Pipe-in-pipe Research Articles

Nonplanar flow-induced vibrations of a cantilevered PIP structure system concurrently subjected to internal and cross flows

Pipe-in-pipe (PIP) structures are widely used in offshore oil and gas pipelines to settle thermal insulation issues. A PIP structure system usually consists of two concentric pipes and one softer layer for thermal insulation consideration. The total response of the system is related to the dynamics of both pipes and the interactions between these two concentric pipes. In the current work, a theoretical model for flow-induced vibrations of a PIP structure system is proposed and analyzed in the presence of an internal axial flow and an external cross flow. The interactions between the two pipes are modeled by a linear distributed damper, a linear distributed spring and a nonlinear distributed spring along the pipe length. The unsteady hydrodynamic forces due to cross flow are modeled by two distributed van der Pol wake oscillators. The nonlinear partial differential equations for the two pipes and the wake are further discretized by the aid of Galerkin’s technique, resulting in a set of ordinary differential equations. These ordinary differential equations are further numerically solved by using a fourth-order Runge–Kutta integration algorithm. Phase portraits, bifurcation diagrams, an Argand diagram and oscillation shape diagrams are plotted, showing the existence of a lock-in phenomenon and figure-of-eight trajectory. The PIP system subjected to cross flow displays some interesting dynamical behaviors different from that of a single-pipe structure.

Lateral Buckling Theory and Experimental Study on Pipe-in-Pipe Structure

With the increasing depth of marine oil and gas exploitation, more requirements have been proposed on the structure of deep-sea oil pipelines. The influencing factors of lateral buckling of a pipe-in-pipe (PIP) structure containing initial imperfections and its critical force were investigated in this study by conducting an experiment, a finite element analysis, and a theoretical derivation. The change laws on the influence of initial imperfections of the PIP structure during thermal loading were revealed through an experimental study by using imperfection amplitude and wavelength as parameters. Appropriate finite element models were established, and the influences of initial imperfections, pipe-soil interaction, and the height and the number of centralizers on the global buckling critical force of the PIP structure were analyzed. The formulas of global buckling critical force of inner and outer pipes and that under pipe-soil interaction was obtained by using a theoretical derivation method. A comparative verification with experimental and finite element (FE) models result was conducted, which provided a corresponding basis for steel pipeline design.

Effectiveness of Using RFHDS Connected PIP System for Subsea Pipeline Vibration Control

Pipe-in-pipe (PIP) system can be considered as a structure-tuned mass damper (TMD) system by replacing the hard centralizers by the softer springs and dashpots to connect the inner and outer pipes. With properly designed connecting devices, PIP system therefore has the potential to mitigate the subsea pipeline vibrations induced by various sources, such as earthquake or vortex shedding. This study proposes using rotational friction hinge dampers with springs (RFHDSs) to connect the inner and outer pipes. The rotational friction hinge dampers (RFHDs) are used to absorb the energy induced by the external vibration sources and the springs are used to provide the stiffness to the TMD system and to restore the original locations of the inner and outer pipes. To investigate the effectiveness of this new design concept, detailed three-dimensional (3D) finite element (FE) model of the RFHD is developed in ANSYS and the hysteretic behavior of RFHD is firstly studied. The calculated hysteretic loop is then applied to the 3D PIP FE model to estimate the seismic responses. The effectiveness of the proposed system to mitigate seismic induced vibrations is examined by comparing the seismic responses of the proposed system with the conventional PIP system. The influences of various parameters, such as the preload on the bolt, the friction coefficient and the spring stiffness, on the RFHD hysteresis behavior and on the seismic responses of PIP system are investigated and some suggestions on the RFHDS design are made.

