DNV GL: CSR BC & OT coming into force on 1July 2015

The new Rules will be replacing the existing Common Structural Rules for Double Hull Oil Tankers (CSR-OT) and Common Structural Rules for Bulk Carriers (CSR-BC), and provide an improved, comprehensive and consistent rule set which will set the standard for oil tankers and bulk carriers.

What is CSR BC & OT?

The IACS Common Structural Rules for Bulk Carriers and Oil Tankers (CSR BC & OT) enter into force on 1 July 2015, replacing the existing Common Structural Rules for Double Hull Oil Tankers (CSR-OT) and Common Structural Rules for Bulk Carriers (CSR-BC).
A large team of technical experts has from 2008 to 2014 harmonised and further developed the two originally independent rule sets – CSR-BC and CSR-OT. The result is an improved, comprehensive and consistent rule set which will set the standard for oil tankers and bulk carriers.

The new rule set consists of two main parts. The first is a common part covering general hull requirements applicable to both ship types, such as wave loads, hull girder strength, buckling and fatigue requirements. The second part covers ship-type specific requirements only applicable to bulk carriers or oil tankers.

Some key features of CSR BC & OT compared with CSR-BC:
■ Extended verification scope, with an FE analysis of all cargo holds including the transition to fore part and engine room

■ More transparent and consistent requirements, including technical background

■ Improved load model

■ Enhanced fatigue standard

■ Hull girder buckling, also including lateral pressure and the combination with shear stress

■ Hull girder ultimate limit state assessment, including damaged condition

■ Compliance with IMO Goal Based Standards (GBS)

■ Increased design grab weight for Panamax and Capesize vessels

Consequences

The application of CSR BC & OT to existing designs shows that there will be some changes. Compared to CSR-BC designs, increases in the range of 1–3% may be expected. This is based on a simplified assessment without any redistribution of scantlings/design iterations. This means that, after iterations in a full design analysis, the actual impact may be reduced. The change in steel weight and scantlings also depends on the size of the vessel, structural arrangement, type of profiles used and amount of high tensile material used.

Prescriptive requirements will normally lead to scantling impact for some members, such as:

■ Bottom longitudinal girders

■ Inner bottom, hopper and stool side plate due to grab loading

■ Bottom plate and stiffener between 0.2L and 0.3L from F.P. due to bottom slamming

In the midship area finite element analysis, the results show small changes. However, some reinforcement due to buckling may be necessary in the side shell and for longitudinal stiffeners in the top wing tank.

Due to the increased scope of the FE analysis, a scantling increase is expected for primary supporting members, i.e., web frames, stringers and girders, for the foremost and aftmost cargo holds, including members connected to the collision bulkhead inside the forepeak tank as well as structures attached to the engine room bulkhead inside the engine room. Scantling increases have been found in local areas for the following structures:

■ Aftmost cargo hold region

– Engine room bulkhead, especially upper part

– cross deck structure

– double bottom loors

– top wing web frames

■ Foremost cargo hold region

– collision bulkhead, especially upper part

– cross deck structure

– double bottom loors

– top wing web frames

– side shell

DNV GL support

DNV GL has expanded its activity world-wide to assist customers in implementing CSR BC & OT. An extensive training programme has been initiated for all approval units, enabling our staff to be well prepared to provide efficient and local support to the industry. We have also been running workshops for owners, yards and designers. Together with the yards, we have performed software training resulting in extensive consequence assessments of existing designs. We want to ensure a smooth transition to CSR BC & OT for our customers.

Source: DNV GL, Bulk Carrier Update No1 2015