Above Water Tie-in

Salah satu analisa yang bisa dilakukan oleh OFFPIPE adalah davit lifting analysis. Davit lifting adalah proses pengangkatan ujung pipeline oleh davit (cable) yang ditarik dari atas barge. Biasa digunakan 3-5 davit untuk proses tersebut sampai ujung pipeline berada di atas permukaan laut, dimana davit pertama letaknya hampir di ujung pipeline lalu selanjutnya berjarak beberapa meter dari davit yang satu ke davit yang lainnya.

Ketika proses pengangkatan ini maka pipeline akan melengkung membentuk kurva dan tentu saja jika tidak diperhitungkan dengan sebaik-baiknya bukan tidak mungkin lengkungan tersebut akan menghasilkan stress yang melebihi batas dan pipeline bisa mengalami kegagalan (patah). OFFPIPE mampu melakukan analisa proses davit lifting ini dan memberikan output berupa stress yang terjadi pada semua daerah pipeline, terutama pada daerah lengkungan yang terbentuk ketika pipeline diangkat ke atas.

Gambar 1 Ilustrasi Proses Davit Lifting (OFFPIPE Manual)

Proses davit lifting biasanya dilakukan sebagai proses penyambungan pipeline, dimana sebenarnya bisa saja dilakukan penyambungan (pengelasan) di bawah laut tapi dengan biaya yang jelas lebih mahal. Davit lifting memberikan harga yang relative lebih murah dari pada proses pengelasan bawah laut. Proses penyambungan dengan mengangkat pipeline ke atas permukaan laut bisa dikenal dengan Above Water Tie In. Penyambungan ini bisa antara pipeline dengan pipeline atau juga antara pipeline dengan riser. Tahapannya cukup sederhana, pipeline diangkat ke atas permukaan laut, lalu disambung dan diturunkan lagi ke dalam laut.

Perhatikan foto di bawah ini (Gambar 2). Dalam foto tersebut posisi pipeline sudah ditarik berada di atas permukaan laut, ada 2 buah pipeline yang akan disambung menjadi satu bagian.

 

Gambar 2 Pipeline Berada Di Atas Permukaan Laut

Foto selanjutnya (Gambar 3), di bawah ini terlihat jelas kedua bagian pipeline yang diangkat dari bawah laut. Terlihat ada kelebihan panjang (gap) dari kedua pipeline, agar bisa nyambung dengan sempurna maka harus dipotong sedikit bagian agar pas hehehe..

 

 Gambar 3 Ada Gap Antara Pipeline

Pipa sudah tidak tampak lagi gapnya sehingga proses penyambungan bisa dilakuan, seperti yang terlihat pada Gambar 4 dan Gambar 5.

 

 Gambar 4 Pipeline Siap Disambung

Gambar 5 Proses Pengelasan Pipeline

Sementara sampai disini dulu, semoga bermanfaat. Terima kasih kepada Mas Martinus Luckyanto (JPKenny) atas semua foto-fotonya.

Source:

https://vladvamphire.wordpress.com/2009/06/16/above-water-tie-in/

Pipeline Construction

Pipeline construction is divided into three phases, each with its own activities: pre-construction, construction and post-construction.

Pre-Construction

Surveying and staking

Once the pipeline route is finalized crews survey and stake the right-of-way and temporary workspace. Not only will the right-of-way contain the pipeline, it is also where all construction activities occur.

Preparing the right-of-way

The clearly marked right of way is cleared of trees and brush and the top soil is removed and stockpiled for future reclamation. The right-of-way is then leveled and graded to provide access for construction equipment.

Digging the trench

Once the right-of-way is prepared, a trench is dug and the centre line of the trench is surveyed and re-staked. The equipment used to dig the trench varies depending on the type of soil.

Stringing the pipe

Individual lengths of pipe are brought in from stock pile sites and laid out end-to-end along the right-of-way.

Construction

Bending and joining the pipe

Individual joints of pipe are bent to fit the terrain using  a hydraulic bending machine. Welders join the pipes together using either manual or automated welding technologies. Welding shacks are placed over the joint to prevent the wind from affecting the weld. The welds are then inspected and certified by X-ray or ultrasonic methods.

Coating the pipeline

Coating both inside and outside the pipeline are necessary to prevent it from corroding either from ground water or the product carried in the pipeline. The composition of the internal coating varies with the nature of the product to be transported. The pipes arrive at the construction site pre-coated, however the welded joints must be coated at the site.

Positioning the pipeline

The welded pipeline is lowered into the trench using bulldozers with special cranes called sidebooms.

Installing valves and fittings

Valves and other fittings are installed after the pipeline is in the trench. The valves are used once the line is operational to shut off or isolate part of the pipeline.

Backfilling the trench

Once the pipeline is in place in the trench the topsoil is replaced in the sequence in which it was removed and the land is re-contoured and re-seeded for restoration.

Post Construction

Pressure Testing

The pipeline is pressure tested for a minimum of eight hours using nitrogen, air, water or a mixture of water and methanol.

Final clean-up

The final step is to reclaim the pipeline right-of-way and remove any temporary facilities.

Source:

http://www.cepa.com/about-pipelines/pipeline-design-construction/pipeline-construction

Stress Analysis for Buried Pipeline

Bending Stresses From External Loading On Buried Pipe

The pipeline industry has long been interested in evaluating the effects of external loading due to fill and surface loads, such as excavation equipment, on buried pipes. This interest stems not only from the initial design of pipeline systems, but also from the need to evaluate changing loading conditions over the life of the pipeline. Variations in loading conditions can arise due to the construction of roads and railroads over the pipeline and one-time events in which, for example, heavy equipment must cross the pipeline.

The pipeline may also suffer corrosion or damage that requires excavation and repair. Heavy excavation equipment is often placed directly over a pipeline during repair work, as shown in Figure 1. Safety while excavating pressurized pipelines is a serious concern for operating companies. Both gas and liquid pipeline companies often specify reduced pressures while excavating and repairing in-service pipelines.

A common issue is determining what pressures are safe during excavation and repair procedures. Design codes, regulations and industry publications offer little guidance on what factors should be considered to determine safe pressures during in-service excavation activities. Surface-loading conditions and soil overburden result in stresses that should be evaluated in determining safe excavation pressures near areas of damage or corrosion. Large concentrated loads, like truck wheel loads, are of primary concern.