An experimental and numerical study on inward integral buckle arrestors for pipe-in-pipe systems

ABSTRACTA type of improved device, designated as inward integral buckle arrestor, is introduced for the pipe-in-pipe (PIP) systems. The experiments have been performed to reproduce the buckle propagation and crossover scenarios of the PIP installed with such devices in a hyperbaric chamber under external pressure. Based upon the experimental observations, dedicated finite element models for test specimens are developed to simulate the whole process of a propagating buckle crossing over an inward integral arrestor to the downstream PIP under the quasi-static, steady-state conditions. Through extensive parametric dependence analyses, the potential buckle propagation and crossover modes are identified, and a reasonable empirical formula of the crossover pressure of the arrestors for the PIP systems is established. The corresponding empirical design formula and lower bound envelope line are proposed to estimate the arresting efficiency of such devices in the preliminary design stage.

On collapse of the inner pipe of a pipe-in-pipe system under external pressure

Collapse of the inner pipe of a pipe-in-pipe (PIP) system under external pressure is studied experimentally and numerically herein. Hyperbaric chamber test results of three PIP systems with identical inner pipes and different outer pipes are presented. It is observed that the geometric and material properties of the outer pipe affect the collapse pressure of the inner pipe. Using validated finite element analyses (FEA), a parametric study is conducted and collapse mechanisms of PIPs with various combinations of outer and inner pipes with practical range of diameter-to-thickness ratios (D/t) between 15 and 40 are discussed. Empirical expressions are proposed for the collapse pressure of the inner pipe (Pci), and its upper and lower bounds. The proposed empirical equation for Pci, is shown to agree well with the experimental results of the tested PIPs. Moreover, two distinctive modes of collapse in the inner pipe are identified and discussed.

Numerical study and parametric analysis of the propagation buckling behaviour of subsea pipe-in-pipe systems

Propagation buckling mechanisms in pipe-in-pipe (PIP) systems with thin and moderately thin carrier pipes with a diameter-to-thickness (Do/to) ratio in the range 26–40 are investigated using 2D analytical and 3D nonlinear (material and geometry) finite element (FE) models. The FE models are validated against hyperbaric chamber tests of a PIP system with Do/to of 30. Using the validated FE model a parametric study is conducted and two distinct buckle propagation modes in PIPs are observed. Empirical expressions for each mode are proposed and are found to be different from previous expressions suggested for PIPs with (Do/to) ratio in the range 15–25.

Subsea Pipeline Lateral Buckling Design—Strain Concentration or Strain Capacity Reduction Factors

A strain concentration factor is typically incorporated in the higher-pressure and high-temperature (HPHT) pipeline lateral buckling assessment to account for nonuniform stiffness or plastic bending moment. Increased strain concentration can compromise pipeline low cycle fatigue and lateral buckling capacity, leading to an early onset of local buckling failure. In this paper, the philosophy of local buckling mitigation using the strain concentration factor is examined. The local buckling behavior is evaluated. Global strain reduction and evolution against buckling are analyzed with respect to varying joint mismatch level. The concept of a strain reduction factor (SNRF) due to joint mismatch is developed based on the global strain capacity reduction with reference to the uniform configuration. It is demonstrated that the SNRF in terms of strain capacity reduction is a unique characteristic parameter. As opposed to strain concentration, it is an invariant insensitive to evaluation methods and design strain demand level, hence more representative as a limiting design metric to maintain the safety margin. The rationale for its introduction as an alternative to the strain concentration factor is outlined and its benefits are established. The method for obtaining the SNRF and its application is developed. The discernible difference and scenarios for application of either factor are discussed, including low and high cycle fatigue, linearity and stress concentration, engineering criticality assessment (ECA), and lateral buckling. Additional causal factors giving rise to mismatch such as pipe schedule transition and buckler arrestor are also discussed. Iterations of finite element (FE) analyses are performed for a pipe-in-pipe (PIP) configuration in a case study.

