Inspectioneering Journal

API-653, A Case Study

By P.E. Myers at Chevron Research & Technology Co. This article appears in the March/April 1996 issue of Inspectioneering Journal.

This case study is an example of an incident that started with a routine API 653 inspection and resulted in a very difficult repair to a tank bottom contaminated with hydrocarbons on the underside. This case highlights the potential risks with performing tank inspections and the consequences of poor inspection practices.

In April 1995 a 211' diameter crude oil storage tank located in western Africa was taken out of service for repairs. Management, which had just adopted a policy of complying with API Standard 653, decided to conduct a complete internal inspection in accordance with this standard. After the inspections and repairs were made the tank was put back into service in late April without incident.

Six months later a catastrophic leak occurred without warning. An estimated release of 10,000 bbls resulted. Oil was observed seeping up from the ground around approximately 1/4 of the perimeter of the tank and from several feet to a dozen or more yards away from the tank. Pools of standing oil several inches deep, covering several thousands of square feet, formed from oil oozing from the soil. It was estimated that the tank leaked for approximately 12 hours. The tank was shut down, drained, cleaned and prepared for entry. An investigation team was to determine the leak cause and why the API 653 inspection didn't prevent this incident.

Management had just endorsed a policy of complying with API 653. Now they began to question the value of API 653 tank inspections as well as any API 653 facility-wide tank inspection.

The tank was an external floating roof type with a cone-up bottom design built upon a ringwall. The tank bottom and 2 feet up the sides were coated with a thick film glass fiber reinforced (FRP) lining. The tank had a 12 foot diameter by 5 foot deep sump used for a 30 inch filling line. Examination of the tank bottom indicated that the large sump was the source of the leak. A 2 inch hole was found in the sidewall of the sump about halfway down. Hammer testing indicated several large areas (approximately 1-2 feet in diameter) of severe corrosion where there was virtually no steel left. The FRP lining had been holding the contents of the tank until the hydraulic pressure finally caused the lining to burst a 2 inch diameter hole which allowed oil to flow under the bottom of the tank.

The tank bottom under the sump which was located near the shell within a radius of approximately 30 feet had sunk anywhere from 4 to 8 inches beneath the original elevations. This was due to washout of the underlying soil and loss of support. Since the tank was on a ringwall the oil took a subterranean path under the ringwall and flowed as a liquid through a porous medium out to the surface.

A thorough review of the inspection report which was prepared 6 months earlier was conducted. The inspection report was thorough and complete in every regard except for one. The report did not mention any specific inspection data for the sump, nor was there any mention of any problems with the sump.

The inspection agency was asked to produce detailed records on the inspection of the sump. They stated that water was in the sump which prevented access at the time the internal inspection was conducted. This is understandable since the tank is an external floating roof tank and the inspections were carried out in the rainy season. However, they could not explain why no mention was made of this in the report. A thorough review of the inspection report would lead one to believe that the sump had been inspected with the bottom but not as clearly documented.

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