11. Natural Vibration of Jack-Up Oil Rig

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It is important to investigate the characteristics of the vibration of jack-up oil rigs in order to ensure their successful design. FEM is available for the accurate calculation of natural frequencies, but there are a few unknown quantities to be studied such as the virtual mass of a leg vibrating in water, and the supporting condition of the sea bed, among others. This paper, first of all, deals with the calculation of natural frequencies and mode shapes, using an idealized model, composed of beam elements and plate elements, and vibration tests of an actual jack-up oil rig which has been constructed during the past two years. In these tests, four oil rigs were excited by the wire cutting method to obtain the lower natural frequency and logarithmic decrement. At the same time, the vibration of the leg due to wind was also measured in order to obtain its natural frequency. The main conclusions are as follows. (1) It may be accepted that the virtual mass coefficient of a leg composed of circular cylinders, C_v, equals 1.0. (2) In calculating the natural frequency of the oil rig in a jacked-up condition, the spring effect should be taken into account. (3) In estimating the dynamic response of the oil rig, the lowest three natural frequencies govern the dynamic response, due to wave force. (4) The wire cutting method is very useful for measuring the natural frequency and logarithmic decrement of the oil rig. (5) The measured lower natural frequencies are approximately proportional to h^<-3/2> (h: platform height) and W^<-1/2> (W: platform weight) (6) The logarithmic decrement indicated is from 0.2 to 0.3, which is larger than that for ship's. (7) The first and second modes of the legs are governed by their bending and torsional deformation, respectively. Their natural frequencies are approximately proportional to l^<-2> (l: leg length) and l^<-1>, respectively.
社団法人日本船舶海洋工学会の論文

11. Natural Vibration of Jack-Up Oil Rig

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