Rules that need tweaking

While most participants in the single-hull versus double-hull debate accept that a second hull is not a panacea, it is acknowledged that it is an acceptable and cost effective way of preventing oil spills in certain types of tanker incidents, such as low energy groundings or collisions. Lloyd?s Register says, however, that concerns have been growing as the industry surpasses a decade of experience with double-hull tankers, particularly with respect to maintenance and survey in the void space between the hulls.

The class society has recently carried out a study that showed that different sizes of tankers exhibit differing modes and locations of failure. “We are looking at about ten years of information and we wanted to see where problems were coming,” says John Carlton, global technology leader for marine technology and investigations. “This represents the first cut of looking at our database because you?ve got to get a reasonable sample of data before you can start to draw some conclusions about the general trends”, he explains. The work is based on a sample of 373 ships with an average service life of 7.3 years.

The class society split the ships into five categories ? product tankers, Panamax, Aframax, Suexmax and ULCCs/VLCCs ? and looked at where the greatest incidences of failure were.

LR looked at whether any categories of ships suffer more than others with particular sources of problems. “For example, if you get the three largest categories of ships Aframax, Suexmax and the ULCC/VLCC range you generally find that they have the least incidence of damage ? somewhere around about 47% of the total ships at risk whereas if you go to the small ones, the Panamax and the product tankers, that statistic rises to about 71%, which in itself is quite an interesting number,” says Carlton.

However, he told The Motor Ship that at present he is not able to give a very convincing answer for this. He explains: “That is where the second part of this work comes along ? having got the initial cut, we are now going to start the analysis as to where in the ship ? which sorts of brackets, which parts of the structure are giving us the greatest problem.” That work is just about to start and he hopes it will be finished in about six months time. “We can then come back and start to look at the new rules and, in particular, the joint tanker rules and put them into some sort of perspective to give us a clue as to where we might need to do a little tweaking,” says Carlton.

If the same analysis were done on single hull tankers, it would result in a different distribution. “You?ve got, to some extent, two different animals here ? you?ve basically got the same thing looking outwardly but internally you have a rather different structural arrangement,” says Carlton. However, it may well be that you get the same course statistic purely by the way the ships are handled and perhaps the same broad statistics because of mechanical damage. “What you tend to find in mechanical set-in type damage is that virtually all the ships have got a very similar pattern with the exception of the Aframaxes for some reason,” he says. The other ships are all in the high 30s to low 60% while the Aframaxes stand out at 27%.

Statistical analysis

“Whenever you go into these sort of statistical analyses you often uncover a whole range of problems ? whether we?ve got something critical in length, or some dimension or other, or susceptibility to damage or just the particular ports that the ships are using, or the weather they are encountering”, he explains. This could be relative to the ship size, which will hopefully be highlighted in phase two of the work. Another thing that was noticed was the excessive wastage and corrosion seen to affect Panamax tankers at the highest rate, with a 57% incidence, a significantly higher percentage than for the other categories, which had incidences lying between 5% and 29%. It is known that these defect modes are influenced by the method of application and integrity of coatings, accessibility of certain areas of the ship for repair and maintenance, cargo heating practices and trade routes. “In the second cut of the data one can start to identify the causes,” concludes Carlton. n