The ALA Guideline for the Design of Buried Steel Pipe presents design provisions for use in evaluating the integrity of buried pipelines for a range of applied loads. (ref: “Guideline for the Design of Buried Steel Pipe,” American Lifelines Alliance/ASCE/FEMA, 2001.) Its methodology offers an approach for evaluating the fill and surface-loading effects on buried pipelines. This approach utilizes the deflection of the pipe, calculated using a version of the classic Iowa Formula, in estimating the wall-bending stresses in the pipe. The wall-bending stress is then combined with other calculated stresses to calculate the overall stress in the pipe.

Smith and Watkins pointed out that the Iowa Formula was derived to predict the ring deflection of flexible culverts, and not as a design equation to determine the wall thicknesses of pipes. (ref: Smith, G., and Watkins, R., “The Iowa Formula: Its Use and Misuse when Designing Flexible Pipe,” Proc. of Pipelines 2004 Int’l Conf., ASCE, 2004.) It is often used to estimate wall stresses, however, and determination of the total stress is important to safety calculations. In this article, the wall-bending stress calculation and some quirks in its behavior will be discussed.

Pipe materials are classified as being either flexible or rigid. A flexible pipe has been defined as being able to deflect at least 2% without structural distress. (ref: Moser, A.P. and Folkman, S., “Buried Pipe Design, 3rd Ed.,” McGraw Hill, 2008.) Materials such as steel and most plastics are considered flexible pipe. Concrete and clay pipes are considered rigid. The Iowa Formula was developed for use with flexible pipes.

Flexible pipes derive much of their load-carrying capacity from pressure induced at the sides of the pipe as they deform horizontally outward under vertical loading. Analysis of the effect of fill weight and surface loading is therefore a problem of interaction between the pipe and the soil. The Iowa Formula describes the interaction of the pipe and soil and the deflection that results from vertical loading.

Figure 3: Effect of wall thickness ratio on the normalized wall-bending stress

In his research of the performance of buried flexible pipes, M. G. Spangler observed that, compared to rigid pipes, flexible pipes provide little inherent stiffness and perform poorly in 3-edge bearing tests. However, flexible pipes performed better than predicted by these tests when buried. He reasoned that the source of strength of the flexible pipe is not the pipe itself, but is primarily the soil beside the pipe. (ref: “Insight into Pipe Deflection Predictions: An Interview with M.G. Spangler,” Sewer Sense No. 17, National Clay Pipe Association, 2004.)

The ability of buried flexible pipe to support vertical loads is based on support from the soil around the pipe and the restraining force induced on the sides of the pipe counter to the horizontal deflection. Coupling these concepts with ring-deflection theory led to the development of the Iowa Formula in 1941. (ref: Spangler, M.G., “The Structural Design of Flexible Pipe Culverts,” Bulletin 153. Iowa State College, Ames, Iowa, 1941.)

The Iowa Formula was developed to estimate the distortion of a buried flexible pipe under vertical loading. A sketch of a deflected pipe is shown in Figure 2. The formula for the deflection can be written as:

The formula has two terms in the denominator, the first of which depends on the pipe stiffness and the second on the modulus of soil reaction, E’. For thin-walled flexible pipes, the modulus of soil-resistance term tends to dominate the equation. This term defines the resistance of the soil to deformation. Unfortunately, E’ is not a true property of the soil, but instead depends upon a number of factors including compaction, texture, and fill depth. E’ is normally estimated from tables or by testing.

It can be shown that the maximum through-wall circumferential bending stress can be determined from Eq. 2:

This stress equation is what is used in the ALA guidelines to account for the stresses due to ovality of a buried pipe.

Along with the Iowa Formula, Spangler also derived a formula for determining the wall-bending stress in a vertically loaded pipe with internal pressure. This is often referred to as the Spangler Stress Formula. In this case, however, he did not include a term to represent the soil support that resists distortion of the pipe. Warman et al. derived a combined equation that includes the effects of lateral soil restraint and the distortion-resisting effects of internal pressure. Inclusion of the pressure term removes some of the conservatism of the Iowa equation when applied to pressurized pipes. The wall-bending stress term proposed by Warman et al. can be written as:

Consider the wall-bending stress without internal pressure, as shown in Eq. 3. If the ratio of the wall thickness to pipe diameter is set to zero, the wall-bending stress goes to zero. The wall-bending stress also approaches zero as the wall thickness ratio increases. A graph showing the calculated wall stress as a function of the wall thickness ratio is shown in Figure 3 using 500 psi for the value of E’. Note that the magnitude of the stress depends on other terms, but the shape of the curve is determined by the ratio of E’ to E.

Again using a value of 500 psi for E’ and the Young’s modulus of steel (2.9×106 psi), the maximum stress occurs at t/D of about 0.01. This roughly corresponds to a 48-inch pipe with a 0.5 inch wall thickness. At thicknesses greater than the critical thickness, the stress equation predicts that the wall stress gets lower as the wall thickness is increased. However, for thickness ratios below the critical value, using a thicker wall results in a higher wall-bending stress. This is interesting, in that the thinner the wall is made, the lower the stress becomes.

Looking at the Iowa Formula in Eq. 1, it can be seen that reducing the wall thickness to zero results in a finite value of deflection. At this point, the wall-bending stress is zero, so the soil is carrying the entire vertical load. It appears that for gravity fed flows, that a hole in the soil does not need a pipe at all! Unfortunately, there are reasons that we can’t get rid of the pipe. The Iowa Formula assumed that the pipe transfers the vertical load to the side walls, so without the pipe, the formula doesn’t work. Also, ignoring the wall-bending stress, the vertical load itself will cause the pipe to fail due to buckling or crushing if the walls get too thin.

Figure 4: Example minimum wall thickness ratio calculation using the wall-bending stress formula without a pressure term.

A maximum value of the circumferential stress can be determined by adding the hoop stress and the wall-bending stress. If the circumferential stress and its Poisson contribution to the longitudinal stress are used to calculate the Von Mises stress, the resulting equations can be solved to determine the minimum acceptable wall thickness ratio as a function of internal pipe pressure.

