Degradation Assessment and Failure Prevention of Pipeline Systems

Strategic infrastructures made of pipelines transporting hydrocarbons across the world are exposed to the risk of failure due to damage accumulation during operation. The degradation process is promoted by material aging and enhanced by harsh service conditions. Severe consequences can be prevented by the long-life monitoring and integrity assessment of materials and components. Structural diagnosis can be assisted by non-destructive mechanical testing. This chapter provides an overview on the procedures at present available for pipeline steels in this context. The information content of hardness and instrumented indentation tests is specifically addressed. The focus is on the reliability of the predictions that can be provided by small sampling sizes when experimental information and numerical simulations are combined. The significance of such methodology for the evaluation of the current properties of exercised pipelines is illustrated together with the relevant validation studies. The gains resulting from the progressive technological advancements are also evidenced.

Long-term operation of structural steels causes an essential decrease of the mechanical properties, especially characteristics of brittle fracture and SCC resistance. General regularities of in-service degradation of pipeline steels are analysed in the chapter. On these base two stages of pipeline steels degradation are distinguished in the chapter. The first one is deformation aging which is characterized by improvement of characteristics of strength and hardness but from the other hand a decrease of plasticity and brittle fracture resistance. The stage II is the stage of in-bulk steel dissipated microdamaging, which is more dangerous with regard to a loss of pipeline integrity. Operational degradation of the mechanical properties of the steels is accelerated by their hydrogenation from the inner surface of the pipe, which indicates the hydrogenating ability of transported hydrocarbons. The accelerated method of pipeline steels degradation is substantiated. It is based on the common method of deformation ageing of steels by plastic strain with subsequent heat treatment up to 250 °C, however, it additionally involves preliminary hydrogen charging.

Long-term operation of natural gas transmission pipelines leads not only to the appearance of macro defects but also to in-bulk damaging of pipeline steels at nano- and micro-scales. In-bulk steel degradation and a decrease in characteristics of brittle fracture resistance of pipeline steels under long-time operation increase significantly a failure risk. Therefore, deterioration of pipelines under operation calls for effective methods for current condition evaluation. The paper is aimed to the development of a prediction method of degradation of pipeline steel in operating conditions based on electrochemical correlation. The low-carbon ferrite-pearlite steels with different strength of gas transit pipelines after long-term operation were investigated. It was shown that mechanical and electrochemical properties of the pipeline steels were deteriorated due to long-term operation. It was found that one of the most sensitive parameters to in-bulk steel degradation among electrochemical properties was polarization resistance. An acceptable correlation between relative changes in polarization resistance and impact toughness of steels caused by long-term service was revealed. It was concluded that mechanical properties changes of pipeline steels caused by degradation under operation can be evaluated by measurements of changes in their electrochemical characteristics. Having initial properties of the steel, its current properties can be predicted. The method enables non-destructive in-service assessment of degradation degree of brittle fracture resistance of pipeline steels. The verification studies of prediction method of pipeline steel degradation were carried out on damaged and operated pipeline steels.

Structural and fractographic features of degradation are analyzed for pipe steels after their operation on main gas pipelines. The structural feature of steel degradation is damaging along the boundaries between pearlite and ferrite grains manifested by more intensive etching the boundaries with extraction of cementite particles from the matrix. The most obvious effect of degradation is revealed for the steel 17H1S, and the least one for the steel X70. It is concerned with steel texture peculiarities, namely, different sizes (thickness and length) of strips of pearlite, and with dispersion of its components (i.e. cementite lamellae) which control to a large extent hydrogen permeability in a pipe wall and its trapping at the ferrite–pearlite boundaries. Structural peculiarities of steel degradation revealed themselves clearer fractographically in a form of delaminations at the fracture surfaces of the operated steels. The hydrogen absorbed by metal during pipe operation and concentrated at the structural defects along the boundaries between interlayers of ferrite and pearlite grains led to the occurrence of these delaminations and their extension. Therefore, these delaminations are considered as the fractographic features of embrittlement for the operated steels. The relationship between mechanical properties of pipeline steels and metallographic and fractographic parameters is obtained. The critical state of degraded steels is substantiated by the change of the crucial element of embrittlement on the fracture surfaces from delaminations (in the subctitical state) to cleavage (in the overcritical one).

