Design of a suction pile multiline anchor system for floating offshore wind turbines

Abstract

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.