As an example, consider a steel pipe 48 inches in diameter. Assume some combination of burial depth, lag factor, and live-loading to achieve a vertical pressure at the pipe of about 13 psi. This is a high value of vertical loading, but it was chosen to illustrate peculiar behavior of the wall-bending stress equation. In this example, we also assume a modulus of soil reaction E’ of 500 psi and constants based on a bedding angle of 30º. Using the wall-bending stress equation given in Eq. 3, the minimum required wall thickness as a function of internal pressure was calculated. The results are graphed in Figure 4. Wall thickness ratios above and to the left of the curve are acceptable, below and to the right are beyond the acceptable stress limit (in this case taken to be 0.6 times the ultimate tensile strength).

The results show that, for pressures between roughly 160-180 psi internal pressure, an S-curve occurs where a range of wall thicknesses are not acceptable while thinner walls are. In our example, at 170 psi internal pressure, a wall thickness ratio of 0.85% is not acceptable, but a wall thickness ratio of 0.45% is okay. Based on the pipe diameter of 48 inches, these correspond to wall thicknesses of 0.408 inches and 0.216 inches, respectively. This behavior certainly appears unrealistic.

Next we repeat the example, but using the wall-bending stress determined with Eq. 4, which incorporates the pressure term. Again, the pipe has a diameter of 48 inches and a vertical-loading pressure of 13 psi.

Figure 5 shows the results based on the Iowa Formula with the pressure term included and the previous results without the pressure term. The minimum required wall thickness calculated using the hoop stress only (neglecting the wall-bending stress entirely) is also shown for comparison. The results for the calculation using the Iowa Formula with the pressure term included do not show the strange behavior observed when the pressure term is not included.

Figure 5: Comparison of minimum wall thickness ratio calculations using several approaches.

Comparing the results shows that not including the pressure effects in the wall-bending stress equation leads to calculated wall thicknesses that become extremely conservative as pressure increases. On the other hand, comparing the results based on the Iowa Formula with the pressure term included to the results for hoop stress alone shows that the effect of the wall-bending stress is not insignificant, and the results become increasingly different as pressure increases. This indicates that neglecting the effects of wall-bending stress could result in non-conservative stress calculations and wall thickness values.

The chart in Figure 5 also shows the calculated wall thickness ratio at the buckling limit with no internal pressure. This is a performance limit based on the pipe walls buckling under the vertical load. Since internal pressure puts the walls into tension rather than compression, this effect will be observed for unpressurized pipes or pipes under vacuum. However, since all pipelines will be unpressurized at some time, this performance limit must be satisfied for pipes that normally operate pressurized as well.

The buckling limit wall thickness ratio for the example pipe is about 0.48%. Therefore, for any design pressure less than about 360 psi, the buckling limit will determine the minimum required wall thickness for the example pipe. Note that the buckling limit is a function of the vertical load pressure, which was specifically chosen in this example to be high.

Conclusions

Smith and Watkins pointed out that the Iowa Formula was derived to predict the ring deflection of flexible culverts, and not as a design equation to determine the wall thicknesses of pipes. It is, however, widely used in stress calculations, and is part of the methodology used to predict the stresses in pipelines due to vertical loading in the ALA Guideline for the Design of Buried Steel Pipe. The use of the Iowa Formula to calculate the wall-bending stresses in a pressurized buried pipe is generally unrealistically conservative, and can, under certain circumstances, lead to results that behave strangely, particularly for high vertical loading.

Inclusion of a pressure-stiffening term in the stress equation appears to improve the behavior and remove some of the excessive conservatism inherent in the Iowa Formula. At high vertical-loading pressures and low internal pressures, the wall buckling limit may be the dominant factor in the minimum allowable wall thickness.

Source:

http://pgjonline.com/2011/06/03/bending-stresses-from-external-loading-on-buried-pipe/

Crack on Offshore Pipeline

Crack detection in gas pipelines

Intelligent pigs, which detect geometry defects and metal loss in long distance pipelines have been around for many years. It has also been possible for several years to detect crack-like defects with the ultrasonic method in liquid pipelines. In gas lines, however, the detection of crack-like defects incurs a high additional cost because the ultrasonic method requires a coupling liquid, and ultrasonic pigs can only be run with a liquid batch. A crack detection pig for gas pipelines was therefore urgently required.

The new EmatScan® CD is capable of detecting crack-like defects in gas pipelines with the ultrasonic method without a coupling liquid. The EmatScan® CD utilises EMAT (Electro Magnetic Acoustic Transducer) technology, whereby the ultrasonic pulse is generated electro-magnetically inside the material by an electric pulse applied to a coil in the sensor. The EmatScan® CD has already successfully inspected several gas pipelines in North America and is currently available in the size of 36 inches.

Introduction

High pressure long distance pipelines transporting gas, crude oil or products are inspected by intelligent pigs for the location of defects. These inspections are an important contribution to the continued safe operation of these pipelines.

Typical defects are geometrical anomalies, metal loss and crack-like defects. Intelligent pigs are measuring robots which are propelled through the pipeline to detect defects, using appropriate measuring techniques.

For geometrical anomalies, pigs with mechanical sensors have been used for many years. It is customary to inspect new pipelines with calliper pigs prior to commissioning.

In the 1970s metal loss (corrosion) was the type of anomaly that caused the development of the first intelligent pigs. For metal loss two technologies are customarily used: the ultrasonic method, which measures the wall thickness directly, or the magnetic flux leakage (MFL) method, which responds to the change of the magnetic field in the presence of metal loss.

The ultrasonic method is the more accurate method, but a coupling liquid is required to apply the ultrasonic pulse to the pipe wall. It is therefore mainly used in liquid pipelines. The MFL method, on the other hand, does not require a coupling liquid and is therefore the preferred method for gas pipelines. Both types of instrument have been operated for many years and play a central role in the upkeep and maintenance of high pressure long distance pipelines.

During the 1990s longitudinal crack like defects began to appear additionally in more and more pipelines causing serious problems. This led to the development of a new generation of crack detection pigs.

Types of Cracks

Even though isolated fatigue cracks have been seen since the 1970s, it was the increased appearance of stress corrosion cracking (SCC) defects in the 1990s that led to some spectacular pipeline failures in Russia and North America. Figure 1 shows typical SCC colony.

SCC develops in pipelines under narrowly defined conditions. These include: susceptibility of the steel, moisture of the soil, soil chemistry, quality of the coating, variable stress and highly increased temperatures. SCC first appeared in the above mentioned areas mainly in high pressure pipelines directly downstream of compressor stations and now also occurs more and more often in liquid pipelines, even though these lines do not display increased temperatures.

