To reveal the influence of free surface variation on the hydrodynamic loads of submarine pipelines with spiral strakes installed and verify the vortex-induced vibration (VIV) suppression effect of spiral strakes under partial detachment, hydrodynamic model tests were conducted on submarine pipelines equipped with spiral strakes. The tests investigated the changes in drag coefficient in the near-free-surface region (with a ratio of immersion depth to pipeline diameter is 1-4) and the near-false bottom region (with a ratio of bottom clearance to pipeline diameter is 1-4) under different Reynolds numbers. Additionally, the VIV suppression effect of spiral strakes was analyzed when 30% of their length was damaged. The test results show that when the submarine pipeline is immersed from a depth of four times the diameter to approaching the free surface, its drag coefficient gradually increases; when the submarine pipeline approaches the false bottom from an initial distance of four times the diameter, its drag coefficient decreases. With the increase in the Reynolds number, the suppression efficiency of VIV by the 30% damaged spiral strake exhibits a trend similar to that of the full-coverage scheme, both showing a decrease. Nevertheless, it still maintains a considerable suppression effect.

Offshore wind power plays a crucial role in energy strategies. The results of traditional small-scale physical models may be unreliable when extrapolated to large field scales. This study addressed this limitation by conducting large-scale (1:13) experiments to investigate the scour hole pattern and equilibrium scour depth around both slender and large monopiles under irregular waves. The experiments adopted Keulegan—Carpenter number (N_KC) values from 1.01 to 8.89 and diffraction parameter (D/L, where D is the diameter of the monopile, and L is the wave length) values from 0.016 to 0.056. The results showed that changes in the maximum scour location and scour hole shape around a slender monopile were associated with N_KC, with differences observed between irregular and regular waves. Improving the calculation of N_KC enhanced the accuracy of existing scour formulae under irregular waves. The maximum scour locations around a large monopile were consistently found on both sides, regardless of N_KC and D/L, but the scour hole topography was influenced by both parameters. Notably, the scour range around a large monopile was at least as large as the monopile diameter.

Cemented riprap underwater technology uses good antidispersion performance and high-fluidity characteristics of underwater self-protecting concrete or mortar to fill the gaps between underwater blocks to form cement with a definite structural strength. This paper covers the first instance of cemented riprap underwater technology being applied to the field of scour protection for offshore wind power monopile foundations. Based on the cemented riprap underwater technology, this paper proposes anti-scouring measures for offshore wind power monopile rock-filled concrete, and uses a 1:13 large-scale wave current physical model test to study the anti-scouring performance of underwater structure around piles. Tests have shown that the plum-shaped pouring method can form a complete cemented riprap underwater protective structure. This protective structure can maintain good integrity and stability. Under extreme wave and current conditions with a return period of 50 and 100 years, the maximum scour depths of the sand around the monopile are 4.00 and 6.24 m, and the scour pit widths are 6.50 and 13.75 m, respectively. After the cemented riprap underwater protective structure, the maximum scour depths of the soil around the pile are 2.96 and 4.42 m, and the scour pit widths are 2.65 and 4.55 m, respectively. The above results provide new ideas for the protection measures and scour restoration of offshore wind power monopile foundations.