Briggs Rock Outfall

Groomsport, Co-Down

This was a design and construct contract for a long sea outfall into the Irish Sea off the East Coast of Northern Ireland, which was 860m long, 900mm diameter.

A summary of the construction methodology is set out below:

1) Large excavator located on jack-up barge used to excavate trench
2) Pipe was pre-fabricated in single length at shipyard, floated into place over the trench, and sunk into place.  Divers were utilised to ensure accurate placement
3) Trench backfilled using hopper bottomed barge (loaded from shipyard).  First finer material used as pipe-bedding, then larger material (450mm) used as scour protection.

Operations were made more challenging by the shallow coastal location, rock seabed, racing tidal conditions and other key challenges such as:-

1) Weather - obviously inclement weather, and high seas are problematic in operations such as this.  However, by selecting the most suitable type of plant, i.e. a jack-up barge, we were able to excavate the trench in moderate wind/swell conditions.  However, when weather deteriorated, jack-up barge was unable to be moved (it is towed by an attendant tug boat).  A suitable weather window had to be identified for the floating and sinking of the pipe.
2) Plant and specialist labour - it was critical that sub-contractors were brought on board very early to ensure that their specialist plant (e.g. jack-up barge and hopper bottomed barge) was available to meet our programme.  Also, had to ensure divers would be available at critical time.
3) Availability of dock space - as the pipe was pre-fabricated on shore, and stone loaded onto barge at dock, early discussions with Belfast Harbour was essential.