Dynamic Modelling of davit launched sea boat

[AlexHewitt]

Hi all

I’ve been asked to investigate modelling the accelerations and loading during the launch of recovery of a large Rhib from either a single or dual point davit or rail system, suspended by cables, mounted on the side of a ship. The focus of the study is to evaluate the loadings and accelerations on the hook points and the effects on the occupants of the crew due to the resulting motions.

The simple model would consist of a full evolution of picking up the craft with the davit and lowering it roughly 10-15m out and over the ship’s side, followed by it hitting the water and the hooks automatically releasing.

I have the following questions:

• Can I model this sort of system in Autodesk Inventor or Simulation? Which package and version would be best for my needs?
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• I have identified that Autodesk Simulation has the ability to model “flexible joints and link” but can it specifically model cables or an approximate but accurate equivalent thereof?
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• Ideally, the inputs would include a set of exciting forces from a sea keeping package, resulting in movement of the suspension points. I have read that Simulation can model “continuous harmonic loading” and a response spectrum analysis but is there any provision for non harmonic loading?
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• It would be preferable to model the moment when the craft hits the water, which I would like to model in the calm and excited states. Is it possible to model a sea state in this way?

I understand Inventor/Simulation’s purpose is quite different to what I want out of them but hopefully it would be sufficient for a simple model. If anyone could think of better software that would be much appreciated.

Alex

[John_Holtz · Accepted solution]

Hi Alex,

I think I understand the general goal (lowering a small boat, the Rhib, overboard) without understanding all of the details (what's a davit? what's an Rhib?), but the software you should be looking at is Autodesk Simulation Mechanical, and the analysis type would be Mechanical Event Simulation (MES). MES is a dynamic solution (think F=m*a) of nonlinear models (large displacement, nonlinear materials, nonlinear elements like cables). Of the things that you described, here's what can be done in MES:

• Suspend the Rhib from cables
• Specify the motion (X, Y, Z displacements) of the suspension points by using prescribe displacements. If your sea analysis software can output the motion to a text file (preferably a comma separated .CSV file), then you can import the motion into an MES load curve instead of re-typing it. Or if you know what the rocking motion of the ship is, and about what "point", you may be able to use prescribe displacements to input the motion of that point (translation and rotation), and use a rough model of the ship from that point to the suspension points.
• Model the boat being lowered. (MES includes pulley elements and cylinders, so at least a portion of the pay-out can be modeled.)
• Model the boat being lowered into the calm water by using a hydrostatic pressure on the boat.
• Depending on the complexity of the "full evolution", it may need to be analyzed is different stages instead of one model with the complete motion.

I do not know how you would be able to analyze these aspects, but maybe someone else has some ideas:

• Model the Rhib hitting the sea when it is not calm. (Of course, if you knew what the forces or motion of the Rhib is versus time, then you could apply that to the Rhib and watch it bounce around.)

Ship ahoy, matey! I'm feeling sea sick already.

[John_Holtz]

I received an email notification which shows that Alex posted another reply, but it does not appear when I look at the thread. The link in the email that should open the post causes an "unexpected error", so it is either a glych in the system or just for me.

The basic question is whether Simulation includes the ability to create models, or does it need a modeler like Inventor.

The answer is that Simulation does include its own modeling capability. You can either model directly in the software, or use Autodesk Inventor Fusion which is provided as a free add-on in the Simulation installer. Fusion is a 3D solid modeler, similar to Inventor, but different as well. The complexity or simplicity of the model will determine if it is easier to model within Simulation, or whether a model should be imported.

By the way, Simulation can read solid models from many applications (Inventor, Pro/E, SolidWorks, etc), surface models from some of the CAD apps, and wireframe "models or drawings" from AutoCAD and wireframe IGS.