.

A 91m Incat Tasmania vessel (image from Incat Tasmania website). Length Overall Breadth Overall Displacement Service Speed

91 metre 26 metre 510 – 800 tonne 38 knot

Model of the 91m wave-piercing catamaran – bridge and topside detail were omitted.

At rest (left), with bow down trim. At speed (right), hydrodynamic lift changes trim. The simulation process begins by allowing the vessel to float freely to find its equilibrium position. The trim of the vessel in this equilibrium position is a demonstration of the hydrostatics of the vessel, and agrees well with design data from a hull design software package. Confirmation of the hydrostatics in simulation can be demonstrated by altering the centre of gravity of the model, in which case a corresponding change in the trim of the vessel is observed in simulation. Hydrodynamics were observed by steadily accelerating the vessel from its equilibrium position to a cruise speed, at which point the dynamic lift from the under-body shape of the Copyright Pacific ESI 2004



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hull was developed, altering the pitch of the vessel, all of which would be expected from the physics, and showing good agreement with actual vessel behaviour. Oblique waves can be modelled in the simulation much like a real tow tank, except that the vessel is free to move in a completely untethered response. As shown here, a vessel with constant thrust approaches a large wave on the starboard bow. The result is significant yaw to port, with corresponding heave and pitch motions. The ship motions can be used to determine indices for sea-sickness, or tie-down requirements for cargo. From a structural aspect, the use of a structural model would reveal ship-loads during this event. Internal tanks could also be modelled, providing information on sloshing of fluids in the tanks. Perspective side view

Top View

The SPH approach allows a completely untethered response to an oblique wave.

The simulation model reveals both ship motions (left) and forces on the vessel due to the waves (right).

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US Navy DDG51 The DDG 51 Class is a US-Navy multi-mission guided missile destroyer. It has a high sprint speed and carries a versatile armament allowing it to undertake a variety of roles in varied conditions. This model was sailed (numerically) into a wave at an oblique angle. As can be seen from the simulation, significant yaw and roll was revealed as the wave passed the vessel Length Overall Breadth Overall Displacement Service Speed

154 metre 20.4 metre 7600 tonne 30 knot

Model of the DDG51, as used in the simulation – without rudders or propellers.




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HYSWAC – US Navy SES200B HYSWAC is a Hybrid Small Waterplane Area Catamaran vessel developed from SWATH research. The vessel, sponsored by the Office of Naval Research, is designed and built by Navatek Ltd, Hawaii. The vessel uses Navatek’s (patented) Lifting Body technology to improve seakeeping

Images of HYSWAC SES200b from the Navatek website. Length Overall Breadth Overall Displacement Service Speed

48.8 metre 13.1 metre 270 tonne 30 knot

The model used in the SPH simulations – the bridge structure was not present.

The effect of the lifting-body is illustrated through SPH simulation. Simulations of the HYSWAC illustrate the lifting of the main hull from the water as the design speed is reached (upper two images). As waves of increasing height are encountered (lower two images), the seahandling performance of the SWATH feature of this vessel is observed, to a point where the waves are too severe, resulting in large motions of the vessel.




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LCS Trimaran – General Dynamics Littoral Combat Ship This vessel is one or two designs that have been selected for the production of two prototypes for the US Navy’s new Littoral Combat Ship, or LCS, a fast, highly manoeuvrable vessel for shallow waters. The trimaran is based on the multi-hull technology of Austal Ships, which is part of the General Dynamics consortium for the LCS. A recently launched commercial passenger and vehicle ferry is the first trimaran of this size to be built. Note that the final shape of the LCS prototype vessel has recently changed slightly from that shown below. Length Overall Breadth Overall Displacement Service Speed

127 metre 30 metre 2637 tonne (guessed, and probably light) 30 knot

Model used for the simulations. A chined hull was used for this model, whereas later research showed the vessel has a round-bilge.

Image of the General Dynamics Littoral Combat Ship based on Austal Ships technology. (image from the US Navy website)

Pitch and heave response is observed in a side view of progress into a headsea. These results seem extreme, suggesting our guessed parameters were not quite accurate.

Encountering a wave on the starboard bow, the yaw, pitch, roll and heave response of the trimaran is easily visualised. Copyright Pacific ESI 2004



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Zodiac RHIB This size of a rigid-hull inflatable boats span the range between recreational, rescue craft, and law-enforcement, or military vessels. The light weight and large power of this type of craft can lead to high pounding forces on both the occupants and the hull, leading to potential health risks, or compromised operational performance to both over prolonged periods. Simulations such as those shown below allow the magnitude of these loads to be predicted at an early stage, enabling design changes to be made or operational guidelines to be proposed, to ensure longevity in, and optimal operation of, both the crew and the vessel. Length Overall Breadth Overall Displacement Service Speed

7.33 metre 2.74 metre 2.5 tonne 30 knot

Model used for the simulations, and image of a Zodiac RIB (image from the Zodiac website).

