The present paper is addressed to evaluate the hydrodynamic forces and motions experienced by two flexible circular cylinders in staggered arrays which are towed in still water. The cylinders were submerged partially into the still water from a towing carriage as hanging-off vertical cantilevers. Each cylinder has length-to-diameter ratio of 34.4 with low mass ratio (= 1.24). The Reynolds number varied from 27,000 up to 37,800. For examining wake interference effects on the forces and motion of the cylinders in the staggered configuration, a centre to centre longitudinal gap between the upstream and downstream cylinders was fixed by 4D with transverse gaps were varied by 1D, 1.5D, 2D, 2.5D, 3.5D, and 4.5D, respectively. Measurement of the hydrodynamics forces and motions in two, in-line and transverse directions to the flows were performed simultaneously. Frequency of the drag force was found approximately twice the corresponding lift force frequency at the related reduced velocity (Ur) for all cases and the forces frequencies ratio was independent from the Ur and LT parameters in the range used. For the upstream cylinder, the LT = 2.5D became a critical value at which the forces coefficients started to jump into the higher values as the LT increased (LT > 2.5D).
Future offshore structures such as OTEC or CO2 sequestration platforms may has multiple cold water pipes (CWP) or CO2 injection pipes in order to increase their productivity. If so, obviously it is need more clearly picture on physical mechanism of their pipes dynamics; because the pipes have distinct design with ordinary riser pipes of oil/gas offshore platforms which are typically terminated on sea bed. Better understanding on the dynamic behaviors of the flexible hangingoff circular cylinders arrays in water flow are required for design purposes of those types of structures.