Scour suppression beneath single and piggyback pipelines using cationic polyacrylamide-treated non-cohesive sediments

Local scour beneath submarine and piggyback pipeline systems remains a major challenge in offshore and hydraulic engineering due to its potential to induce free-span formation, structural instability, and pipeline failure. Although numerous scour protection techniques have been proposed, the application of polymer-assisted sediment stabilization for mitigating scour around piggyback pipeline configurations has received limited attention. This study experimentally investigates the effectiveness of cationic polyacrylamide (CPAM) treatment in suppressing local scour beneath single and piggyback pipelines installed on non-cohesive sandy beds under steady-current conditions. A series of movable-bed experiments was conducted in a recirculating flume under both clear-water and live-bed regimes (U/Uc = 0.92–2.0)). The influences of relative flow velocity, diameter ratio, and gap ratio on scour development were systematically evaluated for untreated and CPAM-treated sediment beds. The results showed that untreated piggyback configurations generated substantially deeper scour than corresponding single-pipeline cases, particularly at small gap ratios and high flow intensities due to intensified local flow contraction and near-bed sediment entrainment. In contrast, incorporation of 8% CPAM completely suppressed measurable scour for all investigated hydraulic and geometric conditions, including severe live-bed cases after 72 h of continuous flow exposure. Regression and sensitivity analyses identified relative flow velocity as the dominant parameter governing scour development, followed by diameter ratio and gap spacing. An empirical relationship for predicting normalized equilibrium scour depth was also proposed and showed good agreement with the experimental data (R2 = 0.94). The findings demonstrate the strong potential of CPAM treatment as an effective scour mitigation strategy for submarine and piggyback pipeline systems installed in coarse non-cohesive sediments.