Bad Vibes Felt at Offshore Platform— Replaced with DRAG® Design

Ever since initial operation in mid-1998, the 8-in. (200-mm) recycle valves in the main oil line (MOL) export system Eastern Trough Area Project offshore platform had been experiencing extreme vibration problems. This North Sea platform is operated by BP AMOCO on behalf of its partners.

Flow-induced vibration was extreme in the valves themselves, causing excessive vibration as well as of the associated piping and structures. The valves were suffering from continual failures in the gland-packing assemblies, instrument and air supply tubing, and the mechanical and electronic components of the position feedback control assemblies.

These MOL export pump recirculation valves were designed for a maximum flow of 900,000 lb/hr (404,000 kg/hr) at inlet pressures as high as 3200 psia (220 BarA) and Ps of 2900 psi (200 Bar) at 150 F (63 C).

After an evaluation of the problem in July of 1999 by JMDynamics, it was recommended that the valves be replaced. However, for operational reasons, this could not be immediately done. A revised-design trim replacement in these three-stage cage/diffuser valves was accomplished, but this failed to mitigate the severity of the vibration problem.

In May of 2001, JMDynamics was called in to again evaluate this continuing vibration problem. The company used Southwest Research Institute’s (SRI) vibration-level criteria as a guide only for the evaluation of results from piping vibration testing. These provide vibration criteria acceptance as a function of frequency and use two predominant levels of acceptance.

Figure 1: The replacement “A” MOL recycle valve

Figure 2: A multi-stage pressure reduction disk stack with individual disks showing the right-angle, tortuous flow paths.

When these MOL pumps startup or shutdown, the control valves are 100-percent open. From a vibration point of view, this is their most critical operating condition, and a maximum vibration level of 25 mm/sec RMS was recorded in the horizontal direction on the control-assembly mounting plate; the dominant frequency was 29 Hz.

At this point, JMDynamics’ original recommendation that these valves be replaced with new CCI severe service valves was instigated. These valves are specifically designed to minimize vibration through limiting trim flow velocity for this severe, very high P service.

Replacement Valve Design

To eliminate the destructive effects and potential dangers of high vibration levels in the severe-service application, the new replacement valves (Figure 1) are designed to limit trim fluid velocities to less than 100 ft/sec.

This is accomplished through multi-stage pressure reduction (over 20 stages) within a stack of tortuous-path, electro-discharge-machined (EDM) disks containing a series of sequential, right-angle turns, Figure 2. In addition, each disk incorporates a pressure-equalizing ring (PER) on its inside diameter to ensure that equal pressure acts radially around the circumference of the plug at any position in its stroke. This design keeps the plug centered at all loads, thus preventing plug vibration that could cause galling and impede free stroking motion.

Also, the actuator control is provided with a snap-action relay which is set to reduce operating time of the valve at very low flows when the plug would be close to the seat. Under the close seat/plug conditions, high, resultant fluid velocities could result over time in seating surface erosion.

Results of Valve Replacements

Vibration levels were recorded not only at many points throughout the MOL recycle valve associated piping and support structures, but of course also on the two valve bodies and actuators involved as indicated in Figure 3. Table 1 shows comparatively the root-mean-square (RMS), vibration reduction by a factor of 10 at these valve locations between the original valves tested in 1999 and the multi-stage pressure reduction valves tested in October of 2001. Figure 4 shows a typical peak actuator vibration comparison under relatively comparable conditions. This vibration level reduction is even more significant since the replacement valves operate at a higher flow and pressure drop than the original valve. In addition, there is now virtually no transmission of vibration energy to the piping and support structure.

Figure 3: A schematic of the two, parallel MOL pump recycle systems showing the points of valve vibration measurement.

Table 1: A 1999 vs. 2001 comparison of the RMS vibration measurements on valve actuators and bodies taken in XYZ directions.
* Pump and valve a operating only, no flow through pump or valve B

Figure 4: A typical plot of before and after actuator peak vibration spectra over a wide frequency range.

Published in SOLUTIONS Summer 2002