Apart from SCC, metal fatigue cracks are becoming increasingly common, mainly due to the increasing accumulated number of pressure cycles in the aging pipeline population.

Cracks, which influence the structural integrity of the pipeline, are mainly longitudinally orientated, caused by the predominant stress distribution in the steel. Fatigue cracks can grow both from the internal or the external surface of the wall. Because of the growth mechanism, SCC cracks are external defects.

Batching with UltraScan® CD

In the early 1990s the UltraScan® CD crack detection pig was developed by GE Energy. It uses angular beam ultrasonic technology to detect longitudinal cracks. The sensors operate in the immersion mode, the transported fluid is used as coupling liquid.

The basic principle is demonstrated in Figure 2. The angular ultrasonic beam is reflected to and fro between the two surfaces at an angle of 45°. If the signal is reflected by a crack it travels back along the same path and is received by the same sensor as the echo signal. The appearance of the echo signal along the time coordinate indicates whether the crack is located internally or externally. As the tool is designed to detect longitudinal cracks the sensors are slanted with circumferential orientation to allow the beam to travel through the wall perpendicular to the longitudinal direction. In order to scan each defect from both sides two sets of sensors are employed, one operating clockwise, the other in an anti-clockwise direction. Each ultrasonic pulse is monitored up to two and a half full reflections (skips), meaning each crack is seen by several sensors from different distances. This results in a redundancy of information which is important to guarantee a reliable detection of the cracks and to differentiate between real cracks and harmless small inclusions in the material.

The multitude of sensors are mounted on the sensor carrier so that the entire pipe circumference is scanned in one pass (Fig. 3). The effective distance between sensors in circumferential direction is about 10 mm. The individual skids of the sensor carrier are mounted in such a way that geometric irregularities of the pipe are compensated and the sensors are always locally orientated with the right angle to the wall.

During the inspection, large amounts of data are generated. During the travel of a 24 inch UltraScan® CD tool through a 100 km long pipeline, 100 terra bytes of primary data are generated. The data is screened in real time for signals relating to crack like defects and only those signals are stored in the on board solid state memory. To achieve this, the most advanced FPGA electronic components are employed in the tool.

The UltraScan® CD detects all defects of 25 mm minimum length and 1 mm minimum depth. The data is displayed as a coloured area scan (C-Scan). The colour displays the intensity of the reflected signal according to the colour code. The intensity of the signal is an indication of the depth of the defect (Figure 4). UltraScan® CD tools have inspected more than 15 000 km of pipeline since their introduction in 1994 and detected a total of 3000 SCC colonies and over 700 fatigue cracks.

The ultrasonic technology is established as the industry’s most reliable and accurate method to detect cracks. In liquid pipelines the UltraScan® CD can be applied directly in the transported medium. This is not the case in gas pipelines, because the coupling liquid is not readily available. To inspect a gas pipeline reliably for cracks the UltraScan® CD tool has been run in a liquid batch in recent years (Figure 5). Even though this batch technology is well proven, it causes interruptions in the production and additional cost. These interruptions not only lead to loss of income for the line operator, but are often simply not possible because of the dependency of the end customer on the delivery.

A solution of this dilemma is now offered with the EmatScan® CD.

EmatScan® CD

For the EmatScan® CD the EMAT technology has been employed. This technology has the advantage that no coupling liquid is required. The ultrasonic pulse is generated inside the wall by an electro magnetic effect.

Principle of operation

Figure 6 demonstrates the difference between the standard piezoelectric sensor of the UltraScan® CD and the EMAT sensor. In the case of the piezoelectric sensor, the ultrasonic pulse is generated by a crystal inside the sensor and is transferred to the wall through the coupling liquid. The EMAT sensor, on the other hand, consists of a permanent magnet and an electric coil. The pipe wall is magnetised locally by the permanent magnet and an electric pulse sent through the coil generates eddy currents inside the wall. An eddy current flowing in the magnetic field gives rise to the so called Lorentz force, causing a deflection of the crystal lattice. Through this movement of the lattice the ultrasonic wave is generated right inside the metal itself.

Based on the orientation of the magnetic field and the eddy currents, ultrasonic waves are induced which travel in different directions inside the pipe wall. This mechanism also works in the reverse for the reception of an ultrasonic pulse.

In the case of the EmatScan® CD EMAT sensor three different waves are generated: the SH (shear horizontal) wave, the RH (rayleigh high frequency) wave and the TS (thickness shear) wave.

The individual waves fulfil different tasks: The SH wave front extends over the entire thickness of the wall and travels in circumferential direction through the wall. This wave provides the basic information, responding to any crack oriented in longitudinal direction. The RH wave only oscillates close to the internal surface and also travels in circumferential direction, responding to internal cracks only. By combining the information generated by the SH and RH waves it is possible to distinguish between internal and external cracks. This combined information is also used to estimate the depth of the crack. The TS wave travels perpendicularly into the wall and is used to measure the actual wall thickness of the pipe joint.

The EmatScan® CD features three sensor heads per sensor carrier equally spaced over the circumference. The sensor acts as transmitter and receiver. Each sensor head transmits an ultrasonic pulse, which, in the case of the existence of a crack like defect, is reflected and received by the same sensor. Part of this pulse also travels on around the circumference and is received by the adjacent sensor head as a very strong transmission signal.

The relevant information for the detected crack is deducted from the strengths of the reflected echo and the transmission wave. The part of the pipe circumference located between two sensor heads respectively is divided into three zones: the near gate for crack echoes which arrive ahead of the transmission signal of the neighbouring sensor head, the far gate for echoes which arrive after the transmission signal and the transmission gate for the reception of the transmission wave itself. Additionally there is a dead zone directly in the sensor head area from which no signals are received.

Based on the fact that each EMAT sensor head is able to scan a large portion of the pipe circumference, the EmatScan® CD tool only needs a total of 12 sensor heads located on four sensor carriers. The individual sensor carriers are mounted with an angular set off to allow for covering the entire pipe circumference.

Mechanical design

The EmatScan® CD is of modular design, similar to any modern pipeline inspection tool (Figure 7). The individual modules travel inside the pipeline on cups or rollers. They are connected by universal joints to allow the passing of bends. The electronic components are housed in pressure tight bodies. Electronically the individual bodies are connected by especially designed pressure tight cables and plugs. The first module houses the batteries for the power supply of the electronic system, while the second houses the electronic components for data treatment and storage. Trailing behind these modules are the four sensor carriers with three sensor heads each.

