Johannes de Wit
Tank specialist
The Hague
The oil and chemical industries use many thousands of steel tanks to store crude oil, oil products, and chemical liquids. The majority of these tanks are 30-40 years old. Tank bottoms are likely to begin leaking in the coming years, as these tanks get older.
The European technique of jacking up a tank and repairing its foundation allows the thorough inspection of the underside of the tank bottom and the removal of saturated foundation material. And the possibility of soil and groundwater pollution is reduced to a minimum.
With good, regular maintenance, the lifetime of a storage tank is very long. But experience has shown that special attention should be paid to the tank's bottom.
Tank bottoms are only 5 or 6 mm thick, and in the last 10 years, an increasing number of leaks in tank bottoms have been reported. Tank foundations are affected by these leaks.
When a leak develops in the bottom plate, the stored product will penetrate the tank foundation. This may pollute the tank foundation and subsoil. When a leak is not detected early and no action is taken, in time, serious pollution of the subsoil and the groundwater may occur.
Problems with tank bottoms are frequently reported in Europe. Consequently, the issue has been studied and a proper and efficient solution sought.
This article describes the resulting procedure, which is used successfully in many European countries, but is not yet common in the U.S.
TANK FOUNDATION
Steel storage tanks are seldom built on piled foundations. They are usually built on a tank pad made of sand covered with a bitumen top layer (Fig. 1).
The tank pad generally rises 500-750 mm above grade level to eliminate the influence of groundwater on the tank. It is important that the shoulder of the tank pad be wide enough and in good condition, otherwise the bottom-to-shell connection may slip away.
When a tank is built on rock or hard soil, the soil settlements will be so small that they may be neglected because they will not influence the behavior or safety of the tank.
In The Netherlands, Germany, Belgium, and Denmark, most tanks are built in locations with weak, compressible soil layers (e.g., clay, peat, silt). Consequently, the tanks in these countries have experienced considerable soil settlement. This settlement is sometimes even, but often the soil layers have different thicknesses and compositions, causing uneven settlement.
Uneven soil settlement under a tank shell causes out-of-roundness of the shell. This may hinder the movement of a floating roof.
Also, the bottom-to-shell connection may be seriously stressed where the settlement is uneven. When this occurs, the rigid, vertical tank shell cannot follow the uneven settlement of the flexible, thin tank bottom (Fig. 2). The soil settlement under these tanks should be measured at regular intervals (Fig. 3). Jacking up such tanks and releveling the foundation is general practice in the aforementioned countries. The Netherlands, in particular, has experienced such serious soil settlements that the procedure is used almost daily.
The Dutch specialist in jacking and releveling tanks is Verwater, of Rotterdam. The organization has used this procedure on about 5,000 tanks. The biggest tanks jacked up so far are some 120,000 cu m floating-roof tanks, with diameters of 80 m.
The procedure is also frequently used in Europe when a tank bottom leaks and the foundation is saturated with product. With the tank jacked up 2.0-2.5 m, special low equipment can be used to remove the saturated foundation material and replace it with new material. This is done safely in the open air, without risk of fire or explosion. Fig. 4 shows a storage tank jacked up 2.5 m above the foundation. This position also allows the underside of the bottom plates to be inspected for corrosion and, when necessary, protected with a coating.
TANK BOTTOM
Storage tanks are built according to international codes. The two most important codes are API 650 of the American Petroleum Institute and BS 2654 of the British Standards Institution.
The thin bottom plates are usually lap-welded on top only (Fig. 5). However, for larger-diameter tanks, thicker butt-welded annular plates (8-15 mm) are installed under the tank shell.
Corrosion of tank bottoms can occur inside the tank, from the underside of the tank where the plates rest on the foundation, or both.
- Corrosion at the topside of the bottom:
One possible cause is that the stored product contains corrosive components. This may occur, for example, in crude oil at oil fields, where sand and water may be present in the bottom layer.
In floating-roof tanks, rainwater can contaminate the stored liquid by passing through gaps between the roof seal and the tank shell. This oxygen-rich rainwater will collect at the lowest areas of the tank bottom and cause local corrosion.
Leaks in a tank bottom may also be caused by movable parts of the roof, such as the roof drain scraping over the bottom plates.
- Corrosion at the underside of the bottom:
One possible cause is that the foundation material is corrosive, for example when salt is present in the sand.
Another possibility is that the mill scale was not properly removed from the underside of the bottom plates before they were laid on the foundation. This creates an uneven covering of mill scale, resulting in local corrosion.
A third, well-known possibility is that the tank shell has penetrated the foundation, causing a kind of gutter around the tank. Rainwater and loose soil will fill this gutter. The rainwater will also penetrate under the bottom annular plates and may cause serious corrosion, particularly when hot products such as bitumen or fuel oil are stored in the tank (Fig. 6).
REPAIR METHODS
Two possibilities should be considered when choosing a repair method: either the tank bottom has no leaks, or the tank bottom is leaking and the product has saturated the foundation.
- The tank bottom has no leaks:
When the bottom plates show serious corrosion, it may be possible to remove them and replace them with new ones. The plates can be removed safely by burning, because no flammable product has penetrated the foundation. When such product is present in the foundation, hot work may cause fire or explosion.
Sometimes it is necessary to replace only the bottom annular plates under the shell. In that case, the tank is lifted 100-200 mm above the foundation. The old annular plates are removed and new ones installed. The tank shell is then jacked down onto the new bottom annular plates and welded. When corrosion is suspected at the underside, it may be necessary to jack up the tank 2.0-2.5 m to allow a complete inspection of the underside and, if necessary, to protect it with a coating.
- The tank bottom has leaks:
In this case, the tank must be jacked up 2.0-2.5 m above the tank foundation. The tank bottom can then be thoroughly inspected and the foundation repaired.
Polluted foundation material should be removed and replaced with new material. Plates with leaks can be repaired or replaced. If the entire tank bottom has to be replaced, the method described for a tank without leaks can be followed after the tank foundation has been repaired.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
