Floating offshore wind turbines: Installation, operation, maintenance and decommissioning

. 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


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 decommissioning 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: Wind float ::::::::: 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)).
Given the nascent nature of the floating wind energy industry and the lack of established best practices, these operations 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 around the world fit into a stability triangle.The installation procedures differ according to the type of the 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 in :: at a commercial level.In the following sections, a review of the projects that have reached the Technology 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 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 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 :::::::: wet-towed, out into a sheltered area using tugboats.Then they were upended using water ballast.Later, the water was pumped out and solid ballast (magnetite) was filled using a rock installation vessel.The tower and rotor assembly were mated with the floater later (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.Figure 3 explains the steps involved in the installation of the Hywind floaters.
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 .Characteristic durations, in-order to perform operations limited by a wind speed of 10 m/s for a period of 48 hours, expected mean durations and 10, 50 and 90 percentiles (Mathiesen et al. ( 2014)) 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 :::::::: Operated :::::::::: Underwater :::::: Vehicle).Scour might also be an 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 :::::: 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 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 ; :::: 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.

Semi-Submersible Type: WindFloat Atlantic & Kincardine Projects
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 in :: 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 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 for towing purposes.One of the floaters was loaded-out and floated-off using a semi-submersible barge, Boskalis 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 of nameplate capacity :::: with : 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)).
Safe havens should be identified along the route of the towing operation to avoid difficulties in case of harsh weather.Some of 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 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 usage of US built :: 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 and dismantled.
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, 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 in :::: 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.towing ::::: speed :: of :: 5 ::::: knots.: There are many upcoming concepts that are designed to allow alternative installation methods.An 175 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.Theses ::::: These platforms do not require specialized vessels for installation since they can be towed using low-180 cost and widely available tugs (Andersen et al. (2018)).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 the 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.
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 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 data (Jacobsen and Rugbjerg (2005)).Moreover, metocean conditions play an important role in site selection, calculation of design loads for floaters and mooring systems also.Wave and wind conditions also present safety concerns during the transfer of ::: the crew to the floating platforms.The uncertainty associated with various environmental variables are : is : of interest to all offshore operations concerning renewables as well as the O & G industry.The high complexity of the ocean environment makes it even harder to predict the operating conditions due to the presence of numerous variables.Also, the availability of metocean data for a particular location and time period affects calculations.It is very important to have a general understanding of the metocean conditions of a particular geographical area to plan the construction of wind farms.There are four main sources of metocean data: 1.In situ measurements 2. Numerical modelling 3. Satellite measurements 4. Pre-existing statistics and reports Among these, the in situ measurements provide the most reliable metocean data.Even though in situ measurements cannot provide long term data and incurs :::: 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 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 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 wave model that can calculate random, short crested waves in shallow waters and ambient current.

Environmental limits for installation, O & M and decommissioning
The installation, O & M and decommissioning activities require precise monitoring and assessment of metocean conditions in the planning phase itself.FOWTs are installed far offshore to utilize the higher wind speeds.As wind speed and wave heights are directly related, the installation and O & M of a floating wind turbine can be more challenging compared to a bottom-fixed wind turbine.Many pre-hook-up activities like site inspection surveys, installation of anchors, cable laying etc. are possible only during calm sea states.Periodic maintenance and repairs also depend on the weather conditions for safe crew transfer.
Special operations, such as the mating of a spar-type platform with : a : wind turbine is difficult when the platform is unstable.
Significant Wave Height limitation during assembly, transit and installation (James and Ros (2015)) 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 than 72 hours.This is the maximum time period for which the weather forecast is sufficiently reliable.Precise weather forecast and continuous monitoring are required for weather restricted operation.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.

