SOMETHING ABOUT THE PETROBRAS 36

The world hardly held its breath as the P-36, possibly the most advanced floating production unit in the world gradually heeled over and on Tuesday 20th March 2001 sank beneath the waves of Brazil's Campos Basin. Ten of the rig crew, either Petrobras employees or contractors died in the catastrophe, there was a moderate level of pollution and the insurance industry has to fork out $500 million.

Those of us professionally involved in offshore safety have followed events by any means possible, and have been generally distressed not only by the event itself, but also by the scant research undertaken and the consequent poor quality of the reporting by apparently reputable news agencies.

The phrase "As tall as a 40 story building" appeared in almost every report and leaning over "three times as much as the leaning tower of Pisa" was used the illustrate the angle of the structure. The part of the rig destroyed by the explosion was described as "one of the supporting columns" and the pontoons as "the buoyancy containers." It was also often described as the largest oil rig in the world, and the largest semi-submersible in the world.

All this must have been very confusing for those unfamiliar with offshore structures since it was pretty confusing for those who are. The truth is that the P-36 was a moderately sized semi-submersible oil rig which was converted for its role in Brazil. it was about 80 meters long and a similar width and about 50 meters from keel to deck. In common with most semi-submersibles, two pontoons - separate hulls looking a little like submarines - supported columns which in turn supported the deck, and the deck housed the mass of equipment required to separate crude from gas and water. The P-36 had only four columns though they were dimensionally large and it was in one of these that the explosion, or explosions occurred. It is rumoured that the first explosion took place in a paint locker and that the casualties were the fire team sent to deal with the resulting fire. It then appears that further explosions destroyed the column.

Most modern semi-submersibles are constructed so that in theory, even if one column ceases to provide buoyancy for any reason, the rig will remain afloat. To achieve this the connecting hatches or doors between internal watertight divisions must be kept closed and the apertures at deck level must also remain closed. It is unlikely that his was the case. It is difficult for the ordinary rig worker to appreciate that keeping hatches doors and vents closed will be of any benefit. They will be unable to visualise an event so disastrous that part of the deck of the rig could be submerged. If they could visualise it they would probably stay at home.

Incidentally the P-36 had one statistic which counts against it ever being recovered from the seabed, and will ensure that its replacement will be difficult to connect and position. It was moored in 1360 meters (about 4500 ft ) of water.