LR scantlings advice

Discussion in 'Class Societies' started by nevilleh, Aug 30, 2015.

  1. nevilleh
    Joined: Sep 2008
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    Location: scotland

    nevilleh Junior Member

    I need a bit of help here . I have a small ship - 36 metres x 9 metres . Built to LR in 1968.

    We are looking at steelwork replacement at present .

    On the shell expansion drawing all the plates thicknesses are noted .

    For instance in alot of the areas the plates will be marked up as 0.25" , fitted 5/16" .

    Ok, so thats obvious that its 5/16 plate that has been fitted .

    Now, what I have been 'told' is that the 0.25" was the rule thickness, and then according to LR rules as I see I can have a 30% allowance in thickness for corrosion deducted off the rule thickness which would take the allowable down to a minimum thickness of 0.25" minus 30% which is not alot .

    I am wondering if the consultant that we have used is wrong and that the 0.25" design did not include any corrosion allowance and that we can only get the 30% allowance off the original 5/16" plate fitted .

    Can anybody advise me in the right direction ?

    Ive been through a couple of consultants last week and sometimes the information i get is watery to say the least .

    Thanks in advance for any help .
     
  2. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    nevilleh

    A corrosion allowance is only added if it is requested. And this is easily (or should be) found by looking at the original specification for the vessel. In the spec, and also on the Class approved stamped dwgs, it should note the Class notation. The Class notation tells you if the "CA"..corrosion allowance is part of the vessels approval. If it is not, then someone has elected to add a corrosion allowance, for whatever reason.

    Since the LR "CA" is only included in the notation if it is required from the outset, otherwise a 3rd party has decided it is required and should not affect the original scantlings approved by LR, other than is is in excess of the LR rule requirement.
     
  3. nevilleh
    Joined: Sep 2008
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    Location: scotland

    nevilleh Junior Member

    Thanks for such a quick reply.

    There is no notation CA on the approved drawings or in the specification but then the vessel was built in 1968. Albeit to A1.

    If there is no CA does that affect the 30% maximum allowable diminuation for this type of vessel (class 3 , other, less than 90 metres).

    I suppose the only way I can find out the exact margins would be to get the scantlings on the drawings checked.
     
  4. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    If there is no CA listed in the spec or on the approved midship section dwg then the corrosion allowance is not part of LR approval. This is usually a client request, not an LR requirement.

    So, if there is 30% added to the scantlings over that which is required by LR as a minimum, this is either a client request or some "other".
     

  5. JRM
    Joined: May 2008
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    Location: UK

    JRM Junior Member

    For all "ships in service" all class societies have an allowable diminution of thickness, before they demand renewals.

    Otherwise, renewals would be needed for every piece which has dimensions less than the originally approved dimension. Fairly early in life for most vessels that would require renewal of a LOT of structure.

    The allowable diminution should certainly be related to the original REQUIRED thickness - not the ACTUAL installed thickness (if greater). So you can certainly sometimes avoid renewals by checking carefully if thicker plate than required was used in the original construction.

    When you are dealing with large ships and thousands of tonnes of steel renewal potentially at issue - this is a big deal.

    Recently had a case where a significant piece of structure built with 9mm plate was corroded to 7.16mm, and a very difficult renewal implied. Examination revealed that actual class REQUIRED thickness as-new was 7mm - so no renewal required.

    The trick in your case is to establish what allowable diminution is applicable to YOUR vessel. That also depends on which rules your vessel was built to. The modern LR rules are quite clear on the allowable diminution that applies for different types and sizes of vessel. But there could be some difficulty with your older vessel.

    Is your vessel still in class with LR ? If it is, then the opinion of the LR surveyor is really what matters (not the opinion of the consultant) - though of course you may have to mount arguments to support your case and sometimes correct them (i.e. remind them of their own rules !).

    If your vessel is no longer in class, then other considerations apply.
     
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