The model describing the corrosion-mechanical fracture of the underground gas pipeline with semi-elliptical external surface crack is developed taking into account the intensification of crack growth in the pipe steel 17H1S by diffusible hydrogen. The model is grounded on the energy approach to fracture combined with the hydrogen accelerated soil corrosion cracking mechanism. The formula for the soil corrosion rate is derived as the sum of two components: the rate of regular soil corrosion due to contact of steel with the soil, and the term characterizing its acceleration by hydrogen. Corresponding mathematical model (differential equation with initial and final conditions) is built up to determine the residual lifetime of a pipe of gas pipeline subjected to hydrogenation from the transported gas, soil corrosion, long term sustained and transient loadings caused, respectively, by gas pressure in the pipe and by start/stop valve operations. As a result of model implementation, the pipe residual lifetimes are evaluated considering pipe hydrogenation from the inner surface, soil corrosion at the outer surface as well as transient loading. From the analysis of results, it is concluded that pipe wall hydrogenation, as well as transient loading, lead to significant acceleration of corrosion-mechanical crack growth in the pipe, and thus, reduce its residual lifetime.

As it is widely known, pipelines can be a safe and environmentally/economically sound means to transport multiphase fluid of different nature; if not well monitored, however, they can pose a serious threat to health and environment. Operational and monitoring methods are improving, but new challenges are also present, mainly due to the quality of transported fluids, especially in the case of hydrocarbons. To quote a few, crude oil often contains H₂S; new pipelines transporting and injecting supercritical fluids are built; new corrosion problems can occur due to the—only partially known—damage mechanisms caused by biofuels, and to external aggressive environments as for instance deep sea water. After more then 40 years of work in the field of corrosion, often in projects specifically related to pipelines, in the present paper the author will try to make a summary of the principal damage mechanisms, the present knowledge and the open threats for the future, from the partial point of view of a former employee of the Oil&Gas industry and at present consultant in that sector.

Over the last decade, there has been a substantial increase in construction projects of pipelines for the transportation of several varied and dissimilar fluids, such as oil and natural gas, fuels and chemicals, as well as water to drink or for irrigation. The need to transport fluids over considerable distances calls for carbon steel as construction material as it guarantees the required tensile strength and toughness properties. Also, pipelines can be laid in very diverse environmental conditions and terrains, thus exposing steel to different risks of damage. Since any deterioration of line pipes can lead to leaks or ruptures, that requires the development of an adequate system for control and monitoring of the structural integrity of the pipeline. Existing inspection and maintenance practices commonly applied by most pipeline operators are formulated mainly on the basis of experience. Such procedures are no longer sufficient and quantitatively risk-based methodologies are required. Analytical tools have therefore been developed for several years with the aim, on one hand, of reducing the economic impact of failures and, on the other hand, of limiting the impact that failures may have on environment, health and safety as much as possible. The present article, by evaluating the rationale behind commercially available risk-based procedures and through a critical analysis of the open literature on the subject, intends to highlight limitations and possibilities to improve a procedure of this type.

The model of the software system for monitoring the integrity of the linear part of a gas main pipeline is considered. The pipeline is considered to be a linear structure formed by series-connected compressor stations and sections of the linear part. The structure model of a section consists of sequentially connected line and nodal elements. The nodal elements represent the technological objects of the linear part, which create small pressure drops between their inputs and outputs. Mathematical models of gas motion through such elements contain ordinary time-dependent differential equations. Gas flow through the line elements is described by partial differential equations, which depend on the spatial coordinate and time. According to this model, the integrity monitoring system of the linear part consists of the integrity monitoring systems of all objects represented by both linear and nodal elements. An object integrity control systems include sensors of informative parameters, logging systems, monitoring database, mathematical models and object integrity checking algorithms, data exchange subsystem and information security subsystem.

