Floating wind turbines: Marine operations challenges and opportunities

. The global floating offshore wind energy industry is rapidly maturing with several technologies having been installed at pilot and demonstration scales. As the industry progresses to full array-scale deployments, the optimization of marine activities related to installation, operation & maintenance and decommissioning presents a significant opportunity for cost reduction. This paper reviews the various marine operations challenges towards the commercialisation of floating wind in the context of spar-type, semi-submersible and Tension Leg Platform (TLP) technologies. Knowledge gaps and research trends are 5 identified along with a review of innovations at various stages of development which are intended to widen weather windows, reduce installation costs and improve the health and safety of floating wind-related marine operations.


Introduction
Wind turbines are moving further offshore to deeper waters and are exploiting higher wind speeds in harsher environments (McCann (2016)). This trend creates additional challenges in the design, installation, operation, maintenance and decommis-10 sioning phases of an offshore wind farm. Numerous investigations for developing efficient and optimum Floating Offshore Wind Turbine (FOWT) platforms and various innovative design concepts have been evolving in the last few years (Uzunoglu et al. (2016), EWEA (2013)). Several pilot and demonstration-scale floating wind farms are now operational in different parts of the world (eg: WindFloat Atlantic -Portugal (25 MW), Hywind -Scotland(30MW)) and there is a robust pipeline of projects which is expected to deliver 250 GW installed floating capacity by 2050 (DNV (2020), James and Ros (2015)). 15 To realize this ambition, significant reductions in LCOE (Levelized Cost of Energy) will be required across all key stages in the development of a floating wind farm. Numerous marine operations are required in the last three stages (Figure 1) of the floating wind farm life cycle (shown in orange). Marine operations represent a significant proportion of the offshore wind energy costs. Approximately 15.2% of CAPEX (Capital Expenditure), 50% of OPEX (Operational Expenditure) and 80% of DECEX (Decommissioning Expenditure) are directly related to marine operations for a bottom-fixed wind farm (Judge 20 et al. (2019)). Comparative figures for the floating wind energy industry are unavailable due to a lack of projects, however, investigations by Castro- Santos (2016) show that approximately 36% of the total floating project costs are incurred during the 1 Figure 1. Stages in the development of a wind farm installation, exploitation and dismantling activities. Castro- Santos et al. (2018a) have revealed that the size of the floating wind farms has a considerable impact on installation costs and LCOE. It was found that the LCOE reduces as the farm size increases.
Given the nascent nature of the floating wind energy industry and the lack of established best practices, these operations 25 present significant scope for optimisation and cost reduction. The FOWT industry has a significant second mover advantage and can benefit from the technical expertise and innovations developed in the offshore oil and gas (O & G) and fixed wind energy industries. However, the deployment of highly dynamic unmanned floating platforms is unprecedented and requires the development of bespoke solutions to reduce the cost and increase the safety of installation, operation and maintenance (O & M) and decommissioning related marine activities.

Installation, O & M and decommissioning of FOWT floater types
Floating wind platforms can be mainly classified into three broad categories according to the restoring mechanism for attaining hydrostatic equilibrium. They can be ballast stabilized, buoyancy stabilized, mooring stabilized or combinations of these (Leimeister et al. (2018), Banister (2017), Booij et al. (1999)). Figure 2 demonstrates how the different FOWTs developed 45 around the world fit into a stability triangle.
The installation procedures differ according to the type of floater used. Generally, floating wind installation requires a greater number of vessels compared to fixed wind, but the vessels are cheaper to hire and easily available (Crowle and Thies (2021)). Even though many floating wind concepts have been developed, only a few have been successfully deployed and commissioned at a commercial level. In the following sections, a review of the projects that have reached the Technology 50 Readiness Level 7 (TRL7) (EC (2017)) or above is provided with a focus on the marine operation strategies which they employed.

