By Steph Barker, Assistant Editor, Pump Industry Magazine

As the focus on renewable energy production continues to increase around the globe, offshore wind farms have become a key part of many countries’ strategies to produce clean, green energy. Here we explore how smart pump technology could play a vital role in this industry, and further our country’s transition to renewables.

Offshore wind farms offer many of the same benefits as land-based wind farms – they provide renewable energy, do not consume water and do not emit any environmental pollutants or greenhouse gases. However, offshore wind farms also offer many benefits that traditional land-based wind farms do not.

One such benefit is that wind speeds at sea tend to be significantly faster than those on land – even relatively small increases in wind speed yield large increases in energy production, with a 24km/h wind generating twice as much energy as a 12km/h wind. Offshore wind speeds also tend to remain more steady than onshore wind speeds, enabling steady and stable generation of energy.

Wind speeds on land tend to decrease into the late afternoon and early evening, which are peak times for domestic energy use. This is not the case with offshore winds, which tend to remain steady through the evening hours.

Another benefit of offshore turbines is that many countries tend to have high populations in coastal areas – Australia is a prime example of this, with more than 85 per cent of the population living within 50km of a coast. Land-based wind farms require large swathes of empty land to install and run turbines, which often means they need to be installed away from urban centres. Offshore farms can be installed at sea closer to densely populated coastal areas, enabling smooth power delivery to domestic dwellings.

Wind in the works

According to RenewEconomy, there are currently 26 offshore wind farm projects proposed across Australia, with the majority concentrated in New South Wales, Victoria and Western Australia. The process of establishing an offshore wind farm is long and complex: although every effort is made to minimise environmental impact, little is known about the long-term effects of offshore wind farms on marine flora and fauna – so ensuring strict compliance with environmental guidelines is crucial.

Projects must go through multiple feasibility, evaluation and compliance stages before beginning construction. A new legal framework has further enabled development within the sector, with the Offshore Electricity Infrastructure Act being introduced in June 2022.

Thankfully, Australia can look to other countries in the world – primarily the United Kingdom, which is currently the second-largest producer of offshore wind behind China – for guidance and inspiration.

As of October 2022, operational offshore wind farms in the UK totaled 2,595 turbines with a combined capacity of 13,628MW. The cost of offshore wind has historically been higher than that of onshore wind, however costs have decreased significantly and offshore wind power in Europe became price-competitive with conventional power sources in 2017.

Almost all offshore turbines currently installed around the globe are fixed foundation structures. Fixed foundation turbines are considered to be the most viable type for areas with water depths of less than 50m and average wind speeds higher than 7m/s. Floating turbines are considered technically viable with water depths from 50 to 1,000m. There are currently only three floating turbines deployed around the world.

Pumps playing their part

Pumps and pumping applications are used at many stages of offshore turbine installation, connection and maintenance. Pumps are especially vital in the installation and securing process for offshore turbines.

Offshore turbines require different types of bases for stability, according to the depth of water. Types of underwater structures used to secure turbines at sea include monopile, tripod, and jacketed, with various foundations at the sea floor including monopile or multiple piles, gravity base, and caissons.

Fixed foundation turbines are generally installed using pile drivers to ensure that the foundation can be placed deeply and securely into the sea bed. The typical construction process for a monopile foundation in sand involves driving a large hollow steel pile roughly 4m in diameter and approximately 25m deep into the seabed and through a 0.5m layer of larger stone and gravel in order to minimise erosion around the pile.

A transition piece is then attached to the driven pile, which generally includes features such as boat-landing equipment, cathodic protection, cable ducts for underwater cables and more. Sand and water are then pumped from the centre of the pile, to later be removed and replaced with concrete. An additional layer of even larger stone, up to 0.5m diameter, is applied to the surface of the seabed for longer-term erosion protection.

Pumps used in this process are generally kinetic hydrodynamic diesel pumps and/or hydraulic pumps. Suction caissons, or suction anchors, are another popular choice for offshore turbine anchorage. Suction caissons are a type of fixed platform anchor, featuring an open bottomed tube embedded in the sediment and sealed at the top while in use so that lifting forces generate a pressure differential that holds the caisson down.

Attachment to the sea bed is achieved either through pushing or by creating a negative pressure inside the caisson skirt by pumping water out of the caisson; both techniques have the effect of securing the caisson into the sea bed. The foundation can also be rapidly removed by reversing the installation process, pumping water into the caisson to create an overpressure.

Hyping hydraulics

Wind turbines vary greatly in the ways in which they are controlled and powered. Every turbine requires some sort of internal mechanism to convert wind power into energy, as well as mechanics to transmit that energy to a generator. In order to take most advantage of the wind generated offshore, turbines need to be large enough to keep up with wind speeds that are much higher than those on land.

According to the European Academy of Wind Energy (EAWE), the drivetrain of horizontal-axis wind turbines (HAWTs) generally consists of a rotor–gearbox generator configuration in the nacelle, which enables each wind turbine to produce and deliver electrical energy independent of other wind turbines. While the HAWT is a proven concept, the turbine rotation speed decreases asymptotically and torque increases exponentially with increasing blade length and power ratings.

