ROSEN Pigging School管道清管及内测技术

Manual

Pigging School

H. Rosen Engineering GmbH pipeline inspection

Pigging School


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1 | Introduction

The purpose of this pipeline pigging school is to enhance the knowledge of the participants in all aspects of pigging so that they can be in control of safe and efficient pigging operations. Pigging operations are safe when they can be executed without the risk of harming equipment, personnel and environment.

1.1 Figure 1: Oilfire in the russien taiga

Pigging operations are efficient when the purpose of pigging is achieved in shortest time at the lowest cost.

Comment:


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1.2 Figure 2: Pigging without disturbing the environment

This can only be achieved when all aspects of pigging are handled correctly but the difficulty is that these aspects are not under the control of one man in oil producing companies. Oil producing companies are organized in disciplines such as planning, engineering, project management, purchasing of equipment/services, commissioning, maintenance and operations. If the design for pigging the pipeline is not entirely handled correctly and normal pigging practice would be applied, it would be more difficult to achieve success.

1.3 Figure 3: "Bath Tube Graph" showing life time of pipeline


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This school intends to train participants for handling their own discipline and understanding the total picture of pigging so that they can: plan and engineer/design a pigging system for safe and efficient use, manage a pipeline project with pigging facilities, purchase pigging equipment/services economically, commission pipelines in a way, that no construction debris will interfere with the

operation of the pipeline or reduce its lifetime, maintain pipelines efficiently at low cost using pigging methods, operate the pipeline when pigging is required to prevent reduction of flow area. Due to this, the design engineer understands what the pigging requirements are, the purchaser can select products and services on technical grounds too, the operator can assess the design facilities before starting on pigging, etc. Todays industry is still back tracking as existing pipelines were not built in accordance with pigging design requirements and even though there is a better knowledge about pigging under pipeline design contractors and oil companies, there is no common standard. This leads to misinterpretations of the pigging requirements and result into unnecessary pigging difficulties in the field. Objectives of this school are, that participants can : Assess existing pipelines designs for pigging requirements. Take corrective action for efficient pigging, even though the design of the pipeline

is not optimal for pigging. Design new pipelines in accordance with pigging requirements for efficient

pigging. Install new pipelines facilitating pigging requirements. Optimize pigging operations during construction and commissioning of pipelines. Optimize pigging operations for pipeline maintenance and operations activities. Manage pigging campaigns for existing pipelines that have not been pigged

before or for special pigging jobs.

1.4 Figure 4: No guesses, facts are the result from pigging This is possible if all people involved understand the whole process of pigging.

Facts noguesses


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Pigging can be a trouble free operation, provided that a proper written engineering plan is made before pigging. In this way, trouble free pigging can also be achieved for pipelines that have not been pigged before. Trouble free pigging means that the purpose of pigging can be achieved without or with the least possible interference with the normal operation of the pipeline. Achieving the purpose of pigging is still very difficult to define: The purpose of pigging is to keep the line clean so that corrosion is minimized. There are no scientific rules to proof the effectiveness of such an activity. The purpose of pigging is to remove condensate from a gas line. We may be able to measure how much debris a pig pushes out of the pipeline. But how much is left in the pipeline is unknown.

1.5 Figure 5:Only a few meters of pipeline is visible and can be inspected from the outside Most of the pigging technology is based on common sense rather than on any scientifically developed program. Design considerations, maintenance practices and operational consequences are mentioned in this booklet for its relation to pigging only. They shall always be checked against existing standards and regulations of clients and local authorities.


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2 | Why Pig Pipelines?

There is only one reason for pigging: To get a better performance of the pipeline in a way that can not be achieved in any other way than by pigging! If pigging does not fulfill this requirement, pigging should not be applied. A better performance for pipelines means : Lower installation cost. Lower energy requirement for moving the product. Lower maintenance cost. More efficient separation of product batches. Longer pipeline lifetime.

2.1 Figure 6: Pipelines require preventive maintenance to combat

wear, tear and degradation


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The pipeline is an asset and the performance efficiency is measured by ; a combination of cost for initial investment and cost over the years for

maintenance and operations, versus financial proceeds for operating the pipeline and cash flow characteristics. Saving on investment cost results quite often into much higher cost in maintaining and operating the pipeline. This trade off should be analyzed before jumping into investment cost savings. If the investment cost are one hundred million US Dollars, saving 5 percent sums up to five million. However, if the yearly cost are ten million US Dollars for 30 years, saving five percent on maintenance and operations adds up to 15 million US Dollars. Saving on investment cost however, may be considered for cash flow restriction requirements. It would be a direct choice and the consequences should be accepted. The following are the various stages, in which pigging could be considered: Construction Removal of construction debris. Test water filling and removal. (If this is the only reason

for pigging, temporary pigging facilities may be considered) Drying of the pipeline before commissioning. Commissioning Introduction of pipeline product leading up to process conditions. Maintenance Pipeline cleaning (removal of aggressive debris from production so that corrosion

is minimized). Pipeline inspection. Operation Pipeline cleaning (removal of obstructive production debris for maintaining

designed flow area; wax, paraffins, asphaltenes, liquid built-up in gaslines, etc.). Batching of different products. Repair Product evacuation and re-introduction. De-commissioning Product evacuation. When it is decided that pigging is required, the pipeline shall be designed for carrying out pigging without interfering with the normal operating activities. The above points are elaborated on as follows: Removal of construction debris This is required to prevent contamination of the final product to be carried by the pipeline with unwanted materials or substances. Examples: Soil like materials in jet fuel transmission lines. Millscale in gas lines feeding power generation facilities.


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Sand in waxy oil lines. Test water filling Using pigs for test water filling is always a requirement to prevent air pocket formation in high points of the pipelines. Besides being dangerous having air in the pipeline during hydrostatic testing, air pockets may cause three kinds of other problems: a. The temperature variation during night would offset the normal pressure

variation causing inconclusive pressure readings. b. If the pipeline has a leak, the pressure reduction over time-rate can not be

used for determining the leak size. c. If the air pockets accumulate in the peak of high range mountains, the pumps

would not be able to pressurize the pipeline for hydrostatic testing. The pressure in the highest peak would be the same in the following lower peaks and the required hydrostatic test pressure will not be reached at the end of the pipeline.

Test water removal Removal of test water is usually required as it is not desirable to have water in the pipeline product, unless the pipeline product is water. Particularly, water is absolutely not desirable if the pipeline product is gas with corrosive agents. The corrosive agents would form a reaction with the oxygen in the water and prematurely corrode the pipeline wall. Although, pipeline drying is not the prime subject here, it is stated here that it is not a good approach to use only so called swabbing pigs for attempting to dry pipelines before final drying processes (vacuum drying or industrial alcohol drying). The capacity of a swabbing pig is very small. It is limited to a couple of liters, being the sponge capacity of the foam volume of the pig. The amount of moisture in the air that is compressed to drive the pig through the pipeline may even outweigh the quantity of water in the foam pig. More water is coming into the pipeline for driving the pig than that is coming out in the foam pig. After a normal pig run with sealing type pigs, the pipeline wall is still wet. The mill scale holds water and the weld penetration breaks the seal of the discs/cups temporarily and allows water to flow back behind the pig. The initial attempt to dry the pipeline using pigs should include running a pig train at steady speed of a minimum of 1 meter per second with at least five high capacity sealing pigs. Three pigs should be launched as good as together with almost no space in between. The pigs number four and five shall follow each at intervals of about 10 percent of the pipeline length. Pressure expansion may be used to drive home the last two pigs to save compressed air and to prevent additional moisture coming into the pipeline. The pig train run may be repeated once again, if felt necessary. This method achieves a maximum push out of water with a minimum amount of moisture input. Pig trains are usually more efficient than the sum of individual pig runs. Swabbing pigs shall be used at short intervals (maximum 10 percent of the pipeline length spacing). This requires a launching facility that can launch consecutive pigs without interrupting the flow. The positive effect of pushing water forward will otherwise be spoiled by stopping the flow. Introduction of pipeline product leading up to process conditions Mainly sealing pigs are used for this purpose to ensure that only the designated product is in the pipeline when the process is started.


