For the installation of suction pile jackets, a new installation method initiated by SPT Offshore could extend the allowable size for suction pile jackets used as foundations for offshore wind turbines. This method involves installing the suction piles separately from the jacket frame. First, the suction piles are installed on the seabed. Then, the jacket frame is placed on top of the suction piles, and the piles and the frame are connected. The installation of the suction piles in the seabed requires a certain level of precision to ensure a proper connection with the jacket frame. This can be achieved by introducing an installation tool called the suction pile installation template (SPIT). The goal of this thesis is to take the first steps towards realizing the SPIT installation method by designing the suction pile installation template and identifying key design challenges for the SPIT. To bound the scope of the research, assumptions are made regarding the installation vessel, foundation dimensions, jacket-pile connection, and site specifications based on a case study and the resources of DEME. To design the SPIT, the use of the SPIT is analysed, considering a wide range of design options. The most significant inputs to the analysis are the limitations of the installation vessel and operational efficiency. Next, the changes to the suction pile and jacket frame are examined. The selected grout connection between the suction pile and the jacket frame creates a jacket frame similar to a standard pin pile jacket. The suction pile requires a stub on top of the top-plate. An optimization study is conducted to determine the size of the stub. These two analyses provide the general design requirements for the SPIT, which is then checked for structural strength, installation tolerance of the suction piles and lift capacity. The checks are based on industry standard codes. Analysis shows that the hydrodynamic loading on the suction piles induces the largest loads on the SPIT. However, if the suction piles are incorrectly placed in the seabed, the interaction between the soil and the suction piles could result in even larger loads on the SPIT. The models used in this thesis should provide conservative estimates. In future research, the analysis of hydrodynamic loading, geotechnical analysis, and dynamic response of the SPIT should be verified and justified using more sophisticated models and/or simulation software. The results from this thesis indicate that the proposed design of the SPIT provides a solution to extent the installation of SPJ for OWT. The research identifies four key design challenges. Each challenge indicates solvable obstacles to the design of the SPIT. Based on the results, the estimated total weight of the SPIT is 240mt. DEME's installation vessel, the Orion, has sufficient lift capacity to perform the installation and deck-space to perform up to 13 installations in one trip.

In the past decade, suction caissons have emerged as a preferred offshore foundation solution for wind turbines due to their silent installation process and potential for recyclability. However, there has been growing speculation regarding the necessity of under base filling, which involves filling the gap between the top plate of the suction caisson and the seabed. Some experts have suggested that under certain conditions, this under base filling may not be required at all. Furthermore, concerns have been raised about the efficacy of under base filling in achieving full contact between the top plate and the seabed, as it has been observed that gaps may persist even after the filling is applied. Consequently, doubts have been cast on the overall need for under base filling. However, there is limited research focused on understanding the behavior of water plugs in the absence of under base filling, at different loading conditions ( Compression , tension , cyclic etc.). This knowledge gap motivates this thesis study, which aims to investigate the behavior of water plugs specifically in dense sand samples, as sand is considered more critical compared to clay in terms of its variability in drainage conditions that can influence the foundation’s performance. To achieve this, a series of centrifuge tests were conducted on suction caissons that were partially installed and some fully installed. The results of the experiments shed light on the role of under base filling in different loading scenarios. Under monotonic compressive loading at higher rates, it was observed that under base filling played no significant role in the load transfer . Both the caissons with and without under base filling exhibited similar load transfer mechanisms, indicating that filling the gap may not be necessary in such loading conditions. Additionally, under tension loading, it was found that under base filling had little to no effect on the development of tensile capacity. By expanding our understanding of the necessity and effectiveness of under base filling, this study contributes to the ongoing discussion surrounding suction caisson design and installation practices for offshore wind turbine foundations.

In this thesis I will show the realisation of a multiline anchor system using a suction pile anchor (SPA). From the literature research it can be concluded that the SPA is a suitable anchor for a multiline anchor system and that the most probable mooring configurations will be either the original single-line, the 3-line or the 6-line system. Furthermore it could be concluded that when using a catenary mooring solution for the 3-line system, that the anchor perceives a lower horizontal net force due to anchor lines coming from different directions and cancelling parts of each other. This would, in theory, make the SPA for the 3-line multiline system smaller and also there would be 67% less anchors needed in a wind farm array. Furthermore the 6-line anchor does seem to have a bigger horizontal net force which requires the SPAs to be bigger but 83% less anchors are needed in an array of wind turbines. The looked at FOWT-system was the University of Maine VolturnUS-S reference floating offshore wind turbine semi-submersible, which supports the IEA-15-240-RWT turbine with a turbine rating of 15MW. With the original mooring configuration being a catenary one, a comparison has been made with a taut system and its implementation into a multiline anchor system has been researched. Lastly three different soil profiles have been chosen: normally consolidated clay, slightly consolidated clay and loose sand. In Chapter 4 Suction anchor concept design, the forces on the anchor are set up using the data set-up by NREL and the University of Maine and the C. Fontana papers. A SPA has more bearing capacity when the mooring-anchor connection point is below the mudline. Because of this feature the mooring line attaches to the SPA at an angle and in this thesis a range of angles of approach are chosen to be investigated: 25 and 35 degrees for the chain and 45 degrees for the taut line. Because of the angle there is a vertical force introduced to the anchor which need to be added together for each line. Because of this the total net force on the anchor is increased for the 3-line and 6-line anchors compared to the single-line anchor. With the mean and maximum forces the calculations for the holding capacity/pull-out capacity can be set up by looking at the Ultimate Limit State (ULS) or maximum forces in the system. These calculations are taken from the DNV guidelines which are the industry standard. To set-up the first parameters estimation a embedment (h/D-ratio) starting value has to be chosen and from literature and in discussion with SPT offshore the starting values were set at 5 for the clay profile and 1.5 for the sand profile. The weight of the SPA must be defined which was done as the mean vertical force applied to the anchors as such they will not be pulled out over time. The installation and removal calculations are an important step in the design and are also set up. Here the under pressures required to fully install the SPAs are calculated. Furthermore, structural failure due to buckling is checked for and different soil failures are analysed. Lastly the removal pressure of each concept is checked which allows for complete removal of the suction anchor. By lowering the embedment ratios of the different concepts the pressures inside the anchors can be minimised and problems can be averted. In the detailed design the full design of a in use SPA is shown and each part is defined. Furthermore, the one-line-broken criterium is discussed and it can be concluded that in case this happens the 6-line multiline system is very dangerous because a chain reaction can be started which can take out large parts of a wind farm array. Also a weight estimation of each SPA is made from which the extra needed ballast is calculated. Subsequently a cost estimation of each anchor concept can be made by calculating the cost of each system from the structural weight, the ballast and the mooring line lengths. At a depth of 200m, at which this study is situated, the taut multiline system cannot be set-up but the single-line taut system can be compared to the catenary single line system. Lastly a parametric analysis is done where changes in different parameters are compared to each other. What can be concluded from this thesis research is that a multiline system is technically feasible for a 15MW floating offshore wind turbines using SPAs. The 3-line and 6-line systems both have larger anchors than the single-line system although they need less anchors in a system. When including the mooring line costs together with these anchor costs it can be concluded that the 3-line anchor is more economically viable but the 6-line anchor is not. What can also be concluded from these mooring line costs and what is discussed in the parametric analysis is that the system works better if the wind turbines are closer together because the lines will be shorter. This distance is dictated by the wake recovery and an optimisation study is recommended for the 15MW wind turbine but it is also recommended that smaller turbines and deeper depths are looked at.