Vortex Induced Vibration (VIV)
For all Tekmar projects supplied with their patented TekLink Mechanical Latch Cable Protection System (CPS), a screening for VIV is undertaken to demonstrate this is not a concern for the TekLink Cable Protection Systems supplied on typical OWF projects.
VIV is typically associated with tense, stiff risers (i.e. steel catenary risers) installed in deep and ultra-deep water locations (i.e. long span, high length/diameter ratio) where the cylindrical free spanning member is subjected to a steady, uniform current flow. The phenomenon is characterised by the formation of a vortex street downstream of the free spanning member. If the vortex shedding frequency matches the natural frequency of the cylindrical member then the deflection of the cylindrical member can be amplified.
When implementing the TekLink Cable Protection system (CPS) within typical OWF project, there are a number of points to consider:
- The combined cable and bend restrictor systems are not tense or stiff like a steel catenary riser. They are also likely to have variable natural frequencies depending on the applied environmental loading.
- The flow field in the vicinity of the foundation structures is unlikely to be steady or uniform. Scour formation and protection, and irregularities in the flow upstream of the foundations (e.g. seabed profile, large rocks, and other foundations) will all act to disturb the subsea current flow. The effect of wave kinematics (both in a free stream and when interacting with the foundation structure) should also be considered as they are likely to disturb the flow field.
- The surface of the CPS is segmented and will become covered with marine growth. This gives the free spanning member a rough, non-cylindrical geometry. This will inhibit the possible formation of any vortex streets downstream of the cylindrical member VIV are, therefore, not of concern when considering the response on OWF cables and protection systems.
As a further check, the screening assessment outlined in Table 1-1 of DNV-RP-F105 can be considered:
L/D < 30 Very little dynamic amplification = unlikely to experience VIV
Tekmar has screened for VIV on over 40no OWF projects to date and within all cases VIV has not been a concern.