Safety Factor

Safety Factor

Sep 09 2014

Using safety factors is getting out of hand. Companies use it to increase their safety in load-outs, or is it because they are not sure of themselves?

When cargo is loaded onto a vessel according IMO Annex 13, acceleration factors will be taken from a table or calculated by using formulas. The achieved acceleration is always very high. This is done to have a single guideline for all sort of vessels. However, if you engineer the acceleration forces they are always a lot less than the IMO. According IMO this is done to add safety in the load-out. Understandable. Now we have to lash the cargo against various forces and when using a chain we can only use 50% of the MBL (Maximum break load), again for safety. Then after this 50%, we again have to apply a safety factor of 1.35 or 1.5 to obtain a calculated strength (CS). Why? Again for safety.

In reality, this means that if you have a chain of 200kN break load, you only have 100kN in MSL (Maximum Securing Load). Again, you have to apply a factor of 1.35 to obtain the calculated strength. This brings the chain back to 74kN. If this chain is applied in transverse direction with an angle of 45 degrees, you will get the following equation 74 x (Cos 45 = 0.707) = 52.3kN what can be used…

Now, using a GP shackle of 85tons SWL for lifting, you can only use it for 85tons. A factor 5 is applied to retrieve the break load so. 85 x 5 = 425tons Break load. When the same shackle is used for lashing & securing, we can use 50% of the break load which is 212.5tons. The safety factor is used in many ways, WLL, MSL, SWL, break load. When it gets to offshore safety it flies to the top. Safety on safety on safety.

We have to be careful that too much safety not becomes unsafe. Are you sure you use the safety factor in the correct way?

Do you add additional safety because you know or because you don’t know…… how to calculate?

safety-factor

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1 Comment on "Safety Factor"


November 26, 2014

Nice article, though it is not that easy as you described. For some ‘safety’ factors, there are other explanations (eg. the CS is not a safety factor, it is a solution for a practical issue). Shackles for lifting or for lashing are different…). The comment box is too small for a full reply