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.
A pipeline could be viewed as being like a human artery, with a number of possible problems that can affect them. One of the most important facets of diagnoses in terms of the arteries is the ability to determine how furred up they have become with fats, thus blocking blood flow. If we were to be in the unfortunate position of having this condition we would insist the medical team performed a proper examination prior to operating. Therefore, the diagnostic techniques described herein can be considered similar to medical technologies on an industrial scale.
Utilising a diagnostic scanning technique used in topside processing for over 50 years, the amount of deposit can be determined from outside of the pipeline by placing a yoke with a small sealed radioactive source and a ultra sensitive detector either side of the pipeline and assessing the detector response. Access problems arise though when the pipeline is buried, as is the case in many subsea applications. Therefore diagnostic tracer techniques, similar to a barium meal used in medical technology, can be utilised to determine deposit location and quantification, essentially providing the operator with 'insight onsite'.
Radioisotope tracing techniques are regularly used for checking or calibrating installed flowmeters, measuring the flow in systems where no flowmeters are installed and real time flow characterisation studies in process systems. The tracer used is designed to follow a particular material through a system. Sensitive radiation detectors are placed on the outside surface of a pipe (or other process system), which detect the tracer presence upon its flow past specific positions. These measurements can be used to directly to measure fluid velocity, flow rate, phase distribution, and deposit inventory.
By measuring the time interval between detector responses and knowing the distance between the detectors, the mean linear velocity can be calculated. If full bore turbulent flow can be assumed then the velocity can be converted to volumetric flow knowing the pipe internal diameter. Accuracy will depend on the precise circumstances but the mean velocity can usually be measured to better than ±one per cent. The total accuracy on the volumetric flow depends on how accurately the internal diameter is known, and is typically three to four per cent working from piping specifications, although this can be improved by measurement of outside diameter together with wall thickness measurements.
The basic requirements of a tracer are as follows:
. It should behave in the same way as the material under investigation
. It should be easily detectable at low concentrations
. Detection should be unambiguous
. Injection and detection should be performed without disturbing the studied system
. The residual tracer concentration in the product should be inconsequential
The criteria can be met by the use of radioisotope tracers (although sometimes inactive molecular tracers are possible) and by careful selection of the most appropriate tracer for a particular application. Frequently, more than one radioisotope can be chosen, and the factors that are important in the selection of the tracer are the:
. Half life
. Specific activity
. Type of radiation
. Energy of radiation
. Physical and chemical form
As little preparation is needed onsite to carry out a measurement, diagnostic scanning and tracers are increasingly being used to provide a rapid and detailed picture of pipeline contents. These techniques are carried out on-line, external to the pipeline, with no interference to normal pipeline operations, effectively allowing the user to 'look' through pipe walls to measure contents and process parameters.
Locating pipeline debris
One example of the tracer method in use occurred when a Middle East offshore operator needed to ascertain the current condition of a natural gas liquids (NGL) pipeline. The initial plan was to inspect it using a magnetic flux leakage (MFL) inspection tool, but prior to deploying the tool, the level of pipeline debris had to be quantified. Tracerco used a radioisotope pulse to measure the velocity of the NGL at various points along the system as this would provide the necessary data to determine debris volumes.
The pipeline, which was designed to allow a NGL flow rate of 4,000 tonnes per day, was 117 km long with a 12-inch diameter and a fill volume of 8,537 cu/m. Extending from an offshore platform to the onshore terminal, the system is partially buried and subsea for 89 km. The lowest point in the system lies within a tanker channel and has a span of 4 km. The NGL flow rate was significantly lower than design limits therefore some restriction was postulated.
Given the NGL flow rate through the pipe, it was possible for Tracerco to compare this to the measured velocities at specific locations and calculate the mean cross sectional area between these points and thus determine any restriction within. A sharp pulse of radiotracer was injected into the pipeline at the platform and the flow rate was then monitored at hourly intervals. The progress of the pulse was analysed using a sensitive, high accuracy radiation detector mounted on a remotely operated vehicle (ROV) that was deployed from a dive support vessel and positioned at strategic locations along the pipeline. A record of the tracer pulse centroid was made along with the relative position of the ROV as the pulse passed the detector.
Accurate locations for the ROV and radiation detector were obtained from the vessel’s dynamic positioning (DP) system. Once the pulse had passed the radiation detector, the ROV would be repositioned at a location downstream of the tracer to await the arrival of the pulse once more. Again, the time of the tracer pulse centroid and detector position were recorded. This procedure was repeated along the entire length of the subsea portion of the pipeline. Onshore, fixed detectors monitored the progress of the pulse up to its arrival at the terminal.
Because accurate measurements of the actual flow rate of NGL through the pipeline were available, it was possible to compare these readings to the measurements taken. Therefore, after the initial tracer injection, the NGL flow rate from the platform was measured using Tracerco’s radioisotope pulse velocity technique to corroborate the flow rate readings supplied.
The tracer pulse velocity was successfully measured over nine days and from the time versus distance records the NGL velocity was calculated. 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 (full-bore flow). 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.
The results clearly showed areas of increased velocity in the first third of the pipeline, indicating that debris had been deposited on the upward slopes due to the inability of the flow to transport solids up these inclines. The velocities measured were used to show the percentage restriction and detailed that over the first 35 km, the restriction was on average 20 per cent of the bore. However, this rose to between 50 and 60 per cent at the 15 km and 32 km areas, with a total amount of deposit in the whole pipeline of 1,600 cu/m. 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.
While tracers are ideal for characterising the flow of produced fluids through a pipeline system, Tracerco’s pipeline scanning technologies are suited to non-intrusively identify issues such as lining integrity, the density of any liquid present, the presence of fluid or gas slugging, the build-up of solids and other anomalies. 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.
The application consists of a sealed radioactive source positioned on one side of the pipeline such that the gamma radiation transmitted through it was measured by an ultra sensitive detector on the other side. The source and detector paths were kept constant along the pipeline system to ensure that the measurements obtained correctly identified the deposit’s density. Hundreds of measurements were taken at approximately one-metre intervals along the exposed pipeline. The data collated was then converted into a 2D tomograph, which provided the operator with an accurate 'map' of the profile and location of the deposited material in the pipeline.
Diagnostic tracer and scanning techniques provide a number of significant benefits when deployed. Process systems are made transparent which removes guesswork as to what may be wrong. This gives the operator accurate information to assist in making decisions in respect of further intervention or mitigation. On-line and real-time results are provided allowing optimisation and troubleshooting to be performed in-situ. Also, as most pipeline and subsea production systems are extremely complex tracer and scanning techniques can essentially break down systems into individual component modules so that individual parts can be assessed.
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. Until recently, these direct non-intrusive measurements of pipeline contents were thought impossible, but many operators have utilised the technology and its use is rapidly increasing worldwide.
By Steve Woolley and Lee Robbins of Tracerco