After developing the technique some five years ago, Aker Arctic has now completed the research required to get its new design methodology for icebreaker hulls accepted into the Finish-Swedish Ice Class Rules. The method has also been adopted to Polar Class rules of several Class Societies (LR, ABS, DNV).
Although the research focused on vessels typically sailing the Baltic Sea, Aker Arctic has also used the methodology in many of its recent newbuilding designs including, for example, Canada’s new icebreaker which is currently under construction. The innovation is not specific to any particular region or ice load, rather it extends the modelling of steel plate and reinforcements such as web frames beyond the elastic properties that help prevent denting to also include the plasticity that ensures they do not fracture after deforming.
Typically, mathematical formulas or direct linear calculations made with finite element analysis (FEM) have considered the elastic behavior only, with the plasticity of the steel beyond yield point assumed to follow the same linear relationship. Elastic behavior is quite linear in nature – the more pressure, the deeper the dent – but the plasticity is not, so predicting structural failure based on linear analysis involves making assumptions. The exact safety margins are unknown.
An image depicting non-linear FEM analysis results.
Image courtesy Aker Arctic“In non-technical terms, we calculate how much the dent is after an impact with ice and determine, can we accept that?” says Juuso Lindroos, Team Leader, Structural Design at Aker Arctic. “That allows us to target what needs strengthening and what doesn’t. We can pinpoint the weakest parts of the structure and optimize the capacity of the steel at that point to ensure a permanent deformation is still safe – within its plasticity limit and is still safe.”
This enables designers to make the hull structure simpler, with fewer brackets, and with a significant reduction in the scantlings of primary structures, lowering the steel weight of the vessel. The icebreakers are therefore simpler in design and simpler to build.
Aker Arctic has found that it is possible to achieve steel weight savings of 100 to 300 tons in a typical icebreaker. The ship can carry more fuel or cargo or a smaller vessel can be designed with less draught for shallower water areas. And as the production of steel emits large amounts of CO2, there are also environmental benefits.
Lindroos says the impetus to develop the new methodology came after experience building icebreakers that meet Polar Class requirements. When the Polar Class rules were established in 2006, they led to excessively heavy primary load-carrying elements compared to the shell and frame structures, even though the older vessels built before the Code had a proven safety record.
Structural engineer Ville Valtonen from Aker Arctic led a group investigating the issue which included Rob Hindley from Aker Arctic and James Bond from ABS. They published their research in the scientific journal Marine Structures in 2020. This documented their new, robust assessment methodology and acceptance criteria using FEM and demonstrated that a nonlinear analysis provides better insight into the behavior of hull structures.
Targeted solutions could be designed to strengthen the structure instead of adding unnecessary steel to meet simple, proscriptive formulas.
While linear FEM methods are straightforward and widely used, they cannot predict what happens beyond yield including post-buckling behavior and whether the failure mode will be gradual or sudden. The non-linear method can accurately predict this behavior, but it requires more parameters and formulas for material modelling.
Permanent deformation of a structure after the design load has been applied and removed.
Image courtesy Aker Arctic
It is a more laborious process than linear elastic analysis, but the cost savings and other benefits, such as being easier to repair, compensate for the extra work, says Lindroos, particularly for high ice class vessels.
Aker Arctic has compared results from the nonlinear calculations to instances of real ice damage. The calculated failure loads and the way a structure fails align very closely with the observed damage, giving confidence in the accuracy and reliability of the method. “We do not lose anything from a safety point of view. Having more confidence about failure modelling means we can mitigate the risks.”
It is generally accepted that small local denting of the shell plating is acceptable as long as there is sufficient margin against other more severe failure modes such as rupture or buckling of plate structures. An accurate assessment of the structure both before and after yielding allows for direct connection between the analysis criteria and the desired outcome, which is that the permanent deformations from ice navigation in normal service remain small and that the structure has sufficient reserve capacity for ensuring safe behavior in accidental overload scenarios.
The first vessel project developed utilizing the new methodology was the Aker ARC 130 S Baltic Sea escort icebreaker for the Swedish Maritime Administration. Lloyd’s Register evaluated and approved the methodology in this case. The procurement process is ongoing.
The Canadian Polar Icebreaker is one of Aker Arctic’s most recent references. Aker Arctic’s nonlinear FEM tools made it possible to use commonly available high strength steel. Without the savings extra high strength steel would have needed to be used which is more difficult to acquire and to weld. Measuring 158 meters long and 28 meters wide, the Polar Class 2 vessel is designed to operate self-sufficiently in the high-Arctic year-round. It will be able to operate farther north, in more difficult ice conditions and for longer periods than any icebreaker in Canada to date.
Meanwhile, a new draft Finish-Swedish rule was written based on the Aker Arctic research findings and circulated among classification societies and the International Association of Classification Societies’ Hull Panel. After receiving comments, the rule was finalized and is expected to be adopted into the next version of the Finnish-Swedish Ice Class Rules as an alternative to the current prescriptive method.
With that work done, Aker Arctic continues its research. This year, Valtonen published another study using the nonlinear methodology to examine how hull shape influences ice loads. Hull shape is important consideration for icebreakers as the ultimate ice breaking force depends on the shape of the hull at the contact location. The study refined design ice loads to optimize hull structure, increase cargo capacity and reduce costs.
It is expected to improve icebreaking vessel design standards, including the Polar Class rules and the Finnish-Swedish Ice Class Rules.
Rendering of the new Swedish icebreaker design that Aker Arctic provided nonlinear FEM analysis for.
Image courtesy Aker Arctic
