Lightest GRP structures

In peoples experience which class society rules (or alternative) produce the lightest standard GRP structures (hand laid polyester/glass) for commercial boats?

We do the main bulk of our composite structures to Lloyds SSC rules, however we have found this increasingly heavy compared to boats designed 15>20 years ago.

We have looked at using ISO rules but these seems aimed mainly at yachts.

We are therefore looking for an alternative design standard which does not lead us down the route of more complicated/exotic methods

 

The ISO 12215 is intended mainly for pleasure boats who need to get the "CE" mark. It ignores some of the stresses that other regulations apply in their calculations, so that lead to smoother scantlings.
As for what Socieded Classification results lighter structures, I think the question is not correct. In my opinion all of them can result in similar structures. It is very important for the designer to know how to use the Regulations and feel confortable with them. With the same rules, playing well with the reinforcements, you can get different weight structures. Getting the "minimum weight structure" is the obligation of the designer and depends mainly on his knowledge. Working with fiber reinforced plastic, for example, one or another arrangement of the layers may result in different weights panels.
A very important factor is the tool that each Classification Society makes available to the designer. The program sells for Lloyd's Special Service Crafts is, in my opinion, very comfortable and will allow you to adjust the results much.

 

This is because the boats made 15>20 years ago over the years have all shown minor niggly issues which have been slowly fed back into the rules. Thus, if you took a boat from 15>20years ago and plugged it into LR of 15>20years ago and then the same but using today's SSC rules, the 2 scantlings will be different. Just because it was lighter, does not means that it is necessarily better. Rules evolve owing to in-service experience.

DNV, in some cases can produce lighter scantlings.

But one must question, what is it you're after, a structure that is "fit for purpose" or a nice light-er structure but may well fail sooner than you expect.

 

Ad Hoc - I agree that some of the changes to class rules are due to experience in service (shingling of bottom laminates in LRS SSC rules for example), but we have boats running around today with over 75,000 hours on them without major structural problems and in my experience it tends to be the design details causing the niggling issues rather than overall scantlings changes.

Like all designers we are in the business of producing a product that is fit for purpose, however there is always commercial pressure to make a boat simpler and cheaper to build. In today’s world material cost are significant and reduction in structural weight goes hand in hand with reducing build cost.

TANSL - you may be correct and all class rules result in similar structures (this is certainly the class for LRS, ABS and BV) and fully understanding a set of rules will always give you the ability to optimise the hull structure. However class rules do differ and my question relates to peoples experience of using different rules and the resulting structure, rather than working within a certain set of rules.

Yes although we don’t tend to use DNV I had heard that their composite rules had the potential to produce light structures, however without spending a lot of time running the calculations I'm not sure whether this is only achievable by using sandwich or exotic materials

 

And there in lies the first crux. You may have 1 or 2 or several boats with 75,000 hours. But LR has literally thousands of boats per year. So their statistical sample is far greater than your product range. Ergo they can spot statistical aberrations much easier.

Secondly, it is usually the QA side that lets most production run boats down.

Actually, doesn't take much at all. DNV rules, in this aspect, are much easier than LR SSC rules. I designed a composite boat recently with DNV rules, for Class approval. Doesn't take much, just a few hours.

 

It's a little more than 1 or 2 boats we have with this level of service hours but I take your point.

However assuming that class calculation methods for structure are valid (which I'm sure we will all agree they are) if a lighter structure is not failing in service when the current rules say it should what are the options?

1, Input design pressures are higher than the vessel actually sees in service or
2, allowable design stresses are onerous and the material has more load carrying capability than assumed
3, ?

Lucky we don't need to get into QA for production boats, but I agree quality is important especially on composites which are a built material.

Interesting that you say DNV is easier than LRS SSC rules, have you developed your own spreadsheet/software for calculation or are you using their software?

 

You present a case to LR, or whom ever. You demonstrate which rules and notations and formulae were used to determine the scantlings. You show the as-built drawings, and provide evidence that it is indeed the actual boat. And then provide in-service evidence of the conditions the boat has been subjected to and then provide evidence from annual surveys showing that there are no structural issues.

