Precommissioning Hydrotest of Pipelines Onshore & Offshore

Offshore Pipelines: Hydrostatic testing of the submarine pipeline system commences only after complete installation works of pipelines, risers, crossings, free-span corrections, trenching/ burial/ backfilling operations, and remedial works, if any. Sectionalization of offshore pipelines is impractical for hydrotesting, and the complete pipeline is tested in one shot. However, sometimes the connected piping such as the riser, topside piping (the section between the riser top and scraper trap), spools, flexibles, hoses, and laterals are excluded from the strength test of the offshore pipeline (but included in the system leak test), depending upon contractual obligation, installation sequence and ease of execution in subsea. Before hydrostatic testing of the pipelines, in-line ball valves located on the lateral connections and PLEMs are kept in the partially open or crack-open position, and the check valve flapper is locked in the open position. During the hydrostatic testing, in-line valves shall not be closed for making pipeline sections, and hydrostatic testing shall not be performed against the closed valves, particularly if the valve is soft seated. Hydrostatic tests shall be conducted in accordance with approved Hydrostatic Test Diagrams (HTD), clearly indicating the number of test sections along with minimum and maximum test pressures in each test section. Any flexible pipe or hoses for permanent installation shall not be included in the pipeline hydrostatic strength testing and shall be tested separately as per their manufacturer's guidelines.

Onshore/ cross-country pipelines: The pre-commissioning hydrostatic test for the onshore pipeline commences only after the completion of mechanical and civil works, i.e., after acceptance of all welds and backfilling of the entire pipeline section. The hydrostatic test also includes those sections which have been previously tested (pretested), viz., pipeline sections installed at Rail/ road crossings, major water crossings by HDD (horizontal directional drilling) etc. A pre-determined/ calculated pipeline section length shall be selected for the hydrostatic test based on ground elevation, water availability, and location of sectionalizing valve stations and/ or terminals. The availability of test water along the pipeline route sometimes dictates the length and arrangement of test sections.

Graph showing the ground elevation of the complete test section shall be plotted to verify that the test pressures at the high and low points are within the specified limits before the start of the test. In addition, it helps divide the total pipeline length into various hydrotest sections, wherein the difference between the highest and lowest ground elevation within the hydrotest section is minimum. The pressure difference between the highest and lowest point within the hydrotest section is equal to the change in pressure gravity head ( ∆P_g ) which is calculated as below:



Where,

∆Pg	=	Change in pressure due to gravity head
∆h	=	Difference in height between the highest and lowest point
ρ	=	Density of test fluid (generally water)
g	=	Acceleration due to gravity

The value of change in pressure due to gravity head ( ∆P_g ) may appear small compared to the required hydrotest pressure. However, it plays a vital role in sectionalizing the pipeline for hydrotest, mainly when the minimum hydrotest pressure is close to the pipe's minimum yield strength (SMYS) e.g., 90% of SMYS. After meeting the requirement of minimum pressure at the highest point of the section equal to 90% of SMYS, only 10% of SMYS of the pipeline material can compensate for the difference in pressure due to gravity head and fluctuation in pressure due to temperature change. Hence, as a rule of thumb, the maximum length of the test section for onshore/ cross-country pipelines is kept within 50 km for thermal stabilization, control gravity head, ease of filling and pressure monitoring during the hydrotest.

Note: If the above-ground or above-water section length exceeds 3 % of the total test section length, then it may result in temperature fluctuation as the exposed section will receive additional heat transfer due to sun’s radiation. Hence, a separate hydrostatic pressure tests for the above-ground/ above-water and the buried/ submerged sections shall be carried out to minimize the influence of temperature fluctuation during the test.

Hydrotest Test Procedure: Before hydrotesting, a hydrostatic test procedure which includes the detailed procedure for the cleaning, gauging, filling, hydrotesting & dewatering of pipeline strings, is prepared and submitted to the Company. A statement of responsibility for the test supervisor and his/ her team is also included in the hydrotest procedure. The test procedure includes, in general, but not be limited to:

1. List of nominated personnel to supervise the pressure testing operations with their qualifications, tasks, responsibilities, and authorities.
2. Detailed schedule giving proposed dates of the main activities, tests, and mobilisation dates of the nominated personnel.
3. Details of the selected test sections, including assemblies and pre-test sections.
4. Identification of potential safety and environmental hazards, including the necessary measures and emergency plans.
5. Details of the line-fill water, including the source, treatment method, discharge/disposal and permits.
6. Details of the test equipment, including layouts and size and/or performance. A valid calibration certificate for the instruments proposed to be used shall also be included.
7. Details of the test section preparation, including cleaning, gauging, and filling.
8. Details of the hydrostatic pressure test preparation, including temperature stabilisation period.
9. Details of the hydrostatic pressure tests, including pre-test, strength test, leak-tightness test and pipeline system test.
10. Details of the post-testing activities, including depressurising and documentation.
11. Details of rectification activities, including the proposal for locating leaks, dewatering and leak rectification.

