Hydrotest on Offshore Pipeline

The hydrotests are conducted mainly to verify the mechanical strength of pipeline or pipeline sections. Hydrotesting is carried out by pressurizing the system to a specified internal pressure and holding it for a certain period of time to check whether or not there is a pressure drop. If the pressure drops within the hold period, it is assumed that a leak or leaks may exist somewhere in the system. After the holding period is over, the pressure is released and a complete visual inspection follows. Standard codes, such as ASME B31.4, ASME B31.8, and API RP 1110, provide guidance on how to perform pressure tests of gas and liquid pipelines.

Pipeline hydrotest

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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, and
• 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. Before conducting the tests, it is necessary to establish the specifications for the test procedures and equipment. The procedure specifications should include the following:

• A description of the pipe sections to be tested (lengths, elevation, tie-ins, etc.)
• Test medium (seawater is used for subsea pipelines)
• Chemicals to be mixed with the test medium (biocide and corrosion inhibitors)
• Mixing process of the chemicals with the test medium including the chemical concentrations
• Test pressures
• Test holding time
• Process of pressurizing
• Description of all testing equipment
• Description of testing instrument
• Monitoring and recording of test pressure
• Procedure for depressurizing and discharge of test medium 

The test pressure is recommended to be set at no less than 1.25 times the internal design pressure for both hydrostatic testing and leak testing. The holding time is recommended to be at least 4 hours for hydrostatic testing and at least 1 hour for leak testing. A complete description of the test equipment and instruments is very important for the success of the tests. The following is a partial list of the devices needed for the tests:

• A high-volume pump that can fill the line at high enough velocity to remove debris and to push the pigs
• A filter that would remove all particles larger than a certain size to ensure the test fluid is clean
• A meter to measure the quantity of water filled
• Injection pumps for chemical injections
• A variable speed, positive displacement pump to pressurize the line. The pump should have a known volume per stroke and should have a stroke counter
• A pressure recorder that would continuously record the test pressure for the whole test period. The pressure measuring equipment should have an accuracy and repeatability of 0.1%
• A temperature measurement device that is properly calibrated and should continuously measure the fluid temperature. The device should be able to read in increments of no less than 0.18F (0.058C)
• A temperature device to measure the ambient temperature
• Pressure relief valves
• Pig transmitter device or remote pig signaling system

When planning the hydrotesting and leak testing, a few issues must be taken into account. The tests should be planned so that nowhere in the test segment does the pressure level produce hoop stress near or above the specified minimum yield stress (SMYS). This will require the test pressure to be determined by taking into full account the effect of the pipeline profile and external conditions.

ASME B 31.8 specifies the test pressure factors for pipelines operating at hoop stress of 30% of SMYS. This code also limits the maximum hoop stress permitted during tests for various class locations if the test medium is air or gas. There are different factors associated with different pipeline class and division locations. For example, the hydrotest pressure for a class 3 or 4 location is 1.4 times the MOP. The magnitude of test pressure for class 1 division 1 gas pipeline transportation is usually limited to 125% of the design pressure, if the design pressure is known. The allowed stress in the pipe material is limited to 72% of SMYS. In some cases it is extended to 80% of SMYS. The position of Pipeline and Hazardous Material Safety Administration (PHMSA) is similar. Thus, a pipeline designed to operate continuously at 1,000 psig will be hydrostatically tested to a minimum pressure of 1,250 psig.

If the test pressure is relatively high because of the high design pressure, the pressure relief valves have to be properly checked and set at the right pressure to protect the pipeline and the involved personnel. When launching a suite of pigs from a test-head launcher and receiving into a similar receiver, there is always concern that not all the launched pigs have been successfully launched or received into the terminal. It is necessary to install a pig transmitter device or some kind of remote pig signaling system on the final pig to confirm the pig launch and reception.


How the test results will be reported and what will be reported should be defined before performing the tests. The test records should include the details of the test operations and details of any failures. The failure report should include the exact location of each failure, the type of failure, the causes for the failure, and the recommended repair methods. When the tests are finished, all waste should be disposed of in the correct manner which should be defined in the company’s waste management and disposal policy.



Source:
Offshore pipelines - Dr. Boyun Guo (2005)
http://pgjonline.com/2009/12/17/pipeline-hydro-test-pressure-determination/