Cost modelling of floating wind marine operations
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 ::: life :::: cycle : of the 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 a 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) considered using Service Operation Vessels (SOV) ::::: SOVs and towing vessels for O & M of a floating offshore wind farm consisting of fifty 8MW semi-submersible wind turbines and investigated the costs and downtime.The availability was found to be 91.4%, which was lower that ::: than fixed-bottom wind turbines, because of the towing of the platform to the shore for major repairs.Detailed : A ::::::: detailed : investigation is pending in this area to improve the downtime by using better vessels 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 :: 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 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)).
The most challenging safety concern is access and egress of personnel to and from FOWTs during repair and maintenance activities.The FOWT and the vessel form a floating-floating couple which makes access and egress challenging.Bump and jump can only be safely accomplished in low wave heights whereas walk-to-work systems increase this wave height limit while providing safer means of access/egress.Precise ::: For :::: crew ::::::: transfer :::: onto :::::: FOWTs :::::: SOVs :::: with :::::: relative :::::: motion :::::::::::: compensation :: are :::::::: preferred :::: over :::::: CTVs ::: due :: to :::::: higher ::::::::: operability ::::: limits ::: and :::::::::: established :::::::: practices.:: A :::::: precise : weather forecast is also required for planning O & M operations to avoid difficulties during the operations.Aboard an : a FOWT, even the small motions of the floater get amplified into large displacements at the nacelles and blades.This contributes to severe motion sickness or reduced efficiency of the personnel working on such structures.In the long-term, : these factors have to be considered in the design phase of the FOWT itself to reduce the dynamic response in waves.Remote O & M can also be considered to reduce the O & M visits.FOWTs can be towed back to shore for O & M which makes them attractive from a :: an : HSE point of view.
After decommissioning, it is important to leave the site in a similar condition as it was before the deployment of the wind turbines.After the wind turbines are transported to the shore, the mooring lines and anchors have to be removed.Certain types of anchors offer easy removal, while others are difficult/impossible to remove.Extreme care should be taken while removing cables from the sea.The complete removal of cables can cause severe damage and disruption to the sea-bed and in most cases, they are left buried at site ::::: at-site (Topham and McMillan (2017)).Post-decommissioning surveys are conducted to ensure that no debris is left behind and the buried remnants are not causing obstructions.

Innovations applicable to Floating Offshore Wind Turbines
Significant research is currently being conducted in the development of efficient and optimal floating wind turbines.To decrease the costs associated with installation, O & M and decommissioning operations, many innovative designs, procedures and technologies along with established best practices are required.Some aspects can be adopted from the O & G industry, however, the unique nature of FOWTs requires dedicated research and development.Some of the innovations applicable in this domain are discussed in this section.These innovations are targeted on :: at improving the operability of vessels, widening weather windows, improving HSE factors and reducing the need for complex marine operations.

Shared mooring and anchoring systems
For large floating offshore wind farms, the mooring lines and anchors can be shared with multiple FOWTs, reducing total 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.
Many researchers have worked on these concepts for the application in the floating offshore wind domain.The technical know-how of integrated mooring systems can be transferred from the O & G industry.They have been employed in the floating O & G platforms and the technology is relatively mature.Musial et al. (2004) has :::: have : observed that individual mooring and anchor costs are significant for single-turbine systems compared to a shared system.Fontana et al. (2016) investigated the hydrodynamic performance and loading analysis of shared anchors for various 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 (Diaz et al. (2016)).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 the anchor forces on the latter was :::: 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 was :::: were : studied.It was found that mooring system cost reductions up to 60% and total system 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 wind turbines in 10 rows and 10 columns configuration) compared to a conventional single line :::::::: 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.
Geo-technical ::::::::::: Geotechnical investigations are carried out at the anchor site prior to the installation of anchors.Dedicated geo-technical ::::::::::: geotechnical investigation vessels are used for this.The cost of geo-technical :::::::::: geotechnical : investigations is a function of water depth, site conditions, pre-existing survey data and method of investigation.The geo-technical :::::::::: 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 geo-technical :::::::::: geotechnical site investigation costs.(Fontana (2019)) compared the costs of installation and geo-technical :::::::::: 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.

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 mooring lines and anchors for offshore structures.
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.
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 a considerable amount : 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 account (Xu et al. (2021)).The concept has not been experimentally evaluated and the economics of the DP system have not been optimised.

