Verification of the integrity of existing pipeline systems
Aug 27, 2007
In the last 50 years many pipeline systems for the transport of oil products were constructed. Due to the developments in the global oil market there have been a lot of changes in ownership of pipeline systems and the current state of the pipeline systems have to be evaluated.
These requirements are comparable to the national technical standards applicable in other countries.
On basis of the surveyed status, and the subsequent check against the target status defined by the standards, the necessary check and rehabilitation measures on the pipeline system shall be defined together with the respective technical experts (Water Management, TÜV) and summarized in a check and improvement measures catalogue. The check and improvement measures catalogue shall be implemented during the project.
Usually, pipeline integrity shall be proved by a water pressure check (stress test) according to VdTÜV “Merkblatt Rohrleitungen 1060”. For some of the studied pipelines many test sections for a stress test would be necessary, due to the frequent changes of the wall thickness and / or the elevation profile. The scope of work for cleaning, emptying, allocation of test sections, filling with water, testing including possible repairs and recommissioning is very extensive and cannot be scheduled precisely. Also there is often no possibility to take the pipeline out of operation for a longer period.
Alternatively to pressure testing, an evaluation of the pipeline condition can be made with an investigation of geometric deviations, wall thickness reductions and cracks and a subsequent analysis.
Before start of intelligent pigging the pipelines were thoroughly cleaned to remove loose deposits and solid accumulations. Depending on the contamination level, several disc and brush pigs as well as a D-Waxing- Pig were available (Figure 1a and Figure 1b).
Deviations from a circular profile or changes of inside diameter shall be detected and measured with geometric measurement pigs (Figure 1c). Buckles, circumferential joint welds and ovalities can be detected with the corresponding software. The measurement of bends and bend radius is partly possible. Moreover, with geometric measurement pigs it can be checked if the pipelines are suitable for pigging with intelligent ultrasonic pigs.
Wall thickness measurement pigs ( Figure 1d ) use ultrasonic technology to measure the wall thickness of the checked pipe wall quantitatively and directly. Location and depth of wall thickness reductions (e.g. inside or outside), laminations, inclusions, buckling, bubbles, weld seams, bows, installations and repair points can be detected with this measuring method.
Longitudinal cracks and crack-like failures inside and outside of the pipeline can be detected with ultrasonic crack detection pigs (Figure 1e). For one crude oil pipeline that was tested, it was not possible to reach a sufficiently clean state of the pipeline to allow measurement with ultrasonic pigs. During the ultrasonic pig run there was often an accumulation of solid particles on the ultrasonic sensors leading to a reduced data quality. Therefore it was decided to run each ultrasonic pig in a separate fuel oil batch in order to avoid an accumulation on the sensors. This resulted in good data quality.
During recent years, regular cathodic protection measurements were carried out (Close Interval Potential Survey – CIPS) and numerous coating damages were repaired. During the project another CIPS survey was done. On basis of these results sections of damaged coating had to be rehabilitated.
Different test methods were required on the non-piggable piping of the pump, block and transfer stations. Some of these methods are described below.
At the nozzles in the block stations and pig traps ultrasonic testing for wall thickness measuring, as well as MPI and X-ray tests were executed.
Pressure tests were executed in order to prove the integrity of station piping including all underground slop lines. The respective test pressures were defined in cooperation with the nominated expert considering every single system component. Pressure vessels were also included in the pressure tests.
Vessels such as filters and slop tanks were inspected internally and wall thickness measurements were executed.
For a buried, non-piggable pipeline, the socalled NoPig-Method was applied (Figure 3). With this method the magnetic field generated by current injection to a buried pipeline was analysed without interrupting operation. With this method, indication of reduction of wall thickness (> 50 %) can be achieved.
Station piping supports located in areas which were not easy accessible were checked for corrosion with the ‘LIMA test’ (electromagnetic ultrasound).
Physical samples were taken from representative pipeline locations in order to carry out material and fatigue tests. These results were also considered in relation to the influence on the pipeline integrity.
In the course of the operating permit renewal, exact as-built drawings and documents are required by authorizing authority. Therefore a complete resurvey of location and depth of pipeline and pipeline cable was executed.
Areas of low pipeline cover could be detected through this resurvey. These were rehabilitated through back-filling, re-channeling of water streams and ditches, or through installation of a geotextile layer as mechanical pipeline protection.
Moreover, sections were identified in which the pipeline axis was not laid centrally within the right-of-way protection strip. For these locations an application was made for approval of eccentric pipeline axis.
On basis of CIPS, damaged coating was localized and removed. In particular in case of polyethylene coated pipes the rehabilitationof coating damage is very important due to increased AC-corrosion risk (Figure 4).
During the pipeline route investigations, areas with low pipeline cover were detected. These low coverage areas were rehabilitated, or a regular re-inspection period was defined, depending on local conditions and reason for low cover. For instance, crossings of dewatering ditches with a coverage less than 1.5 m were cased with concrete pipes.
Low coverage areas were back-filled or provided with geotextile layer (Figure 5) as protection against farming activities (e.g. ploughing).
Defects were detected when checking the fittings in the block stations. These fittings were completely renewed. New nozzles were installed in such a way that there is the possibility to drain the pipeline sections through pumping.
Where pig run records indicated a defect that resulted in restrictions on a safe pipeline operation, such defects were removed and repaired (Figure 6). On the one hand the sections to be repaired were defined by the nominated expert, while on the other hand also defects which limites the pipeline capacity were be repaired.
The upgrade of the control and supervision system to state-of-art was an essential part of a pipeline renewal. For instance, a transient leakage detection and location system was added to the leakage supervisory system, as required by TRFL.
The internals of all welded top entry valves in pump, transfer and block stations were renewed. Motor drives, which did not comply with the ex-protection requirements, were replaced. The manually operated section block valves, usually located at water crossings, were retro-fitted with motor drives and integrated into the SCADA system.
Pressure indicators were retrofitted onto pig traps, where these were not previously provided.
Through modern surveys and checks carried out on existing pipeline systems, the integrity can be proved reliably. The necessary surveys and checks require a high planning and coordination effort.
After a detailed survey of the current status of the pipeline system, this status was brought to the target status which is defined in the currently valid regulations and by “state-of-the-art”.
With reference to the German pipeline systems implementation of the above mentioned measures now enable an unlimited operating permit to be obtained.
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