Buckling up for more robust ships

Three major incidents in three years have woken the industry up to the fact that post-Marpol, buckling is a far more critical design strength factor than pre-Marpol. Erika, Prestige and to a lesser extent Castor have demonstrated that current minimum buckling criteria for decks, bottoms and girders only take strength so far. When it comes to ultimate strength ? the situation where a ship breaks in two ? the criteria fall short. This has encouraged owners and class to cooperate in the development of new rules leading to more robust tankers.

Post-Marpol, tankers are required to carry more segregated ballast. Designers have accommodated this requirement by making the ships wider and higher. This has resulted in a drop in the length to depth ratio from about 11.5 to 10.5. The greater distance between steel deck plate and the bottom has meant a decrease in steel thickness (the section modulus is not reduced) from 24mm on pre-Marpol VLCCs to 20mm on post-Marpol VLCCs. This has meant that on deck plates, the margin between plate thickness at newbuild and the buckling critical plate thickness, identified as about 18mm, has dropped considerably.

“The 2mm margin is not good enough,” says Wilhelm Magelssen, vice president of DNV Maritime and head of the Norwegian classification society?s technical support projects group who believes the above example may be close to reality in some cases. “A Ships in Operation (SiO) requirement ought to be incorporated in the newbuilding requirement.”

Currently, class rules for plate thickness require a corrosion addition to the basic strength of the plate (i.e. t=to + tk). DNV, LR and ABS are developing rules where the minimum basic steel strength criteria take into account tension and buckling, including ultimate strenth etc. and where a revised corrosion addition is used to account for a more robust ship (i.e. t = tmin SiO+tcorr, where tmin SiO != to and tcorr != tk).

“Buckling is critical for the ultimate strength. Tension may give you cracks, but not a breakdown of the ship,” explains Magelssen.

It might seem straightforward therefore to attain rules for more robust ships simply by addressing buckling criteria, but ultimate strength is not as simple as that. There is a corresponding need to avoid stress transfer into the longitudinals and therefore longitudinal collapse.

“Ultimate strength is important to look at at both the newbuilding stage and for ships in operation,” explains Magelssen.

Cracks need to be viewed from a safety aspect and from their potential as a source of oil spills.

DNV, ABS and LR are therefore undertaking a main supporting structure analysis for rules that address plates, stiffening, welding, fatigue, corrosion allowance, materials, hull girder strength (already an IACS UR) and ultimate strength into the requirements for ships in operation.

The aim is to develop rules that account for limited maintenance and result in ships that have a minimum 25-year lifespan. Minimum load strength criteria will increase to 25 years North Atlantic operation as will minimum fatigue strength criteria. The corrosion addition will increase to about 15 years. A coating standard will also be considered within IACS.

Some companies, such as the Papachristidis-controlled Hellespont, have specified and incorporated a very high standard into recent ships. The problem is that this costs money and it doesn?t necessarily give a pay-back.

“There aren?t too many charterers that pay more for higher standard, higher quality ships,” explains Peter Swift, managing director of Intertanko. He reckons that owners will pay for more robust ships, recognising that the additional investment will come back through the longer life of the asset, but only if it is a requirement, through the class rules, for all owners to do this.

Magelssen adds that the most expensive way to operate a ship is to bring it up above minimum strength requirements at each survey – if the ship is built more robustly initially, costs at each survey will be reduced. Plus the ship is more likely to come within the minimum standards (CAP2) accepted by oil majors vetting managers.

While owners and class seem to agree on the need for new rules resulting in more robust ships, the development process has not been without controversy.

That DNV teamed with ABS and LR to develop the rules through LAN, rather than taking the IACS route, has caused some bitter arguments. However Magelssen defends the approach from the DNV standpoint by pointing out that DNV has a strategy of alliances and from a general standpoint by explaining that with only three societies working on the rules, as opposed to 10 within IACS, the new rules can be completed sooner.

However, the speed of rule development and the perception of the input that shipowners can have in the new rules have still upset some owners. However, in this correspondent?s opinion these owners would do well to recognise that the industry does a reasonable job of policing its risk through the class system and that while rules have to respect the needs of owners, they cannot be merely written to accommodate every commercial whim – class is not perfect, but for the owner it is better than the alternative (an EU-run body, for example).

Magelssen reckons the new rules will be “nearing completion” by the end of this year.

Ice and coatings next

Magelssen reckons that within the next 10 years, class societies must become much more involved with coatings because of their impact on corrosion protection and fatigue. He reckons that rules will probably develop to cover both the constitution of coatings and their application by shipyards. The situation will be complicated by the proximity of many shipyards to large urban centres, he says, where different local rules will apply to coatings.

He also believes that recent winter rates of $100,000 per day plus for oil cargoes from Primorsk, combined to a general growth in Russian oil exports, will encourage many owners to adopt ice class standards in their newbuildings. While ice-class addresses hull strength it takes little account of the impact of icing from green seas and spray ? and consequently broader rules are required for the safety of operation in cold climates.

Machinary on Agenda

The second tripartite meeting of Intertanko, Intercargo and the International Chamber of Shipping will have the robustness of machinery on the agenda. The meeting, scheduled for Seoul on September 29-30, will address some of the questions raised by the three organisations? members over the maintenance and reliability of machinery. Questions that are expected are whether competition has squeezed the safety margins built into machinery and whether there is a collective way to build in greater margins if it is agreed that safety margins have been squeezed to unacceptable levels. Representatives of Asian shipbuilders, BIMCO and IACS are also expected to attend the meeting.