EAWE has presented a business case for hydraulic pump transmissions, or drivetrains, for offshore turbines. In an effort to reduce turbine weight, maintenance frequency, component complexity and thus the levelised cost of energy (LCOE) for offshore wind, hydraulic drivetrain concepts have been considered from as early as 1967.

Traditionally, these concepts used oil as the hydraulic medium because of its fluidity and availability – but this presented restrictions, as a drivetrain using an oil pump would need to operate in a closed circuit in order to minimise the need for oil refreshment and avoid environmental impacts.

To overcome this, the TU Delft Wind Energy Institute (DUWIND) developed the Delft Offshore Turbine (DOT). The DOT is a hydraulic wind turbine concept that replaces conventional drivetrain components with a single seawater pump. The single water pump is powered by the turbine rotor and effectively replaces high-maintenance components in the nacelle, which in turn reduces the weight, support structure requirements and turbine maintenance frequency.

Maintenance of nacelle components can be one of the most challenging aspects of offshore wind production. Turbine maintenance often requires manual work, which can be dangerous and costly. DUWIND have also made the case for a prototype pump-powered turbine using off-the-shelf components – the DOT500. When the concept was initially developed in 2018, a low-speed high-torque seawater pump, required for the ideal DOT concept, was not commercially available.

Instead, the study used a Vestas V44 600kW turbine and retrofitted its drivetrain into a 500kW hydraulic configuration. The original Vestas V44 turbine is equipped with a conventional drivetrain consisting of the main bearing, a gearbox and a 600kW three-phase asynchronous generator. The blades are pitched collectively by means of a hydraulic cylinder driven by a HPU (hydraulic power unit) with a safety pressure accumulator.

The DOT500 became a fully functional hydraulic wind turbine, with fully automated and safe operation. After retrofitting the V44 drive train, a 32 per cent nacelle mass reduction was achieved. The novel spear valve torque control technology enables active regulation of the rotor speed. The total power transmission efficiency was predictably low, as a result of the double hydraulic circuit.

As a result of these tests, DUWIND is currently in development with the DOT3000 Power Train System (DOT3000 PTS) – a seawater-hydraulic drive train. The drive train, consisting of a high-torque, low-RPM, multi-MW pump, as well as a hydraulic support system, is an essential part of the innovative hydraulic multi-MW class wind turbine, which is being developed by Delft Offshore Turbine (DOT) company. The project is likely to be completed in December 2023.

Cutting-edge controllers

Another advancement in the renewable energy industry is the proliferation of smart technology. Opportunities for automation, as well as secure communication through digitalisation, are becoming more and more achievable. Offshore turbines especially benefit from automation – turbines need to be in the correct position to take most advantage of wind, and automation of positioning allows for turbines to be maximised with minimal interference or manual input.

While smart technology is already fairly commonly used in smaller-scale, onshore wind energy, offshore farms present new challenges.

Firstly, turbines are unable to connect to traditional infrastructure as this doesn’t extend to deep sea ranges. There is also the challenge of ensuring electrical components are protected from the elements, as turbines can face some hefty winds and seas. Thirdly, controllers must allow for multiple components to work together to optimise turbine performance.

A wind turbine controller consists of a number of computers that continuously monitor the condition of the wind turbine and collect statistics on its operation. As the name implies, the controller also controls a large number of switches, hydraulic pumps, valves, and motors within the wind turbine. Communication between the controller and the operator has traditionally been conducted via phone, radio or fibre optics – but these types of communication can be unreliable and do not provide constant, up-to-date information on turbine performance.

Many companies are now installing wireless infrastructure as part of their turbine components, which enables turbines to be powered and controlled as part of the industrial Internet of Things (IoT). Self-optimisation can be programmed into the turbine controller, enabling the controller to assess measurement data from the turbine in order to maximise performance.

Wind direction and speed can be interpreted by the controller, which can then direct turbine movement to optimal positioning by communicating with the pumps and valves that control movement. Companies have developed such technology – using industrial wireless LAN (IWLAN), ethernet switches – to enable diagnostic functions for evaluation devices within a turbine. IWLAN in the nacelle allows network access via mobile communications.

Another big benefit of using smart controllers in turbines is the ability to instigate predictive maintenance. Predictive maintenance is a technique that uses data analysis tools and techniques to detect anomalies in operations, as well as possible defects in equipment and processes, so that they can be fixed before resulting in operational failure.

Predictive maintenance of pumps is already common in many industries as pump units are frequently exposed to bearing failures, water/oil leaks and electrical faults. Machine learning can be used to assess component data and devise a predictive maintenance schedule. In the case of wind turbines, which use hydraulic and potentially seawater-powered pumps, predictive maintenance is an incredibly valuable tool in reducing reactive maintenance which results in higher downtime, higher repair cost and high safety risks.

Key takeaways

When considering the options available for offshore wind turbine components, a seawater-hydraulic drive train coupled with a wireless communication or IWLAN controller seems to present an ideal solution. The opportunity for turbine automation, predictive maintenance and wireless control, combined with the reduced LCOE and maintenance frequency of a hydraulic seawater pump, takes offshore wind turbines from a cumbersome, difficult prospect to an innovative, easy-to-maintain source of renewable energy. While we are yet to see Australia’s offshore wind industry fully take shape, it may well present an exciting opportunity for the pump industry.

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