Comprehensive integrity verification of the high-pressure DN 400 pipeline

The concrete example of this comprehensive approach to ensuring the integrity was the rehabilitation of 55 km long gas pipeline DN 400 in south-western Poland.

Rehabilitation included the following basic steps:

• to clear pipes for cleaning and smart pigs,
• cleaning of pipe,
• performing of internal inspection of pipelines using intelligent pig MFL,
• testing of pipe material and pressure tests of the pipe body on the part of pipe cut from the pipeline and
• pressure reparation and drying of the pipe.

Basic data on pipeline

Pipeline DN 400 PN 55, which was mainly used to supply the area around Jelenia Gora by natural gas, was built in 1991–1992 and 1997 of screw welded pipes 406 × 6.3 mm made of steel 18G2A. On pipe bends, into protective sleeves and for crossings were used pipes 406 × 8.8 mm made of steel R45. From actual building of construction, however, only part of the inspection documents of pipe material for pipelines made of steel 18G2A was preserved, to the pipe made of steel R45 the documentation did not survive.

The pipeline was designed and built as a cleanable with the entry and exit chamber for launching and receiving cleaning pistons. From the pipeline there are leading out three branch pipelines DN 250, 150 and 100.

Pipeline cleaning and execution of internal pipeline inspection

This first phase was carried out by T. D. Williamson Polska. According to information from the operator the gas pipeline would never been cleaned; neither in operation and probably even nor during construction. Even if there was quite a considerable risk that the pig passing through the pipe stops in the column of impurities or at the deformed pipe, the operator required the cleaning during operation.

In the first process of cleaning the pig after about 40 km stuck at the point of crossing railway line. After the pipeline was excavated, on pipe was found sharp vertical segment bend, which hindered the passing of the pig. After cutting of this segment bending by using technology TDW STOPPLE, the pipeline was cleared for cleaning pistons and during the second cleaning the pig was picked up at the end of the pipeline in the receiving chamber. Afterwards it was made even multiple cleaning. In case of the first runs of the pig, the cleaning chamber at the end of the pipeline was half filled with dust.

After cleaning the pipe on the gas pipeline there was performed internal inspection using a calibration and then intelligent pig MFL. By passing smart pig through gas pipeline there were detected many defects with a thinning pipe wall up to 79%. After the shutdown of the pipeline operation – before the start of pressure reparation – the largest of these defects were dug up and depending on the severity of found defects were either cut out or covered by the sleeve.

Material tests of pipe material and tests of pipeline body

During the cut-out of the mentioned unsatisfactory segment bending that was found in the process of the pipeline cleaning, in the same place was taken out also the sample of pipe with dimensions of 406 × 6.3 mm for testing. From this cut-out pipe section were then the samples taken for material tests and from about 3m-piece was manufactured the test unit for pressure tests.

According to the still existing inspection documents were screw welded pipes 406 × 6.3 mm made of steel 18G2A, where is a guaranteed minimum yield strength R_e min = 350 MPa.

Results of material tests

Chemical analysis

According to the obtained results, it is a low-carbon steel of type ČSN 41 1523, or St 53 in accordance with DIN 17172 or X52 according to API Spec 5 L.

The results of chemical analysis are given in table 1.

Tensile tests

Tests were conducted on flat bars, which were taken at two pieces from the longitudinal and peripheral direction of the pipe.

Results of tensile tests are given in table 2.

In comparison with the mechanical characteristics required for the steel L 360 NB (MB) in accordance with EN 10208-2 the achieved values are satisfactory.

Impact bending test

Samples for these tests were again collected from the peripheral and longitudinal direction of the pipe. Test results of impact bending test are shown in table 3.

The value of impact energy KV* in peripheral direction are at temperatures 20 °C and 0 °C very favourable, above 140 J. The values obtained on samples taken in the longitudinal direction are at both temperatures less favourable – 40 J. Results on samples from the longitudinal direction do not meet requirement of the standard EN 10208-2. Within the standard is at least required the average value KV* 60 J.

Metallographic analysis

This analysis was performed on cut-outs from the longitudinal and also from the peripheral direction of the pipe.

As for the extent of sulphide the micro purity is favourable, in terms of the content of oxides is only at the surface occasionally worsened. As for the content of inclusions the steel has a good to average quality.

It is a steel with ferritic-pearlitic structure, fine-grained (ferrite grain size corresponds No. 8.5 to 7 on the relevant standard). The structure is not linear, pearlite lines occur sporadically. Otherwise, the structure is polyhedrosis. Pearlite is excreted in lamellar form. Along the ferritic grains are sometimes excreted chains of tertiary cementite. Pipe surfaces do not show decarburisation.

Quality of screw welding

Screw welding has according to informative test in cut a good quality. The line and vertical rise on the outer surface are satisfactory. On the inner surface the rise is satisfying, even if the line is not ideal. The structure of the weld metal shows columnar character, it is ferrite-pearlite structure. Weld quality is satisfactory.

