Low-speed roller bearings are generally in the sheave bearings and turret bearings which are used in the offshore industry under heavy loads. One of the challenges to the integrity of these bearings is the presence of subsurface cracking in the raceways. Downtime cost, which is caused by the failure of
these bearings is significant. Based on the potential of the recently-developed monitoring methods for detection of damage in low-speed bearings, this research is focused on the feasibility of the remaining lifetime prediction for roller bearings with existing subsurface cracking in the raceways.
The main approach of this thesis comprises a locally-coupled elasto-plastic damage model for low-cycle fatigue. In the numerical simulations, models with and without crack have been implemented and subjected to heavy loading condition. To avoid the crack singularity in the FEM, and reduce the
mesh sensitivity, a non-local method has been applied to determine the strain data around the crack tip. In a post-processing part, the nonlinear damage accumulation is implemented with a multi-yield surfaces algorithm.
To assess and validate the computational accuracy, the implemented locally-coupled elasto-plastic damage model has been compared with other (published) results. The numerical simulations suggest that the number of cycles to failure is about two times higher when the detectable crack size decreases from 20mm to 5mm. From these results, it is recommended to improve the monitoring system for the detection of smaller cracks. Moreover, further experimental assessment is required to validate and update the estimations in the thesis.

The safety of offshore assets such as wind turbines, offshore cranes, and turret mooring systems partly relies on the integrity of heavy-duty bolted joints. These critical connections can be of different dimensions and exposed to monotonic and cyclic loading. Despite being small components of a structure, bolts need to be maintained appropriately to ensure structural safety and reliability. The integrity of bolted joints can be assured by checking for adequate preload. In addition, fatigue failure resulting from preload loss is of primary concern, as it can occur suddenly without any visible changes. Hence, bolts should be re-tightened periodically to ensure sufficient preload. However, in offshore conditions, this countermeasure leads to high maintenance costs while exposing the crew to unfavourable conditions and risks.

This research focuses on the feasibility of condition-based maintenance of bolts using ultrasonic waves. An energy attenuation method has been implemented for this feasibility research. The main objective is to establish a proper methodology and hypotheses for preload detection. Furthermore, along with preload detection, the feasibility of crack detection was investigated. Finally, the proposed methodology and hypotheses were validated by performing experiments and numerical simulations.

The experimental and numerical results verify the proposed methodology by showing an increasing trend in the energy and the power of the transmitted ultrasonic wave for increasing preload. Also, the feasibility of crack detection using the same setup has been positively evaluated.

The obtained results suggest that the ultrasonic waves can be employed to monitor bolts for condition-based maintenance. Additionally, a number of relevant research activities are recommended based on this study.