Non-intrusive pipeline inspection techniques for accurate measurement of hydrates and waxes within operational pipelines

By Steve Woolley, Pipeline Services Manager of Tracerco UK, and Lee Robins, Operations Director, Tracerco Asia

As remaining reserves of oil and gas become more difficult to access and technology makes older, mature fields more viable, there is an increasing requirement to attain longer subsea tiebacks to production facilities. As such, pipelines are more prone to blockage from deposits such as hydrates or waxes and therefore require regular inspection to generate effective maintenance programs. The use of non-intrusive measurement techniques are essential as a tool to reduce unnecessary downtime, as it is important that pipelines can be examined and deposits quantified effectively and quickly so appropriate action can be taken.

These inspections can be carried out utilizing radioisotope diagnostic techniques used in topside processing for over 50 years. The amount of deposit can be determined by either gamma scanning using a yoke with a small radioactive source and sensitive detector either side of the pipeline, or when access problems arise, (for example when the pipeline is buried), by injecting a tracer and monitoring it progress using externally mounted detectors.

These techniques essentially provide the operator with “insight onsite”.

Tracer Technology
Radioisotope tracing techniques are regularly used for real time flow characterisation studies in process systems.  The tracer used is designed to follow a particular material through a system. Sensitive radiation detectors placed on the outside surface of the system detect the tracer presence upon its flow past specific positions.  These measurements can be used directly to measure fluid velocity, flow rate, phase distribution and deposit inventory.

Figure 1 – Flow rate determined by tracer injection

By measuring the time interval between detector responses and knowing the distance between the detectors, the mean linear velocity can be calculated (see figure 1).  If full bore turbulent flow can be assumed, and the volumetric flow through the pipe is known, then the average cross sectional area of the pipe over the distance between the detectors can be calculated. Comparing this with the actual cross section area of the clean pipe, allows the volume of deposit present over that length to be calculated.

Case Study - Locating Pipeline Debris using radioactive tracer technology
A Middle East offshore operator needed to ascertain the internal condition of an NGL pipeline prior to using a magnetic flux leakage (MFL) inspection tool. Tracerco used a radioisotope tracer to measure the velocity of the NGL at various points along the pipeline, as this would provide the necessary data to determine debris volumes.

The pipeline extended 117km from an offshore platform to the onshore terminal, 89km was subsea and it was partially buried. The NGL flow rate was significantly lower than design therefore some restriction was expected.

A sharp pulse of radiotracer was injected at the platform and its progress analyzed using a detector mounted on a remotely operated vehicle (ROV). A record of the tracer pulse centroid was made as it passed the detector and accurate locations for the ROV and detector obtained from the vessel’s dynamic positioning system. Once the pulse had passed the detector, the ROV would be repositioned at a new location downstream to await the arrival of the pulse once more (see figure 2). This procedure was repeated along the entire length of the subsea portion of the pipeline.

Figure 2 – Detector response as tracer passed various detectors

The tracer pulse velocity was measured over nine days and compared with the actual NGL flowrate. Areas of the pipeline that contained solids build-up or restrictions would exhibit a higher velocity (given a constant flow rate) than areas with no restrictions. As full bore turbulent flow was noted, it was possible to compare these measurements to given flow rate figures and subsequently calculate the effective internal diameter of the pipe. This data was then used to calculate the degree of restriction in the pipe. 

Debris Defined
The results clearly showed areas of increased velocity in the first third of the pipeline, indicating an average solids deposition of 20% of the bore. This rose to between 50% and 60% at the 15 km and 32 km areas, with a total amount of deposit in the whole pipeline of 1,600 cubic meters. Tracerco’s technique allowed the operator to quickly and accurately map a deposit profile and plan a cleaning program prior to the inspection of the pipeline to determine its condition (see figure 3).

Figure 3 – Deposit locations as percentage of restriction

Case Study – Using gamma scanning to detect Hydrates
While tracers are ideal for mapping the location of deposits in difficult to access pipelines, Tracerco’s gamma scanning techniques are well-suited to identify issues such as lining integrity, fluid or gas slugging, the build-up of solids and other anomalies in easily accessible pipelines.

When an operator suspected that a flowline was blocked by a hydrate, Tracerco carried out a scan at exposed sections of the pipeline using an ROV with a fixed yoke scanning system. With a radioactive source positioned on one side of the pipeline the gamma radiation transmitted through the pipe can be measured by a detector on the other side. As long as the source and detector spacing remains constant the measurements obtained can identify the presence and density of any deposity. Hundreds of measurements were taken at approximately one-metre intervals along the exposed pipeline. The data collated provided the operator with an accurate “map” of the profile and location of the deposit, allowing a cleaning program to be planned prior to the inspection of the pipeline.

Conclusion
Diagnostic tracer and scanning techniques essentially make process systems transparent, removing the need for guesswork by giving the operator accurate information to assist in decision making related to further intervention or mitigation. On-line and real-time results are provided allowing optimisation and troubleshooting to be performed in-situ.

The Tracerco technology offers powerful and well-proven inspection techniques for accurately measuring the amount and location of pipeline contents such as waxes or hydrate deposits, in instances where pipeline conditions are uncertain.

For more information on the full range of Tracerco’s products and services please refer to the website; www.tracerco.com.