Submarine debris flows threaten offshore pile foundations, yet their interaction with twin piles on sloping seabed remains unclear. A three-dimensional multiphase numerical model based on the Herschel–Bulkley rheology was established and validated, to investigate the effects of pile spacing ratio G/D (0.5–8, G = center-to-center spacing, and D = pile diameter) and upstream pile aspect ratio h/D (2–8, h = upstream pile length) on twin-pile force and flow field evolution. Two interference regimes were identified: strong coupling shielding at G/D ≤ 3 (downstream pile peak drag coefficient rising by 200% from G/D = 0.5 to 3) and weak coupling independence at G/D 5 and h/D ≥ 6 (downstream steady drag coefficient increasing by ∼2.5% per 1D spacing). Increasing h/D strengthens the upstream pile's shielding effect, fragmenting the downstream steady recirculation zone into discrete recirculation zones of varying intensities (dispersion index F minimum 0.355 at h/D = 6). Impact peak dynamic pressure is 1.67 times the steady-state value for the upstream pile and 2.31 times for the downstream pile.
Guo et al. (Wed,) studied this question.