Our customer & the problem
Pendennis Shipyard Ltd (PSL) is a world-class custom build and refit superyacht yard in Falmouth, that specialises in luxury sail and motor yachts of between 30 and 100m. Their quay dried out at low water and they wanted to be able to work on yachts while they were afloat. The main problem was that the shipyard is located in a Special Area of Conservation, so the potential methods for design and construction were limited and the costs could prove prohibitive. Traditional piling was out as it would have disturbed the sea life, so an innovative, cost effective design that that negated the need for percussive pile driving and which could be built in a limited window of opportunity, was called for.
Ian Granville, PSL’s Finance Director
“Knights Brown was a breath of fresh air compared to other contractors we have employed in the past. Knights Brown listened carefully to our requirements and of the many possibilities of how to approach the build proffered by others in the tender process, they were by far the most thoughtful and cost effective. They understood working in the water in a special area of conservation and not least, working in five metres of tide. They priced accurately and were transparent and gave comfort that it was not ‘bid low, claim high’. Their contractual price was the price paid. In a project that was far from straightforward, there were times when difficulties had to be overcome and tough conversations had to be had, e.g. rock state. Both companies took the decision to deal with it at the time and Knights Brown’s engagement and professionalism in tackling the issues, led to the success of the project and a very happy client. We would without hesitation, work with Knights Brown again.”
Two stage tender
Knights Brown, working with Hemsley Orrell for the detailed design, was selected as the preferred bidder in a two stage design and construct tender. Our solution was for a 7,564 m2 wet dock comprising four main elements:
- East and west harbour arms
- Concrete sill (entrance structure)
- Short-hoist jetty
Together we carried out extensive value engineering during the second stage of the tender process, and so were able to make cost savings of £2.3m. Rather than install a cofferdam to allow construction of the gate sill to take play in the dry, we opted instead to pre-cast the structure on the dock edge, lift it into the sea using PSL’s boat hoist and float it into position.
This brought the project within budget at £7.8m.
The Fal and Helford is a designated Special Area of Conservation (SAC) not least because it’s home to seagrass, which shelters a wealth of wildlife including the UK’s two native species of seahorse.
Its presence within the site footprint presented the challenge of how to conserve it. We worked closely with Dr Ken Collins, one of the world’s leading experts, to formulate an acceptable protection plan. This involved carefully translocating the seagrass by boat to a reception site in the Helford Estuary, leaving behind rhizomes, which were preserved through the construction period
We maintained close contact with the regulatory organisations throughout including the Marine Management Organisation (MMO), Natural England and Cornwall Council.
Silt was a second issue requiring careful management. We undertook real time measurement of turbidity using fixed monitoring buoys, as 1.5m of silt had to be removed. We mounted an excavator on a spud leg barge and enclosed the entire operation in a silt curtain, monitoring turbidity for the duration of operations.
PSL had created a window of opportunity in their calendar for the works to take place but with bookings already confirmed, the completion date was fixed. The accepted programme was 49 weeks.
As the shipyard would continue to operate throughout, we naturally coordinated closely with PSL to ensure we didn’t hinder their day-to-day operations. Given the confined area for the combined operations this wasn’t always easy but was maintained amicably.
Concrete mix & methodology
The build required a total concrete volume of 15,000 m3. We knew that timely delivery of supply on a tidal basis, day or night, would be essential. Onsite batching was the ideal solution but space to accommodate a plant was extremely limited. However, TSL Contractors offered a solution for a mobile plant that could deliver 40m3 an hour. This granted us close control over quality and production at all times, as well as minimising truck movements through Falmouth over the summer.
We planned mass concrete pours under the sea, averaging 120 m3 a day, so our operational requirements on mix design were fairly onerous:
- The design required a concrete with high durability.
- We needed slumps of 150 mm plus to ensure that the mix would flow and self-compact to all areas of the cofferdams from our tremie pipe.