Effectiveness of using pipe-in-pipe (PIP) concept to reduce vortex-induced vibrations (VIV): Three-dimensional two-way FSI analysis

Pipe-in-pipe (PIP) systems have been increasingly used in offshore applications because of their favourable thermal insulation capacity. Very recently, the conventional PIP system was slightly revised by using carefully designed springs and dashpots to connect the inner and outer pipes. This revised PIP system can be considered as a structure-Tuned Mass Damper (TMD) system. It therefore has the potential to mitigate the offshore structural vibrations induced by various sources such as earthquake excitation and/or vortex shedding. This paper carries out three-dimensional (3D) numerical simulations to investigate the effectiveness of the proposed method. The cross-flow oscillation of the conventional and optimized PIP systems are numerically investigated by developing a two-way coupled Fluid-Structure Interaction (FSI) framework for computational fluid dynamics (CFD) analysis. The developed FSI model is validated with the available experimental and numerical benchmark data on a single cylinder. This validated model is then extended to the PIP system to study its efficiency for Vortex-Induced Vibration (VIV) suppression. Numerical results show that the optimized PIP system can noticeably reduce VIV.

A systematic approach to pipe-in-pipe installation analysis

In the present study, the selection of a pipe-in-pipe (PIP) system which includes an installation analysis modelling technique is investigated. The PIP system has become the standard pipeline design for subsea field developments, especially for deep water fields. One of the most important design challenges for engineers is the pipeline installation design. A design underestimation of loads could result in serious damage, while overestimation would result in high operation costs. An accurate or improved modelling method is definitely required for the PIP system. The modelling of a finite element single pipe-lay simulation has been studied and discussed by many authors, and is very much understood by the people in the industry. However, modelling complex PIP systems for pipe laying simulations in order to obtain accurate results is quite challenging. To build the most economic finite element (FE) model for the pipe-lay simulation of PIP systems, various modelling aspects of the installation have been studied separately. In this work, three types of FE models which are two models with different simplifications and one actual PIP model were established and compared. Finally, a systematic approach selection for performing a PIP installation is presented.

Propagation Buckling in Subsea Pipe-in-Pipe Systems

AbstractThis study investigates propagation buckling of subsea pipe-in-pipe (PIP) systems under hydrostatic pressure. Unlike in previous studies, PIP systems consisting of carrier pipes with a diam...

Passive vibration control of cylindrical offshore components using pipe-in-pipe (PIP) concept: An analytical study

This paper aims to propose a design of using the revised PIP system to control vortex induced vibrations (VIV) of cylindrical structural components. Analytical studies are carried out to examine the effectiveness of the proposed method. To this end, the fluid-induced vibration of a single pipe is investigated and the equation of motion of the system is solved and validated by the experimental data. This single pipe system is then extended to the proposed PIP system and its dynamic behaviour under the excitation of vortex shedding is simplified as a Two-Degree-of-Freedom (2DoF) system. The optimal damping ratio and tuning frequency of the revised PIP system are obtained through a series of numerical searching technique and sensitivity analyses. Explicit formulae are also derived for practical ease of use. The governing equation of the system under VIV is solved in the time domain using the MATLAB/Simulink program. The responses of the single pipe system and the proposed PIP system due to vortex shedding are calculated and compared. Analytical results demonstrate that the proposed PIP system can significantly suppress the VIV of offshore cylindrical components. It could be then a cost-effective passive solution to suppress vibration of offshore cylindrical components.

Parameters study on lateral buckling of submarine PIP pipelines

In meeting the technical needs for deepwater conditions and overcoming the shortfalls of single-layer pipes for deepwater applications, pipe-in-pipe (PIP) systems have been developed. While, for PIP pipelines directly laid on the seabed or with partial embedment, one of the primary service risks is lateral buckling. The critical axial force is a key factor governing the global lateral buckling response that has been paid much more attention. It is influenced by global imperfections, submerged weight, stiffness, pipe-soil interaction characteristics, et al. In this study, Finite Element Models for imperfect PIP systems are established on the basis of 3D beam element and tube-to-tube element in Abaqus. A parameter study was conducted to investigate the effects of these parameters on the critical axial force and post-buckling forms. These parameters include structural parameters such as imperfections, clearance, and bulkhead spacing, pipe/soil interaction parameter, for instance, axial and lateral friction properties between pipeline and seabed, and load parameter submerged weight. Python as a programming language is been used to realize parametric modeling in Abaqus. Some conclusions are obtained which can provide a guide for the design of PIP pipelines.

