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| Subsea wireless video transmission. image source: WFS Technologies |
Subsea wireless communication is moving into a new era. The Subsea Wireless Group (SWIG) hopes to help the industry better understand-and use this technology, thereby introducing new standards. Elaine Maslin reports.
In 1854, Scottish-born James Bowman Linsay patented "a mode of transmitting messages by means of electricity through and acriss a body or bodies of water," sending a transmission 2mi. across the River Tay, between Woodhaven and Dundee, Scotland, in the same year.
Lindsay was ahead of his time. It wasn't until 1897 that Giglielmo Marconi took out a patent on wireless telegraphy, through air, with his "black box." Nearly 120 years later, wireless communication is still seen as something of a dark art, able to transmit multi-gigabytes of data through air.
For the subsea industry, it is an even darker art. The challenge is to harness wireless technologies-using sound (acoustic waves) and electromagnetic spectrum (EM) waves (radio and freespace optics)-to enable through-water communication, control, and even powertransfer, between a suite of permanent and mobile subsea hardware and topsides, vessels, or onshore control and monitoring centers.
While acoustic technologies have been used subsea for some time, and radio frequency (RF) is starting to become established, there continue to be developments in both. New technologies are also emerging, such as free-space optics (FSO), offering further capabilities into the field.
However, according to the Subsea Wireless Group (SWIG), an industry body comprising technology firms, service providers and operators, few applications are addressed by just one of these technologies, due to the unique capabilities and limitations of each.
At the moment, there is also limited interoperability between different technologies and manufacturers’ devices. SWIG hopes to improve the situation. The group was formed with two main aims: to enable interoperability by developing standards, and to help those wanting to use such technologies understand the benefits and limitations of each.
There is no easy answer to the question operators want answered; “how fast and how far can it communicate,” says Ian Crowther, a director at Edinburgh-based WFS Technologies. “It is dependent on many things, not least if a particular system can work in a particular operating environment.
“Optical systems can carry very high bandwidth and, provided the water is clear, transmit over relatively long distances,” Crowther says. “Radio is less sensitive to the environment, communicating through high levels of turbidity, inside complex structure and through solid barriers including the seabed, concrete and even metal. But it supports lower bandwidths than optical systems."
Acoustic signals, or sound waves, travel more slowly through water than electromagnetic (RF and optic) waves (although sounds travels faster through water than air), but, because the wave lengths are longer, and frequency shorter, they travel further and are more robust, but have a lower bandwidth, so carry less data.
Wireless signaling works by encoding, or modulating, a sound or EM wave by altering its shape in a way related to the data required to be communicated.
Over time, methods have been developed to create more complex signals, or wave-shapes, by modulating the amplitude, frequency or phase of the signal either in analogue or digital data format. But, the transmission methods for acoustic and EM signals differ.
Standards
SWIG is developing a set of standards to support interoperability between acoustic, radio and optical systems subsea.
Work began on this body of standards in 2012, focusing initially on radio. The draft subsea radio standard was peer reviewed last year, then presented to the American Petroleum Institute (API) Subcommittee on Subsea Production Equipment (SC17) in January. No decision has been taken yet as to which subcommittee it will come under. This will be discussed at an API meeting in June.
“Radio was chosen first because it was the easiest standard to develop given the number of radio standards available for industrial applications,” Crowther says.
“The idea is it would also create a template for the others, with future standards referencing the first. Work has begun on acoustics and free-space optics standards. Work will begin on subsea inductive power transfer in mid-2014.
These standards will be brought together to form a single hybrid technology standard, to provide a platform on which all the technologies can work together on certain levels.”
The initial work on the acoustics standard has focused on investigating how to define an open standard in a relatively mature market with several de facto standards developed by competitors, most of whom are members of SWIG.
Differentiating subsea wireless technologies
There are currently three types of subsea wireless communication technologies: acoustic, radio, and free-space optics (FSO). Each can be used on their own, and together, for subsea wireless communication. Performance factors to be taken into consideration for deployment are: bandwidth, range, efficiency, cost, and reliability.
Each have their own benefits. Acoustic through-water communication, for example, offers a long-range solution, but is limited in the data it can transmit, while optical communications can transmit HD video over a 1-500m range, but its signal is susceptible to turbidity, bio-fouling and light interference.
Here we give and introduction to each, with help from SWIG and its members.
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| A subsea field, with NASNet positioning stations to provide acoustic positioning. image source: Nautronix |
Acoustics
Acoustic subsea communication is the most established through-water wireless communication technology in the offshore oil and gas sector.
It has been used in the industry for more than 40 years, but it has only recently been honed to a level at which operators are confident to use it for primary control on critical applications, such as primary control of subsea well isolation devices.
