Support structures

Loading approaches for offshore wind turbines

For the development of economical and durable offshore wind turbines (WTGs), a sound knowledge of the loads acting on them is essential. Based on oceanographic input data, approaches for the simulation of the resulting loads are developed for different sea states. Special attention is given to directional influence and directional dispersion in order to simulate different long- and short-crested sea states.

The implementation is done by extending the program “WaveLoads” developed for wave load generation. It allows flexible applicability to all slender structures in the hydrodynamic sense, which include WTG support structures (monopile, tripod, jacket). By creating interfaces of a general nature – but also specifically to certain FE applications such as ANSYS, Abaqus, MSC Nastran – WaveLoads is integrated into simulation environments as a load module.

In addition to independent simulations and calculations, the load module is incorporated into the overall integral WTG model. This allows conclusions to be drawn about interactions in the turbine dynamics, especially taking into account the wind, which mainly acts on the support structure via the rotor.

Integrated modeling

Optimization of the support structure is of central importance in the development of economical offshore wind turbines (OWEA). In this context, the consideration of all load variables and the investigation of their interactions with the plant dynamics with special regard to the support structure are indispensable.

The chosen approach therefore links both existing and new program developments within the framework of integral modeling based on multibody simulation. The OWEA models created in this way can be extended at will for detailed investigations or supplemented by flexible structures with finite element discretization for targeted stress analysis.

This simulation environment will enable the participating research projects to incorporate their results into the model for further investigations. The modules for the simulation of impacts developed so far in ForWind thus find their targeted application and at the same time document their applicability in further research projects or external projects.

Lifetime prediction for the support structures of offshore wind turbines

Supporting structures for wind turbines (WT) are subject to highly dynamic loads due to wind and wave loads as well as loads from turbine operation. The service life of a wind turbine is designed for 20 years and more than¹⁰⁹ load cycles must be endured during this time. The verification of fatigue in offshore wind turbines is of particular importance due to the above-mentioned boundary conditions with regard to safety and economic efficiency.

For realistic service life predictions, it is necessary to develop improved methods and design bases for the different design forms of the support structures of offshore wind turbines. The increasing plant sizes and the increasing water depths require special attention.
The aim of the research project is to use the stresses derived from an overall simulation to investigate a number of typical design details in greater depth and to improve the design models for fatigue. These design details include:

• welded connections (tube node connections of tripod or jacket structures)
• Hybrid joints (grouted joints)
• bolted connections (e.g. ring flange connections with large bolts)

The investigations carried out as part of this research project are intended to help increase planning reliability with regard to the service life of the supporting structures. Particular attention should be paid to the economic conditions dictated by the manufacturing and assembly process.

Projects on the research topic “Support Structures

Research focus

Fatigue behavior of bolted joints

Axially stressed bolted connections are widely used in the designs of wind turbines. Typical examples are the ring flange connections on steel tubular towers, the connection of rotor blades to the hub and fastenings of machine components to the machine carrier. Due to the high dynamic stress, the preload force is of particular importance for fatigue strength. The use of the innovative DISC design element should now enable virtually maintenance-free as well as precise torque-, torque-angle- or yield point-controlled bolting. In addition, the use of the Hytorc-DISC is said to allow side-load-free bolting.

The aim of the development project is to increase the nominal pretensioning forces compared with the current standard by means of increased precision in setting the pretensioning forces, thus making optimum use of the existing bolt strengths and ensuring the nominal pretensioning force for the specified service life of the structural element without retensioning.

The development project supports the innovative Hytorc-DISC fastener on its way to practical construction applications in wind turbines. In particular, it is investigated whether DISC can be used to increase the scheduled bolt preload, reduce the dispersion of the preload and make better use of the material strength. This is expected to result in a higher degree of reliability and durability, economical use of materials and improved cost-effectiveness for the joints.

Scour phenomena and scour protection

In the case of foundation structures of offshore wind turbines (OWEA) anchored to the seabed, scouring or undercutting of the support structures (formation of scours) often occurs. These scours are caused by the highly complex interaction between the sea state, the seabed and the structure itself, as well as the resulting changes in the natural flow regime in the vicinity of the supporting structure. The formation of scours always has a direct influence on the stability of the plants.