Using pipe-in-pipe systems for subsea pipeline vibration control

Pipe-in-pipe (PIP) systems are increasingly used in subsea pipeline applications due to their favourable thermal insulation capacity. Pipe-in-pipe systems consist of concentric inner and outer pipes, the inner pipe carries hydrocarbons and the outer pipe provides mechanical protection to withstand the external hydrostatic pressure. The annulus between the inner and outer pipes is either empty or filled with non-structural insulation material. Due to the special structural layout, optimized springs and dashpots can be installed in the annulus and the system can be made as a structure-tuned mass damper (TMD) system, which therefore has the potential to mitigate the pipeline vibrations induced by various sources. This paper proposes using pipe-in-pipe systems for the subsea pipeline vibration control. The simplification of the pipe-in-pipe system as a non-conventional structure-TMD system is firstly presented. The effectiveness of using pipe-in-pipe system to mitigate seismic induced vibration of a subsea pipeline with a free span is investigated through numerical simulations by examining the seismic responses of both the traditional and proposed pipe-in-pipe systems based on the detailed three dimensional (3D) numerical analyses. Two possible design options and the robustness of the proposed system for the pipeline vibration control are discussed. Numerical results show that the proposed pipe-in-pipe system can effectively suppress seismic induced vibrations of subsea pipelines without changing too much of the traditional design. Therefore it could be a cost-effective solution to mitigate pipe vibrations subjected to external dynamic loadings.

Buckling response of pipe-in-pipe systems subjected to bending

The buckling response of pipe-in-pipe(PIP)systems subjected to bending is investigated in this paper. A set of parameterized models are established to explore the bending characteristics of the PIP systems through eigenvalue buckling analysis and nonlinear post-buckling analysis. The results show that the length of PIP systems and the height of centralizers are the most significant factors that influence the buckling moment, ultimate bending moment and buckling mode; the other geometric characteristics, such as initial geometric imperfection and friction between centralizers and outer pipes, evidently influence the post-buckling path and ductility of PIPs; the equivalent bending stiffness is dependent on the length and centralizers. Moreover, the range of equivalent bending stiffness is also discussed.

Validating pipe-in-pipe vibration software model to accurately determine ground-borne noise and vibration above construction tunnels

The London Crossrail project required that re-radiated noise from the construction of the tunnels should achieve exceptionally stringent criteria with respect to ground-borne noise and vibration. The challenge has been to implement robust vibration modelling for the movement of construction trains within the tunnel in order to derive noise levels within sensitive properties. The Pipe-in-Pipe (PiP) software model has been modified and refined empirically utilising the measurement of exceptionally low vibration levels emanating from the temporary underground construction railway track.

Numerical study on upheaval buckling of pipe-in-pipe systems with full contact imperfections

Upheaval buckling may occur in high-temperature and high-pressure subsea pipe-in-pipe (PIP) systems. Finite element method and dimensional analysis method are used to investigate the upheaval buckling response and critical buckling force of PIP systems with full-contact imperfections. Results show that imperfections, along with clearance (the gap between the centralizer and outer pipe) and stiffness ratio of the inner and outer pipe, influence buckling response and buckling force. Two empirical formulas that correspond to two typical imperfection profiles are proposed to calculate the critical upheaval buckling force of PIP systems on the basis of dimensional analysis and numerical results. The proposed empirical formulas show good accuracy.

The fictitious force method for efficient calculation of vibration from a tunnel embedded in a multi-layered half-space

This paper presents an extension of the Pipe-in-Pipe (PiP) model for calculating vibrations from underground railways that allows for the incorporation of a multi-layered half-space geometry. The model is based on the assumption that the tunnel displacement is not influenced by the existence of a free surface or ground layers. The displacement at the tunnel–soil interface is calculated using a model of a tunnel embedded in a full space with soil properties corresponding to the soil in contact with the tunnel. Next, a full space model is used to determine the equivalent loads that produce the same displacements at the tunnel–soil interface. The soil displacements are calculated by multiplying these equivalent loads by Green׳s functions for a layered half-space. The results and the computation time of the proposed model are compared with those of an alternative coupled finite element–boundary element model that accounts for a tunnel embedded in a multi-layered half-space. While the overall response of the multi-layered half-space is well predicted, spatial shifts in the interference patterns are observed that result from the superposition of direct waves and waves reflected on the free surface and layer interfaces. The proposed model is much faster and can be run on a personal computer with much less use of memory. Therefore, it is a promising design tool to predict vibration from underground tunnels and to assess the performance of vibration countermeasures in an early design stage.

