Elsevier

Thin-Walled Structures

Volume 159, February 2021, 107036
Thin-Walled Structures

Full length article
Theoretical investigation on hub structure design of subsea connectors

https://doi.org/10.1016/j.tws.2020.107036Get rights and content

Highlights

  • An analytical model of hub structure's axisymmetric bending deformation is proposed.

  • Stress solutions are acquired for a thick-walled cylinder bearing edge moment and shear force.

  • A case study is investigated to compare the stress design method with the ASME method.

Abstract

Subsea processing of oil/gas is performed by production system facilities, which are linked into a complete system by subsea connectors. High reliability and long service life of the connectors, defined by strict requirement of deepwater oil fields, raise the challenge of the design of the structure components to which the ASME-VIII code has been applied. The theoretical design problems of the main parameters of subsea connector's hub structure are detailed in this paper in comparison with the shortage of the ASME design method. Therefore, a new analytical model is developed, and is named stress analytical method (SAM). The hub structure is separated into two parts, i.e. a thick-walled cylinder and a hollow annular plate, and axisymmetric bending deformation problems of the thick-walled cylinder are proposed. The geometric equations, the constitutive equations, the physical equations and the equilibrium equations are developed to obtain the displacement analytical solution of the hub structure's thick-walled cylinder. The deformation continuity relationships between the thick-walled cylinder and the hollow annular plate are also established, and the analytical solutions of internal forces, displacements, rotation angles and stress components are acquired accordingly. The accuracy of SAM is investigated by comparing stress calculation results with Finite Element Method (FEM) results. A case study is carried out to compare SAM with ASME design method.

Introduction

The increase of oil and gas production comes from offshore fields in the globe in the past decades, and about 90% of the increase is from the deepwater. Subsea production systems (SPS) including subsea pipelines and risers are the main equipment and facilities for the development of deepwater oil and gas fields, and their applications to deeper and remoter waters as well as arctic areas are challenging the scientists and engineers, especially in increasing safety and reliability. It is always the main concern of the industry to reduce the capital expenditures (CAPEX) and operational expenditures (OPEX) of the deepwater fields by developing new technologies of subsea production systems. The technologies cover the whole period of life of the systems from initiation of concepts to design, manufacture, testing, transportation, installation, monitoring and inspection, maintenance, repair and decommissioning. Any technology shall aim for the maximum oil and gas production, the minimum environmental impact, the minimum risk to assets and personnel, the maximum profit and the highest safety of the subsea systems, and their advances are progressing each year.

A subsea production system consists of the seabed wellhead, subsea production X-tree, subsea tie-in to flowline system, and subsea equipment and control facilities to operate the well, as shown in Fig. 1. It can range in complexity from a single satellite well with a flowline linked to a fixed platform, FPSO (Floating Production, Storage and Offloading), or onshore facilities, to several wells on a template or clustered around a manifold that transfer to a fixed or floating facility or directly to onshore facilities. As the oil and gas fields move further offshore into deeper water and deeper geological formations in the quest for reserves, the technology of drilling and production has advanced dramatically. The latest subsea technologies have been proven and formed into an engineering system, namely, the subsea production system, which is associated with the overall process and all the equipment involved in drilling, field development, and field operation. More detailed information about subsea production system can be obtained in the ISSC technical committee report (Duan et al. [1]).