The exact formation and, if necessary, the expected dimensions of scours are insufficiently known so far. This leads to a significant increase in safety factors for foundation dimensions in advance of WTG installation. Therefore, as part of the research activities at the Franzius Institute of Leibniz Universität Hannover, studies are being conducted on local scour development at the base of the foundation structures. The scientists are also looking at measures for effective scour reduction and scour protection. Methodologically, the institute works with a combination of numerical simulations and physical model experiments. The latter can be performed in different model scales in the wave channel and 3D wave pool as well as in the world’s largest wave channel, the Great Wave Channel (GWK) in Hannover.

The investigations on scouring can be used to determine effects on the load-bearing behavior of the overall system. On this basis, suitable scour protection measures for the respective foundation type and the prevailing sea state conditions can be developed, if necessary, to enable more efficient foundations and sustainable scour protection in the future.

Load models for shafts

Approaches to calculate wave loads on offshore structures have existed for decades. However, due to the high consequential costs in case of failure of the facilities and inaccuracies in the load assumptions, in many cases the structures are over-dimensioned.

Wave loads on offshore structures are usually calculated using the Morison equation, whose empirical coefficients have been obtained from tests in wave channels. However, the equation coefficients are not fully applicable to the three-dimensional natural sea state. The expected load is therefore often overestimated.
The current research projects at the Franzius Institute and the Large Wave Channel of Leibniz Universität Hannover aim at optimized load models for breaking and non-breaking waves.

Model tests in the large wave channel, measured data from the Alpha Ventus offshore test field and numerical CFD models are used to develop load models for structures of offshore wind turbines (OWEA). With the help of these models, WTGs can be produced that correspond to the actual loads.

Seabed strength measurement device

The laying of cables for the grid connection of offshore wind farms requires the measurement of the seabed strength. In the shallow subsoil, i.e. 3 to 10 meters below the seabed, cable routes are laid, for example, which transport the generated electrical energy to land.

So far, Germany lacks a flexible, ship-based measuring device that operates from the seafloor with an exploration range of 1 to 15 mbsf (meters below seafloor) and fast measuring intervals to cover as many measuring points per ship day as possible.

The Bremen Institute for Metrology, Automation and Quality Science (BIMAQ) at the University of Bremen is therefore working on the development of a mobile seafloor-based measuring probe to record geotechnical and geophysical seafloor properties. The equipment, to be deployed from small to medium sized vessels, is intended to enable technical exploration of subsurface strength to a depth of 15m into the seabed, which is economically feasible. The device to be developed speeds up and makes cheaper any kind of subsoil investigation in the offshore sector. For predominantly scientifically oriented missions, the measurement system can be equipped with additional sensors and deployed at ocean depths of up to 4000m.

Damage detection by new methods of measurement data analysis

Early and reliable detection of damage to wind turbines is essential for avoiding consequential damage and increasing service life. Therefore, a constant improvement of the recognition performance is required.

The wind turbine vibration quantities recorded by accelerometers and/or strain gauges are examined for changes that indicate damage. The investigations undertaken for this purpose at the Institute of Statics and Dynamics of Leibniz Universität Hannover are to be extended by new stochastic methods. It is investigated to what extent new insights or improved methods for early damage detection can be derived from the division of the time signals into deterministic dynamic components and stochastic components. The aim is to detect damage to the tower and rotor blades of the turbine earlier.

System identification and monitoring for early damage detection

One way to increase the service life and thus the profitability of offshore wind turbines is to detect any damage to the support structure at an early stage and prevent consequential damage. Due to the poor accessibility and reachability of the facilities, regular assessment is very costly or even impossible.

From this situation, the goal for manufacturers, operators and insurers, among others, is to develop an early damage detection system.

In the research project presented here, the solution pursued is to infer the condition of the support structure from the vibration quantities measured at the wind turbine. This results in two main topics: first, the reliable determination of vibration quantities at the offshore wind turbine and second, damage diagnosis based on these measured values.

For this purpose, measurement methods have to be developed which, on the one hand, are sensitive enough to deliver the required measurement quantities in the required quality, and, on the other hand, they have to be simple and robust enough to withstand the harsh offshore conditions for many years. The parameters of the mathematical models must be determined from the measured values and calculation methods must be developed that provide information on whether damage is present, where it is located and what its extent is. Furthermore, simulations on finite element models, measurements on scale models in the laboratory and measurements on real plants are carried out.