Hydrotest Test Equipment/ Instrumentation: Any test equipment pressurized during the test operations shall be designed for a working pressure not less than the test pressure. These components shall have material certificates, and the equipment shall have data sheets from the Manufacturer. The test equipment connected to the test section shall have hydrostatic test certificates and shall have been tested to a pressure of at least 1.25 times the test pressure of the test section. The test equipment shall be internally clean and fit for purpose.

The Equipment / Instruments to be used for performing the work shall include, but not limited to the following:

1. Pigs for filling, cleaning, and gauging:
   • Cleaning pigs
   • Four cup batching pigs
   • Gauging pig with gauge plate diameter equal to 95% of thickest wall pipe in the pipe sections.
2. Water Filling Pumps:
   • Water filling pump with differential head 20% greater than the maximum required and flow rate preferably between 400 m³/h and 1000 m³/h.
   • Positive displacement pump equipped with a stroke counter and capable of exceeding the maximum test pressure by at least 20 bar.
3. Positive displacement meters to measure the volume of water used for filling the line having a valid calibration certificate.
4. Portable tank of sufficient size to provide continuous supply of water to the pump during pressurization.
5. Armoured type flexible hose can be used in short lengths, for pressurising the test section.
6. Pressure gauges of suitable pressure range and accuracy with valid calibration certificate.
7. Pressure recorder to continuously record the pressure.

Test Medium: Cleaning, filling, and hydrotesting is performed using soft non-aggressive water, free from sand and silt, and withdrawn from a suitable source along the pipeline route. Only clean, fresh water from a river, aquifer or potable water system or clean seawater from the open sea should be used. Brackish water from estuaries or harbours are avoided as a source wherever possible.

The water used for pressure testing is selected from a source with a temperature close to the pipeline's ambient temperature. This minimizes the thermal stabilization period required to bring the temperature of the line-fill water equal to the ambient temperature. The water is filtered through 50-μm (2 mils) filters, except for water injection lines, which are filtered through 2-μm (0.08 mils) filters before entering the test section. The arrangement of filters at the test work sites should enable back-flushing without disconnecting the pipe work.

The water is tested in an approved lab for its non-aggressiveness and suitability as the test medium. Depending on the quality of water, the duration of contact of line-fill water with the interior of the test section, the pre-commissioning requirements and the future use of the pipeline, treatment packages containing an oxygen scavenger and biocide is added to the line-fill water. The environmental acceptability of any proposed treatment package, e.g. toxicology, Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD) etc., shall be demonstrated after filtering/ separation and/or neutralization. Environmentally acceptable line-fill treatment packages should be used.

To minimize biological fouling and/ or bacterial corrosion to an acceptable level in the filled pipeline, the above concentrations should not exceed the following limits:

• sulphate concentration 42 mg/kg (42 ppm)
• fatty acids concentration 14 mg/kg (14 ppm)
• ammonium concentration 3 mg/kg (3 ppm)

If water from aquifers is used, the analysis should show all components and their concentrations. Where the line-fill water is to be transferred from one test section to another, the quality of the water should be checked, and the water should be chemically retreated as necessary before it is moved into the test section. In addition, the water should be refiltered through 50-μm (2 mils) filters to reduce the entrainment of debris from one test section to another.

Line filling should not occur if the ambient temperature is below 2°C (36°F) unless a suitable antifreeze has been added to the line-fill water. If a hydrostatic pressure test is to be carried out in freezing conditions, the Contractor shall provide the required data for the pressure/ temperature variation calculation before performing the test. The test equipment required, and the disposal of the antifreeze/water mixture should be included as part of the test procedure.

Preparation of test section mainly consists of three operations viz. cleaning/ flushing, gauging, and filling.

