I have been inspecting a tank which used to have an external pressuriser roof, which has a water trough over the top strake. We have taken some thickness readings on the inside of the tank (the only way to get the actual tank shell thickness readings in this area) and found readings between 2.9 and 3.2mm in some areas.
Drawings show that the plates used for construction were 1/4"(6.35mm) although readings are as large as 7.2mm. The average thickness readings on this strake are still around 6.5mm.
In API653 Cl 4.3.3.1 b) you can work out the t(min) at the locally thinned area. Seismically the thickness only needs to be about 0.7mm thick. this clause goes on to state "t(min) shall not be less than 0.1 in. for any tank course"
My questions are
1 - What is the minimum thickness that we should allow in this area? I believe that the minimum of 0.1 inches relates to the strake (course) thickness which is satisfied (=6.5mm)
2 - What is the extent that this thinning can be considered acceptable? is it up to 2 inches in 8 inches? or up to the length L?
3 - Bearing in mind that the water trough hides the outter (corroding) surface of the tank, the tank needs to be cleaned and NDT done on the internal surface at 1/2 the remaining tank life. If the corrosion rate of 0.11mm/year is used, and only 0.4mm of corrosion remains (2.9mm-0.1 inch), then the tank has to come out in less than 2 years. Is this correct??
In another very similar tank, we cut the trough off the tank completely, now the outter surface has been painted and can be fully inspected from the external, hence problem alleviated. The owner does not want to spend the money removing the trough in this case.
Can any one help with this one?
Drawings show that the plates used for construction were 1/4"(6.35mm) although readings are as large as 7.2mm. The average thickness readings on this strake are still around 6.5mm.
In API653 Cl 4.3.3.1 b) you can work out the t(min) at the locally thinned area. Seismically the thickness only needs to be about 0.7mm thick. this clause goes on to state "t(min) shall not be less than 0.1 in. for any tank course"
My questions are
1 - What is the minimum thickness that we should allow in this area? I believe that the minimum of 0.1 inches relates to the strake (course) thickness which is satisfied (=6.5mm)
2 - What is the extent that this thinning can be considered acceptable? is it up to 2 inches in 8 inches? or up to the length L?
3 - Bearing in mind that the water trough hides the outter (corroding) surface of the tank, the tank needs to be cleaned and NDT done on the internal surface at 1/2 the remaining tank life. If the corrosion rate of 0.11mm/year is used, and only 0.4mm of corrosion remains (2.9mm-0.1 inch), then the tank has to come out in less than 2 years. Is this correct??
In another very similar tank, we cut the trough off the tank completely, now the outter surface has been painted and can be fully inspected from the external, hence problem alleviated. The owner does not want to spend the money removing the trough in this case.
Can any one help with this one?
