ABSTRACT The vortex-induced motions (VIM) of semi-submersibles depends on the shape of the submerged structure, small changes in column geometries may have a significant impact on the VIM response of the platform. The study on the effect of column rounded ratios on the VIM of semi-submersible platforms has important guiding significance for the design of platforms. In the present study, numerical simulations of the VIM characteristics of a semi-submersible platform with different column comer radius at 45° current heading have been carried out by naoe-FOAM-SJTU. SST-DDES was used for modeling the massively separated turbulent flows. The displacements and hydrodynamic forces information in three DOFs including sway, surge and yaw motion are obtained. Based on the advantages of CFD method, the mechanism of the influence of the column rounded comer on the VIM of the semi-submersible platform is studied. Vortex shedding patterns and other flow field information are given for further analysis. INTRODUCTION As the increase of the depth of offshore oil and gas exploitation, semi-submersible platforms have been widely used in the field of ocean engineering because of their strong loading capacity. The heave motion performance is improved by increasing the platform draft. However, the Vortex-Induced Motions (VIM) response increases significantly because the increase of the effective excitation length of columns will lead to higher fluctuating pressure caused by the vortex shedding. As a kind of multi-column floating platform, the interaction of vortex shedding between the multiple columns makes the VIMs of the semi-submersible platforms more complex than those of the single-column floating platforms such as Spars. The VIM not only affect the operation of platforms, but also severely reduce the fatigue lives of risers and mooring systems. (Volnei et al , 2009) Experimental works on the VIM of semi-submersible platforms were carried out in recent years. (Waals et al , 2007; Magee et al , 2011; Goncalves et al , 2012; Liu et al , 2016). With the deep development of studies, many researchers transferred focus on the geometric parameters of the platforms effect on the VIM of the semi-submersible platforms. Studies have shown that the VIM response depends on the shape of the submerged structure and the columns have the most significant impact. (Gonçalves et al , 2015) At present, most of the columns of the semisubmersible platforms are square section with corner rounding, but the studies on the effect of the column rounded corner on the response of the platform are still very few. The study on the effect of column rounded corner on the VIMs of semi-submersibles platform has important guiding significance for the design of platforms.

ABSTRACT This study considers a new method for the ice load consideration based on the ice load spectrum in the time domain. Recently, the interest on the Arctic offshore structures for exploring and exploitation of the Arctic region has been increased by the decreasing trend of sea ice covered area. The ice load consideration in the design of the Arctic offshore structures is essential. In the design and evaluation of the station-keeping system of the Arctic offshore structures, the consideration of the ice load acting on the offshore structures is one of the main vital components. There are several methods to consider the ice load in the time domain analysis. However, these numerical approaches require a significant amount of computation because the number of ice floes to be considered in the calculation is too big. In this study, a new approach to consider the ice load acting on the Arctic offshore structure is suggested. The proposed ice load calculation method in time domain is based on the ice load spectrum concept. From the experimental results, it is expected that the global ice load will have mean components and oscillatory components in the frequency domain. Thus, if the mean components and oscillatory components of the ice load by the angle of attack of the ice drifting with ice parameters are given, then the ice load acting on the offshore structure can be reconstructed. The feasibility of the proposed ice load consideration method is examined with experimentally gained data and numerically calculated data. INTRODUCTION The coverage of the sea ice on the Arctic Ocean has been reduced for the last several decades. The trend of the sea ice decreasing is now commonly accepted as the effect of the global climate change which is regarded as one of the threats to humankind. In other hands, better accessibility to the deep Arctic Ocean will be opening as the sea ice reducing trend keeps its pace. Moreover, it had been established that the oil and gas deposits in the Arctic area are reaching up to 22% of the world's oil and gas deposit (Hearing, 2009). In line with these circumstances, developments of the Arctic offshore technology for exploring and exploitation of the Arctic region are being conducted in many countries. In the development of the Arctic offshore technology, the development of station-keeping methodologies in Arctic condition is the essential and crucial component (see Fig.1). For floating offshore structures in general, the mooring system and dynamic positioning (DP) system are considered as the method for station-keeping. In the design of the mooring system and the DP system, the time domain analysis is frequently conducted to examine the performance and security of the station-keeping system. Consequently, the consideration of ice loads in the time domain becomes one of the essential components in the design of the station-keeping methodology for the Arctic condition.

ABSTRACT: Based on the review of present force coefficients estimation methods, a new method in frequency domain, which is revised cross-spectrum estimation method, Is presented in this paper. Compared to the oases of vertical cylinders, there have not been that much data available for Inclined cylinders. In this paper, some experiments on the wave-current force on Inclined cylinder and the wave current force coefficients estimated by the revised cross-spectrum estimation method are presented. The results show that the wave and current directions have some consistent effect on them. Based on the results, empirical formulas are obtained which are used to modify the wave-current force coefficients on the Inclined cylinders Into unified coefficients. By comparison, It is found that the unified coefficients fit very well with the others results of the cases on the vertical cylinders. INTRODUCTION In offshore engineering, most structures are consisted of circular cylinders. The diameters of the cylinders are usually much smaller than the wave length. For the wave force on small diameter fixed cylinders, an empirical formula by Morison et al (1950) is proposed to be used. According to the Morton formula, the wave force can be divided Into two parts, the drag force and Inertia force. The drag force coefficient and Inertia force coefficient are needed to be determined by experimental results. For the same experimental data, the results may differ when different coefficient estimation methods and different wave theory are used. For regular waves, the peak value method, Fourier and least-square methods are commonly used. For the Irregular wave force on cylinder, Wiegel, Beebe and Moon (1957) analyzed each wave to obtain the force coefficients Co and CM. Although the high order wave theory can be used, the results are very scattered and are not convenient to use In engineering practice.

ABSTRACT: The characteristics of drag and lift forces on a circular cylinder model near a wavy boundary have been experimentally investigated in a water tunnel. In the range of the Reynolds numbers, based on the diameter of cylinder model, from 10 4 to 1.9 X 10 4 , the drag, lift force on the cylinder and lift force frequencies have been measured at the wave crest and trough and various distances from the wavy boundary. Flow visualization experiments revealed the wake structure varying as gap-to-diameter ratios (G / D) and mechanism Interacting between the cylinder and the boundary. INTRODUCTION In the offshore engineering, pipelines are used to convey oil and gas for power generation, sea-water for desalination, sewage for disposal at sea, and for the protection of communication cables. There are numerous problems associated with the design and installation of pipeline in deep-water offshore. The submerged pipelines are often fixed by concrete saddles and buried by dredging and trenching the sand at a seabed. It has been known from the practice that pipeline spans can occur due to local scour of the bed sediment The proximity of a pipeline near a seabed boundary gives rise to hydrodynamic and environmental problems m offshore engineering. Research on flow field and forces on the cylinder at various distances from the seabed boundary are of a great interest to many. There are many investigations (see Davis,1976) on forces on submerged pipelines In flow or wave. A lot of research works also have been carried out about wave forces on a cylinder near a plane boundary (see Sarpkaya,1981, Shankar et al.,1988). However, researches of hydrodynamic forces on a cylinder near a plane boundary in steady flows are relatively scarce. Second, the plane boundary changes the symmetry of flow about the cylinder nearby.