ABSTRACT
With the expansion of offshore development, the application of multicolumns offshore platforms has become more widespread. The slamming on the deck of the platform becomes one of the critical issues relevant to the air gap design. In the present study, a number of factors influencing the surface elevation around array of truncated cylinders are investigated. The influence of the distance between cylinders, wave directions, scatter parameter and wave steepness are examined. Based on the potential theory, the first-order and the second-order surface elevation are calculated, and the nonlinear characteristics of surface elevation around cylinders is discussed. Further investigation of the detailed components of surface elevation revealed that not only the firstorder but also the second-order surface elevation component is significant for some particular incident wave frequencies. The neartrapping phenomenon observed from the numerical simulation further demonstrated that the second-order surface elevation is dominated by the strong near-trapping phenomenon of the surface elevation.
With the continuing development of the offshore structures, the platform with a group of cylinders becomes more widespread. The hydrodynamic interaction between incident wave and the structures attracted much attention in recent decades. The surface elevation amplitude between cylinders of offshore structures plays an important role in the ocean engineering design, in particular to the air gap design of the platform to avoid the slamming. Therefore, the prediction of surface elevation between the cylinders of structures is very important in platform design stage.
There are many factors having impact on the surface elevation between cylinders of offshore structure. To investigate the relationship between these factors and surface elevation, several experiments have been conducted. Niedzwecki and Huston (1992) reported a significant impact on the surface elevation with different leg space between fixed truncated columns. By comparing the experimental results of platforms with pontoons and without pontoons linked the columns, the surface elevation is found to be slightly smaller in the absence of the pontoons. Morris-Thomas and Thiagarajan (2004) investigated the wave runup on the single fixed bottom seated cylinder in gravity waves. It was shown that the second-order harmonic component in the incident wave is important to the runup for a single column. Siow et al. (2013) performed smallscale model tests of air-gap response of a floating semi-submersible. It was revealed that the first-order numerical solution seriously underestimates the wave run up. Xiong et al. (2015) measured the inline force for a single truncated circular cylinder in a wave tank under different submergence depths. The experimental result showed that the submergence depth, wave steepness and scatter parameters have great impacts on inline force acting on the single truncated cylinder. Cong et al. (2015) carried out the experiment on the diffraction of regular waves by four-cylinder structures and reported that near-trapping wave motion was observed inside the structure for a specific incident wave frequency. By analysing the experimental result, it is found that the nonlinearity has great impact on the surface elevation around the cylinders of structure. However, there are many factors impact on the surface elevation. Many numerical simulations are carried out to study the characteristics of surface elevation around the cylinders of platform. Kristiansen et al. (2004) conducted the mesh sensitivity study of columns and free surface for the second-order nonlinear wave run up. Wang and Wu (2010) investigated an array of cylinders in a numerical wave tank. Wave elevation and hydrodynamic force were obtained for both bottom mounted and truncated cylinders. Grice et al. (2013) investigated the near-trapping effects for multi-column structures using potential theory program DIFFACT. The result indicated that near-trapping has significant impact on the surface elevation between four bottom-seated columns.