Spar-type: Hywind Scotland Project
Hywind Scotland is a floating offshore wind farm, which has a rated capacity of 30MW, produced from five 6 MW turbines, which has been functioning since 2017 (Equinor (b)). The spar-type floater, namely the Hywind concept was developed by 55 Equinor ASA (Equinor (b), Skaare (2017)). A 2.3 MW demo (TRL 8) (OREC (2015)), was deployed at the west coast of Norway in 2009 for a test run of 2 years (Butterfield et al. (2007), Equinor (b)). Following the demonstration, a wind farm was installed 30 km off the coast of Aberdeenshire, Scotland. The farm has an area of approximately 4km 2 and an annual average significant wave height of 1.8 m (Mathiesen et al. (2014)).
The spar-type substructures have high draughts which require the use of offshore assembly. The installation phase also 60 requires sheltered coastal waters (maximum significant wave heights are up to 0.5 m and a Beaufort wind force 4 (DNV (2015)) for some operations. This is a challenge, as sheltered waters with high depths are required near the construction sites, due to the high draught of the floaters. They should either be available near the ports or should be made by dredging. The first marine operation was the installation of the suction anchors and the mooring lines. The floaters were first wet-towed, out into a sheltered area using tugboats. Then they were upended using water ballast. Later, the water was pumped out and solid  (Jiang (2021)). This was done using the heavy-lift vessel, Saipem 7000 (Saipem). A notable feature of the Hywind installation was the use of this expensive heavy-lift vessel. Significant cost cuts may be achieved by innovative installation methods in the future, by avoiding the use of such vessels. After the mating, the wind turbines were towed to the location of the wind farm.
They were then connected to the pre-installed mooring lines and final ballast corrections were performed. Metocean assessments were performed before the commencement of the project to check for suitable weather windows for marine operations. The duration characteristics of marine operations were predicted for various limiting conditions of wind and waves for a full year (Mathiesen et al. (2014)). The months from April to September were found to have the widest operational time windows. The inshore assembly (upending, solid ballasting, heavy lift) and offshore installation operations 75 (anchor & mooring, transfer & installation of wind turbine generators, cables) were scheduled and carried out in this period.
The operation and maintenance of the wind turbine mounted on the spar-type substructure is similar to that of a bottomfixed offshore wind turbine. A campaign-based inspection and monitoring scheme is planned for the FOWTs. Maintenance and repairs of the sub-sea systems (foundation, mooring system, cables) will follow a different approach. Periodic sub-sea inspections and maintenance will be performed using ROVs (Remotely Operated Underwater Vehicle). Scour might also be an 80 issue concerning the Hywind spar-type platforms as they employ suction anchors. Heavy component exchanges up to 2 tonnes will be performed using the platform crane and a crew transfer vessel (CTV). Heavier components like the transformer can be exchanged using a dynamic-positioned offshore service vessel. In the case of even larger components, the turbines have to be towed to a sheltered region and exchange will be carried out using heavy lift vessels (H.H.Hersleth (2016)).
The decommissioning of the spar-type platforms could be more expensive compared to other types of platforms. The 85 platform has to be partially decommissioned in deep water due to the high draughts before getting towed to the quayside.
The blades, nacelle and tower can be dismantled using a heavy lift vessel; this incurs additional costs. The mooring lines and anchors also must be retrieved. Easily recoverable anchors can reduce decommissioning costs and time. There is scope for further investigations to develop cheaper and easy decommissioning strategies of spar-type FOWTs.

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In 2011, Principle Power installed a prototype semi-submersible FOWT, namely WindFloat, 5 km off the west coast of Portugal (Roddier et al. (2010)). This was followed by the installation of a 25 MW floating wind farm on the west coast of Portugal.
The wind farm consists of three MHI Vestas 8.4 MW turbines mounted on Principle Power's semi-submersibles (Banister (2017)). The installation operations were carried out with the help of tugs, AHTSs (Anchor Handling Tug and Supply) and ROV-Support cable-lay vessels supplied by Bourbon Offshore (Ocean-Energy-Resources). The towing operation took three 95 days, taking the completely assembled FOWT from the port of Ferrol, Spain to the farm located 20 km off the coast of Viana do Castelo, Portugal. The AHTS (Bourbon Orca (Ulstein)) was used for the operations, which is an advanced vessel with a bollard pull of 180 tonnes and a maximum speed of 17.1 knots. Most AHTSs were constructed to serve the O & G industry, but they make a good choice for floating wind farm installation since there is good availability and can be used for the installation of anchors and towing purposes. One of the floaters was loaded-out and floated-off using a semi-submersible barge, Boskalis 100 Fjord. Compared to the Hywind project, specialized expensive heavy-lift crane vessels were not employed here. Castro-Santos et al. (2013) has observed that the installation of anchoring and mooring systems is less expensive using an AHTS compared to a combination of barges and tugboats.
The Kincardine Offshore Floating Wind Farm is another project which employs semi-submersible floaters for the wind turbines (offshorewind.biz). The wind farm is located 15 km off the coast of Aberdeen, Scotland. It features a wind farm with 105 a nameplate capacity of 50 MW. All of the floaters were constructed in Fene, Spain and transported to Rotterdam for mounting the 9.6 MW wind turbines (Umoh and Lemon (2020)).
Semi-submersibles are buoyancy-stabilized floaters. The stabilizing righting moment is contributed either by the large water-plane area of the hull or small cross-sectional areas at some distances from the central axis (Leimeister et al. (2018)).
They can be fully constructed and assembled onshore. The installation of mooring system is less expensive compared to other 110 types of floaters, which makes semi-submersibles, one of the most cost-effective solutions from an installation point of view (Liu et al. (2016)). However, the construction costs are higher compared to other types of floaters, since they are larger and heavier structures (James and Ros (2015)) The semi-submersible wind turbines are completely constructed onshore and towed to the wind farm location and connected to the pre-installed mooring lines. Light drag-embedded anchors can be used for semisubmersibles. The main challenge in the installation process is finding the right weather window for the marine operations as 115 they are more sensitive to wave heights during towing (Banister (2017)). The installation of the three FOWTs in the WindFloat project took place in October 2019, December 2019 and May 2020 respectively (EDP (a), EDP (b)). It is very important to assess the weather windows and downtime before commencing marine operations. Another important aspect to be considered are the safe havens. Towing of FOWTs is a continuous operation and it is difficult to halt the operation during the process.
Safe havens should be identified along the route of the towing operation to avoid difficulties in case of harsh weather. Some of 120 the floaters in the WindFloat and Kincardine projects were constructed at a different location and transported to the assembly port using specialized heavy-transport vessels. This can be avoided by carefully choosing the right port with shipyard facilities, where the complete construction and assembly of the FOWT can be carried out.
Operation and maintenance activities can be carried-out either offshore or onshore (Banister (2017)). The periodic inspections, preventive maintenance and repair activities will be performed in situ (i.e., at the platform location). In case of large 125 corrective maintenance or repair activities, the platform can be towed to a sheltered location or port. No heavy-lift vessels are required and local vessels can be used for towing, this is beneficial in terms of promoting the use of local content and adhering to regulations such as the Jones Act (B.Cheater (2017)) which requires the usage of US-built ships for marine operations in the US waters. Due to the large size of the floater, helipads can be constructed aboard, which make access by helicopters possible.
During decommissioning, the platform can be towed to shallow water or taken out of the water completely inside a dry dock 130 and dismantled.

