Open access peer-reviewed chapter - ONLINE FIRST

Combined Calculated, Experimental and Determinated and Probable Justifications for Strength of Trunk Crude Oil Pipelines

By Dmitry Neganov and Nikolay Makhutov

Submitted: June 6th 2019Reviewed: August 6th 2019Published: November 14th 2019

DOI: 10.5772/intechopen.89036

Downloaded: 19

Abstract

Within the long-term Russian and foreign practice, deterministic methods of basic strength calculations have been developed and are being developed at the design stage of long-distance pipelines. Occurring operational damages, failures, accidents, and catastrophes show there are no direct substantiations for the prevention of such emergencies in the framework of existing calculations. In order to respond to these situations, the following are developed: additional precise deterministic, static, and probabilistic calculations with linear and nonlinear criteria of deformation and fracture mechanics, complex diagnostics of the state of the pipeline using in-line pigs, and laboratory, model, bench, and field tests of pipelines with technological and operational defects. The results of systematic scientific research and applied developments are presented.

Keywords

  • pipeline
  • strength
  • calculation

1. Statement of the problem

The trunk pipelines for transporting liquid and gaseous hydrocarbons are one of the main types of transport infrastructure in the world.

The most important pumped media are crude oil, gas, broad hydrocarbon fractions, and petroleum products. The total length of trunk pipelines in the world is more than 1.5 million km; in Russia it is about 230 thousand km, and the length of oil pipelines in the world is 170 thousand km; in Russia it is about 70 thousand km.

The length of the largest individual oil pipeline systems is Canada-USA 4700–5300 km with pipe diameters of 450–1220 mm, China-Kazakhstan 2200 km with a diameter of 813 mm, Azerbaijan-Georgia-Turkey 1768 km with a diameter of 1067 mm, Tanzania-Zambia 17,210 km with a diameter of 200–300 mm, and Italy-Germany 1000 km with a diameter of 660 mm.

In Russia, the largest oil pipelines are the Eastern Siberia-Pacific Ocean, 4740 km with a diameter of 1020–1200 mm, and Druzhba, 5500 km with a diameter of 520–1020 mm; eight trunk oil pipelines have a length of more than 1000 km.

The operating pressures in the main oil pipelines range from 2 to 10 MPa (Figure 1).

Figure 1.

Oil trunk pipelines (Russia).

The trunk oil pipelines are operated in a very wide range of climatic conditions (from −70°С to +60°С) and natural hazards (seismic, landslides, geological faults), with ground, underground, and underwater laying.

Despite the large, more than century-old experience of research, testing, construction, and operation of oil trunk pipelines in the world and in Russia, there were large-scale accidents and disasters. These accidents were accompanied by the release of large amounts of oil (up to 100–600 thousand barrels) into the environment (land, water) with great environmental damage, fires, death and injury to people, and pollution of hundreds of hectares of land. Economic damages from such accidents are estimated at $ 10–100 million. The total number of accidents on oil pipelines in the world over the past 20 years is more than 2000, and the number of large oil leaks is more than 4500. For every million tons of pumped oil, 3–5 tons fall into leaks.

In general, the accident rate on the trunk oil pipelines is reduced. However, at present it is at the level of 0.1–0.3 per 1000 km per year (Figure 2).

Figure 2.

Accidents on oil pipelines (Venezuela, China, Russia).

These data indicate the need for further research and practical development to reduce accidents and improve the safety of trunk pipelines.

In recent years, four basic approaches to determining the strength, resource, and safety of oil pipelines have emerged:

  • Deterministic

  • Statistical

  • Probabilistic

  • Combined

2. Solving the problems of strength by basic and calibration methods

2.1 Basic deterministic calculations

The system of domestic and foreign trunk oil pipelines that took shape in the second half of the twentieth and the beginning of the twenty-first century is characterized by multistage creation and development of integrated approaches to justifying their strength [1, 2, 3, 4]. These approaches were initially formed on the basis of the fundamental theories of thin-walled shells, classical theories of strength; they made it possible to form the main computational methods for the selection of computational schemes and computational cases and the assessment of static strength, taking into account the types of stress and limit states.

