The Concept Of Offshore Wind Energy Engineering Essay

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The offshore wind energy is an attractive renewable energy source and this report intends to highlight the challenges in offshore wind energy and the possible breakthroughs. Key challenges are divided into technical, economical, environmental, policy and other aspects.

The technical challenges focus into wind resource estimation, wind turbine designs, wind farm design and layout, support structure and electrical system and the Research & Development efforts carried out. The economical challenges are the high investment cost and possible cost reduction recommendations are discussed. Note that the technical and economical challenges are interrelated where solving some of the technical issues will eventually reduce the cost.

The environmental challenges are mainly to seabirds and the underwater sea creatures. Here the importance of proper environmental assessment is emphasized. Policy challenges are mainly the licensing procedures and perhaps the implementation of the new act for Round 3 projects will see a more efficient flow of the consenting process. Lastly the public opposition issues are briefly discussed.

1. Introduction to Wind Farm

Figure 1: Offshore Wind Warm (4)

Basically offshore wind farm comprises of the followings (4);

(1) Piles driven into the seabed as support structure for the wind turbine.

(2) Blades aerodynamically designed to rotate when wind blows.

(3) Nacelle consists of shaft, gear box, brake and generator that generate electricity.

(4) Subsea transmission cable that transmits electricity from turbines to offshore transformer.

(5) Offshore transformer converts electricity to higher voltage.

(6) Substation at shore.

2. Offshore Wind Energy Technical Challenges

The main technical challenges in offshore wind energy are the followings (3);

Wind resources estimation

Wind turbines technology

External design conditions

Characteristics of prospective sites

Wind farm design and layout

Wind farm operation and maintenance

Some of the critical issues are discussed briefly in the next sub-sections.

2.1 Wind Resource Estimation

The wind energy depends largely on the wind resource to generate power and the offshore wind speed depends on the following factors (5);

Changeable properties of the boundary layer at the coastal area that can results in varying wind speeds, boundary layer profiles and turbulence.

Air flows with different temperatures and origins may be slow to mix and results in unusual boundary layer profiles.

Changes in tide will move the wind farm location in the boundary layer and may impact the mean wind speed.

Day and night temperature effect creates wind from and to offshore respectively. The strength and direction of wind to offshore depends on the existing high-level gradient wind and in some cases the gradient wind can be cancelled out by the sea breeze resulting in no wind.

Backing and veering wind effects at coastal area.

Hence, much emphasize is put into identifying the suitable wind resource for offshore wind and ensure optimum gain for the wind. Several methods can be used for wind analysis such as (5);

WAsP (wind atlas analysis and application program) in combination with the coastal discontinuity model (CDM). However, the WAsP has prediction errors ranging from 0% to 6% (for 1-3km distance) and not suitable for estimating wind speeds at near-coastal mountainous sites (6).

Existing offshore measurements.

Onsite analysis using Measure correlate predict (MCP) methods from a mast offshore to an onshore reference station can be used

2.2 Wind Turbine Designs

Key challenges to offshore wind turbines designs can be summarised as below (5);

To increasing reliability of the turbines and minimising maintenance.

To increase access to the wind turbines for fast recuperations without requiring additional support (i.e. cranes), on the spot repair or maintenance and easy access to components

Hence some of the critical design considerations are the nacelle design, location of turbine (to be inside the nacelle or base of the tower), weight of tower top, internal crane or winch for maintenance purposes, access to the turbines (i.e. helicopter), access frequency, lightning resistance blades and maintenance frequency (5). Larger capacity wind turbines are expected in the future to gain more energy from the wind.

2.3 Wind Farm Design and Layout

Generally the offshore wind farm design involves the following steps before being approved for construction (5);

data-gathering;

preliminary design and feasibility study;

site investigation;

concept development and selection;

value engineering;

specification; and

detailed design.

The significant decisions in the design phase are probably the followings (5);

Site selection taking into consideration of the feasibility, economics, consenting process, grid connection and others.

Selection of suitable wind turbine i.e. model and capacity.