Other upcoming projects
The Floatgen Project by Ideol (France) is another project that employs barge-type substructures (Alexandre et al. (2018), Ideol). They can be fabricated using steel and concrete, which can help promote the use of local content and employment. The In 2008, Blue H Technologies successfully installed a prototype in Italy as a predecessor of commercial-scale TLPWTs (Adam et al. (2014)). Even though many concepts have been developed and a few prototypes have been reported, commercialscale TLPWTs are still not a reality. The installation of a conventional TLP system is a complex process. The tendons, which 145 hold the platform in place, are usually installed before the platform. The TLP floater is constructed onshore and towed to the location using tugboats or transported using a bespoke barge. The platform is ballasted and connected to the pre-installed tendons. Finally, the platform is de-ballasted to a draught where the tendons attain the optimum tension and the TLP is secured.
The nacelle acceleration and the dynamic tension on the mooring lines are the critical operational parameters here. Low nacelle accelerations are required for the reliable operation of the wind turbine and low mooring line tensions ensure low weights of 150 gravity anchors and longer fatigue lives. Sclavounos et al. (2010) investigated the hydrodynamic performance of the TLP in various significant wave height and water depth conditions. It was found that the TLP is safe to be installed in waters deeper than 50 m and significant wave heights up to 14 m. Figure 4 illustrates the main components involved in the installation.
The main challenge involved in the installation of a conventional TLP is its unstable behaviour before connecting to the tendons. For the installation of an O & G TLP, bespoke barges are often used for transportation and positioning of the system, 155 which incurs additional expenditure (James and Ros (2015)). The anchors and mooring lines system used in TLPs are designed to handle high tensions and thereby more complex than the semi-submersible and spar-type platform systems. The tendons are susceptible to fatigue failure, which makes TLPs expensive from an O & M point of view also. Decommissioning of a TLP system is also expected to pose some challenges due to the complexity of the mooring system. The platform has to be carefully released from the tendons and towed back to the shore. The mooring lines and anchors are removed later. The GICON-TLP (Adam et al. (2014), Kausche et al. (2018)) is another innovative concept. The TLP system is placed on a floating slab and dry-towed to the location. The slab is then ballasted which submerges the TLP into the required final draught. Figure 5 shows this installation method proposed by GICON. Analyses by Hartmann et al. (2017) found that the transportation and installation of the GICON-TLP should be performed only at a wind speed below 12 m/s and a maximum towing speed of 5 knots. There are many upcoming concepts that are designed to allow alternative installation methods. An 165 example is the TetraSpar concept (Borg et al. (2020)) which features a tetrahedral floating structure equipped with a keel, which can be ballasted and lowered into a certain depth on site. The keel can be air-ballasted and towed to the offshore location. Once the mooring lines are hooked, the keel is lowered and the system starts acting like a spar-type platform. The TetraSpar has been developed aiming to implement large-scale industrial production, as they consist only of cylindrical tubes bolted together.
These platforms do not require specialized vessels for installation since they can be towed using low-cost and widely available 170 tugs (Andersen et al. (2018)). The towing vessels and the FOWT must satisfy the stability criteria set by a classification society (Hartmann et al. (2017), DNV (2021)). Even though small tugs can be used for towing, this might not be always feasible for installation in deeper seas due to the large motions of the tugs in severe sea conditions. It is advisable to use larger towing vessels for such installations. During the sinking operation of the ballast, the structure will be susceptible to current loads also.
Detailed analysis is required to ascertain the motion and the loads on the tendons during this operation. As mentioned above, different types of FOWTs have advantages and disadvantages when it comes to installation, O& M and decommissioning. Table 1 summarizes them according to the type of floater. Significant wave height, current and wind speed restrictions apply to all marine activities. Some are more sensitive to this, but generally, it is important to overcome this restriction by precise weather monitoring and forecast (Emmanouil et al. (2020)) or using innovative ships and technologies. Marine operation is defined as "a generic term covering, but not limited to the following activities, which are subjected to the hazards of the marine environment: Load-out/ load-in, Transportation/ towage, Lift/Lowering (Offshore/Inshore), Towout/tow-in, Float-over/float-off, Jacket launch/jacket upend, Pipeline installation, Construction afloat (DNV (2011a))". Clearly, many of the operations listed here are applicable for floating wind farms.

