Continuous development in the wind turbine industry leads to increasing size of wind turbines. Allseas investigates the possibility to enter the offshore wind turbine installation industry with Pioneering Spirit.
For Allseas’ preliminary wind turbine installation design, wind turbines are assembled offshore, requiring wind turbine components to be brought from shore to Pioneering Spirit by means of a cargo barge. This operation requires a proper mooring procedure of which the mooring system is an essential part. The mooring system secures the barge alongside Pioneering Spirit where it has to stay for multiple days.

In this research a pre-defined vessel orientation is analysed where the barge stern is extended 40m in longitudinal direction from Pioneering Spirit stern, to increase the barge area reachable by the unloading crane.
This barge position is challenging due to limited shielding from Pioneering Spirit, leading to excessive environmental loads acting on the barge. Due to the barge extension, also properly connecting the mooring system to Pioneering Spirit is a challenge.
The above leads to the main objective of this thesis: improve the mooring system to secure a barge alongside Pioneering Spirit in offshore conditions, including an evaluation of the dynamic mooring loads.

The main mooring system design requirements are that the system Safe Working Load and motion limits are not exceeded.
Evaluation of the mooring systems starts with a multi-body diffraction analysis with hydrodynamic matrices and other hydrodynamic properties as output. This data is imported in a time domain model which enables capturing non-linearities, e.g. the mooring system and wind and current loads. Validation and verification steps are required to assure realistic outcome and understanding limitations of the numerical models.

Before new mooring systems are introduced, a base case mooring system is defined and modelled in the time domain.
Finding the limiting sea state for which the Safe Working Load limit is reached enables to compute the workability for the reference location defined. Two reference locations are considered: a wind sea area and swell sea area. Wind and current speed is assumed to be constant over time and independent of elevation. Workability is defined as the percentage of time the mooring system is able to operate.

Evaluating the base case mooring system shows performance of 67% workability for wind sea areas and 16% for swell areas. Next to the base case, four mooring improvement concepts are evaluated from which the Cavotec Moormaster® system shows most promising results. This system consists of three main components: ‘fixed structure - hydraulic cylinder - vacuum pad’ (Figure 2) that connects the barge to Pioneering Spirit. This system is modelled as a link with constant stiffness and damping properties within its operational limitations.

The Moormaster system, shows perspective to improve the workability for both wind and swell seas. Besides workability, the Moormaster system improves the entire mooring procedure by quick connection and safe disconnection upon exceeding operational limits.

The Poineering Spirit is a heavy lift vessel that can lift offshore platforms in a single lift. Due to the waves, the vessel moves with respect to the platform. To make sure the vessel does not hit the platform, a motion compensation system is installed on the Pioneering Spirit. The maximum allowable vessel motions during
the operation are restricted by the capacity of the motion compensation system. Comparison of the predicted motions with measurements have shown good comparison for the deep water situation. It was found that prediction of the vessel motions in shallow water does not agree with the measuredmotions.
Therefore the prediction of the vessel motions in shallow water is investigated, to find out how do predicted vessel motions compare to measured motions. The goal is to quantify and reduce the uncertainty in vessel motion prediction in shallow water. First the calculation method of comparing the predictions to measurements was investigated and validated. The wave spectrum and as well as the Response Amplitude Operator (RAO) are available in 2D, The vessel response is available as a time series. To compare the predictions to measurements,
both prediction andmeasurement are translated to a 1D response. For the prediction, first the 2D response is calculated, which is then added over all wave directions to get the 1D response. The measured time series of the vessel motions are translated to a 1D frequency domain response using the Fast Fourier
Transform (FFT). The comparison of the 1D response spectra is done by comparing Significant Double Amplitude (SDA) and peak period Tp .
Comparison of the peak period has a relatively large error, with errors exceeding 0.5 s for most of the comparison for heave, roll and pitch. Looking at the SDA, the heave motions are underestimated for most of the time for low amplitudes. As the amplitudes increase, there is a spreading of the predicted motions above and below the measured values. Taking into account the motions with at least 0.4 m measured heave, the SDA is within 10 cm from the measured motions for 25% of the measurements. For the roll,the motions were underpredicted for larger motions, being at an incoming peak wave direction between than 220 and 260 degrees. The heave motions at the location of the topside lift system are dominated by the contribution of the pitch motions. The motions in shallow water are not accurately predicted in most cases, with a deviation from the heave at the sensor location by more than 10 cm. The response deviates above and below the measured motions. A possible explanation for the overestimation is the cushioning and sticking effect due to the presence of the seabed, this needs to be further investigated. The prediction of the peak period is limited by the precision of the buoy data. In order to do a good comparison for the peak period, the precision of the wave spectrum should be increased. This can be done by fitting the measured spectrum, or by using buoy data which has a smaller frequency step.

