Assesing for Racking of a pontoon boat in waves with a moon pool.

Hi All,

The pictures that Ad Hoc posted on 12-16 at 7:04pm EST are what I mean by racking.

I don't mean to say that anyone here was more right than others, but I ended up using Ad Hoc's calculations of Mp, Mt, and Ms to calculate moments that will occur on the frame of the structure. I am now running FEA simulations on the frame using those moments as load cases.

The truss concept is an interesting one and I will admit I have not done a full analysis to see how it would work out, but we are short on time and money(as with pretty much every job). I'm guessing the trusses may save a little in material and weight, but would be much more expensive to make than a simple box frame. The structure is also going to be deployed under a bridge so we do have a limit on the height of the structure.

Thanks for your continued help!

 

Twisting / Racking

I believe the issue in this discussion, perhaps improperly called "racking" is illustrated in post 10, where there is a tendency to twist about the transverse axis of the boat. One pontoon gets most of its buoyancy in its after section, one pontoon gets most of its buoyancy in the forward section. Generally due to asymmetrical wave action. Post 13 also attempts to demonstrate this load condition.

Rosse …"how the numbers come out compared to the conventional (low profile, typical for catamarans) structure connecting the two pontoons" and Ad Hoc comment……….."And what is conventional here and why?

By conventional I mean a structure between pontoons which does not rise above the catamaran's deck very far. Compare the proposed rather tall pyramid shape in post 9 and 13, with the (what I would call) the more conventional structure shown in post 19, where the deck is as high as the strucrure gets.

 

I'm going to stick my neck out on this one. I think racking applies to a load on complex structure such as a space frame and racked refers to permanent deformation at the corners. Whereas torsion and shear are much more specific in engineering usage.

A racked cube can have faces that are twisted and/or diamonded. It just means the space frame has been distorted at the joints. This could be caused by shear or by torsion.

But the OP didn't mean permanent distortion, and I took him to mean deck twist. I don't know why, I just did. I'm used to hearing diamonded for a ladder frame shear, but that's a truck frame term.

 

Question for Ad Hoc. Do most naval architects design to those standards that your showed me or do they add a factor of safety on top of those standards. If so is there a typical factor of safety that you would recommend?

Basically is there a factor of safety already built into those standards?

 

iang

Forget a truss, you need box beams.

As previously noted you first need to see what deflections you get....what material is this, not seen one noted yet?

The loads from the DNV bending moments are the loads, but the response, is down to you - what is designed. What is critical for your operations, do you intend to allow for any movement of the deck from corner to corner, if so how much is acceptable etc. When you apply these global loads, is the deflection greater or lesser than the deflections you consider to be acceptable? Once that is established, the same goes for the stresses you get globally and locally.

We need to establish these simple facts first.

 

Yup, a box beam frame is what I have designed so far.

Right now I am using 1010 hot rolled steel in my FEA simulations. We will most likely use some sort of hot rolled steel. Any recommendations for something that is corrosion resistant and still pretty cheap? We were also thinking of coating everything in coal tar epoxy. Is that the right way to go?

I have mostly been concerned with survivability of the platform. I'll have to think about what the maximum allowable difflection the turbine mounting points can take. That is probably the most critical area.

My FEA results are currently showing that the highest stresses in the frame will be right around the yield strength of 1010 hot rolled steel.

I was basically just wondering is there some conservatives built into those calculations for Mp, Mt and Ms or should I be adding a factor of safety on top of them?

 

What you have to check is that the stresses on the structure do not exceed, at any point, the maximum allowable stresses provided by the Classification Society. At these values the CS has already taken into account safety factors. No need to apply additional safety factors.

 

Iang

I’m not familiar with 1010 hot rolled steel. I did a quick search and the properties seem to vary depending upon the source, nothing new there. Thus I’m wondering what it would be classified as under Classifications society standards. It has 0.1%C which makes it a standard mild steel. Thus, is this 1010 cheaper than normal Mild Steel?...if so, I assume that is your reason for selecting it. IF not..why not select a marine industry standard grade?

Yup…you first need to establish what is YOUR pass fail criterion. That shall dictate to you the course of action, if any, to take.

The loads from Class shall be sufficient, however, any FoS, shall be added to this depending upon the vessels role/duty. For example if the vessel shall experience large waves all day 24hours a day 365days a year, then I would expect an additional FoS to be added to the Class Ms/Mp/Mt bending moments. Whereas if the vessel experiences, in general, benign condition and just the occasional big sea, then Class loads would suffice.

Thus it is your decision based upon the known operational parameters the vessel shall experience coupled to the duty it must perform.

Once you have that then you can assess the stresses and deflections. But, since this is a steel vessel, it shall be a stress driven design not a defection driven design. However, you must still check for completeness.

As to the FEA results, hmmm…you can read whatever you wish into them. If you post some plots showing what you mean, I can give further advice. Since it depends upon how much has been modelled, the mesh size, type of element etc etc.

 

Phil,

As noted above, it is rigid body (whole) movement of the structure. That depends upon the fixity and joint configurations and hence can simply be as described above and shown below pictorially. When pin-jointed there is no twist or warp:

 

Ya I basically mean just your standard hot rolled steel tubes. What would typically be used as the off the shelf, cheap, "workhorse grade," structural steel that could be found in rectangular tubes (box beams)? Also any recommendation on coatings? The platform must last for a minimum of 2 years although getting 20 out of it would be ideal.

The platform is in a relatively sheltered location where the largest waves it will see are from shipping traffic which passes by a few times a week. I'm probably just going to stick with the class bending moments.

 

This should help you. This structural steel has Mn, P and S added, which help with corrosion resistance:
http://www.speedymetals.com/information/material59.html

 

In which case i would strongly recommend using marine grade A mild steel, nothing less. You want/need this to be fit-for-purpose, ergo, do the job right first time. It will pay you later...trust me!

Then any standard marine paint system will suffice. Just make sure you exercise caution with dissimilar metals from fixings etc.

Also

No...not tubing.

By that i mean plating that spans from side to side ..not those small RHS types....unless you weld plating to the upper and lower surface to create a full plate width box beam.

 

OK sounds good. I'm going to check and see if our steel supplier has marine grade A mild steel. Planning on using all stainless fasteners and then electrically isolating the turbine from the frame. OK probably still going to go with the coal tar epoxy.

So you are saying we should have our own box beams welded up out of plates? Seems expensive. They don't just make rectangular tubes out of the marine grade steel?

Just an FYI, planning on filling in the frame with pressure treated joists and then using thruflow decking held down with stainless deck screws.

Merry Christmas all. Your advise has been a fantastic gift!

 

iang

Hmmm..ok. We are at a stage where really if you can post the numbers you calculated with your Ms/Mp/Mt loads and show what structure you have designed or assumed. Since you need to ensure you have minimal deflection between the two hulls as well as low stress and designed with fatigue in mind too. It is hard to explain with just abstract words now, ...so better to explain with what you have and thus what you need with options for lowest cost. Ready made structure like RHS etc are not always the best/cheapest option!

Going for correct marine grade A steel is better for many reasons. However the one that is most over looked, but shall yield you results, is a show of compliance and adherence to standards. The loads you have calculated again, you can demonstrate a means of compliance. This is all necessary when getting the vessel surveyed/approved by a 3rd party. But more importantly should something go wrong, you can show more than a sketch on the back of a fag packet and saying well..some guy on the interweb told me to do XX and YY. It provides confidence in what you have done..and your client should recognise this too.

Wish you all a very merry Christmas too ))

 

iang, is this a university project, a commercial project, or something else?