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DNV (DNV (2011a), DNV (2011b)) classifies marine operations into weather restricted and weather unrestricted. The operation period T R is defined as: T R is the operation reference period, T POP is the planned operation period and T C is the estimated maximum contingency time for the marine operation. Weather restricted operations are marine operations with T R less than 96 hours and T POP less 190 than 72 hours. This is the maximum period for which the weather forecast is sufficiently reliable. Precise weather forecasts and continuous monitoring are required for weather restricted operations. Towing operations must be analysed based on this (DNV (2011a)). For the Kincardine project, the towing took approximately 9-10 days (Bridget Randall-Smith). This can be considered as a weather unrestricted operation (DNV (2011a)). Statistical extremes of metocean conditions must be considered for planning such an operation according to Table 2.

Duration of towing Return period of metocean parameters
Up to 3 days Specific weather window to be defined  There is a lack of consensus on installation approaches, costs and time for each task for floating offshore wind farms.
Multiple installation, O & M and decommissioning approaches have been suggested for bottom-fixed wind turbines and many of them can be adopted for floating wind farms. But FOWTs also present additional challenges, for example: dynamic cables, mooring lines, longer real-world operational windows etc. The most significant challenges and the opportunities for addressing 205 them are discussed in the following sections.

Metocean Assessment and Analysis
One significant challenge associated with FOWTs is predicting the optimum meteorological and ocean conditions for design as well as operation of FOWTs. In general, metocean data is required for planning installation, operation, maintenance and decommissioning activities by predicting the suitable weather window and associated costs based on wind, wave and current Among these, the in situ measurements provide the most reliable metocean data. Even though in situ measurements cannot provide long term data and incur costs, it is advisable to start taking in situ measurements once the location is decided for a wind farm. Predictions using numerical models can be validated and augmented using in situ measurement data or 225 satellite measurements. Long-term metocean data can also be generated using hind-cast data for a particular region. A careful combination of all these will serve as a reliable data-set for planning installation, O & M and decommissioning activities. Wind, waves and current are the most important metocean parameters to be monitored and analysed for offshore wind installations.
Wind, being the primary source of power, can be measured using LIDAR or satellite scatterometer ((Ahsbahs et al. (2018)), (Remmers et al. (2019))), the former has proved to be more accurate and continuous for a particular region of interest. Waves 230 can be measured using multiple buoys deployed at a particular location, micro-wave radar systems (Teleki et al. (1978)) or using satellites. Current measurements are made using current meters mounted below buoys or using micro-wave radars. Large scale geostrophic currents can be measured using satellites (Dohan and Maximenko (2010)). Shore-mounted HF (High Frequency) radars can measure winds, waves and currents in coastal waters over a range of 120 km from the shore (Wyatt (2021)). Various numerical models exist for the precise calculation of metocean conditions. SWAN (Booij et al. (1999)) is a third-generation 235 wave model that can calculate random, short crested waves in shallow waters and ambient current.

Environmental limits for installation, O & M and decommissioning
The Special operations, such as the mating of a spar-type platform with a wind turbine is difficult when the platform is unstable.
The installation of TLPs also pose similar challenges as the platform itself is relatively unstable before the connection to the tendons. Waiting for suitable weather conditions can raise the costs, so a precise weather forecast is essential. Figure 7 shows 245 the metocean limitations during assembly, transit and installation of different kinds of platforms. Towing is another marine operation that is significant during the installation, operation and maintenance and decom-265 missioning of FOWTs. Here the critical response parameter is the tension on the towing line, which has to be kept below the breaking load during the operation. During the towing of fully assembled FOWTs, one large towing vessel provides the required bollard pull for towing and two or three smaller tugs stabilize the motion of the floater and maintain a straight-line course. DNV (2021) has provided the allowable motion and acceleration limits for these vessels used in the operation. In all these cases, it is important to ascertain the loading conditions corresponding to the limiting value of the response parameter.