The problem of corrosion evaluation of subsea pipes after many years under water is a real problem for many companies operating in the oil and gas business. The European Union research action in the frame of the new research program Horizon 2020 has developed a project to monitor deep subsea tube with guided wave manipulated by a Remote Operating Vehicle (ROV). The paper summarizes the main points of the program, the experimental tests, the sensitivity of the system. The target is also to prepare a white paper document to present to ISO for a norm draft, as a specific EU request. The project is at its final stage after the satisfactory tests in the laboratories of I&T NARDONI INSTITUTE in Brescia and at Dacon in Oslo. The final test is scheduled on end of February in OSLOFJORD. VERNE has been presented in the most important exhibition as OTC 2019 Houston, OMC Ravenna, AIPND BIENNALE NDT CONFERENCE Milano 2019 EGYSP 2020 Cairo. Oil&Gas companies have expressed great interest for VERNE being an alternative to intelligent PIG Inspection, only possible from the inside of the pipe, while not all pipelines are inspected for different reason from the inside.

A case study of the Urengoy–Pomary–Uzhgorod main pipeline inspection in the section between Illintsi and Bar compressor stations is described, and the causes for initiation of longitudinal cracks on the pipe outer surface are discussed. Importance of timely and correct reaction to diagnostic results is shown. Experience of the Joint Stock Company “Ukrtransgaz” in taking measures on optimization of the processes of main gas pipeline repair is shown. It consists in establishing precise terms both for assessment of in-line inspection results and for making repair plans, determination of criteria of defect selection for repair and formation of a common approach to the process, in particular, to technical documentation. The recommendations for the repair, replacement and strengthening of dangerous sections of main gas pipelines are developed basing on the data obtained during the diagnostics and the results of calculations. Regular monitoring of these sections makes it possible to assess adequately their current technical state and allows establishing the expediency of further operation and recommendations for the elimination of detected defects. The system of pipeline integrity control has been introduced into operation by Ukrtransgaz. It is based on currently available geographic information system of certification and technical monitoring of the main gas pipelines and also on the analytical hardware and software system that is constantly being developed and improved.

This paper evaluates, by quantitative fractography and image analysis techniques, the hydrogen embrittlement and microdamage in notched samples of 316L steel, the described phenomenon affecting the structural integrity, durability and safety of the pipelines made with such a material. After the hydrogen embrittlement tests, it is seen that microdamage created by hydrogen is concentrated in an external circumferential ring with the same center as the cross sectional area of the notched samples. The microscopical appearance of this embrittled zone or damaged area is very rough and irregular at the micro-scale, with evidence of secondary cracking, in contrast with the smooth surface (at the micro-scale) created by micro-void coalescence (dimpled fracture) in the inner core which is not embrittled by hydrogen and fails by mechanical reasons. In addition, differences are observed in the matter of the appearance of the hydrogen-assisted microdamage area as a function of the notch geometry and of the embrittlement time.

The fatigue crack growth rate diagrams of the carbon pipeline steel were received under the presence of the admixtures of sodium nitrite as the passive component in the basic aqueous hydrogen-containing solution. It has been found that the fatigue crack growth rate \({{\text{d}}a/{\text{d}}N}\) depends ambiguously on the concentration \({C_{{{\text{NaNO}}_{{2}} }} }\) in solution due to the different properties of the passive films formed on the steel surface. The strength of passive films formed under different concentrations \({C_{{{\text{NaNO}}_{{2}} }} }\) was evaluated as a characteristic value of stress intensity factor \(K_{I}^{ * }\), which corresponds to the passive film failure of at the crack tip. For the determination of the parameter \(K_{I}^{ * }\), the special experimental procedure was developed. Received results showed that the dependence \(K_{I}^{ * }\) on the concentration \({C_{{{\text{NaNO}}_{{2}} }} }\) is ambiguous and the maximum exists at some concentration \({C_{{{\text{NaNO}}_{{2}} }} }\) when the value \(K_{I}^{ * }\) is maximal. The study of hydrogen permeation in steel at the presence of the passive film on the metal surface showed on some specific value of \({C_{{{\text{NaNO}}_{{2}} }} }\), at which the formed passive film is the most resistible barrier against electrochemical hydrogen absorption. This value is very close to the above-mentioned value \({C_{{{\text{NaNO}}_{{2}} }} }\), which corresponds to the highest strength of the passive film and also to the maximal deceleration of fatigue crack growth rate. Consequently, it may be concluded that the relationship between passive film strength, its ability to serve as a hydrogen barrier and fatigue crack growth rate exists. Thus, it is possible to decelerate the fatigue crack growth by the targeted variation of environmental composition.

