UK regulator warned Transocean on blow-out valves

* Health and Safety Executive warned in 2005, 2006

* Transocean declines comment on 2005, 2006 criticisms

By Tom Bergin

LONDON, May 6 (Reuters) - The UK's safety regulator warned Transocean (RIG.N), the driller whose rig caught fire in the Gulf of Mexico, leading to a giant oil spill, about violations related to the equipment whose failure caused the accident in 2005 and 2006.

The Health and Safety Executive (HSE) issued Transocean with an improvement notice in 2006, criticizing the testing of a so-called blow out preventer (BOP).

"The multi-purpose tool used in blow-out preventer pressure testing was not so constructed as to be suitable for the purpose for which it was provided: and failed in service, exposing persons to risks that endangered their safety," the regulator said in a notice in June 2006.

A year earlier, the HSE issued an improvement notice criticising the condition of the equipment used to operate the BOP on another Semi Submersible Mobile Drilling rig, which a BP spokesman said was on contract to the London-based oil major.

The regulator's 2005 warning said Transocean had failed to ensure that a remote blow out preventer control panel had been properly maintained.

The notices are available on the regulator's website but the regulator was unable to immediately give further details on the matters on Thursday. On April 20, the Deepwater Horizon, a semi-submersible drilling rig owned and operated by Transocean and contracted to dril a well for BP Plc (BP.L) exploded and caught fire.

The companies believe a blow-out preventer failed to operate properly when the well hit a pocket of high pressure, causing the explosion.

The failure of the BOP to engage means the well is still pumping around 5,000 barrels/day of crude into the Gulf, generating a slick which is threatening wildlife, fishing and tourism across the region.

The cost of the spill is expected to run to billions of dollars.

A spokesman for Transocean declined to comment specifically on the 2005 and 2006 criticisms.

"Any prior incidents on Transocean rigs have been investigated by those responsible," Guy Cantwell said.

BP Chief Executive Tony Hayward said last week that the accident could show that new safety regulations are needed in the industry, and cited the testing of equipment as a possible area where changes could be made.

BP shares continued to regain big losses since the accident, trading up 1.0 percent at 0902 GMT, against a 0.3 percent rise in the STOXX Europe 600 Oil and Gas index .SXEP. (Reporting by Tom Bergin; Editing by Hans Peters)