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These loading conditions should be compared with the metocean forecasts and the decision of whether the operation can be executed or not should be taken. For long-duration projects a weather window analysis is performed to predict the periods when the weather parameters that affect the operations are below the threshold limits. Using hind-cast data of a particular location, the Weather Down Time (WDT) can be predicted and decisions on important marine operations can be made (Lambkin et al. (2019)).
275  (2012)). Raising this limit is essential for all marine activities concerning 280 offshore structures. Table 3 shows the significant wave height (Hs) and wind speed limits for various vessels used for marine operations involving FOWTs. These limits can be raised by developing innovative ship designs that have better hydrodynamic performance in rough seas; Walk-to-work vessel is one example. The possibility of outfitting existing vessels with motioncompensated gangways should also be investigated. At the same time, specialized vessels are expensive to hire, which would raise the costs. Operation Vessels (SOV) and Walk-to-work vessels fitted with motion-compensated gangways should be used considering the harsh sea conditions in the deep seas where the FOWTs will be installed. Many good models exist for bottom-fixed wind turbines. Figure 9 shows the breakdown of costs associated with a floating wind farm compared to a bottom-fixed wind farm.

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They can be extended into floating wind farms depending on the type of floater and the methodology of the various operations.
The floating wind costs are less developed at the moment, hence new and improved models are required for precise calculation of the costs. The installation costs consist of costs incurred for the installation of the wind turbine, floating platform, electric system, mooring and anchoring systems and the start-up cost (Castro- Santos et al. (2016)). The installation costs of the floating offshore 300 wind platform depend on the port and shipyard costs, transportation/towing costs and site installation costs. Draught restriction applies to many ports depending on the type of floater used. The spar-type platforms and TLPs have higher draughts compared to semi-submersibles. In such cases they have to be stored in a special storage area (Castro- Santos et al. (2018b)) or the port is needed to be dredged. This incurs additional costs. The electric system installation costs are composed of cable installation costs and sub-station installation costs. For the installation of mooring and anchoring system, it is economical to use an Anchor

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Handling Vessel (AHV) compared to using barges and tugs (Wayman et al. (2006)). The costs vary according to the number of anchors used in the wind farm. By the end of the installation of all the systems, the wind farm is commissioned and start-up costs are also incurred during its connection to the electrical grid. Installation costs strongly depend on the site location of the wind farm. Maienza et al. (2020) investigated how the factors like distance from the coast, distance from the nearest port, and sea depth influence the total cost of a wind farm in the Italian 310 waters. A cost map for the Italian national waters was produced, indicating the total costs for a wind farm consisting of 12 TLP FOWTs. Preliminary analysis showed that the number of wind turbines has a considerable effect on the total cost. Doubling the number of turbines increased the cost by 35%, but quadrupling or increasing eight folds increased the costs by 60% and 80%, showing a decrease in the rate at which the total costs increase. The LCOE was found to decrease with an increase in the farm size (Castro- Santos et al. (2018a), Myhr et al. (2014)). According to Castro-Santos and Diaz-Casas (2015), the distance from 315 the shore has a 36.3% influence on installation cost and 14.2% on dismantling cost. As the wind turbines move further into the sea, it is expected that they will grow in size too, to fully utilize the higher wind speeds. 15 MW wind turbines are predicted to be used from around 2030 and 20 MW turbines from 2037 in the UK floating offshore wind sector (OREC (2015)). The wind turbine diameter is found to have a 15.9% influence on the dismantling cost (Castro-Santos and Diaz-Casas (2015)).
O & M costs mainly consist of the corrective and preventive maintenance costs throughout the whole life cycle of the 320 wind farm (Castro-Santos (2016), Sperstad et al. (2016)). To minimize these costs, it is important to design reliable systems and efficiently monitor the health conditions of the turbine system, floater, electrical installation, mooring and anchoring system. This is of high importance as the failure of main components could result in considerable downtime, as the FOWT might not be accessible due to a lack of suitable weather windows. A preliminary analysis by Dewan and Asgarpour (2016)  Decommissioning is an environment-sensitive activity. It is better to incorporate decommissioning studies in the design phase to avoid complications in the final years of the wind farm life cycle (20 years or more) ((James and Ros (2015), Castro-Santos (2016)). A reverse-installation approach can be used for calculating the decommissioning costs of a wind farm (Topham 335 and McMillan (2017)). The main components are the dismantling costs of the wind turbine, floating platform, electric system, mooring and anchors. Some anchors, eg: drag-embedded anchors, are easily recoverable, which would save costs and time (González and Diaz-Casas (2016)). There is an additional cost due to the cleaning of the wind farm site which is of environmental concern (Castro- Santos et al. (2016)). Some materials can be sold as scrap eg: steel from the floater and copper from the electric cables, which would result in a negative cost (income) (Castro- Santos et al. (2016)).

