The Studying On Towing Behaviour Of Bucket Foundation Offshore Platform With Air Cushions

As a kind of mobile facility for oil extraction, especially for the marginal oil and gas field development, using the bucket foundation offshore platform can dramatically decrease the cost and operation time during the construction due to the convenient installation and transportation. In addition, the offshore platform is movable. The towing of air cushion supported offshore platform with bucket foundation is a new towing style by virtue of parallel springs consisted of series coupling springs with air cushion and water plug in buckets. Based on systemically scale model tests, the studying on dynamic characters of bucket foundation platform during towing was conducted, supported with three air cushions and four air cushions, respectively. The course stability of this kind of platform, which was not only recycling but also towed with the air cushion support without barge, was explored. Further, the accelerations of the towed platform, the air cushion and water plug pressures in each bucket and tension of the tow line were determined by physical model tests. The wave and spectrum analysis were performed on the test signals. In addition, the action mechanism of the fluid solid coupling of floating body supported with the coupling spring consisted of air cushion and water plug. The influence rules of the intrinsic factors including the draught of buckets and the coupling spring stiffness and so on and extrinsic factors such as the tilt of towed platform, the towline length, the towing velocity, the waves and water depth on the course stability of towed platform were discussed. The following creativity work and achievements are included.Setting an extroverted angle before towing is helpful to improve the stability of towed platform; when the ration of towline length to platform length is less than 3, the course stability of towed platform was in good condition; shallower water depth could make the course stability more stable.Based on the test data and FFT spectrum analysis, the incentive mode between the towline and offshore platform with bucket foundation supported by air cushions was established. The vibratory frequency of the air cushion and water plug and the acceleration of the towed platform in each loading condition equal that of one harmonic component of the towline tension and consists with the result according to the wave frequency, the wave and towing velocity based on the microwave theory. In addition, the waveform recurrence period of the towline tension equals the value of the primitive period.The factor analysis was conducted with the principal component method in towing analysis based on the multivariate statistics. Then the cluster analysis was performed according to the correlation of the variables searched with above method. Finally, the worst towing combination and the typical mechanical parameters were found and transit conditions were classified by evaluation according to the different environments.The concept of the wave drag of air cushion was proposed in order to differentiate that from the wave drag of traditional ship with closed bottom. At lower towing speeds, the wave drag in which the drag due to the air cushion and water plug in the bucket is more important than those outside the bucket is dominant in whole resistances; at higher towing speeds, the form drag is dominant in whole resistances.