Feature ArticleMultibeam Sonar For ROV Control
By Dr. Ioseba Tena • Ed Cheesman
Recent advances in automation have seen the introduction of navigation systems which are able to fuse data from navigation sensors and use the extracted information to control UUVs. Driven initially by the development of AUVs, the solutions have found their way to the workhorse of the underwater space, the ROV.
The ROV has been instrumental in the development of subsea fields. So much so that when new subsea infrastructure is designed an important requirement is to allow seamless ROV operations. From drilling (where the ROV is used to monitor the blowout preventer and riser) through construction support (surveys, touch-down monitoring, interfacing, tooling, etc.), IRM (inspection, repair and maintenance) and even in the decommissioning phase, the ROV presents the easiest and safest way to interact with the subsea environment, enabling truly amazing feats of engineering and ingenuity.
From their initial development to their current form, ROVs have seen many changes and improvements. They have become more capable, going deeper and tackling a greater variety of tasks. Today, there is also a greater range of ROVs, from small, one-man portable systems to large, sophisticated work-class systems operating at thousands of feet from dedicated offshore vessels. The extensive use of ROVs has created a need for expert operators with extensive training. Their skills are required as much to pilot the systems as they are to service and maintain them. Ultimately, the bottom line is tied to how efficiently and expertly an operator can fly the ROV. Until recently, the control of the ROV remained tied to the joystick commands affected by a pilot looking at a video monitor. This is an extremely challenging task, as the pilot is deprived of many sensors and is restricted to the use of 2D video monitors and some gauges.
Thus, the introduction of automation has been a welcome addition. By fusing the navigation data, the ROV user can now command the ROV to hover in one place for extended periods of time, to cruise at a given velocity and even to follow a set of predefined waypoints. All of this is done in relation to coordinates in a local or global set of references, meaning that the ROV can be made to move to a position or distance measured relative to the ground. For the most part, the ROV interacts with objects, not with the ground. Many of the objects are static on the seafloor but others are flexible and may not be in a known position, so the ability to estimate their position and then control the ROV relative to them stands to improve operations radically. From monitoring loads as part of construction support at the initial stages of a field development and inspecting risers from seabed to surface during its life all the way to inspecting the structure before a decommissioning job, object-relative positioning for ROVs adds undisputed value to operations.
In order to achieve reliable control, the user must be able to obtain reliable positioning information, measuring the distance and orientation between the object and the ROV. This challenge has been met with the use of a new generation of sonar known as multibeam imaging sonar (MBIS).
Multibeam Imaging Sonar
MBIS are active devices which use electrically excited ceramic arrays to emit pulses of sound into the water and listen for acoustic returns or echoes. Through a method known as beam forming, the transducer array is divided into a series of pseudobeams which are used to measure the range and intensity of the returned sound signal by breaking it into a fixed number of equiangular channels.
The recorded signal from each channel is passed through an analog-to-digital converter and then through a signal processor before finally being plotted to create a digital 2D image that can be visually interpreted.
As with all acoustics, the usable range depends on the frequency employed (a low frequency propagates further than a high frequency) and the strength of the return from the target; itself a function of the material from which the sound is being reflected. Soft sediment and sand, for example, are poor acoustic reflectors making them difficult to detect against ambient noise, whereas metal and concrete tend to give off a much stronger 'ring' and can therefore be detected at greater distance.
Typical MBIS units in the market range from about 200 kilohertz (which will allow stronger targets to be detected at several hundred meters) to 3 megahertz (which will detect only the brightest targets beyond about 5 meters). To date, the most common MBIS in use operates at a center frequency of 900 kilohertz.
Importantly, unlike their predecessor, the single-beam scanning sonar that creates a single 2D image by sweeping a pencil beam across a given area of interest, an MBIS collects data from many hundreds of beams simultaneously, multiple times per second, creating a series of images every second rather than a single image every four or five seconds. This has the advantage of providing the user with a streaming video-like image rather than a series of captured stills. (For this reason, ROV pilots sometimes refer to MBIS as acoustic cameras).
Furthermore, due to the fast update rate (modern imaging sonar can image at more than 20 hertz), multibeam imaging sonar are motion immune, i.e., they do not smear or blur when operated from a moving platform. There is no longer any need to stop and scan when navigating an ROV by sonar. This makes an ROV a more efficient tool for aiding piloting operations when visibility is poor, e.g., in areas of high turbidity or when operating in the dark such as deep water or night-time operations (during night-time operations, ROV lighting typically limits visibility to just 10 meters in clear water conditions).
Imaging sonar therefore complement optical camera- based navigation by allowing their user to see through suspended sediment and far beyond the limitations of either natural or artificial light, and are rapidly replacing scanning sonar as standard tooling across the ROV industry globally. Sonar images also provide range information where optical imaging systems cannot.
While the MBIS is used today as a standard visual aid for the manual piloting of ROVs, of perhaps greater worth is its pioneered use as a sensor input for the dynamic positioning and automated control of ROVs. Using automated target- recognition routines, range and relative bearing of selected targets (to the vehicular frame) can be streamed continuously to the control system, allowing more complex tasks to be undertaken, i.e., following the track of a riser, surveying around a target or maintaining a constant offset from a chosen target, e.g., a blowout preventer, while monitoring or intervention work is completed. To continue this article please click here.
Dr. Ioseba Tena is SeeByte's sales manager, responsible for the development and delivery of SeeByte's marketing and sales process within the company. Tena joined SeeByte as a founding member of the management team. He graduated with a Ph.D. in engineering from Heriot-Watt University in 2001.
Ed Cheesman is senior manager of sales and business development at Teledyne BlueView. Cheesman holds a bachelor's in engineering from Aberdeen University, where he graduated in 1999. He has worked in a variety of technology and commercial roles in the underwater industry.
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