Acoustic signaling uses pressure (sound) waves in water. Their lower frequency means less data can be transmitted than a radio or optical signal; but it can be transmitted significantly further because water attenuates pressure waves far less than optical or radio wave forms.
One of the best known uses for underwater acoustic signaling is positioning, for vessels or subsea assets, such as remotely operated vehicles (ROVs) or autonomous underwater vehicles (AUVs), via long, short, and ultra-short baseline positioning techniques.
Acoustic’s use has spread to other applications, such as Tsunami detection. This conveys data from pressure monitors on the seabed to floating buoys, which then transmit the signal, and data, via satellite, to onshore receivers. Acoustic communication is also used widely as a back-up to wired communication for divers, and to send control signals or general data from sensors, loggers, or other monitoring equipment subsea.
“Over the last 30 years, hydro-acoustic signaling has evolved significantly,” says Ben Grant, technology product manager, Nautronix, based in Aberdeen. “Early implementations of acoustic communications and positioning systems were seen as being unreliable and susceptible to interference.
“Going back 30 years or so, acoustic systems typically made use of single frequency, monotonic pulses. This form of signaling was relatively straightforward to generate with the available hardware of the time, but it was susceptible to interference. Since then, the underlying signaling has developed dramatically and signal integrity has increased substantially, leading to an overall increase in the robustness of acoustic systems.”
More recently, spread spectrum signaling (sometimes referred to as digital signaling, due to a change from traditional analogue systems) has been developed, producing a more robust signal, with greater range capabilities.
Spread spectrum signaling works by sending symbols (spreading codes) to represent single information bits. Typically spreading codes are phasemodulated onto a carrier wave, and continuously transmitted. This method greatly improves the signal-to-noise ratio, and also improves the timing accuracy for positioning systems.
This type of acoustic communication was developed in the mid-late 1990s, for underwater defense applications, and was first introduced into the oil industry in the early 2000s by Nautronix, who have developed direct sequence spread spectrum acoustic signaling technology for underwater.
It is now being used in positioning and control applications, including wireless control of subsea blowout preventers (BOPs) or isolation devices. Here, it has predominantly been a secondary system, backing-up a primary wired system. But, some are starting to use it as a primary system (see case study).
New acoustic systems are also merging positioning and control capabilities within a shared subsea equipment. An example of this is Nautronix’s NASNet system, which provides field-wide life-of-field positioning and communications.
In 2008, Murphy Oil deployed an acoustic control system for a surface BOP application in 1200m water depth, on the Azurite FPSO offshore Congo. This application made use if a subsea isolation device in conjunction with the surface BOP. The acoustic system provided primary control of the isolation device. it was used for a continuous two years.
The modulation scheme was an acoustic digital spread spectrum (Nautronix’s ADS²). It operated at a central frequency of 10kHz and sent a heartbeat signal every five minutes, to ensure it was still in operation, with data and control as required. Power supply was via a lithium battery. Two transducers were used topside and subsea, for redundancy.
The deployment removed the requirement for a control umbilical, removing a failure point, and reducing effort required to deploy the subsea isolation device.
Radio
Radio frequency (RF) subsea wireless communication is relatively new to the oil and gas industry.
Radio waves lie on the electromagnetic spectrum below 300GHz. Terrestrially, they are used by mobile phones, AM/FM radio, television, cordless phones, Wi-Fi and Bluetooth.
Extremely low frequency (ELF) systems were used for submarine communications during the Cold War. Operating at 76-82Hz, they sent a few characters per minute across the globe, acting as signal to another submarine to surface for higher-bandwith communications using terrestrial radio.
To create a radio signal, electrical energy is changed into electromagnetic energy. The transmitter provides electrical current at an appropriate frequency. The signal is then modulated, to carry data, and launched via an antenna. A receiver antenna detects the signal, which is then demodulated at the remote receiver.
Radio signals are attenuated when propagating through conductive media. Seawater is particularly conductive. Attenuation increases with frequency in the radio spectrum, therefore subsea radio systems use low frequency radio. At very short distances (<cm) radio can support data rates up to 1Gbps (i.e. HD video), using a higher frequency, enabling great data to be modulated into the radio waves.
RF signals are immune to acoustic noise interference, and any negative effects of turbidly and bio-fouling. Subsea RF also doesn’t suffer interference subsea from other radio, i.e. radio stations, or permanent magnets.
Using low power processors and techniques developed for the mobile phone market, the latest generation of subsea radio are energy efficient and designed to operate for many years off battery.
Subsea radio can be subject to interference from nearby sources of EMI (Electromagnetic Interference).
Common sources of EMI include electric thrusters and high voltage transformers.
The resilience of the latest subsea radios to local EMI continues to improve provided good practice is followed it is possible for subsea radios to be deployed on most subsea structures and vehicles. Disturbances can also be created from ferrous or other magnetically active materials, or electrically induced magnetic fields, such has motors or transformers, moving equipment.
