Coping under pressure

In one of the biggest projects of its kind ever carried out, UPS used its under-pressure drilling techniques at a Hampshire reservoir. And, when the drill diameter is 1,200mm, some careful planning is necessary.

Underpressure Pipeline Solutions (UPS) recently won a contract with 4Delivery (4D), a consortium comprising United Utilities, Costain and MWH, because of its expertise in under-pressure drilling techniques.

The work was carried out on behalf of Southern Water at the Budds Farm WwTWs in Havant, and would prove to be one of the biggest projects of its kind ever undertaken.

Making five connections in total under pressure through a 250mm reinforced-concrete wall is no straightforward task. And the diameter of the drillings was 1,200mm.

Under-pressure drilling is usually associated with making live connections into pipelines. But similar techniques can be applied to drilling reservoir or digester walls, minimising cost and disruption to supply.

UPS produced special fittings that could be attached to the activated sludge plant wall and bolted securely to affect a seal. The fitting has a flanged face onto which is normally bolted a gate valve.

In this case, however, a standard gate valve could not be used – the size and weight of such a valve would have been prohibitive. As a result, 4D needed to procure 1,200mm-diameter knife gate valves.

Spool pieces were manufactured to fit in between the gate valve and the new pipework to facilitate enough space to accommodate the drill head containing the concrete core once the drilling was complete.

When the wall fitting, the knife gate valve and the spool piece were in place, the drilling rig could be bolted on to the spool piece. The problem now was that the overall weight of the equipment was in excess of five tonnes, and the fear was damaging the wall itself.

Taking the weight

The design engineers at 4D found a solution in having metal pipe supports manufactured. These would be bolted to a concrete base as a permanent structure and situated under the spool piece to take the bulk of the weight.

Once the drillings had been completed, the five 1,200mm valves were left in place in the closed position, ready for the pipelines to be connected. In the revised scheme layout, four of the valves would be used to connect to a pipeline taking flows to four existing final settlement tanks.

The final one would be connected to a pipeline taking liquors from a newly constructed activated sludge plant to enable these flows to reach another four existing final settlement tanks.

The whole operation was part of a major extension programme to the Southern Water Budds Farm Wastewater treatment works, which will add an extra stage to the existing cleaning process to remove nitrogen from the treated water. This is in order to meet new consents that are being implemented under the Urban Wastewater Treatment Directive (UWTD) to limit the total nitrogen that can be released into the receiving water.

The scheme has a contractual requirement to achieve Southern Water take-over by March 2008, when the plant must be fully commissioned and operational.

Southern Water let the contract to 4D in April 2005, and the schemes were immediately earmarked as necessitating a fast-track design-and-build approach that could only be achieved by a fully integrated team.

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