The cups of the first module seal the tool inside the pipe to allow for the build up of the differential pressure needed to propel the tool through the pipeline.

Since the SH wave front extends over the full thickness of the wall, the frequency of this wave is dependent on the wall thickness of the pipe. For pipelines with wall thickness that differ from the range of 9 to 16 mm sensor heads with different frequencies must be employed.

Test results

Inspection results are displayed as B-Scan and C-Scan. The B-Scan displays the signals of an individual sensor with respect to time (y-coordinate), with the sensor travelling down the pipeline displayed in the x-coordinate. The intensity of the received signal is displayed as colour, with red indicating the maximum intensity.

Figure 8 shows test results of defects with the minimum depth of 1 mm. The group of defects shown in Figure 9 feature different angles with respect to the longitudinal direction – this is clearly seen in the results. Figures 10 and 11 demonstrate the capability of the system to resolve two defects in close vicinity, both in the longitudinal direction (Figure 10) and in the circumferential direction (Figure 11).

By combining the results of the individual types of wave an estimate for the depth of the crack like defect can be determined. In the inspection report the depth is reported in 3 classes:

1) less than 2 mm deep

2) 2 mm to 5 mm deep

3) more than 5 mm deep.

Using the depth and length information the influence of each defect on the safe operating pressure of the pipeline can be calculated.

Economy

The EmatScan® CD tool provides an important contribution to the safe operation of gas pipelines, in that it detects with high probability all defects relevant for the structural integrity of the pipe material. Apart from the aspects of safety and environmental protection this also has positive economical consequences, eliminating the cost a failure of a gas pipeline would generate, not to mention the loss of public opinion connected to such an incident.

Special aspects

The EmatScan® CD can be employed in gas lines only. Due to the fact that the ultrasonic pulse needs to travel over relatively long distances around the circumference of the pipe, the medium or material in direct contact with the pipe wall has a great influence on the propagation of the wave. In liquid-carrying pipelines, a lot of the ultrasonic energy is lost by part of the wave migrating into the liquid, so that the signal amplitude vanishes before the wave reaches the Far Gate or the adjacent sensor.

The coil of the test head must be within 0.5 mm of the internal pipe surface. To achieve this, the coil section of the test head is gently pressed against the wall, causing it to slide along as the tool is progressing through the pipe line. One of the challenges during the development was to find the right material for the abrasion resistant layer on top of the sensor coil that at the same time would influence the strength of the electric signal as little as possible.

Field testing

The EmatScan® CD has completed runs successfully in several gas pipelines in North America. One of the lines was of special interest, because it had already been inspected by the UltraScan® CD running in a liquid batch. As a consequence of this the locations and dimensions of several crack-like defects were known prior to the EmatScan® CD run. All defects found by the reliable UltraScan® CD tool were also found by the EmatScan® CD.

Source:

http://pipeliner.com.au/news/crack_detection_in_gas_pipelines/043294/

Offshore Pipeline Corrosion Prevention

When Frans Nooren founded STOPAQ in 1988, he started a waterproofing contracting company aimed at solving many civil structure water problems, a major concern in a country like the Netherlands. Drawing on his practical experience, he set out to improve sealing technology products and developed an innovative product, which he demonstrated to great success by sealing leaks in the harbour walls at Rotterdam.

Due to erosion of the soil behind the dock wall pilings, corrosion had taken place and corroded the sheet from the rear, leading to perforations. These had to be sealed from the harbour side using novel technology involving a sealing compound applied and cured under water. This product was the start of a new generation of sealants and coatings.

Polyisobutene resin sealants for the offshore industry

The polyisobutene resin technology STOPAQ's product range is based on make it ideal for field joint coating of pipelines. The company's latest innovation of visco-elastic coatings – which are patented innovative polymer technology and available worldwide – can provide pipeline owners and operators with reliable, long-lasting anti-corrosion coatings for field joints and for the repair of damaged areas on the main pipeline coatings.

As the coatings are 100% self-healing, chemical and temperature-resistant, and less likely to damage in service, they can be easily and quickly applied without the need of special equipment or highly skilled operators. This, in turn, means there are fewer application and through life costs.

Advanced corrosion prevention and insulation systems

The offshore world requires advanced corrosion prevention and insulation systems. Corrosion processes can sever offshore equipment and pipe rupture may occur. Hard and tough coatings may break. Very low PH (2-3) electrolyte solutions can cause CUI. Other chemical reactions and or moisture / water penetration must be prevented at great depths.

In response to the offshore industry's demands, STOPAQ and BASF have joined forces. The result of combining STOPAQ's visco-elastic corrosion prevention layer with BASF's PU is a cost-effective solution delivering long-term protection against corrosion by locking out negative influences. We see stopping corrosion as our common mission now. Via our system, we can offer you more control in all process steps from preparation, application and control beyond design life.

Corrosion prevention systems for offshore pipelines and platforms

Pipelines and platforms need to be safe constructions – for people and for the environment. STOPAQ / BASF Offshore can seal spools, under insulation, under fireproofing, J-tube filling, flanges, risers, christmas trees, pp-coating repair, pipe joints, subsea repair, and piles (splash zone). STOPAQ / BASF applications can be found on many important offshore installations worldwide and on offshore pipeline joints.

STOPAQ / BASF offers fully integrated solutions, including service preparation on-board of lay barge vessels. The joint system offers a simple, safe and fast-turnaround job, guaranteeing 100% adhesion. Mechanical protection is ensured by using tapes, shrinkable sleeves or PU infill.

Tailor-made corrosion-resistant coating systems

STOPAQ / BASF's coating system can be tailor-made for each project, and easily applied. Furthermore, it also allows a quicker preparation of steel and adjacent factory applied coating by at least St2/3 brushing method, cold application of the visco-elastic anti-corrosion layer, and immediate and permanent attachment of the impermeable visco-elastic layer to steel, concrete, polypropylene, polyurethanes and polyethylene. There is no risk of osmosis.

