During a Class survey of a tanker, several cracks were found in the deck structure in the midship area, at the connection between the deck longitudinals and the deck openings (access and Butterworth hatches).
Cracks had occurred at several locations, and they appeared to be recurring cracks, see figure below.
The probable cause of the cracks was lack of continuity in the deck longitudinals. These structure members are essential for the longitudinal strength of the vessel. Longitudinal stresses appear in deck longitudinals due to loading conditions (sagging or hogging) and due to the ship’s movement in waves. The stresses are subsequently transferred to the coaming plate, which will cause the cracks as found.
The recommended design, as shown in figure below, will provide an acceptable continuity and, at the same time give ample space for the access openings by changing the direction of a deck longitudinal. It is then very important to provide proper support at all four knuckles as any knuckle left unsupported will tend to move and introduce new cracks.
Lessons to be learned
- Acceptable continuity of the deck longitudinals is very important, as they are essential for the longitudinal strength of the vessel.
- Continuity of the deck longitudinals is of particular importance in the midship area where the longitudinal stresses are highest.
- Modification of deck longitudinals for practical reasons is to be decided in cooperation with Class. Alternative design will normally include knuckles which are to be properly supported.
Source: DNV
Officer of the Watch 
Leave a Comment