The basic strength condition was then recorded in the simplest form:

E1

whereis maximum operating voltage stress andis hazardous stress.

For a thin-walled pipe with a diameter D with wall thickness, ring stresses are maximal:

(Figure 3).

E2

whereis maximum operating pressure (Figure 3).

Figure 3.

The main design scheme.

Since in engineering calculations of static strength according to (1) and (2), a whole set of design, technological, and operational methods remained unclear, permissible stresseswith corresponding safety margins were entered into the calculation:

E3

Dangerous stresses were applicable ones corresponding to:

  • Ultimate strength, which excluded the occurrence of fracture (the first significant limiting state).

  • Yield strength(or conditional yield strength), which excluded the formation of unacceptable plastic deformations (the second most significant limiting state). For modern pipeline systems transporting petroleum and petroleum products, a number of main life cycle stages, as measured up to 30–60 years, are introduced into the strength analysis:

  • Feasibility study of the project

  • Outline and detailed design

  • Construction and testing of pipeline systems

  • Operation of pipeline systems with diagnostic and repair and rehabilitation works

  • The withdrawal of sections of pipelines or pipeline systems from operation

  • For each of these stages and for the entire life cycle, to date, in our country and abroad, certain approaches and methods to substantiate strength have been formed.

These methods are divided into two main groups:

  • Basic strength calculations

  • Verification calculations of strength for the used construction material

The formation of methods of basic and calibration calculations is currently associated with the stages of the life cycle. At the same time, an important role is always played by scientific studies to substantiate the strength criteria, the choice of design schemes and design cases, followed by the introduction of safety margins. This is a scientific basis for solving applied problems of strength—the development of strength standards with their design characteristics (Figure 4).

Figure 4.

Generalized scheme of research and rating of strength.

For all pipe steelsand to fulfill condition (3), the marginsmust satisfy.

E4

The development of pipeline transport during the decades of the twentieth to twenty-first centuries [5, 6] was accompanied by a gradual increase in the mechanical propertiesof pipe steels (200 ≤≤ 800, 350 ≤≤ 950) and a decrease in strength margins (2.8 ≥≥ 1.5, 4.0 ≥≥ 1.8).

Expressions (1)(4) were and remain central to foreign strength standards [2, 3, 4]. In Russian practice [1, 4, 5], expressions (1) and (2) were retained, but the strength marginsin (3) were presented in a differentiated form:

E5

where n, K1, and Kн are the reliability factors for load, material, and purpose (n,K1,Kн ≥ 1) and m is the operating condition ratio (m ≤ 1).

In view of Figure 4 and expressions (1)(5) in the feasibility study of the project, two main parameters are defined and assigned, pmaxэand D, ensuring the specified pipeline performance.

At the stage of preliminary and detailed design, the main calculation is reduced to the calculated determination of the minimum wall thickness of the pipeline according to the given pmaxэand D, taking into account economically and technologically reasonable choice of pipe steel with characteristics(according to technical conditions or standards):

δminpmaxэD2σ.E6

Since the values pmaxэat each of the calculated sections of the pipeline depend on their height position, which determines the hydrostatic part of the pressure, the calculated valuesare variable in length.

At the stage of construction and pre-hydraulic pressure tests pгpmaxэ, calibration calculations with an assessment of the maximum hydraulic tests are carried out in accordance with (2), their comparison with permissible values in accordance with (3) and (5) and confirmation of the absence of destruction or the formation of unacceptable plastic deformations

σmaxг=pгD2δminσопnσг.E7

At the stage of operation for the timethere is a possibility of accumulation of damage, and a decrease in wall thicknessdue to corrosion, erosion, as well as a change in mechanical properties

These parameters are determined according to periodic in-line inspection, as well as according to mechanical testing of samples from damaged sections of pipelines. The verification calculation of the strength for this stage is reduced to the assessment of the strength margin

E8
σэτ=рmaxэDδminτ;E9
E10

If for the analyzed stagemarginпо (8) is not less than inin (3) and (5), then the operation of the pipeline can be continued.