Optimum layout design taking into account technical feasibility, cost (i.e. electrical system, installation) and energy production as well as seascape impact. Major layout designs are as shown in Figure 2 (7).

In general the offshore wind farm and layout design process is as detailed in Figure 3.

Figure 2: Common Offshore Wind Farm Layout (7)

Figure 3: Layout Design Process (5)

2.4 Support Structure

The most common support structure is monopile which is a steel tube in 2.5-4.5m diameter piled 10-20m into the seabed. Monopiles installation does not require seabed preparation but not suitable for seabed with hard rocks. Several other support structure designs are as shown in Figure 4 (8).

Figure 4: The Offshore Turbine Foundations (8)

(a) Gravity based; (b) monopole foundation; (c) suction bucket foundation; (d) tripod foundation; (e) tripod suction bucket foundation.

The selection of support structure depends largely on the seabed conditions and water depth. A simplified guidance for selection of support structure is as in Table 1 (8).

Table 1: The Selection of Offshore Wind Turbine Foundation (8)

Water depth (m) Types of Foundation

0 10 Gravity foundation

0 30 Pile foundation

> 20 Tripod / Jacket

> 50 Floating platform

Challenges for the support structure installation are assumed as the followings;

Transport difficulties and high cost especially for large/long and heavy structures.

Expensive and large vessels and cranes need to be hired. Any delays may incur additional costs in vessels and cranes lease.

Sea conditions need to be suitable for the installation process, i.e. benign wave and wind. This also restricts the time envelope for installation and in seasonal countries delays in schedule is intolerable.

Environmental effects to the seabed and sea animals during piling.

Dropped object and personnel safety risks.

Typical support structure is not suitable for deep water wind farm.

Hence the floating support structures may be beneficial in terms of greater choice of sites and countries for deep water wind farm; more concepts of floating structures to choose from; more flexible construction and installation procedures; and easier decommissioning process (10). The floating structures are designed to be towed to its site with the wind turbines already installed to reduce amount of work on site (11).

The examples of floating structures are presented in Figure X and the proposed types based on water depths are as followings (10, 11);

TLP structure: 50m 300m

Ballast or Spar structure: 150m 500m

Hydrostatic structure: >50m

On the other hand, the floating structures face challenges such as minimisation of turbine motions induced by waves; complexity in designs; construction, installation, operating and maintenance procedure different for previous experiences; and change in design of the electrical infrastructures i.e. flexible cable and may incur additional cost (11). The floating structures require well proven technology for the mooring cables and piles underneath to prevent breakage of cables or topple of the turbine (10).

At present, prototypes for floating offshore wind turbines are tested by Norway and Italy (Blue H group) (10).

Figure 4: The Types of Floating Foundations (11)

2.5 Electrical System

An offshore wind farm electrical system consists of six key elements (5):

wind turbine generators;

offshore inter-turbine cables (electrical collection system);

offshore substation (if present);

transmission cables to shore;

onshore substation (and onshore cables); and

connection to the grid.

The electrical systems for offshore wind farms face the following challenges (5);

Threats to cables due to fishing and anchoring activities and burying the cables cause additional cost.

Technical constraints in connecting to the local network i.e different voltage and strength

Cost of the cable connection to shore translated from distance from shore.

Remote locations of offshore wind farms with limited transmission capacity to other parts of the electrical grid, where electricity is consumed.

Storage of produced electricity.

However in the future, the development of electrical equipment such as switchgear, transformers and reactive power compensation is expected to enhance the electricity supply. Subsea transmission cables specifically for offshore wind farms may be developed at a more feasible cost. Besides new technology such as high voltage DC transmission may be appropriate for wind farms located far from shore. Major reshaping of transmission networks is necessary in several countries to accommodate the upcoming new offshore wind farms. Moreover interconnection between countries are expected to increase to assure supply security or providing international offshore transmission network dedicated to some offshore wind farms (5, 11).

2.6 Research & Development

Areas of research and development that may further promote offshore wind energy summarised as the followings (7);

Wind conditions efficient and accurate sitting of wind turbines; further understanding of wake effect; understanding of offshore environment for optimisation of wind energy; and short-term forecasting of wind prediction and electricity grid management.

