HYDROTEST ON OFFSHORE PIPELINE

Hydrostatic testing has long been used to determine and verify pipeline integrity. Several types of information can be obtained through this verification process. However, it is essential to identify the limits of the test process and obtainable results. There are several types of flaws that can be detected by hydrostatic testing, such as:

• Existing flaws in the material
• Stress Corrosion Cracking (SCC) and actual mechanical properties of the pipe
• Active corrosion cells
• Localized hard spots that may cause failure in the presence of hydrogen

There are some other flaws that cannot be detected by hydrostatic testing. For example, the sub-critical material flaws cannot be detected by hydro testing, but the test has profound impact on the post test behavior of these flaws.

Given that the test will play a significant role in the nondestructive evaluation of pipeline, it is important to determine the correct test pressure and then utilize that test pressure judiciously, to get the desired results.

When a pipeline is designed to operate at a certain maximum operating pressure (MOP), it must be tested to ensure that it is structurally sound and can withstand the internal pressure before being put into service. Generally, gas pipelines are hydrotested by filling the test section of pipe with water and pumping the pressure up to a value that is higher than maximum allowable operating pressure (MAOP) and holding the pressure for a period of four to eight hours.

Reference Code, Standards and Specification

These are few reference code, standards and specification of hydrostatic testing on submarine pipeline:

• ANSI B 31.8 : Gas Transmission and Distribution Piping Systems
• ANSI B 31.4 :  Liquid  Petroleum  Transportation  Piping System.
• API RP 1110 :  Pressure Testing of Liquid Petroleum Pipelines.
• ASME Sec VIII :  Boilers and Pressure Vessels Code Div.1
• DNV 81 :  Rules for Submarine Pipelines
• IP Part :  Institute of Petroleum Model Code of Safe Practice

Equipment and Instrumentation

These are all necessary materials, equipment, instruments and consumables for performing the work. Materials and equipment shall be in good working conditions and include, but not be limited to the following:

1. Pigs for filling, cleaning and gauging . The Contractor shall provide sufficient number of pigs, including spares. Unless otherwise specified, all pigs shall be capable of negotiating a minimum bend radius of 5 times pipe diameter. In case any full port check valves are installed in the pipeline. Contractor shall ensure that the distance between the driving cups of the pigs are of sufficient length to prevent bypassing while passing through the full port check valve.
2. Fill pumps : Filling pumps shall be capable of filling the pipeline at the volume rate required to maintain pig speeds as specified in section here in after.
3. Variable speed positive displacement pumps equipped with a stroke counter to pressurize the line with a known stroke and capable of exceeding the maximum test pressure by at least 20 bar.
4. Two positive displacement meters to measure the volume of water used for filling the line. These meters shall be provided with a calibration certificate not older than one month.
5. Portable tanks of sufficient size to provide a continuous supply of water to the pump during pressurizing.
6. Bourdon pressure gauges of suitable pressure range and accuracy.
7. Dead weight testers with an accuracy of 0.01 bar measuring in increments of 0.05 bar provided with a calibration certificate not older than one month.
8. Two 48 hours recording pressure gauges having an accuracy of + 0.1% of the full scale value, with charts and ink gauges tested with dead weight tester prior to use.
9. Two temperature recorders for the continuous recording of water temperature with a sensitivity of 0.1ºC and a range of 0-40ºC.
10. Thermocouples having an accuracy of + 0.2ºC for measuring the temperature of the pipe wall.
11. Injection facilities to inject hydrotest chemicals into the test water in the required dosages.
12. Voice radio communication set-up to monitor and control the operations continuously between the beginning and the end of the test section.
13. Temporary scraper traps/test heads along with piping and valve arrangements to allow launching and receiving of pigs. Pinger transmitter for mounting on pigs alongwith pig detecting/tracking equipment.
14. Portable submersible in-situ pipeline leak detectors (flourimeters) to detect the leakage of fluorescent dye (Rhodamine B or equivalent) along with light source, detector electronics and power supplies.

Hydrostatic test procedure

The procedure manual shall include the details of all materials, equipment and procedures etc. as given below:

1. A diagram indicating all equipment, instruments, fitting, vents, valves, thermocouples, temporary connections, relevant elevations and ratings. The diagram shall also indicate injection locations and test water intake and discharge lines.
2. Laboratory test results of the test water, estimated amount of test water including required dosages of oxygen scavenger, bactericide corrosion inhibitor and fluorescent dye; procedure for chemicals and dye injection and control of dosages.
3. Filling and flushing procedures, including a complete description of all proposed equipment and instruments (including spares) their location and set-up.
4. Direction of pigging for the filling, cleaning and gauging operation.
5. The type and sequence of pigs and the pig tracking system for cleaning and removal of air pockets. Pig inspection procedures, including procedure to be followed in case the gauging pig indicates damage.
6. Procedure for thermal stabilization after filling.
7. Pressure testing procedure including a complete description of all proposed equipments and instruments (including spares), their location and set-up and proposed system for observation and recording of data during the pressure test.
8. Theoretical calculations for temperature corrections and entrapped air volume calculations.
9. Procedure for hydrotest acceptance
10. Procedure for detection/location and rectification of leaks.
11. Formats for logging/recording the test data
12. Safety precautions proposed during the test.

Source :

http://www.pipelineandgasjournal.com/pipeline-hydro-test-pressure-determination

http://www.lntenc.com/9wpp/Bidding%20Doc_RSPPM/Vol.II/2022.pdf