To calculate the estimated timeof the next in-line inspection, it is necessary to have data on monitoringandon the basis of previous in-line inspections operations.

If such initial information is absent, then the construction of calculated curves is possible:

E11

where—If such initial information is absent, then the construction of calculated curves is possible [6, 7] (0≤≤ 0.03 forin hours).

2.2 Statistical strength analysis

In actual practice, in the manufacture and testing of pipes, the construction of pipeline sections and the operation of pipeline systems, all specified parameters of expressions (1)(11), are statistically variable, despite the determination of the main calculations in the design of pipelines throughout the system of design expressions.

The statistical analysis of the calculated parameters in the framework of the basic calculations of the strength of pipelines is aimed at establishing:

  • Minimum (min), average (m), and maximum (max) parameter values

  • Comparability with the values adopted in the project documentation

  • Deviations of the calculated parameters to the dangerous and safe side in comparison with the statistically determined.

On this basis, two decisions are made about the possibility or impossibility of further operations of pipelines.

In the first case, the main requirement for the strength of pipelines must be met; in the second case, the strength is considered not ensured if the maximum operating stresses exceed the allowable.

σmaxcэσ;σmaxcэ>σ.E12

For the scientific substantiation of the need and possibility of extending the operation of pipelines in cases of failure to meet the strength conditions, it is possible to reduce the operating pressure to level.

рсэ<рэ,whenσсэ<σэσE13

Simultaneous fulfillment of conditions (1)(3) requires the mandatory calculation of the strength of the pipeline—its pipes and sections, where the realization of expression (12) is detected.

This calculation should include:

  • Maximum values of operating pressure рmaxэ

  • Maximum values of the diameter Dmax of the pipe or section of the pipeline

  • Minimum wall thickness δmin

  • The minimum values of the characteristics of mechanical properties σтmin,σвmin.

In this case, you can write

σсэ=FσpmaxэDmaxδminσ=minσвminnвσтminnтE14

The strength condition according to expression (14) should be checked according to the statistical analysis and when conditions (12) are fulfilled.

At the same time, both for condition (12) and for conditions (13) and (14), it is advisable to give an assessment of the strength according to (1):

  • With average values of all parameters рmэ, Dm, δm, and σтm,σвm

σсэ=FσpmэDmδmσ=minσвmnвσтmnтE15

  • At extreme (extreme) values рmaxэ, δmax, Dmin, and σтmax,σвmax

σmaxэ=FσpmaxэDminδmaxσ=minσвmaxnвσтmaxnт.E16

Thus, according to (12)(16), the calculated (average, minimum, maximum) values of рэ, D,δ, and σт,σвare due to a whole range of design, technological, and operational factors.

2.2.1 Operating pressure

The statistical nature of operating pressures рэ satisfies inequalities.

рminэрmэрmaxэE17

due to changes in the actual pressures in a given pipe or in a given section of the pipeline due to

  • Actuation systems to maintain the specified working pressure at pumping stations

рнminэрнmэрнmaxэE18

  • Deterministic design and actual operational differences of hydrostatic pressures Δргfrom changes in the profile of the heights of laying pipelines

ΔргminэΔргmэΔргmaxэE19

  • Deterministic design pressure changes Δргс

  • Deterministic design pressure changes due to changes in hydraulic resistance to the movement of oil and oil products (due to changes in flow areas, viscosity, and temperature of the transported working fluid)

ΔргсminΔргсmΔргсmaxE20

  • Deterministic design and actual operating pressure changes due to external effects on the pipeline (seismic, temperature, vibration, aero-hydrodynamic)

ΔрвminΔрвmΔрвmaxE21

In the basic calculations using expressions (1)(11), for deterministic and statistical estimates of the static strength of pipelines, pressure components should be included when the pipelines are operating at maximum design conditions:

ррэ=рэ+Δрэ.E22

Deeper in scope, cyclic pressure changes due to software changes in pipeline operation modes (start-up, shutdowns, performance change—throughput) are subject to accounting for cyclic strength and durability calibration calculations. Statistical information on the change in pressure is obtained from the registration data at pumping stations.

