Storage Tank Hydrotest

[mylexicon]

Suppose having a series of 6 floating roof Storage Tanks, each 100 meters in diameter and 14 meters tall. Each capable of storing around 500,000 BBls (79500 cubic meters) of Sweet Crude Oil. If the plot of land for this Storage area is located on an underdeveloped Island (about 20 km from mainland), what means of providing water for Hydrotest would you recommend other than sea water? The client insists that seawater will have adverse effects on the tank shell.
With Sea Water as an altenative they want the shell corrosion allowance to be increased to 3mm (currently 1.5mm)which in turn will signficantly increase the load on the foundation.
Are there any other economically sound alternatives for testing the tank other than performing a Hydrotest?
thanking you in advance,

drno

 

[metengr]

There are nondestructive testing methods, like remote field Eddy current testing that can look surface and subsurface regions of shell course welds, but this might not be feasible because of the location and it would be costly. A second option is to hire a Certified Weld Inspector that would follow construction of the storage tank to assure compliance with API standards, there still would be no guarantees.

I would still use clean or filtered sea water to perform a final hydrotest of the constructed storage tank. You would not need to increase the corrosion allowance of the storage tank. If you can convince the customer that the hydrotest is of a very short duration; fill, check for leaks, empty and completely dry, this would have no adverse affects on the corrosion or corrosion resistance of the steel tank. I would draft a detailed hydrotest procedure for your customer to review and approve. The problem is leaving the sea water in the storage tank for days on end.

 

[NozzleTwister]

Let me address another issue with large tanks like this.

Depending on your soil conditions, you could get significant settlement of the tanks when you fill them with liquid. Normally the total settlement is split between the initial settlement that occurs in the first few weeks of filling the tank with liquid and the settlement expected for the duration of the tank life.

Since the tank settlement has an adverse effect the piping attached to the tank and the tank nozzles, it is desirable to limit this as much as possible.

What is normally done is to fill the tank up with water and allow it to sit full for a few weeks (I've seen 4-6) or so to allow the initial settlement to occur before all of the piping is hooked up. This way your Piping Designers only have to deal with the remaining settlement that will occur over life of the tank.

Hopefully, you've already provided your designers with this settlement data. If not, your Civil Engineer can calculate it from the data provided in the soil boring analysis report.

I know this doesn't answer your corrosion problem but I wanted to give you a heads up of other problems you'll have by limiting, or eliminating the hydrotest time before the tank is put into service.

Good luck,


NozzleTwister
Houston, Texas

 

[unclesyd]

You might want to contact some of the corrosion prevention additives people and see what they might provide to add to the clean sea water mentioned above.

There are quite a few companies that have additives for such a test.

Here is one that I've used several times with good results. give them a call and see what they have to offer.

http://www.cortecvci.com/Products/products.php

 

[JStephen]

Does that complicate the disposal, unclesyd?

 

[mylexicon]

Gents,
thanks for all your replies. Very helpful. Some of it I already knew, but it was still good to hear. All things considered, I think we will end up using filtered sea water but as far as settlement is concerned NozzleTwister is right. We will have to keep the tanks filled for a few weeks to achieve short term settlements before connecting the piping.

regards,

drno

 

[unclesyd]

JStephen,
No, most of the products are now biodegradable and can be easily and legally disposed of without any further treatment.

mylexicon,
I would defiantly consider the addition of some type of inhibitor if as you say the intent is to leave the hydrotest seawater in the tank for a period of time.

 

[JStephen]

Don't forget that sea water will have slightly higher specific gravity, which should be accounted for in the design.