Aberthaw Tidal Power Station

2021 Outline Design by Edward Grist

(11) Barrage Security

Contents
  1. Security Areas
  2. Malicious Access
  3. Ship collisions – Barrage & Pier
  4. Barrage Buffer Ponds
  5. Pier Lagoons
  1. Security Areas

  2. Security experts need to identify and consider from the outset the special circumstances relating to defence of this huge power generation asset in its marine environment. Aberthaw Tidal Power Station security measures described here are in sufficient detail to provide a basis for their validation (or otherwise) later and for establishing a realistic estimate of the associated cost.

    The three security level areas 'A', 'B' and 'C'
    The system described caters for the worst. A risk assessment identifies that the most serious cause is an organised malicious attack. Damage is inevitable. The barrage design limits this damage to a maximum 20% of the total tidal electricity generating capacity in a way that permits it to be restored within 180 days.
    Three security areas A, B and C are shown. Area C is deliberately made accessible to the public from the water. This responds to the needs of persons in distress in the estuary particularly those who have entered the water accidentally (e.g. from a capsized sailing craft or a 'man overboard'). Ladders are fixed at intervals along the side of the barrage to enable those who can to climb out.
    The ladders give ready access to a roadway running the length of the 'sea wall'. When safely off the water those in distress follow a well signed route to the nearest First Aid Shelter. Facilities are provided for contacting barrage security personnel. These shelters are well illuminated at night. First Aid facilities can be administered in the shelter whilst awaiting professional services.
    Self-evident is the potential for misuse that access to Area C provides. At first sight it presents an opportunity to persons intent on mischief. They might range from relatively harmless political activists with banners through to terrorists with explosives. The adjoining secure zone, Area B, is used to isolate and control most of such challenges.
    The only exit from Area C is through the First Aid Shelter. The building extends to the walls of the adjacent buffer ponds making Area 'B' only accessible to those who pass through the inside of the building which is normally securely locked. Beyond this the only roadway is edged by buffer ponds. The roadway then runs to a transit channel for either marine life or shipping. This distance - over 100 metres - gives security personnel watching on CCTV time to plan a response. Prior to reaching this point the only items that can be damaged are the First Aid Centre and the buffer ponds. Destroying them has no effect on tidal power generation.
  3. Malicious Access

  4. Climbing onto the barrage from the estuary or travelling through an access tunnel to get to a Machine Hall raises security alarms in the control rooms. Travelling the circuitous route from Security Area 'B' through the First Aid Shelter and gaining access to a Machine Hall will take time. However, access will be gained by those prepared and determined. The destruction of some machinery is accepted as inevitable. The electricity generating machinery is at the lowest point in T-G boxes. Each of these boxes are fifty metres apart. This makes taking out larger amounts of power time consuming which, in turn, gives security staff more time to devise an appropriate response.
    Terrorists that are able to enter a Machine Hall and place some explosives may also attempt to enter the adjacent Machine Hall. Entrances to Machine Halls are deliberately 'paired'. They face each other. The tedious time-consuming activity of placing explosives in two Halls is possible. However, the entrances to the next 'pair' of Halls is over 1700 metre away via circuitous roadways. Restricting an attack to two Machine Halls limits maximum possible loss to below 20% of the electricity generating capacity.
    Additional Notes
    1. Access to the doors of the Machine Hall from a boat in the Marine Transit Channel is deliberately made difficult. The walls are sheer. Even at the most advantageous time, Spring Tide, when the bascule arm is in its raised position the climb out is at least three metres. At other times it is far more - typically over five metres.
    2. The crane rails between Machine Halls are five metres above the transit channel roadway. Only the rails pass between the two buildings. There is no floor beneath them at this point. So, whilst travelling between 'non-paired' buildings is possible using the crane rails it is extremely difficult, very time consuming and limits the size and weight of items that can be carried.
      A malicious attack through one of the tunnels that run under the shipping transit channels first has to defeat or bypass the Security Control alarms. The drive under a shipping channel to the 'island' takes several minutes. This gives security personnel an opportunity to close the gates at the far end of the tunnel.
      Terrorists might attempt to damage the electrical power lines from the generators. By design this is made near impossible. The two power lines connecting the barrage to the National Grid are buried in the sediments beneath the Machine Halls. They emerge only at points beneath and close to their entry to water-turbine / generator (T-G) boxes. The connector lines are on opposite sides of the Machine Hall - one leads to Aberthaw and one to Minehead. This means that a successful attack on one of the land-based electricity sub-station to which the barrage power output is delivered on one estuary bank does not interrupt output.
    3. Emergency power is provided for the Control Room in the barrage. Emergency lighting for the barrage generally is provided in the same way but independently.
  5. Ship Collisions – Barrage & Pier

