Pipe Stiffness Research Articles

Mechanical analysis of un-bonded flexible pipe tensile armor under combined loads

Marine un-bonded flexible pipes wrapped with fiber-reinforced polymer are essential in offshore oil and gas exploration because of their high flexibility and reliability. These flexible pipes may be subjected to tensile loads, compressive loads, and torsional loads, causing the armor wires to undergo large displacements that may result in failure. This study develops a nonlinear simplified finite element model to investigate the mechanical behavior of flexible pipe armor wires under various types of load. The stiffness change rule of flexible pipe armor wires is obtained from numerical simulations of the flexible pipe tensile armor layer under combined loads. The results reveal that the flexible pipe stiffness under combined loads varies significantly from that under single loads, and the combination of different load types can either increase or decrease the stiffness. This study provides essential reference data for the further design, fabrication, and application of flexible pipes.

Leakage Estimation in Water Distribution Network: Effect of the Shape and Size Cracks

The definition of the relationship between the leak outflow, the total head at the leak and other relevant parameters such as pipe stiffness, leak dimension and shape has been object of extensive studies. The attention to a correct estimation of leakages, leak law, is crucial for the management of water distribution systems. Water utilities, in fact, can reduce leakage levels through the leak detection or more usually by pressure management. In the last cases, the known of the relationship between leak discharge and pressure is fundamental. In recent decades, the use of the Torricelli equation has been questioned, because some experimental results showed that it can yield unsatisfactory results, and other formulations have been suggested to model water leakages in water distribution networks. To investigate the effectiveness of the formulations suggested by different authors, an experimental campaign was carried out at the Environmental Hydraulic Laboratory of the University of Enna (Italy) for leaks of different shape and size in polyethylene pipes. The results of the laboratory experiments contribute to clarify the applicability of the leak law for circular and rectangular leaks and suggest that Torricelli formulation is valid in absence of leak area deformation. Furthermore, the analysis contributes to the knowledge of the coefficients of the leak laws used to estimate the leakage outflow.

On The Stiffness Prediction of GFRP Pipes Subjected to Transverse Loading

The main objective of this study is to predict the stiffness of GFRP pipes subjected to compressive transverse loading. An experimental study is performed to measure the stiffness of a composite pipe with a core layer of sand/resin composites. Then, a simple analytical modeling constructed on the basis of solid mechanics is used to estimate the stiffness of the investigated pipe as the back-of-envelope technique widely used by industrial sectors. The simulation of stiffness test is conducted using finite element modeling wherein both large deformation and inelastic behavior of material is taken into account as the sources of nonlinearity. The results reveal that a very good estimation with high level of accuracy can be reached by proper selection of the element and performing nonlinear analysis.

Mechanical performances and evolution of stiffness of thin-walled strain hardening cement-based composites pipes during cyclic loading

Four series of pipes such as reinforced strain hardening cement-based composites (SHCC) pipe (RSHCCP), plain SHCC pipe (SHCCP), reinforced cement mortar pipe (RMP) and plain cement mortar pipe (MP) were investigated by means of Three-Edge Bearing Testing (TEBT) under monotonic and cyclic loading. Results show that RSHCCP had the highest load-carrying capacity and deflection ratio among them. The synergy reinforced effect of PVA fiber and reinforcing steel to improve load-carrying capacity and deflection ratio of pipe was validated. Under cyclic loading, the irreversible deflection at the crown of pipe increased gradually. The envelope of the load–deflection response of cyclically loading pipe was consistent with the load–deflection curve of monotonically loading pipe. The pipe stiffness of concrete pipe under TEBT was characterized using the ratio of load to the reversible deflection at the crown of pipe during cyclic loading. The pipe stiffness of RSHCCP decreased continuously after the applied load exceeded the elastic range while the pipe stiffness of SHCCP, RMP, and MP remained stable unless the imposed load was close to the ultimate load. The pipe stiffness of RSHCCP could satisfy the requirements for the installation flexural stiffness ratio of semi-rigid pipe as the applied load was more than 70% of its ultimate load. The deflection ratio of RSHCCP was greater than the minimum deflection ratio level (i.e. 3%) of semi-rigid pipe.

