A complex combination of jetty rehabilitation and harbour works in Portsmouth will ensure a safe home for the UK’s largest ever air-craft carriers.
All eyes will soon be on two £3bn aircraft carriers – the largest vessels ever built for the Royal Navy. As they cruise into Portsmouth Harbour more than 680 crew will be aboard and many more people will line the shore. Each of the ships is 65,000t, longer than the Houses of Parliament, and wider than a football pitch – docking them is a hell of a parallel park.
Thankfully, quite a bit of money has gone into making this a smooth manoeuvre.
One £34.4M contract is based around getting the site’s Princess Royal Jetty, formerly Middle Slip Jetty, ready for the first of the ships, HMS Queen Elizabeth, to arrive followed by the second, HMS Prince of Wales. The jetty is at the northwest corner of Her Majesty’s Naval Base which is on the eastern side of the harbour and 1.75km in from the harbour mouth.
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The government’s Defence Infrastructure Organisation (DIO) chose VolkerStevin as principal contractor for this list of upgrades: a stronger jetty was needed – removing, strengthening and reinstating sections and piles for a reinforced concrete jetty that date back as far as the 1920s, using a mix of piles, precast beams and cast insitu concrete.
In addition, 14 bespoke offshore navigational light towers had to be built to guide the ships in; not to mention supplying enough electrics for what are effectively two rechargeable, floating small towns.
A lengthy tender process started in 2013, with care taken on the government’s part due to the jetty’s importance as a one-of-a-kind national strategic asset.
“It does require a level of assurance and approval, right the way up to the minister and prime minister,” says DIO senior maritime engineer Mike Preston. “How the infrastructure operates is significant, but in terms of how the country operates, it’s even more significant.”
Middle Slip Jetty has played a significant part in the UK’s history. German Second World War bombs were found scattered nearby during dredging work for the current programme, and the original (no longer structural) sea wall dates back to the 18th century.
In the 1920s the jetty was structurally altered, with two large sections added, north and south, extending about 20m seawards. This created a jutted structure with two large inlets. In the 1950s a narrow frontage to these “juts” was added. Then in the 1970s a major addition integrated the entire length of the jetty and extended it a further 15m, into a long, single frontage.
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At the same time, vessel sizes in Her Majesty’s fleet were getting larger and, as the ships went from dreadnoughts to destroyers to aircraft carriers, jetties were transformed from small inlets, to massive broad platforms.
Fast forward to the design stage circa 2014. While there were big ambitions for the space, there was also heavy pressure on public expenditure. These budget constraints, combined with the salvageable state of the 1970s jetty section, led to an economical decision to restore the entire structure, rather than demolish it entirely and start again.
“It (the 1970s structure) was one of the key drivers, knowing it was an asset that still had life in it and we could take it forward,” says Preston.
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One restriction on the expansion project was that the diameter of new piles was limited to 1080mm, due to limitations on marine activity during sea trout migrations.
“We were aware it was going to be quite a challenging programme and last thing you want to do is box yourself into a corner, where you only had a limited window to get the thing built,” says Preston.
The cap at 1080mm diameter piles allowed a handy benchmark for the designers.
“The challenge you had here is stitching together 40-year old infrastructure and threading through new increased load paths. And trying to build all that in a quite challenging environment, surrounded by hundreds and hundreds of years of history. And we don’t always have the historical records,” says Preston.
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The array of piling, use of precast beam, insitu poured concrete, caps and plugs is hugely complex. About 220 new piles went in. Most went to about 34m, with a few raked piles to about 40m.
Openings were made in the 1970s structure to place piles and strengthen the deck, so that it can handle the cranes and containers on the operational jetty.
Meanwhile, the 1920s section, north and south, was completely demolished, and precast beams were used to create a new deck. This new structure had to be stitched in and around piles, and existing service tunnels, making use of pre-piling, jacking and grouting to create an integral structure. A reinforced concrete topping up to 1m thick covers the entire structure, serving structural and aesthetic functions, making sure that, after the work is complete, all of the clever design is completely hidden from view.
