| Marine seismic exploration is now the most popular and effectiveapplication of the geophysical techniques. During the practical process ofsurvey, the stability and validity of signals ask for a certain requirement to thecontrol of towed seismic streamers both in depth and horizon, which is noteasy for the underwater towed cable in several kilometers long. The crux ofsolving this problem is to find a way to forecast the attitude of towed seismicstreamers accurately and master the dynamic characteristics of towed seismicstreamers.Based on the review and summary of towed seismic streamer system inthe recent decades, in this thesis, three-dimensional nonlinear dynamic modelof towed cable was built up, dynamic performance of the towed cable wasstudied through numerical and experimental method respectively, embeddedcontroller of towed cable was designed and robust adaptive attitude controlstrategy was studied.Considering bending rigidity of towed cable, this thesis analyzedexternal loads of towed cable and deduced its motion equation of each cableby differential element method under Lagrange-coordinate system. Asimplified approach was carried out to study the motion equation withoutconsidering bending rigidity.Using Box method to solve nonlinear equation of towed cables wasunstable and depended heavily on initial value. In view of that, generalizedmethod was employed for temporal discretization in numerical calculationprocess. The accuracy of generalized method was demonstrated when thegeneralized method was applied in simple nonlinear problem. Also, usingspectral radius method, the stability of Box method, backward differentiationformulae, generalized trapezoidal algorithm and generalized method wereresearched respectively.Mean coefficient matrix of complex nonlinear dynamic equations werecalculated by adopting same weighted method. Generalized method wasemployed to solve complex nonlinear dynamic equations. Box method wasused in spatial discretization and generalized method was used intemporal discretization.New-Rahpson method was adopted to solve two–point boundary valueproblems in calculation program of tree-dimensional nonlinear dynamicequations. In this calculation program, based on variation method, an adaptivespatial meshing method was deduced to reduce computational time andimprove calculation efficiency. In the time domain, adaptive time step methodwas used to improve non-convergence caused by strong nonlinear. Throughpractical calculation of towed cable, a comparative analysis was made withother algorithms.We edited the calculation program of the low-tension cable. Theadvantages were verified by comparative analysis of classical examples.Experimental research of the cable dynamic performance was carried out intowing tank. In the experiment, the velocity of the cable were set as0.55m/s,1.01m/s,1.5m/s, respectively, to study the dynamic response of thelow-tension cable. Cable tension and shear forces were measured at the topend and the middle span. Acceleration was measured at the fixed end of thecable for the effect of towed carriage vibration. The tension and shear forcewere measured at the top end and the point about one meter from the free end.Strengthened coefficient method was used to carry out steady state simulation.Both variable resistance coefficient method and constant resistancecoefficient method were used to study transient dynamic response of towedcable. Motion control and attitude optimization of towed cable are hot pointsand trends of seismic exploration technology. In order to control the towedarray effectively, this thesis designed an embedded controller of towed cable(SJTU-bird). Firstly, the mechanical structure, communication and controlmode were designed and a kind of pressure sensor with high precision andanti-jamming ability was developed to meet design requirement. To decreaseflow-induced noise and increase lift-drag ratio, the shape of hydrofoil wasdesign as ellipse. Also, dynamic performance of hydrofoil test, precision andanti-jamming ability performance of pressure sensor test, pressure integritytest and pivoting friction torque measurement test were carried out,respectively.The vibration of towed seismic streamers is a main reason for efficiencyreduction of the seismic survey and tail tangling. A vibration suppressionscheme of towed seismic streamers was investigated in this paper. The towedseismic streamer is divided into two spans, a controlled span, i.e., the lowtension span, and an uncontrolled span, i.e., the high tension span, by acontroller. Usually, the controller is called bird. The system model includes ahyperbolic partial differential equation with variable coefficient describing thetowed seismic streamers, and an ordinary differential equation describing thedynamic of bird. Robust based-model knowledge and adaptive controllers,based on the Lyapunov method, were designed to isolate the vibration oftowed seismic streamers caused by the course deviations of towing vessel，thevariations of towing vessel velocity and the variations of towed seismicstreamer density. The stability analysis of the whole system was carried outunder the action of robust based-model knowledge and robust adaptivecontrollers respectively. The finite difference scheme was used to validate theefficiency of the control law under different external disturbances.The conclusions from the researches above are gained as following:1)Theoretical analysis showed that generalized method has secondorder accurate and has good computational stability when the value rangeof is-0.7to-0.3. Spectral analysis results show that, compared with Box method, backward differentiation formulae and generalized trapezoidalalgorithm, generalized method that used in calculation program can notonly improve accuracy of solution, but also decrease computational time.2)The FFT spectrum analysis of low frequency accelerometer shows thatthe low order natural frequencies of carriage are not the same as that of thetowed cable. The fluctuation and FFT spectrum of the tension and shear forceunder steady towed velocity demonstrate the effect of vortex shedding and themain role of moment at the top end. The comparative analysis of numericaland experimental results shows that strengthened coefficient method hasbetter agrrement at steady state simulation.3)The results of dynamic performance of hydrofoil test, precision andanti-jamming ability performance of pressure sensor test, pressure integritytest and pivoting friction torque measurement test verified that all kinds oftechnical indexes can satisfy requirements under extreme sea conditions.4)The robust based-model knowledge and adaptive controllersexponentially and asymptotically drive the controlled span-towed seismicstreamer displacement to zero, respectively. The robust adaptive controllerprovides improved damping but not sacrifice isolation performance. Therobust adaptive controller can isolate the vibration from the vessel when theperiod external disturbance does not act on the controller. The robust adaptivecontroller can control the streamer under a small bound when the periodexternal disturbance acts on the controller.5)The adaptive scheme can tolerate the lack of knowledge of someuncertain parameters and directly execute the online adjustment of theparameters. Meanwhile, the proposed control law is robust and can resist themodel uncertainty due to parameter incertitude, model error and unknowndisturbance and so on. The control law only includes the velocity and slope,while it does not need the displacement which is difficult to measure inpractical operations. Thus it is easy to implement. The proposed control lawsare not only used in horizontal control, but also in depth control. |
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