FEM analysis

[Smircio]

Hi to everyone. I'm studying by finite elements the structural strain on a trimaran freighter, I have calculate the right pressure to load (for exemple the pressure due to the impact af waves....) but I don't know how and where to constrain the ship.Can anyone help me?

Thank You

[Sands]

Although I haven't done FEA on a boat, I have done quite a bit on industrial equipment - including non-linear impact. What you are talking about seams to be impact? Are you looking at wave impact on surfaces or structural loading of the hull attachment members?

The problem that you are going to run into is a boat is statically indeterminate since it's not naturally attached to or contacting anything solid (as long as it doesn't sink...! ) Essentially, it's not going to be easy. The whole garbage in, garbage out routine.

If you are talking about wave loading, the only *real* way is to build a model or prototype and test it. 2nd best is to look at similar designs of similar class and intended use and see how strong/flexible those designs are built.

If you are talking about structural members, then I suppose the outriggers should be able to support themselves suspended? An open ocean (?) freighter should be able to crest hurricane waves and not break it's back.

Some more information on the basic dimensions and intended uses would help.

[Smircio]

Thank you, Sands, for your so fast reply.
Yes, I know, the problem should be statically indeterminate because allow rigid displacement...But that's my question: Is there a manner (using for exemple the so called "Inertia Relief"...) for simulate the impact of the ship with a wave? I've tryied to constrain for all the 3 degree of freedom(3 translations) the "kell" of the central hull (in this way I can see the torque on the wet deck), but I've obtained too high value of von Mises stresses near the constrained nodes.

[Sands]

You can put in springs (package dependent) to simulate a "fixed" location. I'd ignore the stresses near the fixing points as you can get all sorts of weird results. But since your fixing points are artificial anyway, the whole analysis is suspect.

Any further comments are going to require pictures, etc. to get a better idea of what you are doing.

[water addict]

Originally Posted by Smircio

Hi to everyone. I'm studying by finite elements the structural strain on a trimaran freighter, I have calculate the right pressure to load (for exemple the pressure due to the impact af waves....) but I don't know how and where to constrain the ship.Can anyone help me?

Thank You

Smircio,
It depends on the load you are looking to design to. Are you interested in the local (stiffened panel) stresses, or global (hull bending/shear) stresses? This is important to determine first. I have done ship FEA associated with my job for many years, and there are ways to approach both, depending on the details.

[MikeJohns]

You really need a specific package for this. You have accelerations in 6 degrees of freedom that dictate the experienced hydrodynamic pressure distribution. Also there's the wave type which will complicate the analysis further (and that's why we end up with scantling rules :-) ).

FEA is good for static panel analysis, for static frame loads and for global strength but once we start to get into the dynamics it is hard to check for sensible results. Set up and analyze worst case scenarios and I would assume that the hull will not "give" significantly (wrt impact time) to the wave impact by translation. But what size vessel are we talking about? What package are you trying to use?

[Guest625101138]

I have NOT done FEA on ships. On the other hand I have done it on machines and I do a lot of thinking about the forces on small boats and some simple related stress analysis.

I would think 'static' conditions would be the control on the design on a trimaran freighter. [Although I cannot think of the benefits of a multi-hulled freighter - maybe high speed for length and easily driven.] The advantage of the trimaran should be best achieved by making a long narrow entry on the hulls to avoid slamming into waves and thereby keeping decelerations low. I

The static load cases that concern me with my lightweigt trimarans are:
1. The bending stress on the main hull when going downwind or upwind and the centre of the hull is in a trough. I keep buoyancy in the ends low to keep the stress down.
2. The vertical bending moment on the outrigger arm. The worst case is in a beam sea with the main hull suspended or the outriggers fully submerged. I do not have more buoyancy in the outriggers than that required to give adequate stability.
3. The torsional stress in the outrigger support arm when only the bow or stern of the outrigger is fully immersed.
4. Localised stresses around the supports for the heavy bits and propulsion unit.

The 'static' conditions have to be analysed with a fatigue life in mind as there is constant cyclic loading up to these extremes. I aim to design stresses to be lower than the fatigue limit (typically 10E8 cycles) for the material I use. I have no idea how fatigue criteria are applied to ship design.

I use very narrow entries on all 3 hulls so the impact forces from waves are low relative to the static load cases above. I also keep the outrigger arm high to keep it above the wave level as much as practical. With larger ships there would be local stresses due to dynamic conditions. To analyse these I would fix the hull at a bulkhead a good distance from the loaded area being analysed. Discount any high stress around the point of fixing.

If you provided some idea of the shape and speed of the ship I might come up with more applicable load cases.

How do you discharge a trimaran freighter? Maybe RO-RO!

Rick W.

[PI Design]

Hi Smircio,

There should be several papers by QinetiQ, and possibly VT (formerly Vosper Thornycroft) on the design of the experimental trimaran RV Triton. These should be available from RINA (UK, not Italian!).

[PI Design]

You can still find Triton on Google, but for some reason QinetiQ have erased all mention of it on their website. But they did loads and loads of research and analysis into trimaran hull structural analysis, and published the much of their work.

[Smircio]

First of all, I want to thank everyone for the support....
RINa(Italian) give information about catamaran and specified how to calculate the pressure on the hull for different types of condition and also give information about the way to constrain the ship (Hidrostatic pressure in still water, presure due to max velocity in still water,loads due to cross sectione acceleration....) but it say just the formula of the pressure for the so called "combinata 2" that is the pressure due to the impact af the hull on the wave when it sag and hag not to mention the constrain conditions. All that is valid for catamaran....for trimaran I've done some simulation to investigate the worst situation due to "combinata 2", angle of incidence of the wave and wavelength, constrainng the ship for all the 3 traslation in corrispondence of the keel, but in that nodes I've too high stress concentration...