Gain insights from Dr Jon Tore Lieng, Managing Director of Deep Sea Anchors AS, as he discusses more about Dynamically Installed Anchors for offshore floating wind turbines
Floating wind turbines are a more recent innovation. As the name suggests, the turbines are mounted on floating structures, which are moored and anchored rather than directly fixed to the seabed. The first large, industrial-scale floating wind farm (HiPR-Wind) was established in Scotland in 2015. Ongoing and future projects have even more ambitious plans to take advantage of deeper and more challenging waters and to put greater numbers of wind turbines in place.
Dr Jon Tore Lieng of Deep Sea Anchors in Trondheim, Norway, is experienced in anchoring systems for offshore floating structures such as oil and gas platforms, drilling rigs, and wind farms. He explains that various anchor designs for floating wind turbines are available, each with specific installation requirements. These include drag embedment anchors such as fluke anchors, where the anchor is dragged across the seabed until it realises sufficient resistance.
These are extensively used with temporary installations but may also be used for single-line permanent mooring. As they are drag-embedded, they need to be proof-loaded up to design requirements, since the anchor’s precise embedment depth is unknown. Proof loading may require several vessels and or additional marine operations and equipment to fulfil the prescribed loading capacities.
Another design is a suction anchor, where a cylinder, closed at the top end, is lowered onto the seabed, where it self-penetrates to a certain depth and then is suction-embedded (therefore coined suction anchor) by pumping out the enclosed water. Suction anchors may have up to three mooring lines attached (anchor sharing), usually require large crane vessels and may take from six to twelve hours to install, making marine operations relatively expensive.
Alternatively, a driven pile can be used, which involves the installation of a steel pipe that is driven into the seabed to act as an anchor point. However, deep waters, say over 150 metres, require lengthy and massive hydraulic hoses and large hydraulic hammers, which require ditto large installation vessels and equipment. Whilst suction and driven pile anchors can take large loads, the installation is evidently not only complex and time-consuming but also costly.
Dynamically Installed Anchors (DIAs)
Dr Lieng has a novel approach to reduce the complexity and cost of installing anchoring systems for floating wind farms and to support the move of oil and gas exploration into deeper waters. To this end, he favours the concept of Dynamically Installed Anchors (DIAs, also known as gravity installed anchors), which is used where the seabed soil sediments consist of cohesive material such as clay or silt.
These anchors are dropped from a certain height above the seabed, i.e., typically from a height of 70–80 metres, allowing gravitational forces to effectively embed them in the seabed some 20–30 metres, depending on the strength of the seabed sediments. This makes their installation much easier to accomplish than for other anchor designs, especially in deep or challenging waters.
His extensively tested Deep Penetrating Anchor (DPA) is one type of DIA. It is similar in shape to a dart and will vary in size and mass depending on capacity design requirements. This size, shape, and weight ensure that when the anchor is released, it builds up enough momentum to penetrate deep into the seabed.
Capacities of more than 1,000 tonnes can be realised, which is sufficient for large 15-megawatt wind turbines. The DPA is approved by Det Norske Veritas (which sets standards for offshore structures) and is a qualified technology within Equinor, an offshore wind developer and operator that has supported the development of the concept.
To ensure that these anchors are installed as designed, a monitoring unit is mounted on the side of the anchor. This unit is designed to be recoverable and provides the team on the surface with information about how the anchor descends through the water phase – recording any pitch or yaw, velocity, anchor verticality and acquired penetration depth.
From these data, one can document if the anchor is installed as designed, i.e., has reached minimum penetration and verticality. Interestingly, the condition of the seabed that the anchor penetrates is less important than the geometry and mass of the anchor. Dr Lieng tested his design in two regions with differing seabed soil shear strength and found that the theoretical pullout capacities were very similar for both regions.
One of the most significant benefits of this anchor design is its simplicity of installation. Dr Lieng explains that this means that the requirement for large construction vessels is reduced, as smaller vessels of type Anchor Handling Tugs equipped with Anchor Recovery Frames (ARF) will be able to install DPAs with relative ease.
Effectively harnessing wind energy
The DPA designed by Dr Lieng and his team will allow offshore wind farms to be built in places that were previously economically unviable or physically too challenging. This opens massive amounts of wind energy that can now be effectively harnessed while negating the need for the construction of large wind farms, either on or close to land.