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A high wear rate on the pig discs/cups shall be expected as the pipeline is new and the wall will be abrasive due to the presence of mill scale. For longer pipelines (above 80kms long), it may be required to provide supportive components (e.g. brushes) on the pig to prevent premature excessive wear on the discs/cups. Pigs subject to excessive wear on discs will loose its sealing ability, allowing fluid to bypass and air staying behind it. Some processes or pipeline products are very sensitive for mixing with air or water. Particularly fluids such as propylene, ethylene, etc. Ethylene requires particular attention as it is a gas under atmospheric condition and it goes into supercritical condition above 80 bars pressure when it is a liquid gas. The density is about 300 kg/per cubic meter, which is nor gas (100 kg/per cubic meter) nor liquid (850 - 1000 kg/per cubic meter).

2.2 Figure 2: Dust in an ethylene pipeline

Nothing can be done, if a pig is run in ethylene and it will get stuck. It is not possible to weld on the pipeline. Pressure variations can cause operational problems and it is better to empty the pipeline before pigging and pig with nitrogen. Even then there is the present of dust or ethylene grains, that may accumulate in front of the pig and cause the pig to stop.


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Pipeline cleaning (Maintenance) There are two reasons for pipeline cleaning in a maintenance program:

2.2.1 Removal of aggressive debris from production so that corrosion is minimized. Examples are; water in sour -crude oil, -product or -gas pipelines, etc. Quite often, the effect of corrosion activities is only discovered after inspecting the pipeline by intelligent pig. The best pig for the job can be tried out and established by keeping good records on efficiency and pig wear as these maintenance type cleaning pigging operations are usually carried out at frequent intervals. In cases of neutralizing the aggressive corrosion agents, frequent cleaning and scraping is required for proper preparation of the pipewall for treatment with chemicals for neutralizing the aggressiveness of corrosion agents. Money spent on chemicals is only efficient, if the chemicals are applied to the clean wall of the pipeline. The chemicals are either injected into the pipeline by dosing pumps or they are applied by pumping a batch, dissolved in a solution of liquid of the same specific gravity as the chemical, in between two pigs.

2.2.2 Removal of accumulated debris over a longer period of time after prior inspection cleaning. This may be an irregular pigging operation requiring special attention. The removal of the accumulated debris may interrupt the normal operation of the pipeline. This type of job is usually new or strange to the operating team of the pipeline. If organized by maintenance department, detailed plans for carrying out such a pigging campaign shall be written including possible consequences for the operation of the pipeline. The total plan shall be reviewed by all company departments involved prior to pigging. In certain instances, the plan should further consist of back up procedures for the contingencies response plan. These instances may be : The line has not been pigged since commissioning (e.g. 28 years ago) and there

are worries about the piggability. A lot of debris is expected to have accumulated in the pipeline and there is

concern about blocking the pipeline.

2.3 Figure 3: Wax removal


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The back up plan may include hot tap and by pass equipment or standby of sufficient nitrogen capacity for expelling the gas from the line section. Pipeline inspection Pipeline inspection is required for keeping the finger on the pulse for the integrity of the pipeline. The subject is further elaborated on in section 9, Pipeline Inspection. Pipeline cleaning (Operation) Removal of obstructive production debris for maintaining the designed flow area free from sand, waxes, paraffins, asphaltenes, liquid built-up in gaslines, etc. These pigging operations are usually carried out at regular intervals. The best pig for the job can be tried out and established by keeping good records on efficiency and pig wear. During the design stage, attention shall be paid to the downstream equipment for handling the extra ordinary process condition created by running a pipeline pig. Batching of different products Batching of different products in pipelines is a common practice for pipelines carrying refined products. The interface of different products is reduced, economizing on the transport efficiency. The pigs used for this purpose must have a good sealing ability. In many cases, however the practicality of using pigs is diminished by ill functioning pigging installations; pig-sigs that do not work, quick opening doors that do not open quickly, logic control systems that malfunction, etc. Batching is also used for sending batches of chemicals in solution as described in Pipeline cleaning (Maintenance). For special jobs batching can also be used to run an Ultrasonic Type Inspection Pig in a batch of liquid of a gas pipeline. More elaborate information is in Section 7, Special Pigging Jobs. Product evacuation and re-introduction (Pipeline Repair) Pipeline pigs may be used to economize pipeline repair activities. Pigging in opposite direction may be required for this activity. Product can be evacuated from the pipeline or a part of the pipeline can be freed of hydrocarbons. High friction pigs can be used to batch a slug of water to the area of pipeline repair and hold the hydrocarbons away. More elaborate information is in Section 7, Special Pigging Jobs. Product evacuation (De-commissioning) At the end the lifetime of a pipeline local authorities might demand that the pipeline is not just abandoned but the pipeline is cleaned, the product is evacuated and the line is filled with a product (water) harmless to the environment. Pigging in opposite direction may be required for this activity.


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3 | Planning

Consideration should be given to the pipeline loading growth for planning of pipelines. Certain pipelines are designed for transmitting full load from the very beginning. Other pipelines are designed in anticipation of increasing throughput loading. They reach the ultimate design throughput later in their lifetime. The effect of initial low flow shall be given attention. Example : The pipeline will be carrying crude oil from an offshore field to an onshore terminal and the field is presently producing only 15 percent of its anticipated design throughput that will be reached in 5 years time. The produced crude oil contains traces of water with H2S or other aggressive components that require regular removal from the pipeline. The pipeline requires pigging for maintenance purposes. The planner shall consider building two pipelines. One for the initial 3 years and a later one to share the ultimate field production capacity with the firstly installed pipeline. The added advantage in addition to the low initial investment cost is, that we end up with two lines, giving ultimate flexibility. Building a full scale pipeline for the ultimate flow requirements may result into operating the pipeline in the initial 3 years below minimum flow conditions. The initial flow will be too slow to perform efficient pigging to remove the aggressive components that settle down at the bottom of the pipeline creating irreversible internal corrosion at the bottom half of the pipeline. Pipelines, especially those operating at low laminar flow conditions, are known to be very good gravity separators. The aggressive components will settle down at the bottom of the pipeline, especially at such low flow rates. The minimum flow for effective pigging of liquid lines is normally 0.2 meters per second. The final result may be that the pipelines lifetime has been shortened by 15 years in the initial 3 years of operation.


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This is an example where the planner should be aware of the effect of different loadings on pipelines that require pigging for maintenance purposes.


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4 | Design Aspects for Piggable Pipelines

The design of piggable pipelines must aim at trouble-free and efficient pigging for all the aspects of cleaning, commissioning, maintaining, inspecting, operating, and de-commissioning activities. Trouble free means that the pig can pass the pipeline all the way from the beginning to the end without any obstructions that might stop, slowdown or make the pig accelerate dramatically. Efficient means that the purpose of pigging is achieved in the most economical way. If the pipeline is existing and a pigging program is to be planned, the following design notes shall be checked against the as-built situation. Any differences between the design documents and the as-built situation shall be highlighted and assessed before starting pigging. An engineering report shall be made to address the differences so that precautions can be made in terms of selecting the type of pigs or whether pipeline modifications are required. By carefully preparing this, pigging can be trouble free. For new pipelines, the following notes shall be included in the basic design package to be submitted to engineering contractors prior to awarding the contract for engineering the pipeline. The engineering contractor shall advise whether these notes contradict with any of the other standard documents issued by the client. It is recommended to assign one individual mechanical engineer to the project with the responsibility to safeguard the piggability of the pipeline throughout the entire engineering, procurement and construction process of the pipeline, starting with the feasibility phase of the project. The engineering contractor shall be requested to appoint a pigging engineer too. Many existing pipelines have irritating pigging problems due to minor design deviations from the original concept. There are many examples of this :


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The pipeline is built under a turn key type contract and the barrel diameter of the

pig-trap is not specified in the project brief. In order to save cost, the oversized pig-trap barrel is insufficient. It is a struggle to get the pigs in and out from the traps every time the line has to be pigged.

