Spotted on 14 November 1993, this crude proved to have leaked from a joint between the concrete wall of storage cell number 5 and its cap. This is one of seven such cells distributed around the bottom section of the shaft which rises from this gravity base structure (GBS) to support the Draugen topsides.
The seven outer cells are designed to store crude oil produced from the field before it is discharged to a shuttle tanker for shipment to land. Production was halted immediately, with oil in cell 5 pumped to the other storage units. By then, about 10 000 litres (10 tonnes) had then leaked out.
Draugen produced some 30 000 barrels (4 769 619 litres) per day at this time, and had not reached full output. This meant the six unaffected cells had enough capacity for operator Shell to resume production after 36 hours. The leak was caused by two holes in the cell joint, which were so small that the measuring instruments had failed to detect them during pressure testing of the GBS.
Although the cells were pressure-tested with water before production began, this medium was not as visible as oil when it leaked out.
Establishing why the holes had arisen in the first place took a long time. Since builder Norwegian Contractors (NC) did not know the reason, determining the right repair method was also hard.[REMOVE]Fotnote: Dagens Næringsliv, 6 January 1994, “Ukjent sprekkgrunn på Draugen”.
NC devoted a great deal of time, money and brainpower to coming up with the solution. This involved injecting an epoxy blend into the holes to seal them.
Epoxy is a form of liquid plastic which forms a bond with the substrate, and thereby hardens into a fixed material which functions like glue.
Before the actual operation was initiated, NC conducted injection trials with various epoxy blends and under different pressures below water in the Gands Fjord outside Stavanger.[REMOVE]Fotnote: Aftenposten, 14 January 1994, “NC tester tetting av Draugen”.
Work to perform the actual sealing began on 22 February 1994. The biggest challenge was getting the area around the holes completely tight. A specially made box was installed over the porous area, and epoxy injected. This work took place in 210 metres of water with the aid of a remotely operated vehicle (ROV).[REMOVE]Fotnote: NTB, 23 February 199, “Draugen tettes – resultatet klart til helgen”.
Although a few decilitres of oil per day continued to trickle out after the work had been completed, these final small holes eventually sealed themselves.
The leak attracted big headlines in the media. This was not long after NC had lost a GBS during construction when the Sleipner A concrete structure sank in the Gands Fjord in 1991.
Twenty-two people had been aboard the GBS when it sprang a leak during testing, and everyone fortunately escaped to boats or barges. However, the structure could not be saved.
A leak in the Draugen GBS accordingly created a big furore in the Norwegian media. Everyone on the platform was warned ahead of a press visit that only the communications manager could talk to the journalists about the issue. But the visitors accepted no such restrictions. They put questions to whoever they met. One of them asked production head Per Sælevik whether for his assessment of the shaft leak.
When he admitted that his views on the issue were clear, he found both camera and microphones directed at him and the question being put directly.
“I’m convinced that it’s due to concrete rats which came aboard [at the construction site] and have gnawed holes,” came the strictly tongue-in-cheek answer. The interview was not broadcast.[REMOVE]Fotnote: Sælevik, Per. Stories from Draugen, unpublished.
Illustration: see Teknisk Ukeblad, 3 March 1994.