Cleaning/ Flushing: Someone may raise a question: when the pipeline is newly installed and in pristine condition, why clean it? To clarify this doubt, we should acknowledge that there is a long journey which line pipes must go through, from fabrication/coating to the installation at the site, during which the dust and debris are deposited on the internal surface of the pipes. Various construction wastes such as electrodes, grinding wheels, safety gloves etc., along with sharp and impetuous objects such as guns and dead bodies of snakes, crocodiles etc., have been recovered from the slug during the cleaning of the pipeline.
Initial cleaning may subject the pig to more severe forces and process conditions than regular operation due to the potential presence of debris. Therefore, a more robust design is required than that intended for normal operating conditions. The first pig run can be propelled by water or compressed air at a pig speed between 0.5 and 1 m/s (1.6 and 3.3 ft/s) followed by further pigging runs using brush or foam pigs, depending on the extent of cleaning required. Bi-directional pigs are generally used during initial cleaning, before the gauging run, to confirm that the line can be pigged successfully because bi-directional pigs can be reversed if they get stuck. Compressed air to drive pigs shall be used at pressures below 7 barg (100 psig) with a maximum pressure differential of 7 barg (100 psig) applied if the pig gets stuck.

Soft seated valves and control valves should not be installed until the lines have been thoroughly flushed to prevent any damage to the seat due to flow of debris during cleaning. A magnetic cleaning pig may be used after the completion of brush cleaning for the final removal of metallic debris.

The number of pig runs depends on the cleaning results. The quantity of debris received at the end of each pig run is assessed. The quantity of debris received should decrease with each run, and pigging should continue until it is judged that further pigging will not result in a significant reduction of the debris. Cleaning is considered completed when the quantity of debris directly ahead of the last pig is less than one percent of the total volume of the pipeline.

Gauging: After acceptance of the cleaning and backfilling of the trench, internal gauging is carried out using either a gauge plate pig or an instrumented caliper pig to confirm that there are no unintended intrusions (dents, gouges, etc.) into the pipeline and confirm that the ovality of the pipeline is within acceptable limits. It is mostly a contractual requirement to prove the successful laying of the pipeline.

A commercial grade aluminium gauging plate is fitted, located in front of the last set of discs on a bidirectional pig with two sets of separate guiding and sealing discs. The plate should have 8 radial incisions at 45-degree intervals to allow the plate to buckle in the presence of a pipe bore reduction or ovality. The radial incisions should extend from the outside diameter of the gauging plate to the outside circumference of the locating plate of the pig. The gauging plate thickness should be:

• 3 mm (1/8 in) thick for pipeline diameters up to DN 100 (NPS 4)
• 6 mm (1/4 in) thick for pipeline diameters from DN 100 (NPS 4) to DN 300 (NPS 12)
• 12 mm (1/2 in) thick for pipeline diameters above DN 300 (NPS 12)

The gauging plate shall have a minimum diameter of 95% of the minimum nominal pipeline internal diameter or greater & not less than 25 mm (1 in) clearance for pipeline nominal internal diameter of 20 inch or greater depending on the specified ovality tolerance. As an alternative, following formula can also be utilized to firm the minimum diameter of the gauging plate:
A column of clean water is pumped in front of the pig for lubrication and to flush out the remaining dirt (if any) inside the test section. The gauge plate is permanently marked with a unique identifier or jointly signed by the Owner/ Consultant and Contractor Representatives before insertion with relevant information written on the plate. Gauging plate measurement, insertion of the gauging pig into the test section and removal of the gauging plate from the pipeline is made in the presence of the Company Engineer. A photographic record of the gauging plate is taken before gauging plate insertion and as it is removed from the pipeline.

The gauge pig run test results are analyzed to evaluate the internal status of the pipeline. A deformed, bent, severely nicked plate or damaged pig shall be evidence of gauging pig run failure. In such a case, the cause is conclusively determined, and necessary repairs to the pipeline are carried out. If the gauging operation cannot locate the defect, the Contractor must run a geometric inspection pig to locate the defect. After repair, the gauging pig must be re-run with new gauging plates of the same diameter. Filling the pipeline for hydrotest shall commence only after an acceptable gauging run has been completed. If necessitated, the gauging pig is fitted with a pig detector to monitor its position in the line. A pig tracking plan, be it for an onshore pipeline or offshore pipeline, should be prepared explaining the following:

• the means of tracking
• the support operations required,
• the provision for pipeline access to be made, and
• details of anticipated pig progress as a measure of pig location

Launch procedures shall be carefully defined in writing and implemented to avoid damage to the plate (particularly the bottom) during the launch process.

