The development of offshore resources has traditionally relied on floating production systems, such as floating production storage and offloading (FPSO) units and semi-submerged ships [1
]. The hydrocarbons produced by an FPSO or from nearby subsea templates are transported through a pipeline or offloaded onto a tanker [2
]. In terms of offshore pipes, submarine pipelines, which are buried in a trench or laid on the seabed, are commonly used [4
]. Compared with conventional steel pipes, flexible pipe systems have the characteristics of higher flexibility, greater applicability, and enhanced recyclability [6
]. Flexible pipes can be classified into two primary types: bonded flexible pipes and unbonded flexible pipes [7
]. An unbonded flexible pipe usually comprises an outer polymeric layer, helical tensile armor, anti-wear layers, pressure armor layers, and an inner carcass layer [9
], as shown in Figure 1
. With the rapid development of techniques for exploiting deep-water resources, unbonded flexible pipes now play a significant role in transferring oil and gas resources from offshore platforms to onshore facilities.
Connecting the subsea infrastructure to surface facilities and transporting hydrocarbon products are the major applications of flexible pipes in the offshore oil and gas industry [10
]. However, the harsh deep-sea environment imposes significant challenges on flexible pipes, necessitating higher mechanical response and performance characteristics [11
]. According to the American Petroleum Institute (API), the terminations of a flexible pipe are defined as end fittings (as shown in Figure 2
), of which the functions are: (1) to provide a transition between the pipe body and the connecting component and (2) to transmit the loads acting on the pipe without allowing the pipe to fail [12
]. The widespread use of flexible pipes under more demanding operational conditions makes the safety performance of end fittings particularly important [13
Experiences in offshore environments have shown that the end fitting of a flexible pipe may be the weakest point [13
]. In service, end fittings will be subjected to similar environmental loads and conditions as the pipe, such as axial tension, inner pressure, and external hydrostatic pressure [14
]. Apart from mechanical load, the end fitting has to offer thermal insulation and be leak-proof [13
]. Therefore, if the sealing capacity of the end fitting is insufficient, there will be a risk of oil and gas leakage, which can have serious consequences. Because the composite structure of flexible pipe consists of many independent concentric metallic and polymeric layers, the structure of the end fitting is also multifaceted and complex [15
]. To ensure that the end fitting has adequate sealing performance, it is necessary to investigate its sealing capacity.
In general, existing studies related to the sealing performance of a structure have concentrated on the sealing rings [16
]. Typical examples are “O” rings, although these are different from the structure of an end fitting sealing assembly. In addition, a number of studies have focused on the mechanical behavior of flexible pipe, such as the instability of the armor wire [17
], the collapse of the carcass layers [18
], and fatigue reliability analysis [19
]. Although the sealing behavior of the end fitting is unlike the mechanical behavior of the flexible pipe body or the layers inside the end fitting, which have been extensively investigated [8
], there has been relatively little research on the sealing performance of the end fittings themselves.
The composite materials used in flexible pipe have different properties to metallic materials in terms of anisotropy, thermal expansion coefficient, thermal conductivity, and stiffness. Indeed, the structural properties of composite materials are more complex than those of metallic materials. This may lead to interface failure, such as when the composite material separates from the metallic material, thus losing the ability to maintain leak-tight integrity. Hatton et al. [15
] studied the design of sealing assemblies in different types of end fittings using finite element (FE) analysis and laboratory testing.
To understand the sealing performance of a mechanical connector in a subsea pipeline, Wang et al. [21
] investigated the critical condition of the sealing structure and created a new method to analyze the contact pressure of the sealing surface by examining the static metal sealing mechanism. An optimized design for a new subsea pipeline mechanical connector was proposed and an approach for determining the contact pressure of various dimensions was provided.
For an end fitting in a high-pressure pipe, it is challenging to create the necessary sealing performance. Fernando et al. [22
] developed an FE model of a flexible pipe end fitting and presented a method of evaluating the sealing performance of the sealing assembly and the design requirements for the sealing assembly of the end fitting. In their work, FE analysis was conducted using specially established leak criteria. In addition, an ultrasonic technique was used to measure the contact pressure at the metal-to-metal interface, which showed that their method had significant promise.