LABORATORY EVALUATION ON PERFORMANCE OF GLASS FIBER REINFORCED PLASTIC MORTAR PIPE CULVERTS

This paper investigated the performance and behaviour of glass fiber reinforced plastic mortar (FRPM) pipes under different loading conditions. FRPM pipes with inner diameter of 1500 mm were prefabricated in factory. Mechanics performance testing (ring and axial compressive strength and elastic modulus), stiffness and fatigue test were carried out in laboratory. Ring stiffness test provided pipe stiffness (PS) which is a function of geometry and material type of pipe through parallel plate loading test (PPLT). The fatigue test and micro-structure measure method were used to evaluate the durability effects of FRPM under repeated compression load. Results indicated that FRPM pipes had better mechanic performances as the road culverts under soils. It may be helpful for the design and construction of FRPM culverts.

A comparative study of the response of buried pipes under static and moving loads

Abstract The buried pipes should be designed properly to withstand the loads imposed by the backfill soil weight and traffic loads. However, a thorough literature review has shown differing opinions on the effect of static and moving traffic loads on buried pipes. Some studies have shown that moving loads produce higher displacement in buried pipes compared to static loads, while other studies have shown contradicting results. These differing opinions have created confusion among researchers who are studying the response of buried pipes under traffic loads, where most of the studies have been conducted using either static or moving loads without proper justification to the selection of the loading type. To clarify this confusion, this paper presents a rigorous study on the behaviour of buried pipes under static and moving traffic loads using a robust finite element analysis. The static and dynamic finite element models have been developed and validated using high-quality field data collected from the literature. The developed models were then used to investigate the effect of the truck speed, pipe stiffness and loading conditions on the maximum displacement of buried pipes. The results showed that the displacement of buried pipes due to static loads is always higher than the pipe displacement due to moving loads. In addition, it was found that the ratio of the static to dynamic pipe displacement decreases as the pipe stiffness increases and increases to a lesser extent as the truck speed increases. Hence, future studies should consider the static loads in designs as these are the most stringent loading condition. This is actually very helpful for designers if they are using numerical methods in their designs, because static analyses are much more straightforward to conduct and less computationally demanding compared to dynamic analyses.

Influence of Support Stiffness on Dynamic Characteristics of the Hydraulic Pipe Subjected to Basic Vibration

The basic vibration generated during the tunneling process of hard rock tunnel results in the change of the support stiffness of the hydraulic pipe, which causes the problem of reduction of pipe transmission efficiency. A transverse motion equation of the hydraulic pipe was established by correlating with Hamilton’s principle under basic vibration. In consideration of the fluid‐structure interaction (FSI), the support was simplified as an equivalent and a spring. The bidirectional fluid‐solid coupling analysis method was used to investigate dynamic characteristics of the pipeline under different support stiffness conditions. The finite element method (FEM) and the experimental analysis are applied to verify the proposed methodology. The numerical results show that the maximum displacement of the pipe decreases with the increase of the support stiffness; the maximum stress of the pipe decreases first and then increases with the increase of the support stiffness; the amplitude of the fluid pressure fluctuation at the outlet of the pipe increases with the increase of the support stiffness. But the fluid pressure fluctuation with the higher stiffness is first stable, which can indicate that the support stiffness can increase the damping of the pipe system. This study can get a significant access to the structural design of the pipeline under basic vibration.

The stiffness of axial pipe–soil springs and axial joint springs tested by artificial earthquakes

This study proposes an experimental method to obtain the stiffness of axial pipe–soil and axial joint springs in time and frequency domains. The proposed method can determine axial pipe–soil spring stiffness by using the pipe strains and slippages between pipes and soil. It can also determine axial joint spring stiffness by utilizing pipe strains and joint deformations. The pipe–soil spring stiffness values of ductile cast iron (DCI) and welded steel (WS) pipes are obtained and analyzed through artificial earthquake tests on a 24m × 24m buried pipe network. Artificial earthquakes are produced with trinitrotoluene explosives. Three theoretical models are discussed, and their results are compared with the test results. The comparisons indicate that the proposed experimental method is valid to obtain the stiffness of axial pipe–soil and axial joint springs. The stiffness values can be a benchmark to study the pipe–soil interaction and flexible joints. The effects of axial pipe–soil spring stiffness on the joint deformations of DCI pipes and pipe strains of WS pipes are also discussed.

