DNV Design Pressure

Hi,
we are designing a HSLC (High Speed Light Craft) with the assistance of DNV rules.
We came across the design pressure (Pt. 3 Ch. 1 Sec. 2 C201) for slamming and I am not sure how to proceed.

From my initial understanding, the design pressure is the maximum pressure which the craft can withstand during slamming . I am confused about this:

1. The design pressure increases as the spacing between stiffners reduces. This makes sense. Then why does the design pressure not depend on the material properties?
Does it mean that a craft made of wood has the same design pressure as a craft made of steel, if they have the same weight.

2. Is there some way we can calculate the acting pressure on the craft? Do the acting pressures and the design pressures have the same values?

Regards

 

Design pressure in some area of the vessel, is the pressure, for example, the water exerts on the bottom or sides, just to be at a certain depth. It just depends on liquid density and height to the waterline, does not depend, therefore, on the material of the hull.
The designer can (or must) select a design pressure exceeding the statutory, if circumstances warrant. For example, if there are locomotives on the upper deck, the design pressure is not the one that says the DNV, but pressure from the locomotives(considering accelerations due to ship's motion in waves).
Classification Societies have formulas to calculate the design pressure in each case. When you try to calculate the thickness of a plate, one of the factors involved in the formula is the design pressure.
If you are calculating a recreational boat, you better use ISO 12215-5.

 

But how does this pressure exerted by the water on the bottom/sides depend on the distance between the "stiffners"?

 

If you want to know why design load increases when panel size decreases, read my explanation in this old thread.

http://www.boatdesign.net/forums/boat-design/hull-water-loads-4085.html#post27687

 

The pressure is a force divided by an area. The pressure can not depend, therefore, on the distance between reinforcements. I think you're not understanding the concept of design pressure.
The total load (design load, not design presure) on a plate which is equal to the design pressure multiplied by the dimensions of the plate itself which depends on the separation of stiffeners, decreases logically where separation decreases.

 

So, where do you see material properties in this formula?.....you don't. The only input is the displacement...thus the answer your question about this formula is yes same pressure.

There are varying pressures along the vessel which you need to calculate. The dominant one though, is slamming, generally.

You may find this comparison of slamming pressure values of interest too.

And also this discussion on accelerations and slamming pressures.

 

Actually, the hypothesis of bottom impact pressure dependence on panel size if originally based on 'equal stress' or 'equal deformation' approaches. This is well explained in Allen R.G., Jones R.R., “Considerations on the Structural Design of High Performance Marine Vehicles”, SNAME New York Metropolitan Section, 1977.

Unfortunately, this approach is not evident for composite structures, where criteria other than stress or deformation are used, say strain or TsaiWu. So, the TS is right - in such case the design pressures should depend not only on panel size, but also on material.

Recently at windfarm vessel conference we presented a paper 'EXPERIMENTAL ASSESSMENT OF IMPACT LOADS ON CATAMARAN STRUCTURES' where we touch the accelerations and bottom impact issues by comparing our measurements with calculations from different rules. I hope this paper will be admitted for RINA's Journal of Small Craft Technology.

 

You're only partly correct.

Slam pressures are the same no matter the hull the material. A slam load is simply that. You're now confusing the applied load with the response.

The response of aluminium structures generally indicates that simple bending and deflection is sufficient for the analysis of the structure.

In applying the same assumptions for composites does not yield the same. The reasons being is how the load is applied to the structure and thus analysed. Composites tend to fail with shear core failures and the adhesives bonding the laminates to the core. Thus if you take those exact same slamming loads being applied to an aluminium vessel and apply them to a composite hull and only check for bending and deflection it will most likely fail. Simply because what is missing is the shear load.

In aluminium stiffened panels shear loads do not dominate the pass/fail, but they do in composites. Correct analysis of the shear loads from the exact same slam loads on a composite structure, then yields a different structural arrangement to one using only bending and deflection.

Thus partially correct.

First is the load application...second is the response. The response is dictated by the material not the other way around.

