The invention relates to a device for measuring the movement of a subsea deformable pipeline in relation to a sea bed.
One envisaged application field is that of on-bottom pipelines, or “flowline” pipes, that extend over the sea bed. They are intended to connect a wellhead, which projects from the sea bed, to a riser, which, from the sea bed, extends as a catenary to join a surface installation. The on-bottom pipeline, which is supported on and extends over the sea bed from the wellhead, has a connecting end for connecting the on-bottom pipeline to the riser, or to another on-bottom pipeline.
Therefore, a hydrocarbon which flows from the wellhead is brought up to the surface installation via the on-bottom pipeline and the riser.
Other technical fields are envisaged where a flexible pipeline is liable to deform under the effect of the thermal and/or mechanical variations of a liquid passing through it.
The hydrocarbons flow from the wellhead at a pressure and a temperature that vary over time. Moreover, when the flow has stopped, for any reason due to the operation, the pressure and temperature conditions of the on-bottom pipeline change dramatically. As a result, the on-bottom pipeline then expands or contracts when, for example, flow restarts. An on-bottom pipeline that is a thousand meters, for example, can be subject to meter-scale longitudinal dimension variations.
Thus, during the life of an oil field, which can be some years, the on-bottom pipeline is subjected to numerous expansion and retraction cycles with consequential amplitudes that bring about large stresses on the pipeline and the connecting parts.
It is known to minimise the stresses placed on the structure by designing structures that are capable of absorbing these stresses. To this end, the connecting ends are mounted on metal structures that can slide on a foundation anchored in the sea bed. In this manner, the connecting end can accommodate longitudinal movements. However, residual friction remains at the connecting ends and it is important then to assess these excursions to ensure that these stresses are compatible with the structure of the on-bottom pipeline.
It can also be envisaged to continuously monitor the behavior of the structure by recording data in real time. Thus, the lengthening of the structure can be monitored in real time and it can be determined if it is compatible with the maximum lengthening values that the connecting parts in particular can tolerate. However, this requires an expensive fragile device and a connection to the surface for processing the data.
Therefore, a problem that arises and which the present invention aims to solve is that of providing a device which enables the movements of a subsea deformable on-bottom pipeline to be measured and inspected, and at a favorable cost.