The International Association of Class Societies’ (IACS) Common Structural Rules (CSR) effective from 1 April 2006, applies to all bulk carriers, single or double side, equal to or above 90 metres in length, and to all double hull oil tankers equal to or above 150 metres in length.

More robust ships are being built as a consequence of CSR

CSR are now in force for both bulk carriers and oil tankers

What is CSR and why CSR?
Many of the rules of IACS Class societies are largely prescriptive, a result of an evolution from purely empirical require-ments to a combination of empirically and theoretically derived requirements that compensate for the limitations of solely experience-based rules.

The new Common Structural Rules are a welcome initiative for the industry, as they have been developed using a consistent engineering approach that applies advanced analytical and numerical methods based on engineering first principles to establish the requirements. This is in line with how the DNV Rules have been formulated for a long time already. In short, CSR may be summarised to yield the following characteristics:

• Rules covering structural requirements for bulk carriers and tankers.
• A Rule set utilising state-of-the-art computational methods for more extensive direct strength calculations (Finite Element Analysis).
• Vessels built to CSR shall have overall safety of the hull structure equivalent to or better than that currently achieved by any current rules by any Class society.

The reason for introducing CSR may be summarised as follows:

• To obtain improved control over the minimum safety level during the operational phase.
• To eliminate competition between class societies with respect to structural requirements and standards.
• To employ the combined experience and recourses of all IACS societies to develop a single standard, or set of Rules.
• To ensure that a vessel meeting this new standard will be recognised by the industry as being at least as safe and robust as would have been required by any of the existing Rules.

Left: Loads generated automataically from CSR.
Right: Cargo hold with integrated meshing

Benefits of CSR
The new Common Structural Rules represent a significant step forward in the establishment of technical standards that will result in a new generation of robust, safe ships. In summary, the new Rules offer several obvious benefits for the shipping industry:

• The new Rules will allow shipbuilders and designers to work with one common set of Rules, instead of one set
  from each class society. All IACS societies will cooperate to maintain a single Rule set.
• More transparent technical background of the Rules.
• The intention of the new Rules is to encourage the design and construction of robust oil tankers and bulk carriers to eliminate competition on scantlings.
• CSR-compliant ships will be designed to rigorous analytical criteria.
• Extensive prescriptive requirements have been put into effect with respect to the application of the new CSR and the manner in which the central finite element analysis is undertaken, so that consistency in application and common scantlings result.
• Stringent and clarified requirements to critical areas.
• The minimum fatigue design life has been upgraded from the 20-year worldwide trading to 25-year North Atlantic wave environment.
• The Rules include design standards for the net scantlings of a vessel, meaning that the renewal thicknesses for the inservice time is checked and known at the newbuilding stage.
• The corrosion additions that have been established take into account the location and the environment to which the structural member’s surface is subjected.

New Nauticus software with unique support for CSR
The new Rule requirements for bulk carriers and oil tankers introduce a radical shift towards more computerisation of the rule formulations and structural assessment. Hence, good software support is critical for any classification society to provide timely and rational support to shipowners, designers and shipbuilders. As a response to this, DNV Software’s wellestablished

Nauticus Hull program has undergone a major upgrade to provide efficient support for the new Rules. Nauticus Hull is a powerful software package for the design and strength assessment of ship structures. It provides a highly efficient environment for ship design, with capabilities ranging from simplified rule-check and scantling calculations to advanced tools for extended calculation procedures, including wave load and finite element structural analysis.

In January 2006, DNV was the first company to launch complete software support for CSR for oil tankers. During April 2006, DNV again made a new release of its Nauticus Hull software, thus again being first, this time with software support for the new rules for both ship types.

The software has been developed in close cooperation with major shipyards to ensure it supports the design process in a most efficient way. Therefore, the software has been developed not only focusing on cutting modelling and input time, but rather cutting the total design and approval time.

Especially, new tools have been developed to make finite element analysis more efficient through automated post-processing and code-checking, as well as generation of loads, net scantlings and boundary conditions.

Practical consequences of CSR
The IACS Common Structural Rules for bulk carriers will provide benefits for both shipyards and owners. The new Rules will result in more robust ships, with an increase in steel weight.

The CSR for bulk carriers will provide benefits for the shipyards in terms of the number of Rule sets and software packages they need to become familiar with. Whereas the existing regime provides ten sets of Rules, the new regime will provide only one. Another effect of common Rules is to avoid the chance of the classification societies being played off against each other in a process that could lead to unsafe scantling optimisation. The application of the CSR to existing designs shows that there will be changes.

The CSR will influence steel weights and design details. Compared to current DNV designs, typical increases are in the range of 3–4 per cent and occasionally even somewhat higher.

The change in steel weight and scantlings depend on the size of the vessel, type of profiles used and degree of high tensile material. In general, the majority of the changes are seen in the cargo hold region and, to some degree, the forward region of the vessel. For the cargo hold region, the new Rules are seen to give a slight increase in hull girder sectional modulus – mainly due to the new Ultimate Limit State check.

Further it can be noted that certain increase in deck longitudinals, plate thickness of lower stool, lower part of hopper plating, inner bottom, side shell plating, bottom plating, and webframe plating due to buckling and yielding, and side and bottom longitudinals due to fatigue can be expected.

It is further seen that, although high tensile material may be used, this cannot be fully utilised in areas dominated by dynamic stresses due to the higher fatigue standard. This is particularly so for NV36 steel. A typical example is the deck area, where the dynamic loads account for two thirds of the total loading and, consequently, the fatigue requirement limits the allowable stress level.

Author: Erik Hansen