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

Thanks, Ad Hoc, as always, for your wonderful lectures and for your kindness.
You may have given a solution, an answer is not necessarily a solution, the OP will decide that. But I do not think your awesome calculations will serve him much.
As for my English, you can judge. As for my technical ability, I do not think you trained to, or be the right person to put in doubt.
Greetings, and end of my conversation with you in this thread.

 

wow thanks for all the great suggestions everyone! Ad Hoc I have found a DNV manual that talks a bit about racking on semi submersible oil drilling rigs(a very similar case). I'm going to see if I can work through their calculations today.

 

Det Norske Veritas as CS no longer exists because it merged with GL, so its regulations is likely to be outdated. A good professional, active, has a responsibility to always keep up to date. Although it does not have much importance in your case, why not have the latest regulations available ?.
Precisely the Lloyd's Register has announced a few days ago the publication of its 2015 Rules : http://www.lr.org/en/energy/compliance/rules-supporting-software-and-guidance/rules-and-guidance/

 

iang,

It is a straight forward calculation, and you should be using these three global laods. Also as I previously noted, assume an acg = 1.0 or 9.81ms-2



From DNV Pt.3 Ch.1 Sec.3, which you can find here.

 

From DNV rules for HS craft it follows that, since the boat speed is zero, the value of acg also be equal to zero. That would be great because it meant that there is no racking due to that moment. Am I wrong in something?

 

There's those minimum Acg's quoted.

 

Ad Hoc should intervene here as he is who has advised this method.
I do not see that applies to your structure. It has nothing to do with it. In any case, acg not be equal to 1, as Ad Hoc says, but equal to 0,5. He knows why he says you apply the value of 1.
Sorry I can not help more. I leave you under the protection of Ad Hoc and his explanations, if any.

 

iang

You plug in the value for 'g' = 1.0 or 9.81ms-2.

Once you have the bending moments, you then need to apply these loads in the load cases noted for each.
What you need to be careful of, is the local attachment of the structure to ensure no stress concentration and adequate shear paths. You can never have enough shear paths!

Then you need to check the deflections. For this a simple linear relationship is suffice. Thus if the length (depending upon which direction the loads are being applied) so for the Mp and Ms it is 42 feet, not sure of your beam, thus the Mt will be subtly different.

However for the Mp, being 42 feet = 12.8m, you should ensure you do not get any more than 12-13mm (1/2") deflection at the maximum extent of the vessel from the datum.

 

iang, could you give us an e-mail to address you and to exchange information?. Thanks.

 

Truss Arrangement

It may be that the truss type structure (four booms that essentially form a pyramid shape over the pontoon deck) is considerably more economical in terms of material requirements. The geometry here with relatively larger dimensions for moment resistance allows structural elements with much less strength required. This method was used for decades before the high strength of steel became available, when wood steamers had large hogging trusses built into the hulls.

Even more so if the apex of the four booms is used as a lifting point for the water turbine, rather than another independent structure.

Has any of this been examined and quantified? If so please let us know how the numbers come out compared to the conventional (low profile, typical for catamarans) structure connecting the two pontoons.

 

Without knowing the loads the layout and the budgets etc...it is hard to stay what will or will not be best. At this unknown stage, anything is possible.

And what is conventional here and why?

 

iang: In the US "racking" usually means when a flat structure changes the angles at the corners, but stays on the same plane. Do you mean "twist"?

 

Whatever it is that "racking" means in the US, when a flat structure experiences "racking" is difficult to remain in its plane.

 

1) Twist/Torsion
2) Warping
3) Racking

1) Sections remain in-plane. Such as a shaft that is twisting.
2) Sections do not remain in-plane. Such as an I-beam that is fixed at one end and has a torsional moment applied at the free end.
3) Is rigid body movement, and may or may not experience 1) and/or 2) depending upon the fixity and arrangement. Such as a frame work of 3 members which are all pin-jointed. 2 are vertical ( l and l) and the 3rd is horizontal ( _ ) to connect the 2 vertical members together. Apply a horizontal load at the top, in-plane, to the horizontal member, and the whole frame work racks, as a parallelogram. All members do not experience 1) or 2), but it ‘racks’ to one side.

 

Exactly, that is the same definition I have. If it is racking, the deck will remain flat, or in a plane. From the drawing, twisting may be a major concern.