Filling: Filling operation includes backpressure control, a steady, controlled filling rate, utilization of a break tank, and at least two bi-directional pigs with water in front and between. Filling is carried out to evacuate air and fill the line with test water before hydrotesting. On acceptance of gauging, the air pigging headers are cut and the temporary test headers (already pretested at a pressure 1.25 times higher than the pipeline test pressure) are welded to the test section.

• If block valves are installed, it is ensured that all block valves are correctly positioned in the fully open position prior to filling. Provision shall be made to equalize the pressure between the valve body cavity and the line. It is ensured that piping and facilities for the disposal of treated water are installed and functional before filling the test section. The Contractor shall take all necessary measures to remove air from the line during filling.
• The filling end test header is pre-loaded with three numbers of four-cup batching pigs.
• The filling operation begins with pumping uninhibited water equal to 10% volume of the test section in front of the first pig. The pressure in front of the pig is carefully monitored and controlled at not less than 2 bar (g) (30 psig). Venting is carried out repeatedly at points in the test section where air might accumulate, e.g., at ancillary piping.
• The first pig is launched by pumping uninhibited water equal to 1.5 km of pipe length behind the pig. The water column before and after this pig acts as a cushion to control the movement of the filling pig and hence minimize air entrapment.
• The second pig is launched by pumping in the actual line-fill water behind the pig. Filling should be carried out with a range of 0.6 m/s (2 ft/s) to 1.8 m/s (6 ft/s) and maintain an average rate of around 1 m/s (3,3 ft/s). This is required to control the pig speed and prevent water column separation when the pig travels downhill. It is preferable to fill the pipeline from a low point if possible. Pre-test sections and testing of associated pipework within stations and depots may be filled without pigs, provided the piping configuration is such that all air can be displaced by water. During the filling operation, the valves at the receiving end of the test headers are throttled suitably to maintain adequate backpressure and thus control pig movement.
• The filling continues till the second pig reaches the other end of the test section, and the pressure at the test end rises to the static head.
• Required quantity of corrosion inhibitor is injected during the filling operation through any of the nozzles provided on the test header or into the break tank. The amount of inhibitor is controlled by adjusting the flow rate of the dosing pump.
• The first two columns of water are drained out from the receiving end of the test section.
• To assist in controlling the line filling and water treating, the following measurements are recorded:
  
  1. inlet flowrate of line-fill water
  2. inlet pressure of line-fill water
  3. inlet temperature of line-fill water
  4. pressure at the receiver head
  5. chemical injection rates (if carried out)
  6. dye injection rate (if carried out)

Note: No filling should take place if the air temperature is 2°C (36°F) and falling or in freezing conditions unless antifreeze precautions are taken.

Chemical Inhibitor Dosing Rate: The dosing rate is decided based on the estimated period the line is going to remain filled with test water i.e., the period between the line-fill and dewatering of the pipeline (depending on pre-commissioning activities) after the hydrotest. In case the period between the hydrotest of the section and dewatering is less than 6 months, then the dosing rate should be 500mg/L (500ppm) and if it is more than 6 months, then the inhibitor dosing rate should be 750mg/L (750ppm) of line-fill water. In case the line-fil water is transferred from one section to another section, an additional dosage of 60 ppm of inhibitor is added to the already-dosed water. Furthermore, for any reason, if the time period exceeds six months for the sections which are added 500mg/L of inhibitor, an additional dosage of inhibitor shall be added for the estimated extended period.

Thermal Stabilization: Pressure and temperature of any fluid are proportional to each other. If the water filled in the test section is hotter or colder than the surroundings, then there can be an unaccountable decrease or increase in test pressure due to heat transfer between line-fill water and surroundings during the test. Hence, thermal stabilization of the water filled in the test section with the surrounding is critical before the start of the test, and its temperature should be within 1.0°C (1.8°F) of ground or seabed temperature. It is determined as the difference between the average pipeline temperature and average ground temperature over the test section length. Thermocouples are attached to the surface of the pipeline's test section after removing the external coating. For the above-water/ above-ground test section temperature measurements, the thermocouples are firmly installed into the line-fill water or firmly attached to the bare pipe and covered with insulating materials. The bare line pipe is recoated upon completion and acceptance of hydrostatic pressure testing.