The 6 inch size pigtrap has a forged type reducer size 6 by 10 inch with an

extreme small internal diameter. It requires hazardous high differential pressures to get the pig passed this reducer. Small diameter reducers shall be rolled rather than forged.

4.1 Figure 1: Rolled Reducer

The internal diameter was not specified for pipeline fittings in a 1500lbs rating, 6

inch pigtrap. Pigging turned out to be impossible due to extreme internal diameter differences.

Reduced bore ball valves were chosen for a low pressure gas pipeline as pipeline

separation valves. As a result the inspection pig accelerated to high speed, making it difficult to assess the inspection data in the pipeline section downstream of each of the valves.

The outlet of a pigtrap was so small, that the cup of a normal cup type pig blocked

it completely. The impact on cost is very minor if a pigging engineer is assigned to the project, but it would prevent these types of mistakes. The following notes are to be followed for designing piggable pipelines:


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4.1.1 Design the pipeline for bi-directional pigging for maximum flexibility at very low cost.

4.1.2 A pipeline system consists of two parts; the pipeline itself and the pigtraps. The pipeline design has to guarantee an effective running of the pigs, while the pigtrap design shall be aimed at safe and efficient handling of the pigs for launching and receiving.

4.1.3 The pipeline shall have a constant internal diameter over its full length. This means,

that changes in wall thickness should affect the outside diameter and not the internal diameter. An area of concern is road crossings and off-shore platform riser piping.

4.1.4 For pipeline inspection purposes using intelligent pigs, the pipeline should be markered with pipe pups with a length of 2 meters, spaced every one or two kilometers. These pups should be installed at the location of the above ground kilometer markers for future reference.

4.1.5 Also, the chosen internal diameter shall be specified for all fittings such as elbows, tees, valves, flanges, etc. of the pipeline and the pig trap piping. Supplier's quotations shall be checked against the specified internal diameter. Also, the factory inspection for fittings shall include this checking procedure. Elbows in pipelines shall have a minimum radius of 10 times the diameter for pipelines of 4 inch size, 5 times the diameter for pipeline sizes up to 14 inch and 3 times the diameter for 16 inch and above.

4.1.6 Tees with a side outlet bigger than 50 % of the pipeline size shall have guiding bars or

preferably, shall be sphere type tees (see sketch).

4.2 Figure 2: Sphere type tees

4.2.1 Valves shall be full bore and shall be checked for piggability before purchasing.

4.2.2 Pig passage indicators (pig sigs, pig alerts, pig flags, etc.) shall be specified and

marked for the wall thickness of the specific pipeline section in which they are to be mounted. Otherwise they may penetrate too far into the pipeline and obstruct the passing pigs. It is best if bi-directional pig passage indicators are specified. This will prevent any mishap in case of a pig reversing, intentionally or unintentionally,


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4.2.3 Check valves or non-return valves shall not be used in piggable pipeline sections. In case of booster pump station arrangements without pigtraps, this arrangement is recommended. Sizing of the check valve arrangement can be based on maximum velocity so that the diameter is usually two sizes smaller than the diameter of the pipeline (designed on pressure drop).

4.2.4 The piping layouts shall avoid swanneck arrangements (back to back elbows). The

minimum distance between two fittings shall be at least three times the pipeline diameter.

4.3 Figure 3: Fitting Arrangement

One specific requirement with reference to the layout of pipelines is mentioned here. Reference is made to the example in Section 3, Planning, about a pipeline with low flow and aggressive components; in these cases steep inclines shall be avoided, if possible. Following the contours of the landscape can of course not be avoided but pipeline crossings should be installed with a maximum 5 degrees uphill incline. This is an experience figure. Steeper incline will cause internal corrosion of the bottom of the pipe upstream of the pipeline crossing due to debris accumulation. Pushing pigs through small diameter pipelines (< 16 inch) requires a much higher differential pressure across the pig than in pipelines of a larger diameter. The required force for pushing the pig forward is determined by the friction between the pipewall and the cups/discs. In small diameter pipelines, the ratio cross section/circumference is low.

| Pipe dia. | Cross section | Circumference| Ratio | Delta p


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| (inches) |(square inches) | (inches) | | up to (PSI)

| 4 | 12.56 | 12.56 | 1 | 175

| 6 | 28.26 | 18.84 | 1.5 | 125

| 8 | 50.24 | 25.12 | 2 | 75

| 16 | 200.94 | 50.24 | 4 | 15

| 32 | 803.84 | 100.48 | 8 | 3

An additional reason for higher differential pressure requirement in small pipelines is the stiffness of small diameter pig discs/cups. In addition to the high differential pressure requirement for running pigs, there is also a relation between the starting pressure requirement in small pipelines. If a 4 inch diameter pig happens to stop with the cup against severe weld penetration, it might take more than 800 PSI differential pressure to overcome the resistance before it restarts. Pigging is most effective if a relative constant speed can be maintained and it is clear that this is quite difficult pipelines of a smaller diameter. In view of the above, special care shall be taken for designing the internal geometry of the pipeline. Avoid internal diameter changes especially between fittings and pipe. Especially for very high pressure systems (above 900 lbs rating), the internal diameter for fittings must be specified. The factory inspector can then check it before assuming ownership. Designing small diameter piggable piping systems requires more than usual attention from designers and engineers. The design cost to make a small diameter pipeline piggable is higher than with a large diameter pipeline. The basic design philosophy for pigtrap arrangements is recommended to be in accordance with this piping and instrument diagram.


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Notes on the diagram: The design of the pig trap and piping, up to and including the main line valve and the bypass valve, shall be considered as piping in accordance with the applicable cross-country pipeline code. The pig trap shall not be considered a pressure vessel. Two pressure indicators, one located near the door and one in the pipeline section. By this, the pipeline pressure can be monitored during pigging and the pressure in the trap is measured at all times. A pressure equalizer line is installed over the reducer. This line is a safety feature to prevent accidents resulting from trapped pressure during opening of the trap. The line also prevents shifting of the pig in an uncontrolled manner and hitting the valve during pressurizing of the trap. The kicker or bypass valve size shall be two sizes smaller than the pipeline diameter with a maximum of 16 inch. The type of valve recommended to be used is a soft seated ball valve. The kicker/bypass valves of 8 inch and larger shall have a smaller size bypass valve for pressurizing the trap. The small size valve shall be designed for throttling service. Two drain connections avoid unnecessary spills to open air as the trap is drained on both sides of the pig. The piping arrangement of the traps is recommended to be basically in accordance with this sketch.