- The maximum pump line length was 120 m.
- Low heat was required to ensure minimum thermal cracking.
- Alkali Silica Reaction (ASR / concrete cancer) had to be considered as well as resistance to sea water.
- Construction joints were generally underwater so the mix had to minimise laitance.
Working with TSL, Tarmac Cement and Sika, we selected a CEM IV pozzolanic cement from Aberthaw for all the mass concrete and structural mixes. This was specially delivered into the south west via a holding depot in Liskeard. We trialled various options on site and finally adopted a granite coarse aggregate with recycled sand from the china clay works in St Austell.
Using the CEM IV and a combination of water reducing agent and superplasticizer, we set up a full scale underwater trial using a 5m long shuttered pour. With a bespoke 10″ adjustable tremie pipe we successfully established that our mix would flow a minimum of 5m underwater with very little laitance appearing. Pumping characteristics and strength were excellent and strength gain continued on average 20% between 28 and 56 days.
The harbour arms comprised mass concrete contained within sheet piles acting as shutters only. We drove the piles to 200 mm using soft start techniques. Cell sizes were approximately 10 m x 6.5 m x 11 m deep.
We placed concrete via a static pump using the underwater tremie method. Temporary pile support was provided by steel frames placed from a flat barge. After three concrete lifts, the de-bonded legs of the frame were pulled out and moved forward for the next cell.
Adjacent to the gate abutments, the harbour arms were strengthened with fibre reinforced concrete. Four hundred cubic metres was successfully pumped along the tremie pipe into position.
Critical to the success of the project were the key tasks that could only be achieved at low spring tides, most significantly being the ‘float and sink’ operation to install the concrete sill structure that would accommodate the caisson gate. We constructed the sill on land and making use of PSL’s own 600T boat hoist, launched and floated it into position before sinking and anchoring it in place.
Working on land, we cast the sill and lower abutments, accommodating a 19.7 m entrance. The abutments were made up of match cast removable sections at each end.
Once again the choice of concrete was critical to success. It had to be stiff and light enough to be lifted by the boat hoist and sufficiently buoyant to float into position. Yet it needed sufficient draught to settle onto prepared pads in position at low water.
Using a barge mounted excavator and underwater hydraulic breaker we excavated the rock trench that would accommodate the sill and cast the pads with underwater shutters.
In squally conditions, we lifted the structure with the boat hoist, launched it complete with buoyancy bags and positioned it by tug. As the tide went out the sill structure was dropped by the guide frames onto the pads. Once it had settled to line and level, the flood valves were opened. Concrete was pumped underneath and inside the structure before placing the match cast abutment sections in place. Rock anchors were then drilled 30 m into the rock head.
Making slower progress than we had planned on the harbour arms, we began construction of the short hoist jetty early under tidal conditions. As soon as the basin was pumped dry, the jetty was finished using traditional formwork.
The steel caisson gate was designed by Geoffrey Austen (Pebbles Consultancy) and manufactured on site by PSL. Once it was launched and positioned the basin was pumped out, allowing the dock to be excavated to level.
The deck finishes and wave walls were a combination of precast units and in situ concrete. The works were finished off with precast deck work, mooring bollards, capstans, fenders, ladders and car parking before the site handed over to PSL.
- Steel sheet piling (permanent formwork) | 600 tonnes
- Concrete batched on site | 15,700 m3
- Rock anchors at the gate abutments | 850 tonnes capacity
- Excavation of raised beach and entrance floor | 4,200 m3
- Precast entrance structure (reinforced concrete) | 400 tonnes
A successful collaboration
Less than 11 months after starting work, the first superyacht entered the wet basin, four weeks ahead of programme.
The final account was settled prior to the completion of the works without significant alteration to the budget figure.
Our entire team worked together to deliver a sizeable new facility safely, ahead of time and to budget, with minimal effect on the surrounding environment.
The project was highly commended at the British Construction Industry Awards.
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