2.2.2 Diameter of pipelines in operation

The diameter D, which is included in expressions (1), (4)(6), and the pipeline, is characterized by the scattering of its actual values. It is due to pipe manufacturing technology and is reflected in the maximum and minimum technological tolerances on the diameter ΔDт:

DmaxтDminт=Dп±ΔDт,E23

where Dп is design diameter.

Values ΔDтin either direction may be the same or different.

The diameters of Dэ pipes in various parts of pipelines that are fixed during operation during inspections and diagnostics of pipelines may differ from the diameters Dтafter the manufacture of pipes:

DmaxэDminэ=DmaxтDminт±ΔDэ.E24

Values ΔDэ, as a rule, have a positive value due to the possible deformation under the action of test or operating modes with increased pressure.

The second factor of change in diameters Dэcan be ovalization of the cross-section during transportation, construction, and operation (usually while maintaining the length of the perimeter of the pipeline):

Dэ=FDDminэDmaxэ,E25

where Dminэ,Dminэare minimum and maximum diameter in the zone of ovalization.

For ovalized sections, the calculated determination of stresses according to (1) should take into account their increase.

The calculated justification of static strength in the framework of the basic calculations according to (1)(21) should be mainly oriented:

  • To the maximum values in Dmaxin (23) and (25)

  • At maximum operating stress σmaxэ

2.2.3 Pipeline wall thickness in operation

Pipeline wall thickness δhas the most significant effect on operating stresses σэand strength conditions.

The statistical variation of values, as well as diameters D, is due to:

  • Rolling sheet technology, which is a blank for pipes:

δminтδmaxт=δп±Δδт,E26

where δпis design wall thickness and Δδтis technological tolerance for thickness.

The change in wall thickness during pipe rolling can be neglected, taking into account the main bending deformations.

When testing pipes at the manufacturing stage and during construction, it is possible to change the wall thickness downwards:

  • Due to plastic deformations from high-pressure tests ри

Δδи=FδриΔDэμ,E27

where μis Poisson’s ratio (0.3 ≤ μ≤ 0.5) and ΔDэis a possible increase in the diameter Dэof the pipeline, defined by (25).

  • Due to corrosion and erosion damage

Δδкэ=Fδрэτэскэ,E28

where τэis operation time and скэ is the rate of corrosion erosion damage.

In the basic calculations of static strength according to the basic calculations of static strength according to the expressions (1)(28), the minimum value of the wall thickness should be used:

δminэ=δпΔδтΔδиΔδкэ.E29

Expression (29) under condition (14) will mean the maximum increase in operating stresses σэ.

2.2.4 Characteristics of mechanical properties

Mechanical properties with strength characteristics (σв,σт), (R1, R2), as well as рэ,D,δare stochastic. In order to ensure and justify the static strength of pipelines ,their minimum values should be entered into the calculation. The statistical variation in the characteristics of the mechanical properties of pipe steels is determined by a set of technological factors:

  • Chemical composition and structural structure (grain size d)

  • Modes of thermal and thermomechanical (tт) processing

  • The level of preliminary plastic deformations еn during sheet rolling, rolling of tube blanks, and testing of pipes, sections, and sections of pipelines

  • Temporary factors of aging and degradation in time τ

σвэσтэ=Fσd3tпеntэ.E30

For widely used pipe steels, the increase in strength characteristics σт,σвis usually combined with a decrease in ductility.

In the main calculations of the strength of pipelines, it is recommended to use statistical data on the reduction of strength characteristics:

σвэσтэ=minσвσт.E31

2.2.5 Reflection of statistical factors of strength in margin

Use in domestic and foreign basic regulatory calculations of the strength of the system of strength margins nσ(when calculating the permissible stresses σ) and reliability coefficients K1, K2, m, and n (when calculating the limiting states and resistances) makes it possible to obtain a connection between them in the form of expression (5).

All coefficients of expression (5) in a deterministic form, taking into account the statistics of design parameters for expressions (12)(31), reflect the general variation of design, technological, and operational strength factors.

