Marine pipelines are often operated at high internal pressure and temperature. This loading condition results in the development of axial compressive forces that can cause the pipeline to buckle globally. Global buckling occurs when significant lateral motion is present in the pipeline and excessive feed-in occurs at that specific location, which in turn forms into a sharp curve that can initiate destructive structural failure. Research has been conducted addressing the control of buckle development. The "buckle initiation" techniques were invented to mitigate the uncontrolled buckling of the pipelines on the seabed. These techniques involve the creation of less stiff sections in the pipeline (imperfections), so that buckling occurs in these locations. The most common buckle initiation techniques used in S-lay installation so far, are the "snake lay" method, the "artificial vertical out-of-straightness" and the "distributed buoyancy" method. However, these techniques require the addition of subsea structures on the seafloor or larger pipeline length, which can increase the pipelaying cost dramatically. A beneficial buckle initiation technique is the "residual curvature" method. The residual curvature method (RCM) principle is based on creating intermittent residual curvature sections in the pipeline so that buckling can be initiated at these locations. The curvature sections are created by adjusting the settings of the already existing installation equipment. So far, this method is only used in reel-lay installation. It is particularly urgent to examine if the local residual curvature method can successfully be applied in pipelines laid by S-lay vessels, since S-lay is considered the most common and frequently used technique due to its adequacy on different water depths and pipe diameters. The scope of this master thesis is to assess the feasibility of creating local residual curvatures in the pipeline by lowering the stinger during S-lay. The assessment is accomplished by simulating numerically the pipelaying process and the creation of the residual curvature, by analysing the behaviour of the pipeline while being lowered (in particular, looking at its twist/rotational behaviour) and by verifying if the alterations to the normal pipelaying procedure still respect the integrity of the pipeline and installation equipment. ... Read more

Railways and other sources of environmentally induced vibrations often lead to annoyance and sometimes property damage. In this framework, transition zones are deemed as the most sensitive locations on the railway track, especially to high vibrations and impact loads. This causes high maintenance costs and is often very annoying, as the availability of the track gets limited during maintenance processes. One of the main reasons of this problem is the stiffness change of the railway supporting structure which is sometimes abrupt. This thesis will shed some light on the factors playing roles in altering the vibration intensities in level crossings. Moreover, the effects of these factors on ground vibrations will be highlighted. Further on, a model is presented which incorporates all these parameters. This model can be used to predict the change in ground vibrations in level crossings. The objective of this thesis is to determine the influential parameters in terms of the response vibrations in the surroundings of railway tracks. On that account, following an in-depth literature study, and based on the analysis of the measurements performed at Movares, it was suggested to investigate three factors being the ambient temperature effect, the hanging sleeper scenario and the inhomogeneity of foundation and analyze their impacts. The in-situ measurements illustrated a correlation with temperature, where the vibrations are amplified during hot periods and in the summer season. Besides that, the reports indicated the presence of the hanging sleeper scenario, where the suspension between the rails and the foundation is no longer active. It was also reported that locations with inhomogeneous foundations experience higher vibrations than those with quasi-homogeneous foundations. On those grounds, it was decided to study the effects of the hanging sleeper scenario, the ambient temperature and the presence of a slab-track crossing on the track. A complete vehicle/track/soil numerical model has been used in the parametric study in order to assess the factors affecting the response vibrations induced by trains and to quantify their impacts. Along with that, the numerical results have been validated, to some extent, with analytical results. More specifically, the steady-state response obtained with both approaches are compared present a good agreement. The results exhibit that the hanging sleeper phenomenon has a substantial effect on the vibrations. Furthermore, the existence of a level crossing (representing a foundation inhomogeneity) proved to be of significance as well. Lastly, the ambient temperature possesses a limited impact which could turn severe in extreme conditions. ... Read more

