Abstract: Ocean floating platforms are often equipped with a “C” type tube bundle passive residual removal systems that includes the horizontal heat transfer section. Under the influence of oceanic conditions, due to the change of device space position and the additional inertia force introduced by rolling, the vapor-liquid distribution and heat transfer performance in the horizontal tube may be affected, which in turn affects the normal operation and safety of the passive residual removal systems. Therefore, this paper mainly focuses on the experimental study of steam condensation flow pattern in a horizontal tube under rolling conditions. In this paper, the variation rule of steam condensation flow pattern in a horizontal tube with a diameter of 25 mm and an effective heat transfer length of 1 050 mm under the rolling motion was investigated by experimental methods. In the experiment, the mass flux was 20-50 kg/(m²·s), the vapor quality was 0.1-0.9, the rolling angle was 0°-20° and rolling period range was 10-20 s, respectively. The experimental method involved high-speed photography to observe and classify flow patterns. The results indicate that rolling motion significantly complicates flow patterns, categorizing them into composite types (intermittent/stratified, semi-annular/wavy stratified, stratified/reverse stratified) and simple types (semi-annular, annular). Even slight rolling alters flow field distributions, particularly under low liquid-gas shear stress conditions. As rolling angle and period increase, intermittent/stratified flow regions expand, while semi-annular and annular flows remain minimally affected. The predictive capability of existing flow pattern maps for steam condensation flow patterns in horizontal tubes under rolling conditions was evaluated. First, rolling test data were analyzed based on static flow pattern maps. The results indicate that static flow pattern maps accurately predict annular flow and can indicate potential intervals for composite flow patterns, but they cannot precisely identify composite flow patterns. This suggests that optimization is needed to adapt to rolling operating conditions. Further examination of rolling flow pattern maps reveal limited discriminatory capability due to experimental scope constraints and prediction deviations for certain flow patterns. In summary, existing flow pattern maps struggle to accurately predict condensation flow patterns under rolling conditions, necessitating the development of more suitable identification methods. By analyzing the experimental data of 489 rolling flow patterns, it is found that the existing adiabatic flow pattern map is not applicable to the rolling condensation flow pattern prediction. For the analysis of condensate force under rolling conditions, the rolling Froude number (F_roll) considering the additional inertia force was proposed, and the condensate flow pattern transition criteria applicable to rolling conditions were obtained by combining the experimental data. This study proposes transition criteria for horizontal pipe condensation flow patterns under rolling conditions, providing crucial guidance for optimizing marine heat transfer system designs.