Flow Induced Motions on Multi Column Floaters

TLDR

Offshore floaters exhibit low‑frequency responses to currents, as demonstrated by tow‑test data on Spar VIMs, semi‑submersibles, and TLPs. The study investigates how multi‑column floaters behave in current, focusing on column shielding, mass‑ratio effects, and the resulting increase in mean current loads from transverse motions. Model‑test motions are employed to analyze mooring and riser systems, examining column shielding and the mass‑to‑displacement ratio. Lower mass ratios lead to larger sway responses, and transverse motions increase total mean current loads.

Abstract

Recent years have shown an increasing interest in low frequency response of offshore floaters in current. Spar VIM behavior and Semi Submersible flow induced behavior is known from calm water tow tests. Recent tow test projects have also shown low frequency TLP response in steady current. The motions from the model tests are used in the global analysis of the mooring systems and risers for these platforms. This paper discusses the dynamic behavior in current of multi column floaters and the associated complex flow patterns. Shielding between columns is addressed as well as the effect of mass ratio (i.e. floater mass divided by displacement). It is shown that lower mass ratios such as for conventional TLP’s may result in larger sway response than for deep draft semi submersibles. The motion behavior is discussed as well as the increase in total mean current loads due to transverse motions.