Investigation On Couplng Dynamic Response Of The Floatover System

With the oil and gas resources on land exhausting and demand for energy increasing, in recent years the offshore oil industry flourishes. As essential equipment for offshore oil and gas development, offshore platforms are in a growing demand. However, the comparatively lagging technology and equipment for platform installation have restricted the development of oil and gas exploitation greatly. Larger and more integrated offshore platforms lead to the increasing weight of platform topsides, whose installation is both risky and high value-added, and has become a research focus in ocean engineering.Floatover is a new method for integrated topside installation. In this method, the topside is constructed as one large integrated module and assembled completely on shore, then is transported to the installation site by a barge, and finally is transferred from the transportation barge onto the supporting structure through barge ballasting. Floatover has some substantial advantages, such as high capacity, short project time and low costs. It has been applied to various kinds of platforms in many sea areas. The method is becoming mature and sophisticated, and therefore has become the first consideration for topside installation.The floatover system is a complex multi-body coupled system. The barge and topside interact with each other through LMUs and DSUs, while subjected to loads from the jacket and seabed by means of mooring lines, mating lines and fenders. In addition, the mean draft and ballast of the barge and the dynamic characteristics of floatover system are also changed continually in the whole installation process, which makes the problem much more complex. This paper adopts a combination of theory study, numerical calculation, model test and prototype monitoring to investigation the dynamic response of multi-body coupling, hydrodynamic performance for a barge and hydrodynamic interaction between multiple barges.In the second chapter, force analysis and numerical simulation are performed to study the core problem(mating between multiple bodies and load transfer) in floatover installation. The coupling mechanism of LMU and DSU is analyzed, and relational expression between the coupling force and the relative motion of the topside and barge is deduced. The simulation of time-varying mass distribution of the barge from changing ballast is also carried out. Based on the second law of Newton, the equations for multi-body coupled motions in calm water are established, and the motion responses in the load-transfer process are simulated successfully both in calm water and under sine excitation.In the third chapter, the appropriate calculation method is selected to calculate hydrodynamic loads of the barge. Considering that the barge goes down slowly, an assumption is put forward to simplify the problem. First, multiple wet surface boundary value problems are solved in the frequency domain based on the three dimensional potential flow theory, then the hydrodynamic coefficients obtained are used to estimate the corresponding value at any draft by interpolation, and finally the hydrodynamic loads in time domain can be obtained through time-frequency transformation.In the fourth chapter, multi-body coupling time-domain motion equation is established by considering all the external forces and interaction between the barge and the topside, and is solved to get their movement and load on each connector. The calculated results are compared with model test results to verify the reliability of the numerical calculation method and accuracy of the computing program. In addition, the whole dynamic process of load transfer is simulated in the time domain, considering the effect of draft changing. In the simulation, the ballast of the barge is simulated by point distribution, and the buoyancy is calculated by integral directly on the changing wet surface.In the fifth chapter, a series of comprehensive model tests are carried out to investigate the dynamic response of the floatover system used in Liwan 3-1 platform installation. The models for the barge, jacket, topside, and the connecting mechanism are established, and various environment conditions are considered in the model tests. The whole floatover process is decomposed into several steps and studied respectively, to get the dynamic characteristics and laws of installation system.In the sixth chapter, for actual operation on the sea, a corresponding real-time monitoring integrated system is set up, including the hardware of the measuring device, data transmission, data analysis and 3D simulation. The content of measurement includes environmental condition, barge motion, and stress on the fender. Using this monitoring system, the whole installation process of Liwan 3-1 topside is monitored and analyzed, and the significant measurements provide guidance on the model test study and numerical calculation.In the seventh chapter, based on the previous study on single barge floatover, coupled dynamic response for the twin-barge floatover is also investigated. First, hydrodynamic interaction between multiple bodies is focused on, and the system of two side-by-side barges is chosen as the analysis model. Corresponding model tests are carried out to obtain the motion responses of the two barges and to explore the mechanism of multi-body interaction. Modification of the hydrodynamic results for the two barges obtained by the potential flow method is realized by comparing with the experimental results. Then both the hydrodynamic interaction between multi-floating bodies and coupled effect of various connectors are taken in to consideration. Time domain calculation program of this paper is used for simulating dynamic response tri-barge load-transfer, and relative motions of the barges and the forces acting on them are obtained. By comparing with model test results, the calculated dynamic response is validated, which shows that the simulation program is suitable for coupling analysis of multi-floating bodies.In summary, multi-body coupling response, dynamic performance of a floatover system and hydrodynamic interaction between barges are investigated in this paper. Load transfer process is simulated by numerical calculation, and the simulation is validated by model test and real-time monitoring. Through analyzing the dynamic response characteristics under different conditions, the risky stages and dangerous parts of installation system are concluded, which provides valuable guidance for engineering design and offshore operation. In addition, the real-time monitoring integrated system in the our achievement has been directly applied in many marine operations. It accurately monitored many important data and reduced the risk of offshore installation effictively, which is of great importance.