Industry teams up to advance structural safety
Class societies and software suppliers are working to ease the introduction of the harmonised common structural rules for tankers and bulk carriers, and there will be changes to come for container ships, says Wendy Laursen.
The push for harmonisation of the Common Structural Rules (CSR) began soon after their publication in April 2006. One of the big issues was the harmonisation of buckling assessment methodologies, but the harmonisation work has covered a range of topics including load calculations, corrosion and fatigue.
It has been a major project for IACS, says Roberto Cazzulo, chairman of both IACS and RINA Services. The details will continue to be clarified as they prepare for entry into force of the rules for tankers and bulk carriers contracted for construction on or after 1 July 2016. Opinions still differ between shipowners and yards on the net thickness of some structural elements and on the finite element calculations to be used on the fore and aft parts of a ship.
“These calculations are quite demanding from the design office point of view,” says Mr Cazzulo. “The comments that we received from, for instance, the builders were to justify the reasons for having these calculations for the fore and aft parts. On the other hand, the comments from the owners were to require the calculations for the full ship, based on their own experience.”
There are some claims that the new requirements will lead to a significant increase in total steel weight, in particular for large bulk carriers, which has implications for their ability to meet EEDI regulations. However, Mr Cazzulo has stated publicly that these arguments are a little exaggerated as new ship designs have not yet been fully optimised against the Harmonized Common Structural Rules (CSR-H).
IACS decided that CSR-H should also conform to the IMO Guidelines on Goal Based Standards (GBS) defined in 2010 and to enter into force on 1 July 2016. “It is the first time that we have carried out this exercise and it is very important that the work is successful from both points of view. It is important for IMO to be assured that the structural standard meets its safety goals and also for class to have further assurance that the rules to be applied to the future generation of ships will meet the standard,” says Mr Cazzulo.
Plate thickness requirements demonstrate the benefits of GBS. While additional thicknesses are currently considered including a corrosion addition and the coating, the goal-based approach allows for future innovations such as sandwich plating. “It is really an open door to future innovation that considers both safety and energy efficiency,” says Mr Cazzulo. “Even very traditional ship types like bulk carriers and tankers have plenty of room for design changes and energy efficiency changes to the hull form and bow.”
Initially released in July 2012, ClassNK’s PrimeShip-HULL design support software is claimed to be the world’s first to incorporate the requirements of CSR-H. A new version released in April 2013 incorporated the changes found in the second draft of the rules, but the process for all software designers and class societies is on-going as the industry continues to provide feedback to IACS on the requirements.
IACS members are independently developing their own rule checking software and ClassNK and legacy GL have undertaken a project with design software supplier NAPA to develop a 3D model with seamless integration to their rule checking software. NAPA aims to share tasks between the various tools. For example, NAPA holds the primary 3D ship geometry and compartment information and creates the finite element meshing which is then exported to the rule checking software. This software holds the primary information for loading and boundary conditions and does proscriptive rule checking. Only changes required to the NAPA model need then be exported back and this is now done automatically.
“The new CSR-H requirements offer challenges to ship design in integrating information with class rule check software and product models used in ship design,” says NAPA product manager, Tapio Hulkkonen. “CSR-H will present new challenges for structural design, and without any improvements in the design process, they will also increase the time spent for structural analysis.”
The requirements for cargo hold strength analysis require that a three cargo hold finite element model is used where the mid hold is the target for the analysis. This is an extension beyond what was required for CSR. The modelling must now represent the geometry of the actual hull form rather than the simpler use of prismatic cargo hold models. This means that 3D hull geometry needs to be defined for virtually the entire ship.
Local and hull girder loads are now to be applied simultaneously without any superimposition of simplified beam theory. Shear forces must be balanced on a case by case basis and therefore specialised software is needed to generate the proper loadings, says Mr Hulkkonen.
“The finite element assessment must include the corrosion margin, and the as-built thickness is used in 3D modelling to ensure that lightweight is calculated correctly and to produce drawings. Therefore, automatic corrosion reduction is essential when large finite element models are created in order to minimise the possibility of human errors if the corrosion reduction is submitted manually and to save man-hours during the modelling process,” says Mr Hulkkonen.
Intergraph has released Intergraph Smart 3D 2014, a new single design software solution created from the consolidation of SmartPlant 3D, SmartMarine 3D and SmartPlant 3D Materials Handling Edition. The software has optimised the design of tankers and bulk carriers with a close integration with CSR and now CSR-H by providing an out of the box interface of Neutral XML. This allows the exchange of the model as well as its related properties (for all plates, profiles, compartments etc) so that a detailed check can be conducted by the tools provided by the class societies such as ABS, DNV GL or Lloyd's Register. “This will allow shortening of the design process and check cycles since it allows the engineers to continuously check their Smart 3D design and validate it against the CSR rules,” says Marcel Veldhuizen, vice president of business development at Intergraph. “Therefore when the designer is finished and provides the class drawings for approval he can rest assured it is compliant with the latest CSR rules.”
Bureau Veritas headed some of the harmonisation workgroups and has continuously updated its software suite to be fully compliant with CSR-H. Technical director for BV, Jean-Francois Segretain, has been involved in the work done which has included ensuring the loads used in hydrodynamic calculations are harmonised across vessel types and also buckling calculations.
“Two different methods were in use, a closed form method for the CSR for bulk carriers and a non-linear buckling capacity software for the CSR oil tankers,” says Mr Segretain. The buckling project team made a comparison between the two methods and the results of a non-linear finite analysis program. 144 stiffened panels made of different plate thicknesses, stiffened with flat bars, bulbous bars, angle bars and tee bars of several scantlings were tested for load cases corresponding to compression of the plate panel edges, shear and/or pressure. “Although the closed form method and the non-linear buckling capacity software provided similar results which were close to those obtained from the non-linear finite analysis program, IACS decided to keep only the closed form method after some improvements.”
BV already has tanker and bulk carrier newbuilding projects underway in Korea, Japan and China. “One reason owners and yards come to Bureau Veritas for their bulk carrier class is its expertise with the CSR and the forthcoming Harmonised rules. Bureau Veritas has been at the heart of the harmonisation process within IACS. It has used that expertise to constantly update its tools including MARS and VeriSTAR Hull software and to train teams in shipbuilding countries.”
With CSR-H ready, IACS is now looking to the structural safety of large container ships and the dynamic effects that they experience moving through waves as a result of their long, slender hull form. One of the motivators for this came from the Marine Accident Investigation Branch (MAIB) following the MSC Napoli accident investigation. Some recommendations were made to IACS regarding longitudinal strength, whipping dynamic effects, buckling and hull stress monitoring.
“We are not arguing the fact that the safety standard of the ships being designed and classed today is an issue,” says Mr Cazzulo. Considering the statistics, he sees no reason to think that they are below standard. “But what is an IACS area of interest is to verify the standards and to adopt some international requirements that apply equally to ships being designed, built and operated around the world. This is our main scope of activity. Otherwise you may have cases that are specific to a particular ship or rules. Through IACS, we offer an international set of regulations.”
Mr Cazzulo is also aiming to improve the transparency of accidents and incidents and will submit a paper to IMO in 2014 that he hopes will be the result of feedback from class, industry and flag administrations. “There is a wide variety of information in public and private databases that have some of the information, but there is not a harmonised approach. There is not a common definition of the kind of information that is considered to be the most vital for the feedback from these incidents with a view to the future development of regulations.”
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