Ullage

The quantity a container requires to completely fill it, i.e., the space in the container remaining above the liquid level.

Ultra slim hole drilling

The application of mineral drilling concepts and techniques for the purpose of low cost petroleum exploration. See slim hole drilling.

Ultra-large crude carrier (ULCC)

An extremely large ocean-going tanker of over 300,000 metric tonnes deadweight used to transport crude oil from its source to a transhipment storage facility or direct to a refinery.

Unconformity

A tectonic feature in which strata are eroded and subsequently overlaid with younger strata (sometimes angular), which are said to lie unconformably on the older strata; the actual boundary between the two sets of strata.

Unconformity trap

A trap in which the escape of hydrocarbons is prevented by impervious rocks unconformably overlaying the reservoir. See unconformity.

Underwater acoustics

A special field of technical acoustics dealing with the physics of sound propagation in the water.

Underwater position fixing

A method of position fixing for surface vessels, submersibles or towed devices in relation to a network of underwater acoustic beacons on the sea-bed with known relative positions.

Unit operator

The oil company in charge of development and producing in an oil field in which several companies have joined together to produce the field.

Unitisation

When owners of oil and/or gas reserves pool their individual interests in return for an interest in the overall unit, which is then operated by a single company on behalf of the group, thus increasing efficiency.

Universal time (UT)

Time as defined by the rotational motion of the Earth and determined from the apparent diurnal motions which reflect this rotation. Because of variations in the rate of rotation, universal time is not rigorously uniform. Also called Greenwich Mean Time.

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