In the case of onshore pipelines, the following guidelines can be utilized for the placement of thermocouples:

• One pair of thermocouples shall be installed 500 meters from the pumping test headers.
• Temperature probes shall be installed to measure the pipe and soil temperature at every 2.5 km of the pipeline, and spacing may be increased to a maximum 5.0 km based on the terrain and nature of subsoil along the alignment of the test section.
• One pair of thermocouples shall be installed 500 meters from the terminal headers.

The calculation of the temperature stabilization period based on the expected line-fill water temperature and the ambient temperature is detailed in the test procedure. Once the test section is filled entirely and a minimum pressure of 1 bar has been achieved at the highest point, the thermal stabilization period should start.

The test section temperature and ambient temperature (ground/air/water) is plotted against time during the temperature stabilization period. Pressure and temperatures, including ambient, are recorded every hour (can be increased to 4 hours) during the stabilization period. If this is not practical for subsea pipelines, they should be filled for a minimum of 24 hours before the hydrotest begins.

Thermal stabilization is achieved when a temperature difference not higher than 1°C is attained between the average pipe temperature and average soil temperature.

Pressurization
Pre-pressurization Requirements: Prior to pressurization, checks are carried out to confirm the following:

1. The test section is isolated from all other pipeline sections.
2. All temporary piping, test heads, and other equipment connected to the test section are tested at least 1.25 times the test pressure.
3. The test section is completely filled, and the air has been vented-off.
4. If installed, the block valves are returned to the half-open position so there is no differential pressure across the seats/seals.
5. All instruments and gauges used during pressurizing and testing have been calibrated, and valid test certificates have been submitted to the Company.
6. All warning notices, marker tapes, protective barriers and other safety equipment are suitably positioned, and the appropriate authorities are informed.
7. All relevant personnel at the affected stations have been informed of the commencement and duration of the hydrotest.
8. The communication channels have been tested and established, and backup equipment is available.
9. The personnel engaged in the testing work are briefed on the emergency plans.
10. The person operating the pressurizing equipment is instructed by the test supervisor regarding the limiting pressure, which shall apply to the test section.

After the above checks, high-pressure pump and test equipment at the pressurization area are mobilized.

Pressurization Cycle: After thermal stabilization of the line-fill water, pressurization is carried out at a constant rate not exceeding 1 bar - 2 bar/ minute. One pressure recording gauge is installed in parallel with the dead weight with sensitivity of ± 0.05 bar. Volume of water added to the pipeline section is measured through a positive displacement meter and recorded periodically throughout the pressurization as follows:

In case, during the pressure hold period indicated above, a decrease in pressure is observed, the above operations must not be repeated more than twice, after which the test section should not be considered capable of test, until the cause for the lack of water tightness is isolated and eliminated.

On acceptance of the air volume test the pipeline pressure shall be raised to the test pressure and pressure stabilization period shall commence

Air Volume Calculation: During the pressurization to full test pressure, one test is carried out to calculate air volume in the pipeline when the pressure is at 50% of the test pressure. The air content of the filled test section is determined using the pressure/ added volume plot. The linear section of the curve is extrapolated to the volume axis, which corresponds to static head pressure. The volume of air can be read from the intersection of the line with the volume axis and is used to calculate the air content thus:

Air Content Percentage	=	Volume of air	×	100
		Volume of test section

In case the air content is found to exceed 0.2% of the volume of the test section, further pressurization is terminated. Pressure in the test section shall be released, and the air in the test section shall be vented-off. If, after successive trails of venting, the air content remains more than 0.2% of the volume of the test section, then additional batching/displacement pigs shall be passed, and the test section shall be emptied and re-filled to remove the air pockets.

The procedure for establishing the air content is repeated till the air content in the test section is not found below the allowable limit of 0.2% of the volume of the test section. The pressurizing can continue if the air content is within 0.2 % of the test section.

Test Pressure: The pipelines are hydrotested to the pressures specified below.

Minimum strength test pressure can vary from 1.25 times the design pressure to the pressure equivalent to 96% of the specified minimum yield strength (SMYS) of the line pipe material with the lowest wall thickness in the test section depending on pipeline design code and Operator’s safety philosophy.

