The Marianas Trench is the deepest part of the planet, at over 36,000ft (or 12,000m) and the PPC-1 cable system crosses it to reach Sydney from Guam.
The cable finally reached the seabed at 0625 this morning on 2nd May, some six hours after it left the ship. It reached a final depth of 9440m. At the point that the cable touched down, the ship was a further 26km south in ‘only’ 7000m of water.
This photograph shows two graphs of the seabed; the top one is a side profile and the bottom one charts the slopes and dips in percentage terms.
The Trench is about 175km south of Guam and leaves the Tyco Decisive with another 2100km of cable to lay.
Given the depth of the route across the marianas trench is 9440m, is a different type of cable used in this area than the rest of the route? Is it possible to give a little information about the route either side of the trench – over what distance does the route get 3000m deeper? what are the slopes like on either side? what is the seabed like around there? rocky, sandy etc?
Cheers
Stevie B,
The Trench is approached in exactly the same way as any other valley to be crossed. We’ve used a cable type to suit the seabed but we didn’t load a ‘Trench Special’ for this section.
The trench profile is fairly reasonable, the run in from the north (Guam side) is steeper than the south side. The depth drops fairly quickly and reaches the bottom. The picture above is taken directly from the survey data and is an accurate depiction of the slopes. The left side of the charts are the Guam side and the right side of the charts is towards Sydney.
This involves a question I’ve had for some time. When crossing large underwater trenches like this, how do the cable cross, exactly. Do they just dangle with some amount of slack? I would think that the cable would be fairly heavy as it dangles. Are there also not any significant currents that low to put any pull on the cable that’s crossing the chasm?
Chris,
The layback for the cable (the distance from the stern of the ship to the point where the cable touches down) can be seen in the picture above. It’s the reddish diagonal line on both charts. The cable engineers know the types of slopes that they will encounter and are able to adjust the ship’s speed and cable payout to ensure that enough slack is laid to fill all of the dips. The key factor here is to avoid suspensions.
The cable does have a fair bit of weight on it but the computers model this and factor it in to the laying.
As for currents, it’s likely that there are a number of currents that affect the cable at all depths as it sinks to the bottom. To accurately model these in real-time and adjust the lay accordingly is very difficult. The cable route is always planned to provide a safe area around the cable. We don’t have to planned it within a few centimeters of the planned route.
Thanks for the response. I was more interested in the cable as it rests, not as it lays. Your response seems to indicate that the cable actually goes all the way down into the trench and lays on the floor of the trench itself. Perhaps this is nieve, but I was thinking that the chasm was like a canyon that needs to be bridged with a dangling cable. Is the slope of the trench gradual enough that the cable is draped down to its deepest depths?
Chris,
OK. I understand where you’re coming from. The cable is designed to land on the seabed at all times. That’s why we lay with slack to allow the cable to fill in all the dips and hit the seabed at the bottom of an upslope (even though the ship could be many kms further ahead of the touchdown point).
If we were to dangle the cable across the Trench, then to two points at each side would be under a lot of tension and, over time, this could lead to a cable failure at these points. We work really hard (or I should say Tyco worked realy hard…) to avoid such suspensions. The steepness of the slopes in and out of the Trench were not an issue for the cable laying and were easily dealt with. In fact it was much less dramatic than we were expecting!
Is the speed of the cable layout changed a lot due to the trench, or just slowed down a bit to allow the additional time for the cable to sink?
Also whats the normal laying speed (or distance per day) once the trench is behind?
David,
Thanks for your comment. The speed of the ship is changed depending on the seabed we’re going across. The speed changes are there to set up the angle of the cable to cope with the changes in terrain. We went across the trench at 2 knots. The maximum ship speed when laying cable is 6 knots (or about 11.5km/h).