Triaxus, a 3-D Underwater Undulating Towed Vehicle

Undulating towed underwater vehicles have been used for a number of years, mainly in the ocean science community, hydrographic industry and by navies. When looking at the systems currently in use, there are several issues and problems to be addressed such as size and weight of vehicle and winch/cable, sufficient space for payload sensors, payload sensor power supply and data transfer, tow cable breaking strength/drag/diameter, 2-D undulation / 3-D undulation.

Undulating towed underwater vehicles have been used for a number of years, mainly in the ocean science community, hydrographic industry and by navies. When looking at the systems currently in use, there are several issues and problems to be addressed such as size and weight of vehicle and winch/cable, sufficient space for payload sensors, payload sensor power supply and data transfer, tow cable breaking strength/drag/diameter, 2-D undulation / 3-D undulation.

Some of the existing systems use large electrical or hydraulic motors to drive the control flaps. This means big heavy metal frames resulting in a relatively heavy vehicle both in air and in water. Furthermore, the large motors leave limited space for payload sensors protected within the vehicle body. In order for a heavy vehicle to obtain an acceptable undulation envelope, especially at relatively low tow speed, the tow cable needs to be faired with hard fairings. This results in a complicated winch and launch / recovery system requiring more deck space and higher power consumption.
Another big issue when designing underwater towed undulating vehicles is the outer shape of the hull. In order to obtain stable attitude of the vehicle, both at high and low forward tow speed, it is necessary that hull design will allow the payload to be installed inside the hull as far as possible. If large payload sensors are installed external to the vehicle hull, it may have a major impact on the vehicle’s performance, especially at high forward tow speed. One way to overcome this is to use a neutral or slightly positive buoyant vehicle design. This will result in a stable attitude behaviour over an extended speed range (2-12 knots) and, depending on the detailed hull design, it will allow generous room for payload sensors inside the hull.
Installing payload sensors on a vehicle should be easy for the user, especially on cruises where the speedy change of sensors for other work tasks is critical for the cruise programme. Another important issue when discussing payload sensor interfaces, is the ability of the system to accept sensors with a high data rate, especially when the user requires real-time data transfer to the surface processor unit for all the installed individual payload sensors. All of the above mentioned features can only be obtained if a fibre-optic transmission and multiplexer systems are implemented. Furthermore, it must be possible to supply all the required AC and DC power to the individual payload sensors from the vehicle control system; and it is also a big advantage to be able to monitor the individual power supplies and even switch each individual power supply line on and off via the control system in order to avoid interference between sensors.
The use of fibre-optic transmission technology is also having a huge impact on the tow cable design, especially on the outside diameter of the cable. The cable will typically have two or three power conductors, two or three optical fibres, all covered with a dual steel armour and the diameter will be typically between 8 and 10mm. By utilising the right armour package design, this cable will have excellent breaking / working strength capabilities as well as low drag coefficient figures.
All of the existing underwater towed undulating systems on the market are based on 2-D vertical undulation (Y and Z), fulfilling most user requirements, but there are various cases where a third dimension (X) is required. One of the obvious applications of using the 3-D undulation, is in shallow water, when undulation directly behind the surface ship will result in distorted data due to the ship’s propeller stirring up the water. In this case, the 3-D underwater towed undulating vehicle can be offset horizontally relative to the ship,
and undulation can take place in undisturbed water. Another application is using the vehicle at an auto depth and then undulate horizontally from side to side to collect data in a certain horizontal water layer. Finally, the 3-D vehicle can be set to undulate vertically and horizontally simultaneously for maximum water column coverage.

System Description
The new 3-D underwater towed undulating system includes the following main elements:

• Surface control unit
  
• Winch and tow cable
  
• Underwater vehicle