Structure Defects on Double Hull Tankers

Double Hull tankers built in the past have experienced certain structural deficiencies. The statistics indicate that a significant number of defects, especially fractures, occurred in double hull tankers less than 10 years old. Information in this post are from a paper on “Structure Defects on Double Hull Tankers” by Philip G. Rynn of ABS, which was presented in Tanker Structure Cooperative Forum (TSCF) 2007 shipbuilding meeting. The statistical data below illustrates the type of defects known to occur in the early double hull tankers by the application of Rules prior to the IACS Common Structural Rules (CSR) and hence where the prior Rules could have been improved.

The IMO in 1992 adopted MARPOL Annex I Chapter II Regulation 13F which made double hull tankers mandatory. Thus the United States requirement of the Oil Pollution Act of 1990 mandating double hull tankers effectively became the world standard. The initial view of the Tanker Structure Cooperative Forum in the publication “Guidelines for the Inspection and Maintenance of Double Hull Tanker Structures” (TSCF DH Manual) published in 1995 identified locations that may be prone to fracture in double hull oil tankers. These locations are selected from experience from the number of double hull oil tankers operating at that time, and also from other types of vessels in operation with similar structure. The TSCF DH manual mentioned above identified the following areas as ones where defects are more likely:

Cargo Tanks:

1. Under deck vapor spaces in cargo tanks,
2. Termination of primary support structure (webs and stringers)
3. Panel instability where panel stiffeners have not been properly arranged
4. Joints of the inner hull in way of lower hopper and upper shelf construction.

Ballast Tanks:

1. Same locations as cargo tanks
2. Connections of longitudinal stiffeners to web frames, floors and bulkheads
3. Vertical stiffeners on transverse bulkheads to deck, horizontal stringers and inner bottom.

The above gave to the industry some early insight into the problems that may occur in double hull tankers.

For damages found in double hull oil tankers, crack type defects and wasted type defects account for over 80% of the reported defects. The wasted type defects reported are approximately 20% of all the defects found in the sampled reports. The wasted type defects can be controlled by improving the protection of steel. The majority of the defects, almost 70%, found in the sampled reports are crack type defects. Therefore it is important to evaluate these closely to determine the probable cause. Cracks in the deck account for about one third of the crack type defects. The attention to detail for both longitudinal connection to transverse webs and transverse bulkheads and deck outfitting will be necessary in order to minimize, or eliminate, the reported crack type defect found in deck  structure.

The data for the upper deck and stiffeners in is noted to be greater than that for either the inner bottom or bottom shell plate and stiffeners. As a consequence there have been questions regarding the cause since the damage to upper deck and bottom should be closer than the ratio 6 (21.2%/3.4%~ 6) in the table above from the records sampled from the service history of double hull oil tankers. Discussion with those that assembled the data reveals many of the defects are in plating due to outfitting details and do not involve the end connections of longitudinal to either the web frame, or transverse bulkhead.

As many of the defects reported for the upper deck plate and stiffeners are due to details, where outfitting has not provided appropriate supports for the above deck structure, or has selected poor details for construction, or has been located on soft plate. When the defects other than the end connections are eliminated, the defects for end connection of deck longitudinal are similar to that reported for the bottom longitudinal. The number of defects occurring in the longitudinal and stiffener connections located on the side shell, longitudinal inner skin, inner bottom and transverse bulkhead are found to be the highest.

The table below shows data on the type of defects that are found on different types of oil tankers.