Fatigue tests of axially loaded butt welds up to very high cycles

Fatigue strength curves that are established from fatigue tests provide a basis for the fatigue assessment applying nominal stress approach. In the codes valid for steel structures, like the EC 3, the fatigue strength curves for constant amplitude loading have a knee point in the transition region. The fatigue strength curve beyond this knee point is commonly assumed to be a horizontal asymptote. However, the behaviour of the fatigue strength curve in the area of very high cycles and more importantly the existence of an endurance limit are much discussed. In the case of welded joints the experimental data beyond 107 load cycles is limited due to the possibilities in testing. Testing techniques with high frequencies are necessary to obtain experimental data with very high cycles in a reasonable period of time. In this scope a testing device with approximately 390 Hz operates by alternating current magnets and using resonance amplification, which was developed by a third party. This testing device was investigated and advanced for the application of long term tests reaching 5·108 load cycles. Fatigue tests on axially loaded butt welds with constant amplitude loading are conducted in three test series until very high cycles. The fatigue tests include the area of high and very high cycles. The influence of test frequency and stress ratio is investigated

Protected steel and composite connections in fire, Simulation of the mechanical behaviour of steel and composite connections protected by intumescent coating in fire

Actual developments in numerical simulations of the structural behaviour in fire situation are focussed on taking into consideration the interaction of all structural members in a global approach. Therefore it is necessary to simulate the load bearing behaviour of connections.With this motivation, the authors conducted experiments and thermal FE-simulations on two different connection types. In this paper, the accompanying mechanical FE-simulations of both investigated connection types will be described. The joints are defined as an end plate connection in a steel structure and a fin plate connection in a composite structure. Besides the validation of the numerical models, the results of the described investigations show that it is possible to activate a significant moment resistance within fin plate connections of composite structures. The main requirement for this activation is sufficient reinforcement strength

Probabilistic Fatigue Analysis of Jacket Support Structures for Offshore Wind Turbines Exemplified on Tubular Joints

AbstractThe design of offshore wind turbines is usually based on the semi-probabilistic safety concept. Using probabilistic methods, the aim is to find an advanced structural design of OWTs in order to improve safety and reduce costs. The probabilistic design is exemplified on tubular joints of a jacket substructure. Loads and resistance are considered by their respective probability distributions. Time series of loads are generated by fully-coupled numerical simulation of the offshore wind turbine. Especially the very high stress ranges that rarely occur during a period with constant conditions are decisive for the fatigue design. The peak-over-threshold method is applied to find the probability distribution of the very high stress ranges. The method of maximum-likelihood estimation is used to determine the parameters of the underlying generalized Pareto distribution. Further analysis shows that especially the number of the very high stress ranges, scattering for different time series, has a significant impact on the resulting fatigue damage

Early age movement in offshore structures with various bearing conditions

Grouted connections (GCs) consist of two infiltrating steel tubes of different diameters and a grout material bonding the two steel tubes together. Predominantly, the GCs are used to connect the foundation piles with the jacket support structure. Additionally, it can be applied for monopile substructures. A challenging and scientifically still marginally analysed detail of grouted connections is the influence of early age movement (EAM), also known as early age cycling (EAC) on the final load bearing behaviour of the connection. After grouting and until the end of the curing of the material, the offshore structure is partially supported by fluid grout material for several hours. During this time waves and current affect the soft connections. This leads to relative movement during the curing of GCs, which is known as EAM. Previous investigations showed grout material damage due to vertical EAM. These results form the basis of the current offshore guideline limits. According to DNVGL-OS-J101 from 2014, it must be ensured that during the curing process the vertical EAM does not exceed 1 mm. Since 2016 the limit value also applies for the lateral EAM according to DNVGL. However, the influence of lateral EAM on the load bearing behaviour is barely experimentally nor numerically investigated. Thus, the authors are involved in a comprehensive research project (GREAM) addressing this topic. This paper focusses on Finite Element Analysis of different support structures under varying boundary conditions and sea states to analyse the amplitude of lateral EAM of grouted connections

Influence of Structural Redundancy on Fatigue Life of Offshore Wind Turbine Jacket Structures

The concept of structural redundancy is implemented in the fatigue analysis of an offshore wind turbine jacket structure. The analyzed jacket is a real life example. Time domain analyses are performed for the most representative design load case. The uni-directional and multidirectional simulations of the offshore wind turbine system are carried out using a coupling of the aero-elastic code and the finite element code. Fatigue analyses are performed using hot spot stress approach and Miner's rule. Comparative studies show that considering structural redundancy leads to expanded fatigue life of the offshore wind turbine jacket structures. © 2017 ISOPEEC/Horizon202

Advanced representation of tubular joints in jacket models for offshore wind turbine simulation

Jacket substructures for offshore wind turbines show strong potentials in water depths from 25 up to 70m. A review of state-of-practice and enhanced state-of-the-art modeling of offshore wind turbine jackets is conducted regarding detailed joint properties. The state-of-the-art approach takes advantage of an accurate description of the local joint behavior by use of superelements, enabling more accurate load simulations. Studies conducted in the past highlighted both strong benefits as well as shortcomings of this approach, whereas the drawbacks were mainly related to the size of superelements and the application of local wave loading. This work develops a smart sizing for detailed joint models taking into account the loading and location of the jacket joints. A concept of local wave loading is presented as well. Advice on recommendable parameters is given and enables an optimized superelement application for jacket substructures. As an example the potential for fatigue load reduction is shown using the NREL offshore 5-MW baseline wind turbine and the OC4 reference jacket. The predicted fatigue lifetime was increased by about 15%.Research Council of Norwa

