Growing low-cost subsea vehicle market enabled by sensor innovations
The development of small, low-cost subsea vehicles is facilitating commercial and research missions previously beyond the budget and capabilities of many potential users. But getting the most out of them requires a new wave of sensors too
The subsea vehicle market is no longer the preserve of companies and institutions with deep pockets and highly specialized expertise on tap. There is a new subsea community keen to use small, low-cost and easy-to-operate ROVs and AUVs that would have been out of their reach before now.
Right-priced, compact subsea navigation sensor for lower-cost vehicles
That’s great news. But it’s not all plain sailing, because, for these vehicles to maximize their usefulness, the vehicle pilot or operator needs to know where the vehicle is at all times, and once it arrives on location the operator must exploit sensor-aided vehicle control. This need is exemplified when using ROVs to inspect fish farms, maintaining navigational consistency for offshore subsea survey vehicles, or deploying ROVs to perform visual inspection of a wind turbine in high currents.
Both of these operational requirements – navigation and vehicle control – require a suite of sensors. The challenge is composing this suite of sensors in a form factor small enough for the space available, and they need to be cheap enough to be affordable in the context of an overall mission budget. Keep in mind that both are likely to be quite tight.
A Doppler velocity log (DVL), for example, is a necessary sensor for many subsea vehicle activities. It is a critical component for providing accurate navigation, station keeping and altitude measurements. But unless the DVL is small enough and cheap enough, the disadvantages may outweigh the benefits for low-cost ROVs and AUVs.
“Low-cost vehicle operators are not looking for the survey-grade DVLs Nortek traditionally has offered. What they want is something that makes their life a lot easier – a device with all the functionality they are familiar with, but at a size and cost that is complementary to the vehicle,” says Torstein Pedersen, Product Manager – DVL at Nortek.
“If one digs a little deeper, the desire is for a complete sensor package that saves cost and time for system integrators, who often have to integrate, test and calibrate several sensors, and this is something we have been thinking about for some time,” he adds.
The ideal solution is obviously something more compact than a standard DVL and at a more affordable price. That requires compromises: reducing the size of a DVL while keeping costs low generally means it will have lower accuracy than a larger sensor of the same frequency.
But that’s not necessarily a deal breaker for the sorts of uses for which people tend to deploy smaller subsea vehicles. Navigation sensors on low-cost vehicles don’t necessarily require the same high level of locational and velocity measurement required for survey-grade vehicles. Sensors just need to be part of a good-enough, all-round solution, size-adjusted to their needs, providing adequate vehicle control and navigation.
“They want to have something that is small, in their price class – and the performance has to be proven and sufficient for their needs,” Pedersen says.
Subsea navigation sensors need to offer ease of use for a broad range of users
Ease of use is a more and more important part of the package. The increasing ubiquity and usefulness of smaller subsea vehicles means more people from a broader range of backgrounds are involved in the sector. AUV and ROV operation is no longer the exclusive preserve of operators with months of specialized training.
Anything that helps inexperienced users to control vehicles is becoming highly desirable. So, vehicle and sensor manufacturers are putting increasing emphasis on making their products as user-friendly as possible.
Sensors, such as DVLs, have a major role to play here, because they make it easier to carry out potentially tricky tasks such as station keeping, operating at a constant depth, or navigating to a site in areas of unpredictable current or complicated seabed topography.
“We can imagine that, in the future, many coastal facilities or vessels will have a small vehicle that any of the personnel or crew would have the ability to launch and use to conduct an inspection,” says Pedersen.
Providing a subsea navigation sensor suite in an ultra-compact instrument
Integrating the many different sensors in as energy-efficient a way as possible on a vehicle is important to provide the best navigational results and data. In many cases, the job of integrating the altimeter, attitude and heading reference system (AHRS), pressure sensors and temperature sensors will be that of the vehicle manufacturer.
Nortek meets the needs of this burgeoning market with its Nucleus1000, a three-transducer 1 MHz DVL that provides a suite of sensors for control and navigation in a compact instrument that weighs just under 300 g in water and measures 90 mm in diameter by 42 mm in height. It has a depth rating of 300 m. Average power is less than 4 W. The Nucleus1000 is unique in having all the sensors necessary for navigation and control in such an ultra-compact package.
A temperature sensor together with an estimate of salinity helps improve the DVL accuracy by estimating speed of sound in water. The position in the water column is provided through the pressure sensor (depth), along with a dedicated altimeter – a vertically oriented transducer that provides altitude estimates directly below the vehicle. The Nucleus1000 incorporates a high-grade MEMS-Magnetometer AHRS.
The sensor also offers water tracking when the bottom is out of range. It can be used for current profiling using alternate pings.
The Nucleus1000 is offered with two levels of accuracy. One is export-license-free with around 1 percent accuracy – good enough for a wide range of tasks. If the user needs greater accuracy for higher-grade applications, there is also a 0.3 percent accuracy option (which is export controlled).
Nortek helps make the integration process easier by pre-calibrating and pre-aligning the DVL and AHRS in its calibration facility. All the skill that is found in the survey-grade DVL is also in lock step with the firmware that runs on the Nucleus1000.
If the instrument is not pre-calibrated then a system integrator with an AHRS and DVL installed on their vehicle will need to take the vehicle out in the field and do the calibration and alignment process, which is time consuming and expensive. So, pre-calibration is a major benefit for the user.
After reviewing the capability of the Nucleus1000, you will quickly realize that it saves the end user countless hours of integration and calibration. It is also a cleaner configuration with a single power demand and communication line, as opposed to several that require synchronization and management.