Leak detection with layers

Figure 2. True and estimated deposit thicknesses from a single DILI test run.

To avoid direct contacts with pipeline walls and damages to the electrodes, soft brush-type centralisers are used.

Sizes of pipelines where DILI sensors can be used vary in the range of 8 - 28 in. Design pressure of 100 bar can be reached even for the largest sizes. Common stainless steel grades are preferred in vessel manufacturing but the requirement of neutral buoyancy may necessitate the use of low-density materials such as highstrength aluminum and titanium alloys, especially in high-pressure operating environments to achieve both sufficient strength and sufficiently low overall density.

Operating time of about four hours can be achieved with relatively small-sized battery packs, but operation times of several days can be achieved if sufficient space can be made for larger battery packs. Electronics components have limited tolerance against high temperatures and the maximum temperature of operating environment for the DILI tool is approximately +60˚C, to ensure that heat produced by electronics can be dissipated into the surrounding liquid to avoid overheating of critical components.

The DILI tool can be operated in hazardous environments and there are mechanisms to ensure its safe use. In such environments sensor vessels are charged with inert gas to prevent hazards due to potential electronics failures, and two redundant power controlling systems make sure that power is switched on only when the pressure in the pipeline has reached a pre-set threshold value. For the case of a severe battery failure or other incidents that can pressurise the vessel(s), each sensor module is equipped with PRVs or bursting discs so that excessive pressure is released out in a controlled and safe manner.

During an inspection run, no external communication or data transfer is possible but the powering state of the system can be checked in the launcher or receiver from the signal sent by a dualrate EM transmitter. ET measurement data and data from other sensors are stored by the data logger, and data packages can be downloaded for the analysis phase via an external connector. In the data analysis phase the main objective is to extract deposit characteristics and properties from ET measurements. An extra challenge in data processing is that the position of the DILI tool is not tightly centralised but it is in random movement with respect to the central axis of the pipeline. In such cases the computational cost of conventional ET imaging algorithms would be very high. However, Rocsole has developed an AI based approach for DILI data processing, which enables very efficient data processing and quick reporting after inspection runs.

Rocsole’s latest DILI tool was designed for 12 in. pipelines (nominal ID 303.2 mm) and it is required to pass 1.5D bends. Thus the system was divided into two modules, namely battery unit and measurement unit, and they were mechanically connected by a flexible towbar (Figure 1). The total length of the system was approximately 1300 mm and its maximum hard OD was 220 mm. The objective in field deployment is to run the DILI tool in water and characterise wax deposits in 0 - 10 mm thickness range.

Performance validation Before field tests, the DILI tool was tested and validated in a controlled flow loop environment simulating field test conditions. The flow loop consists of a 12 in. pipeline with three 66 cm test sections that can be easily removed and equipped with artificial deposits of various thicknesses. Artificial deposits of 8 mm and 4 mm were built into two test sections, while the third section and rest of the pipeline was without any deposits.

After filling the flow loop with tap water and setting flow speed approximately to 0.5 m/s, the DILI tool was switched on and launched into the pipeline. Actual deposit thickness and estimated thickness from a single test run are shown in Figure 2. It can be seen that estimated values are very close to the true thicknesses, and estimation error in this case seems to be well less than 1 mm. In the regions where deposit thickness changes, there can be some inaccuracies as the electrodes have a physical length of 100 mm and the effect of both deposits is measured at the same time. This particular DILI tool was not equipped with an odometer so the results are as a function of time, not function of distance as it would normally be.

Conclusion Validation tests showed that the proposed DILI tool can provide valuable information on deposits in pipelines and is therefore a noteworthy technology to complement deposit monitoring techniques. Cost-efficiency, inspection coverage and reliability are clear competitive advantages of DILI tools as the need for information on deposits is continuously increasing. Understanding deposition conditions can enable significant cost and energy savings in optimising maintenance procedures. Additionally, deposit control and management is becoming an increasingly important question in flow assurance, since the focus in oil exploration and production is shifting towards unconventional reserves where crude oil properties and production circumstances are prone to cause more severe deposition issues.

References

1. OLAJIRE, A. A., Review of wax deposition in subsea oil pipeline systems and mitigation technologies in the petroleum industry. Chemical Engineering Journal Advances 6, 100104, 2021. 2. ROSTRON, P., Critical Review on Pipeline Scale Measurement Technologies. Indian Journal of Science and Technology 11(7), 1-18, 2018. 3. DRUMMOND, K., Nonintrusive Pipeline Internal Deposition Mapping Service Provides Insight to Operators. Pipeline Technology Conference 2018, Berlin, Germany. 4. WANG, W. and HUANG, Q., Prediction for wax deposition in oil pipelines validated by field pigging. Journal of the Energy Institute 87, 196-207, 2014.