By Tim Baker, Technical Director – Marine Energy, Black & Veatch Europe
As the first multi-megawatt tidal array, MeyGen Phase 1A is a trail blazer. To further the development of tidal energy as a commercially viable source of renewable power, lessons learnt from Phase 1A were recently shared in a new assessment authored by Black & Veatch’s marine energy team.
The 6-megawatt (MW) demonstration array, comprised of four 1.5MW tidal turbines, entered its 25-year operations phase in April 2018. To-date the array, in Scotland’s Pentland Firth, has successfully exported 21 gigawatt hours to the grid, with an average turbine availability of circa 95 percent during its operational phase.
MeyGen Phase 1A was partly funded through a £10 million grant from the Department for Business Energy and Industrial Strategy (BEIS), with a requirement that lessons learnt from every aspect of the project were collated and shared. The result is the most comprehensive lifecycle assessment to-date of a megawatt-scale array in the UK.
This article shares some of the findings of overall significance to the near-term development of tidal energy. The full learning is presented in the Black & Veatch authored report.
- Integration of turbine and power train suppliers: By integrating the entire power train system with the turbine supply workpackage, correct component system integration would be ensured.
- Interventions: The majority of early stage issues that required interventions were as a result of faults in standard components, rather than novel components specific to the tidal industry.
- Pitch system: MeyGen encountered fewer technical issues with the pitch system of those turbines employing an electrical pitch system in comparison to those with a hydraulic pitch system.
- Foundation type: MeyGen Phase 1A uses gravity foundations which have three feet that each require a suitably level seabed. MeyGen has found it extremely difficult to find locations that satisfy the foundation requirements for all the feet of all the foundations. MeyGen should have given a higher weighting to this issue when deciding between the use of gravity base or monopile foundations in the early engineering stage.
- Cable stability: No suitable guidance existed for stability of cables on a fractured rock seabed under the action of significant tidal and wave loading, which meant that MeyGen had to develop a custom approach which relies on frequent cable monitoring but allowed significant cost savings. A cable stability standard drawing on MeyGen’s experience is in preparation and will be published by the British Standards Institute.
Dry-mate vs wet-mate connectors: MeyGen found that the use of dry-mate connectors significantly increased the complexity of the turbine installation and retrieval, due to the need to handle the cable tails attached to the turbine, which increased the mobilisation time required and also required standby periods between operations.
- Handling cable tails on deck, which could be under tension, also significantly increased the health and safety risk compared to the use of wet-mate connectors. Dry-mate connectors also restricted the permissible current velocity for installation operations, as the vessel remained connected to the sea bed, which limited installation windows to neap tides, making it more difficult to negotiate on vessel rates due to the lack of flexibility, exacerbated if multiple turbines required installation/retrieval.
- Vessel capability: One of the most impactful lessons learnt by MeyGen from a cost perspective is that currently available dynamic positioning vessels do not work reliably in currents stronger than 6 knots. The safe use of a Jack-Up Vessel at a high velocity tidal site was proven to be possible (this approach had previously been questioned within the industry for a number of years). MeyGen’s view is that for foundation installation a Jack-Up Vessel can be cost effective; however, MeyGen would still expect to use a dynamic positioning vessel for turbine and cable installation.
- Real-time onsite Metocean data feed: Having real-time metocean data feeds on site can be invaluable as it allows detailed operational planning.
- Marine Warranty Surveyor: The Marine Warranty Surveyors (MWS) were not initially familiar with the kind of operational procedures in often strong currents required for a tidal energy project and the successful engagement with the MWS has been a key component to the success of MeyGen Phase 1A.
- Energy-based availability: Most O&M contractors applied the same philosophy as for offshore wind in terms of attempting to maximise overall availability but in tidal energy priority should be given to maximising availability during times of peak flow (time-based vs energy-based availability).
- Turbulence: Turbulence variations across site can significantly influence performance of individual turbines if they have a narrow design envelope. Having a machine that can be remotely adapted to different environmental conditions would negate the need to decide between operating a turbine at sub optimal parameters or choosing to mobilise an unplanned intervention.
- Performance estimates: MeyGen attempted to conduct the power performance assessment according to the International Electrotechnical Commission Technical Specification (IEC TS) on Tidal Power Performance Assessment, but in practice encountered a number of issues which made this difficult. In particular, the site not meeting the required sea bed slope parameters for instrument deployment, and the number of instruments stipulated by the IEC TS was cost prohibitive. The MeyGen experience is feeding into the second edition of this IECT TS.
- Health & Safety: The onshore and offshore works had different lead parties responsible for ensuring the Construction, Design and Management regulations (CDM) were met. This introduced a lot of interface complexity which could be greatly simplified if only one party was responsible for both.
- Stakeholder engagement: There have been no local stakeholder complaints or incidents throughout the construction and early operations phases of the project. Due to the novel nature of the project potential stakeholder impacts had been identified as a possible area of concern during the consenting process.
To read the report’s key findings, click here to download the Executive Summary. The full lessons learnt throughout the design, construction and initial operations phases of MeyGen Phase 1A are available here.