Having spent the past 11 years or so working on integrity management projects as part of riser engineering experts 2H Offshore, we have encountered a lot of strakes and have seen it all. Here are the most common queries that we have received from clients over the years, along with some guidance on what to keep in mind.
1. The risers were designed for VIV, so why should I worry?
- Most riser designs assume perfectly clean strakes with little to no marine growth.
- Marine growth has the potential to reduce the effectiveness of VIV strake suppression efficiency in high current regions.
- In recent months loop currents in excess of 4 knots have been observed in the GoM – on the order of a 100+ yr event. Can you remember what your design current speed was?
2. What about anti-foul strakes?
Unfortunately, the design premise that ‘anti-foul strakes will be used to mitigate VIV’ just does not hold up. In similar marine environments, ships have to regularly maintain and renew their anti-foul coating and, in our experience, significant marine growth even with anti-foul strakes does occur – albeit at a slower rate than strakes without anti-fouling.
3. Is marine growth a problem?
Maybe not. Some risers are less susceptible to VIV anyway and strakes were only added as additional insurance. The engineering design, specifically the predicted fatigue life, can be used as an indicator of whether VIV will be an issue when strakes lose efficiency due to marine growth.
4. Does the marine growth roughness help reduce VIV (like golf ball dimples)?
No, quite the opposite. Hard marine growth reduces a strakes ability to stop VIV as the depth of marine growth approaches the height of the strake fin. According to a study done by MARINTEK [OMAE2008-57587], strake efficiency reduced inversely to increased marine growth. The simulated hard marine growth took more height (>60%) compared to soft marine growth (>35%) to reduce the efficiency, but both have their limits.
5. How much is too much marine growth?
If the actual growth is more than predicted in the original design calculations, then your design predictions are no longer valid and you should consider cleaning. Marine growth over 1/3 of the fin height is a good rule of thumb for deciding when to clean. When the fins can no longer be distinguished from the growth, they are past due for cleaning.
6. But isn’t VIV analysis conservative?
Maybe. Riser specialists 2H Offshore [OMAE2008-57046] and others have written papers on the topic which indicate that analysis predictions are conservative. However, VIV is not a linear phenomenon and system specific responses are based on the structural natural frequency and damping in a particular current profile.
7. What can I do?
First off, gather information regarding the extent of marine growth coverage during the next inspection, specifically the height of the growth compared to the height of the fin. Secondly, confirm if the design evaluated the effect of marine growth on VIV fatigue performance, or commission a short engineering study to do so. Finally, review the current data that is collected at the facility via the ADCPs. Evaluating current speed against the design predictions will at least give an indication of over or under conservative design. If after doing all the above, you’re still worried or the analysis suggests there will be a problem, then a strake cleaning program can be implemented, or a monitoring system can be retrofitted to determine if VIV is occurring.
8. How do I clean the strakes?
Cavitation blasters seem to do less damage to the strakes (or strake bands) than water jets but both have been used effectively. Try and plan the cleaning with your UWILD inspections as the tools will already be onsite.
9. But the risers have fairings; is this still an issue?
Fairings also suffer from degradation of VIV suppression due to marine growth [OTC 19373]. But fairings seem to perform better overall than strakes until the marine growth restricts the ‘weather vaning’ of the fairing. Fairings that are stuck perpendicular to the current flow increase the apparent diameter and may lead to galloping. Make sure the fairings are not only clean, but also free rotating during inspections.
In summary, keep an eye out for marine growth on your strakes, maintain and clean when necessary, and this should help secure the long-term integrity of your system.
John MacDonald is vice president at Clarus Subsea Integrity. He holds a BS in ocean engineering from Texas A&M University and is both a Chartered Engineer and a certified project management professional. He has accumulated 14 years of engineering experience in riser analysis, naval architecture, verification, acceptance testing and integrity management.
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