Working to develop stronger and larger steel structures at sea

17 January 2017 • 3 min reading time

The Dutch offshore industry wants to show that the next generation of ships and platforms are safe. At the same time, its aspiration is to create ever stronger and larger structures. If these are to be safe, more needs to be known about the mechanics of steel and how it behaves. TNO’s Structural Dynamics expert group is working on reliable testing and measuring methods for identifying and combatting material failures. It is also working on software that will be able to predict the toughness of steel.

TNO is providing insight into material failures in offshore steel caused by collisions and fractures. For example, fractures can occur due to brittleness at low temperatures. Dr. Carey Walters, senior researcher at TNO, points out that “When steel gets cold – and we are talking about exposure to air at temperatures of -10°C to -40°C – it can fail quite suddenly. While current standards should prevent such incidents, this is not a definitive solution to the problem. The rules concerning the brittleness of offshore steel are an obstacle to the innovative designs that the Dutch offshore industry has in mind. This is because these designs require thicker and stronger steel than are typically allowed in standards.”

Modernize rules on testing

The aspirations are clear: the proposed new ships and platforms will be larger than ever. That will require a greater knowledge of materials. On the one hand, this will be needed to show that cutting-edge structures are safe while, on the other hand, it will provide evidence that various rules used by classification societies are in need of modernization. “Our goal in carrying out this study is to develop test methods that make it possible to safely use thicker and stronger types of steel,” Carey explains. That, in turn, will open up avenues of opportunity for the Dutch offshore industry.

“The rules concerning the brittleness of offshore steel are an obstacle to the innovative designs that the Dutch offshore industry has in mind”

More cost efficient

The system of regulations governing various thicknesses and strengths of offshore steel is currently undergoing rapid development. Carey notes that “The rules we have been using up until now are based on a test that dates back to the last century. We are only able to interpret that test based on experience.” There is a scientifically-based alternative that is suitable for the new steel, but it is far too expensive for routine use. Carey is currently working on a more cost-effective method for routine use, to replace the older, experience-based method.

Scientifically-based table

To ensure that the safety of the weld demonstrably complies with the classification societies’ rules, TNO experts are also researching the post-weld treatment of steel. “After welding, a great deal of stress is locked into the material, and this can cause failure to occur earlier than would otherwise be the case,” Carey says. “The use of heating as a post-weld treatment rearranges the atoms, reducing the forces within the material while ensuring that the weld is strong and that it will remain so.” Here too, practical applicability is a crucial factor. Carey says that “If everything goes according to plan, in one or two years we will be able to offer the offshore industry a scientifically supported table based on concepts that are recognized by classification societies. Using this table, designers and shipbuilders can see at a glance whether the material they are using needs to be heated.”

“The Netherlands has a prominent position in the offshore industry, but we will need applied knowledge if we are to retain that market position”

Assessing the degree of toughness

In just a few years, if Carey has anything to say about the matter, experienced materials engineers will also be able to determine whether the steel’s toughness meets the requirements, based on how the steel was made. “Some types of steel are tougher than others, which is determined by the geometry of the material at the microscopic level,” Carey says. “We are currently cooperating with Delft University of Technology to develop a software tool that engineers can use to assess the degree of toughness, based on the steel’s small-scale geometry. This involves many different parameters. The new software tool will be able to process a large number of these simultaneously. Ultimately, we want this software tool to be able to handle welds too.”

Maintaining a reputation

TNO works with a range of partners, including Allseas, one of the world’s largest companies in the construction of offshore pipelines and subsea structures. Bin Hu, a senior R&D engineer at Allseas, underscores the importance of having a Dutch knowledge institute in this field. “As our predictions become more accurate, quality will increase, as will the financial savings involved. The test methods developed by TNO closely match our own objectives. This allows us to anticipate developments more effectively, and to operate along very practical lines. The Netherlands has a prominent position in the offshore industry, but we will need applied knowledge if we are to retain that reputation.”

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