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Notes on the sketch: The internal diameter of the neck of the trap shall be the same as the chosen internal diameter of the pipeline. The length of the neck shall be 3 meters for all sizes, making the trap bi-directional. Any type of pig can be received without the risk of having the tale end stuck in the trap valve. The reducer shall be eccentric (bottom flat installed) and preferably be manufactured from plate. Forged reducers, in particular sizes up to 8 inch, often have reduced internal diameters which make them not suitable for pigging. The barrel shall be oversized by: 2 inches for pipelines up to 10 inch, 4 inches for pipelines up to 30 inch and 6 inches for pipelines over 36 inch size. The length of the barrel shall be 4 meters. The dead end connections shall be self draining and as short as practical to prevent stagnant flow. The practical length between the centerline of the pipeline side connection and the main line valve is recommended to be 2 times the pipeline diameter. This will prevent operational problems in case the valve is accidentally kept closed during pig receiving. Two pig passage indicators are recommended. One at the reducer in the neck and the other 5 diameters away from the mainline valve. The bypass/kicker line connection should be located midway between the reducer and the door. A flanged connection is recommended between the trap and the bypass/kicker valve. It is recommended to standardize the pigtrap layout, so that the risk of mis-operating the valves is kept to a minimum. Platforms shall be installed, if necessary, to operate the valves safely. The trap area shall be hard covered with a concrete slab and protected by railing against traffic. H2S service


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The pigging trap unit shall have double block and bleed valve arrangements for the trap valve and the kicker/bypass valve. If the kicker/bypass valve arrangement has a second valve for throttling, this valve may be used as double block and bleed. The bleed valve shall be double block too and shall be opened only for testing the seals on the main valves. They should normally be closed. The trap should have permanent connections for purging before opening of the trap. The trap arrangement operations shall only be carried out when all the safety requirements for H2S installations are in place. Quick opening doors Quick opening doors have actually three features that require attention during technical evaluation : The sealing reliability The easiness of operation Standard size spares The sealing reliability is of course very important and attention should be paid to the sealing arrangement; the seal shall be a large diameter o-ring that should not be squeezed for sealing. But it should seal in a chamber that is irrespective of the alignment of the door. This is especially relevant for the larger sizes. The hinge arrangement may be of influence to the alignment of the door seal. The technical evaluation for choosing the door type should concentrate on this feature. Any seal arrangement, that is depending on alignment will sooner or later be mis-aligned. It is better to justify some more money during the purchasing proceedings than to be stuck with a troubling door ever afterwards.

4.4 Figure 4: Pig Trap Doors

The easiness of operation (not necessarily the quickness of operation) is important for the pigging crew. The easiness of operation is determined by the number of people


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required to open the door, or whether special tools are required (if the tool is lost or broken, the door can not be opened or worst closed). The spares should be easily available and easily installed. Standard size and shape o - rings, Standard size threading in the door for the pressure warning valve, Standard type pressure warning valve, Large size threading on the plug of the pressure warning valve so that it is not

spoiled easily after a few times of use. The screw type door requires quite some force to break the seal. If the door is not used for a long time, the force is tremendous and the lugs on the door can give way sometimes due to the high forces. Similarly, the key to operate the door can get easily spoiled, if high forces are applied for breaking the seal after long time of operation. Similarly, the frequent use wears out these keys fairly quickly. If they are used forty times, they will be worn out.

4.5 Figure 5: Clamp Door

Some clamp type doors have safety catches so that they can not slam open all the way when perhaps little pressure has built up during the process of opening the door. This may be caused by a slightly leaking valve. The safety catch will allow the door to open little only to relieve the pressure. The clamp type doors should not have an arrangement, that requires the clamps to pressure on to the seal. The seal shall be made by the o - ring in the chamber and the clamps should just be holding the collars of the door and the trap together without pressurizing the sealing o - ring. Some designers like the non-venting type pressure warning valves for H2S service. It is a general experience that the non-venting type safety pressure warning valves are


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not really reliable at low pressures. The arrangement is usually, that over pressure in the trap prevents opening the door. How little over pressure is over pressure? If any of the valves to the trap are leaking, they will be leaking. And if the leaking rate is low, we may be able to open the door before the non-venting pressure warning valve kicks into action. The situation would be more dangerous if the impression is given, that there is no leak while, actually there is one. With the venting type pressure valves arrangement, the procedure is to sniff the outlet of the valve after purging for H2S content in the gas. If there is no presence of H2S, there is no leak. According to safety practices, continuous readings shall be made during the time the trap is open. These are the most important tips for reviewing different designs for doors. Pig Passage Indicators Pig passage indicators (PPI), pig sigs, pig alerts, etc. are relatively inexpensive. They tend to be ignored or their importance is underestimated during the technical review. But they are very important for the success of pigging operations. They are usually considered as unreliable. When they have not signaled at time that the pig is expected to have passed by, generally they are ignored. There can be three causes for malfunctioning: Poor design of the PPI Wrong installation No maintenance The design of the PPI can be checked for: The length of the finger penetrating into the pipe The angle at with the finger switches the top work of the PPI The sealing arrangement between the pipe connection and the top work The ability and the easiness to exchange the internals of the PPI under pressure.

What spares are required to do this. Is it foolproof? Can it be installed wrongly and the finger brakes off when a pig passes? Do you need different models (and internals) for different wall thickneses? Is it bi-directional? Can it be hooked up to the instrument systems for remote monitoring? What corrosion resistant materials are used for inside or outside corrosion? Pipeline companies usually require a lot of these PPIs and since they are inexpensive, it would pay off to actually buy one of each on the selected vendors list and let engineering study them. The wrong model could be installed on the wrong wall thickness. The result would be, that the finger would not sufficiently penetrate the switch, when the pig passes, or the finger would penetrate too much and damage the pig. The finger penetrates into the pipe sideways rather than in longitudinal direction. The PPI has been sitting on the pipe and has been painted three times and nobody looked at it before using it. A little solvent and grease plus some movement does wonders. Effect on downstream pipeline facilities caused by pigging operations


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Condensate in gas lines Condensate separates very well from gas in gaslines especially under laminar flow conditions. The separated condensate will accumulate in the bottom of the pipeline in the low spots of the pipeline. Especially in the case of offshore lines, that run from platform to platform or from platform to coast, the middle section is low and a lot of condensate accumulates there. The amount of accumulated condensate can only be calculated by computer modeling as it is depending on many factors: pipeline length, inlet/outlet pressure, flow, amount of condensate in the gas, vapor pressure of the condensate, topography of the pipeline route and accumulation of condensate. Different flow regimes in different parts of the pipeline will occur at different times due to the restriction of accumulated condensate and the movements of condensate. Some time after start up, if no flow changes occur, a status of equilibrium will be reached and the product composition of the inlet flow will be equal to that of the outlet flow of the pipeline. A pig however will disturb the equilibrium and a slug of liquid will puke over into the downstream facilities. The downstream facilities should be designed for such an event. The normal gas/liquid separation facilities will usually be insufficient for the slug of liquid coming in at gas rate. Gas/liquid separation facilities are normally designed for the liquid production as part of the gas production, say 1,000 cubic meters per day. When the slug comes in, the requirement may be as high as 1,000 meters per hour. Slug catchers are the usual solution to this problem. The size of the slug catcher can be optimized by choosing a certain pigging frequency. Gas in oil lines The phenomena of slugging may also occur in pipelines from wells, in which the associated gas has not yet been separated from the crude oil. The gas/oil ratio at the downstream end will be disturbed tremendously when a pig is run. Initially during the pig run, only gas will arrive and towards the end of the pig run only oil will come in but at a rate that could be 5 times higher than the design rate for the gas/oil separator. The separator will also have insufficient hold up capacity to handle the slug of oil. And if no special pre-cautions are taken, the plant will shut down on high oil level in the separator. The inlet flow into the pipeline has to be reduced. History case: A 12 diameter, 36kms long pipeline carrying untreated crude oil with associated gas from an offshore production unit to an onshore treatment plant. When the line was pigged for cleaning prior to inspection, the separator would flood the plant. The field had to be shut down. The shutdown could have been prevented, if the production had been cut back by 50 percent when the pig reached the 28 km mark. This was calculated by time and flow. Following this procedure, the separators could handle the increased oil flow at the incoming rate and the field was cut back by 50 percent for maximum 4 hours only. After pig arrival, only gas would come in as the low spots in the pipeline were filling up with oil before the status of equilibrium was reached again.


