By Will Brindley, Project Engineer at ORE Catapult
For the majority of offshore wind turbines in the UK, operations and maintenance is carried out by fossil fuel-powered vessels. As both a producer and user of clean fuels, the offshore wind industry can act as a first mover in the deployment of zero-emissions vessels, and as a springboard for broader maritime decarbonisation.
One avenue that ORE Catapult is looking into is the ability to charge crew transfer vessels (CTVs) offshore. Providing a charging capability offshore would boost investment in electric vessels, which would be able to operate in the field for longer periods, and significantly reduce operations and maintenance costs. Decarbonising the offshore fleet is vital if the UK is to meet its net-zero targets by 2050.
Earlier this year, I was part of the ORE Catapult project team that supported MJR Power and Automation on the design, build and testing of a first-of-its-kind system to charge offshore wind turbine electric Crew Transfer Vessels (eCTVs). The eCTV’s on-board battery is plugged into a charging cable, which is deployed from the offshore wind platform. The vessel then taps into the renewable power generated by the offshore turbines, providing a system that is low cost, easily maintained and operated, and highly scalable.
The in-air charging cable solution was selected for the project (pictured below), which places the bulk of the battery charging infrastructure on the transition piece deck of an offshore wind turbine, out of the waves, and in an accessible location for maintenance. The eCTV can then be charged from the offshore wind turbine power grid between crew transfers. To keep the operation as simple and efficient as possible, vessel mooring capability is integrated within the power transfer cable.
This concept integrates the critical system components within existing structural and electrical power infrastructure. This offers substantially lower system deployment and maintenance costs over a standalone system. The technology can be scaled up to be deployed on any fixed-bottom turbine where CTVs are used for maintenance.
Although developed for offshore wind industry crew transfer vessels, once proven the technology can be applied to other scenarios such as larger service operation vessels (SOVs), oil and gas, fishing and maritime craft.
The most technically challenging aspect of the project was to ensure that the charging cable could be safely connected, used as a mooring line for the charging period, and then safely disconnected.
The designed cable charging system used for the eCTV is pictured below, but how does it work in practice?
Figure 2: Charging system setup in the MJR workshop
The charging cable contains both the electrical power conductors and tension members to provide both power transfer and mooring functions. It is terminated into a bespoke connector that is used for both electrically and mechanically connecting the turbine to the vessel.
The connector is lowered from the turbine under remote control and docked into the vessel receiver bellmouth. The connection and disconnection is designed to be done without crew having to handle the cable on deck. The boat then pulls away from the turbine with the cable reeler paying out automatically. Once the vessel has reached the mooring and charging position, power can be fed to the vessel. The design also includes an emergency release and a motion compensation system.
The demonstration was designed around a 250kW charging converter and battery, which can fully charge in an hour. As the system is scaled towards the necessary higher battery capacities of around 2MWh for a practical vessel, the charging system can be scaled accordingly.
In order to understand in what wave conditions the charging system can be safely operated, an OrcaFlex software simulation model was developed by ORE Catapult.
It is very difficult to accurately predict the response of such vessels without physical test data. The hydrodynamics are just too complex. To resolve this issue, a full-scale trial was performed with a CTV moored to a turbine. The measured vessel response in the trial was compared with the simulated response to allow us to refine and build confidence in the software model.
The refined model (shown below) was then applied to simulate and optimise the design of the full charging system. The simulations were also used to demonstrate that the vessel could safely connect to the charging system in a reasonable range of weather conditions. This was done by running a whole year’s worth of measured weather data in the simulation. The integrity of the cable, charging system, vessel and wind turbine structure were then checked against a maximum allowable safe design load.
Video: OrcaFlex simulation model with simplified spring model of cable compensation system
Whilst the system is undergoing final stages of testing with offshore deployment expected imminently, further refinements are planned for wider deployment to increase the flexibility and capability of the system.
SOVs are capable of operating further offshore and in harsher sea conditions and an offshore charging solution for these vessels is needed to allow a fully electric fleet to service all wind farms. The principal technical challenge in providing this solution relates to the size and energy requirements of SOVs which are larger than CTVs.
MJR was recently awarded funding from the Clean Maritime Demonstration Competition Round 2 (CMDC2), to accelerate development and prototyping of the system to allow SOVs and other vessels to connect to offshore wind turbines at sea.
To find out more about this project and our work within the clean maritime industry, get in touch today!
The on-turbine electrical vessel charging project was led by MJR Power and Automation. Vessel operator Tidal Transit, vessel designer Artemis technologies, wind farm operator Xceco, and the Offshore Renewable Energy (ORE) Catapult were project partners.
The project is part of the Clean Maritime Demonstration Competition (CMDC), a program funded by the UK Department for Transport and delivered in partnership with Innovate UK to support the research and development of zero emission technology for the maritime sector.
A video of the operation of the system is linked below:
A full detailed report on the content of this post is available by email request to: paul.cairns@mjrpower.com