In this thesis, the friction performance of polyurethane tensioner pads used in offshore pipe-laying was investigated. Tensioner pads are used in pipe-lay to create tension in a pipe. The need to understand the pads better is because of the increased use of pipe coatings and pipe lay in deeper waters. Deep water leads to more pipe tension, and polymeric coating reduces friction.

A literature review was conducted on the friction of polyurethane. From the literature review, a few variables that influence polyurethane friction behaviour were summarised into the following research question: What is the influence of normal force on the pads, pad hardness, pad geometry, and pad temperature on the friction performance of polyurethane tensioner pads?

Experiments were performed at a full-scale setup. Polyurethane has a nonlinear coefficient of friction, making it difficult to upscale results from laboratory testing. In the full-scale setup, existing pads can be tested to find the influence of the different variables from the research question. The test setup was found to be flawed. Not only did it deform due to the massive forces, but the data gathered from the hydraulic pressures was not robust and accurate enough to estimate the static coefficient of friction, which is important in tensioners. Therefore, only trends in the pad behaviour were observed.

Increased normal force on the pads was found to increase the coefficient of friction. This contradicted the literature but was likely due to the geometry of flat pads versus round pipes, where the contact surface increased when the pads were loaded. Softer pads were found to yield higher friction than pads made from harder grades of polyurethane. This effect was also found in the literature. The influence of temperature was difficult to test. Since the pads could only be heated externally before testing, temperatures were inconsistent, and test results were inaccurate. The temperature did affect the friction significantly, though, so some newly produced pads were fitted with internal heating.

Next to the experiments, different pad geometries were tested in finite element analysis. A new pad shape was found that decreased stresses, leading to higher friction.

Next to the variables of the polyurethane itself, the relative humidity was recorded. The relative humidity changed significantly during the different tests, and it was found to affect friction more than anticipated.

Three sets of new pads were produced for future testing to validate the influence of geometry and further investigate the influence of the pad temperature. Moreover, a new test setup was designed to test the coefficient of friction more accurately.

Two-bladed offshore wind turbines regained interest in finding the most profitable way of generating wind energy. Wind industry companies demand the safe operation of two-bladed offshore wind turbines. To guarantee safe operation, the companies perform operational modal analyses to investigate modal properties variation which might be allocated to damage. However, the operational modal analysis of an operative two-bladed offshore wind turbine faces multiple challenges. (1) Fundamental operational modal analysis assumptions about the applied loads are violated by environmental and operational loads. (2) The closely spaced modes of an offshore wind turbine are hard to identify. (3) An operative two-bladed offshore wind turbine is a linear time-variant system. This paper introduces an enhanced operational modal analysis procedure to overcome some of the mentioned challenges. The enhanced procedure incorporates a post-processing technique in a transmissibility-based approach. A developed representative model of an operative two-bladed offshore wind turbine is used to compare the enhanced procedure with the frequency domain decomposition method. Based on the comparison, this paper proposes a new operational modal analysis method that combines a transmissibility-based approach, the post-processing technique, and the frequency domain decomposition method. This paper proves that this proposed combined method is a promising new operational modal analysis technique that outperforms the enhanced procedure and the frequency domain decomposition method in identifying the modal properties of a two-bladed offshore wind turbine.

The slip joint connection is a relatively new alternative method for connecting offshore wind turbine towers to prior installed monopiles. It consists of two overlapping conical sections that together form a connection. There are several challenges left to be solved before the slip joint can be applied on a commercial scale. One of those challenges lies in the decommissioning process of the slip joint connection. It is proven that build-up settlement can increase the friction force within the joint over its lifetime and complicate its disconnection. One potential method for reducing the friction force in the slip joint is the excitation of one of the structures shell modes localized around the slip joint. Good estimations of the shell modes of the structure are therefore essential. Models for precise estimation of the shell modes of a wind turbine with a slip joint are difficult and expensive to develop due to the complexity of the joint. This thesis is a study into the possibility of using simplified finite element models to estimate the shell modes of a wind turbine connected with a slip joint. This is done by estimating the shell modes of the structure using a reference model and seven simplified models in a modal analysis. The estimated shell modes are compared based on their eigenfrequency and mode shape. Based on this comparison, conclusions about the applicability of simplified models are drawn. The first 300 eigenmodes of the structure are estimated with both the reference model and simplified models. Of these eigenmodes, ten shell modes of interest are selected based on their eigenfrequency and location at which the modes are localized. The modes of interest are than compared for their eigenfrequency and mode shape. The mode shapes are compared based on the Modal Assurance Criterion (MAC), which is calculated at a specific location of interest. In addition to this, the angle in which the modes are orientated is measured and compared, as this can be useful information for the decommissioning process. Results showed that three out of seven simplified models studied resulted in a estimation of the shell modes which was sufficiently accurate. The simplifications used for these models consist of the averaging of wall thickness for the upper and lower slip joint and a simple model for the upper slip joint. From these results it can be concluded that some of the simplifications can be applied when estimating shell modes of a wind turbine tower connected with a slip joint, as they lead to only a small decrease in accuracy of the shell mode estimation. However, using these results to make predictions about other potential simplifications is challenging as each different simplification has a unique influence on the estimation of the shell modes.