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An additional problem in this line was that the oil is wet and water separated too. After inspection, internal corrosion was found and it was decided to sweep the line regularly. Dust in gas lines Another problem is dusty gas lines feeding into power generation plants. Dusty gas lines are generally a problem for both operational and pigging activities. OPERATIONAL PROBLEMS The problems of dust, during normal operation, manifest themselves mainly in the downstream facilities of the pipeline i.e. power generating turbines, transmission compressors, etc. Running of pigs usually increase the downstream problem during arrival of the pigs when a large cloud of dust comes in with the pig. Most of this dust cloud will leave the pig receiving trap through the outlet connection into the downstream facilities and only a small portion will stay in the pig trap. DUST FORMING The origin of dust is not exactly known but it is generally believed that it is formed in a reaction caused by H2S (even if it is only present in very small quantities) with the pipewall. Oxygen is required for this reaction and this may be the explanation why dust is formed at a high rate in new pipelines. The mill scale is rich in oxygen and the reaction of dust forming continues at quite a high rate until all mill scale is converted into small size dust particles that may be pyrophoric. The rate of dust forming slows down after all mill scale is converted. Dust forming does not completely stop after this stage due to the fact, that all produced gas contains, at least, traces of water which contains the necessary oxygen keeping the H2S reaction active. Dry gas does not exist. Up to 5 milligrams of water was found per cubic meter of dry gas with a Dew point of minus 42 degrees Celsius. Although dust is being formed immediately after commissioning of the pipeline, it may not manifest itself immediately in downstream facilities. This will only happen, when the flow in the pipeline is sufficiently high to loosen the dust from the pipewall and move the dust to the downstream facilities. The theory of preferential route in piping systems may explain why dust particles seem to accumulate in side connections such as instrument tubing connections which cause problems to operations. PREVENT DUST FORMING Considering the foregoing, intensive cleaning pigging using brush type pigs, immediately after commissioning is the best solution to prevent the development of a dust problem before it can go out of proportion. In this way, the mill scale can be removed before it is converted into small size dust particles. The cleaning activities must be continued until several pigs are received without any debris present. Cleaning the pipeline before commissioning is good. It is however more expensive and it is impossible to apply a quality criteria for the cleanliness of the pipewall. Before commissioning, pigs are usually propelled by low pressure compressed air for economical reasons. Low pressure is not ideal for running pigs in stable conditions and there will be a tendency to cut down on the number of pig runs before the pipeline is clean. The dust problem may still develop without it being known.


page 28

It is recommended, that the pipeline be cleaned before commissioning for removing normal construction debris and cleaning the pipeline again after commissioning to remove all mill scale. A train of 4 brush pigs in front of hydrostatic test water will give good results provided that a constant flow of water at a minimum velocity of 0.3 meters per second can be achieved. Since it is more difficult to remove construction debris than to prevent it from entering into the pipeline, it is highly recommended that the pipejoints be equipped with plastic caps immediately after fabrication. These plastic caps should be kept in place during transportation from the pipemill to the jobsite until the pipejoints are welded together. Install nightcaps during construction of the pipeline. Usually the dust manifests itself when the flow through the pipeline system gets up to speed and the downstream filter units have to be changed at an unacceptable high frequency. The design of filters protecting downstream equipment is based on catching dust particles larger than 5 or 10 microns. These filters clog up when the differential pressure reaches about one Bar but they do not have any reasonable storage capacity for dust. Getting rid of dust that is accumulated in the pipeline system by filtering is not practical. The average quantity of dust measured per filter element is 0.6 kg. These elements cost about US$50 so that the material cost for removing one kilo of dust is US$80. Diverting the gas flow to the flare during pigging may not be acceptable from an environmental and economical standpoint. REMOVAL OF DUST Removal of dust in gas pipelines from a pigging point of view does not differ much from removing any other debris from pipelines but there are two specific difficulties: fast wear on the discs by the abrasive dust bringing-in manageable quantities of dust with each pig run

(usually, the quantity of dust exceeds the push-out capacity of the pigs by many times and the danger of an excessive amount of dust accumulating in front of the pig is not unusual)

HANDLING OF DUST DOWNSTREAM OF THE PIGTRAP The pig receiver is not capable of separating the dust from the gas and most of the dust will leave the pigtrap through the outlet line. Handling the dust during pigging downstream of the pigtrap is significantly different from the conditions of gas/dust separation under normal operational conditions. At normal gas/dust separation operations, filters are normally used and the dust present in gas at a certain density is filtered and the filter is changed when the pressure drop reaches the maximum level. At pig arrival, a large amount of dust arrives at once and such a quantity is too large to be handled by filters. Filters do not have storage capacity and they would be blocked before the dust cloud is finished. For a dry method of separation, a cyclones unit must be installed in the outlet of the pigtrap for pigging operations. The unit shall have a high dust/gas separating ability and a large dust storage capacity.


page 29

The unit has to be emptied after each pig run and many pig runs may be required to clean the line once dust is formed.


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5 | Pipeline Construction Aspects for Pigging

Besides the normal construction requirements for laying pipelines, there are some special requirements for trouble free pigging; With reference to the notes on small diameter pipelines in section 4, Design aspects for piggable pipelines, care should be taken to avoid having excessive weld penetrations and misalignments in small diameter pipelines up to 8 inch. Pipelines are usually marked every kilometer. The marker, however, is situated above ground and has no physical connection to the pipeline. This means, that the marker position cannot be recognized by any intelligent pig unless a feature is installed in the pipeline itself marking the above ground marker location. It is suggested to introduce a pipeline feature by installing a short pup-joint (say 2 meters long) at the above ground marker location. The short pipejoint welds will be recognized by all the different types of intelligent pigs and together with the above ground marker joints, it can be used as a landmark for finding pipeline features such as corrosion defects accurately in the future. With regards to freeing new pipelines from construction debris, it is better and easier to prevent debris entering into the pipeline during construction rather than trying to clean the pipeline afterwards with cleaning pigs. During transport and construction, pipejoints should be capped with a plastic cover. The pipeline should also be night-capped daily. Pipeline pigging during construction for cleaning the pipeline of debris should be carried out in accordance with written plans. These plans should be presented in the construction quotation for review before order placement.


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5.1 Figure 1: Pipeline Construction


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6 | Considerations for Purchasing of Pigs

Looking at a bit of history, pigging started to be used first, early this century and in those days there was not really any sophistication in the design of pipeline pigs. Pigs developed over the years, following a trial and error method. What that really means is that when somebody, with a little more courage than others, would come out with a new model and this model was successful (commercially) it would be copied by every company that is in the pig business. No real research or scientific developments were done until feedback from instrumented pigs was used for optimizing pig designs. This feedback included confirmation that pigs do rotate in pipelines. They only stop rotating when they wear excessively high on the discs/cups and they wear out one sided. It was the company of ROSEN, that looked at the basics of pig behavior and came up with the innovative idea of separating the sealing discs from the supporting discs. Until then, the bi-directional pigs had only square discs and when they worn out the sealing was lost and the pig stopped.


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Similarly, the design of conical cups is always a trade off between: a. sufficient flexibility to maintain a seal for driving the pig and b. providing sufficient rigidity to center the pig in the pipeline. The idea of separating the two functions of sealing and supporting the pig into two different discs has proven to be very effective and efficient. The guiding discs are thick and rigid for supporting the pig and for driving debris from the pipe with less tendency then conical cups to climb over debris and leaving it behind. The sealing discs are thinner and made of a softer grade polyurethane, they are also oversized so that they shape up as a conical cup in the pipeline featuring wear compensation over long distances. The concept of separate sealing and guiding discs was further developed for long run pigs for the 800kms long single run Zeepipe pipeline. Special emphasis was given to the material quality of the discs and their mounting design in terms on internal pipeline /mounting flange diameter ratio.

