Frequently Asked Questions:
How reliable is wave powered profiling?
Compared to battery-powered crawlers and buoyancy driven devices, the Wirewalker profiler is extremely reliable. The Profiler efficiently harvests energy from the high-frequency end of the surface-wave spectrum, enabling profiling in 1-3 kt wind waves. When conditions are totally “glassed off”, with neither wind waves nor swell, profiling might cease. These conditions are rare in the ocean perhaps less so in lakes. When the waves return, profiling is quickly resumed.
What is the chance that I’ll lose all my gear?
Essentially, the Wirewalker is just a smart way to attach your instruments to a surface mooring wire. Instead of being damaged by mooring motion, your data record will now be enhanced by it. While the attachment concept can be executed inexpensively, users quickly found themselves motivated to attach $100K or more of instrumentation to the profiler. At this point, the design of the Wirewalker evolved to protect the payload, on-deck on research vessels, over the side, on small boats, and in the relative tranquility of the deep sea.
The principal risk to your sensors will be associated with the integrity of the mooring to which it is attached. Poaching by local fishermen is a concern in some operating areas.
The DMO Wirewalker package comes standard with an Iridium Tracking system as well as strobe to aid in locating the Profiler.
Does it operate as a fixed mooring?
The Wirewalker profiles along 3/16” galvanized wire that can be easily moored in coastal waters. DMO has a variety of mooring designs that we can suggest. In extreme coastal conditions where a lightweight mooring is inadvisable DMO can work with you to devise a suitable mooring.
Similarly, the Wirewalker surface float is too small to support the 4-5 km of wire necessary to attach to the deep sea floor. Tethering a Wirewalker to a standard deep-sea surface or sub-surface mooring is quite feasible and DMO will be happy to discuss design approaches with you.
Does it operate in free drifting mode?
Free drifting deployments are the forte of the Wirewalker.
What are the principal failure modes?
The galvanized wire is trustworthy for 10-50 thousand profiles. In shallow (10m) water, the Wirewalker might make 50 thousand profiles in a month. Used cable can also corrode while in storage between deployments. It’s best to plan to replace the cable after every deployment. Compared to replacing the batteries in your instruments, this is a minor concern and expense.
Will it work in a strong current?
The tilt of the Wirewalker suspension cable significantly influences profiler performance. The more-nearly vertical the wire, the more predictably the Wirewalker performs. The 20kg weight that is supplied standard with the Wirewalker is sufficient to guarantee good profiling in currents up to 50 cm/s. In stronger currents more bottom weight can be added, although as weight is increased, the load on the Wirewalker Profiler increases.
Being wave powered, does it only work near the surface?
Rather the reverse. The Wirewalker derives power from the differential motion between the suspension cable and the surrounding water. This difference increases with increasing distance from the sea surface, as the Wirewalker can harvest power from progressively lower-frequency components of the wavefield.
For deep profiling operations, it is necessary to use floatation that can tolerate high pressure. (Buoyant foam rated to 300m comes standard.) The buoyancy of foam floatation decreases as its pressure tolerance is increased. So deep profiling Wirewalkers require more foam and /or a smaller payload.
Our deepest experience to date is a 500 m free drifting deployment in the Indian Ocean. The Wirewalker made the round-trip in 50 minutes.
Will seaweed / kelp quickly inhibit profiling operations?
Fouling is a fact of life in coastal waters. In tests off San Diego, kelp frequently engulfs moorings. However, the vertical motion of the Wirewalker suspension wire has proven very effective in sawing through even thick kelp. A bad entanglement will interrupt profiling for roughly three hours in moderate sea states.
Stray fishing gear is more of a concern in some regions. Here, the ability of the Wirewalker to cut itself free is difficult to predict.
The worst-case situation is that you get a standard fixed-depth data record from your sensors, rather than the desired two-dimensional data set.
How long can a WW operate unattended?
Sensor battery life typically determines mission duration. The problem is that your favorite sensor, which you used to sample at 5 minute intervals, will now be recording at 2 Hz, in order to resolve the beautiful detail of the vertical profiles that “fine-structure contaminated” your data in the past. So you will be burning through batteries more quickly than previously.
With long-duration low-power sensors, bio-fouling can become an issue. In many profiling scenarios, the instruments experience a large range of temperatures and light levels every few minutes, providing an inhospitable environment for bio-growth. Still, fouling remains a concern for long-term operations.
The Wirewalker suspension cable should be replaced every 10-50 thousand cycles.
Can I get my data back remotely, in real time?
Soon. Inductive modem-based data transfer systems can be supplied by instrument makers such as RBR Electronics and Seabird Instruments. We are working with these and other suppliers to offer a turn-key system. Final reliability tests of the Wirewalker using inexpensive insulated cable are underway. The initial tests are extremely promising.
How hard is it to get started? Do I need to send my tech to a three-week training school?
If you already have experience deploying instruments at your site of interest, converting to Wirewalker profiling will be straightforward. We can help you to get started.
Famous beginners errors include:
Adjusting the ballast in your fully loaded profiler while there are still air bubbles trapped in the profiler body. On deployment the profiler will make a few profiles and then sink comfortably to the bottom of the wire.
Designing a coastal mooring such that the suspension weight settles on the bottom at every low tide. Profiling will cease until the tide comes in and the wire is free to oscillate with the waves.
Deploying the Wirewalker is like flying a kite: tricky the first time but straight-forward after that. Still, preparing any measurement system to survive an extended marine deployment requires enormous attention to detail. The profiling aspect is the easy part.