Numerical and experimental analysis on motion performance of new sandglass-type floating body in waves
Introduction
As ocean oil and gas exploration expands towards the deep sea in harsh environments, FPSO (floating production, storage and offloading unit) has played an important part in the development mode, due to its large capacity of oil storage, multifunction, strong adaptability and convenience for maintenance and repair. In the future, FPSO will have wide and good application prospects [2], [14], [4].
Traditional ship-type and cylindrical FPSOs exhibit some performance shortcomings [17], [19]. Firstly, the traditional ship-type floating body is extremely sensitive to the wave direction. Secondly, with the complicated environment and single point mooring system, FPSO would be usually in the oblique (beam) sea, which may result in poor heave and roll performance. Next, because the longitudinal scale is extremely large, the phenomenon of wave impact and green water is common and would damage the deck structure. Furthermore, the vane effect will cause frequent yaw motion, which results in the serious wear and tear of turret structure. As a result, regular maintenance and repair would significantly increase production costs. With a greater longitudinal scale, the relatively large bending moment of hogging and sagging will bring about the severe deformation and fatigue failure. Finally, the natural period of heave motion for the cylindrical floating body is still in the centralized area of wave energy and thus the heave motion response is very large.
Therefore recently some new concepts of ocean engineering structures are proposed. On one hand, based on the cylindrical FPSO, the auxiliary structures (such as skirts, heave plates and moon-pool, etc.) are used to improve heave motion performance. Therein the MPSO (mono-column production, storage and offloading system) is one of typical concepts [5], [6], [7], [16]. On the other hand, a new concept of floating body with an innovative sandglass-type shape was presented by Huang et al. [9], [10], [11] Yao et al. [20] and Wang et al. [18], to solve the performance limitations of traditional ship-type and cylindrical FPSOs.
In this paper, firstly the new concept of sandglass-type FPSO is introduced. Next, by using classic boundary element method (BEM) based on potential flow theory, the effects of shape parameters on motion performance of sandglass-type model are studied. Furthermore, in order to provide guideline and scheme for the shape design of new floating body, the engineering estimation expressions of wave frequency versus minimum heave motion RAO for the new floating model are theoretically and mathematically deduced. On this basis, the basic functions of cylindrical FPSO “Sevan Piranema” (i.e., the design values of load capacity, displacement, storage space, topside area, etc.) are incorporated into the design for new sandglass-type FPSO vessels. Finally, the new sandglass-type floating body is compared with cylindrical model by numerical method and experimental analysis to verify the accuracy of the numerical method in this paper and show the advantages of hydrodynamic performance for the new sandglass-type FPSO.
Section snippets
Description of new sandglass-type FPSO
The new sandglass-type FPSO has an innovative floating body with sandglass-type shape, which has not only larger spaces of oil storage than traditional ocean platforms but also better hydrodynamic performance and adaptability to extreme sea environment than traditional ship-type and cylindrical FPSO [18]. On this basis, various modules (such as production, processing, storage, offloading, heating, living functions, etc.) are equipped on the upper deck, respectively, as shown in Fig. 1. Thus,
Effects of shape parameters on motion performance of sandglass-type model
Firstly, sandglass-type shape of new floating body is introduced and its numerical model is created. Then, based on wave potential theory [1], [14], [21], here numerical boundary element method of this paper is taken to study the effects of shape parameters on motion performance of sandglass-type model.
Further discussion on heave motion performance of sandglass-type floating body
Here, in order to search a convenient and available design scheme for model shape to control and improve the heave motion performance of sandglass-type floating body, further discussion on hydrodynamic parameters of heave motion is made.
Due to the difficulty of calculation accurately on viscous damping and coupled heave-pitch nonlinear stiffness, firstly the wave frequency versus minimum heave motion RAO, which not only decides the position of maximum peak and second highest peak but also can
Validation of test cases and discussion
In order to better verify the validaty of design guideline and scheme of new sandglass-type floating body, two test cases are designed and calculated by numerical method and engineering estimation formula. Furthermore, by taking cylindrical floating model of “Sevan Piranema” FPSO for test case, the new sandglass-type floating model with the same basic functions (displacement, storage space, upper deck area, etc.) is designed by the guideline and scheme in this paper. Finally, the hydrodynamic
Conclusion
In order to solve the performance limitations of traditional ship-type and cylindrical FPSO, this paper presents a new concept FPSO with innovative sandglass-type floating body. Then the hydrodynamic performances of new floating model are studied by the qualitative and quantificational analysis. Furthermore, based on the design scheme and functions of cylindrical FPSO “Sevan Piranema”, a new sandglass-type floating body can be achieved and discussed by numerical and experimental methods.
Acknowledgment
The authors are grateful to all organizations that funded the research in this paper, which was financially supported by the National Innovation Team Foundation under grant no. 50921001 (China), the National Natural Science Foundation of China (Grant No. 11202047), the Scientific Research Foundation for Introduction of Talent under grant no. DUT13RC(3)46 (China) and Natural Science Foundation of Liaoning Province under grant no. 2015020157 (China).
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