تحقیقات نظری بر روی هیدرودینامیک از لحاظ اسپار هندسی در فرکانس و زمان دامنه

Considering the coupling effects of the vessel and its riser and mooring system, hydrodynamic analyses of a geometric spar were performed both in frequency- and time-domains. Based on the boundary element method, the 3-D panel model of the geometric spar and the related free water surface model were established, and the first-order and second-order difference-frequency wave loads and other hydrodynamic coefficients were calculated. Frequency domain analysis of the motion Response Amplitude Operators (RAO) and Quadratic Transfer Functions (QTF) and time domain analysis of the response series and spectra in an extreme wave condition were conducted for the coupled system with the mooring lines and risers involved. These analyses were further validated by the physical model test results.

Spar platforms are mainly used in the sea areas of 500 m-3000 m water depth. Since the first installation of the Oryx Neptune Spar in the Gulf of Mexico in 1996, the spars have been increasingly popular due to their high stability, excellent motion performance and low cost of construction and maintenance. By far, as many as 14 spar platforms have been fabricated and applied in different regions of the world and they have become efficient facilities in deepwater oil and gas exploitation [1]. Along with the further development of the ocean engineering technology, the spar platforms are moving towards a period of flourish. In the early stage of spar platform development, two basic generations of spar platforms, which arerespectively classic spar and truss spar, were proposed successively [2]. Moreover, during the recent years, some new concepts of spar platforms have been put forward, such as the geometric spar [3], cell spar[4] and cell-truss spar[5]. The concept of geometric spar was proposed by Novellent Offshore LLC of USA. It combines some good qualities of the classic spar and truss spar designs, and it is hoped that improvement on motion performance and reduction in installation cost would be achieved. The model test of the geometric spar was conducted in the State Key Laboratory of Ocean Engineering at Shanghai Jiaotong University for its hydrodynamic performance [6]. The geometric spar has a unique configuration which is different from other types of spar platforms. In order to make a deeper research on this concept theoretically, a 3-D hydrodynamic vessel model with the mooring system and risers included is created, and numerical simulation of its wave loads and motion responses in a specified wave condition is carried out with the commercial program SESAM. The mooring lines, risers and the main body of the platform are regarded as a whole coupled system[7,8]. Thefrequency domain first-order Response Amplitude Operators (RAO), wave excitation forces, added mass, damping coefficients, second-order Quadratic Transfer Functions (QTF), and responses in time series in the wave of 100 year return period storm in the Gulf of Mexico are simulated and calculated. The calculated results are analyzed and compared with the model test results, thus the hydrodynamic performance of the geometric spar can be obtained.

By means of numerical simulation, the 3-D model of the geometric spar in the specified sea state has been established and the coupled hydrodynamic performance of the geometric spar is calculated and analyzed both in the frequency domain and in the time domain. The following conclusions as follows are reached: (1) Regarding the hull, IBC and the water in the moon-pool as a new integrated body, a simplified numerical model of the geometric spar is constructed, and the numerical simulation in both frequency- and time domains is performed. The calculated results include the frequency domain first-order RAO, wave excitation forces, added mass, second-order QTF, total damping coefficients and time domain response time series in the wave of 100 years return period storm in the Gulf of Mexico, and so on. (2) The characteristics of heave motion are independent of the incident wave angle, pitch and roll are symmetrical with regard to the incident wave angle, so are surge and sway, the yaw motion keeps inconspicuous and is close to zero at any incident wave angle. With the longitudinal incident bi-directional waves, the longitudinal second-order difference-frequency forces, i.e., forces for surge and pitch motions, are obviously larger than the forces of the other modes, all of the second-order difference -frequency forces are symmetrical about the line where the angular frequency 1