Submarine pipelines are an industry rapidly changing in innovation and technological advances and developments, particularly in challenging operation conditions and deep water applications. Integral buckle arrestors have proved to be an essential device to limit damages induced by a propagating buckle and to reach and ensure very high performances both during the pipelines laying and the service life. New forged integral buckle arrestors are applied especially in case of large diameters and high wall thicknesses and under extreme external load conditions. Their main goal is to guarantee and improve the integrity of the offshore pipelines. After a brief description of the status of the art, the objective of this paper is to show the efforts in progress to better satisfy the requests of the oil & gas industry. The paper analyses the manufacturing steps to obtain the final product including material, forging and mechanical machining processes, intermediate and final tests and inspection.

The in-service degradation of pipeline steels, affecting performance of natural gas transportation infrastructure, is now comprehensively investigated with the usage of various approaches. Steel degradation implies embrittlement and decreasing the mechanical properties, increasing a failure risk of pipelines. The mechanical properties of strength and plasticity, which can be changed due to degradation under long-term operation of pipeline steels, are usually evaluated by tension tests based on the obtained stress–strain diagrams in nominal values, i.e. without taking into account a change of cross-section of specimen during tension. In this paper it is proposed to use true stress–strain diagrams for an assessment of in-service degradation degree of gas pipeline steels. The low-alloyed X52 pipeline steel in as-received state and after 30 years of operation on the gas transit pipeline was investigated. The obtained results demonstrating the advantage of a usage of true stress–strain dependences instead of nominal ones are discussed.

To study a full range of the mechanical properties on small-sized flat specimens from the pipe steel, including the characteristics of crack resistance after the realization of certain loading modes, the authors used the method of complete stress–strain diagrams. This method was previously substantiated both theoretically and experimentally by professors A. A. Lebedev and M. G. Chausov for assessing the fracture kinetics of plastic materials on small specimens subjected to various types of preliminary loading. The effect of preliminary plastic deformation under static tension on changes in the crack resistance and mechanical characteristics of the pipe steel is extreme at a strain level ε_pl = 6.3%. Test results were compared at identical levels of the preliminary plastic deformation under static tension and impact-oscillatory loading at strain levels of 4.5–8.8%. The comparison showed significant differences in the effect of impact-oscillatory loading on changes in the mechanical properties and crack resistance of the pipe steel. Nanotechnologies were used to analyze the original results on the effect of preliminary plastic deformation under static tension and identical levels of dynamic deformations caused by impact-oscillatory loading on changes in the mechanical properties and crack resistance of the 17G1S-U pipe steel in the initial state. This was especially noticeable when tungsten carbide and carbon nanoparticles were used under impact-oscillatory loading. To confirm the revealed mechanical effects, detailed fractographic studies of specimen fractures were conducted along with investigations into the surface macrohardness of the steel.

Analytical researches of growth of internal surface cracks in oil pipeline pipe wall under real conditions of operation and determination of its residual service life were carried out. The analysis of the operating conditions of the pipeline was carried out during the research. It is believed that the flow of oil is turbulent with possible hydraulic shocks; produced water is collected at the bottom of the pipe, which causes corrosion in contact with a crack in the pipe wall. An important point in these studies is to consider the corrosion-hydrogen degradation of the pipe material (X60 steel) when calculating its residual life. Such calculations are based on a mathematical model of corrosion crack growth in metallic materials under appropriate loading conditions, in particular time variables (turbulent oil flow with hydraulic shocks), the action of the corrosive environment (groundwater) and the change in the characteristics of X60 steel over time as a result of its degradation. It is shown that the turbulence of the oil flow and the shocks significantly reduce the residual life of the pipeline. The degradation of its material (X60 steel) over time puts the value of this resource in the interval between the values of the residual life of the degraded and not degraded pipe.

