Contractors winched the Singapore end of the ASC cable to shore in December. Peter Harrison, Vocus Head of Major Projects, says it is a significant project milestone. "The work completes the first stage of the marine programme,” he says.
Landing a cable in the world's second busiest harbour is a complex operation. Harrison says not only do the engineers need to cope with a huge amount of shipping traffic passing through the area, but Singapore has a strict requirement that the 5.5km of the 10km submarine cable within the Singapore port limit has to be buried sufficiently deep enough below the seabed to protect it from damage.
He says burying 10 metres deep, the height of a two or three-story building, was the tricky part of the operation. This depth is necessary because huge ships with heavy anchors often dock at Singapore. It is a major oil terminal. If a ship drags its anchor, it would damage a surface laid cable. Harrison says while it is a legal obligation to deep-bury cables in the harbour, it also helps ensure it doesn't fail.
Burying a cable that distance below the seabed brings challenges. The cable has to be laid using a special machine and there are only a handful of them in the world. The tool had to make at least three passes across the 5.5km km length of the cable's Singapore harbour segment.
Where the seabed is soft, the machine uses an injector/jetting tool to loosen the soil. In harder areas it uses a rock-cutter to cut a deeper trench. The removed seabed soil sits loose. On the final pass the jetting/injector tool is used to move the soil out of the way as it inserts the cable at the bottom of the trench created from the previous passes. The loose soil then settles back into the trench, burying the cable.
The task is made harder because there are already dozens of cables in the area. Each runs parallel from the foreshore, with one extending out every 100m or so along the length of the harbour.
Before starting work ASC carried out a precise marine survey to map the route. It then had to negotiate agreements with the owners of other assets that the cable would cross. Five of the 43 crossings along the entire 4,600km length of the ASC cable occur within Singapore Harbour.
Harrison says a horizontal drill was used for the last 800m or so of the Singapore harbour cable segment where it crosses the shore line. He says: "It drilled a bore hole in an arc 20 to 30 metres below the seabed to meet the 5.5km trench."
At the point the bore hole meets the trench, the cable is carried on a barge. It is then pulled from the barge by a rope that was already threaded through the bore hole. At first it is connected to floats on the surface.
Then the cable is winched through the bore hole to a large beach manhole on shore, where it is anchored. The manhole has to be large enough to accommodate several coils of spare cable length in case it is needed later.
Often a submarine cable terminates at a special purpose landing station. Harrison says at Singapore the ASC terminates in a data centre 18km away from the point the cable reaches the shore.
Harrison says the Singapore harbour landing site is one of the hardest engineering tasks in the overall ASC programme, so success there represents an important step towards completing the overall project.