Verification of the integrity of existing pipeline systems

The construction of the pipeline systems was carried out in accordance with different national and international standards. Due to the improvement of standards this has resulted in additional requirements in technology and safety.

The evaluation of a pipeline system is based on the verification of the integrity of the whole pipeline system including all related pump, block and transfer stations. ILF Consulting Engineers GmbH (ILF) has carried out evaluation of several pipeline systems in Germany and other countries resulting in upgrade to state-of-the-art and enabling these systems to receive unlimited operating permits, so that the pipelines remain approved for oil transport long-term.

The necessary checks and rehabilitations for extension of the operating permit in Germany are described using these pipeline systems as an example. ILF has prepared the approval applications completely, or partly, including the environmental impact assessments, planning of check and rehabilitation measurements, as well as the construction supervision. Some of the referenced projects are still not completely finalized, so that some of the described measures are still being implemented.

Standards

The erection and operation of pipelines in Germany has to be executed according to “state of the art”, as per the applicable pipeline code (“Rohrleitungsverordnung”, § 3 (2)). The “Rohrleitungsverordnung” accepts that “state-of-art” is achieved when the pipeline system complies with the specifications of the TRFL. The TRFL is the succeeding document of the TRbF 301. Moreover, the considerations of the German environmental law (UVPG) shall be considered.
These requirements are comparable to the national technical standards applicable in other countries. 

Survey of the current status

At project start, a detailed survey of the current status of the pipeline system shall be executed. On basis of the documentation, the design manuals and the registration of important data on-site, the state of the system can be established.

A check list, prepared in cooperation with the relevant authority (TÜV), shall be used for the survey in the pump, transfer and block stations.
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. 

Preparation of the permit application

The planned scope of the environmental impact assessment according to UVPG and the list of contents of the permit application shall be communicated to the approval authority in a so-called scoping document.

On this basis, the scope and level of detail of the permit application shall be coordinated with the overall approval authority, the specialized authorities and the technical experts.

Basically, the technical design is described in the permit application with details regarding design, calculation, operation, security devices and supervision. A further focus is given to the environmental impact assessment and an analysis of possible disruptions.

Also attached to the permit application shall be: description of safety measures; presentation of various pipeline route and station drawings; diagrams showing potential leak quantities; operating manuals; emergency plans; and comparison of actual status and target status with the resulting rehabilitation measures.

Scope of investigation of the pipeline system

For the renewal of an unlimited operating permit for a mineral oil pipeline it is necessary that the pipeline system is in correspondence with the current state-of-art. The integrity of the whole pipeline system shall be verified.

When checking the whole pipeline, a significant factor is often the fact that the pipeline system is in operation and only a temporary stop in delivery may be allowed.

A study was prepared at the beginning of the rehabilitation project, which defined the necessary technical and economical surveys which shall serve the experts as proof of the technical integrity of the pipelines.

In the following the most important parts of the investigation program are explained taking an oil pipeline in Germany as an example.

Stress test of pipeline
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.

lntelligent pigging
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.

For this purpose, a test program with intelligent pigs was prepared with the technical experts (TÜV and Water Management authority). It was agreed with the specialized authorities that hydrostatic pressure testing with water does not need to be carried out, if acceptable records achieved by pig runs are available. The pipelines were tested to determine possible failure points using a geometric pig, an ultrasound and wall thickness measuring pig and a crack detection pig.

The rehabilitated main pipelines are all piggable with permanently installed pig traps. Since commissioning, cleaning of pipeline was done regularly. The pig traps sometimes have to be extended in order to execute the testing with intelligent pigs.

In the following a short overview on the pigging program is given, with reference to the pigs and detectable defects:

Cleaning Pig
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).

Geometric Pig
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.

Ultrasonic Wall Thickness Pig
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.

Ultrasonic Crack Detection
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.

These pig runs provided very high-quality data, so that an integrity assessment could be done without any problem. The analysis of defects and the calculation of the restricted safety values were carried out by the nominated expert. On this basis, it was concluded that the pipeline could continue to be operated in a safe manner.

The pigging results were verified by numerous excavations at indicated defect locations. All defects which could lead to a reduction of the allowable operating pressure were incorporated into the line of the maximum allowable operating pressure on the hydraulic profile. A comparison of the line of maximum pressure (MOP) in the most unfavourable operating state, and the line of the maximum allowable pressure (MAOP) amended to incorporate the effect of all identified defects, indicates those locations where defects limit the allowable operating pressure of the pipeline (Figure 2).

Cathodic protection analysis
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.

Examination of non-piggable pipeline sections
Different test methods were required on the non-piggable piping of the pump, block and transfer stations. Some of these methods are described below.

Manual US-, Magnet- and X-ray Testing
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 Test of Station Piping
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.

NoPig Testing
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.

LIMA Testing
Station piping supports located in areas which were not easy accessible were checked for corrosion with the ‘LIMA test’ (electromagnetic ultrasound).

Sample
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.

Survey of the pipeline route
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.

Rehabilitation measures

All necessary measures which were identified from the ‘actual’ and ‘target’ status comparison, and survey implemented, were carried out in the framework of a rehabilitation program. The most significant aspects are described below.

Cathodic protection analysis (CIPS)
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).

Rehabilitation of low pipeline cover areas
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).

Rehabilitation of pipe fittings
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.

Rehabilitation on basis of pig run records
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.

Renewal of the SCADA System
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.

Renewal of fittings and engines
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.

Pig traps
Pressure indicators were retrofitted onto pig traps, where these were not previously provided.

Summary
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”.

The condition of the pipeline system is now better known to the Operator than at the time of erection. For instance, pin holes and defects in the pipe material were detected, which were already existing since the erection of the pipelines. Through regular checks, such as execution of intelligent pigging, the pipeline integrity shall be monitored regularly. The technical experts (e.g. TÜV) guarantee compliance with “state-of-the-art” through regular supervision of the pipeline system.

With reference to the German pipeline systems implementation of the above mentioned measures now enable an unlimited operating permit to be obtained.