The influence of hydrogen factor and residual stresses on the sum of the magnetoelastic acoustic emission (MAE) signals amplitudes during magnetization of the oil and gas pipeline material is investigated. The regularities of change of the MAE signals parameters by the thickness of the pipe wall and around the longitudinal welded joint are established. The largest sum of the MAE signal amplitudes is recorded in the material from the internal layer of the pipe wall, which is evidently caused by the presence of hydrogen in the material that under long-term operation of the pipelines has penetrated into structural defects and facilitates jumping of the domain walls under samples magnetization. For the longitudinal welding of oil and gas pipelines, the MAE signals with the largest sum of amplitudes are generated in the material from the weld zone and the smallest—from the heat-affected zone. This effect is caused by the peculiarities of the restructure of the material structure under the influence of high-temperature fields under welding in different parts of the welded joint. It is found that residual stresses around the welded joints reduce the intensity of domain wall jumps and cause decrease in the sum of the MAE signals amplitudes. The use of the obtained research results in the newly developed methods of non-destructive testing makes it possible to quickly assess the local damage of the pipes of long-term operation.

The relationship between the hydrogenating of steels 20, 30CrMo and 17Mn1Si and their resistance to corrosion-fatigue fracture under static and cyclic loads in NACE solution has been investigated. It was found that the volume of adsorbed hydrogen does not determine the corrosion cracking resistance of steels. Steels 20 and 17Mn1Si absorb about the same amount of hydrogen under static loads, however their corrosion cracking resistance is different. The threshold tension for 20 steel is less than for 17Mn1Si steel. Corrosion fatigue resistance of 20 steel under cyclic loads is higher than of 17Mn1Si. Absorption of hydrogen by steels increases under the action of tensile static stresses and decreases at the symmetric cyclic loading. The threshold tension for 30CrMo steel at static stresses is high enough (440 MPa, σ_th/σ_0.2 = 0.8), but threshold average tension does not reach at cyclic asymmetric stresses with amplitudes σ_a = 0.2σ_0.2 and samples fail before 720 h. It indicates a higher sensitivity of this steel to the action of cyclic asymmetric stresses compared to 20 and 17Mn1Si steel. Reducing of resistance to corrosion-fatigue fracture of 30CrMo steel under asymmetric cyclic loads correlates with the change in the hydrogenating nature of environment. Corrosion-mechanical failure of steel 20 occurs mainly as a result of hydrogen embrittlement, and the failure of steel 17Mn1Si caused by the simultaneous action of corrosion and hydrogen factors.

The problem of changing the spatial position of pipelines at different stages of their life cycle is described. The stages and conditions of pipeline construction or operation as well as different modes of technological equipment operation mainly effect on it. In various applied researches of these processes, an attention is mainly paid to recommendations on how to achieve the optimal mode of bending the pipeline. However, in the practice of pipelines construction and operation, the optimal load mode of equipment rarely becomes the subject of interest. In the first place becomes the issue of ensuring the load regime within the permissible limits. To define them a finite-element model of a hypothetical pipeline was created. After defining loads and impacts occurring within the typical pipeline construction sequences as well as under pipelines operation appropriate pipeline deformations were obtained. The processes of pipeline constructing both onshore and offshore (so called S-method) was taken into account. It is shown that the range of permissible pipeline displacements during pipeline lowering on the seabed is an order of magnitude lower than the one on land. It was found that bending stresses in the pipeline are mainly determined by the angle of inclination of the stinger and the axial tension force of the pipeline laying ship. Concerning onshore pipeline construction process they are determined by the position of the first pipelaying crane (so called side boom) from the laying side. Dependence of critical values of process managing parameters from the pipe laying conditions was obtained.