Crude Oil Storage

Grades, gravity and sulfur

API gravity expresses density on an inverted scale: the lighter the crude, the higher the number, and water sits at 10 degrees API. Sulfur content decides whether a crude is sweet or sour, and sour crudes usually bring more dissolved hydrogen sulfide (H2S) into the tank vapor space. Together, gravity and sulfur decide the tank, the vapor controls and the price differential.

Floating roofs and tank design

We store crude in floating-roof tanks built to API 650. An external floating roof (Annex C) rides directly on the product, open to the weather, with primary and secondary rim seals closing the gap to the shell. An internal floating roof (Annex H) sits under a fixed roof or dome that keeps out rain and snow. Both shrink the vapor space that drives evaporation loss and air emissions.

A floating roof only works while it floats. When the level drops to the height of the roof legs, the roof lands and a vapor space forms beneath it. We plan draws to keep roofs floating in normal service and treat any landing as a planned event, with emission and safety controls in place.

• Rim seals inspected and gap-measured on a set schedule
• Roof drains checked so rainwater neither sinks a roof nor enters the product
• In-service inspection to API 653 and overfill protection to API 2350
• Secondary containment sized for the largest tank plus rainfall

H2S management

Hydrogen sulfide is heavier than air, numbs the sense of smell at high concentrations and can reach dangerous levels on a gauging platform or at an open hatch. Our crews wear personal H2S monitors, gauge sour tanks from the upwind side and use closed gauging and sampling where the tank allows it. Fixed detectors cover low points where vapor can collect.

Mixing, heel and BS&W

Crude left standing stratifies and drops sediment and wax. Side-entry mixers keep the tank homogeneous, slow the build-up of tank bottoms and make upper, middle and lower samples agree. Sediment that settles anyway becomes part of the heel, the volume below the lowest level the suction can draw. We record heel quantity and ownership at the start of every contract.

Basic sediment and water (BS&W) is measured by centrifuge, and water alone by Karl Fischer titration or distillation, under API MPMS Chapter 10. Free water is drawn off the tank bottom and gauged separately. Many pipeline tariffs cap BS&W at 1 percent or less, so we test before every delivery, not after.

Blending to refinery specification

Refiners buy crude to run a crude unit, not to hold a certificate. We blend grades in tank or inline to targets for API gravity, sulfur, total acid number (TAN), vapor pressure and metals. Specific gravity blends close to linearly by volume and sulfur blends by mass, so we calculate on those bases rather than averaging API gravity numbers.

Adding light crude or condensate to heavy crude causes shrinkage: the blend occupies less volume than its parts. API MPMS Chapter 12.3 sets out the calculation, and we agree the method before blending so shrinkage is not booked as a terminal loss. We also screen paraffinic and asphaltenic crudes for compatibility, because some mixes drop asphaltenes as sludge.