The Testing of Subsea Valves at Petrobras
Petrobas's Valve Qualification Test
Many of the valves used by Petrobras must pass the company’s stringent qualification test. This assessment is broadly based on well-established standards set by the American Petroleum Institute(API), but also contains a range of extensive tests that exceed these requirements.
The additional tests provide a more comprehensive evaluation of the subsea valves' performance in a wider range of simulated conditions - these tests are conducted to guarantee the reliability of the subsea valves.
Over the past year, Oliver Valvetek has carried out the tests on its hydraulically-operated subsea gate valve. This is a unit that has been designed to function either as a stand-alone valve for methanol injection processes – commonly used to prevent formation of hydrate – or as part of a Christmas tree for annulus lines.
Tests Exceeding the Standard API Checks
Firstly a series of lock tests are performed whereby the valve is operated both hydraulically and using the torque override, against a ‘stuck gate’. During the test, the internal slab gate is physically restrained to simulate the valve ‘icing up’ in service.
The test ensures that even if this occurs there will be no mechanical damage to the system that will prevent functionality returning once the blockage is cleared. For the second phase, the valve is de-greased. This ensures that the unit’s performance does not depend on being artificially enhanced by lubricants, which will inevitably be gradually removed from the valve during its life by the fluid flowing through it.
After the de-greasing, the valve is operated through approximately 3,800 cycles – a process that takes 4 months. This number of cycles was calculated based on the expected service life of a Wet Christmas Tree. Gas seat leakage is constantly monitored, as well as fluid pressure across the valve, hydraulic pressure, and torque required to cycle the valve. Any changes are collected using digital pressure and torque meters and the detailed data is logged at a frequency of 8 samples per second, for analysis and inclusion in final reports. The third stage is for the valve to be installed in a temperature cabinet.
The valve then undergoes a further series of cycles whilst at the required maximum and minimum temperatures +121 Celsius at the hottest and -18 Celcius at the coldest. For added safety, these comfortably exceed the temperatures that will be encountered in operation. Stage four is testing under simulated external and internal pressures. For this, the valve is installed in a hyperbaric chamber which simulates the external seawater pressure at a water depth of 2,500 meters. The valve is operated through over 300 cycles, some with hydraulic operation and some using the manual override mechanism. Finally a series of ambient cycle tests are performed to see what effect all of the extreme pressure and temperature cycle tests have had on the operation of the valve and actuator. After a year of testing, the 1” gate valve was finally approved and is now being installed on a number of subsea christmas trees.
To some, a year-long testing process may seem excessive for such a small component. However, it is only by collecting such a large number of signatures over such a long period of time and under simulated conditions that manufacturers and clients can understand the way a valve will perform over it’s lifetime once in service. Petrobras’ tests are notorious among valve manufacturers for their stringency, and only a small number of suppliers around the world, including Oliver ValveTek, have passed the latest edition of the specification.
The intensity of the analysis to which each component is subjected is testament to Petrobras’ commitment to safety. Ultimately, the cost of a valve failing in situ can be potentially devastating both in terms of financial losses and – in the worst case scenarios – harm to people or the environment. Because of this, it is impossible to be too careful when it comes to testing subsea equipment.
What measures would you like to see implemented into offshore valve testing? Leave your thoughts in the comment section below.
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