S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Program in Offshore Plant Engineering (협동과정-해양플랜트엔지니어링전공) Theses (Master's Degree_협동과정-해양플랜트엔지니어링전공)
A Study on the Ultimate Strength of Flexible Pipes using Numerical Analysis
수치해석을 이용한 유연식 파이프의 극한강도 평가에 관한 연구
- Yim Ki-Ho
- 공과대학 협동과정해양플랜트엔지니어링전공
- Issue Date
- 서울대학교 대학원
- 학위논문 (석사)-- 서울대학교 대학원 : 협동과정-해양플랜트엔지니어링전공, 2015. 8. 장범선.
- A riser is the section from the production unit to the subsea pipe line that can transfer the fluid.
The design of riser must concern about strength and fatigue damages of the whole length due to the dynamic motions depending on the location. Careful concerns are required on connections and touch-down area with sub-surface.
Floating production units which are ship-shaped such as FPSO, FLNG and FSO always experience dynamic loadings and have large motions relative to subsea. In this case, the flexible pipe is suitable for the riser since the SCR is highly concerned about failures of fatigue and buckling.
The flexible pipe is a complex structure which consists of several layers that have different functions, materials, and geometries. The size of the flexible pipe is relative to rate of fluid, and high external &internal pressure involves increments of both thickness and the number of layers. The variations cause the change of stiffness in flexible pipes. In case of existing flexible pipes, the stiffness and the strength are well-known, however it can only be provided to a new design by the structural analysis.
To verify the safety of the riser, global riser analysis has to be performed. This analysis evaluates the motion of floating unit and the riser including configuration in terms of specific environments. The axial and bending stiffness is essential for this analysis
Although flexible pipes are assessed safely, unexpected environmental conditions or uncontrollable situations may suffer the flexible pipe during operation. Therefore the flexible pipes might expose the structural failure due to its likely cause.
Based on the report from MCS. Kenny and PSA on flexible pipes, failure causes may be categorized into ageing and excessive loads which raise deformation, buckling, rupture, crack and so on. Each failure cause takes almost 50% of total failures, respectively. The failures by excessive loads are closely related to the ultimate strength. Hence, the failures can be evaluated by the analysis of ultimate strength for flexible pipes.
Recent offshore projects are going deeper and bigger, accordingly a new flexible pipe design is required and faced challenges to overcome a harsh environmental condition. Consequently, the reliable structural analysis is essential for the safety of flexible pipes.
Theoretical (analytic), numerical and experimental approaches are available for structural analysis of the flexible pipe. In the theoretical manner, the equilibrium matrix is derived from relations of virtual works versus internal energy and strains versus displacement for each layer. Then, total stiffness of the flexible pipe is computed by combining contributions of relevant stiffness of each layer in regard to the deformation taking into account gap between layers.
The finite element model can also provide a solution. Each layer is modeled proper elements including contact. The final solution is computed by the iterative Newton-Raphson method. Non-linear contact, complex geometries, non-linear material property can be considered. However, due to the high degree of freedoms, the size of model is limited.
The experimental approach, the most reliable method, has also limitations for the length and the number of specimens.
Many of comparative studies have been introduced. Most of their result was focused on the behavior of flexible pipes, while comparisons of stress, strength are not easily found.
Thus, this study quantifies stiffness and stress in tensile armours obtained from both approaches with respect to various loadings, based on linear assumptions. Furthermore, practical manners for the computation of ultimate strength considered non-linearity of flexible pipe are provided.