The subsea connector, located on the end of the jumper, is the key connection facility in the subsea production system (shown in Fig. 2). The main function of the subsea connector is to achieve locking and sealing between two different subsea facilities. If the sealing fails, oil and gas leakage accidents will occur. This will impact the safe operation of subsea production system. In the former study (Zhang et al., [2]), the sealing working principle of subsea connectors has been expressed, and the theoretical design method of the gasket, a key component of subsea connectors, has been developed. In the present work, the theoretical design method of the hub, another key component of subsea connectors, will be investigated. On the one hand, the hub component directly contacts with the gasket component and provides a compressive load on the gasket component to achieve sealing performance. On the other hand, the hub component contacts with the outer claw components, and cooperates with the claws to lock the locking mechanism. The design of hub component is constrained by the sealing performance. Meanwhile, the design of hub component will influence the locking performance of the subsea connector. Therefore, it will be of great significance to study the design methods of the structural parameters of the hub component. Among all the structural parameters of the hub component, the wall thickness dimension is the most significant. The reason is that the hub structure can be considered as the joint of two parts, i.e. a thick-walled cylinder and a flange ring plate, and the change of the two parts’ wall thickness dimension seriously influences the global size of hub component. In order to ensure enough structural strength, the wall thickness dimension is designed very large in some cases, which will increase the global size of hub structure and make the whole structure of subsea connector much larger and heavier. Therefore, it is necessary to investigate the design methods for such key parameters of wall thickness of thick-walled cylinder and flange ring plate.

As a central part of the subsea connector, the typical locking system is composed of claws, an actuator ring, a gasket ring, upper hub and lower hub, as shown in Fig. 3. It can be seen that the upper hub structure plays a key role in defining the geometrical dimensions of other components of the whole connector system, and the wall thickness of both the thick-walled cylinder and the flange ring plate shall be first set. Fig. 4 presents the loading conditions of the hub structure respectively in locking and unlocking states. Zhang et al. [2] made a detailed presentation on the geometrical relationships and load transmission from the components in locking and unlocking conditions. Appendix A shows the calculation of such loads which are the basis of the derivation of the equations of this paper.

The ASME Boiler and Pressure Vessel VIII 2 [3] code is the general design method for the pressure vessel flange structure, where various sealing connection mechanisms have been elaborated, such as flat gasket sealing, double-cone gasket sealing, Uhde sealing, Casale sealing and clamp sealing. The design method is to calculate the stress components on critical sections and to make a judgement whether the design parameters meet the strength criteria or not. The hub structure of the subsea connector is similar to the flange of the clamp connection mechanism [4], and the terminal of the flange structure presents an outward expansion tendency under the combined actions of axial loads, axial moments and radial loads. As a result, an axisymmetric bending deformation takes place in the flange structure. The advantage of ASME design method is that the mechanics analytical model and the solution process are easy to be conducted. However, the biggest disadvantage is the approximations of the analytical model, which assumes the flange as a beam structure and neglects the continuous deformation consistent condition in the joint section between hub's thick-walled cylinder and hollow annular plate. Therefore, it is inevitable to cause deviations if ASME design method is used to design the hub structure, and a more precise method should be developed to, especially to present the stress discontinuity.

The flange stress analysis method is the most widely used method to design the flange. In this method, the flange is separated into three parts: a hollow cylinder, the neck of the flange, and a ring plate [5]. The hollow cylinder, the neck of the flange and the ring are taken respectively as a thin-walled cylinder shell with constant thickness, a thin-walled cylinder shell with varying thickness, and a circular plate with a central hole. The axisymmetric bending equilibrium equations, the constitutive equations, the geometric equations and the boundary condition equations of the cylinder shell and the ring are established under the action of edge moment and the edge shear force. The conventional approach is to assume the bending moment and the shear force as unknown variables and to utilize these two unknown variables to express the displacement and rotation angle equations of the cylinder shell edge, then to impose the compatibility conditions of deformations to obtain the two unknown variables. In this way, analytical solutions of the displacement, rotation angle and stress components of the thin-walled cylinder shell edge are obtained, as can be seen in many literatures [[6], [7], [8], [9], [10], [11], [12], [13], [14]]. Such a stress analysis method not only takes into account discontinuous stress generated from the junction between cylinder and flange but also makes up for the drawbacks in ASME design method. However, the cylinder part of the subsea connectors’ hub structure is comparatively large and thick, and a simple application of the stress analysis method of cone neck flange is limited. Therefore, the axisymmetric bending of the thick-walled cylinder should be investigated under the action of edge moment and edge shear force, rather than the axisymmetric bending of thin-walled cylinder shells. Some recent literatures about functionally graded rotating thick-walled hollow cylinder with variable thickness [[15], [16], [17], [18]] also have to develop new models to obtain the analytical stress on the basis of thin wall-walled shell theory.