Maximum strength test pressure is the pressure equivalent to 95% - 100% of the specified minimum yield strength (SMYS) of the line pipe material with the lowest wall thickness in the test section depending on pipeline design code, operator’s safety philosophy, specification requirement etc.

Minimum leak tightness test pressure can vary from 1.25 times the design pressure to 95% of the minimum strength test pressure depending on pipeline design code, Operator’s safety philosophy, specification requirement etc.

Test duration (Hold Period): The strength test commences after thermal stabilization and pressuring of the test section. During the strength test, pressure and temperatures are recorded at least every half hour using calibrated and certified test gauges or instrumentation and plotted against the time maintained. In addition, the volume of any water added or removed to maintain test pressure during the mechanical strength test is also recorded. The test pressure for the strength test is maintained within ±1 bar (±15 psi) for the specified test pressure duration varying between 2 hours and 8 hours depending on Operator’s safety philosophy.

The leak tightness test is followed directly after the strength test. The pressure pump connections are removed, and the test section is completely isolated and all connections at the test heads is checked for leakages. No water is added or removed from the pipeline during the leak tightness test. The minimum hold duration for the leak tightness test is generally 24-hour. During the pressure hold period the following data is recorded / reported:

• Every one-hour pressure measurement from dead weight tester
• Every two hours the ambient temperature, pipe and soil temperature from the thermocouples

Acceptance Criteria: The pressure, the test section temperature and the ambient temperature are plotted against time during the period of the hydrotest using calibrated and certified test gauges or instrumentation and plotted against time. The hydrotest is considered successful if the pressure has kept a constant value throughout the test duration, except for changes due to temperature effects. The pressure change value as a function of temperature change is algebraically added to the pressure value as read on the meters. The pressure value thus adjusted is compared with the initial value, and the test is considered acceptable if the difference is less than or equal to 0.3 bar.

The pressure/ temperature changes can be calculated from the pressure/temperature equation Formula 1a for the restrained test section or Formula 1b for the unrestrained test section to determine that any pressure variation is due to temperature changes or whether a leak is present:

Where,

∆ p	=	incremental pressure
∆T	=	incremental temperature
D	=	pipeline outside diameter
E	=	Elastic modulus of steel
t	=	nominal pipe wall thickness
ʋ	=	poisson's ratio of steel
B	=	bulk Modulus of water (it is very sensitive to temperature and to anti-freeze)
γ	=	volumetric expansion coefficient of water (for fresh it changes significantly at low temperatures)
α	=	coefficient of linear expansion of steel

On successful completion and acceptance of the hydro-test, the pipeline is slowly de-pressurized at a moderate and constant rate. All the thermocouples installed in the pipeline are removed and damaged corrosion coating is repaired using approved procedure & material.

Testing of Special Section: In some test sections, it is difficult to correlate the pressure changes with the temperature fluctuation because the temperature fluctuations of the test section are difficult to measure, e.g., in above-water or above-ground test sections (effects of the sun, rain and wind) or test sections of pipe bundles and insulated pipelines (slow heat transfer). In such cases, the leak test is carried out, wherever possible, based on a visual examination to check the leak tightness of the test section after the satisfactory completion of the 4 hours strength test. Visual examination is commenced after the pressure has been reduced to the leak test pressure and held for a minimum of 4 hours.

If it is not possible to check the leak tightness of the test section by visual examination, then leak tightness shall be demonstrated by a 24-hours leak tightness test. The influence of temperature fluctuation should be considered in specifying the location and number of temperature measurements. In addition, it may be necessary to protect above-ground or above-water test sections against sun radiation during the test.

A 24-hours leak tightness test for pre-tests of assemblies is not necessary. However, the influence of temperature fluctuation should be considered in specifying the location and number of temperature measurements and the stabilization period.

Depressurizing: After the satisfactory completion of the strength test and leak tightness tests, the test section is depressurized to the hydrostatic head plus 1 bar (15 psi) so that air does not enter the test section. Pressure let-down valves are opened slowly, and depressurizing continues at a rate that does not generate vibrations in associated pipework. The depressurization rate should be at most 1 bar (15 psi) per minute until the pressure is reduced to 40 % of the test pressure. Then depressurization is continued at less than 2 bar (30 psi) per minute.