Some of the system advantages are:

• Increasing the speed of application
• Eliminating the need of flame torch
• No need for primers
• Cost-effective: reduces inventory requirements eliminating diameter specific solutions
• Outstanding impact resistance
• Cold flow; providing corrosion protection by penetrating into the finest pores of the substrate
• Very surface tolerant; no grit blasting, only wire brushing or hand tool cleaning required
• Higher temperatures resistance

Pipeline Commissioning

As submarine gas pipelines get longer and more remote, the challenge of pre-commissioning becomes greater. No project demonstrates this better than the groundbreaking Nord Stream pipeline, which comprises two 760-mi long, 48-in. diameter twin gas transmission pipelines running from Russia to Germany through the Baltic Sea – a delicate marine environment that needs to be protected and preserved.

In September 2011, the pre-commissioning of the Nord Stream Line 1 was completed ahead of schedule. Line 2 pre-commissioning was completed one year later. These are the world's longest single-section offshore pipelines

Nord Stream pipeline route. (Image courtesy Nord Stream) The following discusses some of the challenges experienced during the planning, engineering and preparation for the pre-commissioning of this pipeline, and relate the relevant field experience collected during execution. A review of the schedule and lessons learned is also provided.

Nord Stream

The Nord Stream pipelines are defined as the offshore system which exports gas from Vyborg, Russia, crossing the Gulf of Finland and the Baltic Sea, to a receiving terminal in Lubmin (Greifswald), Germany. In addition to crossing Russia and Germany, the pipelines also cross the Exclusive Economic Zones (EEZ) of Finland, Sweden, and Denmark. Each pipeline has a capacity of 84 MM cm/d (2.9 bcf/d), with a total yearly capacity (for both pipelines) of 55 bcm (1.9 tcf).
The pipelines are designed with a segmented design pressure concept in accordance to the gas pressure profile along the route. There are no intermediate platforms along the route.
Each of the three sections was installed with offshore subsea terminations (start-up and laydown heads) designed for both the start-up and laydown operations and subsequent pre-commissioning activities. This was required because each of the sections had to be pressure tested separately.
Subsequently, the sections were joined by hyperbaric tie-ins at KP 297 and KP 675 thus creating a single 760-mi (1,223-km) pipeline.
Saipem was selected as the contractor for construction activities, while Baker Hughes was the selected contractor for the pre-commissioning work. The design and construction concept had a significant impact on the pre-commissioning execution.

Pre-commissioning concept

The initial concept was based on water filling from onshore to onshore, using large-diameter crossovers at the subsea tie-in locations for flooding. High-pressure crossovers would then be installed for pressure testing. Thus, all three sections of the pipeline had to be laid before commencing pre-commissioning operations.
Since the German area consisted of an almost closed bay with very shallow water and low currents, flooding was to be performed from Russia to Germany with dewatering in the opposite direction. This plan required a water winning and injection spread to be installed at the Russian landfall, with an air spread installed at the German landfall.
In an effort to reduce the environmental impact of the water treatment as much as possible, caustic soda (NaOH) and sodium bisulphite (NaHSO3) were initially selected as additives; the first to stifle anaerobic bacteria activity, the latter as an oxygen scavenger. The use of caustic soda generated significant concerns because of the possibility of precipitation of carbonates and blockage at crossover locations. Consequently, detailed water sampling and analysis trials were performed.
All pre-commissioning activities on the Nord Stream project were essentially "one-shot" operations that could not be repeated or reversed without major schedule impacts or affecting the permitting limitations for water disposal.
The pre-commissioning philosophy was further discussed and modified during the tendering phase with consultations between Nord Stream, Saipem, and Baker Hughes.
The resulting adopted philosophy was based on subsea intervention at the wet end of the sections from a subsea construction vessel (SCV). This philosophy enabled each section to be completed independently of any other sections, and enabled Section 1 (closest to Russia) to be pre-commissioned earlier in the year, while the sea at the Russian coast remained frozen and while Section 3 pipelay was still ongoing. This resulted in significantly increased schedule flexibility. The final pre-commissioning concept was established as:

• Pre-commissioning spreads onshore were located in autonomous areas separate from the construction sites used for the permanent facilities
• Pre-commissioning pigs for the subsea pipeline would not traverse the permanent pig traps or permanent valves
• Flooding, cleaning, and gauging (FCG) were performed offshore onboard the SCV. All subsea handling was performed by ROV
• Water was treated with sodium bisulphite and ultra violet light (UV) only
• Pressure test of sections 1 and 2 from SCV
• Pressure test of Section 3 from the receiving terminal in Germany (to reduce vessel time and risk from waiting on weather)
• De-watering from Germany with water discharge in Russia, after completion of subsea hyperbaric tie-in at KP 297 and KP 675
• Drying from Germany to Russia
• Nitrogen as a barrier between air and gas during commissioning of the pipelines.

The flooding, cleaning, gauging, and pressure-testing spread were installed onboard the Saipem vessel Far Samson and included suction pumps, a water treatment system, flooding and pressure test pumps. In addition, pressure test pumps for Section 3 were installed at the German landfall.

 

Flooding, cleaning and gauging activities were performed by the crew aboard the subsea construction vessel. (Image courtesy Nord Stream)

The dewatering and drying spread was located at German landfall and included 15 x 760 cu m (530 x 26,839 cu ft) steel water tanks. The Russian landfall was developed as a water receiving facility, and included a settling pond and a temporary 20-in. floating discharge line.
The FCG pig train was designed to ensure that the operation could be completed in a single pig run while providing contingency pigs to account for wet buckle scenarios during pipe laying. For each section, four bi-directional pigs were used. The pigs were back loaded into the subsea test head and installed on the seabed up to one year before the operation.

Flooding, cleaning and gauging pig. (Image courtesy Nord Stream)

The dewatering pig train was designed to ensure that water removal and desalination could be completed in a single pig run:

• The first batch of four pigs was separated by slugs of potable water designed to dilute to an acceptable level the residual salt content remaining on the pipe wall
• The second batch of four pigs was separated by dry air to pick up water remaining after the desalination pigs
• The pig train was spaced so that the first four pigs could be received and removed before the arrival of the second set of four pigs.

Dewatering pig. (Image courtesy Nord Stream)

Key challenges

The Nord Stream pre-commissioning work scope provided several key challenges because of its size, geographic location and environmental sensitivity. These challenges included:
Pipeline length. This was the world's longest offshore dewatering operation and the world's longest sealing tool run. Experience from the pre-commissioning team and the pig vendor was important in ensuring that pig integrity could be maintained along the full pipeline length. It was also essential to establish the location of possible events, particularly in case of gauge plate damage or stuck pigs. This required the development of a carefully managed pig tracking system.
Pipeline volume and water depth. Each 48-in. pipeline had an internal volume of 1.3 MMcm (343 MMgal). The maximum water depth, combined with losses, required a dewatering pressure of 29 barg. Therefore, a very large air compressor spread was necessary in Germany.