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Various stakeholders are involved in the construction of an offshore wind farm. Proper planning and optimum utilization of resources are very important in bringing down costs. Weather restrictions during the marine operations would cause the logistic costs to increase exponentially (Lange et al. (2012)). Often hiring offshore support vessels from abroad invites large  Theoretically, the installation of FOWTs is less environment-sensitive compared to fixed offshore wind turbines, as most of them are constructed completely onshore and towed to the offshore sites. Again, the type of floater is a deciding factor 355 here. The installation of anchors and mooring lines can have a negative impact on the environment. Spar-type platforms and semi-submersibles employ catenary moorings with long mooring lines that have a larger footprint on the seabed, while TLPs don't, because they employ taut mooring lines (James and Ros (2015)). Heavy-lift vessels are used during the installation of spar-type platforms which invites additional safety issues, but there are well-defined guidelines and standards available for this operation. The installation of drag-embedded anchors has less environmental impact compared to suction or pile-driven 360 anchors. The noise impact during hammer piling is also an issue. However, the underwater noise during the installation of FOWTs is lesser compared to bottom-fixed wind turbines (Koschinski and Lüdemann (2013)). The noise during hammer piling can be mitigated by carefully engineering the piling process. By using the right specification of piles and drivers the noise can be reduced. Bubble curtains can be created using underwater bubble pipes to attenuate the blast and piling noise.
Vibropiling is another technique employing vibratory pile drive machines that drives the pile by applying rapidly alternating 365 forces, which are quieter than impact piling machines (Nedwell et al. (2003)). Long-term post-installation noise measurements are required to understand the effects of operational noises of large wind turbines fitted on floating foundations (Farr et al. (2021), Koschinski and Lüdemann (2013)). The effects on fish populations and marine mammals need to be studied separately.
Protected marine areas, migratory birds' paths and migratory marine life paths should be avoided while selecting sites for offshore wind farms (Díaz et al. (2019)).

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The most challenging safety concern is access and egress of personnel to and from FOWTs during repair and maintenance activities. The

Shared mooring and anchoring systems
For large floating offshore wind farms, the mooring lines and anchors can be shared with multiple FOWTs, reducing total 395 mooring line length, saving construction material for anchors and reducing the need for marine operations by optimising utilisation of installed infrastructure. In a shared mooring system (Figure 10), the FOWTs are inter-connected using mooring lines, reducing the frequent connections to the sea bed using anchors. In a shared anchor system, a single anchor takes multiple mooring lines and the number of anchors can be reduced. Both these systems are practical for large wind farms and significant cost reductions are achievable due to the savings in material and installation costs. FOWT configurations and found that shared anchors must be structurally strong enough to handle loading from unexpected directions. Anchors with a directional preference in their holding position and capacity are generally not suited for multi-line moorings but they can be adapted by extra structural outfitting for handling mooring loads in various directions ). The type of floater is also found to have a significant influence on the anchor forces. (Balakrishnan et al. (2020)) analysed and compared the anchor forces on a semi-submersible floater system and a spar-type floater system and found that 410 the anchor forces on the latter were lesser. This is because spar-type platforms have a lesser surface area interacting with waves compared to that of a semi-submersible. Goldschmidt and Muskulus (2015) investigated the performance of coupled mooring systems involving 1, 5 and 10 floaters in various configurations. Semi-submersible floaters were arranged in row, triangular and rectangular configurations separately and the system dynamics were studied. It was found that mooring system cost reductions up to 60% and total system 415 cost reductions up to 8% were achievable using shared moorings. However, it was noted that the displacements of the floaters were higher when the number of floaters in the system increased, which would be a problem for large wind farms. Further investigation is required to improve the behaviour of floaters in larger wind farms. One of the main challenges in adopting a multi-line system for FOWTs is the reduction of system reliability. Failure of the multi-line system components can cause a large number of FOWTs to detach and stay adrift. Hallowell et al. (2018) investigated the reliability of a multi-line system (100 420 wind turbines in 10 rows and 10 columns configuration) compared to a conventional single-line system. It was found that the reliability of a multi-line system reduced considerably in extreme load conditions due to progressive failures. Further research is required in this area to increase the reliability of multi-line systems for FOWTs.
Geotechnical investigations are carried out at the anchor site prior to the installation of anchors. Dedicated geotechnical investigation vessels are used for this. The cost of geotechnical investigations is a function of water depth, site conditions, pre-425 existing survey data and method of investigation. The geotechnical site investigation costs are also dependent on the number of anchors (Fontana (2019)). In a wind farm with shared anchors, there is a significant reduction in the number of anchors. This would help to bring down geotechnical site investigation costs. (Fontana (2019)) compared the costs of installation and geotechnical site investigation for different farm sizes. It was found that a multi-line configuration reduces the installation cost for wind farms with 36 or more wind turbines. The geotechnical site investigation costs also get reduced with an increase in the 430 size of the wind farm but taper due to the diminishing of the perimeter effect (anchors in the perimeter share less FOWTs). In a shared mooring system, mooring lines can be inter-connected instead of connecting to the sea-bed. The critical parameters are the tensions in the mooring lines and the loads acting on the anchors. As mentioned before, the system is susceptible to loading from multiple directions and the reliability of the system is dependent on the line taking the maximum tension. The tension on the mooring line must be below the breaking load in all environmental conditions as well as extreme events. The pad-eye on the 435 anchor is also subjected to loads in various directions ). Detailed analyses are required to ascertain the dynamic performance of a shared mooring system and estimate the increase in extreme and fatigue loads (Hall et al. (2014)).
The feasibility of using shared mooring system for pilot-scale floating wind farms was investigated by Connolly and Hall (2019). It was found that significant cost reductions can be achieved by proper selection of wind farm layouts, mooring line properties and platform displacements. The Hywind Tampen is an upcoming floating wind farm that employs shared anchors 440 (Reuters). The optimized array-scale installation procedure for the shared mooring and anchoring systems and its effect on costs and time are yet to be investigated.

