Dynamic characteristics of subsea structures being lifted out of water in irregular waves

Abstract

The exit of structures from water bodies in the presence of irregular waves is a key issue in offshore lifting operations. This study investigates the dynamic characteristics of structures during the water-exit process in irregular waves using experiments and numerical simulations. The analysis focuses on cable tension and structural motion. The results show that irregular waves significantly affect the structural motion, with notable pitch motion occurring after the structure has fully exited the water. The maximum cable tension increases as the sea state becomes more severe. The ideal environmental conditions for lifting operations occur when the significant wave height H_s ≤ 1.5 m. When H_s > 2 m, the cable tension increases significantly, and large fluctuations occur. The dynamic effect of cable tension, which follows a normal distribution, increases as the severity of the sea state rises, and reaches its maximum value when the structure passes through the splash zone. This study provides a reliable basis for assessing the safety of offshore lifting operations in irregular waves.