Fairings have historically been known to achieve in-line drag coefficients (Cdx) of approximately 0.60 across the Reynolds number (Re) range of 100,000 to 1,000,000, typical for the offshore environment . The recent development of helically grooved drill riser buoyancy was shown to achieve Cdx values of 0.65 for this Re range , presenting a strong alternative to fairing products especially considering the additional installation, storage and maintenance requirements of fairings.
Therefore it is the purpose of this paper to investigate possible fairing designs capable of achieving even lower Cdx values where fairings can still be beneficial in further reducing drag loading. This paper proposes a non-parallel reduced chord horseshoe (RCH) fairing design and is analysed using computational fluid dynamics (CFD) in 3-d using the transient k-epsilon (Reynolds-averaged Navier-Stokes) turbulence model. The modelling approach is validated against tow tank test data of a previous teardrop-shaped (TD) fairing design which showed good agreement with published, peer-reviewed literature.
It was found CFD simulations with axially continuous fairings provide artificially low Cdx values due to the absence of fairing end-effects and gaps between fairing sections. In essence, an infinitely long and uninterrupted fairing in the riser axial dimension is not realistic. Incorporation of this discontinuity sees a significant increase in Cdx compared to the axially continuous fairing configuration. Although this is the case, it was found Cdx of approximately 0.48 or lower is achievable for the entire offshore Re range for the discontinuous fairing configuration (assuming a chord/diameter ratio of 2.0). Larger chord/diameter ratios would provide lower Cdx at the cost of a longer chord length which may impact fairing installation efficiency. Longer axial lengths would also achieve lower Cdx but with the risk of flutter instability.
This development in RCH fairing design sees a possible option for further fairing applicability to offshore drilling operations where lower drag is desirable beyond that offered by the helically grooved buoyancy.