The subsea environment creates specific inspection and surveillance problems:
- accessibility for inspection and maintenance (e.g., no human intervention is possible at depths >300 m).
- the pipeline configuration may prevent external inspection; for example, pipe-in- pipe systems, riser caisson assemblies, etc.;
- most pipelines are producers with no redundancy for alternative route (i.e., if they fail there is an increased cost of lost production);
- increased cost of remedial works;
- third party interaction – fishermen, and other users of the sea are in continual contact with pipelines.
The effects of all these special considerations are:
- Inspection and surveillance costs can be greater than 10 times the onshore cost.
- The subsea environment can lead to ‘special’ types of threats to the pipeline; for example, pipeline spans, pipeline exposures, external damage from dropped objects, buckling (lateral and upheaval), etc..
- Different types of inspection methods and tools are needed.
Subsea pipelines, or water crossing, can be surveyed by either:
- hydrographic surveys;
- ‘remotely operated vehicles’ (ROVs); or,
- ‘smart’ (‘intelligent’) pigs.
Hydrography is the measurement of various physical characteristics of the oceans (or other waters) such as bottom depth, currents and waves. Hydrographic surveys map, chart and record underwater environments. These surveys use:
- acoustics, seismic, magnetic, and electrical survey;
- a wide range of survey vessels of differing sizes; and
- a wide range of equipment (ROVs, AUVs, etc.).
Types of Surveys
Subsea pipelines receive numerous inspections/surveys:
- General Imaging Surveys (GI) (uses acoustic (side scan sonar), wide seabed coverage, fast and relatively cheap);
- General Visual Inspections (GVI) (uses visual methods to inspect: ‘work class’ ROV, cameras, and additional sensors);
- Corrosion Protection Survey (often undertaken as part of a GVI survey with a CP probe and visual inspection of anodes);
- Close Visual Inspections (CVI) (specific features identified using divers or ROVs);
- Landfall Surveys;
- Riser Surveys (ROV, etc.).
General Imaging Survey
A General Imaging (GI) Survey provides an overview of the pipe and surrounding seabed:
- identifies extent of burial, exposure and spanning;
- indicates condition of trench;
- identifies presence of any debris or signs of third party interaction local to the pipeline;
- identifies areas of degradation since last survey;
- identifies areas of pipe movement.
A GI survey can be used to identify features which require additional investigation; i.e., General Visual Inspection. ‘Side scan sonar’ (creating an acoustic image of the seabed) can be used.
Side Scan Sonar is generally deployed on a Remotely Operated Towed Vehicle (ROTV), but can be also be mounted on an ROV or Survey Vessel. It is towed parallel to the pipeline at a fixed offset. An acoustic pulse, narrow (1.5 degrees) in the horizontal plane and broad (60 degrees) in the vertical, is transmitted to hit the seabed at an angle. The strength of the reflected signal is plotted against time with the scale resetting to zero with each new pulse. With each received signal a picture is built up analogous to an aerial photograph.
The recorded reflected signal can be interpreted to provide dimensions of objects, spans and areas of movement along the pipeline route. Resolution and range are limited by the frequency of sound wave employed. The lower the frequency, the higher the range, but the poorer the resolution.
Frequency of side scan sonar can vary from 50 kHz to 650 kHz.