Pipelines are being laid over longer distances in remote areas affected by geohazards, harsh environmental conditions and possible third party intrusion. Deep water flowlines and Arctic pipelines have introduced new challenges in terms of pipeline integrity management as the assets are submitted to seabed erosion, migrating bedforms, and permafrost thaw settlement.
Pipeline monitoring has often been restricted to visual inspection and mass/volume balance measurements. As a result, pipeline failures are usually noticed when either the output flow is affected or the surrounding environment is severely affected. It is widely recognised that pipeline failures have huge operation costs, environmental and image impacts forcing the oil and gas industry to look for new sensing techniques to perform permanent and real-time integrity monitoring.
Switzerland-based Omnisens has developed DITEST fibre optic-based monitoring systems to be used to monitor onshore and offshore pipelines. The systems have been implemented over the last five years, showing good pipeline integrity monitoring performance.
Omnisens’ approach is to use standard telecommunication grade optical fibres as sensors, and to deploy fibre optic cables alongside the pipeline in order to perform a continuous uninterrupted monitoring. Once connected to an Omnisens DITEST measuring unit, the optical fibres provide information about temperature and strain conditions with metre resolution along the pipeline. Fully distributed temperature and strain profiles are recorded at regular time interval of a few minutes over up to a 100 km distance, without compromising the performance of the monitoring.

The occurrence and location of leakages is determined by analysis of the temperature profiles, and the leak detection limits are within 0.01 percentile of the total throughput and even lower for pressurised gas; more than two orders of magnitude better than that of conventional mass/volume balance systems.
At the same time, the fibre optic strain profile is used to detect and locate ground movement and pipeline strain, enabling the early detection of increased stress due to external effects such as geohazards, ground movements, permafrost thaw settlement or even third party intrusion. Specific fibre optic cables have been developed, demonstrating ground movement sensitivity down to the centimetre. Pipeline strain monitoring can also be performed with sensitivities as low as 20 micro-strains, provided that the cables are bonded to the pipeline. A variety of cables for either or both leakage and ground movement detection is available and can be selected with respect to different soil characteristics and pipeline installation procedures.
Since the monitoring is external to the pipeline, the DITEST technique is applicable to any kind of pipeline and the monitoring performance can be maintained despite flow rate and operational changes. The combined information about pipeline temperature and structural conditions is transferred to SCADA systems.
Subsea component integrity monitoring using fibre optic sensing
The number of offshore production facilities continues to grow as the quest for oil extends into deeper water environments, exposing subsea components and structures to harsh operating conditions.
Components such as mooring ropes, submarine cables, umbilicals, risers and flowlines have design limits set by fatigue accumulation, which may – during installation, commissioning or in operation – be significantly larger than anticipated, reducing their lifetime and putting the operations at risk. Moreover, subsea components are susceptible to corrosion and damages caused by dropped objects, extreme storms, fishing gears, and contacts from vessels.
Periodic remotely operated vehicle inspections are used to assess their structural integrity conditions over the entire length. However this method is expensive and unreliable since visual inspection is only capable to detect external anomalies and the system is unable to detect internal structural deterioration.
In recent years the need for effective tools to manage the safety, environmental and financial risks associated with the operation of offshore production facilities has been emphasised. To ensure the safety and integrity of new and existing subsea structures, non-destructive testing techniques and improved permanent monitoring solutions have been developed. Among these techniques, fibre optic-based monitoring systems have been proven effective to perform permanent and real-time structural integrity monitoring.
One of the advantages of the Omnisens DITEST solution is that it is fully compatible with subsea fibre optic components such fibre optic rotary joints and wet-mate connectors. The monitoring performance is not affected by optical losses, making the solution robust and reliable for long-term monitoring. Fibre optic instrumented structures provide the operators with information about abnormal operational changes, occurrence and location of damages, leak and excessive strain, generating alarms and status reports regardless of changes in structural integrity. The complete information about structural conditions is transferred to SCADA systems and eventually helps operators make executive decisions based on actual operational and structural conditions.
Submarine cables, umbilicals and risers
Submarine cables, umbilicals and risers must withstand huge mechanical stresses and strains during installation and operation, especially in ultra deep water environment. Cable and umbilical systems have limited access for maintenance after the installation is completed. To minimise the risk of costly recovery and repair operations, permanent and real-time monitoring enables the early detection of problems and allows the operator to take adequate measures in a timely manner to avoid catastrophic failures.
Subsea processing requirements as well as ultra deepsea production have forced manufacturers to design umblicals integrating high voltage power elements and a capability to withstand larger tensile loads. Today most umbilicals and submarine cables integrate multiple singlemode fibre optics for telecommunication purposes and several spare fibres (so-called dark fibres) can be made available for real-time temperature monitoring. The benefit of the temperature monitoring is the ability to pin point any ‘hot spot’ associated to abnormal operation. Examples include overloading of the integrated power lines, cable damage leading to risk of short circuit, and the overheating of cables due to multiple cables at buoyancy location.
In umbilicals integrating flowlines, temperature monitoring allows the early detection of hydrates and wax formation. Optimised dosage of hydrates can then be used to prevent complete blockage of flowlines saving large running costs and protecting the structure from the risks of buckling. The monitoring technique is applicable to flexible risers in which optical fibres in a metal tube integrated directly in the armoring can contribute to efficient flow assurance via a real-time temperature monitoring. These strain sensors connected to an Omnisens DITEST monitoring system can provide complete and accurate structural strain over several kilometres, pinpointing the strain to within metre resolution.

The availability in real-time of the local strain information at every location along the structure, from the date of the installation, allows the computation of actual fatigue accumulation. It also enables the detection of any motion (due to vessel excursion, weather conditions) resulting in stresses that may exceed the design limits and to evaluate their impact on the lifetime expectancy.
Offshore pipelines and flowlines
The challenges associated with the design and the operation of subsea pipelines or flowlines varies depending on the pipeline type and route. The failure risks are in most cases associated to: the modification of the pipeline environment, seabed topology, as well as pipeline crossing, and dropped objects (such as ship anchors or fishing gears). A modification of the pipeline’s direct surrounding due to seabed erosion or seabed migration can lead to additional cooling of the exposed pipeline section and possible hydrates and wax plugging. The extent of the hydrate or wax formation problem increases with pipeline length through the effects of cooling and the challenge is significantly greater when assuring flows in deep water and remote subsea locations, emphasising the need of pipeline permanent monitoring.
Additionally, subsea migrating bedforms subject the pipeline to large strain, and eventually the risk of pipeline upheaval buckling. Events can be detected based on the differential temperature between a pipeline and its environment. A standard subsea fibreoptic cable laid along the pipeline has proven effective to provide an early warning of such events before they develop into catastrophic pipeline failures. Examples include erosion monitoring of shallow water, shore crossing and offshore buried pipeline sections. Being able to monitor seabed erosion helps identify and remediate erosion conditions similar to those that may have contributed to shallow water or river crossing pipeline failures. If necessary, the fibre optic temperature monitoring system can be combined with fibre optic strain measurements in order to map in real-time bedform migration and to detect and localise pipeline strain. In addition, temperature-based fibre optic can be used to detect and localise pipeline leaks through the associated temperature change.