The space axisymmetric bending deformation theory is usually applied to the calculation of axisymmetric bending deformation of the thick-walled cylinder. The space axisymmetric bending deformation is one of classical elastic mechanics problems. The solution method of this deformation is generally finding a displacement function which is suitable to the bi-harmonic equation. Iyengar and Yogananda [19], Malova et al. [20], Vendhan and Archer [21], Ogaki and Nakajima [22], Chandrashekhara and Kumar [23], Li et al. [24], Ren [25], Zhu and Redekop [26], Chau and Wei [27], Meleshko and Tokovyy [28], and Wu et al. [29] have implemented such a method for solving practical engineering problems. The simplified models of these problems are all space axisymmetric bending deformations, and the axial stress boundary conditions of axial section is assumed to meet with some certain function distribution, and the shear stress is neglected. However, the axisymmetric bending deformation of the thick-walled cylinder of subsea connector is forced by the action of the edge load and edge shear force, and the shear force on the axial cross section will also take its effect. The above mentioned methods of solving such problems of axisymmetric bending deformation of thick-walled cylinder could not be simply applied when the shear force on the axial section could not be neglected.

To address the difficulties of the design of hub structural wall thickness in solving the problem of axisymmetric bending deformation of thick-walled cylinder under the action of the edge load and edge shear force, this paper will propose a new analytical model by critically analyzing the deformation theory of thin-walled cylinder shells. Taking the effect of the wall thickness into consideration, the geometric equations, the constitutive equations, the physical equations and the equilibrium equations are developed to obtain the displacement analytical solution of hub structure's thick-walled cylinder. Then the deformation continuity relationship between the thick-walled cylinder and the hollow annular plate is established, and the analytical solutions of internal forces, displacements, rotation angles and stress components are acquired accordingly. In this way, the analytical solution of the thick-walled cylinder axisymmetric bending deformation will be acquired under the action of edge loads. This analytical method is verified by FEM model, and a comparison investigation is also made between the analytical method and ASME design method.

Section snippets

Overview on the ASME design method

In the ASME design method, the hub structure is simplified into two parts, i.e. the thick-walled cylinder and the hollow annular plate (shown in Fig. 5). Firstly, the internal loads, i.e. edge bending moment MH and edge shear force QH, on the joint section are calculated. Then the stress components are calculated by the internal loads, and finally the wall thickness parameters’ design ranges are determined according to the strength criteria.

Validation and discussion

To evaluate the accuracy of stress analytical solutions of SAM, finite element modeling analysis is firstly adopted at the present stage as thick shell elements or solid brick elements can be easily used to model thick shells in the commercial finite software. A 2D axisymmetric finite element model is developed by using structural parameters of a given subsea connector. The axial stress and hoop stress are calculated for the thick-walled hub structure with loading the forces shown in Fig. 4(b).

Concluding remarks

An analytical model of the hub structure's axisymmetric bending deformation is established in this paper, which can provide the solution to axisymmetric bending deformations of the thick-walled cylinder and the hollow annular plate under the action of edge bending moment and edge shear force. The displacement analytical solution to hub structure's thick-walled cylinder is obtained by the deformation continuity relationship between the thick-walled cylinder and the hollow annular plate. The

Author statement

Menglan Duan: Conceptualization, Methodology, Investigation, Writing- Reviewing and Editing. Kang Zhang: Methodology, Investigation, Validation, Writing - original draft. C. Guedes Soares: Writing - review & editing. Jeom Kee Paik: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Ke Research and Development Program of China (Grant No. 2016YFC0303702), the 111 Project of China (B18054), the Natural Science Foundation of Jiangsu Province of China(Grant No. BK20200165), the National Science and Technology Major Project of China (Grant No.2016ZX05028-003-005).

References (35)

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