Wind turbines advanced aerofoils designs and light weight blades from carbon fibre-reinforced plastic (CFRP); Faster rotational speed and two bladed wind turbines; efficient control system; and better operation and maintenance strategies.

Integration improve wind farms plant capabilities to fulfil grid code requirements and possible grid code harmonisation.

Offshore deployment and operations structural designs of wind turbines to extend lifetime and reduce cost; more efficient assembly, installation and decommissioning of wind turbines; manufacturing and installation of electricity infrastructure; larger and more efficient turbines; and optimisation of operations and maintenance costs and procedures.

3. Offshore Wind Energy Economical Challenges

The wind energy economics are determined by the following factors (12);

Investment cost or capital cost

Operation and maintenance cost

Electricity production

Turbine lifetime

Incentives/discount rate

The two major factors are the investment cost and electricity production (12). Offshore wind farms capital investments are generally 50% more expensive than onshore wind farms. The wind turbines and foundations cost are 20% and 2.5 times more expensive than onshore wind farms. Table 2 below shows the breakdown of offshore wind farms capital investment cost (1).

Table 2: Average Investment Cost per MW (1)

However these costs vary on project basis and strongly influenced by factors outlined below (11);

Cost of connecting cable from offshore to shore increases with distance from the shore.

Cost of foundations increases with sea depth in addition to higher installation cost in the ocean.

Costs of offshore turbines are higher than onshore turbines due to higher protection requirements against the marine corrosive environment.

Costs of operation and maintenance are higher for accesses to turbines are more difficult especially during bad weather.

Nevertheless offshore wind farms are still favourable as the total electricity production is much higher than onshore wind turbines. Wind availability and quality is much better offshore than onshore. For an onshore wind turbine the utilisation time is about 2,000 to 2,500 full loads per year but an offshore turbine may reach up to 4,000 full load hours per year (10). Electricity production also depends on site selection that can give most of the invested cost (12).

In order to make offshore wind energy more economically practicable, manufacturers could increase their efficiency and produce items in large quantities to benefit from increased production. Ongoing R&D is could further improve the technological aspects of wind turbine to reduce the capital cost (11). The majority of turbine cost is due to high cost of steel and perhaps alternative lighter material could be used in addition to better fatigue resistant and reliability. This would reduce dependence on steel, improve turbine designs and eventually save cost (13). Moreover the sizes of offshore wind farms could be larger and accommodate higher number of turbines to benefit from the high cost of cable connection to shore besides latest designs of larger turbines. Reduction in infrastructure cost is also expected to facilitate growth in offshore wind energy (11).

A typical offshore wind farm development would require an investment cost of 2.0 to 2.2 million/MW and after much learning from previous projects offshore wind farm investment cost is expected to decrease by 15% in 2015 to 1.8 million/MW (1). In another study based on costs in year 2006, the cost is expected to rise from 2.37 to 2.6 million/MW in 2011 and fall by 20% of the total cost by 2020 (13). In conclusion the cost of offshore wind energy is expected to decrease over years.

4. Offshore Wind Energy Environmental Challenges

Offshore wind farms could potentially affects the ecological systems of underwater sea creatures and seabirds. Potential affects and effects to the underwater sea creatures are as listed below;

Noise during installation of wind farms could drive away sensitive sea creatures from their habitats or potentially impair hearings or cause injuries or death (14). This could affect certain fish species populations, variety of fish in an area and damage to fish eggs during physical interruption of seabed (15).

Electromagnetic fields around submerged cable results in disruption of orientation for migratory marine species such as eel and salmon; and impediment of foraging activities for shark and ray for instance (14, 15).

Sediment plumes and heating of the seabed by electric power transmission pose danger to seabed creatures and benthic communities (14). Fine sediments from the seabed arise during installation of cables and reduce fish visibility and clogging of gills. Disturbance of the seabed also stirs other pollutants in the sea (15).