The strength margins nσof (5) in the deterministic basic and calibration statistical calculations take into account the level of the necessary reduction of operating stress σэcompared to dangerous stresses σmaxэ<σоп:

nσ=σопσmaxэ.E32

At the same time, dangerous stresses σопare understood not only as deterministic but also as statistical limits of strength σв(to exclude one-time static damage) and plasticity σт(to exclude one-time static damage) and plasticity (to prevent the formation of unacceptable plastic deformations):

σоп=minσвσт.E33

The statistical information about the values σэis obtained on the basis of the analysis of the stress–strain state by statistical parameters рэ,D,δon the basis of the entire system of expressions (1)(33).

In combination with the statistical data on the hazard values of the criterial characteristics σопin the form of tensile strengths σвand yield strengths σт(or design resistances R1, R2), a scatter can be obtained nσnвnт:

nσmin=σвminσmaxэσтminσmaxэ;nσmax=σвmaxσminэσтmaxσminэ.E34

To make decisions about the admissibility of safety margins, nσshould be estimated, and their average values

nσср=σвmσmэσтmσmэ.E35

The strength of the pipeline, determined by the allowable stresses in the statistical interpretation, can be considered as secured if the normatively specified margin nσнnвнnтнsatisfies the inequality

nσmaxnσmnσminnσн.E36

According to these statistics, it is possible to quantify statistical variations of the coefficients m, n, K1, and Kн. On this basis, you can make a conclusion about the strength of the pipeline, if combinations are performed:

mminKIImaxKнmaxnmin>mK1Kнnн.E37

Failure to comply with conditions (36) and (37) requires making decisions about conducting refined basic and calibration calculations by deterministic and statistical methods.

2.3 Basics of probabilistic strength calculations

The accumulation of statistical information in the form of histograms of the main design parameters of strength makes it possible to proceed to a probabilistic analysis in the form of a distribution of strength. They are reflected in regulatory calculations for limiting states and limiting resistances [5] through the safety factors for the material, load, working conditions and purpose, and load in expression (25):

The essence of this analysis [1, 6, 8] is:

  • to obtain the probability density functions р of external and internal effects (the number of pressure рэ) and the corresponding design stresses σэand design resistances (yield strength σТand strength σв) with the subsequent determination of the probability of failure Ррin areas, where areas with extremely low probabilities (Рр107);

  • to construct probability functions Рσэand Р{σТ,σв} with the definition of the relationship between strength margins {nТ,nв} and given probabilities Рσэ, Р{σТ,σв}, corresponding to the volume of the initial statistical and probabilistic information.

There is a simple relationship between probability Р and the amount of initial statistical information:

n=i0,5P,E38

where i is the sequence number of the measured value and n is the total number of measurements.

With a commonly used sample of 20 measurements, the value is Р = 2.5·10−2 (or 2.5%).

To estimate the values of Р at the level of 10−2 (or 1%) it is necessary to make already 50 measurements, and for the probability of 10−4 – 5000.

In statistical and probabilistic studies of the mechanical properties of structural steels, the volume of samples n is in the range of 20–22.000 [6, 10]. According to the histogram of the limit distribution functions σт,σв, the functional F is obtained for the strength margins nт,nв:

nTnв=FPσэР(σТσв)E39

The number of laboratory samples of steel 17G1S, cut from pipes in the initial state and after 40 years of operation is 28.

To solve probabilistic problems of strength in terms of expression (39) in the zone of small probabilities of destruction Рр, a large amount of statistical information is needed with samples measured in the hundreds and thousands, which is practically impossible in many real cases. In this connection, it is more promising to use expressions (38) and (39), which allow nTnвestimating reserves for a given probability Р of calculated characteristics, corresponding to the availability of experiments on the distribution functions, Рσэand Р{σТ,σв}, with the choices of tens and hundreds.

Figure 5 shows the scheme for the implementation of a probabilistic analysis of reserves: along the ordinate axis, the probabilities Рσэand Рσвon a scale corresponding to the normal distribution law. Then by the median values σmэиσвmfor the probability Р = 50% and for other values of Р (Р < 50%).

