Submarine gas main: free-span inspection and support
Underwater inspection of a section of submarine gas pipeline left spanning over a scoured seabed, and a plan to support it and fill the scour with aggregate.
- Year
- 2012
- Client
- The Hong Kong and China Gas Company
- Location
- Hong Kong waters
- Status
- Completed
- Category
- Underwater & marine
Currents can wash the seabed out from under a submarine pipeline, leaving a length of pipe hanging unsupported: a free span. Left alone, it can bend or fatigue.
In 2012 we inspected the free span underwater and surveyed seabed levels all around it on a grid, in HK Grid coordinates. The drawings mark where the pipe lifts off the seabed, where mud samples were taken, where temporary supports go, and the area to be filled with 40 mm aggregate to give the pipe back a continuous bed.
From the drawings
The free span was 16 m long, at Pak Sha Tau near Tolo Harbour, with the pipe at −19 to −21 mPD. The repair removed about 18 × 5 m of rock and mud from above the pipe by airlift and crane, filled the scour with 800–1,000 m³ of 40 mm aggregate to pipe-top level, and covered it with grout-filled concrete mattresses of 6 × 3 × 0.3 m. In 2013 we also drew a tie-in proposal for Mirs Bay using a dry welding habitat.
Drawings
