Compliance

Aboveground Storage Tanks: Inspection and Repair Under API 653

At a tank battery, the big round tanks hold the oil and water the wells produce. They look simple next to the pumpjacks, but keeping them sound is steady work. Steel corrodes, tank floors thin out, and a tank that is never inspected will eventually leak. This article explains how aboveground storage tanks are inspected and repaired under API 653, and how that work ties into the federal spill prevention rules for facilities that store oil.

Atmospheric tanks versus pressure vessels

Most tanks at a tank battery are atmospheric tanks. An atmospheric tank breathes to the outside air through a vent, and the liquid inside sits at roughly the pressure of the air around it. The tank is not built to hold internal pressure. It stores liquid, and the shell, floor and roof are sized for that job.

Pressure vessels are a different class of equipment. A pressure vessel is closed and built to contain gas or liquid at pressure well above the atmosphere, so it carries heavier metal and follows its own design and inspection rules. This article covers the atmospheric tanks: the oil and water tanks that make up the tank battery, not the pressurized equipment elsewhere on the lease.

Why tanks get inspected

Corrosion is the reason tanks get inspected. A tank floor and shell can be attacked from both sides. Underneath, the floor sits on soil or a foundation, where moisture and oxygen work against the steel day and night. On top, water drops out of the oil and settles on the floor, and that tank-bottom water carries salt, bacteria and dissolved oxygen. The shell corrodes from the inside where product and water touch it, and from the outside where weather works on the coating.

Corrosion thins the steel. Thin steel is weaker steel, and at some point the floor or the lower shell can no longer hold the weight of the liquid above it. That is how a tank develops a pinhole or a seam leak.

A leaking tank is a cost, not a nuisance. Oil that escapes through a hole is product lost, and it does not stop there. The oil soaks into the soil under the tank, it can reach groundwater, and a spill that reaches the ground or the water has to be reported and cleaned up. Cleanup runs well past the value of the oil that leaked. Finding a thin spot with a gauge and repairing it on a schedule is far cheaper than digging out contaminated soil.

What API 653 covers

API 653 is the American Petroleum Institute standard for inspection, repair, alteration and reconstruction of aboveground storage tanks. Operators across the oilfield treat it as the rulebook for keeping existing tanks fit for service.

The standard is written for tanks that are already standing. It is not the code for building a brand-new tank; new construction follows a separate standard. API 653 tells an operator how to find out whether an existing tank is still sound, how to repair it when it is not, how to alter it when the service changes, and how to take it down and rebuild it when that is the right call. The aim is a tank that stays leak tight and safe for as long as it is used.

The standard also settles who does the work. Inspections are carried out by qualified inspectors who know tanks and the standard, and repairs and alterations are planned and approved by engineers. A tank is not judged by whoever happens to be on site with a gauge.

How an inspection program runs

A tank inspection program runs on three tracks. The first is the external check. While the tank stays in service, an inspector goes around the tank on a regular cycle, looking at the shell, roof, welds, vents and seals, and watching for bulges, coating failure and signs of weeping. These checks catch problems while the tank is still running.

The second track is the internal inspection. The tank is emptied, cleaned and made safe before anyone goes inside to look at the floor and the underside of the roof up close. Climbing into a closed tank is confined space entry into tanks, and that work carries its own permits and its own hazards.

The third track is thickness measurement. An inspector uses ultrasonic testing to measure how much steel is left in the floor and shell, and the readings are drawn up as maps so thin spots stand out. Comparing one round of readings with the next shows how fast corrosion is eating the metal, which is how an operator knows how long before a repair becomes due.

Tanks and the SPCC rule

Tank integrity connects directly to the SPCC rule. SPCC stands for Spill Prevention, Control and Countermeasure, and it is part of the federal oil pollution prevention regulations. It applies to facilities that store oil and could reasonably be expected to discharge it, and a tank battery that holds oil sits squarely inside it.

The rule asks two things of a facility. First, prevent discharges: keep the tanks, piping and equipment sound so oil stays where it belongs. Second, plan for them: write down where oil is stored and what the facility will do if something lets go. EPA's spill prevention program expects tanks to be in sound condition and checked on a regular schedule, and most facilities spell those checks out in their SPCC plans.

That planning rests on records. A facility shows its tanks are sound with inspection reports, thickness readings and repair records, and that is the kind of document trail kept in OpsFlo: the ticket, dispatch, timesheet, approvals and document software built for field crews. When an inspector asks when the last floor scan was done, or a regulator asks how a repair was handled, the answer has to live somewhere it can be found.

Sources and further reading

• API 653: Tank inspection, repair, alteration and reconstruction
• eCFR 40 CFR 112: Oil pollution prevention (SPCC)
• EPA: Oil spill prevention and preparedness