Finite element analysis of a sandwich pipe joint

Sandwich pipes in which the core material performs both thermal insulation and structural function are viewed as a lightweight alternative to conventional pipe-in-pipe systems in which insulation material carries no loading. Developing a suitable method that permits the joining of sandwich pipes in an efficient manner is essential for their successful application. In this paper, the mechanical response of a swaged field joint between sandwich pipes subjected to bending is investigated using a series of finite element models. In order to gain a thorough understanding of the response of the joint components to installation based loadings, parametric studies are carried out to establish the effect of the inner pipe thickness, cutback length, and stiffness of the field joint filler on the strain concentration at the joint, with particular focus on the swaged weld region and the girth weld region. The influence of interface adhesion properties and weld metal yield strength on the variation of strain intensity is also evaluated. Numerical studies show that increasing filler stiffness and maintaining a cutback length less than 2.5 times the radius of the inner pipe could produce lower strain intensity at the two regions of interest.

Earth pressures on the trenched HDPE pipes in fine-grained soils during construction phase: Full-scale field trial and finite element modeling

Abstract High density polyethylene (HDPE) pipes have been widely used in civil engineering applications. The peaking deflection of the buried HDPE pipes caused by the compaction of the pipe sidefill during the construction phase is not considered in the current pipe design standards. Fine-grained soil may be an alternative backfill when relative high quality coarse-grained soil is absent in-situ or difficult to transport from a long-distance away in urban areas. The effect of construction on the earth pressures around HDPE pipes buried in fine-grained backfills has not been well understood. In this study, a full-scale field trial was undertaken to monitor the earth pressures acting on HDPE pipes buried in fine-grained soils during the construction phase. Three HDPE pipes with norminal diameters of 600, 600, and 300 mm, respectively, were used in the field trial. Numerical modeling which was able to consider the peaking deflection was conducted to investigate the effects of pipe diameter, pipe stiffness, degree of compaction of the backfill, and trench width on the earth pressures acting on the pipes. The measured earth pressures and deflection were utilized to verify the effectiveness of numerical modeling procedures. The finite element analyses indicated that the trench width had a more significant effect on the earth pressures, as compared to other factors. Two empirical equations were proposed, based on the numerical results, for predicting the earth pressures at the top and the springline of the pipes during the construction phase. The validity of the proposed equations was evaluated using the field data obtained from both this study and published studies.

Performance of buried HDPE pipes – part I: peaking deflection during initial backfilling process

Peaking deflection caused by compacting the sidefill, referred to as the maximum change of the pipe diameter divided by the undeformed diameter, is an important parameter in the design and safety check of buried pipelines. However, quantitative equations on the deflection useful for engineering practice are very limited. In this paper, a two-dimensional finite element analysis is used to investigate the peaking deflection of high-density polyethylene (HDPE) pipes. In the analyses, the pipe–soil interaction is rationally modeled. A field trial is conducted and the finite-element modeling is evaluated by using the data measured in the field test. Parametric studies are also conducted to investigate the effects of pipe diameter, pipe stiffness, soil modulus, trench width, and compactor type on the peaking deflection of buried HDPE pipes. A new estimating tool is developed that considers the major influencing factors: pipe diameter, pipe stiffness, soil modulus, and compactor type (vibratory plate or rammer) to predict the peaking deflection of HDPE pipes, and the proposed method is finally verified by data reported in published studies. The comparison of the calculated and measured peaking deflections demonstrates a reasonably good prediction of the peaking deflection.

