Mar 23, 2021

Enabling Persistent Maritime Surveillance

Over a two-week period in late 2020 in the protected waters of Vancouver, Canada, International Submarine Engineering Ltd (ISE) tested its prototype of an underwater dock. The dock is a platform for autonomous underwater vehicles (AUVs) to upload data and re-charge their onboard batteries all while staying submerged. Setting aside the technical achievement, in this article we’re going to discuss why this is important and what this means for general AUV operations as well as with regard to maritime surveillance.

Monitoring and mapping the waterways has always been a priority for sea-faring nations. Whether it be for defence purposes or for scientific research, understanding these places has been critical for success to the civilizations that surround them. The knowledge of ones own coast includes the ability to defend against foreign entities before they have the chance to do harm. This is best achieved by using preventative measures such as constant monitoring of the coastal waters.

Currently this monitoring is done in a variety of ways, the two most common being the use of fixed systems such as hydrophone arrays and secondly, ship-based monitoring, of course both have their pros and cons. In a nutshell, fixed systems are easy to deploy and can monitor continuously as long as they have shore power of some sort. The catch is that they require shore power, meaning they need a power grid to stay continuously on. Furthermore, as they name implies, they are fixed, so if someone knew of their location, they can easily be avoided.

Ship based systems are mobile, and that is their greatest asset. Also, every major coastal country has some form of navy which of course would possess some ships. Adding some sensors or even some marine robotics to an already existing infrastructure is fairly simple. Conversely, ships are costly to operate and also they are easy to spot; therefor, minimizing some of the desired requirements in coastal surveillance.

AUVs are increasingly becoming more common in their use as survey and subsea monitoring vehicles. They can have high quality sonar systems installed on them and can operate a lot closer to the survey target than the surface ship can. Furthermore, they are much quieter than a ship which would allow hydro-acoustic systems to perform much better. The main downside of AUVs has always been the energy capacity that they are able to carry. An AUV can only travel so far on a single charge, and this has generally meant missions under twenty-four hours duration. Once the energy is depleted the AUV must be recovered so the data can be downloaded, and the batteries recharged. The physical act of launching and recovering an AUV off of a support vessel for this purpose is often regarded as the point of the mission with the highest likelihood of failure. Adverse weather conditions further increase the risk of failure and can lead to damage to the assets or the personnel involved. Underwater is where an AUV is designed to operate and it is there that it is safest.

In water docking eliminates the need for launch and recovery onto a ship or surface platform. Simply put, when the AUV runs low on energy, it will autonomously navigate itself to the subsea dock where it latches to be charged and have its data offloaded. This can all be done regardless of the weather at the surface. With the ability to charge AUVs on the fly, (much in the same way that an air force refuels their planes while in flight), continuous subsea AUV missions can be achieved. This is a step forward in subsea surveillance and is a significant force multiplier to ones defence strategies.

The ability to charge an AUV on the fly also changes the approach that one could take with their AUV fleet. Currently AUVs are increasingly getting bigger so that they can handle more batteries; this is all done with the goal to increase mission durations. By adding the ability to recharge oneself during a mission, energy capacity onboard an AUV is less critical and smaller battery banks can now be used. This would mean that smaller AUVs can be utilized which of course would mean less costly AUVs.

One step further, consider the idea of replacing one large surveillance AUV with several smaller AUVs that can recharge on the fly and thus achieve continuous monitoring. This is a big step with regard to surveillance.

The next big question with the dock must be, what is the dock attached too? Will it be towed or will it be fixed? There is much flexibility with this question because of the very nature of what the dock is. Towing the dock can be done using either a ship or an autonomous surface vessel (ASV). Both have the ability to provide power down the line to the dock as well as providing a surface support station to upload the data too.

With the technological gains that AUVs have made in the last few decades, so have ASVs. It is little wonder that they are the future for surface vessels as they are cheaper to operate and can navigate in harsher climates without the downside of endangering a crew. Pairing an AUV or even a fleet of AUVs with an ASV is a perfect match as ASVs are ideally suited to play a part in persistent maritime surveillance. The ASV can tow the dock along a coastline for example, all the while the AUV is doing its survey. As needed, the AUV can then find the dock to recharge and upload its latest data.

While a mobile dock is very versatile, the use of a subsea fixed dock platform might better suit some applications. The idea is to fix the dock in one location that the vehicle would again return to as needed to charge or upload data. In this configuration the dock can be wired directly to shore power but also hidden because it would never have the need to surface. This could possibly be best suited for areas where constant monitoring is important such as around a port or along popular shipping routes.

Aside from defence strategies, the constant monitoring can also be a great asset to the science community for the same reason, persistent surveillance. Current scientific stations are, for the most part, fixed systems. While these have many benefits, they can only collect data on points of interest that are in close proximity to the sensor. A mobile platform has the obvious advantage of scanning larger areas. In the past this may have been too costly for private institutions to afford as it may typically need a support ship, crew and a way to launch and recover the AUV. With a permanent dock, there is no longer a need for daily support thus reducing the cost of the surveillance.

Persistent subsea maritime surveillance is an important key to ensuring a safe coastline and can add valuable data to the science community. AUVs have proven to be very effective in gathering this data in a costly and timely manner. Of course they have had their limitations, with the greatest challenges attributed to their battery range and launching and recovery of the vehicle. The dock provides a forward-thinking solution as it eliminates the need for constant launching and recovering of the vehicle by providing an in-water power charging and data transfer station.

Find out more about ISE’s docking project here:

This article was written by Dougles Ramsay of ISE and was originally posted on ROV Planet: