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Semi-submersible platforms have been widely used in offshore oil and gas exploitation due to their excellent performance, but wave impacts in extreme sea states remain a major concern. For the design of offshore structures with sufficient structural capacity, scholars have pointed out the wave pressure impulse obtained by time integration is more crucial compared with the peak impact pressure. In this study, wave basin experiments were carried out under a series of irregular waves to study the spatial variations of pressure impulse on a semi-submersible. To capture more wave impact events and get more accurate local impact pressures, a large number of impact load measurement units were compactly installed on two adjacent surfaces at the corner of test model. The probability statistical method is adopted to analyze the nonlinear features and spatial variations. The results show that the pressure impulse on a semi-submersible in irregular waves has strong variability and its spatial distribution presents local concentration features. It is found that wave impacts on the column are more serious and the variations of pressure impulse on the column are greater than that on the side of deck box under head sea state. Spatial variations of pressure impulse on a semi-submersible are severely affected by significant wave height Hs and spectral peak period Tp of irregular waves. Meanwhile, the number of wave seeds used in model tests have a decisive influence on the estimation and spatial variation of wave pressure impulses. The perspectives given in this study may provide references for the structure design of semi-submersible platforms and lay a foundation for improving the laboratory experiment of wave impacts.

Offshore platforms operating in harsh ocean environments often suffer from severe wave impacts which threaten the structural integrity and staffs safety. An experimental study was carried out to investigate the wave impact load and its effect on the global response of a semi-submersible. First, two typical wave impact events occurring successively in the wave test run are analyzed, including the characteristics of incident waves, relative wave elevations and the spatial distribution of the wave impact load. Subsequently, the corresponding global response of the semi-submersible under these two wave impacts are investigated in time domain. It reveals that compared with the incident wave, the relative wave elevation has a more straightforward relationship with the wave impact load. The relative wave crest height is associated with the spatial distribution of the wave impact load, while the local wave steepness matters more in the magnitude of the wave impact load. The impulsive effect of the wave impact load on the motion behaviors is not obvious. But severe wave impacts can introduce excessive horizontal accelerations and nonlinear behaviors like ringing in the acceleration response.