The ratio determines the flexibility/stiffness of the pig. The smaller the mounting flange, the more flexible the pig is in terms of passing dents and other obstacles in the


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pipeline. A smaller mounting flange increases the pig cost as the thickness of the disc has to increase to maintain stiffness and polyurethane is more expensive than steel. This could have quite a large effect on the yearly expenditure of spare parts when many pigs are run per year. The mounting flange diameter is referred to as the hard core diameter of the pig and together with the thickness of the discs or cups, it determines the minimum passage diameter at pipeline dents. There are two parameters for minimum pipeline diameters for passing pigs: a. The smallest free opening at the dent location. b. The smallest reduced diameter in the pipe the pig can pass. Both should be referred to in millimeters or inches for specific pipelines rather than in percentages of the diameter. The diameter can be many different things; the nominal pipe diameter, the outside diameter, the inside diameter, the minimum inside diameter of the pipe or even the minimum inside diameter of the smallest pipe fitting. The mounting flange diameter determination is really a trade off between sufficient flexibility to cope with internal diameter changes/obstructions and minimum cost in terms of polyurethane. For specific jobs, low cost pigs are developed with larger diameter mounting flanges that require less polyurethane. Selecting one or the other design is really an engineering decision rather than a commercial decision. For smaller pigs, some companies have marketed all urethane pigs that do not have a steel body. In some cases, this design is more commercially attractive than pigs with replacement discs. The real comparison is whether the whole all urethane pig can compete with the replacement discs on extensive pigging programs. Beware of small size hollow type all urethane pigs. When they get stuck, the front may be blown out and the remainder of the pig will be very difficult to remove.

6.1 Figure 1: Special Pig

Looking at the consumption of pigs and replacement spares and considering their application efficiencies, it has been concluded that no real scientific research has been carried out to optimize the wear rate of the replacement discs/cups for pigs.


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Some attempts were made to compare different pigs at consecutive runs but no conclusions can be drawn from such an exercise. Consecutive runs, namely are not identical runs. If the attempt is to compare the aggressiveness for cleaning the wall, two requirements should be considered: a. the effectiveness of scraping debris of the wall. b. the effectiveness of bringing the scraped off debris into the downstream pig

receiver. These two requirements can not be evaluated/compared in a couple of consecutive pig runs: The amount of debris found in the receiving trap may not be all the debris brought

in (an unknown quantity will have left the trap through the bypass connection into the downstream facilities).

It is impossible to establish whether the amount of debris brought in by the second pig may have been scraped of the wall by the first pig and left behind in the pipeline just in front of the receiving trap. Pigs have a capacity limit for pushing debris and the amount of debris may have just exceeding the first pigs capacity just upstream the pig receiver. The second pig performance would be overrated if it was judged by the amount of debris in the pig receiver.

Pig wear is very must related to the amount of debris it is pushing. When the pig pushes a lot of debris, the wear is very high. A pig in a clean pipeline has a much lower wear profile.

Evaluation and assessment on the efficiency of different types of pigs can only be established if all the pigging activities in the world would be recorded in a properly designed data base for a three year period. Even if we would have this feedback, it would be debatable if a pig with a higher push out capacity and a high wear rate would be more efficient than a pig with a lower push out capacity and a lower wear rate. For the sake of both pig users and pig manufacturers, it would still be a good idea if a data base built up for publication by a large group of pig users. For the time being however, pigs can be selected on the basis of the following analysis: Establish the application for the pig together with the frequency of use. Compare the quality of the different pigs for the application for final selection. There are basically two kinds of pigs; Instrumented pigs for inspection of pipelines. Mechanical pigs for cleaning of pipelines and for separating fluids. Instrumented pigs are usually not for sale. Mechanical pigs shall be assessed for their efficiency and the efficiency is determined by the application requirement. The application requirement shall be determined first and this is a summary of the most common applications: 1. Scraping the pipewall for removing debris from the wall. 2. Separating fluids.


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The manufacturer shall provide a dimensional drawing. The manufacturer shall indicate the maximum running distance (Foam pigs are

effective for a very limited distance only).


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7 | Special Pigging Jobs

Special pigging jobs are usually low cost activities with a high risk factor carried out on pipeline facilities with an extremely high value. Example: The pipeline has never been pigged, it is 33 years old, there are no drawings, it is the only pipeline available to carry the product and it needs cleaning for inspection. The special pigging job cost around Fifty Thousand US Dollars. The pipeline on which the job will be carried has an investment value of Two

Hundred Million US Dollars The pigging job on the pipeline is risking production loss of up to Three Million US

Dollars per day on revenues, if the pig blocks the pipeline. The job has never been done before and it is debatable whether the job can be

completed as planned due insufficient information on the conditions of the pipeline in question.

The pigging contractor can not give any other guarantees than assurance that he will present a sound engineering plan, bring proper equipment and competent engineers.


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7.1 Figure 1: Gas Dust Pipeline Cleaning

The risk factor is the factor for the chance of successful completion of the job successfully. Compared with a normal job, where all the parameters and all the conditions are known, the risk factor is high. These are not the kind of jobs one would enter into without proper engineering investigation and planning for what-if situations occur. Special pigging jobs must be pre-engineered and all eventualities shall be taken into consideration. What are the main areas of concern: The pig gets stuck and blocks the pipeline. The pig gets stuck but the flow continues. The pig brings in a quantity of debris that can not be handled by the down stream

facilities. There are only two reasons why a pig should stop: 1. Pig stuck - flow situation.

The pig has lost seal and the bypass area is large enough to let the flow pass by without creating the required force to move the pig forward.

2. Pig stuck - no flow situation. The differential pressure is not enough to move the pig passed the obstruction.

In the first case, there is no immediate emergency influencing the operation of the pipeline but there is an unwanted obstruction in the pipeline. No other pigging is possible and that may be a problem when regular operational or maintenance pigging is required. Pig stuck - flow situation can cause losing seal due to severe wear on the discs/cups. Likely in long dry pipelines (> 100kms.). In the second case, there is an immediate problem as the flow is stopped. Pig stuck - no flow situation can cause lack of required differential pressure to move the pig forward because of : Insufficient pressure in the line to move the pig through normal pipeline fittings

and the pig stops at a bend or at a severe weld penetration. Likely in small diameter pipelines (< 14 inch diameter).


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Obstruction in the pipeline.

7.1.1 In the category : dents, wrinkles, short radius elbows, reductions in diameter, the obstruction must be found and the pipeline has to be cut.

7.1.2 In the category : huge accumulation of pipeline debris in front of the pig, the obstruction has to be found and there may still a possibility that the debris can be handled without cutting the pipe. In any of the above cases, if the pig has been equipped with a pig locator, the location of the problem area can be determined. Up to this point, the causes for trouble and amount of trouble has been described, now prevention and resolving the problems will be concentrated on. Prevention of trouble The best way of preventing trouble is to analyze the pipeline in detail and design a pigging program that minimizes the risk to an absolute minimum while still being prepared for trouble. Why risk the trouble? The only answer is and must be: There will be bigger trouble ahead, if the pipeline is not pigged. Do not pig for the sake of pigging. Analyzing the pipeline involves study into the: Design parameters. Mechanical properties. Routing of the pipeline. Operational details. Construction pigging and results. Hydrostatic testing pigging and results. Commissioning pigging and results. Historical events that may be relevant to the pigging program. Give a specific name to the pipeline section and the pigging program. Design parameters Design basis (Local standards, ANSI, etc.) Design pressure Design temperature Maximum allowable operating pressure (MAOP) Maximum flow These parameters are required to set the maximum limits for operations. The pig in the line may cause a condition that is foreseen in the original design parameters of the pipeline. Mechanical properties Length Diameter Material specification(s) Pipe wall (seamless, ERW, Spiral weld, etc.)