4.2 Dynamic positioning for FOWTs
The FOWTs are generally positioned in the deep sea using mooring lines. They have to be pre-installed using offshore support vessels which are expensive and the process is time-consuming. Precise engineering calculations are required to design the 445 mooring lines and anchors for offshore structures.
The water depth is a decisive factor that determines the length of mooring lines and the costs associated with them.
In many cases, the sea-bed has to be analysed and prepared depending on the type of anchors used for installation. Semisubmersibles and spar-type floaters are moored using a longer, but simpler mooring systems compared to TLPs, but the TLPs employ complex anchors and vertically loaded tendons with high load-bearing capacity. The conventional steel tendons are 450 heavy and thereby costly and impractical for ultra-deep installations (Cummins and McKeogh (2020)). Large expensive vessels will be required for transporting and installing them in an array-scale wind farm, which would increase the costs. A mooring system is considered impractical and uneconomical for water depths greater than 1000 meters (Yamamoto and Morooka (2005)). At the moment, researchers agree that the practical water depth limit for floating wind turbines should be between 700 m to 1300 m (Musial et al. (2016)). Developing a cable system for transferring electricity from such depths is difficult 455 and impractical. But with the development of better electricity storage technology, eg: efficient hydrogen production and storage (Cummins and McKeogh (2020)), FOWTs can be deployed in deeper waters. Mooring systems will be less effective for station-keeping in such cases.
Dynamic positioning has been often used for station-keeping of ships and offshore structures in the O & G industry for years. Offshore support vessels actively employ Dynamic Positioning Systems (DPSs) for a variety of offshore activities.

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Employing DPS can be considered for FOWTs in deeper waters. The water depth and sea-bottom conditions do not affect the DPS, hence it can save mooring line, sea-bed preparation and anchoring costs. The disadvantage of the DPS is that it consumes considerable amount of energy the turbine produces. Xu et al. (2021) proposed the feasibility of a dynamically positioned semi-submersible 5 MW floating wind turbine in their paper. It was found that the DPS utilized approximately 1/2 of the total generated energy when the current forces are not considered and the ratio becomes 4/5 when current forces are taken into 465 account (Xu et al. (2021)). The concept has not been experimentally evaluated and the economics of the DP system have not been optimised.
Another scenario where the DPS can be useful is during the crew transfer from offshore support vessel to the FOWT during operation and maintenance activities. Even with the mooring lines attached, the platform is subject to motions which make the transfer of personnel difficult. In such cases, the DPS would help to stabilize the FOWT and help the safe transfer

Walk-to-work for FOWTs
Operation and maintenance of offshore platforms are often challenging due to the difficulty in accessing the platforms. Safe transfer of crew is essential for all kinds of maintenance activities concerning offshore structures. Access to O & G platforms 475 and bottom-fixed wind platforms have always been challenging. In the case of floating platforms, it is even more challenging as the platform and the crew transfer vessel are in relative motion during transfer. Since floating wind technology is relatively new, it is necessary to use the knowledge acquired in the O & G and fixed wind domain for floating platforms ).
Walk-to-work (W2W) vessels are equipped with motion-compensated gangways (MCG) that enable crew transfer in 480 higher wave heights. They help to widen operational weather windows and eliminate the waiting time, which saves time and expenses. They can also be employed during installation activities for the transfer of crew. The MCGs employ a technology known as active motion compensation (Chung (2016)). Accurate motion sensors detect vessel motions and counteract them using a sophisticated hydraulic hexapod on which the gangway is mounted. Motion detection and hydraulics are governed by an automated control system. MCGs allow crew transfer in significant wave heights up to 3.5 m and wind speed up to 20 m/s 485 (Salzmann et al. (2015)). Guanche et al. (2016) investigated the hydrodynamic performance of a catamaran crew transfer vessel (CTV24) with a fender and a Platform Support Vessel (PSV80) with an MCG and the operating significant wave height conditions were compared. It was found that access was theoretically possible with wave heights up to 2 m using the catamaran and 5 m using the PSV in head seas (Guanche et al. (2016)). In a separate study, accessibility was calculated for a semi-submersible located 490 off the coast of Portugal using hind-cast data from 1980-2013. Safe access was possible up to 20% and 76% per cent of the times using the catamaran and PSV respectively (Martini et al. (2016)). Li (2021) further investigated the operability of such systems by taking second-order drift motions also into consideration. It was noticed that the general wave height-limit reduces to 2-3 m when nonlinear drift forces were involved. Both investigations revealed that beam seas are to be avoided in all cases when employing W2W vessels for access. However, all these studies indicate that W2W vessels equipped with MSGs are 495 considerably advantageous over conventional access systems for floating wind platform access. The complex motion of the multi-body system requires further studies to ensure the safety of the crew during such operations.

