Helical strakes consist of one or more fins that are spiraled around a tubular. Traditional helical strakes designed for applications in wind had three starts (three fins each 120 degrees apart around the tubular), pitch per start of approximately 5 times the tubular diameter, and a fin height of 10 percent of the diameter. Early tests of helical strakes for deepwater tubular applications found that the traditional wind parameters were inadequate and that the strakes performed best with a fin height closer to 25 percent and a pitch per start in the range of 12 to 20 times the tubular diameter. The configuration utilizing three starts was generally maintained.
Helical strakes are effective when they break up the correlation of vortices along a tubular and produce random alternating forces along the tubular length. An exception to the optimal geometry for deepwater tubulars has been made for short cylinders, such as spar production platforms, where considerations such as drag and the need for shorter correlation lengths (due to the limited length of the structure) have driven helical strake designs closer to the geometries of tradition helical strakes for wind applications.
Because helical strakes are relatively bluff to the incoming flow and often produce early separation of the incoming flow, they are associated with higher drag. Depending upon various parameters such as the Reynolds number, the surface roughness or the presence of marine growth, etc. the drag coefficient for helical strakes typically varies from about 1.3 to 2.0 for most deepwater tubulars (due to the short aspect ratio, the drag coefficients can be a little lower for spar platforms). However, helical strakes can be very effective at reducing VIV for a wide range of applications and remain a popular choice. Fairings have some advantages over helical strakes, such as producing substantially lower drag (often one-third to half the drag of helical strakes) and producing better performance with a rough surface (due to marine growth) of with other structures downstream, but helical strakes can be quite sufficient for a large range of applications.
While marine growth can substantially reduce the effectiveness of helical strakes, there are coatings which can reduce, or eliminate, marine growth on the helical strake surface for many years. These coatings are expensive, but economic analyses often find that they are a prudent investment.