Investigations of steel elements with intumescent coating connected to space-enclosing elements in fire, Fire tests on intumescent coated steel members

The current status of the research program “Optimized use of intumescent coating systems on steel members” (AiF 17200) is presented.The aim of the project is to quantify the influence of space-enclosing elements on the thermal behaviour of supporting steel members. Those elements cause partially restrained expansion of the fire protection system. Experimental investigations on beams and columns directly connected to space-enclosing elements are presented. Additionally, numerical simulations are performed for temperature field calculations of steel elements with intumescent coating. As a new development, the numerical model takes into account the expansion process of intumescent painting. The setup of the numerical model is introduced

Axially loaded grouted connections in offshore conditions using ordinary portland cement

[EN] A grouted connection (GC) is a hybrid connection which joins two telescoped steel tubes by filling the annulus between the steel tubes with grout. GCs are frequently used to enable a force fitted connection between piles and substructure of offshore wind turbines. At latticed substructures this connection is located at mudline level in wet ambient conditions (AC). Nowadays special grout materials are used to achieve not only best mechanical properties but also a good performance during grouting in offshore conditions.To reduce production costs the use of ordinary portland cement (OPC) is investigated as an alternative filling material within this paper. OPC has a much lower tendency to segregate, as there are no aggregates added. This leads to more simplified, stable and cheaper production processes offshore. Further focus is put on the failure mode of OPC filled GCs in submerged condtions.For an appropriate use of OPC offshore a feasible mechanical performance needs to be ensured. Investigating this, small and large-scale laboratory tests were performed at Leibniz Universität Hannover. Using the experimental test setup of previous investigations for special high performance grouts (HPG) [1, 2], enables a direct comparison of HPG and OPC. Documenting liquid and solid OPC properties, like slump flow and compressive strength confirms a stable material quality. Small-scale ULS-tests showed significantly lower ULS-capacities and a more brittle failing process compared to HPG. Lagre-scale tests confirmed the observed failure mechanisms of Schaumann and Raba for OPC filled GCs in submerged conditions [3]. Carried out tests showed significant influence of grout material and confirmed influence of grout annulus size on fatigue capacity.The presented results were achieved within the research project ‘GROWup - Grouted Joints for Offshore Wind Energy Converters under reversed axial loadings and upscaled thicknesses’ funded by the German Federal Ministry for Economic Affairs and Energy (BMWi, funding sign: 0325290). The research partners were Institute for Steel Construction and Institute of Buildung Material Science, both at Leibniz Universität Hannover, Germany. The authors thank the BMWi for funding and all accompanying industry project partners (DNV GL, Senvion SE, Siemens Wind Power, Wilke & Schiele Consulting GmbH) for their support. Additionally the authors thank the material manufacturers for their support. Concluding thanks goes to Prof. Lohaus and his team from the Institute of Building Material Science for an excellent project collaboration.Schaumann, P.; Henneberg, J.; Raba, A. (2018). Axially loaded grouted connections in offshore conditions using ordinary portland cement. En Proceedings of the 12th International Conference on Advances in Steel-Concrete Composite Structures. ASCCS 2018. Editorial Universitat Politècnica de València. 541-547. https://doi.org/10.4995/ASCCS2018.2018.6944OCS54154

Sensitivity Analysis of Monopiles’ Fatigue Stresses to Site Conditions Using Monte Carlo Simulation

The design of support structures of offshore wind turbines contains high number of design variables that influence load characteristics and structural responses. These variables are stochastic and cause many uncertainties. Some of them are examined in this study. It is investigated how scattering of site conditions and load parameters affect the structural response. It is exemplified in terms of stresses that contribute to the accumulated fatigue damage within a monopile substructure. Random sampling of combinations of site conditions and load parameters is performed in order to classify the effects of parameter scattering on the stress variability by means of Sobol’ indices. Analysis shows that the highest influence on stress outputs have the variations in the load parameters. The reason is the sensitivity of the structural dynamical response to the wave height increase and decrease of distance between the wave peak frequency and the structural eigenfrequencies. © 2017 ISOPEEC/Horizon202

Sensitivity Analysis of Material and Load Parameters to Fatigue Stresses of an Offshore Wind Turbine Monopile Substructure

Steel monopiles are support structures mostly applied for offshore wind turbines. Their installation is straightforward, in particular, in shallow and medium waters. While the wind turbine tower is primarily affected by wind, the wave loads are dominant for the monopile, as it is submerged to a large extent. This study deals with the influence of uncertainties in material and load parameters on the behaviour of those structures. It is investigated how the scattering of material properties (namely Young's modulus of elasticity) affect the structural response. In addition, loads with different characteristics are applied, and it is examined how the changes in loads influence the structural response. The analysed output data of interest are the extreme stresses leading to the accumulation of fatigue damage. In order for a realistic modelling, wave loads are considered with irregular sea states with different wave characteristics (significant wave heights and wave peak periods). The final aim of the analysis is to classify the effects of specific wave characteristics on the stresses by means of a sensitivity analysis. The analysis shows that variations in the wave peak period have the strongest influence on stress outputs. This effect results from the strong sensitivity of the structural dynamical response to the decrease of the difference between the values of the wave peak frequency and the natural frequencies of the structure. © 2017 The Authors. Published by Elsevier Ltd