Open Class 3D Exchange (OCX)

The vessel-specific XML standard for model-based class approval in shipbuilding.

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How OCX uses UnitsML

OCX is an XML-based format for exchanging ship structural and geometric data between shipyards’ design tools and classification societies’ rule-checking platforms. Every physical quantity in an OCX document — lengths, masses, centres of gravity, panel thicknesses, tank volumes — must carry an unambiguous unit reference.

OCX encodes these quantities using UnitsML. In an OCX file, a numeric value is paired with a unit attribute whose values are defined by the UnitsML schema:

<ocx:CenterOfGravity>
  <ocx:X numericvalue="376.1" unit="Um" />
  <ocx:Y numericvalue="0" unit="Um" />
  <ocx:Z numericvalue="16.7" unit="Um" />
</ocx:CenterOfGravity>

The unit="Um" references are UnitsML identifiers (here, the metre). This lets OCX consumers — whether a CAD exporter or a class-society calculation engine — resolve every measurement to a machine-readable definition rather than an ambiguous string.

OCX was developed through the APPROVED joint industry project (2016–2020) led by DNV, with participation from Aveva, Hexagon, Siemens, NAPA, Kongsberg Maritime, Ulstein, Chantiers de l’Atlantique, and Digitread. The OCX Consortium was founded in 2021 to maintain and promote the standard, which is used today for 3D model-based class approval by major classification societies and shipbuilders.

What UnitsML gives OCX

  • Unambiguous units — no risk of mm vs m vs in misinterpretation across tools.
  • Dimensional analysis — UnitsML’s dimensional references let OCX validate that a length is truly a length, a mass truly a mass.
  • SI alignment — UnitsML’s foundation in the SI system matches the maritime industry’s metric standards.
  • Machine readability — class-society engines can automatically convert and verify units without manual review.
  • Standards credibility — using a NIST/OASIS/CalConnect-backed schema strengthens OCX’s position as an authoritative exchange format.

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