Articles & Case Studies

Harrowside Outfall replacement

Posted: Monday 30th November 2015

Land & Marine is currently on site finalising a project to design, build and commission a replacement of Harrowside Outfall in Blackpool located just south of Blackpool Pleasure Beach. The project for United Utilities is being delivered by Murphy subsidiary Land and Marine Engineering. The works are part of United Utilities’ £100m investment to improve the sewer network along the Fylde Coast over the next five years.

The existing cast iron outfall, constructed in 1938 and measuring 500 mtrs, has failed in many places and is now at the end of its useful life. To improve the bathing water quality a new outfall is being installed. Connected to the Lennox gate pumping station it will increase the capacity of the existing sewer network. Just over 1,000 mtrs long and with a diameter of 2.1 mtrs the new pipe will provide an efficient way of dealing with any excess stormwater in the region.

The project has seen Land & Marine install the new 1,000 mtr pipe, which was constructed in Norway as two 500 mtrs sections before being towed, via Land & Marine’s facility in Holyhead, to site. Once off shore, the two sections of pipe were floated into place using a 30 tonne winch and then placed above the pre-dredged channel, 7 mtrs wide by 4 mtrs deep.

The offshore end of the pipe has been connected to a diffuser. Constructed of four sections it features 8 duck bill vales that allow the water to exit the pipe and into the area without allowing water or debris back into the pipe. The diffuser, which measures 4 mtrs by 4 mtrs square and 5 mtrs deep was formed in Ireland and then shipped to Hollyhead where it was put on barges and delivered to site. A 400 mtr long sheet piled cofferdam was then created from the sea wall to mean low water (sheet piles were designed to extend 1 mtr above the beach level to take into consideration any sand movement). Guide piles were also installed to ensure the new pipe stayed in place during installation at all states of the tide.

Once the pipe had been placed in position by barges on site, it was flooded and concrete kennels were added to keep it in position and weigh it down onto the floor of the dredged channel. The channel features over 500 steel sheet piles that will be extracted, returning the sea bed to its original natural state and burying the new pipe.

The project has required the team to closely monitor the tide and the weather. The project team has had to work during low tide windows, often at night, to deliver the project on time. This has seen the creation of a slick process for mobilising equipment and plant from a site yard further along the beach to maximise outputs during these periods.

Key Project Points

· Highly challenging project, working with the tides and weather

· Design through to delivery all managed by Land & Marine

· Use of highly skilled supply chain – Dutch dredging company, Norwegian pipe extruder

· Challenging installation of large pipe using offshore techniques – creation of trench; installation of sheet piles; installation of diffuser; installation of concrete kennels

· Challenge of working on beach front location and managing the public

· Need to construct a 45 degree diversion from the existing outfall chamber to the new pipe alignment (new pipe is located 30 mtrs north of the existing pipe position)

· In house modelling and calculation by Land & Marine to ensure the new pipe could be safely floated in to place during installation and then flooded to sink it

· Design of concrete weight collar to aid the installation of the new pipe

· Construction of sheet-pile safe-refuge for site equipment located adjacent to new pipe (at high tide the beach floods up to the sea wall)

· Creation of shift pattern to enable maximum on site productivity

· Creation of 400 mtr long cofferdam and guide piles

· Assembly of pipe (fitting of two lengths and addition of reinforcing collar at joint) at Hollyhead before being towed by Land & Marine barge for installation

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