Pipe Stiffness Prediction of Buried Glass Fiber Reinforced Polymer Plastic (GFRP) and Polymer Mortar Pipe

Recently, glass fiber reinforced polymer plastic (GFRP) pipes are widely used in the water-supply system because of their advantages such as light-weight, corrosion resistance, etc. In previous study, we present the equation to predict stiffness factor (EI) of GFRP pipe with two tape-winding FRP layers and polymer mortar layer in between two FRP layers. As a result, it was able to predict in the range of -3% to +7%. In addition to previous study, we attempted to predict stiffness factor (EI) of GFRP pipe by the finite element method (MIDAS Civil 2016). From the study it was found that the finite element method can be used to predict the pipe stiffness of GFRP pipe.

Failure analysis of glass-reinforced polyester mortar pipes with different cores subjected to combined loading

In this paper, the failure of glass-reinforced polyester mortar pipes with different cores subjected to pure and combined loading are analyzed. To enhance the stiffness of fiber-reinforced polyester pipe, filler layer is incorporated in between fiber-reinforced polyester plies. Thus, in this study, new nanocomposite materials are proposed as a core layer in the glass-reinforced polyester mortar pipes. The performance of the pipes in the presence of nanosilica/resin, nanosilica/fiberglass/resin and multi-walled carbon nanotubes (MWCNTs)/resin and sand/resin cores are investigated. Finite element method is used to analyze the effects of extensive design parameters on the first ply failure and functional failure of glass-reinforced polyester mortar pipes. Maximum normal stress, Tsai–Wu and Tsai–Hill criteria are employed for the failure analysis. Two end boundary conditions are considered in the simulations. The effects of core material constituents, core thickness, filament winding angle and lay-up configurations, layers thicknesses and pipe radius are investigated. The results are also obtained under pure and combined loading conditions. The stress concentration around circular and rectangular holes in glass-reinforced polyester mortar pipes with sand/resin core is also studied. Accordingly, the stress concentration factors are reported for each case. Detailed discussions are done and brief design guidelines are given.

Pipe Stiffness of Buried Glass Fiber Reinforced Polymer Plastic (GFRP) and Polymer Mortar Pipe with Symmetrical Cross-Section

Recently, glass fiber reinforced polymer plastic (GFRP) pipes are increasing trend in using in the water-supply system because of their advantages such as light-weight, corrosion resistance, etc. GFRP pipes discussed in this paper have polymer mortar layer for core sandwiched between two tape-winding glass fiber reinforced polymer plastic layers. So, GFRP pipe properties for the design such as pipe stiffness (PS) and equivalent modulus of elasticity (Eeq) is complicated to predict or to measure. In this study, we proposed the equation that can predict the equivalent pipe stiffness factor (EI) in relation to PS and Eeq using thickness of each material layer. The predicted result obtained by the equation proposed in this paper is compared with experimental result ASTM D 2412(2010). As a result, it was in the range of-3% to +7%. So, this proposed obtained according to equation can be used to design GFRP pipe used in practice.

A new device to measure permeability evolution under pressure loading: Application to CFRP pipes

Abstract This paper presents an experimental setup to measure permeability evolution induced by mechanical loading. The experiment consists in pressurising a pipe-specimen. Internal pressure is used to load and to measure permeability. The external face of the pipe is accessible, so that leak paths can be localised and quantified. In order to avoid premature cracking of the specimen due to the device, sealing components were designed using Finite Element Analysis with respect to the stiffness of pipes to be tested. Although the current design is used here to test carbon composite pipes, it can be easily adapted to other materials. The design was validated by measuring permeability on an impermeable pipe. The experimental setup and method were applied to a composite filament-wound pipe. Results give substantial quantitative information on the relationship between the number of leak paths and the increase in material permeability.

Soil Stresses on Carbon Steel Pipe Undergoing Uplift

This paper presents a three-dimensional numerical study to evaluate the variations in stresses in the soil mass surrounding a carbon steel pipe class API 5L X60 submitted to uplift due to ground elevation. Analyses were carried out for soil relative density, pipe stiffness and surficial surcharge loading. Results have shown that stress variations due to uplift are lower for looser backfill soils and flexible pipes. Stress variations in pipe invert are meaningful in the vicinity of the region between stable and unstable soil masses.

