Surveyors like to be both precise and accurate and they clearly understand the difference between the two. Modern offshore survey techniques mean high standards of precision and accuracy are not only possible, but required.
Whilst the global positioning service (GPS) units available for cars and leisure activities offer positioning to within a 10m to 20m radius, it can be a requirement in the offshore industry to position something as large as a ship or drilling rig accurate to a centimetre.
The seven-figure log tables may have gone but the space-age equipment to do this has arrived. It exists partly in space and on earth as part of the GPS satellite network, partly on vessels and rigs in the form of powerful computer systems that leave even the fastest modern PCs standing, and partly underwater as acoustic positioning systems which use sound waves to provide remarkable standards of accurate positioning for equipment which can be kilometres underwater.
The skill of the surveyor is to be able to use this technology to best advantage, distinguishing between good and bad results, recording data in an economical, recoverable and readable fashion, and then fashioning and delivering the complex reports that clients can understand and use.
SURVEYING IN THE OFFSHORE ENVIRONMENT
In the offshore world there are added complexities for the team and for the equipment. On a vessel, solving problems can rely solely on the expertise of the team onboard.
The marine environment creates further challenges, as delicate instruments and computers must be hardened against the motion of the vessel, and equipment used above deck, like antennas, must be able to withstand the effects of wind and salt water.
As Fugro Survey’s survey engineer, Ian McKenzie has developed a passion for this precision work. I asked him for a run down on his role and why he still is still passionate about it 20 years down the line.
“I joined UDI Group Ltd as a survey engineer in July 1976, after graduating from Robert Gordons in Aberdeen and I must admit the decision was based on the financial package offered, which had beaten all others by the grand sum of £200!” McKenzie explains.
“UDI became Fugro Survey Ltd, and survey has become my career. Back in the mid ’70s the North Sea survey industry was in its infancy with several companies eager to achieve the technical high ground. Some things never change it seems, survey engineers and hydrographic surveyors were in short supply then, just as they are today.
“Over 30 years later I still get the same reaction when asked “You work in survey? What do you do?” with a puzzled look. The word ‘survey’ covers a wide range of services conducted offshore from survey vessels, rigs and barges.” The main operations are:
- Positioning, which is the provision of precise offshore positioning – today, using GPS (global positioning satellites)
- Geophysical surveys, which provide detailed information and assessment of the seabed and subseabed in preparation for pipelaying, the installation of subsea equipment or even a platform
- Construction support providing support services during the positioning of offshore infrastructure like pipelines, subsea equipment and production platforms
- Inspection services are acoustic and visual inspections of subsea pipelines and associated structures, sometimes using sonar, sometimes, advanced underwater digital video cameras
- AUV surveys conducting deepwater pipeline route surveys
Positioning offshore is provided by using the US Department of Defence’s global positioning system which utilises a ‘constellation’ of over 20 satellites in orbit around the earth. Other similar systems are the Russian GLONASS, Galileo (European), Compass (China) and IRNSS (India), but they are not all yet complete.
GPS makes most people think of their in-car satellite navigation systems but in the oil industry a much more precise solution is required using land-based reference stations providing differential GPS.
DGPS is used in seismic survey, seabed survey, moving drilling rigs, dredging, sub-sea acoustic positioning and installing the offshore infrastructure.
Geophysical surveys are carried out from vessels to acquire seismic, sonar, bathymetry, oceanographic and environmental data. The data gathered is used by the oil, telecommunications, dredging and subsea mining industries.
The surveys identify drilling hazards and establish corridors for pipelines and cables as well as the seabed conditions for sub-ocean mining. This requires cutting-edge technology as the water depths vary from 5m to 3,500m – that’s 3.5km straight down. Typical clients are offshore oil and gas or renewable energy companies, port and harbour authorities and telecoms companies.
Construction support provides installation support and ROV operations and includes acoustic positioning, platform and subsea structure installation, pipelay support, as-built survey, trenching support and project management.
Some truly exciting new challenges lie ahead in construction support as oil is discovered and extracted in ever deeper waters, which creates positioning problems, inspiring the development of inertial navigation systems to aid acoustic positioning at this depth.
Inspections services include ROV television surveys on pipelines, subsea structures and platforms. Data is gathered and processed to give the owner a condition status of the infrastructure whether it is old or new. Initially an acoustic survey is conducted by towed high-resolution sonar using conventional tow fish or ROTVs (remotely operated towed vehicles). Both are towed from a support survey vessel. The acoustic method is approximately four times faster.
Most data is processed offshore on the survey vessel comparing the results with data gathered in previous years. The high resolution sonar gives local seabed conditions adjacent to the pipelines and can monitor fishing activity.
If potential damage is identified it may require further visual inspection / investigation by an ROV. The ROV survey is predominantly visual and the vehicle is tracked acoustically so that precise positioning of any incident is accurately known.
The ROV will also carry instrumentation to monitor the cathodic protection given by sacrificial anodes attached to the structures and pipelines. Should any pipeline be buried then a remote sensing device called a pipeline tracker can be used to locate its position. This is a very sophisticated underwater metal detector which uses sensing coils to detect the magnetic field changes due to the steel pipe.
AUV stands for autonomous underwater vehicle. These vehicles work on their own and have no tether or electrical cable attaching them to a surface support vessel unlike the tow fish, ROTVs and ROVs that I have already described. The vehicles must be self-contained robot devices, not dissimilar to the space probes currently being used to explore the solar system.
AUV technology has developed as the offshore oil industry has ventured into much deeper waters. Conventional survey techniques with towed sensors were not practical due to the length of cable required to enable the tow fish to be as close to the seabed as possible to achieve the data resolution required. With the longer cable lengths positioning accuracy also deteriorated and the length of time to conduct the surveys increased dramatically caused by the vessel line turns required for grids
to be surveyed.
A typical AUV will carry acoustic side-scan, profilers and swathe systems as well as Inertial Navigation Systems and acoustic telemetry for communication with the mother vessel, all leading edge technology. AUV propulsion is supplied by on-board batteries or fuel cells which have to be able to operate for up to 60 hours without changes. These are not like typical torch batteries. The electrical power is provided from the reaction of chemicals stored in the vehicle.
All this equipment has to be strong physically to withstand the pressures at depths of 3,500m. The technologies being used are right at the cutting edge and are complex, sophisticated and expensive, but the survey results being achieved from the sonars, profilers and swathe systems are startling.
OFFSHORE SURVEY OPERATIONS
Obviously, McKenzie has not described all of the survey operations conducted offshore but he has offered an insight to the incredible variety that exists in this space. The client’s end product will always be the final report which is now delivered in the format of CAD charts and databases with integrated positioning and video information.
Modern integrated computer networks, access to the web through satellite broadband, and the availability of high-tech underwater digital cameras mean that what used to be delivered as hand-drawn charts, boxes of VHS tapes and endless paper reports can now be delivered straight to the client’s desktop as a multi-media presentation.
The offshore survey industry provides many career opportunities. The variety of work delivers daily challenges and offers careers involving travel all over the world. Not only does the industry require offshore personnel but also the onshore staff to back up the field operations.
Individuals often have both onshore and offshore careers – some prefer to work offshore and some will eventually come onshore. But there are always opportunities both offshore as vessel managers, party chiefs, geophysicists, electronic and mechanical engineers, hydrographic surveyors and data processors; or onshore as business line managers, project managers, as well as specialists in IT, marketing, software, data management, reporting and interpretation.