Salvage workers and shipping experts were studying a possible mid-sea transfer of nearly 30,000 tons of gasoline from a damaged tanker at risk of exploding, the ship's operators said. Spanish authorities insisted the tanker would not be allowed back into the country's waters to carry out the potentially risky operation. A tugboat carrying equipment and specialists for the operation had joined the Greek-owned tanker Castor, around 56 km (35 miles) off the Spanish port of Cartagena in the Mediterranean Sea, the ship's operator Athenian Sea Carriers said in a statement. The 1977-built Castor sought refuge in ports in Morocco, Gibraltar and Spain last week after developing a large crack in its main deck but was refused entry amid fears sparks could set off an explosion.