Possible contamination from chemical or hydrocarbon releases during installation and maintenance activities (15).

On the other hand, wind farms could benefit as fish habitat where they act as artificial reefs and provide protection over fishing activities (15).

Wind farms could affect the seabirds in coastal areas and birds migrations mainly due to collision with the rotating blades. Birds migration patterns could be changed as well when birds are distracted, disoriented or obstructed by wind farms although this is not scientifically proven. A study of birds movement in Blyth offshore wind farm found that there are 5000 birds movements daily and only 31 deaths were recorded over 3 years (15). This shows that the birds collision risk is relatively low.

Hence thorough Environmental Impact Assessment (EIA) for proposed offshore wind farms is essential in accordance with Guidance Note for Environmental Impact Assessment (16) for UK requirements.

An EIA can be performed by visual surveys or radar studies. Visual surveys include boat-based surveys, aerial surveys or land based surveys. In the UK, most EIA are from boat-based and land-based visual surveys (17). Brookes (17) in her research noted that a complete guidance on methods used in these surveys can be found in European Seabirds at Sea protocols. The research also concluded that impact of offshore wind farms installation to seabirds are difficult to assess against background environmental variation as the environmental variations also contributes to cumulative impacts on the seabirds. Number of seabirds at a location is not the best indicator of impacts to seabirds and it is more necessary to study the birds behaviour to conclude if offshore wind farms affect the seabirds (17).

Thus execution of EIA should be carried out with proper tools, methods and expertise to ensure all aspects have been taken into considerations. New technologies to minimise impact on the environment should be explored for example the acoustic mitigation devices (AMDs) to deter and keep away marine mammals from loud noises at pile driving areas (18).

5. Offshore Wind Energy Policy Challenges

All offshore wind farms should be licensed and most of the round 1 and round 2 projects were based on 1989 Electricity Act which was later amended to Energy Act 2004 (19). However the Scottish government has a different guideline for the consenting process.

However the legal framework for offshore wind farm consents has implications on project development where significant delays are seen on the Round 2 projects. The figure below shows the government target of offshore wind farm projects versus the actual projects (19).

Figure 5: Cumulative Offshore Wind Generating Capacities (19)

It is argued that the delays are caused by the following challenges (19);

Environmental assessment insufficient information and additional survey and supplementary information.

Lengthy consent process lack of planning or schedule, ineffective communications between government bodies, personnel turnover and insufficient resources.

Other arising issues discovered during the consent process impact of the offshore wind farm project to other sea users, aviation radar and helicopter movement.

Delays in the project consents result increase in project risk profile and economic viability; rescheduling resources and milestones impact relationships between different parties; and time limitations on next consequent processes such as construction, installation and grid connection. However delays in project are not only due to the consent processes; some developers may choose to hold a project for various reasons, investors may wait for the suitable economic climate or deficient in supply chain (20).

The Round 3 projects will be based on the Planning Act 2008 and Marine and Coastal Access Bill which is more efficient where all consent processes will be managed and consented by a single body within a defined time frame. The new guideline requires environmental assessment to be completed prior to submission of an application (19). This may reduce resubmissions and unexpected additional supporting documents or surveys.

Even so, all procedures must be made clear and the authorities must ensure that the Infrastructure Planning Commission (IPC), the Marine Management Organisation (MMO) and statutory consultees have enough resources to manage the overwhelming number of applications. It is also important to clarify role of local authorities in the consent processes (19).

Other than that, obtaining consent for grid connection possibly results in lead time.

6. Offshore Wind Energy Other Challenges

An offshore wind farm project could be as controversial as an onshore wind farm project and public opposition to an offshore project is influenced by the following factors (20);

Strong emotional tie to the place.

Wind farms nearby or at places of recreation by local residents and visitors.

Perception that industrial scale of wind farms will threaten the natural beauty of the place.

Lack of efforts by developer to encourage local support by explaining the role of wind energy in addressing climate change issues.

Lack of trust in project developer.

Opposition group is more active and influential.

Hence offshore wind farm planning should take into consideration the above factors to gain support from public.

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