This paper presents an analytical scheme for predicting the collapse strength of a flexible pipe, which considers the structural interaction between relevant layers. The analytical results were compared with a FEA model and a number of test data, and showed reasonably good agreement. The theoretical analysis showed that the pressure armor layer enhanced the strength of the carcass against buckling, though the barrier weakened this effect. The collapse strength of pipe was influenced by many factors such as the inner radius of the pipe, the thickness of the layers and the mechanical properties of the materials. For example, an increase in the thickness of the barrier will increase contact pressure and in turn reduce the critical pressure.
End fitting is an essential component of flexible pipes enabling their connection between moving structures and pipes to create a complete pipe infrastructure. Designing the end fitting requires careful consideration of many factors including creating an effective barrier seal, termination of all the polymer and metal layers and anchoring armor layers so that the external loads of the pipe are transferred to the connecting structure without disturbing the seal. The most difficult challenge in designing end fittings for ultra-high pressure pipes is creating the necessary barrier sealing performance. In general, the seal needs to perform to approximately twice the design pressure of the pipe so that it can sustain the burst condition of the pipe. This requires the sealing components to be subjected to extreme loading where most of the sealing components are subjected to severe plastic deformation. Understanding seal deformations and quantifying and demonstrating sealing performance are important in the development of a reliable seal. This paper describes a method of evaluating the sealing performance of end fitting seals. Design requirements of end fitting seals are also described. The barrier seals in end fittings are normally created by swaging a metal seal ring in between the polymer barrier and the body of the fitting. This creates two leak paths, one between the metal ring and the barrier and the other at the metal-to-metal contact interface between the seal ring and the body. Achieving sufficient compression on the polymer barrier and creating sufficient contact pressure over a reasonable distance at the metal-to-metal interface are the key requirements in developing a reliable high pressure barrier seal. During this study, finite element models have been used to evaluate the contact pressure at seal contacts. These results, with a specifically developed leak criteria, have been used to evaluate the sealing performance. A brief description of the models and some specific results are presented. The FE model predictions are compared with the actual contact pressure measurements of a sealing setup.
In ultra deepwaters, helical tensile armour wires under certain loading conditions may exhibit localized deflection in either the radial (out-of-plane) or lateral (in-plane) direction when subjected to significant high axial compression and bending. This phenomenon is often referred to as birdcage buckling. This paper presents the development of a total strain energy approach for modeling the buckling and post-buckling behaviour of these wires and for illustrating the characteristics of such behaviors. The paper presents the summary of the full scale offshore (Deepwater Immersion) DIP tests performed by Wellstream to date, all of which have been successful in resisting the failure modes. In addition, the results of pressure chamber tests are also presented and discussed, especially some of the key test constraints which can have a significant influence on the final result as observed on a recent 10-inch structure chamber test. The presented model provides a tool to define and implement the design criterions against such failure modes. The further validation of the model through comprehensive tests is planned, which is critical to ensure the ability to economically design and optimize flexible pipe structures to prevent this phenomenon, and to facilitate more cost effective products that meet the ultra deepwater design challenges.
The polymer barrier is the most important component in unbonded flexible pipe, providing the leak-tight boundary for transporting hydrocarbon medium. Premature failure of the barrier during service can be costly and may lead to disastrous environmental consequences. Design of the barrier for 25 years’ service integrity is therefore a major requirement in the flexible pipe design process. However, the API design code does not give a specific procedure for the design of the barrier and is mainly concerned with the design of other layers in the pipe which are intended to provide integrity to the polymer barrier. The selection of barrier material depends on many factors including the service temperature/pressure range and pipe bending requirements. Polyvinylidene fluoride (PVDF) is used as a barrier material in cases where high pressure and relatively high temperature applications are involved. However, a hard polymer such as PVDF can be susceptible to crazing and cracking under specific conditions and therefore the use of PVDF in flexible pipe barriers requires critical consideration of the above issues. This paper discusses the general design requirements of a single layer barrier, and different barriers in relation to static and dynamic applications. The details of a qualification test program performed to establish service integrity of single layer Solef 60512 PVDF barriers is discussed. The unique testing facilities developed to test the integrity of the barrier are presented.
The primary aim of the present composite development program is to enhance access to deepwater fields in the Gulf of Mexico, Brazil, and West Africa. To accomplish that goal, composite materials are being incorporated in unbonded flexible pipelines to lower mass and enhance the overall system performance to expand the operational design envelope. In addition, the use of composite materials will allow a significant improvement in pipe operating pressure (>70 MPa), pipe operating temperature (>125C) and due to increased CO2 and H2S resistance, will improve sour service performance and lifespan. Composite materials are well known for their low density and high specific strength, stiffness and fatigue performance. These properties are desirable and will certainly enhance pipe performance, but the overall performance of the pipe during all stages of manufacture and deployment must be considered, as well as a conservative approach to introducing these new materials. Some of the key factors that need to be assessed are material failure modes under varied pipe loadings, dynamic interactions and exposure to severe oil field environments. There are several individual standards, specifications and joint industry projects (JIPs) focused on composite pipes that address some of these issues, but there is also a general lack of consensus with regard to testing standards and understanding of the long-term performance. As flexible pipe suppliers, the industry must aim to provide performance assessments and address all key challenges to allow the flexible pipe industry to build confidence in the new and enabling composite pipe technologies. In a previous paper, we presented design concepts and a toolbox approach to construct different composite pipe solutions to meet all the aforementioned performance parameters. The present paper selectively highlights important failure modes and design considerations, demonstrates an understanding of behavior in the matrix and fiber phases, and addresses concerns related to the chemical performance of composite materials. The present paper also highlights and addresses some of the concerns of composite pipes and focuses on areas for future development and testing. These results will help support the selection and standardization of analysis tools and testing methods across the industry. Bespoke testing capabilities to address the relevant failure mechanisms and installation strategies for composite pipes will also be discussed.
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