The Ampelmann system occasionally starts vibrating unexpectedly, especially while the system is placed on a pedestal. These vibrations are believed to be caused by the eigenfrequencies of the system and/or amplification caused by the motion control algorithm. In this research an investigation in this phenomenon was done. This investigation was done via an analysis of the eigenfrequencies of the active controlled hexapod. A finite element method model was created to determine the eigenfrequencies of the system. This model was made using MATLAB and the toolbox StaBIL 2.0, created by the university of Leuven. All the elements of the system are modeled as beams except the hydraulic actuators. The properties of the elements which represent the hydraulic actuators are calculated separately using a modelling study on stiffness characteristics of hydraulic cylinder under multi-factors.
Possible causes for the unexpected vibrations have been investigated via measurements performed on Ampelmann systems. Data which was readily available is analyzed. Based on this data three possible causes have been determined. These are: the influence of the pedestal, residual motions due to limitations of the Ampelmann system and vibrations in the bottom frame due to compensating for gangway motions. To investigate the influence of the pedestal and the vibrations in the bottom frame two experiments have been performed. The residual motions have been investigated via calculations based on data already available. The amplitude of the response in the results from the first experiment performed to investigate the vibrations in the bottom frame due to gangway motions is negligible over the entire test period. The data from the calculations done to investigate the effect of the residual motions show no amplification. The results of the experiment and the calculations lead to the conclusion that these do not cause unwanted behavior. The second experiment to determine the influence of the pedestal shows four peaks in the frequency domain of both the signals. The first is directly caused by vessel motions. The other three have a cause which is not directly related to vessel motions. The data from the sensor on the Ampelmann system, at the top of the pedestal, does not contain a peak which is not present at the ship deck. From this it can be concluded that the eigenfrequencies of the pedestal do not have a relevant influence. For one of these peaks the amplitude of the graph related to the top of the pedestal is higher than the one corresponding to the ship deck. A possible explanation for this phenomenon could be the eigenfrequencies and corresponding eigenmodes of the ship deck. The pedestal functions as a leaver arm amplifying the rotations related to eigenmodes of the ship deck. This may cause the unexpected vibrations.

The Ampelmann system offers a safe and reliable solution for offshore access. As safety is a key factor, an extensive safety and warning management system is employed which accommodates for different fault types that may occur. Accommodation of sensor failure in the Ampelmann system is currently done through switching to a redundant component. However, there are several faults that are left undetected in the system. Detection only occurs when the faults exceed a critical threshold. Exceeding this threshold immediately results in shut-down of the system as safety is no longer guaranteed. For sensor equipment critical to the motion compensation, this leads to a code black in the system. The occurrences of code blacks should be limited where possible due to the fact that these lead to downtime.

A critical sensor for the motion control is the position transducer in the hydraulic cylinders. The measured lengths are used for feedback purposes in the control system. The position transducer is redundant in each cylinder. The redundant sensor is mainly utilized for checking the main sensor. However, when the measurements from both sensors deviate too much from one another the system will shut down.

Therefore, in this thesis, the possibility of a model-based fault detection method for the position transducer in the hydraulic cylinder is explored. Firstly, an accurate model of the hydraulic cylinder is derived and identified. Then, the model is combined with an observer to generate accurate estimates of the cylinder lengths. Furthermore, the estimates are compared with the actual measured cylinder lengths from the position transducer to generate residuals. Finally, the residuals are evaluated in order to make a decision about the health of the sensor.

Three different fault types have been defined, which are expected to cause sensor degradation/failure. For each fault type, the residuals are evaluated. Prior to this a threshold has been defined based on a fault-free case. The threshold determines whether the system is healthy or not. Ideally, when there is no fault in the system, the residual is close to zero. Whereas, when there is a fault present the residual will be much larger. Whenever the threshold is exceeded, the detection system knows that there is a fault present, which allows it to sent out a warning. There are three model-based fault detection estimators which generate three different residuals. These three estimators are combined into one fault detection architecture.

The results developed throughout this thesis have provided new insights for the fault monitoring system in the Ampelmann system. Currently it has only been applied for the position transducer. However, it can be extended to other critical components in the system. Furthermore, the work presented in this thesis is valuable for predictive maintenance purposes. Finally, the detection estimators can be used for the implementation of fault tolerant control in the system.