Offshore wind energy is one of the solutions to meet the growing demand for renewable energy. The offshore wind turbines producing this energy keep increasing in size and, as a result, the monopile foundations are becoming larger and heavier. The traditional jack-up installation vessels have limited crane capacity and many of these vessels are unable to install the XXL monopiles. Therefore, the offshore industry is currently investigating a new installation method using a motion-compensated gripper frame on floating vessel with a dynamic positioning system. The gripper frame is attached to the vessel and encloses the monopile with a ring to keep it vertical during the installation. In addition, the gripper frame compensates for the vessel motions such that the vessel motions do not influence the monopile motions. The purpose of this thesis is to investigate the feasibility of such a motion-compensated gripper frame and to determine what control settings minimise the monopile inclination and the force exerted on the monopile. The system is composed of three main bodies: the vessel, the gripper frame and the monopile. The monopile and PID controller, which controls the amount of force exerted on the monopile to keep it vertical, have been modelled in the frequency domain to gain insight in the effect of changing the control parameters. To model the dynamics of the coupled system an OrcaFlex model has been set-up. The system has been tested for various values of proportional and derivative gain, kP and kD respectively, in various wave conditions. First the perfect control system is tested, where the force to keep the monopile vertical is applied instantly and the vessel motions are fully compensated. However, as the real world is never perfect, the system tested for sensor lag and imperfect motion compensation as well. The results are judged based on three criteria regarding the maximum monopile inclination, actuator force and actuator stroke. Resonance is observed in case a value of kP is selected such that the natural frequency of the monopile and controller matches the wave forcing frequency. Adding derivative gain kD limits the monopile motions and force exerted in this case. To limit the monopile motion the proportional gain should be selected such that resonance is avoided. Two different control settings are investigated and it has been found that a relatively high value of kP of 10,000 kN/m in combination with a kD of 11,000 kNs/m is a suitable setting based on the three criteria. Furthermore, bow quartering waves is the favourable wave direction compared to head waves for the system considered in this thesis, as the force on the monopile is more evenly distributed over the actuator in x- and y-direction. Introducing a sensor lag into the system results in higher monopile motions and forces on the monopile. If the sensor lag exceeds 0.3 s it leads to instability of the monopile for both settings. The effect of not fully compensating the vessel motions is found to be limited due to the fact that these motions are slowly varying. The results of this work contribute to a better understanding of the dynamics of the system in various wave conditions. Furthermore, it provides insight in the effect of sensor lag and imperfect motions compensation, contributing to the design of a motion-compensated gripper frame for the installation of XXL monopiles. ... Read more

As the offshore wind industry grows, the demand for larger wind turbines and foundations increases. The most common foundation type for a wind turbine is a monopile which is currently installed by large hydraulic hammers. This installation method generates a lot of underwater noise which may harm marine life. To solve this problem and its unwanted consequences, GBM aims to implement a new silent installation technique. By applying fluidization, jetting and by inducing vibrations by harmonically exciting the bottom of the pile, both the dynamic tip resistance and the shaft resistance are reduced so that the monopile can penetrate the soil. These techniques are aimed to produce less harmful underwater noise than conventional hammering. Little is known about the penetration performance of a pile which is harmonically excited at the bottom. This thesis aims to provide more insight on the penetration performance when exciting a pile at the tip. A literature study is performed on existing pile penetration models. From this study it is concluded that, currently, there are no available penetration models capable of describing the penetration performance of the pile when exciting the system at the bottom with varying harmonic excitations. Therefore, the aim of this thesis is to develop a new penetration model. The purpose of this model is to describe the penetration performance at different harmonic force parameters. The developed model is based on finite elements by using the FEMAP software. The pile is represented by shell elements. The interaction between the pile and the soil is modeled using multiple spring-damper-slider elements which are spread along the pile surface. The slider elements allow the relative motion between the pile and the surrounding soil. The different soil-structure interaction elements are uncoupled and the sliding resistance is assumed linearly elastic, perfectly plastic. Energy radiation due to elastic waves is captured by simple dashpot elements.
The developed model is used to analyze the effect of certain parameters on the penetration of the pile. This is done by changing the amplitude, the frequency or the direction of the harmonic force for a specific set of soil parameters. For each variation, the model calculates the pile displacement at a certain depth from which the penetration speed is determined. This penetration speed is then compared to the other results to determine the effect of each chosen parameter. Also, the location of excitation is analyzed. The model is used to analyze a pile which is excited from the top or at the bottom. From the results it is concluded that an increase in the amplitude and the frequency of the excitation has a positive effect on the penetration speed. The dependence of the direction of the vibration on the pile penetration is complex. Therefore, a clear correlation between the two could not be obtained within the time framework of this research. This research provides is a first step towards understanding of the Vibro-drill system performance. As this is an investigation which is still in progress there are some recommendations for further research on this topic. One important recommendation is to improve the soil reaction in the model to a coupled system where it is now uncoupled. Furthermore field tests can be performed providing more knowledge on the effects of the soil and to validate the model. ... Read more

The main objective of this dissertation is to compare the small-scaled model test by the flume with the full-scaled model test carried out by the towing tank and to find out the differences between the test results and the test facilities to give proper suggestions for the design and operation on small-scaled testing facilities.
The methodology used in this dissertation is carried out on the small-scaled model test in the flume. Three main steps are carried out, the first is comparing the test results of the flume and the towing tank for both the stationary test and the free vibration test, the second is looking for the theoretical explanations of the differences between the test results, the third is finding out the advantages and disadvantages of the small-scaled model testing facility compared with the full-scaled model test results which have been carried out by the towing tank in the previous research. After the model test, the wake oscillator model is also used simulate the real model test and find out the influence of different parameters on the test results.
The main conclusions of this dissertation is that for small-scaled model test carried out in the flume, the free vibration tests match the full-scaled model well, which can be used to evaluate the vibration suppression efficiency of the strake model. In the stationary test, the small-scaled model provides higher drag coefficient due to the difference of Reynolds number. In the numerical model, the parameters which may influence the oscillating amplitudes are the tuning parameters and the different series of tuning parameters are given according to the spring stiffness.
... Read more