Seismic Response Of Subsea Casing String For Deep Water Production

Recently,with the development of oil and gas industry, oil and gas production is gradually pushed forward from land to ocean area. The geotechnical environment of deep sea is very complicated, especially in district of South China Sea.There are a variety of unsteaty fault strutures, which cause frequent earthquakes.Therefore, the securiety of casing string shuold be considered.Thus, the seismic response of casing string for deepwater production is payed much attention to by researchers. In this paper, the land form and geolocgic structure of South China Sea are firstly introduced in Chapter one. Then the law of earthquake distribution and activity are analyzed, which predicts the place of major earthquakes in fifty years. At last, the intensity distrit is obtained, which can be helpful to the engineering construction in South China Sea.According to the theory of dynamic response of pile foundations, the casing structure is simplified as Timoshenko Beam. while he soil is simulated by string and dashpot. The mathod of reverberation-ray matrix is adopted first to calculate the seismic response of casing struture.In general, the surface soil of seabed is mud. When earthquake happends, the shear velocity and thickness of surface soil layer will greatly effect the seismic response of casing string. The results of computation indicate that with the decrease of shear velocity and incease of soil thichness, the influence of shear wave to casing-soil system is greater. While drilling in deep wate, the conductor is injected by water force without cementing in avoid of crack of soil layer. This method has been put into practice in South China Sea successfully already. When earthquake occures, the stress concentration at bottom of conductor may happen, which can destroy the cenment. Then the soil and gas of high pressure in shallow soil layer will seep into the condutor and move up, which will cause great disaster. In this pape, the effects of conductor lengthon shear force and bending moment of condutor tip during earthquakes are discussed. The result indicates that shear force and bending moment of conductor tip will reach the maxmum when it is set down to the interface of soft and hard soil around.The weak interbed under seafloor has great influence on the stability of subsea casing string for deepwater production, especially during earthquakes, which will cause a large bending moment and shear force on the cross section of casing string. How dose the property of soft interlayer influence the seismic response of casing string? It is a question worthy to discuss. In this paper, the bending moment and shear force of different casing section is discussed from the aspect of the shear velocity and thickness of weak interbed. We can find that, while the shear velocity of soft interlayer decreases, the bending moment and shear force of casing string at interface increases.When the thickness of soft interlayer h changes, there may be a critical value hct. When h<hcr, the shear force and bending moment of casing string at interface will increase with the increase of thichness h. When h>hcr, the shear force and bending moment of casing string at interface will decrease with the increase of thichness h.The results in this paper can provide reference and guidance for the engineering construction, design and security assessment.