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.that enable crew transfer in 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.The motion :::::: 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 (Salzmann et al. (2015)).
Several challenges are yet to be addressed to reduce the costs concerning spar-type platforms and TLPs.It is very important to consider the ease of installation, O & M and decommissioning in the design phase of the floater concepts for the practical realization of these concepts.There is also plenty of room for innovation to cut costs, reduce environmental impact and improve :: the : safety of the personnel associated with the industry.
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 weather conditions should be used 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.

Figure 1 .
Figure 1.Stages in the development of a wind farm

Figure 3 .
Figure 3. Stages in the installation of the Hywind Scotland Spar-type platforms (Jiang (2021)) Figure ?? and figure ?? show examples of the predicted characteristic durations for a limiting significant wave height of 2 m for 48 hours and wind speed of 10 m/s for 48 hours respectively.Such calculations were performed for different significant wave heights and wind speeds (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 (anchor & mooring, transfer & installation of wind turbine generators, cables) were scheduled and carried out in this window period.:::::: period.Characteristic durations, in-order to perform operations limited by a significant wave height of 2 m for a period of 48 hours, expected mean durations and 10, 50 and 90 percentiles (Mathiesen et al. (2014)) damping pool technology developed by Ideol , is designed to stabilize the floater in harsh sea conditions.Two demonstration projects have been in operation since 2018 and pre-commercial and commercial projects are expected to be launched in the coming years (Ideol).Another floater concept is the Tension Leg Platform (TLP) which is widely used in the offshore O & G industry.This concept has also been adapted from the O & G industry for the FOWTs.Tension Leg Platform Wind Turbines (TLPWTs) are well suited for intermediate water depths.The depth varies from 70 m, which is an approximate upper limit for fixed wind turbines and 200 m, beyond which the spar-type platforms are considered as the most economical option (Bachynski and Moan (2012)).

Figure 8 .
Figure 8. Important factors to be considered for the financial modelling of FOW marine operations (Judge et al. (2019))

Figure 9 .
Figure 9.Comparison of component level LCOE contribution for fixed-bottom (top) vs floating wind (bottom) farms operating for 25 years (Stehly et al. (2020)) (eg: Walkto-Work vessels), helicopters (Maples et al. (2013)) etc. for different types of floaters.Easy coupling and decoupling systems should be developed to facilitate easy attachment and detachment of FOWTs during major repairs.Advancements in robotics have paved the way for :: the usage of robots for inspection and maintenance.iFROG (Offshore-Engineer) is an amphibious robot developed by InnoTecUK, ORE Catapult, TWI Ltd. and Brunel University London, which can clean corrosion and bio-fouling from monopile surfaces.

3. 4
Health, Safety and Environment (HSE) One of the non-technical challenges that needs :::: need to be addressed, is improving the HSE conditions during the complete life-cycle ::: life ::::: cycle of floating offshore wind farms.HSE factors must be considered during all phases of the development of the wind farm, but only the installation, O & M and decommissioning phases are considered in this section.Since the floating wind technology is in infancy, there is a shortage of dedicated applicable rules and standards.Until the development of such standards, : it is advisable to follow component/process specific :::::::::::: process-specific : standards used in the fixed wind ::::::::::: bottom-fixed :::: wind :::::: turbine : industry or O & G industry (Hutton et al. (2016)).Marine operations in general , can follow the various safety standards, codes and classification society rules accordingly.

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Specialized ships for FOWT installation :::::: 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 using tugs.An exception to this are : 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.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 than ::: that can transport a fully assembled spar-type platform to the location and unload by flipping.The turbine is loaded in an almost horizontal :::::::::::::: almost-horizontal position and the barge is towed to the offshore location.The aft side of the barge is then ballasted taking the whole bargeturbine system into a vertical position.The turbine is then connected to the mooring lines and the barge is towed back to the quayside for reloading.Figure ?? explains the main stages in the installation of a wind turbine using the barge.Windflip AS claims that the vessel can operate in shallow waters which is an advantage when it comes to the installation of spar-type platforms.Installation of wind turbines using a WindFlip vessel (Maritime-Journal) 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 spartype platform.Hydrodynamic evaluation showed that the floating dock was able to reduce the platform-pitch :::: pitch :::: and ::::: heave response of the spar-type platform which aids mating of the blades.