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Wall thickness(es) Internal diameter(s) Coating specification and application (Internal and / or external) Bends radius and inclined angle Fittings (internal diameter and geometry, material specification) Pipeline layout check (any back to back fittings or bends, closely located tees that

can create bypass, etc.) Valves, valves type and valve internal geometry (ball, gate, globe, check valves) Flanges used (raised faced, ring type joint, others) Routing of the pipeline Topographic elevation differences Uphill inclination Downhill declination Water crossings Crossings that may be of critical nature (railroad crossings, water crossings, water

win areas, historical valuable monuments, nature reserves, national parks, etc.) Operational details Temperature and temperature differences in seasons or due to any operational differences, likely temperature during the pigging program. Pressure(s), normal operating pressure, possible maximum operating pressures with corresponding flows, minimum pressures, likely pressure during the pigging program. Fluid composition and possible changes due to flow, temperature or pressure changes. Wax/parafins/asphaltenes presence and melting temperature. Flow restrictions, minimum flow, minimum daily flow, no shut down allowed in winter, maximum pumping time and quantity per pigging run(s), limited storage capacity at the end of the pipeline, pumping frequency or any operational condition that might influence the continuity during the pigging program. Construction pigging and results Get the construction pigging records with type of pigs used, number of pig runs, running times, propelling fluid and pressure and debris analysis. If these written records are not available, it may be possible to find somebody who can remember, although time plays strange tricks on memory. Historical events that may be relevant to the pigging program The pipeline may have been subjected to out of the ordinary conditions that need considering before sticking a pig into the pipeline. Or the operational conditions have changed tremendously and the previous conditions may have created a situation that needs considering before sticking a pig into the pipeline. Examples of out of the ordinary conditions: The weather conditions prevented continuation of construction and the pipeline

was left open and was flooded for a while before construction continued. The pipeline was left without any preservation for two years before

commissioning. The initial flow was very much less than presently. Other types of crudes were transmitted (waxy, parafins, asphaltenes, etc.) The line was used to transfer sludge after a well blow out, 5 years ago.


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These type of events are very important for the selection initial pigs in special pigging jobs. The collected data on the pipeline shall be summarized for analysis. SUMMARY OF DATA ANALYSIS The presentation of the collected information shall be specific to the planned pipeline pigging job. The analysis should aim at concentrating on giving the most realistic information:

Relevant information for pigging that is not known for this pipeline should be

highlighted as such. If difficulties are expected but it can not be proven that they are really present, a

probability factor can be used. As an example, the valves may not be piggable in the way pipeline valves usually

are and they are supplied by an unknown vendor, there is a high probability that the valves may not be piggable.

Generalities shall be avoided. Field checks are always helpful. One example is that the line was specified with

possible presence of reduced bore ball valves. The ball valve on the launching station was labeled with a factory name plate stating 24 x 22 inch. It is better to know for sure so that the pigs capable of negotiating reduced bore valves can be selected.

Efficient pigs for the pipeline pigging program can be selected for dealing with specific problems or difficulties. It is difficult and most of the time unrealistic to have to have pigs that are supposed to handle all thinkable obstructions. The term efficient means here that the pig can meet its target (facts finding) which may be just to pass the pipeline without bringing in any debris. It is best to attach this analysis to the tender documents and request the bidders to comment on it. Furthermore, the bidders should be requested to justify there choice of pigs for the pigging program. It will give good information for the technical bid evaluation process. Examples: Matters of pipeline geometry If short radius bends are expected, run a pig with a slim hard body for passing

previously un-pigged pipelines and with gauging plates for detecting short radius bends. Although these pigs can not indicate the location and the number of the short radius bends but they do indicate whether or not short radius bends are present. This is important for the choice of type of cleaning and inspection pigs.


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If dents are expected, run a pig with a gauging plate and determine the minimum free area at the biggest dent in the pipeline and chose cleaning pigs that can negotiate such a reduction. Matters of debris in the pipeline If a lot of debris is expected, pigs shall be selected to bring in quantities that can

managed or downstream facilities shall be adapted to handle the amount of debris.

Special pigs can be selected to measure/estimate the quantity of debris in the pipeline (see section 9, Pipeline Inspection).

Examples for debris handling: After commissioning, construction debris was left in the pipeline and has to be

removed but the product turbine meters are installed immediately downstream of the pipeline receiver. The filter arrangement shall be checked for capacity to separate and store debris. It may be required to install a filter plate in the pig receiver or the flow, at pig arrival, shall be routed to a temporary installed pit.

The gas pipeline feeds direct into a power generation plant and the piping has to

be re-arranged for preventing dust to go to the power plant (flare the gas, install dust separators, check the filter arrangement for hold up capacity, etc.).

An advanced fast track pigging program may be designed to clean the line during

shutdown.


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CONCLUSIONS The pigging program shall be based on the extend of the job to be done under realistically estimated conditions. The conditions that can not be assumed realistic shall be marked with a probability factor. The pigging program shall design a back-up plan. The back-up program for the emergency response plan shall be based on economical considerations.

Here are some of the possibilities: | Emergency | Stand-by | Services | Remarks | Response Activities | Services | available within | | acceptable time |

| Hot tap & Bypass | | |

| Hot tap & Stopple | | |

| Evacuation of gas by inert gas | Between two | | | | block valves

| Evacuation of product by water | | | Between two | | | | block valves?

| Arrange reverse flow possibility | | |

| Any of the above combinations | | |

For economical reasons, some of the above combinations for may be; select the hot tap & stopple concept but have only the hot tap & bypass services on stand-by. The flow can be re-routed and stopple services can be arranged if and when required. The expenditure of emergency response plans should be considered as insurance policy costs. Special pigging jobs may be split into two stages: 1. Fact finding stage 2. Pipeline cleaning stage The pipeline cleaning stage can be bid in unit prices and the scope can be determined after completion of the facts finding stage. Latest Development in Pipeline Cleaning


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ROSEN has developed a Scale Detection Pig (SDP). The pig makes use of the already developed system of the Electronic Geometry Pig (EGP) combined with sensor shoes that run over the deposited scale and measure the thickness. The EGP technology can measure the distance of the pipeline and the out of roundness. The sensor shoes are spaced less than 200 millimeter so that the deposited debris can be located and estimated in quantity. The configuration of the driving cups is aimed low efficiency for moving the scale. Using this SDP, the cleaning program can be designed together with the downstream facilities for handling the incoming debris/scale. Leak Detection Systems There are leak detection services in the market, using pipeline pigs. They are mainly used during hydrostatic testing of the pipeline. Some leaks do not require specialized pigging services:

7.2 Figure 2: Leak Detection Figure 3: Leak Detection


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8 | Field Pigging Operations

Field pigging operations shall only be carried out following written operating procedures complete with back up procedures (see attached example for pigging procedures). The operational activities for running pigs shall only be carried out by experienced operators of the pipeline owner. It is best if the procedures are discussed in a coordination meeting with the up-stream and down-stream operation staff. Selection of pigs, pig tracking, loading and extracting of pigs from pig traps may be done by the pigging contractor but is not advisable to let the contractor operate the valves without supervision of the pipeline owner. Pigging operations shall be recorded accurately. Records of individual runs will ultimately end up to a complete picture of the pipeline on cleaning, development of corrosion, built-up of wax, behavior of condensate in gas lines and down stream facilities, etc. It is recommended to record the performance of the pig and discs/cups in a data base with the following input: Pipeline information: Pipeline identification {diameter(inches) - length(kms) - number - from - to -

telephone country code number of pipeline start} Example: 24 - 120 - 1 - Amsterdam - Rotterdam - 31) In case of one pipeline, the pipeline number is 1. Year of commissioning. In case of dual diameter give length and diameter order of sequence. Off-shore/on-shore (give length and sequence of length on-and off-shore). Pipeline operational information: Fluid Pressure in / out Pressure in / out with the pig in the line Temperature in / out Pigging information


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Pipeline is pigged regularly yes / no, frequency in weeks. Pigging started since. Any new pigging shall be preceded by a check on the pigging equipment on the pipeline:

Smooth working of pig passage indicators (PPI). Sealing of pig trap valves. Smooth operating and sealing of pig trap doors. Proper indication of pressure indicators on pig traps. Exact position of the pipeline block valves. Make sure all spares are available for PPIs, pig trap door sealing o-rings. Fire fighting equipment at the trap stations. Any other pipeline or downstream equipment that may be actively involved in the

pigging operations. A check list type of arrangement might be useful forging people to sign for the checking proceedings and the OK of the equipment.


