During the exploitation of offshore oil and gas,it is easy to form severe slugging which can cause great harm in the riser connecting wellheads and offshore platform preprocessing system.The flow pattern and pressure fluctuation of severe slugging were studied in an experimental simulation system with inner diameter of 0.051 m.It is found that severe slugging can be divided into three severe slugging regimes:regime I at low gas and liquid flow rates with large pressure fluctuation,intermittent flow of liquid and gas in the riser,and apparent cutoff of liquid phase,regime II at high gas flow rate with non-periodic fluctuation and discontinuous liquid outflow and no gas cutoff,regime III at high liquid flow rate with degenerative pressure fluctuation in form of relatively stable bubbly or plug flow.The results indicate that severe slugging still occurs when the declination angle of pipeline is 0?,and there are mainly two kinds of regimes:regime I and regime II.As the angle increases,the formation ranges of regime I and regime III increase slightly while that of regime II is not affected.With the increase of gas superficial velocity and liquid superficial velocity,the pressure fluctuation at the bottom of riser increases initially and then decreases.The maximum value of pressure fluctuation occurs at the transition boundary of regimes I and II. During the exploitation of offshore oil and gas, it is easy to form severe slugging which can cause great harm in the riser connecting wellheads and offshore platform preprocessing system. Flow pattern and pressure fluctuation of severe slugging were studied in an experimental simulation system with inner diameter of 0.051 m.It is found that severe slugging can be divided into three severe slugging regimes: regime I at low gas and liquid flow rates with large pressure fluctuation, intermittent flow of liquid and gas in the riser, and apparent cutoff of liquid phase , regime II at high gas flow rate with non-periodic fluctuation and discontinuous liquid outflow and no gas cutoff, regime III at high liquid flow rate with degenerative pressure fluctuation in form of relatively stable bubbly or plug flow. These results indicate that severe slugging still occurs when the declination angle of pipeline is 0 ?, and there are mainly two kinds of regimes: regime I and regime II.As the angle increases, the formation ranges of regime I and regime III increase slightly while that of regime II is not affected .With the increase of gas superficial velocity and liquid superficial velocity, the pressure fluctuation at the bottom of riser increases initially and then decreases. maximum value of pressure Precipitation occurs at the transition boundary of regimes I and II.