ROGUE ACCESS POINT (AP)

4.1.3.1 Periodic pressure testing of the barrier philosophy 

For the periodic pressure test of the PMV and PWV no complex model is necessary. The evaluation criterion simply consists on evaluating the hold pressure and visually detecting if the pressure varies. This process could be automated, but since pressure tests usually require witnessing of the procedure to create quality control records, a manual verification is sufficient.

4.1.3.2 Leak detection system 

Detection of hydrocarbon by fluorescence or acoustic signals is done by detecting presence or not of hydrocarbons within the sensor range. Thus, the use of predictive models is not applicable for leak detection.

4.1.3.3 Partial stroke testing and valve signature analysis 

A simple pattern recognition model is recommended for gate valves and hydraulic actuators. This model will be based on empirical results obtained when new valves are tested after assembly in the XT. The result of these tests is the original valve signature, which will be used during operation to compare the values in working conditions vs. the values of the original signature. It is worth to mention that in addition to internal forces of the actuator i.e. spring force, pressure on the return side or piston side, piston seal friction and stem packing friction; the wellhead pressure also adds resistance to the opening/closing of the valve during normal operation; however these forces are negligible when compared with the high force exerted by the actuator and will not be taken in consideration.

For partial stroke testing the analysis is similar to the one previously described, with the difference that the valve opens only for a short period of time with the purpose of obtaining approximately 20% of the closing span. Since the valves in question do not have position indicators, other methods should be used to estimate when the valve is 20% closed. The author suggests using the valve signature for reference of closing time. One can say for example that if a new valve takes 0.2 seconds for the gate to come off the seat (breakaway) and then 2 seconds to close fully from the breakaway point, it would take approximately 0.6 seconds (breakaway time + 20% closing time) to close 20%. This value is not exact since the closing of the valve is not linear due to the spring action, but in this case that is not so relevant because the accuracy of the closing range is not critical for the test. The objective of this partial stroke test is to plot the pressure decay of the valve closing up to 20% and compare it to the original signature, to see if there are any major discrepancies that could indicate some issues with the valve or actuator such as gate obstruction or spring failure.

4.1.4 Discussion 

At first glance a subsea gate valve with hydraulic actuator might seem a relatively simple device, especially due to the lack of electronics and the limited range of motion. But when the analysis of failure modes was carried out, the results contradicted the initial estimation. The valve and actuator analyzed in this case study can have many failures of different nature. Consequently, the monitoring techniques necessary to ensure proper predictive maintenance can be many and represent a considerable capital investment.

The FMSA is probably the most important step in establishing a CM program. This document provides important and relevant information to determine the type of sensors to be used, crucial data for establishing a “best in class” CM program. It is very important to get input from experts with vast experience in fault management of subsea systems, as well as use reliable historical data.

The key for effective implementation (both economically and technically) of CM lies in using the sensors and equipment already available in existing SPS’s in a smart way. This has been demonstrated with the valve signature analysis proposed earlier, where simple signals currently measured in any SPS are used to determine the health of the valves/actuators. There are many other ways to do signature analysis. For example, by using position detectors coupled with electronic valve position analyzers, but the question the designers of the CM system have to ask themselves are:

 Do we really need a new sophisticated system?

 Will this new system provide much more useful information than others?  Can we determine the health of the equipment by other –and simpler‐

means?

Regardless of the simplicity of the valve profile analysis, the efficacy of this method relies on the quality of the original signature; if the latter cannot be trusted it will not be able to provide an accurate baseline to be compared against the valve profile during operation. This highlights the importance of having a good factory

acceptance test of the trees when they are fabricated, so reliable valve signatures are generated for further use. Another interesting finding is that in many cases there is no one size fits all solution available. That is the case of leak detectors. Even though subsea leak detection is a relatively small technology niche, there are a handful of different technologies that can be used in different scenarios depending on the type of fluid being monitored, leak rate detection sensitivity, and effective area of sensor coverage.

It is also significant to notice the importance of keeping to a minimum the equipment shutdowns for testing the integrity of the systems. One way to deal with this is by executing the partial stroke testing explained in the previous section, which allows for testing the opening and closing function of the valves without stopping production.

4.2 Case study 2: Power supply unit