A multiline ring anchor (MRA) system has been developed as a cost-effective alternative for
securing arrays of floating offshore wind turbines (FOWTs) to the seabed. Multiline attachments
can improve the economically competitiveness of FOWTs by reducing the capital cost of the
support system for the floating structures. FOWTs can be subjected to severe wind and wave
conditions resulting in extreme loads to the anchor system. Thus, the reliable design of the anchor
system requires proper determination of the extreme mooring line loads acting on the anchor
needed to secure FOWTs to the seabed. Previous studies showed the MRA in soft clay has clear
advantages over existing anchors under the extreme horizontal loading conditions imposed by
catenary moorings; however, its performance relative to conventional anchors under extreme
vertical loading imposed by taut mooring systems requires further investigation. This study
presents predictions of extreme loads on floating structures secured by taut mooring systems and
evaluates the potential for developing an economical anchor for resisting these extreme loads.
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Uplift Resistance of a Multiline Ring Anchor System in Soft Clay to Extreme Conditions
A multiline ring anchor (MRA) system has been developed as a cost-effective alternative for securing arrays of floating offshore wind turbines (FOWTs) to the seabed. Multiline attachments can improve the economically competitiveness of FOWTs by reducing the capital cost of the support system for the floating structures. FOWTs can be subjected to severe wind and wave conditions resulting in extreme loads to the anchor system. Thus, the reliable design of the anchor system requires proper determination of the extreme mooring line loads acting on the anchor needed to secure FOWTs to the seabed. Previous studies showed the MRA in soft clay has clear advantages over existing anchors under the extreme horizontal loading conditions imposed by catenary moorings; however, its performance relative to conventional anchors under extreme vertical loading imposed by taut mooring systems requires further investigation. This study presents predictions of extreme loads on floating structures secured by taut mooring systems and evaluates the potential for developing an economical anchor for resisting these extreme loads.
more »
« less
- NSF-PAR ID:
- 10313328
- Date Published:
- Journal Name:
- Geo-Extreme 2021
- Volume:
- ASCE GSP 328
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The trend of offshore wind energy in deeper water that is expected to shift from fixed to floating platforms requires a cost-effective anchor solution for floating offshore wind turbines (FOWTs). Multiline ring anchor (MRA) has been developed as a cost-effective solution for FOWTs due to its capability of anchoring multiple mooring lines, its high efficiency, and its availability to a wide range of soils and loading conditions. While previous preliminary studies on the anchor performance provide useful insights on how the potential advantages of the MRA can improve load capacity, these studies are limited to focusing on optimizing the anchor design in certain soil and loading conditions. By contrast, the MRA will be installed in seabeds under more complex conditions that depend on geological location, water depth of at-place, and environmental conditions, of which wind, current, and wave are major components. These may result in additional substantial extra capital costs, delays in the projects, and safety issues, when the complex conditions are not properly considered. Specifically, the installation time and expenses of the offshore anchor are very susceptible to anchor types, installation methods, and environmental conditions. For this reason, this paper compares two existing offshore anchor installation methods and different anchor types on the basis of their performance under the same severe environmental condition. In evaluating the installability of the MRA, this paper conducts a comparative scenario study. The results show that the anchor installations and anchor handling vessel (AHV) operations are sensitive to weather conditions and AHV sizes. In view of total weather standby, the results show that anchor types or installation methods have little effect on it due to their relatively shorter duration than other work sequences. However, the MRA can benefit in substantially reducing transport time and costs due to its compact size. The MRA can be more efficient and cost-effective than other alternatives under complex and severe weather conditions.more » « less
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The multiline ring anchor (MRA) was devised as a cost-effective means for securing floating offshore wind turbines (FOWTs) to the seabed. FOWTs occurring in arrays create the possibility for attaching mooring lines from multiple units to a single anchor. Additionally, the deep embedment of the MRA into relatively strong soil permits high load capacity to be achievable with a small and lighter anchor, thereby reducing anchor material, transport, and installation costs. However, since the MRA is shorter than a conventional caisson, features such as wing plates and keying flaps are needed to achieve parity in load capacity with a caisson having a comparable diameter. Preliminary studies show that attaching wing plates to MRA in soft clay is highly effective in enhancing its horizontal load capacity, but only marginally effective in improving vertical load capacity. This motivated the current study investigating the use of keying flaps to further enhance vertical load capacity. Two-dimensional finite element analyses were conducted to understand how keying flaps impact on the failure mechanism of the stiffeners and provide reliable evaluations of the uplift resistance of the MRA. The results show that the thickness of the stiffener, flap length, and flap angle can affect the failure mechanism and bearing factors. For the optimal design of the stiffener, a comparative study was carried out to compare the effects of keying flaps and thickness of the stiffener. The studies show that introducing keying flaps can have comparable load capacity with thicker stiffeners, and that it can be an economical solution for achieving high vertical load capacity while containing material and fabrication costs.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1061/9780784483688.041