Figure 5.

Scheme for assessing the impact of probability P on the strength margin.

nσвm=σвmσэm;nσвр=σврσэр.E40

If, according to the results of statistical processing of values σэandσвthe parameters of their probability distributions are obtained—(the coefficients of variation VσэandVσвand their average values σmэmandσвm, then the calculated values σврand σэрfor a given probability P are obtained from the expressions.

σэрσвр=σэmσвm1ZpVσэVσв,E41

where Zpis distribution quantile depending on Р.

For coefficients of variation in the range of VσэиVσвin the range of 0.03–0.1 the calculated probabilities Рр are obtained when the margin factors nσв>1,8are in the range of 10−15 to 10−5.

With the currently existing banks of data on operational load σэand mechanical properties of pipe steels (σв,σТ), it is more reasonable to consider not determining the values of Рр in the area of their low values, but determining the strength margins from (40) using the specified probability parameters Р in the range of 10−4 to 10−5 and above.

On the basis of (5) and (41), it is possible to analyze changes in the regulatory strength margin taking into account the probabilistic characteristics of the operational loading σэand the limits of strength σв:

np=nв1ZpVσв1+ZpVσэ=nвn¯p,E42

where np is margin of strength for a given probability Р and n¯pis margin reduction ratio. The relationship between n¯pand Р in (42) with Vσв=0,05and Vσэ=0,08is shown in Figure 6. From the data in Figures 5 and 6, it can be seen that the greatest influence on the allowable change in the strength margins is observed when Р decreases from 0.5 to 10−3. Refinement of probabilistic calculations of strength at lower Р does not make much practical sense.

Figure 6.

Relative decrease in strength margins n¯ with changing probabilistic characteristics of loading and mechanical properties.

The probabilistic approach acquires its practical relevance in the critical sections of trunk pipelines:

  • At their intersections with other transport systems (rail, high-voltage, pipeline), with non-compliance with the allowable distances from other facilities and infrastructures

  • On water transitions

  • In zones of geological faults, landslides, and seismic effects

This approach becomes significant and necessary for those cases when the assigned service lives and estimated durability are developed, and the in-line inspections show increased defectiveness.

3. Implementation of combined methods to substantiate strength

3.1 Formation of the structure of the combined methods

The generalized structure of the standard basic and calibration determination of the strength parameters of main oil pipeline systems discussed above (Section 2) reflects the theory and practice of computational and experimental substantiation of strength developed in our country and abroad for six to seven decades. The focus is on the trunk pipelines for the transportation of oil and oil products. The calculation of strength analysis is based on two methods—the method of calculation for permissible stresses (adopted in foreign practice) and the method of calculation for limiting states and limit resistances (adopted in Russian practice).

The most developed and applied is the deterministic strength calculation at the design stage. This solves the direct main problems of determining the wall thickness of the pipeline for given pressures, throughput of pipes and selected pipe steels. The same method is used at the stage of calibration calculations of the strength of the pipelines under construction and the majority of the pipelines being operated.

In those cases when it is necessary to calculate the substantiation of the strength of functioning pipelines with deviations from the design decisions and when defects in pipes occur outside the established norms, it is necessary to carry out calibration calculations using actual statistical information on all the calculated parameters. One of the tasks solved at the same time is the appointment of all the main design parameters according to the obtained statistical information. In these cases the preservation of regulatory reserves is typical.

For the most critical sections of pipelines, statistical strength analysis may be insufficient and unacceptable. Then probabilistic estimates of strength are required using the functions of the distribution of operational loading and the mechanical properties of pipe steels by the parameter of operation time. For these situations, it becomes possible to change the safety margins for the required probabilities of the occurrence of dangerous states.

The scientific basis of these calculations is the entire system of calculation expressions (1)(42) (Figure 7).

Figure 7.

Block diagram of regulatory foreign and domestic calculations.

