Rules scantlings

Hi naval architects
i am just asking about the scant lings which we drive from LR rules is it containing corrosion addition or we have to add it after formula that i notice in buckling check we subtract a an allowance from the thickness which we get from equations can any one help me in this?- the vessel which i design is oil tanker

 

It often depends on area of service and expected longevity.

 

absolutely not.

 

There are 2 ways you can have a corrosion addition to the scantlings.

1) You can specificity a nominal value above Class min rules and this will be noted in the class notation as +1, or +2, where +1 = 1mm above class min.
or
2) Using the corrosion resistance steel standard noted in the rules:
"..This Standard is based on specifications and requirements which intend to provide a target useful life of 25 years, which is considered to be the time period, from initial application, over which the thickness diminution of the steel is intended to be less than the diminution allowance and watertight integrity is intended to be maintained in cargo oil tanks. The actual useful life will vary, depending on numerous variables including actual conditions encountered in service.."

But either way, this should be noted in your technical specification for the vessel that has been agreed with the client. If it is not there, then it is your choice whether you wish to have an additional corrosion allowance built into the structure.

 

Fresh or salt water use also are taken into consideration for corrosion prediction calculations. The area of service influences that greatly. For example, the St.Mary's cement bulk ship has been in operation in the Great Lakes since 1904 and has little corrosion. The same ship in salt water would be a hunk of rust by now. The type of steel also makes a difference depending on its corrosion resistance characteristics. As Ad Hoc points out there is a system to indicate how much over you calculate to.
The area of service also influences the scantlings. Any place with ice will determine thicker plating or an tougher alloy at the wear areas.

 

Gonzo, you've heard bells but do not know where. In my opinion, you need to work a bit more with the regulations. But the discussion may help the OP.
What you say can not be said that is not true, but it has nothing to do with the increase in thickness due to corrosion (speaking of rust, no corrosion by acids, electrolytict or other) that CS requested. It is true, indeed, that if a plate must be in contact with fresh water will be applied smaller increase in thickness in the case of salt water, but nothing else.
The cement carrier, which you put as an example, have a problem of abrasion due to the load it carries, much higher than the corrosion problem. And this problem is more evident in the internal bulkheads and in the inside than outside. Therefore, in my opinion, the fact that sailing in fresh or salt water, after 100 years, is not very relevant. The plates of these ships, considering that they had not a double hull, is not calculated based on seawater.
The best we can say to the OP is to study the regulations with which the SC is working to find out where and how much increase is requested.
The CS use to have tables that indicate, according to the zones (the areas of the ship, no region of the world) and liquids in contact the required increase.
You can also agree with the SC a good paint scheme that allowed not to apply the correction for corrosion (I have done, in case anyone has doubts). Anyway, as always, knowledge of specific regulations and dialogue with CS is the best the OP could, and should, do.
The increased thickness, in my opinion and answering what the OP asked, should be added to the thickness derived from the formula. What do you think, Gonzo?
I do not know, but I can be wrong, any factor applicable to the increased thickness of the plates, which depends on the type of steel used (I do not mean to stainless steel or other metals).
I hope, thanks to the "opportunity" offered by Gonzo, having helped the OP.

 

Where do you find data about cement and abrasion? This ship has no problem with the cargo holds or the bulkheads. Fresh or salt water, type of alloy, life expectancy and service area are all considerations for plating and framing. This is a link to steel types. It is one of the very basic concepts necessary in design. http://www.leecosteel.com/offshore-and-marine-steel-plate.html

 

Thanks, Gonzo, for valuable information. Why not read what the Rules of Lloyd's Register tells about how, how much and where the corrosion addition is applied. This, I think, is what is being asked in this thread, and not other considerations.
What the LR says concerning the increase in thickness due to corrosion, believe me, has nothing to do with what you say.