Bolt & Beautiful

WHY

In a wind turbine, various parts in the tower, the connection to the foundation and the blade root connection to the hub are secured with large bolts. The size of the offshore wind turbine, monopile and blades are expected to grow strongly in the coming years. This makes it also necessary to use larger ring flange connections and possibly larger bolts. Currently, the size of the most used bolts is M72, i.e. the nominal thickness is 72 mm. However, there is reluctance within the industry to increase the bolt size to M80 or higher.

A GROW workshop on bolted connections has learned that current design procedures, manufacturing tolerances and installation, operation and maintenance procedures are insufficient to apply larger ring flanges optimally. Misalignments in the ring flanges potentially cause too large stress ranges in the bolts. There have been cases of bolt damage in existing structures due to insufficient preload and material degradation, leading to expensive repairs and rigorous inspection regimes. Several actions are required to minimise material usage in bolts and flanges and to optimise inspection intervals. However, this requires an innovative package of measures rather than individual incremental steps.

The industry has indicated a strong need to improve the assessment methods for ring flange connections, considering design, manufacturing and maintenance aspects. Optimisation means, on the one hand, using less steel and, on the other hand, leading to greater tolerances in the production of flanges and/ or less installation and maintenance effort.

WHAT

In the Bolt & Beautiful project, we will reduce the costs of bolts and flanges manufacturing and maintenance and increase bolting safety both for existing and future large wind turbines. We will ensure that the installation procedures are in line with the design and production of the bolts and flanges. This will lead to the lowest possible total cost while ensuring the structural integrity of the critical components. We ensure that the chosen solutions are in consultation with and supported by relevant stakeholders.

• We develop a deterministic model for the ring flange and bolt fatigue. The intended bolt fatigue model will quantify the geometric and material parameters that influence the fatigue strength of bolts/ studs . The ring flange model will quantify the effect of preload, external load and tolerances on the load distribution in the bolts/studs.
• We develop sensors for (pre)stress measurements. The first type will be based on active dual-wave mode ultrasound waves, using a shear wave (S) and a compression wave (P). The method calibrates representative bolts/ studs in a laboratory environment. This ensures that no additional calibration of each bolt in the field is required. In addition, we are studying an innovative contactless magnetic inductance bolt measuring system. Both methods are particularly interesting for measuring bolt loads in existing connections.
• We perform calibration experiments at different scale levels: (i) individual bolt, (ii) bolt and segment, (iii) medium-scale ring flange in a lab and (iv) validation of field test data.
• We are developing a method for a structural safety assessment through a probabilistic framework. We focus on the bolt level and evaluate the uncertainties in the design (fatigue) resistance, considering the fatigue limit, to define safety factors for the bolts/ studs in ring flange connections. We will establish construction tolerances regarding inspection intervals for ring flange connections. We will also integrate the developed structural safety assessment with our stress measurement system.

EXPECTED RESULTS

In this project, we aim for the following results:

• We provide a calibrated probabilistic model for determining the fatigue strength, including the required partial safety factor, the effects of bolt preload (M36-M80) and the effects of hydrogen embrittlement on crack initiation and crack propagation in the context of the application and fabrication processes.
• We provide a calibrated probabilistic model for new and existing ring flange connections with tolerance limits for new design and improved (remaining) life prediction for maintenance of existing structures.
• We improve installation and maintenance procedures for bolted connections with innovative (pre)tension measurement techniques.