* Third failed well this year
* Scarabeo 9 drilling rig leaving Cuba for West Africa
HAVANA, Nov 2 (Reuters) - Cuba's offshore oil hopes suffered a major blow on Friday with the announcement of another dry hole in its still untapped fields and word that the drilling rig used in the project will soon depart the communist island.
Communist Party newspaper Granma reported that a well drilled off western Cuba by Venezuelan state oil company PDVSA had been plugged because it ``did not offer possibilities of commercial exploitation.''
It was the third unsuccessful well drilled this year in Cuban waters, where the country says it may have 20 billion barrels of oil and the key to future energy independence.
The U.S. Geological Survey has estimated a more modest 5 billion barrels.
Cuba gets two-thirds of the nearly 150,000 barrels of oil it consumes daily from oil-rich socialist ally Venezuela.
Granma said that because of the ``technical experience and valuable geological information obtained,'' in the failed well, PDVSA planned to ``continue its participation in the exploration campaign in Cuban waters.''
But it will have to do so without the Scarabeo 9, the massive Chinese-built drilling rig owned by Italian oil service firm Saipem SpA that has been used in the Cuban drilling.
It was brought to the Caribbean island in January under contract with Spanish oil company Repsol SA.
Industry sources said the semi-submersible rig, which can operate in waters up to 12,000 feet deep (3,660 meters), will leave Cuba by mid-November and go to West Africa for exploration there.
It will not be easily replaced in Cuba, where technology restrictions imposed by the longstanding U.S. trade embargo against the island greatly limit the number of deepwater rigs available.
The Cuban wells have been drilled in water more than a mile (1.6 km) deep.
Repsol is pulling out of Cuba after drilling an unsuccessful well earlier this year.
It passed the Scarabeo 9 to Malaysia's Petronas which, in partnership with Russia's Gazprom Neft, then drilled another failed well. Petronas continues to do seismic work in Cuban waters, searching for more drilling prospects.
The Scarabeo 9 then went to PDVSA for the third dry hole.
Repsol drilled a previous failed well in Cuba in 2004 using a different rig.
One problem the companies have encountered is very hard rock, which quickly wears down drilling bits and is so dense that oil does not easily flow through it. Petronas has said it found oil, but it could not be produced.
Cuba's best offshore hope now lies with Russia's Zarubezhneft, which is set to begin drilling this month about 200 miles (320 km) east of Havana.
It will be drilling in shallower depths, using a rig from Norwegian firm Songa Offshore SE capable of working in waters up to 1,200 feet deep.