Annual ROV pipeline survey specifications tend to place considerable emphasis on visual records and the commentary made by the survey crew However, the latter is based largely on the former which are, in turn, restricted by the field of view and the uncertainty of orientation of the boom cameras Thus a slow and laborious process yields only limited and subjective results In addition, re-visiting old records is not only extremely time-consuming, tedious and costly, but does not facilitate any comparison from year to year This paper, which is based upon three recent studies, looks at the limitations of current survey techniques and explores, from the point of view of the pipeline engineer, some alternative and innovative approaches aimed at improving pipeline survey By taking advantage of recently-developed techniques, it will be possible to obtain and present more pertinent data The way these data can be used to advance the understanding of the dynamic interactions between pipelines and the seabed are discussed
Every year, hundreds of kilometres of subsea pipeline are surveyed to ensure, on behalf of the operators and, ultimately, the relevant regulatory authorities, that the pipelines are neither damaged nor in a perilous state There are two principal survey methods used
Side-scan sonar survey, and
Video survey by ROV
In general, the former is used to conduct a preliminary survey of the whole length, usually annually, while the latter is used only to carry out more detailed investigations of areas of particular interest or worry However, full video surveys are generally carried out every fifth year at least
Once a pipeline has been installed, the specialist pipeline engineer tends to be called In only when a particular problem is identified By this time, the information available from the various annual and major surveys may not present the most useful picture, and the resulting analysis may have to be quite subjective
This paper looks at the capabilities and limitations of each survey method, from the point of view of the pipeline engineer, and then considers how alternative techniques could enhance the presentation of survey data The longer term aim would be to utilise survey results for prevention rather than cure
Annual pipeline surveys are required to identify, as far as possible
[1] :
Pipeline movement,
Freespans,
Exposure of previously buried lengths;
Loss of weight coat;
Damage to the pipeline, or fittings,
Local debris,
Excessive marine growth, and
The state of the cathodic protection (c p) system.
These regulatory requirements tend to result in surveys being conducted in the sense of "is it still alright?"
Annual surveys are usually conducted by side-scan sonar These are performed by towing the fish parallel to the pipeline at a set altitude and offset. Shadow techniques are then applied to identify any exposure or significant lateral movement. Local debris will be highlighted, while the existence of freespans or localised damage may also show up Side-scan sonar surveys are relatively simple, fast and inexpensive, but do have disadvantages
