C-Job Naval Architects is always looking for ways to improve its designs and empower our clients’ business. Thanks to our Research & Development department, we are able to create innovative tools that can be used for current and future projects. A recent addition is the Buckling Tool developed by Lead Naval Architect Nikos Papapanagiotou and his structural engineering team.
What is buckling?
Buckling is a mathematical instability leading to the structural failure of a material. It occurs when a structure is subjected to compressive loads. In the shipbuilding industry most of the structures are made of steel and aluminum plates. “Plated structures may fail by yielding due to material failure or by buckling due to structural instability,” explains Nikos. “If the compressive stress on a plate structure exceeds a certain limit, the material will fail. Depending on the slenderness (geometrical characteristics) of the plated structure buckling may occur prior to the yield.”
As an example, standard shipbuilding steel plate can withstand a stress of 235 MPa – any stress greater than this and the material will yield. However, the load at which buckling occurs depends on the stiffness of a component and the choice of structural grid, not upon the strength of its material.
The vast majority of ship construction utilises steel plating of this sort – both flat and curved – strengthened by primary and secondary stiffening elements. It is the unstiffened plate panels in between the stiffening elements that, when exposed to compressive forces, could buckle. Potential causes are the stresses imposed due to the global bending of the ship (in-plane) and also the lateral pressure caused by seawater or fluid in a tank pressing perpendicularly on the structure (out-of-plane).
Introducing ‘C-Job’s Buckling Tool’
During the ship design process it is the role of the structural engineer to analyse structures and materials to see if they are strong enough to meet the requirements stipulated by the classification society. For this purpose, structural engineers employ software to calculate the stresses in a given structure.
“To optimise this process, we have created a unique Buckling Tool that can be linked to the structural analysis software. In particular, when the analysis involves shell elements composed of flat or curved surfaces.”
In the past, such analysis involved vast amounts of data calculated manually and by means of very large spreadsheets. “This has always been a very labour-intensive process, with the limitations of an Excel spreadsheet. What’s more, it was never very good for presenting an overview of the results.”
Optimising ship structures
Responding to clients’ needs concerning their classification society requirements, C-Job’s aim with the 100% in-house developed Buckling Tool is to meet standards laid down by Bureau Veritas (BV), DNV GL and IACS, the International Association of Classification Societies.
“The Buckling Tool gives us information about the stresses on each elementary plate panel and further calculates the buckling capacity of each of the panels. It is possible to analyse both unstiffened or stiffened panels. With these results, we can create a colour-mapped 3D FEM model that identifies areas more susceptible to buckling as well as the areas that are actually underutilized. In this aspect we can optimise ship structures faster resulting in vessels designed for their purpose having less weight.”
Adaptable to multiple software
C-Job has developed the Buckling Tool in such a way that it may be used outside of the FEM software. “At this moment part of the process takes place within ANSYS FEM software and another part takes place in an open source coding language. This gives us vital flexibility with the FEM software used in the industry. For example, in the near future we can offer the Buckling Tool analysis using input results created with a client’s own choice of FEM software. This is very important because it keeps us close to the needs of our clients .”
Current status of the Buckling Tool
“We finished testing the beta version in September, and we are happy with the results – which have been accurately verified according to BV and DNV GL calculation tools.”
In fact, the results were satisfactory enough that the Tool has been successfully used on its first project. “We are now in a position to identify the concrete advantages that the Tool has to offer. Structural engineers always have to check structures but, with the Buckling Tool, we can now do it more efficiently. By utilising the Tool from the earliest stages of concept design, we can define the optimum slenderness of the panels used resulting in optimized understanding of plated structures.”
“And it certainly reduces the amount of time taken to perform analyses – which, in turn allows us to increase our competitiveness in numerous ways. It enables simultaneous analysis of multiple plates, which wasn’t possible with the previous method. It also allows us to present our results in a better way, instead of seemingly endless tables of numbers.”
More to come
The story of C-Job’s Buckling Tool is only just beginning. “In the future we want to add additional classification society codes, like ABS and LR, and take advantage of the tool irrelevant of the choice of FEM software” concludes Nikos. “Which will give us the capability to respond to more diverse market requests.”