To overcome such limitations, digital signal processing can be used. Data is converted to code-words or symbols, which are applied to the carrier wave using a digital to analog converter. Recent developments have seen RF bandwidths increase,
due to advanced digital signal processing and signal compression techniques. RF can transmit with greater band widths, more quickly than acoustics, is able to cross the water to air boundary, as well as through solid objects, such as pipe walls or ice. However, due to signal attenuation (degradation) through water, its range is limited.
RF has been used in the oil and gas industry since the 2000s. In 2006, WFS launched it’s the Seatooth S1510 Medium range communications system, which is able to transmit signals of up to 16kbps over 20m in seawater, and the Seatooth S5510, able to transfer 1-10Mbps up to 1m through water.
Successful trials using an HD camera clamped to an asset with a 3-6m range to give multiple viewing angles during subsea construction operations have been run by WFS with Technip, Canyon, Fugro, and Subsea 7.
In the North Sea, a new generation of pipeline/flowline flow assurance solutions radio-enabled subsea instruments is being deployed.
This could be used to monitor pressure and or hydrate build-up, for example, building on existing internal pipeline inspection using pipeline inspection gauges. This would utilize wireless communications through the pipe wall, using modems on the PIG and ROV to receive the data, across the higher bandwidth.
WFS Technologies recently delivered wireless communication systems to Baker Hughes for a pipeline precommissioning project.
The firm’s Seatooth S100 system was used for wireless data logging during pipeline pre-commissioning on a project in the South China Sea, in 1000m water depth. The data transfer rate was 2.4kbps over 5m range through seawater (the system can operate at up to 4.8kbps up to 5m through water).
The transmitter included a Seatooth S100 connected to a hydro-test skid. The receiver comprised a Seatooth S100 mounted on the remotely operated vehicle (ROV), but key was its lower power use, relying only on batteries.
Both units were bidirectional and no configuration was required; the system was ready to plug in and deliver serial communications wirelessly between the test skid and ROV. Hydrotest data was downloaded at high speed, despite the high levels of salinity and turbidity.
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| A pipe logger logging temperature/pressure data. image source; WFS Technologies. |
Freespace optics
Free-space optical (FSO) communication offers the greatest potential for high data-rate communication, including HD video in real time subsea, but it also poses challenges and there have been few commercial applications to date.
FSO can have a higher data rate than any other approach because its beam is more collimated (light whose rays are parallel, and therefore will spread minimally as it propagates) and its short waves (higher frequencies) can carry more data.
It uses modest antenna size of about 10cm, with modest power consumption. It suffers less from interference from electromagnetic fields, acoustics, and, in deep-water, background sunlight. But, subsea FSO is challenged by high extinction and the immense variability in the optical properties of ocean waters.
FSO uses visible light, in the bluegreen region of the visible light spectrum, between blue and green on the electromagnetic spectrum, to communicate underwater wirelessly. This is because seawater is light absorptive, except around a 400-500nm (nanometer) wavelength window—the blue-green region of the visible light spectrum.
Blue-green light is therefore able to be transmitted as a continuous wave or pulsed wave, using semi-conductor light emitting diodes (LED) and laser light sources, and detected using highly sensitive, PIN, APD (avalanche photodiode), or single photon detectors.
With today’s technology, FSO range is limited to less than 1km at high datarates (sub- Mb/s), due to beam scattering and beam absorption in water.
Ocean water also has widely varying optical properties, depending on location, time of day, organic and inorganic content, as well as temporal variations, such as turbulence and surface motion. Irradiance levels, even in clear water at 0.5 - 1 km distances, are comparable to those predicted for interplanetary laser communications.
FSO can also suffer from inter-symbol interference (ISI), a form of distortion of a signal, in which one symbol interferes with subsequent symbols, which has a similar effect as noise, and the transmit signal can be dispersed, due to power scattered in water.
To circumvent some of the limitations of free-space optics, optical link concepts have been proposed, using underwater
repeaters (transceivers/relays) to transfer data to and from life of field AUVs, or along a network of links, spaced 0.5km apart, to send a signal. Significant research and testing has been carried out in this field, however, to date, there are few commercial systems available.
Woods Hole Oceanographic Institute engineers recently developed and patented a free-space, underwater optical communications system using light to transmit data through water.
The system, BlueComm, provides 1 Mb/s bandwidth at ranges up to 200m. The system could be combined with underwater acoustic communication technology to provide more modest bandwidths over longer ranges.
BlueComm has been demonstrated in a 2010 project, in which it was used to control an ROV that was installing equipment on the seabed at the Juan de Fuca Ridge in the Pacific Ocean.
Woods Hole has been working with UK-based subsea communications firm Sonardyne International, to get the technology ready for the marketplace.
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