Hydrostatic testing of the pipeline including riser should follow the below procedures:
- The pressure should be raised to the test pressure and allowed to stabilize then held for 15 minutes. The rate of pressure increase should not exceed 100 kPa (15 psi) per minute during all operations.
- The pressure should be lowered to approximately 25 percent of test pressure, allowed to stabilize then held for 15 minutes. Rate of pressure decrease should not exceed 200 kPa (30 psi) per minute during all operations.
- The pressure should be raised to an amount approaching the test pressure and again allowed to stabilize before it is finally raised to the test pressure. The pressure should not decrease more than 0.2 percent after compensating for temperature. None of this or the foregoing should be included in the test duration. The allowable, unaccounted pressure loss of 0.2 percent is based on a maximum allowable air content of 0.2 percent. Air content can be evaluated by precisely measuring the volume of water (Vi) injected between the measured pressures P1 and P2 and comparing Vi with the theoretical volume that would cause a pressure change from P1 to P2. The difference between the measured and calculated volumes can be attributed to air content. The same evaluation can also be made by withdrawing water.
- Water should be injected or withdrawn from the test section to maintain pressure within the specified limits. The volume of these injections and/or withdrawals should be measured and recorded.
- Pressure-volume-temperature (PVT) calculations should be performed to show any observed pressure loss due to a temperature change or a leak.
- A pressure recorder and temperature recorder should be used to continuously record the test pressure, fluid temperature and ambient temperature. In addition, pressures should be read and recorded to the nearest 7 kPa (1 psi), using a deadweight tester furnished by the Contractor. The pressure should be recorded every 15 minutes until it has stabilized; then it can be recorded every 30 minutes.
Note: Hydrostatic tests for flexible pipelines should require longer pressurizing times and longer test periods. Flexible pipe will expand more and trap air pockets, making it difficult to reach a stabilized test pressure. Also, the pipe is subject to creep that will result in a slow reduction of test pressure over time. The test should begin at a pressure high enough to ensure that the pressure at the end of the test will still be above the minimum test pressure. Flexible pipe materials are subject to cold extrusion. There are documented cases of field failures after successful shop hydrostatic tests. Therefore, the test pressure should be maintained for 12 to 24 hours to ensure that the pipe will not fail from cold extrusion in the field.