Houston BP and Transocean lacked clear guidelines for key tests to ensure that the ill-fated Macondo well was safely sealed before it ruptured and triggered the 2010 Deepwater Horizon disaster, US safety investigators said.

The preliminary report from the US Chemical Safety Board, to be delivered at a public hearing in Houston on Tuesday, puts the spotlight back on missteps highlighted in prior investigations as BP faces potential civil and even criminal penalties over the incident.