Li et al. [23
] considered the sealing performance of the sealing assembly in a deep-water flexible pipe end fitting and established an FE model using the ABAQUS software (6.11). They studied the key parameters under different conditions, providing further references for research on flexible pipe end fittings. By summarizing the general sealing criteria, Zhang [24
] introduced the concept of “contact pressure amplification factor” to evaluate the sealing capability of end fittings, while Marion et al. [25
] investigated the suitability of end fittings for high-temperature thermal cycling conditions using specially designed pipe samples and facilities that satisfy the API specifications.
Previous studies have analyzed and optimized the sealing criteria and the geometric parameters of the sealing assembly. However, there have been few studies related to the sealing behavior. In general, research on the sealing performance of the end fittings is not comprehensive. In this study, FE methods were used to develop a two-dimensional axisymmetric numerical model of the sealing structure of an end fitting, including the temperature field. The pressure penetration criteria were applied to this model to analyze the performance of the sealing structure.
3. FE Modeling Procedures
Considering the geometry and axisymmetric load characteristics of the sealing system, a two-dimensional plane axisymmetric solid model is employed to predict the seal performance. This section describes a thermal coupling sealing structure model of a flexible pipe with a design pressure of 20 MPa. This model was developed using FE with the ABAQUS software. Note that the factory acceptance test pressure is 1.5 times the design pressure, so the critical pressure acting on the end fitting is 30 MPa in service [12
]. The model comprises the inner sleeve, polymeric sheath, sealing ring, and end fitting body, from inside to outside. The inner and outer diameters of the flexible pipe are 139.7 mm and 209.5 mm, respectively, and the thickness of the polymeric sheath is 5 mm. The basic physical properties of each component are listed in Table 1
In addition, the polymeric sheath of the flexible pipe is usually made from high-molecular-weight polymeric materials, such as high-density polyethylene (HDPE) [35
]. In this study, the polymeric material parameters were taken from the work of Malta and Martins [36
], and the elastic–plastic properties are illustrated in Figure 4
To model the incompressible or quasi-incompressible characteristics of these materials, the planar axisymmetric hybrid element CAX4H is selected. Structured and sweep meshing techniques are used in each part of the model, and the mesh is refined around the contact area to improve the accuracy of the simulation. Because of the nonlinear contact characteristics of metallic and polymeric materials, the Mohr-Coulomb friction criterion is employed to describe the contact relationship (i.e., normal contact is “hard” and tangential contact incurs a penalty under a friction coefficient of 0.1) [37
To simulate the sealing process of the sealing ring, full constraints are applied to the inner sleeve, end fitting body, and polymeric sheath of the flexible pipe, while the sealing ring is free to undergo axial displacement. Pressure penetration is then applied to predict the effectiveness of the sealing. When analyzing the parameter sensitivity of the sealing structure, a temperature field is applied to the model. In addition, an implicit solver is used to obtain improved solution convergence and performance. The FE model of the sealing structure is illustrated in Figure 5
A classical unbonded flexible pipe is a combination of polymeric and metallic layers. An end fitting with reliable sealing properties is a precondition of a successful flexible pipe application. In this study, a hydraulic-thermal FE model was developed to investigate the sealing performance of a flexible pipe end fitting. The FE model employed the pressure penetration criterion and considered the temperature field, which is suitable for real applications. Using this model, the sealing principle was simulated and the influence of thermal effects on the sealing capacity was investigated.
The results showed that the maximum von Mises stress occurs on the sealing ring during the sealing process, whereas the stresses on the other components are relatively small. In terms of the contact pressure distribution, the maximum value appears in the sealing region, and is higher along path 2. By introducing pressure penetration, the sealing performance could be predicted and the dynamical pressure critical node was identified. In the model described in this paper, the critical fluid pressure of the end fitting is 35.5 MPa, which means that leakage occurs when the working pressure exceeds this value. In previous studies, thermal effects were usually omitted. The results in this paper, however, show that temperature is an important factor in the sealing performance of the sealing assembly, and should not be neglected. Thermal effects cause the components of the sealing structure to deform and expand. By increasing the contact length and contact pressure, the sealing ability of the sealing structure can be improved. Of course, very high temperatures are not appropriate, because the strain on the sealing ring should be considered in actual applications.