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Marine: The Island lifeline

On the Orkney Islands contractor Bam Nuttall is building a new dockside for the burgeoning renewable energy industry. Paul Thompson reports.

For the port town of Stromness on Orkney, construction of a new dock to accommodate ships loaded with equipment to service the renewable energy sector cannot finish soon enough.
The town’s local authority is keen to build on recently acquired renewables expertise so that it can attract energy firms.

“It is an industry which is of increasing importance to this part of Scotland and we are happy to be working on a scheme that will help cement the region’s reputation as a centre of renewable energy excellence,” says Bam Nuttall site agent Graham Hopper. The contractor is currently building the new dockside, which will be capable of landing the large loads associated with the industry.

The Coplands Marine Renewables Service Base scheme is one of a series of projects that Bam Nuttall has worked on for client Orkney Islands Council, and Hopper is teaming up with some of the council staff he previously worked with on a marina project in the Orkney Islands’ biggest town Kirkwall.

“It’s been good to link up with them. In my experience the council has proven to be a good client with lots of technical knowledge so they are able to understand exactly what it is we are doing. We have developed a long, clear relationship with a client we like working with,” he says.

“Working in an area where most supplies have to be shipped in, it is important to make sure everything is well organised”

Graham Hopper, Bam Nuttall

Under an £8M design and build contract with Orkney Islands Council Bam Nuttall is providing new docking facilities along a three section pier as well as a new 500m long, full depth construction service road running to the new quay. This service road is 6.25m wide with a 2m hard shoulder on either side. It has been designed by the client and consists of a 500mm deep sub-base, 60mm binding course and 50mm surface course. The piers and quayside have been designed by engineer Royal HaskoningDHV.

Bam Nuttall was awarded the contract in July 2012 and started on site last November.

“We took our time making sure we had everything planned and lined up. Working in an area where most supplies have to be shipped in, it is important to make sure everything is well organised. It also allowed us to progress the final design,” says Hopper.

The dock is split into three main sections: the approach pier, the north pier and the west pier. Each is around 100m long and, with the exception of the approach pier, will provide berthing space for the large vessels of up to 3,000 gross tonnes that supply renewable energy firms as well as smaller local supply boats and fishing fleet.

The site team has used a stone causeway built alongside the line of the approach pier to provide access to the construction site regardless of tidal conditions.

“There is quite a tidal range. We have a beach at low tide and around 3m coverage at high tide,” says Hopper. He says the contractor did look at using a spud leg flat barge, but this was uneconomic.

Environmental engineering

The work in and around the marine environment at Stromness has meant the Bam Nuttall team has had to be extra vigilant in ensuring the project does not have any impact on the immediate environment.

The team is on permanent watch for whales, dolphins and porpoises that might swim close to the project. It also has to look out for other marine mammals, such as seals and otters, and must work to guidelines from the Joint Nature Conservation Committee.

These guidelines help to minimise the risk of disturbance and injury to marine wildlife from the noise and pressure impacts of the site work, particularly during the pile driving phase.

A 750m exclusion zone has been set up around the site and is monitored for at least 20 minutes before work starts. Only when site staff have been given the all clear can work begin.

“There is an agreed strategic observation point where we watch for seals and other marine mammals. They are particularly sensitive to noise propagating through the water from our work. We can’t start before we have been given the all clear and have to stop as soon as we see any that come within the exclusion zone,” says Bam Nuttall site agent Graham Hopper.

Instead it has opted for 80t mobile crawler cranes positioned on the causeway to reach over and install the pairs of 473mm diameter reinforced concrete filled driven steel piles at 6.5m centres along its length using a CX60 5t hammer.

On the two seaward piers the team has used a crane barge equipped with a 160t crawler crane and CX110 9t drop hammer to install the piles with another 180t machine working directly from the pier deck.

The essential criteria for the piling work was to achieve ultimate pile load. Piles must also be embedded a minimum of 1m into the underlying granite bedrock. A layer of weathered rock above the dense granite has enabled the team to reach the minimum targets on all but two of the piles. These two are closest to the shore and have been excavated and installed through a concrete socket to help meet the piling criteria.

“It was always anticipated that the tidal zone would be more onerous,” says Hopper, “But we have managed to meet all those targets.”

The mobile cranes have also been used to lower in the 30t precast concrete cross heads which help support the 8m wide deck of the approach pier.

Precast concrete is being used widely across the scheme for the pier cross heads and for the longitudinal beams and parapets. In all, a total of 5,000t of precast concrete will be shipped across from the Republic of Ireland by supplier Concast.

The units have been delivered in two stages to Kirkwall before being driven over the island on fleets of trucks and stockpiled at the Stromness site.

“It’s a bit of a chicken and egg situation,” explains Hopper.

“Stromness doesn’t have a dock capable of handling that size of load and ship - at least not until we finish!”

The north pier and west pier are being built using barge mounted crawler cranes to install the 720mm diameter reinforced concrete filled driven steel piles. These are being placed in rows of three on the north pier to support the 14m wide deck and in rows of four on the west pier to support a 20m wide quay deck.

It’s a race against the elements for the Bam Nuttall site team to complete all the work. Weather windows that allow construction to continue can be frustratingly short in the Orkneys.

Fortunately the summer has been kind to the team and, with a contract completion date set for February 2014, it is well on target for completion.

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