Part of the solution was using 3D laser scanning and building information modelling to work in the precast beams. “We wanted to put a precast solution in for durability, health and safety,” says CH2M designer Roger Walsgrove. “It wasn’t a unique precast solution – we used the same units all along (the jetty sections), which gave us ease-of-build, and cost savings.”
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All this will be tested by forces exerted by the carrier on the jetty. As the carrier is 56m at its tallest, it will act as a massive sail, pulling and pushing on the structure. And as the new aircraft carriers are 40m wide at water level, and 70m wide at deck height, there is a significant overhang to consider.
Design and build of steel fender spacer units, which float between ship and jetty, was another part of the contract.
Surprisingly, the jetty will only need about 20 people on land to help the ships dock. That efficient use of labour is down to the Ministry of Defence’s (MoD’s) first ever self-tensioning winch system; at two points on the vessel, two lines are cast ashore. Each is tied through a mooring point, and taken back up to vessel, where it is tightened up with the winch.
Another challenge is rust. In Portsmouth, corrosion induced by the aggressive marine environment can eat away at exposed steel at 1mm/year or more, without proper protections in place. The jetty has a guaranteed 35 year design life, and key to this is the impressed current cathodic protection system for the deck, and the sacrificial anode system for the piles.
About 30% of the project spend was on the mechanical and electrical challenge of hooking up the carrier to 11,000V.
NATO rules demand the carriers use the United States’ 60Hz electricity standard, so a transformer had to be built to convert current from the grid from 50Hz. “You can use solid state (transformers)… but the electronic method is too fragile,” says senior project manager Gerrit Smit. The hulking transformer of about 100t in steel and copper is custom made and supplied with electricity from Portsmouth city centre via 1850m of new cables and connected to the ship via a sensitive sensor-equipped telescopic cable boom – another bespoke piece of technology.
Guiding the aircraft carriers into the harbour requires 14 navigational towers. Each is 25m tall in all, the concrete piles are fitted with pre-constructed steel sheaths which contain lights, solar panels, batteries and control equipment.
Each steel sheath section weighs about 22t and was lifted into place using a 350t crane barge. The lights will only be lit when the carriers are either approaching or leaving, with all other navigational towers in the harbour turned off.
The carriers’ decks are 70m wide, while Portsmouth Harbour is just 110m at its narrowest. Enough space is needed not only to enter, but slow down and stop. To add pressure on this tricky parallel park, the carriers’ bridges do not sit straight but are placed off-centre.
“We sat in the simulator,” says DIO project manager Philip Wise. “And as you come through the entrance everything close to the vessel disappears, all you can see is the aircraft carrier deck. It’s quite an eye opener.”
Preston says: “They’ve [the carrier crew] been working on it non-stop, simulation after simulation, so that they’ll be ready when the carrier comes. And that’s the reason the lights are so far away, because there’s no point putting them close, below, you’d just lose sight of them.”
Overall, VolkerStevin’s Smit says the MoD and DIO were good clients and the wider stakeholders in Portsmouth showed an “unheard of” amount of support.
“They (MoD and DIO) are very focused on delivery. Everybody wants to get the project finished. For the city, having the vessel here boosts the economy.”
Beyond the VolkerStevin contract, to get the carriers into the jetty has required more than £100M total investment on the harbour, including extensive dredging.
About 3M.m³, or an area the size of 200 football fields of material has been removed specially for the vessels that extend about 11m underwater.
The dredge extended and deepened existing shipping channels. This required the relocation of local oyster growers, and avoiding environmental restrictions on bird reserves– not to mention busy harbour traffic.
“It’s like trying to do works by the side of a major arterial motorway. There’s a lot of leisure traffic. Plus there’s the second busiest ferry port behind Dover,” says Preston.
Despite a successful project finish, and with Princess Anne officially renaming the Jetty in late March, one item is missing.
HMS Queen Elizabeth is currently being fitted out in Rosyth, Scotland, and was due to make its maiden voyage in March, with rumours of its arrival in Portsmouth being around May/June. But this programme has been scrapped, and sea trials are now expected in June, and arrival in Portsmouth before the end of the year. HMS Prince of Wales is due to follow in 2019.