 

Allen and Jones in their approach use the response as a measure to define the design load. As the response in alu and composite is different, the loads might be different.

I agree that loads do not depend on the material, and actually on panel size as well. In reality! But in the Rules they define equivalent pressure based on distribution of impact peaks over the area, deflection and stress. So there should be the dependence, but right now the dependence on material is ignored.

 

The loads are not different, only the response.

It is simply how the loads are analysed that is different. The analysis looks at the response of the structure - which is its EI - to a load. If I apply a load to a steel flat bar, and the same load to a flat bar made from composite, the response, - the effect - will be different between the 2 FBs. The load remain the same. The analysis yields a different response between them because their EI for each is not the same. Structural engineering is all about understanding the EI. The load...well, that is just that, the load...no matter where it comes from.

It is the job of the structural designer to analysis the correct response of a load on a given structure. All low modulus materials are driven by deflection checks for example, not stress. Whereas steel, a high modulus by comparison, is driven by stress. Thus why ignore the shear load effects on composite when it is a the main driver?

A slam load can be split into the peak impulse load...like a point load on a beam...and a simple increasing uniformly distributed load. There is bending, deflection and shear from these loads on any structure whatever it is made of. The analyst must investigate all load scenarios as appropriate, and not assume.

 

Yes, fully concur. The DESIGN loads might be different, not the physical loads. This is what I meant.

 

May be it is a translation thing, since we concur.

I think what you mean, is the design of the structure using the applied load is different?

 

The design load.

In the Rules, the design load does not depend on material, but depends on panel size. We understand this is artificial, as in reality the physical impact pressure will not depend either on material and on panel dimensions.

The peak of impact during slamming is distributed on very small area. On larger panels, this local peak would be spread over large surface, causing lower average pressure. Thus, the design pressure is taken as a function of panel area.

This design pressure is based on equivalent approach - the design pressure on entire panel is assumed causing same stresses and deformations (response) as physical slamming impact acting on small area within the panel. There are illustrations in Allen Jones paper clearly explaining that.

Unfortunately, this approach is lacking accuracy if we look at composites. Say, why one should assign equivalent pressure based on deformation or stress, if for the composite panels other criteria are used - say, strain, interlamia shear, core shear, TsaiWu, etc.

We did FEA study of sandwich panels applying local impact pressure and equivalent pressure from Allen-Jones and the Rules, and studied results obtained for different structural criteria. The result is far not the same, the approach does not fully work for composites thus the design pressures should actually depend on material. But currently they do not. Right now, this is compensated by very high safety factors sued for structures in composites.

Thats' the point for further research, I think. The study I mentioned is not yet published.

 

Hmmm...may be not lost in translation after all.

No one is telling you to assign the slam pressure based upon stress or deformation. Nowhere does it say that.

The pressure/load is independent of the material. Just as in the FB scenario above, the load is the same.....no one is saying you must design the structure of the steel FB in the same way as the composite FB.

Your pass/fail criteria for composite is different for that of an isotropic material. That is the role of the designer to apply the correct analysis to a structure from a given load.

Since all you appear to be saying is that when designing a composite structure the result is not very satisfactory. If that is the case why??..if it is because you are analysing the structure using only the bending stress and deflection checks as one would generally do for isotropic materials, then clearly the analysis is incorrect, not the load.

You can analyse any structure how ever you wish. But using just EI.d^2y/dx^2 = M(x) is clearly insufficient for a composites - look at the design checks within DNV rules for example, it is not just bending.

But if you are doing direct calculations alone, then the T-W can be your only guide owing to its strain based approach for the ply failure. However, one would not use the T-W approach on an isotropic material as a pass/fail, so why would one expect the general Eqn of bending (above) to be the only method of analysis for a composite?

It seems you're getting confused between calculating a design load...and how to apply that design load. It is all relative.

 

I think to continue further in this discussion, one needs to look at the origins of approach currently used in all the Rules, including classification societies and ISO. I recommend to refer original Allen-Jones paper which states the background of the method before we talk any further.