Fundamental investigations of violent wave actions and impact response

Ocean structures are typically designed to withstand the worst storms occurring within a 100-year period. Scenarios where 100-year waves break and crash into columns and hulls have long been a load case associated with significant uncertainty. The uncertainty of the load leads to uncertainty regarding structural design. New research in the SLADE KPN project reveals that current design procedures are quite conservative and that there is potential of cost reductions associated with new designs and re-qualification of existing structures. That is if more accurate design procedures can be developed. Design-critical wave impacts are challenging physical problems to accurately quantify, often requiring model tests in wave basins to study them. For model tests a scaled model of the structure is built and placed in a basin where it is subjected to waves, currents, and winds. The material most often used in ocean structures are steel and concrete. Both these materials were studied here. In the SLADE KPN project, model tests and response calculations were conducted according to existing design practices. Wave force sensors were attached to the offshore structure in the area just above the waterline where the largest wave impact pressures are measured. The pressures are measured to assess the capacity of steel or concrete wall that will absorb these loads at full scale. The measured loads from the largest wave impacts are then applied to a finite element model to calculate the design responses. The challenge with such analyses is that the calculated load responses are often very large, which is related to the enormous wave loads measured. The industry suspects that the calculations are conservative and that this leads to oversized designs. The measured loads from the basin tests are obtained on a completely rigid structure, while real structures deform during wave impacts. In the SLADE KPN project, we have studied how the deformation of the structure dampens the wave impact and reduces the loads. We have done this by finding appropriately scaled elastic models of the most common types of steel and concrete structures. Scaled model tests in waves follow Froude scaling. When designing elastic models to represent general structures, scaling laws indicate that the construction should be represented with different materials than the full-scale material. Furthermore, model tests of ocean structures in large wave basins require the model to be 40 to 60 times smaller than the actual offshore structure. Relevant model materials for steel and concrete are different types of polymers like thermoplastics. It is important that the model material behaves linearly elastic enough within the stress range. In SLADE KPN, we used modern 3D printing combined with advanced material testing and modeling to develop new elastic models. The scaled elastic models of steel and concrete were tested in wave tanks where they were exposed to hundred-year storms, each lasting three hours. The results show that the current calculation methodology gives higher design strains than what was experimentally measured for both steel and concrete. Current design practices use forces measured on a rigid structure, while real structures will deform slightly when the wave hits. The results from the SLADE KPN project are the first of their kind to quantify this effect and demonstrate the conservativeness of existing practices. Furthermore, in the SLADE KPN project, we have worked on mathematical models that can describe the effect of structural deformations on the hydrodynamic wave loads. These load modifications caused by structural deformations can be represented as the effect of hydrodynamic added mass but also a far less known effect that we have called hydrodynamic slam damping. We have quantified the effect of slam damping and found that it significantly reduces the structural responses. The hydroelastic model tests conducted in the SLADE KPN project are the first of their kind, providing new knowledge on the accuracy of calculated strains based on wave force measurements. The project shows that there is potential for cost reductions associated with both new structures and requalification of structures if more accurate design procedures can be developed. In SLADE KPN, we have studied the hydroelastic physics in depth and have developed models that can help explain the conservativeness and serve as a basis for new and less conservative calculation procedures. Norway has the ambition to develop a significant amount of wind power. Therefore, the cost per installed wind turbine is crucial. Working on reducing conservatism, where conservatism can be documented, provides opportunities for cost reductions. In the SLADE KPN project, we have quantified the conservatism and identified the physics that can explain it. Work remains to establish new and more accurate design procedures based on this new knowledge.

SLADE has documented that the current calculation methodology gives higher design strains than what was experimentally measured for both steel and concrete. Current design practices use forces measured on a rigid structure, while real structures will deform slightly when the wave hits. Furthermore, the SLADE KPN project has developed mathematical models that can describe the effect of structural deformations on the hydrodynamic wave loads. The hydroelastic model tests conducted in the SLADE KPN project are the first of their kind, demonstrating the conservativeness of existing practices and providing new knowledge on the accuracy of calculated strains based on wave force measurements. The project shows that there is potential for cost reductions associated with both new structures and requalification of structures if more accurate design procedures are applied. SLADE results have already been applied in the design of the FPSO for the Wisting field, where the inclined section above waterline was exposed to large wave impact forces. Analyses following the procedure developed in SLADE documented significant hydroelastic effects and avoided costly re-design had present design practices been followed. Norway has the ambition to develop a significant amount of wind power. Therefore, the cost per installed wind turbine is crucial. Working on reducing conservatism, where conservatism can be documented, provides opportunities for cost reductions.

One of the fundamental unresolved problems in design of large volume ocean structures is the accurate prediction of structural response due to wave slamming. Establishing a practical and reliable solution to this problem would make a significant contribution towards safer and more cost-efficient marine operations and installations. Real progress within this field cannot be achieved without the systematic study of practical experience, combined with the development of experimental, numerical and analytical methods. The critical waves causing the largest horizontal slamming loads on ocean structures originates from typical 100 year storms in steep and high sea states. These wave conditions contain massive wave breaking. The present project does not aim at modelling the full problem numerically. The present project will study the local structural response due to steep wave impacts. The emphasis is structural response due to wave slamming on column structures, like offshore and offshore wind structures. The emphasis is on the local structural response due to this type of impacts. This means areas covering parts of a column or walls of the living quarter. The present project will contribute to new knowledge in the field of hydroelasticity and fluid structure interaction. The knowledge on local hydroelastic response of steel structures due to slamming on calm water will be extended in order to describe large plastic deformations. The studies will be compared with original drop tests at a large scale. The knowledge from the drop tests will be used to define original wave tests with flexible local models. These model tests will be compared with a new numerical procedure for calculating response based on slamming force measurements. This in turn will qualify a new procedure for response calculations which can be used by industry.