A RHIB is travelling from right to left into oncoming waves, causing the craft to become airborne with resulting high impact forces on landing back into the water. Copyright Pacific ESI 2004



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Ocean Racing Sailing Trimaran These high-speed sailing craft are considered the Formula 1 of the ocean. They are raced either fully crewed in a sheltered stretch of water, or single-handedly across oceans. The speed of such craft regularly surpasses 30 knots. In 2002 [4], a few of these yachts were caught in severe and confused waves. To preserve the boats, the sailors reduced sail and speed. Even so, a number of these vessels still suffered structured failure due to wave impacts. The simulation that is shown below attempted to duplicate these conditions. Length Overall Breadth Overall Displacement Service Speed

18 metre 17 metre 3.5 tonne 30+ knot

This simulation represents a confused sea-state, where the waves are approaching from different directions at different speeds and heights – resulting in violent pounding of the vessel. Copyright Pacific ESI 2004



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Wash Prediction of Catamaran Hulls Using the INCAT 91 metre wave-piercing catamaran from earlier, a demonstration of wash analysis was performed using the SPH technique. Although this vessel is not a low-wash design, the results demonstrate the capability of the technique. In both examples here, the wash is observed to a distance of 50m from the hull centreline, and is displayed with a vertical of resolution of the water surface of 100mm.

Wash at 10 and 30 knots for a single demi-hull. The next page shows the wash prediction for the full catamaran vessel.




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The wash from the complete vessel shows a more complex wash pattern that results from the interaction of the wash from the two demi-hulls.

The channel used for this example was relatively shallow, and did not produce transverse waves in the wash pattern. The reduction in wake is a critical design requirement for many commercial vessels operating in ecologically sensitive waterways. In their operations on lakes or up and down rivers, the erosion damage and any associated pollution caused by the vessel wash needs to be minimized. The ability to reliably predict such wash is critical to the design process. Being able to visualize the wake generated by a given hull-shape gives the design team the tools to evaluate alternative designs to arrive at the one most suitable. Closure In many respects, the concepts are quite simple. The utilization of a fully coupled solution means that modelling complexities of the combination of the fluid and the structure simply do not exist. It is a case of build the model and run the simulation. Even so, there is the issue of a long simulation time. While it is of the same order of magnitude as say, a train impact during a crash situation, computation times are of the order of days for complex problems, especially when a non-rigid model for the boat is introduced. This work comprises an ongoing development program between Pacific ESI and ESI BV with support from ESI Software. The project was initially concerned with the behaviour of Copyright Pacific ESI 2004



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composite multi-hull vessels in extreme sea-states although it has since grown beyond that. The current and ongoing work includes the consideration of large tankers, with a special interest in those carrying LNG, and some military applications. Finally, Pacific ESI would like to acknowledge some enthusiastic support from the Australian Defence Science and Technology Organization and ESI BV correspondingly from the Marine Research Institute of the Netherlands. References 1. Clauss, G F, “Task Related Rogue Waves Embedded in Extreme Seas”, 21st International Conference on Offshore Mechanics and Arctic Engineering, February 2002, Oslo, Norway. 2. Fontaine, E, “On the use of Smoothed Particle Hydrodynamics to model extreme waves and their interaction with a structure”, Ifremer and IRCN workshop on "Rogue waves", November 2000, Brest, France. 3 Gingold, R A, and Monaghan, J J, “Smoothed Particle Hydrodynamics – theory and application to non-spherical stars”, Mon Not R Astron Soc, 181, 375-389, 1977 4. Hollam, D, “The time has come…”, Part 2, pp 45-47, Seahorse Magazine, November 2003, Royal Ocean Racing Club, UK. 5. Monaghan, J, “Simulating Free Surface Flows with SPH”, pp 399-406, Journal of Computational Physics 1994, 6. Pentecote, N, Kohlgrueber, D and Kamoulakos A., “Simulation of water impact problems using the Smoothed Particle Hydrodynamics Method”. ICD’03 conference, Lille, France, December, 2003. 7. Peterson, O, Wiklund, K M, “Det Norske Veritas Requirements for Direct Calculation Methods of High Speed and Light Craft”, pp 399-407, FAST ’99 Conference Proceedings 1999. 8. Servat, D, Léonard, J, Perrier E, and Treuil, J-P, “The RIVAGE project: a new approach for simulating runoff dynamics”, pp. 592-601, Proceedings of the International Workshop of EurAgEngís Field of Interest on Soil and Water 1999, Leuven, Belgium. 9. Thomas, G., Davis, M., Whelan, J., Roberts, T. (2001), “Slamming Response of Large High Speed Catamarans”, FAST 2001, Conference Proceedings. 10. Tulin, M, Landrini, M, “Breaking Waves in the Ocean and Around Ships”, pp 713-745, 23rd ONR Symp on Naval Hydrodynamics, Val de Reuil, France 2000. 11. McCarthy, M A, Xiao, J R, McCarthy, C T, Kamoulakos, A, Ramos, J, Gallard J P, and Melito V, ” Modelling of Bird Strike on an Aircraft Wing Leading Edge Made from Fibre Metal Laminates – Part 2, Modelling of Impact with SPH Bird Model, Applied Composite, , Materials 11, pp 317-340, 2004. Copyright Pacific ESI 2004



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