“Lock-in” is traditionally known as a phenomenon that occurs when the vortex shedding frequency is at, or near, a structural natural frequency of a tubular. Lock-in is associated with a significant increase in vibration amplitude, which can be extremely detrimental to a tubular’s anticipated fatigue life. For many researchers, this is the classical definition of vortex-induced vibration, or VIV. The effect of VIV on the fatigue damage of a tubular is a function of many variables, but one of the more critical parameters is the mode number of vibration.
To illustrate the concept of mode number, consider a vibrating beam fixed at both ends. If the beam has a mode number of 1, then its mode shape is half of a sine wave with the maximum displacement occurring in the center. This point of maximum displacement is known as the “anti-node” and oscillates back and forth at, or near, the modal natural frequency (the natural frequency associated with this mode). If the mode shape consisted of a full sine wave it would be vibrating in mode 2 since it would have two anti-nodes. Points of zero displacement are called nodes, and points of maximum displacement are called anti-nodes. For mode 2, there are two anti-nodes and three nodes. The three nodes consist of the point at the center of the beam as well as the one point at each end.
A deepwater tubular can exhibit vibration at mode numbers that are quite high. It is not uncommon for a deepwater production riser in 3000 ft. of water to experience VIV at a mode of 25-30 or more. (The actual mode number depends on a number of parameters including the current magnitude and shape, the tension in the tubular, and the outside diameter of the tubular.) VIV of a deepwater tubular may occur at a single mode, multiple modes simultaneously, or even a single mode that changes the actual mode that is vibrating over short periods of time.
The addition of well-designed VIV suppression devices helps reduce the motion of the riser by altering the vortex shedding forces. The magnitude of the VIV displacements can be minimized, resulting in improved fatigue life and overall system integrity.