  6. The barrage has to be capable of resisting a collision from any ship in the Bristol Channel both from downstream or from upstream of the structure. A worst-case collision scenario is a ship deliberately driven at speed into the barrage - such as a vessel hijacked as part of a terrorist attack. Designated 'exclusion areas' and other forms of maritime policing are unlikely to prevent this happening. Avonmouth docks plan to accommodate vessels 'up to 150,000 tonne dwt', Cardiff docks currently harbour up to 20,000 tonne dwt. Larger ships ply their trade at ports in South Wales downstream of the barrage. Clearly, the barrage must be of a very robust design and incorporate engineering features that enable it to repel collisions from any such vessels whilst at the same time minimising the loss of hydroelectric power generation. The Bristol Channel accommodates vessels up to 150,000 tonne dwt. The largest vessels, even when ballast loaded, keep to shipping lanes and sail near to high tide. The Aberthaw-Minehead barrage must incorporate a rapid replacement time for plant following operational failure. This serves equally well in challenging the possible logic of terrorists planning to launch an attack to damage the plant. A likely objective - to cause a loss of power to the National Grid that takes a long time to replace - is easily addressed by design. The Aberthaw-Minehead design seeks to make it abundantly clear at the outset that appropriate measures are in place to make it unlikely that such an objective could be reached.
    The Aberthaw-Minehead barrage and pier designs recognise and accommodate the very real possibility that in its 70-year design life a ship will collide with it. The collision could be accidental or, a reflection of modern times, malicious. Ships up to 150,000 tonne dwt can pass through the barrage on their way to Avonmouth. The largest ships tend to have to travel near high tide. The point of contact in a collision on the barrage from a large vessel is, consequently, high up on the barrage or pier. In a design of the 'concrete dam' variety a ship colliding high on the barrage produces an enormous force on the estuary foundation. The suspended MFU structural steel design avoids this.

    Ship collision - Load v draft data
    Notes :
    1. In recent decades the size of ships travelling in the Severn estuary has increased. In 2014 Avonmouth claimed a future capability of accommodating vessels of 150,000 tonne dwt.
    2. Gaining control of a ship and propelling it into a barrage as an act of terrorism is simple to conceive and execute.
    3. Tides at the Aberthaw-Minehead barrage periodically reach over eleven metres. As illustrated above, at tides of eight metres or more vessels less than 33,000 tonne dwt can traverse and impact at any chosen point that was under water at low tide. Expert navigational skills are unnecessary.
  7. Barrage Buffer Ponds


  8. Following the installation of all the power generation machinery the MFU and the central workshop are next flanked on the upstream and downstream sides of the barrage by upper and lower buffer ponds. These ponds are manufactured, launched into the estuary and then towed to the barrage site. They are then sunk into their final position. When filled with water well above maximum estuary depth the ponds are virtually immoveable so require minimal attachment to the estuary bed. There is no demand for accuracy of location and small slopes acceptable to roadway users are the only requirement.
    The water passages from the estuary pass beneath the buffer ponds until they reach the central well that forms an enclosure underneath the MFU.
    Coarse debris screens are situated on the barrage outer walls. Water passes through them and on underneath the lower buffer pond where, after some 40 metre, it encounters a one metre high step above which it passes through a 60 metre bank of five metre diameter tubes that form part of the upper buffer pond containment. It then reaches the well underneath the MFU.
    The weather-battered Bristol Channel demands a practical response to repairing damage and replacing parts including those that are subject to sedimentary attack or salt water erosion. Such activities have to be carried out no matter how inclement the weather. Beneath the buffer ponds the ‘step and well’ are designed to give debris and stones that have passed through the debris screen an opportunity to settle out and be removed. Access for cleaning equipment can be made from the roadway between the two buffer ponds.
    A large ship collision has the potential to produce a catastrophic breakthrough of the barrage and release extremely destructive water flows through the breach. The solution adopted is not to try to absorb forces arising from the forward movement of a ship but to deflect it sideways towards the shoreline (where it will do less damage) using buffer pond technology.
    Buffer ponds make use of the buoyancy and the strong non-symmetrical fluid forces that naturally occur when the bow of a ship encounters water at a much higher level. Where the magnitude of the increase in level is comparable to the vessel draft the bow of a ship readily rises and turns about its metacentre.

