Anchoring expertise provides bespoke solution for seismic marine application
The Project
The project at Babcock International’s facility within HMNB Devonport – the largest naval base in Western Europe – involved the replacement of corroded anchors that were securing a large cast iron brow hanger to the dock side. These hangers enable the secure attachment of the naval brow – a linking structure between the dock wall and the vessel – which accommodates the high-pressure hoses and pipes used to test the propulsion systems of vessels in dock.
The dock is part of the extensive infrastructure at Devonport which has been supporting the Royal Navy since 1691. Spread over more than 650 acres, the dockyard has 15 dry docks, four miles of waterfront, 25 tidal berths and five basins. Its vast scale makes it a major part of Plymouth’s economy, employing 5500 people, directly supporting 400 local firms and representing 10% of the city’s income.
The brow hanger requiring refurbishment is located in a dry dock used for the maintenance and refit of HM Navy’s submarine fleet, including its nuclear-powered Trafalgar and Vanguard-Class submarines. All eight of the hanger’s existing anchors, which were installed to an embedment depth of up to 550mm, had become corroded through long term exposure to the atmospheric conditions, but the process of replacing them had to navigate a number of complexities.
Most significantly, it was vital for the anchors to be extracted without damaging the rebar within the dock wall. Any interference with the existing rebar would affect the structural and seismic integrity of the dock side and in doing so lead to extensive and potentially costly remedial work. This could, in turn, disrupt the vital work undertaken 24/7 to keep the UK’s naval submarines operational.
But this was only part of the challenge. It was also crucial that the proposed solution could be fully assessed in a reliable test environment to give Babcock International’s team complete confidence in its integrity throughout its service life.
With its extensive anchor design and manufacturing expertise, EJOT UK was ideally placed to help with a bespoke LIEBIG solution as Paul Papworth, its structural anchoring specialist explains.
“We provided engineering consultancy over a period of weeks, including four site visits,” says Paul. “In order to arrive at the most effective anchoring solution, our engineers undertook a series of remote calculations before preparing installation methodology and providing assessments on the suitability of varying alternatives.
“By partnering with one of the industry’s leading diamond cutting and coring specialists, Core Cut Ltd, we were able to conduct off site trials in order to evaluate and prove the effectiveness of the different alternative solutions.
“These trials were an invaluable part of the investigation and solution preparation. They allowed us to replicate the proposed process, evaluate its accuracy and forecast the time involved for the removal and replacement of the anchors. By utilising the expertise of Core Cut Ltd, we could ensure the removal of the corroded anchors could be completed with total precision and with no damage to the dock’s existing rebar.
“We were able to confirm this using a microscopic camera (a borescope) inserted into each of the holes, which provided the basis for a report on the condition of the substrate and condition of the existing rebar after drilling.
“The final proposals and results from trials was presented for comprehensive review by a three-tier approval chain culminating with the dock board who ultimately accepted our solution.”
Tests also sought to provide evidenced basis for the type of anchor replacement critical to the testing of nuclear submarines after major re-fits. The seismic standard to which this dockyard was originally constructed during the Cold War had to be maintained, so any anchoring approach needed to be able to demonstrate structural performance to this level.
The Solution
The solution provided by EJOT UK utilised the EJOT SE800 epoxy resin in combination with specially made resin studs. These 8 x M36 large diameter HCR (high corrosion steel) studs were manufactured in 1.4529 steel in order to meet life expectancy in this harsh environment as well as the load capacity of the application. They were selected based on the need to reduce the possibility of contacting or cutting the rebar during insertion which surrounded the existing installed anchors.
EJOT SE800 is a two-component injection resin based on pure epoxy resin for high performance and structural applications. Designed for anchoring threaded studs into cracked and non-cracked concrete, it was the ideal solution for the Devonport project because it is suitable for use in wet and flooded areas.
Paul adds, “The solution we developed for Devonport dockyard demonstrates our flexibility to be able to go way beyond standard products, meet the complex requirements of the application and partner with experts in other technology fields to provide totally bespoke solutions. We can do this because of the wealth of anchoring technology and construction expertise we have within the EJOT Group, particularly within our LIEBIG business. This ensures we can offer our customers a more innovative approach to overcome specific challenges.”
The large cast iron brow hanger on the side of the dock. The eight anchors, which you can see here, were installed to an embedment depth of 550mm. Thats just short of 2 feet.
Once the anchors are installed, they need to be tested to make sure they can withstand the required tensile loads. This is done using a hydraulic-digital testing device.
The results are in… each anchor easily resists the required 39kN test load which it was designed to do. That’s just under four tones and there are 8 of these anchors.