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9 | Pipeline Inspection

Pipeline inspection using intelligent pigs is now a well accepted method of monitoring the integrity of pipelines effectively. The methods of pipeline inspection that are most commonly used for locating and sizing corrosion defects are : Ultrasonic (US) Magnetic flux leakage (MFL) US System The US system uses an Ultrasonic beam for wall thickness measurement. The sensors are distributed around the circumference of the pipe wall at small spacing and nowadays the smaller defects can even be detected. The system differentiates between defects on the inside the pipe wall as well as on the external and internal surface of the pipe wall. MFL System The MFL system introduces a magnetic field into the pipe wall and measures the disturbance of the magnetic flux at defects in the pipe wall. The system is presently offered to the market in two versions; high resolution and low resolution. The low resolution system is on the way out as the market requires to compare consecutive inspection surveys to monitor corrosion growth which is only achievable with the high resolution systems. The high resolution system is elaborated on later.


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Generally speaking, both methods can equally well find and size defects. US has the special ability to size and identify inclusions and blistering inside the pipe wall. US requires a relatively low speed, extreme clean internal pipe wall surface and a homogeneous liquid between the sensor and the pipe wall. The latter make the method less able to inspect gas lines or pipelines with a mixed flow (Gas / Oil, Oil with parafins, Water / air, etc.). MFL does not have any of these restrictions and can be used in water, gas, crude and mixed flow pipelines at speeds up to 5 meters per second effectively.

Wall thickness wise US can not inspect thin wall but has no limit on thick walls and MFL does not have a thin wall limitation but the thick wall is limited to 18 millimeter for diameters up to 16 inch and 30 millimeters up to the larger size pipelines. Cracking can not be accurately sized by both systems although both of them have a limited ability to find cracks.

BatteryUnit

ComputerUnit

Odo-meterUnit

SecondarySensor

Unit

PrimarySensor

Unit


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There are special crack detection systems on the market but the exact method is kept propriety information. The type of defects that threatens the pipeline integrity and that shall be detected are listed as follows: Corrosion at the internal pipe wall surface due to: Wet sour medium (H2S) attack. CO2 attack. Bacterial (SRB Sulfate reducing bacteria) attack. Salt water. Cracks on the inside pipe wall surface due to: SCC (stress corrosion cracking). Inside pipe wall defects due to manufacturing faults: Laminations. Inclusions. Weld defects Cracks in the weld. Cracks in the heat effected zone. Improper welding Mechanical damage to the pipe wall: gouges dents buckles wrinkles ovality Corrosion on the external pipe wall surface due to: Incidental coating damage during construction. Incorrect pipeline coating material. Incorrect pipeline coating application. Incorrect coating material for weld areas. Incorrect application for coating at weld areas. The above listed defects have a degrading effect on the life span of the pipeline that leads to early pressure de-rating of the pipeline followed premature shutdown. In the past, corrosion allowance was fashionable but it is now considered to be an un-necessary waste of materials. Pipeline walls are designed accurately for the required service. Any degradation of the pipe wall threatens the pipeline integrity. The integrity of the pipeline is threatened, if defects are so large that they cross the Estimated Repair Factor (ERF) curve. The ERF curve is similar for all pipelines percentage wise but unique for each pipeline in terms of defect sizes.


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36" LAUNCHER TO RECEIVER ERF VS WALL LOSS AND FEATURE LENGTH

WT: 10,30mm MAOP : 58.1 DES. PRESS. : 58.1

020406080

100

0 100 200 300 400 500

LENGTH OF METAL LOSS FEATURES IN [ MM ]

DE

PT

H O

F M

ET

AL

LOS

S F

EA

TU

RE

S

[%W

L]

The Estimated Repair Factor (ERF) is calculated with a formula based on

ASME B31.4 respectively ASME B31.8:

If G 4.0 then:

P P

ct

ct G

in

n

=-

-+

* .. *

. * *11

1 0 67

1 0 671

12

If G > 4.0 then:

P Pcti n

= -

* .11 1

ERFMAOP

P=

| P | = calculated pressure

| Pi | = internal design pressure supplied by client

| M | = MAOP supplied by client

| c | = max. Depth of the corroded area in mm

| tn | = nominal wall thickness in mm

| G | = 0.893L/Dtn

| L | = axial length of the cluster

| D | = nominal outside diameter of the pipe in mm


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Note: ctn

is wall loss in %.

The defects in the pipeline wall that are situated above this curve weaken the wall strength to below design level and de-rating of the design pressure is required or repairs have to be made. It seems like that every pipeline wall can sustain quite large defects before the integrity is threatened. This would indeed be the case if these defects would be stationary. This however is not the case, defects are growing defects and it is not known when they will grow though the ERF curve. Obviously, looking at it from this angle, it is required to find them when they are really small so that maximum time is available to fix them before they go through the ERF curve. For external pipe wall surface defects that is relatively easy. When they are located, the coating can be fixed and the corrosion growth is stopped and the defect becomes stationary. Although it is slightly more difficult to get to off - shore pipe walls it is still possible. Internal pipe wall surface defects are more difficult, if not impossible, to become stationary. The best that can be achieved is, slow down of corrosion growth and that requires close monitoring. A special note is made for metal loss in the same location as mechanical damages. The mechanical damage has stressed the pipe wall material over the yield strength level and the original properties of the material are changed. In addition, cracks have usually formed. Which is way, mechanical damage to the pipe wall in combination with metal loss require immediate attention and usually urgent repair. For that reason, some inspection contractors carry out Internal Pipeline Geometry Inspection as part of metal loss inspection surveys. USE OF REPORTS PRODUCED BY INTELLIGENT PIGS The reported results of pipeline inspection services can normally be used for pipeline repair activities. Many pipeline inspection services companies have the ability of linking sizing of the reported defects to a computer program that calculates the ERF. The question is of course : What is the reliability of the claimed accuracy level for the reported defect sizing !!!


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The claimed accuracy and reliability of inspection equipment from the different contractors can be evaluated during the presentation of bids for inspection jobs by asking the contractor to specify his method of inspection and equipment features. This statement will be elaborated on later. Irrespective of the defect size, all metal loss defects are serious. The ones above an ERF of 1 and the ones that are deeper than 60 percent wall loss are urgent. All others are too serious to be ignored, otherwise, one day they become urgent automatically. The reported information is restricted to information on metal loss from the pipeline wall only. It does not provide any information on the cause of metal loss or on the effect of this particular type of metal loss on the specific pipe material for the pipeline in question in terms of growing rate or other weakening abilities. Therefore, the reported metal loss information can not be used as the only input for pipeline condition assessment studies. Pipeline assessment studies provide information on ways of effective preventive maintenance activities and rehabilitation as well as remaining lifetime estimates. These assessment studies are usually carried out by specialized engineering offices who take much more data into consideration. RISK ASSESSMENT STUDIES

in-house

survey data in-housedata evaluation

customer databaseand survey data

customer

Rosoft


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In recent years an increasing number of pipeline operators are evaluating the potential of assessing the risk factors to safely operate their pipeline system. The risk assessment strategies vary from a rather theoretical to a more practical approach. However, in both cases a great number of parameters have to be gathered, and a database for the entire length of each section has to be established. While some of the data might be readily available, others will have to be established with significant cost and time effort involved. Most of the risk assessment models currently being discussed do not consider data from smart pig runs as a primary source for the assessment. The following will provide information

ROSEN Pigging School管道清管及内测技术

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