A question of rules

By now, most are aware of what exactly the new generation of classification rules for oil tankers are. The new Common Rules for Tankers are aimed at establishing a global standard, increased structural durability, longer asset life and the elimination of competition on minimum structural scantlings. The intention is also to initiate a common approach to direct calculations and provide ship owners with the benefit of definite structural renewal thickness.

The new Rules will apply to all double hulled tankers of 150m in length and greater. They cover the entire hull structure in a consistent and logical manner. This includes the hull envelope, transverse and longitudinal bulkheads, main supporting members and local structure, and the fore and aft ends. The intent is to publish standardized text that will translate into common scantlings regardless of which classification society is selected.

Following the release of the draft rules, a series of seminars is now underway in principle shipping areas worldwide to explain in detail the new criteria. A final version of the tanker Rules will be prepared for presentation to IACS Council in late 2004 for adoption 1 January 2005 and the Rules are scheduled to take effect on 1 July 2005.

The Rules form a single, robust structural design standard drawing upon the accumulated new-construction and in-service experience from each of the three societies ? Lloyd?s Register, ABS and DNV ? working jointly to establish the common criteria. Throughout their development, the new Rules have also been discussed with an external review group that includes shipbuilders, shipowners, designers and independent consultants.

So, having established the ?what?” the next question is why the need for new rules?

Well, traditional prescriptive Rules were based on empirical experience and so fell on side of caution. Tankers or bulk carriers built thirty years ago had significantly higher amounts of steel than a comparable vessel built today. While the traditional system had worked well for many, many years, the emergence of computer-based structural and hydrodynamic methods gave way to increasingly sophisticated ship design and evaluation software.

In being able to optimise a design steel weight was reduced rather than re-allocated to the areas of greatest stress. Consequently, Classification societies were drawn into competition and yards began to pit against another for approval of a new design. In turn, the lack of robustness of vessels lead to growing shipowner discontent, which was exacerbated by shipbuilders imposing significant cost penalties for any deviation from the approved standard, minimum steel weight design.

So, the traditional Rule-making approach, which was considered to encourage technical innovation, was seen to be abused by ship owners? and not certainly in their favour.

Response

Class response in March 2001 when ABS, DNV and LR jointly announced that they would work to establish

common criteria for ship structural design, including hydrodynamic loads and corrosion margins for standard ship types, beginning with tankers. The IACS Joint Tanker Project (JTP) and the new Common Structural Rules

for tankers are the culmination of that project.

The IACS Joint Bulk Carrier Project (JBP) is a comparable development, for double side skin bulk carriers, undertaken by BV, CCS, KR, NK, GL, RS and RINA. Following initial development and harmonization of these two “pilot projects”, IACS Council intends to adopt the resulting Rules as IACS Common Rules for tankers and bulk carriers.

In technical essence of the Joint Tanker project Rules concerns loads, net thickness, longitudinal strength, hull girder ultimate strength, prescriptive rule formulations, fatigue, Finite element procedure and buckling. The objective, which must fully embrace the intentions of the anticipated IMO goal-based standards, is a vessel with a design and fatigue life of 25 years in a North Atlantic environment. Moreover it must be recognised by industry as being at least as safe and robust as would have been required by any of the existing rules.

Far greater detail about the

technical implications of the Rules is

available to those with further

questions. The biggest single question, however, is what impact the Rules have with regard to cost and benefits to ship owners and shipbuilders.

Longer asset life

Senior Principal Surveyor and Global Technology Leader at Lloyd?s Register, Alex Johnston explains more about longer asset life, definite steel renewal thickness and a new global standard. “Owners ordering ships to the new Common Rules for Double Hull Oil Tankers will have ships designed to a 25 year service life in lieu of current 20 year standard,” he says. “Combine this with 25 years in a North Atlantic fatigue environment, which is the minimum design standard, then you can see that a post JTP vessel will undoubtedly have a longer asset life especially when you relate the specific steel renewals in mm and not percentage terms.”

He adds that current allowable renewal criteria is more ambiguous than post JTP Era, which bases its

corrosion margins on a large database of specific measurements produced for IACS by ClassNK. “This new global standard ensures that the steel increases are required in areas which have traditionally shown poor service experience and is coupled with a need to renew steel at better defined

margins taking account of both local and global considerations than currently allowed.”

Of course, as Johnston explains, more steel means more initial cost. “In similar fashion to Q1 I would say that these initial steelweight increases will undoubtedly need to be paid for by new owners,” he says. “Discussions with several owners has indicated, however, that they are willing to pay for this increased robustness and durability.”

All costs and benefits resulting from the new Rules are clearly based upon vessels being maintained and operated in proper fashion. And, Johnston says, schemes such as hull planned maintenance and machinery planned maintenance can only add to this. This is not to suggest, though, that there is any particular failing in this area, as all class vessels are subject to a defined survey regime aimed at preventing such areas of corrosion and coating breakdown escaping the attention of Classification Societies.

Duty of care

“However, class societies have a ‘duty of care’,” says Johnston, “especially on older vessels where the original owner has sold on to a less discerning owner who does not employ such attention to detail. Hence the need to renew at more specifically prescribed wastage. Allowances will, hopefully, ensure that areas of neglect are highlighted sooner.”

For shipbuilders the benefits resulting from the new Rules can be measured in design cost and time savings. “It is, I believe, self evident that vessels designed and constructed to a single common rule set will benefit shipbuilders who have long complained about the need to design vessels to several differing standards,” continues Johnston. “The design time savings are obvious and also it is likely that production costs in terms of similar cutting tapes/steel ordering etc. will assist in further reducing costs.”

A further benefit, not be ignored, is that of not having to have each new basic design approved by individual Classification Societies. “When each class society has gained familiarity with the use of these new rules and it’s associated software,” concludes Johnston. “There will, hopefully, not be any reason for any IACS member to re-examine, say, a vessel designed to the CTRs and approved by another IACS member.” n