Thus you are presenting a "picture" from design to build to proven in-service structure being satisfactory. Once you do this, Class cannot ignore this and can provide you with a dispensation, but it is only on a case by case basis. How do I know this? 1) I am on LR's technical Committee and 2) I have done just this procedure to LR, DNV & ABS on several boats in the past.

I don't design frequently enough with composites to spend the time making my own excel calculation software. But using DNV rules, their formulae are straight fwd and doing it by hand, doesn't take long at all. LR...on the other hand, requires a lot of data input into their SSC rules before you can get anything out to produce scantlings.

 

The major difference between ISO and Class Societies rules are the design stresses. Say, ISO12215-5 is using about 0.5 of ultimate tensile/compressive stress and 0.6 for shear stress. For Societies, those are 0.25-0.4 depending on element location, average for normal stress being 0.3-0.33. For long-term loads such as docking, Societies are even using 0.1.

BUT there is another point: ISO is using structural properties of materials that are reduced by 20% (method C), so at the end of the day it comes very close to Classification Societies. For example, we do a lot of calculations to ISO12215-5 at initial stages of design (it is fast!), and then either use LR of GL rules for final design. The results are always similar; for LR stiffeners are heavier; for GL there almost 100% match.

As to the design criteria, they are differ for different Societies and I would not say that ISO12215-5 is too liberal. Say, LR us using stresses but GL is using strain as main criterion, plus some additional checks. Deformation criterion is often critical as well, The most difficult issue with LR is min skin thickness criterion that results heavy structures for craft below 60'.

We increasingly use GL loads/criteria in our work.

I can show some comparison of design loads if interested; actually it was in my paper at SURV-8 conference.

 

I hope I was not mistaken as I have reviewed the calculations a few times.
I calculated the same bottom panel, the same boat, according to Lloyd's SSC, and according to the ISO 12215-5 and get the following thicknesses:
SSC: 6.41 mm
ISO: 5.96 mm considering the EL-c, which is the worst.
The difference does not seem a lot but it is a savings of 7%, which is much.

 

You wanting lightness ?? Why??
or a lighter lay up ?? with a change of resin type and glass type you could make it lighter !!!
What sized boats are you referring to ??
what is there intended use??
what power ?
how many motors ??
how fast ??
What kind of seas are they used in??
Solid glass lay ups or cored ??
if cored what thickness and what density ??
If solid what glass you using ??
give us a look at an actual glass laminate you using !!!

 

This is not surprising because 0.3 for bottom in LR SSC and 0.5 in ISO can not be fully covered by 20% margin - this is evident. Actually, I was talking primarily of sandwiches as we almost never use single-skin bottom in high-speed craft, with sandwich this difference is much less.

Still, 7% is within technological tolerances under ISO12215-5 and might be less than one layer. Also need to check which category are You using in ISO, we usually apply higher category in ISO.

 

There are some comparison of design stresses and design loads, also with Russian Register rules - we did for one of designs to prove those guys that their request to use their rules are not adequate for 50', 42kts patrol boat. Shown are design stresses and distribution of bottom slamming loads.

 

Alik, I have no doubt that everything you say is true.
I am amazed by the depth of your research.
What I can say, without having more than a very simple check is that some panels on my boat, with SSC are thicker than with ISO 12215-5

 

Sure it is true, it is used on dozens of boats that are in service

The reason for research is simple: dealing with such aboriginal classification societies as RS or IRS we spend a lot of efforts not for design itself, but to prove those dummy surveyors that we are doing the right things. Thus, we have plenty of graphs and comparisons for every issue! Say, we are discussing anchor size for 12m catamaran with RS for last 3 mounts; and are trying to explain to IRS that displacement of 15m boat can not be measured with 2% accuracy using load marks... Hard job.

 

Yes, I know the problem, the government officials are always "helping"