Wave Energy Converters – What Fails and Why?

Even though Wave Energy Converters (WECs) have undergone significant developments in recent years, they have still not yet reached full commercialisation. If we are to step closer to this goal, we need to obtain more accurate information on which parts of WECs fail and why.

An abundant, predictable and reliable energy source

Wave energy is an abundant, highly predictable, and reliable source of energy. Wave Energy Converter (WEC) devices harvest this energy from ocean waves to generate green electricity. While there has been significant progress in the development of WEC technologies over the past few decades, WECs have still not reached full commercialisation.

WECs need to be able to survive the harsh conditions of the marine environment. This presents considerable challenges regarding their construction and maintenance costs as well as their life expectancy and reliability. Increasing the reliability of WEC devices and reducing the downtime (hours that a WEC is not operational) are essential performance characteristics for implementing the technology at a commercial scale. Therefore, we need to understand which parts of WECs fail and why.

Identifying and analysing information on WEC failures

The “Failure Modes, Effects and Criticality Analysis” (FMECA), e.g. MIL-STD-1629A, is one of the most widely used semi-quantitative methods for analysing failures by dividing systems into functional elements, such as subsystems, components of subsystems and parts of components.

For example, WEC subsystems include the Hydrodynamic, Power Take Off (PTO), Transmission, Control and Instrumentation, Reaction and Positioning subsystems. Components of the PTO subsystem include the heat exchanger, manifold, piston, and generators. Parts of a Generator include the cooling system, wiring looms, brushes, axle, etc.

Example of an FMECA approach. The FMECA will investigate the probability of failure occurrence, failure modes (descriptions of the way fails happen, e.g. piston breakdown), failure mechanisms (direct causes of the failure modes, e.g. vibration) and failure effects (immediate consequences of the failure on components/subsystems, e.g. economic loss), and will identify which component fails the most and which component will have a higher economic loss, and by extension, are more critical.

As per the FMECA requirements, the subsystems and components of the WEC system need to be categorised according to their functions and their failure data, which includes the failure rate, failure mode and failure mechanism.

The failure rate is the number of expected failures per year for a subsystem, component or part. The failure mode is the way that the failure happened, e.g. loss of oil pressure. The failure mechanism is the process responsible for the failure, e.g. wear and tear on a joint.

Subsystem functions in generic WECs

Despite the diversity of WECs technologies, from the functional point of view, all WECs consist of a few subsystems that work together. 

The Hydrodynamic subsystem absorbs the wave energy. The type of absorber can vary; it can be a heaving buoy, a hinged flap, an oscillating water column, a large surging body, etc. A common absorber is the oscillating body, which interacts with both the Reaction subsystem and PTO subsystem. The technology of this subsystem is proven, but due to its interaction with waves and the PTO subsystem, there are uncertainties that remain in its use in WECs.

The PTO subsystem converts the energy captured by the hydrodynamic subsystem into electricity. A wide variety of technology can act as the PTO, such as a hydraulic PTO, a direct drive train, a linear generator, an air turbine, etc. The reliability of the PTO and the choice of the PTO type and components directly affects the energy production of the machine. Therefore, the PTO is a critical subsystem for a WEC. 

The Transmission subsystem transfers electrical power into an output that is suitable for the grid. It mainly consists of transformers, switchgear, connectors and power cables.

The Control and Instrumentation subsystem is the intelligent part of the system that controls the WEC and records measurements of its behaviour. It consists of sensors (e.g., monitory sensors, alarm sensor and control sensors), data acquisition, processors, communication and data transfer, and the human interface.

The Reaction and Positioning subsystem keeps the WEC in its position relative to the seabed. It provides a reaction point for the PTO subsystem and a support for the hydrodynamic subsystem.

Relevant failure information for WECs

In order to examine failures using FMECA, failure data is required. This is a key challenge, as due to insufficient experience in real sea operations of WECs, as well as confidentiality issues, this kind of data is not available.

Instead, the available reliability data from other sources, such as offshore oil and gas reliability data (e.g. OREDA), the relevant System Reliability Toolkit and literature sources, can provide failure information for components similar to those in WECs. This information is used to identify the high-risk components of WEC subsystems. However, while this enables us to run a criticality analysis for components of a generic WEC, more targeted research on the extent of WEC reliability is required in order to make the failure identification more accurate.

An example of a relative criticality analysis for some components of a generic WEC (source: IMPACT Techno-Economic Report). Criticality is a combination of failure occurrence and failure severity and failure effect (risk and economic loss). An appropriate mitigation measure can be prioritised for the components with a higher relative criticality factor. For example, in the Hydraulic PTO subsystem above, the ‘Valve’ component is relatively more critical compared to the other components. The dominant failure mode (not shown in the figure) is seal leakage. Therefore, the design mitigation priority will be given to the ‘Valve’ component, for instance, by using a fluid with good lubricating properties and an appropriate viscosity index or by considering the use of a redundant valve component, etc.

The FMECA analysis conducted as part of the IMPACT project’s Techno-Economic and Environmental Impact Evaluation work package, tells us that:

In the case of the Hydraulic PTO subsystem, the pump/motor, valve, filter and accumulator account for 87% of the total annual failures of this subsystem. Seal or subunit leakage (e.g. lubrication) is the most critical failure mode.

In the Rack & Pinion mechanism, failure of the rail assembly component due to fatigue and foreign material in the assembly accounts for approximately 59% of the criticality number.

In Ball Screw PTO mechanisms, the screw nut assembly component accounts for more than 74% of the criticality number, and the most common failure mechanism is fatigue and foreign material in thrust bearings.

In Transmission and Control subsystems, the frequency converter and switchgear account for around 61% and 36% respectively of the overall criticality number. The most common parts to fail are the pump and motor starters. Electrical failure and faulty signal indication are the most common failure mechanisms.

Joints and connections as well as mooring lines (and rope) account for around 69% and 28% respectively of the overall potential criticality number for the Reaction subsystem (due to their high failure rate). However, they have a medium risk level (i.e. their lower repair costs balance the higher failure rate in the risk matrix). Fatigue, corrosion and marine growth are the dominant failure mechanisms that cause the material failure of these components.

A floating support structure (e.g. the mooring and anchoring system) does not have a high criticality number nor does it have a high failure rate. However, there is a medium to high risk level for this component, indicating that the repair cost or the economic consequence of the component failure is high. The most common failure mode is the loss of support, which is mostly caused by the Ultimate Limit State (ULS) failure mechanism.

WEC test rigs are needed for accurate information

It is possible to present a relatively detailed analysis of where and why failures happen using failure information from other technology sources that share the same marine environment and reliability challenges. However, this is not ideal. WEC test rigs are important for obtaining accurate information about components’ reliability. 

The test rig being developed by the IMPACT project will produce test-derived metrics to characterise key aspects, such as reliability, performance, and survivability, as well as perform a techno-economic and environmental impact assessment of WECs. IMPACT can therefore contribute vital information for the development of WECs specifically and marine renewable energy in general.

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About Mitra Kamidelivand

Mitra Kamidelivand is a research fellow in University College Cork, MaREI, a partner in the IMPACT project.