As the oil and gas industry and international shipping companies push their assets into high latitude marine environments, it is important to thoroughly understand the performance of construction materials exposed to extremely low temperature and / or subjected to high ice loads. In the past, the design considerations have addressed the uncertainty in these areas by adopting conservative approaches. While proven effective, there exists a need for additional testing to gain insight into the safety margin levels that have been implicitly included. The work presented has the potential to lead to a better understanding of how steel materials behave in long-term Arctic conditions.
This paper presents the results of mechanical property tests of aged steel samples from the Kulluk, an ice-class rated drilling barge. This barge was exposed to the Arctic environment for almost 30 years. The Kulluk was built in Japan in 1983, specifically for exploration drilling operations in the Arctic environment. When the barge was scrapped in 2014, hull steel was selectively harvested. The intent was to conduct tests to better understand the behavior of shipbuilding steels that have endured long-term exposure to Arctic environments.
As an initial project phase, four sample groups of hull plate, from four different locations of the barge (one below the waterline, one in the ice belt region and two above the ice belt), were chosen for testing. The laboratory tests included tensile tests, Charpy impact tests and hardness tests. The test results indicate that the yield strengths, ultimate strengths, toughness and hardness of the aged steels continue to satisfy the ABS Rules requirements. A degradation assessment of these samples was also performed using these testing results and the limited data available as tracked from the barge's construction stage.
It is observed that the yield strengths and ultimate strengths remain consistent within the scatter of original or time period data. With respect to Charpy toughness values no conclusions concerning degradation can be made conclusively as the scatter in the data is substantial, especially at low temperatures. The steel in this study that has aged in the Arctic environment appears to maintain its original mechanical properties.
Based on the study presented in this paper, further studies could be performed such as additional sample tests to increase the reliability of results, material properties test for steel samples with butt/fillet welds to understand the variation of the heat affected zone (HAZ) and Crack Tip Opening Displacement (CTOD) tests to get a better understanding of the material toughness.