Based on tests, the steel quality can be evaluated in terms of mechanical characteristics and structure as the standard, without any major defects.

Figure 1 – Test unit

Pipe testing

Based on the results of material tests – such as chemical analysis, tensile test, impact test and metallographic analysis – the pressure values were determined for the pressure reparation of the test unit.

For the steel R45, from which were made tubes 406 × 8.8 mm, is the guaranteed minimum yield strength of only R_e min = 255 MPa. The lower value of yield strength, however, compensates the bigger wall thickness, and therefore the pressure value on the yield strength of tubes 406 × 6.3 mm and 406 × 8.8 mm is identical. Material tests of R45 steel pipes were not carried out, because the operator did not provide the possibility of sampling.

Test unit (see figure 1) with a length of 2.9 m and the volume of 360 l was at both ends closed and set with lugs. These lugs were fitted with valves for filling pipe with water, pressure recording and connecting of high pressure hose for supply and draining of water under pressure. The test unit was in the presence of the operator and general contractor gradually pressurized to the yield strength up to a value of permanent integral peripheral deformation of ε 0.05% and then to the ultimate strength until the destruction of the material. Summary of values achieved during pressurization of test unit is shown in table 4.

During pressure tests – when increasing pressure or pressure holding times – were on-line monitored and measured dependences p-V and p-t (see figure 2), the flow of water pumped in, the total volume pumped in, the size of pumped plastic volume and the value of the permanent integral deformation ε.

Figure 2 – On-line measurement of dependence p-V while pressurizing the test unit

At a pressure approaching the computing pressure of the ultimate strength of the test unit the peripheral weld was destroyed at pressure of 16.632 MPa (see figure 3 and figure 4).

Figure 3 – Destruction of peripheral weld

Based on the progress and the results of performed tests, was determined the maximum pressure of pressure reparations P_max = 10.450 MPa. At the same time the maximum value of permanent peripheral deformation ε should not exceed 0.05%.

Figure 4 – On-line measurement of dependence p-V shortly after the destruction of the weld

Pressure reparation

Pipeline DN 400 was divided into four working units that were one by one put out of the operation and subsequently after execution of repairs, cut-outs of defects, pressure reparation and drying, putting again into operation.

Each working unit was further divided into working sections, at which were performed pressure reparations. Division of the gas pipeline into working sections was extremely difficult, because the territory, which the pipeline went through, resembled rather the Bohemian-Moravian Highlands or the foothills of the Giant Mountains than Polish plane. This also follows from the longitudinal profile, where the vertical axis of the chart shows the elevation difference or altitude in the range 100–600 m and horizontal axis the length of the pipeline in the interval from 0 to 55 km (see figure 5).

Figure 5 – Longitudinal profile of the pipeline

The timeframe for the execution of pressure reparation was set by the operator as maximum of 4 months, which was really a very tight deadline, if taking into consideration:

• the division of the gas pipeline into four working units and out of it resulting large number of disconnections, reconnections and interconnection on the “live” pipe,
• execution of cut-outs or repairs of internal inspection of detected defects,
• applicable sources of water for filling pipeline with water, passing of water and the subsequent draining of used water,
• execution of pressure reparation, and
• drying of the pipeline.

The entire project therefore required perfect planning, organization and uninterrupted work flow including holidays, Saturdays and Sundays.

Number of working sections was after a thorough reconnaissance of the terrain and the performed tests set at 16. The referred number of the sections, however, was with difficulty obtained compromise between the request of investor and operator in one person to minimize costs and requirements of the contractor to the elevation and length of the working sections.

Currently in Poland there is no rule or standard for carrying out the pressure reparation. For this reason, was within the project developed the Implementation Working Process (RPP), where for the execution of pressure reparations were used two pieces of legislation and made out of them their combinations.

Figure 6 – Mobile laboratory for on-line measurement during the execution of pressure reparation

For carrying out the pressure reparation including also strength tests was applied the Standard technological procedure S05 CEPS Carrying out of pressure reparation and pressure test in accordance with Inspection Report ITI No. 706/02.05/06/15.07/1 dated 3. 3. 2006. For the subsequent evaluation of tightness was used the Polish Code for carrying out stress-tests and subsequent pressure tests of new pipelines, the standard ZN-G-3900 Proby specjalne. The introduced RPP was subsequently approved by the operator and the organization of the Polish state professional supervision UDT (The Office of Technical Inspection), which performs a similar function in Poland as TIČR in the Czech Republic.

Figure 7 – On-line measurement of p-V during the first and second cycle of pressure reparation

During the execution of pressure reparations were on-line monitored and measured (see figure 6) dependence p-V and p-t (see figure 7 and figure 8), the flow of pumped-in water, total pumped-in volume, the size of pumped-in plastic volume and the value of permanent integral deformation ε, which the standard Proby specjalne does not require. However, the Polish gas professional public, which participated in the implementation of this action in various stages of approval or checks, welcomed this progressive way of measuring, and at the same time also recommended in the implementation of other similar works.