Specialized ships for FOWT related marine operations
Even though many FOWTs do not need specialized installation vessels, innovative vessel designs that could simplify the installation processes are worth investigating. Many of the floater types can be constructed onshore and towed to the location by 500 using tugs. An exception to this is the spar-type platforms. They need heavy-lift vessels for the platform-turbine mating process.
There is potential for developing innovative ship designs that can facilitate cheaper and safer marine operations compared to the existing methods. One public competition we can examine to give insight into this, happened in 2014 when Statoil (now Equinor ASA) presented the 'Hywind Installation Challenge (Equinor (a))' that sought out innovative installation technologies and methods for the installation of the Hywind wind turbines.

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Stavanger based Windflip AS proposed an innovative vessel design that can be used for the installation of floating offshore wind platforms (Maritime-Journal). The proposal was based on a specialized barge that can transport a fully assembled spartype platform to the location and unload by flipping. The turbine is loaded in an almost-horizontal position and the barge is towed to the offshore location. The aft side of the barge is then ballasted taking the whole barge-turbine system into a vertical 23 position. The turbine is then connected to the mooring lines and the barge is towed back to the quayside for reloading. Windflip 510 AS claims that the vessel can operate in shallow waters which is an advantage when it comes to the installation of spar-type platforms.
Another concept was proposed by Jiang et al. (2020) to aid the installation of spar-type platforms. A floating dock was specifically designed for shielding a wind turbine during the installation of the tower, nacelle and rotor onto a spar-type platform. Hydrodynamic evaluation showed that the floating dock was able to reduce the pitch and heave response of the 515 spar-type platform which aids mating of the blades.  Figure 13 explains the various stages in the installation of a spartype platform using the catamaran vessel. Jiang et al. (2018) studied the hydrodynamic performance of the system involving the catamaran, a spar foundation, mechanical couplings, mooring lines and dynamic position system using the multi-body approach. The success of the mating is largely dependent on the relative motion between the spar and the wind turbine tower.
The relative motion radius (distance between the spar and wind turbine tower), gripper forces and the mooring line tension 530 were the critical parameters in this process. The system was tested under various significant wave height, peak period, wind speed and blade pitch conditions. The significant wave height was found to be the most important environmental parameter that affects the performance of the system. A maximum significant wave height of 3 m was used for the analyses and the mating process was found to be successful. The disadvantage here is the complex mechanical structural arrangement and various dynamic loads acting on the system. 535 Vågnes et al. (2020) proposed an improved system using winches and wires, instead of grippers. This system can reduce the structural requirements of the lifting system and improve the handling of the wind turbines. The system will be installed aboard a catamaran vessel which can be used for the installation of spar-type platforms. The low height lifting system is controlled by an active heave compensation system and tension tugger wires to ensure the balance of the tower during the mating process. Preliminary hydrodynamic analyses of the system under various weather conditions and operational layouts 540 showed that the system is feasible and can be efficiently used for the installation of spar-type platforms. The system was tested for various significant wave height and peak period conditions. The tensions on the lifting, stabilizing and attachment wires were the critical parameters. The natural periods of the OWT were unlikely to be present at common installation locations, 25 but those of the catamaran and spar are likely to be present. The analyses were focused only on the positioning of the wind turbines, detailed analyses of the lowering and mating processes are necessary to evaluate the feasibility of the system. The 545 performance of the system under various wind, wave and current conditions and experimental validation of these results are also pending. Many challenges need to be addressed and much research is pending in the FOW domain. Some of the challenges are specific to the type of floater but others apply generally. There is much research to be done for the easy and cost-effective as limiting cases for the metocean assessments. As the industry matures, it is very likely that there will be a huge demand for Offshore Supports Vessels (OSV), AHTSs, SOVs etc. in the near future. The industry should be able to supply the vessels according to the demand or else the costs will rise. Marine operations and vessel utilization must be optimised to reduce the LCOE of floating offshore wind energy and increase competitiveness with other energy sectors.
Author contributions. RCR did the reading and literature review and wrote the paper, CD reviewed and edited the initial drafts of the paper 575 and contributed actively, FJ also helped review the drafts, JJS and JM supervised the work Competing interests. The authors declare that they have no conflict of interest