Studying the Life-cycle Performance of Gravity Sewer Rehabilitation Liners in North America

Pipe rehabilitation and trenchless replacement technologies have seen a steadily increasing use and represent an increasing proportion of the annual expenditure on operations and maintenance of the water and wastewater infrastructure around the world. Despite the large public investment in use of these technologies, there has been little quantitative evaluation of how these technologies are performing. A retrospective study conducted by the U.S. Environmental Protection Agency collected 25 samples between 2009 and 2013 of in-service gravity sewer liners from 11 cities in North America. Testing of the various liners included thickness, annular gap, ovality, specific gravity, porosity, flexural strength, flexural modulus, tensile strength, tensile modulus, surface hardness, glass transition temperature, Raman spectroscopy, environmental stress crack resistance and pipe stiffness as appropriate to the liner type and condition. Cured-in-place pipe (CIPP) liners comprised 18 of the 25 samples. Other liners included were PVC fold-and-form, HDPE deform-reform and sliplining. The samples had seen from 5 years to 34 years in service. A number of areas where the QA/QC of liner installation should improve were noted in the studies but, overall, the liners were in excellent condition and the physical test results indicated that these liners are expected to last their 50-year design life and probably well beyond.

Characteristics of Structural Behavior of Unplasticized Polyvinyl Chloride (PVC-U) Pipe Buried Underground

The industrialization and urbanization forced to increase the density of pipelines such as water supply, sewers, and gas pipelines. The materials used for the existing pipe lines are mostly composed of concretes and steels, but it is true that the development for more durable and efficient materials has been continued performed to produce long lasting pipe lines. Recently, underground pipes serve in diverse applications such as sewer lines, drain lines, water mains, gas lines, telephone and electrical conduits, culverts, oil lines, etc. In this paper, we present the result of investigation pertaining to the structural behavior of unplasticized polyvinyl chloride (PVC-U) flexible pipes buried underground. In the investigation of structural behavior such as a ring deflection, pipe stiffness, 4-point bending test, experimental and analytical studies are conducted. In addition, pipe stiffness is determined by the parallel plate loading tests and the finite element analysis. The difference between test and analysis is about 8% although there are significant variations in the mechanical properties of the pipe material. In addition, it was found by the 4-point bending test there is no problem in the connection between the pipes by coupler.

Road surface permanent deformations with a shallowly buried steel-reinforced high-density polyethylene pipe under cyclic loading

Steel-reinforced high-density polyethylene (SRHDPE) pipe is a new type of pipe used in roadways for drainage due to its obvious advantages, such as good corrosion resistance and light weight. In some projects, they are shallowly buried. To investigate the effects of the shallowly-buried pipe on the permanent deformation of road surface under cyclic traffic loading, two laboratory tests of an unpaved road with a buried SRHDPE pipe under cyclic loading were conducted. A 610 mm diameter SRHDPE pipe was buried in a compacted sand trench covered by aggregate or sand base courses in Tests 1 and 2, respectively. A Mechanistic-Empirical (M-E) model was calibrated with the laboratory test data and then adopted to conduct a parametric study on the road surface rutting. It was found that the SRHDPE pipe has very small deflection under the cyclic traffic loadings. The M-E model could predict the permanent road surface deformations well under cyclic loading before the road failed. Parametric studies showed that the road surface permanent deformation was intensified as the stiffness of buried pipe increased. The road surface deformation decreased as the pipe embedment depth increased and an optimum pipe buried depth existed to get the same road surface deformation between the road sections with and without a drainage pipe.

Numerical analysis on Buried pipes protected by combination of geocell reinforcement and rubber-soil mixture

A numerical simulation of laboratory model tests was carried out to develop an understanding of the behaviour of pipes in a trench prepared with 3-Dimensional reinforced (namely "geocell-reinforced" in the present study) sand and rubber-soil mixtures, under repeated loadings. The study reports overall performance of buried pipes in different conditions of pipe-trench installations and the influence of pipe stiffness on backfill settlements, stress distribution in the trench depth and stress distribution along the pipe's longitudinal axis. Good agreements between the numerical results and experimental results were observed. The results demonstrate that combined use of the geocell layer and rubber-soil mixture can reduce soil surface settlement and pipe deflection and eventually provide a secure condition for buried pipe even under strong repeated loads.