Sealing tool. (Image courtesy Nord Stream)

Vessel limitations. Since the flooding spread was large, the SCV had to comply with challenging criteria including deck space, accommodations, ROVs, cranes, and power generators. The fitting of all necessary equipment for a safe operation required input from specialists and extreme attention to detail.
Weather. Most of the Baltic Sea freezes in winter. This limits the offshore operational window. Water winning and water disposal could not be performed while the sea was frozen.
Water treatment. The water treatment philosophy was refined to provide the most environmentally friendly approach, in compliance with the applicable international and local regulations, while maintaining corrosion protection and minimizing the possible formation of precipitates inside the pipeline.
Noise pollution. Strict noise restrictions required the purchase of a custom air-compressor-spread to ensure compliance.
Diesel handling and storage. Large diesel volumes for the compressor spread in Germany required a custom diesel storage and handling system to receive and distribute 100 cm/d (26,417 gal/d) of diesel with no containment loss.
Waste management. A comprehensive waste management system was implemented to separate, track, and manage all the waste produced during the project's execution.

Schedule and execution

The contract for the Nord Stream pre-commissioning was awarded in August 2009. Tendering commenced early, allowing sufficient time for multiple concepts to be considered before settling on the preferred concept upon which the pre-commissioning contract was based. The tender period ran for about nine months concurrently with other project approvals to allow for immediate commencement.
The engineering procedures prepared for pre-commissioning totaled some 110 documents over an 18-month period, and which required review and approval by both Saipem and Nord Stream. The 18-month period was necessary not only for the base scope engineering, but also to evaluate all possible options and changes.
To meet the flow and pressure demands while achieving the strict environmental and noise targets set, a large percentage of the pre-commissioning equipment was procured new for the project. The early award afforded a 12-month period for equipment building, testing, and delivery. Despite robust contracting strategies, some vendors did not meet their delivery schedules; however, the window allowed sufficient float for this to not affect the project's schedule.
The success and performance of the flooding, cleaning, gauging, and pressure testing operations can be attributed to 100% contingency of critical equipment and full onshore spread function testing, including all interconnection piping. It took 41 days to complete Pipeline 1 and 38 days to complete Pipeline 2.
The success and performance of the dewatering, drying, and nitrogen packing operations can be attributed to a combination of excellent pigs and reliable compressor spread performances.
The full pre-commissioning of each line was completed in less than 150 days from commencement of FCGT to the completion of nitrogen injection.
From the formation of the Nord Stream pre-commissioning team in early 2008, to the completion of pre-commissioning operations on Line 2 in August 2012, the following milestones were achieved:

• RFQ issued for pre-commissioning operations, November 2008
• Pre-commissioning contract awarded to Baker Hughes, August 2009
• Engineering and procurement operations commenced, September 2009
• Contracts placed for all Wet Buckle Contingency (WBC) equipment, December 2009
• WBC equipment mobilized and function-tested, March 2010
• Contracts placed for all pre-commissioning equipment, June 2010
• Pre-commissioning FCGT equipment mobilized, February 2011
• Operational period for FCGT on Line 1, March to May 2011
• Dewatering and drying equipment mobilized, June 2011
• Operational period to dewater, dry, and N2 pack Line 1, July to August 2011
• Line 1 gas in work completed, September 2011
• Operational period for FCGT on Line 2, March to May 2012
• Operational period to dewater, dry, and N2 pack Line 2, July to August 2012
• Line 2 gas in work completed, September 2012
• Pre-commissioning sites reinstated, October 2012.

Results

The subsea heads, the hot stabs, and the pig tracking system worked flawlessly during the FCGT operations.
All the FCG pigs performed as expected and no damage was observed. The water filtration, additive system, and pumping spread operated consistently at or above their specified duty.
Success of the flooding operation was demonstrated during the pressure test where air content was confirmed to be well within the DNV requirement of less than 0.2%.
The cleaning operation removed less than 2 kg (4.4 lb) of construction debris in each section, supporting the belief that almost all the construction debris was removed. Some iron oxide, small amounts of sand, and some red-colored dust were also removed.
The gauging plates confirmed the internal diameter to be within the design requirements. Out of six smart gauge runs, only one gave a damage indication (which was proved to be false).
The pressure test operations were all accepted after only hours of pressure stabilization prior to the mandatory 24-hour holding period. The main reason for the quick and successful pressure test was the favorable spring weather conditions, whereby the temperatures were similar at the surface and at the bottom of the Baltic Sea.

Dewatering and drying

The use of temporary onshore pig traps, together with the temporary 48-in. valve, resulted in a very smooth and controlled dewatering operation. This made it easier to control the operation and to keep water and air separated during the launching of the train. The train was launched with Pig 4 as a "perfect" barrier between the desalination water and the air.
The compressor spread and dryers, as well as all support systems, met or exceeded their specified duties throughout the operation. As planned, air injections ceased when the dewatering pig train had traveled 60% of the pipeline length, with the remaining pig travel driven by the expanding air. This was implemented to save fuel and to minimize the depressurization requirements after receipt of the pig train in Russia.
Throughout the dewatering operation, the pig tracking vessel followed the different pigs all the way to the Russian coast. First, the two sets of sealing tools (one set from KP 297 and one set from KP 675) arrived, and then the dewatering train. Such accurate pig tracking was necessary when diverting the water in front of each pig to the settling pond.
During the receipt of the pig train, the desalination water was checked for chloride content. The analysis demonstrated that the final chloride content was well below the specified limit of 200 ppm. The amount of water received in front of the swabbing pigs was very small.
Based on experience from Pipeline 1 (very little water), the flow in front of the swabbing pigs was routed through the silencers for Pipeline 2.
Based on calculations and observations, the amount of water in front of the swabbing pigs was less than 1 cu m (264 gal). This is an impressive result and occurred because of good pigs, internal coating, and very smooth operations (with only the use of the discharge control valve toward the end of the operation to maintain maximum 1 m/s velocity).
The desalination pigs showed little wear after traveling the length of the pipeline. The swabbing pigs showed greater wear, but still maintained sealing integrity. That indicates that they had been running mostly dry and confirmed the excellent results.
The drying operation for Pipeline 1 was completed after 18 days, and included a soak period of 24 hours. An atmospheric water dewpoint of better than -35°C (-31°F) was achieved and confirmed by a 24-hour soak period. Based on this, pipeline 2 was dried to better than -35°C and accepted without a soak period.
During the drying, 4.5 cu m (1,189 gal) of water was removed (calculated based on dew point readings and air volume). This corresponds to a water film thickness of 1 micron on the pipeline wall, and demonstrates the best dewatering results ever achieved (regardless of pipeline size and length). Dryness was also confirmed post-commissioning where gas dew point levels were recorded.

