The location and strength of the Aleutian low is the major controlling factor for weather in the Northeast Pacific. During the summer months it retreats poleward and weakens allowing a high to develop in the region. This tends to deflect passing storms northward, resulting in warm sunny weather on Vancouver Island weather.
World map marked with ocean currents and winds moving around semi-permanent high-pressure cells typical for the month of January. Aleutian Low marked.
This year, the Aleutian low stayed strong well into summer and now appears to be making an early comeback. When the low is in place, we typically expect a 4-day cycle of storms, steadily propagating one after the other across the northeast pacific. As they approach, these storms generate waves from the southeast, which back to the west-southwest after storm passage. The result is a seemingly perpetual high sea state, with waves criss-crossing in multiple directions.
However, during the fall transition period, there is a lot of variability as the winter regime develops. I think we still have good reason to expect breaks in the weather pattern before the Aleutian low establishes itself.
That is where we find ourselves now, waiting for a weather window to deploy our heavy spool of cable to the seafloor, so we can connect the Mothra hydrothermal vents to Endeavour node and the rest of our network.
The 8km spool, which weighs in at about 3200kg, is fastened into ROCLS, light aluminum frame and mechanical apparatus for cable laying beneath ROPOS. ROCLS is lowered first to the seafloor independently of ROPOS, then ROPOS descends and latches onto ROCLS at the seafloor to lay cable.
Two ROCLS frames at the CSSF/ROPOS facility in Sidney, BC, 6 September 2011.
ROCLS with the 8km Mothra-Endeavour Node cable spool, September 2011.
On an oceanographic research vessel like the R/V Thompson, there are two options for deploying ROCLS:
- off the stern through the A-frame, or
- off the side utilizing the ship's crane.
The first option is, I think, much safer for crew; however, wave-induced pitching is substantially greater at the stern of the ship, which further limits seas in which we can successfully deploy. With this in mind, we have already positioned ROCLS on the starboard side within reach of the crane, just aft mid-ship where pitching is minimized. With the wave field we are riding on now we would not be able to move it safely over other equipment back to the A-frame, so this positioning was done in anticipation of deteriorating weather. The question now is, when will the sea state be calm enough to launch?
There are two major areas of consternation for deploying ROCLS, getting it off the ship into the surface waters, and landing it on the bottom. Moving a 3200kg object on a heaving, pitching, rolling ship, is no mean feat. Our launch procedure involves roping tag-lines to all four corners and placing cleats in appropriate multiple locations to attempt to control the load. But how many wraps are needed on the cleats for human power to hold 3200kg in rough seas? If you have too many wraps, you can't release the load properly and risk losing control, but with too few wraps you have no control over this enormous load. Wrapping and unwrapping a heavy load is dangerous, only experience can guide you.
The deployment procedure also includes inserting an acoustic release (more about this below) between the ship's wire and ROCLS. In order to mitigate damage to the cable by the acoustic release after ROCLS is dropped, we attach floats to the acoustic release so it rises up and away. The floats are made of syntactic foam, which does not compress under pressure, and weigh about 14kg in air. For this operation, we need 12 of them (weighing 160kg) strung along a line. These floats are hand balmed over the side concurrently with the launch of ROCLS. During the previous launch of ROCLS in calm weather conditions, we needed 11 people on deck for this operation.
As ROCLS goes over the side, our main concern, after safety, is keeping ROCLS from smashing into the side of the ship. It is an aluminum frame and is quite fragile when fastened to a 3200kg cable spool. So the deployment crew try to get it into the water quickly once it's overboard.
After ROCLS enters the water, our main concern is snap loading on the ship's wire and the ROCLS attachment point. With 8m waves that are often out of phase with the ship's heaving, there will be times when the phase mismatch can put sudden extreme strains or "snap loads" on the cable. Snap loads, not static loads, are the times when cables snap and equipment typically breaks.
Once ROCLS is lowered into the water to depth where we can control it and snap loads are reduced, we need to attach a transponder to the line. To do this, the ship's wire needs to be brought to the side of the ship, where some brave people are required to attach a transponder to the wire that is part of the USBL (Ultra Short Base-Line) navigation system we use for positioning ROCLS at the seafloor.
The second area of concern occurs when we have navigated to the surveyed landing location and lowered ROCLS to the bottom. The acoustic release holds fast while ROCLS is lowered, until an acoustic signal is sent to it from ship and it unfastens. We try to release ROCLS just above the seafloor.
Currently, measurements indicate a trough-to-crest wave height of about 8m. So we need to estimate our distance from the bottom carefully and time the waves so we minimize the free fall and potential damage to ROCLS and our $300,000 cable.
And again, the key question is what sea state/ship dynamics make this achievable? At present, the sea state we are confronting is too rough to risk deployment.
A further confounding consideration is making sure we will be able to lay the cable. Before we can lay cable between the Mothra hydrothermal vent fields and Endeavour node, ROPOS must be deployed with the Mothra Instrument Platform. Such a dive and the subsequent cable lay are also weather-dependent, but it is imperative that they be done if ROCLS is deployed. ROCLS is made out of aluminum, stainless steel and ferrous materials (basically a large battery in seawater) and if left at the seafloor would rapidly corrode over the winter, becoming structurally unsound by next summer. If we deploy ROCLS now but are unable to lay the cable this cruise, both ROCLS and our $300,000 cable will be lost.
So as you can see, there is a high level of complexity to installing a subsea cabled network. Not only are good weather and relatively calm seas required, but also a well-equipped and expertly staffed research vessel like the R/V Thompson is essential. A little good luck doesn't hurt, either!
(NOTE: No facts have been verified in the writing of this blog post, it is based strictly on the author, Steve Mihály's opinions.)