Exploding Supertankers

While the 207,000-ton supertanker Marpessa was steaming southward past the coast of Senegal to pick up a load of crude oil from the Persian Gulf, her crewmen were routinely spray cleaning her empty oil tanks with jets of sea water. For no apparent reason, an explosion ripped through the hull, sending the brand-new ship to the bottom. Two weeks later, an oil hold of the supertanker Mactra blew up in the Mozambique Channel; next day a blast blew apart the Kong Haakon VII off Liberia. Last summer there were two more tanker explosions. Scientists and oilmen were at a loss for an explanation.

The mystery may now be solved. Physicist Edward Pierce reported to a recent conference on lightning and static electricity in San Diego that spraying sea water into holds can generate static electricity at the same rate as a summer thunderstorm. Though the resulting spark would be no lightning bolt, it could touch off a highly explosive mixture of air and oil vapors.

Pierce's conclusion grew out of some unorthodox experiments he conducted four years ago in an unusual laboratory: his bathroom (TIME, May 6, 1966). During the first five minutes of a shower, he observed, the electrical field in his bathroom steadily built up in intensity. The effect was too small to present any danger, but that might not be so in the case of the million-cubic-foot tank of a supertanker. There, Pierce has calculated, the electrical charge could easily exceed 10,000 volts per meter after about 45 minutes of spraying.

As a precaution against such an electrical buildup, some tankers now carry inert gases, like nitrogen, in their empty tanks, but that is an expensive technique. As an alternative. Pierce suggests using chemicals that would prevent the buildup of a dangerous charge, and monitoring the tanks with electrical sensors. Some action is clearly necessary. There are about 4,000 tankers at sea, and shippers are ordering construction of even larger ones to increase their profits. The bigger the tanker, the greater the potential for explosion--an unfortunate example of more bang for the buck.

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