Nitrogen

To avoid explosive mixtures in the pipeline (as set out in DNV OS-F101) during commissioning (gas filling), there was a need to use an inert gas as a barrier between the air and the natural gas in the pipeline. For various reasons, nitrogen packing differed between lines 1 and 2:

• Line 1 was completely filled with 99.9% pure nitrogen from Germany
• Line 2 was partially filled from Russia (gas filling end) using a 99.9% pure nitrogen batch equal to 10% of the pipeline volume.

For both pipelines, the mixing zone between air and nitrogen was approximately 1.5 km (1 mi). The mixing zone between nitrogen and gas was approximately 2 to 3 km (1.25 to 1.86 mi).
It was important to maintain the interface velocity above the critical minimum to obtain these results.

Water treatment

Treatment of the sea water was carried out onboard the SCV where the seawater injection pumps were installed. Pre-commissioning water was pumped into the pipelines at KP297 and KP675.
The water treatment included the following steps:

• Filtration through 200 µm and 50 µm cartridge filters
• Online injection of oxygen scavenger (OS), a commercial solution of sodium bisulphite and iron-based catalyst
• UV light treatment.

The key analytical parameters of seawater for the control of the treatment operations were obtained in a laboratory onboard the SCV.
Metered amounts of OS were dosed and adjusted daily to match the measured oxygen concentrations of filtered seawater. Based on the results of the test program, the OS dosage rate was set at the stoichiometric value, equal to 6.5 mg OS/mg O2.
Dissolved oxygen concentrations in the filtered seawater were generally at saturation values (over-saturation concentrations were also measured at times), ranging between 12.5 mg/l and 15.0 mg/l.
Special attention was given to the potential environmental impact of oxygen depleted water at the discharge location. A special water diffuser was designed and installed. The purpose was to achieve a high re-oxygenation effect in the proximity of the discharge point. This was a requirement of the water discharge permit from the Russian authorities. The effectiveness of the diffuser was confirmed by field measurements during dewatering.
A UV treatment unit was also installed onboard the pre-commissioning vessel. The unit had a design "killing rate" of more than 99% of the initial bacteria count at effective UV dosages of 40 ÷ 60 mj/cm². Bacteriological analysis was carried out in the laboratory onboard the SCV during the FCG operations for both total anaerobic and total aerobic bacteria before and after the UV package.
The calculated "killing rates" for anaerobic bacteria were generally in-line with expectations.
The results and observations confirmed the management of the water treatment achieved the project targets. There was no measurable impact on the marine environment at the discharge location and preservation of the integrity of the pipelines was confirmed.

Environmental considerations

Most of the water was discharged directly into the sea. The water treatment, as presented above, was acceptable for direct discharge.
The discolored water in front of each pig was captured and settled before being discharged back to sea. This water was diverted to the water settlement pond. Water stored in the pond was discharged to sea through filters after a minimum 24-hour settling period. All water discharged to sea was clean and contained no oxygen.
The discharge point was 600 m (1,968 ft) offshore and was fitted with a diffuser nozzle to ensure rapid oxygenation of the water.
The discharge water was continuously monitored by the environmental authorities and showed compliance with local and international regulations (oxygen levels were found greater than 7 ppm well within 100 m (328 ft) distance from the diffuser as required by the regulations).
The noise generated in Germany was monitored by a third party. The compressor spread complied in full with the stringent noise limitations for the project.
The sound proofing of all individual units also improved the working environment for the operational personnel and improved safety, as normal verbal communication was possible within the area of the compressor spread.

Lessons learned

A summary of the lessons learned during the execution of the work underscored the importance of:

• Early identification and focus on long-lead items
• Early establishment of a pre-commissioning concept
• Early selection of main water source and water treatment regime
• Early start of engineering and planning
• Early involvement in permanent design work (identify pre-commissioning requirements)
• Early establishment of any additional local authority requirements
• Early establishment of pre-commissioning environmental basis
• Early identification of risks and maintaining a focus on them
• Maintaining a risk register with regular reviews and updates
• Maintaining focus on equipment and function tests
• Carefully selecting key subcontractors and suppliers
• Careful and comprehensive follow up and control of critical supplies and suppliers
• Approving procedures well in advance of field operations.

In addition to these, it was found that pig tracking was very useful for control of operation and for accurate information to stakeholders. And in general, the project also underscored the importance of maintaining continuity of key personnel.

Conclusion

Given the considerable challenges, pre-commissioning of the Nord Stream pipelines was remarkably successful. Besides being concluded within budget and ahead of schedule, the technical achievements were impressive. They included:

• World's longest, single-section dewatering operation
• World's longest travel distance for tie-in sealing tools
• Combined dewatering/sealing tool removal operation
• Effective dewatering operation confirmed by the quick-drying operation
• Quick and effective pressure test operations (favorable temperatures)
• Effective water pumping through two 6-in. LFH (2,500 cu m/hr or 0.66 m/s pig speed in 48-in. pipeline)
• Effective cleaning and gauging operations
• Effective pigs specifically designed for the work
• Successful pig tracking for good control and operational confidence
• Effective water treatment concept with practically no effect on the environment.

These results were obtained by experienced personnel working as a team and focusing on:

• Early engineering and planning
• Early involvement in pipeline design requirements
• Early focus on long-lead items (e.g. pipeline head)
• High-quality equipment and experienced personnel
• Continuous attention to safety, risk, and environment
• Correct procedures prepared early by involved personnel
• Professional operational execution, monitoring and control.

Source:
http://www.offshore-mag.com/articles/print/volume-73/issue-5/pipelines-flowlines/pre-commissioning-the-nord-stream-pipeline.html