This research investigates how the current friction design standard should be improved, specifically for monopile sea fastening with polymers. Literature and results from tests performed previously to this research are investigated in order to obtain a complete overview of the factors that should be considered in the design standard. Additional tests are carried out on factors for which literature and previous test exercises are inconclusive. Friction test methods are compared to determine the best method for testing monopile sea fastening friction. The total test variability is analyzed to determine why safety measures are required. The validity of these measures is studied with a large set of test results. Finally, modelling as an alternative for testing is investigated.

Due to increasingly efficient engineering of the last decades, the design process for civil structures is heading in the direction of optimal solutions. Among other things, the reduction in self-weight enhances the susceptibility to dynamic loads. As the process for the determination of realistic modal parameters from a digital model often diverges from the reality, the uncertainties should be investigated. This research aims to gain knowledge on the accuracy and stability of such modal parameter estimates.

A case study is investigated to determine modal parameters for a realized tied-arch railway bridge. The desired modal properties include the natural frequencies, mode shapes and damping parameters. The frequency domain decomposition (FDD) method is applied on a set of accelerational measurements to determine the modal parameters.
Four dominant frequencies are identified and investigated, from which the most dominant operational deflection shapes are extracted. The overall response of the structure shows great similarities with the expected mode shapes for similar bridge types. The computed modes show signs of complex behaviour, due to the characteristics of the load or the structure. An effective method to reduce uncorrelated noise is the application of an autocorrelation function (ACF) to the time domain signals, before execution of the FDD. Prior to the computation of the final results related to the case study, a simplified test case is considered to validate correctness of the FDD implementation.

To reach the Paris Agreement goals, switching from fossil fuels to renewable energy is inevitable. Wind power has proved to play a critical role in this transition. To speed up this transition, the wind industry must keep innovating. The major part of Wind Turbine Generators (WTGs) is installed by a jack-up which not only has its limitations from an engineering point of view, such as water depth, soil characteristics and lifting capacity but also from a financial point of view. To overcome these limitations, this research aimed at developing an innovative concept design for the installation of offshore WTGs.
A vessel dynamic analysis showed that the Mighty Servant 1 is the most suitable Boskalis vessel to install WTGs from. The same dynamic analysis also highlighted that a motion compensation system is required to mitigate wave-induced dynamics and enabling WTG installation in floating conditions. After developing multiple concepts of motion compensation systems, a multi-criteria analysis in the form of an analytic hierarchy process is performed. It is found that the most promising concept should rely on hydraulic actuators. To find the optimal values of the design parameters of the chosen concept, a kinematic multi-objective optimization is carried out by means of a genetic algorithm. The optimal set of design parameters is selected by taking advantage of the Pareto front, which is computed by using three chosen performance indexes. A model is developed in MATLAB and Simulink to simulate the dynamics of the motions compensation system. The simulated results showed that a controller which uses velocity feed-forward and position feedback is the preferred way to go. The results also highlighted that the required amount of hydraulic power has a great impact on the economic feasibility of the system. Therefore, to reduce the hydraulic power and guarantee the feasibility, a passive system based on a hydraulic accumulator is added. Based on the results of this research, it can be concluded that the next generation offshore wind turbine generator can be motion-compensated with hydraulic actuators including hydraulic accumulators. Further research could be carried out by assessing more installation phases in combination with a wider range of load cases.

The aim of this research is to increase the workability of a rescue craft launch and retrieval system on FPSOs in heavy sea states. Although a rescue craft’s purpose is to improve safety, multiple incidents recorded in the GISIS database have proven that the launch and recovery operation can be dangerous or even deadly. The problem research shows that the davit system is the main cause for incidents, which are related to design flaws, lack of maintenance and human errors, the three parameters for a successful launch. The opinion of the author is that a different type of launch and recovery system can lead to improvement of these three parameters and to an increased workability of the system. To compare a new design with the conventional design, a computational model of both designs is built in Matlab to simulate the launch of a rescue craft from an FPSO in various sea states. The conventional model shows dangerous accelerations in high sea states and the risk to collide with the hull of the FPSO. The concept design aims to reduce these motions and, in addition, provide a more redundant design with easy maintenance and focused on reducing human errors. After comparison, the concept design proves to reduce dangerous motions during launch and increase the workability in high sea states, while being very simplistic, robust and easy to operate. However, in the splash zone the concept model does not mitigate dangerous motions sufficiently.
To further substantiate this conclusion, it is advised to further investigate the splash zone model and to develop a more detailed model of the concept design.