Dewatering & swabbing: Dewatering of the pipeline after hydrotesting is taken up just before the pre-commissioning activities. Until the Contractor is ready to start pre-commissioning activities, the pipeline/ section after hydrostatic testing is left filled with corrosion-inhibited water. With some pipelines, such as those designed to carry unprocessed sweet crude oil or water injection, it is possible to displace the test water during the initial line filled with the process fluid. However, this depends on the availability of oil/ water interface storage and separation facilities. In case of dewatering the pipeline where ambient lay-up is not specified and there is more than a month gap between the dewatering and commissioning operation, the Contractor must lay up the pipeline with a suitable medium after the swabbing operation to prevent any pipeline corrosion.

It is imperative to make necessary arrangements to dispose of line-fill water safely. Upon readiness to start pre-commissioning activities, the section is dewatered by propelling four-cup pigs first and then foam pigs driven by compressed air. The speed of the pigs is adequately controlled during the dewatering operation. The volume of water displaced is measured during the dewatering pig runs to assess the effectiveness of the dewatering operation and determine the need for additional runs. The line is considered dewatered once the last foam pig flushes out a negligible amount of water.

If the line fill is sea water and there is a requirement to reduce the salt content to specified limits, freshwater batches of approximately 5% of the pipeline volume are driven through the pipeline between two high-sealing disk pigs. Upon completion of the testing and dewatering operation, any provisional traps for pigging and all other temporary installation relating to the test are removed, and the valve assemblies are installed at the respective locations. Subsequently, the individual sections of the line already tested are joined in accordance with the requirements of relevant specifications.

Drying: Drying of the pipeline may be required to preserve the pipeline until the introduction of process fluids or, for example, on gas pipelines to prevent hydrate formation or on product or aviation fuel pipelines to minimize product contamination. Drying the pipeline by blowing dried air inside it is the preferred method as it is generally the quickest, cheapest, and most readily available method. Dry nitrogen may also be used in place of dry air, but this may present logistical challenges in transporting to remote sites. In addition, the effectiveness of these techniques becomes limited on branched pipelines or pipelines with dead legs. Vacuum drying may be used when there is a lack of dry air or nitrogen available or when pigging is not possible. It may also supplement air or nitrogen drying to achieve low dew points or dry branches or dead legs.

To use the air for drying the pipeline, first, the air is dried typically to a dew point below -80°C (-112°F) in a drying unit and then blown into the pipeline pushing a foam pig. The water in the line evaporates into the pre-dried air, which is carried to a point outside the pipeline. Pipes are typically dried to a dew point in the pipe of -20°C (-4°F). However, if required, particularly for process equipment, dew points down to -80 °C (-112 °F) can be achieved. Following are carried out during drying operations:

• The pipeline is initially pigged with foam swabbing pigs using the dry air as a propellant.
• The pigging speed is maintained between 1.0 – 1.5 m/s (3 – 5 ft/s).
• Pigging continues until the dew point of the drying propellant at the receiving end remains below the required dew point for at least 2 swabbing pig runs.

Drying using dry nitrogen generally follows the same procedure as specified for dry air. The dew point of nitrogen used for drying must be maintained below –50°C (–58°F) at atmospheric pressure. Glycol or methanol plugs can be used to minimize the risk of hydrate formation during initial line fill but are unlikely to achieve similar low dew point values to air or nitrogen drying. This technique is valuable in low-temperature environments where achieving low dew points with air or nitrogen may take significant time. However, glycol or methanol drying should only be used if other techniques are not feasible due to the hazards of handling and disposing of these fluids.

Documentation: Upon completion of a successful section test a "hydrostatic test certificate" is completed and signed by the Contractor and Operator. This is supported by original hand-written test data of which photocopies shall be handed over to the Operator. A separate certificate shall be completed for each test, including pre-test and assembly testing.

For each complete pipeline, the Contractor compiles a "final hydrostatic test report", with a general introduction including all relevant pipeline data, detailing each hydrotest. The contents, as a minimum, includes the following:

1. originals of all “hydrostatic test certificates”
2. originals of all recorder charts
3. all pressure readings
4. all temperature readings- volumes of water added or bled-off
5. instrument test certification
6. air content plot and calculation
7. pressure and temperature plots against time
8. pressure/temperature correlation calculation
9. details of line-fill treatment packages
10. pig register of cleaning, gauging and filling
11. photographic record of the used gauging plates
12. leak locating (if carried out)
13. pig register of dewatering (if carried out)
14. details of line-fill water disposal (if carried out)
15. rectification records (if carried out)
16. any special features of the test
17. test procedure