A structural health assessment method for shield tunnels based on torsional wave speed

Following recent rapid developments in tunnel engineering in China, the heavy structural maintenance work of the future is likely to pose a great challenge. Newly developed vibration-based health assessment and monitoring methods offer good prospects for large-scale structural monitoring, hidden surface detection and disease pre-judgment. However, because the dynamic properties of tunnels are sensitive to the coupling and damping effects of the surrounding soil, there is little relevant research on tunnel structures. Using the PiP (pipe in pipe) model, the intrinsic tunnel modes and their response characteristics are investigated in this paper, and the degree to which the identification of these characteristics is influenced by mode superposition and the soil coupling effect are also considered. The response features of these flexible wave modes are found to be barely recognizable in a tunnel-soil coupled system, while the phase velocity of the torsional wave can be determined by combining phase spectrum analysis and the HHT (Hilbert-Huang transformation) method. A new structural health assessment method based on the torsional wave speed is therefore proposed. In this method, the torsional wave speed is used to determine the tunnel structure’s global stiffness based on a newly developed dispersion algorithm. The calculated stiffness is then used to evaluate the tunnel’s structural service status. A field test was also carried out at a newly built tunnel to validate the proposed method; the tunnel structure’s Young’s modulus was obtained and was very close to the designed value. This indicates that this method is an effective way to assess tunnel service conditions, and also provides a theoretical basis for future applications to health assessment of shield tunnels.

Validation of the Pipe in Pipe vibration software model to determine ground-borne noise and vibration levels above construction tunnels and the determination of end corrections for different train operating scenarios

The London Crossrail project requires that re-radiated noise from the construction of the tunnels should achieve exceptionally stringent criteria with respect to groundborne noise and vibration. The challenge has been to implement robust vibration modeling for the movement of construction trains within the tunnel in order to derive noise levels within sensitive properties (including recording studios) above the tunnel. The Pipe in Pipe (PiP) software model (which is based on elastic continuum theory) developed by Hunt and Hussein has been modified and refined empirically utilizing the measurement of exceptionally low vibration levels emanating from the temporary underground construction railway track. The paper describes the extensive and challenging monitoring and modeling which been carried out. The study represents the first rigorous method of determining noise and vibration at properties above the tunnel from construction trains for the Crossrail project, with an associated detailed validation exercise in line with ISO 14837-1 `Mechanical vibration— Ground-borne noise and vibration arising from rail systems'. A combination of modeling and vibration measurements were used to inform the predictions of groundborne noise levels, which can be adjusted to replicate the behavior of other track and rail support types.

SIMULATION OF WAVE PROPAGATION OF FLOATING SLAB TRACK-TUNNEL-SOIL SYSTEM BY 2D THEORETICAL MODEL

Presented herein is a computationally efficient 2D theoretical model for simulating the steady response of a floating slab track-tunnel-soil system. The track-tunnel coupled system is simplified as a beam-spring system and embedded in soil layers. The tunnel is modeled by a Timoshenko beam with its interaction with the soil layers accounted for by two transfer matrices, with each derived for the soil layer above and beneath the tunnel. The approach as proposed herein has been referred to as the Timoshenko beam-transfer matrix method (TTMM), that allows one to analyze the response of the coupled system, including the tunnel motion and soil stresses. The results obtained were compared with those furnished by the pipe-in-pipe (PIP) approach, and were found to be consistent for exciting frequencies smaller than the tunnel second-mode cut-on frequency. The origin of discrepancies was investigated by the dispersion characteristic analysis, which is attributed to the absence of several in-plane modes when the tunnel is simplified as a Timoshenko beam.