Conversion procedure
The ifcOWL ontologies in http://ifcowl.openbimstandards.org/ are generated directly from the IFC EXPRESS schemas that are available at http://www.buildingsmart-tech.org/specifications/ifc-releases. The recommended conversion procedure is entirely open and documented in an LDWG Recommandation. The procedure follows the principles displayed in the below schema.

expresstoowl

More information and full detail can be found in:
Pieter Pauwels and Walter Terkaj, EXPRESS to OWL for construction industry: towards a recommendable and usable ifcOWL ontology. Automation in Construction 63: 100-133 (2016). http://dx.doi.org/10.1016/j.autcon.2015.12.003.

Tool support
The above procedure is implemented in a number of tools:

BuildingSMART IfcDoc tool
UGent – Aalto IFC-to-RDF converter
Walter Terkaj EXPRESS-to-OWL converter

These tools all result in the ontologies made available at:
http://ifcowl.openbimstandards.org/IFC4_ADD2#
http://ifcowl.openbimstandards.org/IFC4_ADD1#
http://ifcowl.openbimstandards.org/IFC4#
http://ifcowl.openbimstandards.org/IFC2X3_TC1#
http://ifcowl.openbimstandards.org/IFC2X3_Final#