P.3 ~ Helical strake testing suggests path to VIV solution

Vibration in subsea risers caused by vortex shedding is a challenge to operations in deepwater. Proper management of vortex induced vibration requires a balance between acceptable movement and the cost of suppression. To address the questions involved in planning configurations for marine risers, and to investigate the impacts of typical conditions affecting VIV, two programs were conducted consisting of tests along a tubular (just under 100 ft long) that was towed under a rotating arm at high speeds to simulate conditions similar to those a deepwater riser may encounter.

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Coverage density and location

While it may be attractive to simply strake a specific coverage length to obtain the desired suppression efficiency, this may not be possible due to the presence of connectors, anodes, etc. or it may be uneconomical. In these cases it is important to understand the impact of a lower density of coverage. Additional tests were conducted with 12 ft, 9 in. of triple start helical strakes on the outer end of each of the two outer joints (so that the total coverage length was 25 ft, 6 in.).

If the outer two joints had been fully covered, the coverage length would have been 35 ft, 6 in. While covering only the outer 12 ft, 9 in. of each 17 ft, 9 in. joint (thus, the outer end of the outer joints were covered) is not as effective as covering the entire joint length, the effectiveness with partial coverage was still high. Accelerations at the inner end of the test pipe were reduced by a factor of about four to five, while accelerations at the outer end of the test pipe were reduced by a factor of eight to 10. It is not critical to fully cover each joint with helical strakes to achieve a relatively high efficiency.

Even if the strakes are not located in the most desirable area (in this case, after the outer joint is straked, the inner most joint would be considered the worst possible joint to cover and the next to outer joint would be considered the most desirable joint to cover), they can still provide a significant amount of VIV suppression. The addition of strakes on the inner most joint reduced the accelerations by at least one-third.

Just because strakes are present locally does not imply that stronger acceleration reduction will accompany that presence. The decrease in acceleration by adding strakes to the inner most joint is not much larger at the inner end than it is for the outer end. Interestingly, with strakes on the inner end, the accelerations are still a little larger at the inner end than at the outer end. This also indicates that the addition of strakes on the inner most joint has mostly contributed only damping to the entire system. Most of the vortex shedding excitation is located closer to the outer end and not the inner end, thus, while the inner joint strakes suppress vortex shedding, they are not suppressing vortex shedding at the frequencies that are most influencing the cylinder response.

Strake alignment

In practice, most helical strakes are made in short segments 5 ft to 6 ft long. Usually it is simple and quick to visually line up adjacent strake sections; however, there are some instances when lining up adjacent sections may be problematic (such as when retrofitting with an underwater vehicle). To examine the importance of strake alignment, an experiment was performed with strakes glued to the outer two joints of the test cylinder in three different arrangements: 1) The strakes attached in a continuous helix 2) The strakes attached in 15-in. sections and staggered 60° apart so that the next section started exactly halfway in between the previous section (the most severe misalignment possible) and 3) The strakes attached in 30-in. sections and staggered in similar fashion to the 15-in. section length strakes (note that since the pipe OD was 2.5 in., the 15-in. section is 6D long, while the 30-in. section is 12D long).

Results from this experiment showed that while the intentional strake misalignment produced a small degradation in the strake performance, all three alignment configurations were effective at suppressing VIV.

Surface roughness

The majority of VIV analyses for deepwater tubulars use computational models built upon smooth cylinder data for both the bare tubular as well as for the suppression devices.

Overall, surfaces simulating soft marine growth (i.e., foam and carpet) had very little effect on the helical strake effectiveness, indicating that the strakes tolerate a modest amount of soft marine growth well.

While helical strakes can effectively suppress VIV using a range of coverage lengths, densities, and groupings, they also usually increase the mean drag forces on the tubular. This larger drag can sometimes be problematic (since it increases deflection of the tubular) and the choice of a VIV suppression device with lower drag such as a fairing may be worth considering.

Acknowledgment

Based on a paper presented at the ASME 33rd International Conference on Ocean, Offshore and Arctic Engineering, held in San Francisco, California, June 8-13, 2014.

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