Q&A: Tidal engineering

Materials World magazine
1 Feb 2018

Mike Unsworth is the director of engineering and construction at Tidal Lagoon Power. Here, the mechanical engineer tells Ines Nastali what materials will be used to build the planned tidal power lagoons in Wales and explains how to maintain them.

What elements does the Swansea Bay tidal lagoon plant contain? And what materials will you use to build it?

This project is our pathfinder, it will be the first of a programme of tidal lagoon projects in the UK and overseas. We secured planning permission (a Development Consent Order) for the Swansea project in June 2015. It is a 320MW tidal lagoon project which will cost approximately £1.3bln.

It consists of a lagoon or sea wall, which is connected to land at two points, and comes out from land and creates a U-shape into Swansea Bay. The lagoon wall is 9.5km long and is primarily a typical breakwater or sea defence wall. The wall itself is built using a dense sand and gravel core, which prevents seepage of the water through the wall. The dense sand core is protected on either side of the wall using rock armour. We’ll use around 3m tonnes of rock armour, which protects the sea wall from any wave action or storm events. The rock is a granite material quarried in the UK and overseas. 

Within this wall, we have a power house, which contains 16 low head hydro bulb turbines and eight sluice gates. The actual structure is made of steel reinforced marine grade concrete to manage any corrosion of structures sat in a salt water environment supplemented with an active impressed current cathodic protection system. Energy is generated four times a day on the rise and fall of the tides, which are entirely predictable.  

What other components does the plant consist of?

The other components include induction generators, one per turbine. We have to convert the energy to generate electricity. They comprise copper windings within the rotor and the stator, while the core material is laminated low-loss non-electrical steel. We also have electrical switch gear, which is manufactured from the combination of painted or unpainted steel.  

There are four power transformers – the generators operate at a voltage of 9.9kV – and we transform that up to 275kV before it connects into the National Grid. These again, use low-loss non-oriented steel for the core, copper for the windings and a paper wrap and mineral oil insulation and the transformer tank is manufactured from steel. Additionally, the power house contains a combination of various auxiliary systems like accumulators, compressors, pipe work, pumps, controllers and building services. These are manufactured from a range of materials including galvanized steel and aluminium. 

The sluice gates will be made from structural carbon and stainless steel and they will be protected with a cathodic protection system. To enable the de-watering of each turbine for maintenance purposes, we have steel stop logs, with a sacrificial anode cathodic protection system, located at either end of the turbine’s hydraulic passage. They are the main elements that form the tidal lagoon, all of which generally employ standard materials and there are no specific precautions that we take into account, except when they come into contact with seawater.  

What are the turbines made of?

There are a number of components that are submersed in seawater during normal operation, including, for example the runner cone, runner blades, discharge ring, the turbine housing and draft tube.

As the client, we didn’t specify the exact materials, instead we specified the functional and performance requirements we need and allowed the manufacturer to specify the material. Consideration for the choice of the submerged components included ability to prevent corrosion and minimise marine growth, the machinability, procurement availability, weldability and tensile strength. These parts are made of the same steels as mentioned before and some of them are specialist alloys. Those materials have a natural protection against the electrical chemical process of corrosion, but we also incorporate paintings, coatings and cathodic protection.

Where were the materials sourced and manufactured? 

The joint venture of Andritz Hydro and GE, as turbine and generator manufacturers, choose the supplier, but we have mandated that 65% of the cost of the project must be spent within the UK. Both companies have been working within the UK supply chain looking at which manufacturers are already operating in the marine or renewables sector and can provide components for the tidal lagoon projects or alternatively, machine shops, forgers and fabricators, who might not currently operate in the marine sector but have the capabilities to transition into it.

We built up a database of over 1,000 companies that have an interest in supplying components. We then had awareness meetings so that the suppliers understand who their buyer will be and what expectations we have as client. These companies were then married together with the Andritz Hydro/GE joint venture. 

The lower tier suppliers are developing detailed designs and material specification in readiness for the project proceeding. The vast majority of the turbine and generator components have been mapped to a UK based supply chain.

What about turbine maintenance?

We don’t actually need to build a cofferdam to de-water the turbines. That would be cost prohibitive. To be honest, it’s very simple. As I described earlier, we insert stop locks and then we pump the water out of the hydraulic passage to create a dry and safe working environment to allow for access into the passage to maintain the components that come into contact with seawater. You can also access the generator without going into the hydraulic passage as it sits within the turbine bulb, and some maintenance can be done without having to de-water the chamber. This is common practice on operational hydropower plants across the world. 

With this Swansea Bay pathfinder and future tidal lagoon projects within the UK and overseas, we have an opportunity to create a new UK based industry with the UK at the forefront as a world leader. Following the Swansea Bay project, we plan to develop the first full-scale tidal lagoon in Cardiff with an installed capacity of circa 3,200MW, ten times the size of the Swansea Bay project. The Cardiff project would consist of up to 108 turbines and 24 sluice gates and cost approximately £8bln. Delivery of a programme of tidal lagoons in the UK will realise significant cost savings, will create up to 70,000 UK jobs as well as research and development opportunities. 

With this and the other projects, we can build a new industry, as the next step will be planning a full-scale lagoon in Cardiff.