Figure 8 – On-line measurement of p-V at the end of the first cycle of pressure reparation

In the course of the pressure reparation came about twice to the destruction of the pipe. The first time on the part of pipeline, that was built 13 years ago since the implementation of the work – in 1997. At a time delay after reaching the maximum pressure in the first cycle of the pressure reparation came to a significant drop in pressure. On the basis of the first accomplished cycle of pressure reparation and based on theoretical calculations, was set the change in the volume of water in the section respectively the size of the water leak in an amount of approx. 411 litre per hour. This relatively large water leak can be in “favourable conditions” (defect located at upper part of the pipeline and the relatively dry terrain in the area of defect) also found during a walking route along the pipeline. After a thorough moistening of the terrain in the area of the defect that happened after repeated pressurization and subsequent spontaneous depressurization of gas pipeline, the fault was after a few days really found. After digging away of the pipeline in the place of discovered spring area on the edge of rapeseed field, it was no longer necessary to search for a long time the place of perforation on the pipe line (see figure 9).

Figure 9 – Fountain in the rapeseed field

After reducing the pressure at position 12 was found impressive assembly longitudinal weld (see figure 10), where on the covering layer the welder certainly did not economize on the material. However, root at the perforation you would seek for nothing. It is surprising that this weld with at first sight striking deficiencies could go through all the checks carried out during the construction of this pipeline (B2 according to TPG 702 04) in 1997. For comparison – in the Czech Republic at that time was built a pipeline B2 DN 1000 from Lanžhot towards Mount St. Catherine, where radiography tests, among other tests and inspections, were carried out on all longitudinal site welds.

Figure 10 – Site weld with continuous defect

There was immediately carried out echo testing of results of the internal inspection of this site, but even this thorough checking did not reveal any anomalies in record of the smart pig. This finding again confirmed the long experience of CEPS that intelligent pig of the MFL type is not able to detect pipeline defects of this type and scope. The referred experience is based on pressure reparations conducted by CEPS subsequently after the internal inspection that used MFL smart pigs at more than 200 km of gas and product pipelines.

The second time a leak was detected in the section, built in 1991–1992. In the course of performed pressure test turned up a pressure drop of 2.7 bar during 24 hours. This pressure drop was caused by the leakage of water from the pressurized section in quantities less than 6 litre per hour. The route of the tested section in the length of 4.6 km led from one half through water-soaked forests and from the second half through the field. At the request of the operator, this section was divided into three sub-sections, and these were again re-pressurized to the pressure of the pressure test. Two sections were evaluated as tight, on the third was detected the untightness with a volume 2.5 litre per hour. Reducing the size of a water leak from the pipeline is a phenomenon that occurs almost regularly after the division of the tested section into sub-sections. The leaky place or places in the wall of the pipeline at high pressures and with such sizes of leakage is very small and almost undetectable by the human eye. When exhausting water from the pipeline and in the process of re-filling frequently occurs – during passing of the smart pig through the incriminated place – to “clog or sometimes even block” of straight-through space by particles of rust or sludge. At the request of the operator was leaks section was divided into three more sub-sections. At pressure testing of these three sub-sections no leak appeared. The continuous defect was probably clogged with dirt or could be cut out while cutting out the pipe at places of the division into sub-sections.

Drying of pipelines

After the performed pressure reparation and after the connecting of the working sections of each working entity was the gas pipeline dried to dew point temperature (TRB) of water in the air leaving from the pipeline −20 °C.

The drying was performed by the use of a high-dry air having TRB −80 °C, which was in the quantities of 1,200 m³/hour blown into the dried pipeline. The source of this dry air was the mobile drying unit PSA (see figure 11).

During the drying was the remaining water from the pipeline was firstly forced out by plate cleaning pistons. Subsequently the pipeline was wiped dry by foam pistons. In both cases, the pistons were always pushed through pipe by a high-dry air with TRB −80 °C. The last foam piston was inserted into the pipeline at the temperature −20 °C TRB of the water in the air leaving the pipeline. After passing of this piston through, the dried pipe was flushed with at least one geometric quantity of air at TRB −80 °C and subsequently handed over to the operator.

Figure 11 – Mobile drying unit

Summary

Serious defects were before carrying out of the pressure reparations cut out or repaired by sleeve. This ensured that the defects on the pipeline, which would certainly appear during pressure reparations by destruction, were already removed or sanitized by sleeves. By eliminating of these critical bottlenecks were subsequently possible to carry out the pressure reparation in a very short period of four months and with no additional costs for a repetition in case of the pipe destruction. The successive pressure reparation verified the internal integrity of pipe with a demonstration of maximum level of security of operating gas pipeline compared to pipe.

During the pressure reparation even when pressurizing the test unit, the critical defect (destruction of pipelines) of peripheral weld at the point where internal inspection no defect detected, nevertheless it appeared.