Allseas Group S.A. is a leading offshore contractor in the field of pipeline installation, heavy lifting and subsea construction. One of Allseas’ vessels is Pioneering Spirit. The main activities of Pioneering Spirit can be subdivided into pipeline installation, topside installation/removal and jacket installation/removal. The equipment of the first two activities has been successfully put into operation. The equipment to install and remove jackets (Jacket Lift System (JLS)) is currently under development. The mechanism to upend/tilt-over the Jacket Lift System is the subject of this graduation project.

The design challenge of Pioneering Spirit’s Jacket Lift System is to install or remove a jacket with a height of at least 70 meters and a mass of up to 20 000 mt (in air) in a single lift/operation. In this thesis project, an additional design solution has been investigated and developed. The objective was to investigate the feasibility and favourability of various principles and concepts to upend/tilt-over a jacket using Pioneering Spirit. Key topics in the development of the design solution were the controllability of the operation, the compatibility of the system in the current appearance of Pioneering Spirit’s, the complexity of the operation and the investments costs to construct, operate and maintain the system.

The design solution found in this graduation project consists of a tilting system that rotates over the stern of Pioneering Spirit. The system is driven by a pushing system installed on the reinforced transverse frames on the aftdeck of Pioneering Spirit. The system must be movable to relocate the centre of gravity before upending or after tilting-over of the system. The system is controlled by two winch systems, one attached to the tip of the tilting lift beams (Derrick Hoist system, consisting of 10 winches) and the other to the upper pivot point of the pushing system (Upend/Tilt-over Mechanism, consisting of 8 winches). The system can be controlled in both rotational directions using the two winch systems. The tilting lift beams and pushing system are connected by means of a roller/slider connection.

The maximum forces, stability and controllability of the system were checked with a coarse dynamic mathematical model. The natural frequency of the system with and without jacket appeared to be in the same frequency range as the excitation response spectra. This was solved by stiffening the system by increasing the effective diameter of the winch systems and by applying pre-tension. Subsequently, the system appeared to be possibly instable during the first/last 18 degrees of the tilting operation. This was solved by applying an auxiliary construction during the first/last 30 degrees of the tilting process. The maximum response amplitude of the system was calculated by means of a calculation of the maximum excitation in the frequency domain (regular waves) and for time series (irregular waves) for the positions in which the Jacket Lift System can be positioned and all incoming wave directions. The maximum response amplitude of the system occurs in beam waves when the system is positioned vertically. Although the maximum response amplitude of the system is small, mainly because of the stiffness, the maximum forces in the system are exorbitantly large. To give an indication, the maximum tension in the Derrick Hoist System is 4125 mt. At a certain moment in the tilting procedure, the entire mass of the Jacket Lift System (15 000 mt) and Jacket (20 000 mt) is applied to the pivot points at the stern of Pioneering Spirit. In general, the static forces deliver the greatest contribution to the total force. The maximum loads are considered feasible, although strengthening measures must be taken.