    The height of the wall of water needed by a buffer pond to deflect a 150,000 tonne ship is of a magnitude that puts smaller ships and pleasure craft at great risk from swamping. Collisions with the barrage that are not borne of a malicious intent will inevitably occur so a very wide spectrum of possibilities has to be catered for. A two-level buffer pond arrangement is required in the Aberthaw-Minehead barrage design. Buffer design requirements follow from this.

    Installing the buffer ponds is simple. Both upper and lower ponds retain water when in service. Empty they are able to be floated (with ballast water) to their position where they protect the columns of a Machinery Hall. The design is not conducive to being towed in the estuary in bad weather. It is prudent to plan for near low tide so that sinking (by pumping out ballast water) and fixing to the estuary bed can be undertaken in the shortest time. Obviously, it is easier if a day can be chosen when sea conditions are relatively calm. There are sixty buffer ponds to be placed.

    The size of a large ship compared to the size of the barrage needs to be appreciated. The Aberthaw-Minehead barrage is very big. However, the size of some ships using the estuary is in comparison HUGE. The inadequacy of a barrage without buffer ponds is illustrated.
    A typical barrage of the 'single dam' design is easily breached by a large ship travelling into it at speed. Many designs rely on the strength of a concrete structure where the impact point is much nearer the foundations than that on any Bristol Channel Barrage. The protection that a concrete 'dam' provides in the event of a 150,000 tonne dwt ship colliding with it is totally inadequate. Buffer ponds are essential to deflect an errant ship sideways with the aim of minimising consequential damage.
    All buffer ponds stand alone. They are not fixed to the barrage in any way. Only the roadways that connect them to the marine life transit channels or to each other are affected. No particular problems arise if a pond is not absolutely level. It is visually nice to see but is not an engineering requirement. Minor slopes in the estuary bed can easily be accommodated. Roadway supports included in the design can be adjusted to provide a near level transport platform throughout the barrage. Several of the buffer ponds include additional features. Those adjacent to the two shipping channels have, rising from the tunnel beneath the workshop/store complex, the upper sections of the access roadways.
    A ballast loaded ship of 33,000 tonne dwt requires only an eight metres draft at high tide so on most days it can traverse the whole of the estuary that was covered at low tide. The Aberthaw-Minehead barrage design caters for a collision under such circumstances. Two buffer ponds are provided. A vessel has to cross both of them before it can impact on a Machine Hall. The buffer ponds are designed to deflect the ship to landward. Should the columns supporting the Machine Hall become collateral damage they can be replaced. They are designed so the upper column section fails without irreparably damaging the root-tubes.
  9. Pier Lagoons

  10. Two piers reach out to the barrage from the UK coast. One is from Aberthaw and one is from Minehead. Each contains a lagoon formed by two containment walls. On top of these walls is a roadway. The road on the upper estuary side provides public access to a Visitor Centre that overlooks a Shipping Channel at the end of the pier.
    The lagoons meet important security needs. A collision by a ship that, intentionally or otherwise, results in damage can provide a flow path that bypasses the barrage. This must be prevented by design. The massive flows that could occur through such a break would immediately prevent power generation and become the route of unstoppable massive water currents with erosive properties. A lagoon over 150 metres wide and filled to one metre below the lowest yearly tide value provides a simple answer.
    The largest ships will immediately be grounded on crashing through a pier arm. The ‘highest’ low-tide depth value during a full year is about 3.7 metre more than the lowest yearly value. Smaller ships must be prevented from breaking through by making the roadway/lagoon walls strong enough. Of course, the lagoons could be completely filled with aggregate. Perhaps a more imaginative alternative is to include an ‘island’ in the centre of the lagoon to provide a secure home for wild life.

    The Pier Lagoon
    Preventing unauthorised persons accessing the barrage from the UK coast .has to be addressed. A Security Post is contained in the Visitor Centre building. This guards the entrance to the roadway tunnels that pass under the shipping channels so all persons entering it can be monitored. Any entering without permission can be brought to the attention of to the main Security Centre high on the barrage adjacent to the main Control Room. A car-park / coach-unloading point is provided outside the Visitor Centre. Mini-coach trips through the tunnel and up to the viewing platform in Machine Hall 15 will perhaps be preceded by a presentation relating to the power station. The road arm on the estuary side carries visitors. It is furthest from potential spray in inclement weather. The ocean-side road on each lagoon should be private and, below road level, carry the electrical power lines from the barrage to shore and on to where it can join the National Grid. In the future a permanent rise of up to five metres in Atlantic Ocean level may occur. The pier design has to provide for this. The load-carrying capability of the road/sea-wall facing oceanward must be designed at the outset with this in mind.