Analysis of the Root Causes of Damage to the Edges of Tank Manholes on the Main Deck of Handy-Size Bulk Carriers

This study analyzes the root causes of cracks in the deck plating around tank manholes. Four handy-size bulk carriers built in one shipyard were analyzed. In all cases, deck cracks were found near manholes, and the average time from the commencement of operation until the occurrence of cracks was 1356 days. Due to this short wear-life of the vessel’s structural material, the authors believed that it was unlikely to be caused by corrosion fatigue. The authors hypothesized that main decks cracked around manholes because of very poor-quality welded joints and poor-quality steel (large amounts of non-metallic impurities) used to make the manholes. In order to verify this hypothesis, on each of the vessels, material samples were collected from near the cracks and then examined thoroughly. Each sample was subjected to the macroscopic examination of the natural surfaces of cracks and their vicinity, microscopic examination of the material, mechanical property tests, and scanning electron microscope fractography for samples obtained after impact tests. The examination and test results were used to draw detailed conclusions for each case study. The general conclusions based on examination of the whole damage population validated the authors’ hypothesis that main decks cracked around manholes because of very poor-quality welded joints and poor-quality steel used to make the manholes.

Structural Failures of Bulk Carriers

Structural failures of bulk carriers continue at a steady and unacceptable rate, claiming some 150 lives per year. This paper explains why bulk carriers are vulnerable to structural failure either because side shell plating falls out allowing ingress of seawater or the deck cracks open. Inadequate detail design, material without guaranteed fracture toughness and limitations of current inspection programmes are identified as fundamental weaknesses. The paper includes important information from a case study of six oil bulk ore carriers prone to deck cracking. One of the group of six which sank in 1980 was traced 4200 metres down on the floor of the Pacific Ocean in June 1994 and the results of this initial search are reviewed. There is a genuine need to go back to basics. Designing from first principles to determine local stresses is an important first step rather than the current use of prescriptive rules. All subsequent steps in bulk carrier construction and maintenance should be subject to a penetrating review.