Hull crack in bulker suggests wider problem with brittle grade-A steel, TSB report warns

The hull crack that nearly sank a bulk carrier in the Gulf of St. Lawrence could represent a wider problem posing the threat of catastrophic failure in other vessels, the Transportation Safety Board of Canada has concluded.

Lake Carling. Investigators believe the crack was the result of steel that became brittle in ice-cold water.
   Image Credit: Courtesy Transportation Safety Board of Canada

A massive crack in the bulk carrier Lake Carling occurred because the grade-A steel in the vessel’s side shell was susceptible to brittle fracture in cold water, the TSB said in its report on the accident.

Currently no standards exist for the fracture resistance of grade-A steel used in the construction of hulls. The lack of such standards means that other ships may be built of steel susceptible to the kind of crack that nearly sank Lake Carling.

“Admittedly most of the grade-A steel is good and very tough,” said Paul Drouin, a senior marine investigator for the TSB and the lead investigator of the Lake Carling fracture. “But we don’t know which is good and which isn’t. It is a Russian roulette game, and sometimes you get lucky, and sometimes you don’t.”

A minor crack in the hull of Lake Carling as it was crossing the Gulf of St. Lawrence on March 19, 2002, became a 21.5-foot-long hull fracture because of brittleness in the steel exposed to temperatures near 32°, according to the TSB report.

The crack was patched while the ship was still in the Gulf of St. Lawrence. The repairs allowed the Marshall Island–registered ship to make it safely from the Cabot Strait, about 140 miles northwest of Sydney, Nova Scotia, to the lee of the nearby Magdalen Islands.

The incident illustrates the need to create fracture toughness standards for all grade-A steel used in ship side-shell construction, according to Drouin.

Of the four grades of normal ship steel, grade A is the least impact-resistant at a wide range of temperatures. For example, grade E, used to build military warships, is tested for fracture toughness at minus 40° F.

About 95 percent of all ships have some grade-A steel in them, according to Dr. Jim Matthews, a scientist with Defence Research and Development Canada. Grade-A steel is used to construct the side shells of most ships. But the toughness of grade-A steel is unknown, because the International Association of Classification Societies does not require fracture toughness testing for the grade-A steel less than 50 millimeters. The side shells of most commercial ships are 25 millimeters or less, according to Drouin.

While tests show that the average fracture toughness of grade-A steel is often quite high, with no actual standards, there is no way to make sure this steel is tough enough for the cold conditions many vessels encounter.

Drouin said the report is not advocating a ban on grade-A steel for shipbuilding. But there should be an established minimum fracture toughness for this steel, set by the IACS, he said.

The 19 crewmembers of the 17,464-gt bulk carrier Lake Carling got lucky. While transporting 25,000 tons of iron ore from Sept-Îsles, Quebec, to Trinidad, the ship developed a severe crack that flooded the No. 4 hold. With help from the Canadian Coast Guard and a salvage company from Halifax, Nova Scotia, the crack was patched and the vessel reached the relative safety of Baie de Gaspé. Lake Carling was then escorted to Quebec City, where repairs were made.

At Quebec City, the entire section of the hull that contained the crack was cut out and taken to the TSB lab for examination. The main fracture was on the port side at frame 91, extending upward and forward from the toe of the weld at the base of the side-shell frame, according to the report. The fracture extended into frames 92 and 93, through H and J strakes, ending just short of frame 94 in the No. 4 upper water ballast.

Two other cracks, each about 3 inches long, were discovered in the No. 4 hold.

Four other cracks were found in the No. 2 hold, all of which had been covered by superficial weld repairs. It is unknown who did this work, or when, because the ship’s classification society, Det Norske Veritas of Oslo, Norway, had no record of the repairs, according to the report.

Lake Carling was built in Turkey in 1992 to DNV 1A1 and Polish Registry specifications.

Image Credit: Courtesy Transportation Safety Board of Canada

A stress facture more than 20 feet long developed as the bulker Lake Carling was crossing the gulf at St. Lawrence in 2002.

But the presence of minor cracks in a bulker isn’t the primary concern, according to Drouin. “It doesn’t matter where or how they originate; it’s a fact of life in the bulker trade,” he said. “What’s important is the damage tolerance … How big can those cracks grow before they go critical? And that is linked to toughness.”

Fatigue cracks develop slowly over time and can be noticed. Brittle fractures are deadly because they happen instantly.

There are three conditions that contribute to a brittle fracture in a ship: steel that is low in toughness, vessel stress and low temperatures.

In the cold, “these seemingly benign cracks may go critical,” Drouin said.

That is what was most disturbing about the Lake Carling incident. The length of the original crack was not determined at the time, but calculations showed it could have been as short as 10 centimeters (4 inches) at the time it became critical, according to the report. “That is not a damage tolerance that can be considered reasonable, because a 4-inch-long crack, you’d expect that to grow slowly and remain intact,” Drouin said.

In Lake Carling, some of the vessel’s cracks were not noticed and not repaired, and some cracks were repaired but not to classification society standards.

The vessel was properly loaded and was in calm seas when the crack became dangerous. The crack in frame 91 went critical because of the steel’s physical properties and the cold temperature, the report stated.

Tests showed that the grain size and fracture toughness of the grade-A steel in Lake Carling were “well below the de facto standard when compared to average values of grade-A steel available worldwide,” according to the report. “This steel performed below expectations and did not provide a reasonable damage tolerance in all its operational conditions.”

By Professional Mariner Staff