This system has been and remains basic in all international practice [1, 2, 3, 4] to the present time with the development of methods for the design, construction, and operation of trunk pipelines to ensure their strength and deformability expressed in a gradual decrease in margins n (1.8 ≥ nT ≥ 1.2; 2.5 ≥  ≥ 1.7) и and an increase in the strength characteristics σт,σв(200 ≤ σТ≤ 800; 420 ≤ σв≤ 920 MPa).

All uncertainty factors included in the calculations and reflecting the operating conditions, design, and construction technologies were taken into account by the coefficients (nσ,nT, ) and the standard purpose of guaranteed mechanical properties (σв,σТ).

A generalized analysis of trends and parameters of the development of pipeline transport of oil and oil pipelines and methods for calculating the strength is made in [6, 7].

Expressions (1) and (2) are initial in assessing the strength of pipelines at all the main stages of the life cycle—design, construction, operation, and decommissioning. Currently two tasks are being solved:

  • The direct task of a deterministic basic calculation of the wall thickness δof the pipeline at the design stage with a preliminary feasibility study of the diameter and pressure р as well as with the selected structural material σоп,(σв,σТ) and assigned margin nσ,nTnв:

δрDв2σ=рDвnσ2σоп.E43

Under these conditions, the wall thickness δcannot be less than the value calculated by expression (6) (Figure 8).

Figure 8.

Regulatory basic and verification calculations for different stages of the pipeline life cycle.

At the stages of construction, operation, and decommissioning on the basis of (43), deterministic calibration calculations are performed with the following objectives:

  • Check of permissible operating pressure [р] at specified

р2σопnσ.E44

  • Validation of selected and assigned mechanical properties σоп,σТσвwith known р,Dв,δ,nσ

σопpDв2δnσ.E45

  • Check by (6) the permissible wall thickness δat

δрDвnσ2σоп.E46

  • Checking the allowable strength margin[nσ]with known

nσσоп2δрDв.E47

In deterministic calculations according to (1) and (2), a systematic analysis of uncertainty factors affecting the quantities nσ, n, m, KI, and is carried out. These factors [6, 7, 8, 9, 10] included such factors as:

  • The effect of the absolute dimensions of the sections Dвδ

  • Type of stress–strain state (components of the main stress σ1,σ2,σ3)

  • Temperature–time change in mechanical properties σТ,σв, which determines the processes of aging and degradation

  • Availability of welded joints with altered properties

3.2 Databases for calculations

On the basis of statistical measurements and estimates of all specified design parameters (pressures р, mechanical properties σТ,σв, geometrical dimensions δand with variations within {min, max}), first of all, the determination of their average (median) values becomes important

Rпm=1nпRпi,E48

where nп is the number of measurements of calculated parameters Rпi.

According to the obtained statistical information on the parameters Rпi, the corresponding histograms are constructed by the intervals of their values. For example, Figure 9 shows the change in the main design parameters—pressure р and ultimate strength σв[10].

Figure 9.

Histograms of pressures and strengths.

Inequality (47) with the parameters included in it, as well as the data from Figure 9, are the basis for calculating the determination of Rp taking into account the statistical and probabilistic dispersion characteristics. At the same time, the assigned parameters should correspond to the inequality systems

PminPmPpPmaxE49
σTσвminσTσвpσTσвmσTσвE50
DminDmDpDmaxE51
δminδpδmδmaxE52

For the design stage, statistical analysis of the design parameters using expressions (8)(12) is done using factory test data for sheet blanks for pipes (δ), pipes (δ,D), and laboratory samples for static tension (σТ,σв).The values obtained δр,Dp,(σТ,σв)p are entered in the technical conditions or standards. They are the basis of deterministic calculations.

If these measurements are carried out at the stage of construction or operation, then the data obtained from (49)(52) are included in deterministic calibration calculations and expressions (44)(47) (Figure 10).

Figure 10.

Statistics on the relative decrease or increase in operating time.

Technical diagnostics of trunk pipelines (mainly using in-line diagnostics [11]) shows that the most significant from the point of view of strength is the decrease in time τwall thickness parameters δdue to such processes as uniform and uneven corrosion, formation and development of cracks of corrosion, and cyclical nature. These processes, as a rule, increase the variation of values δin (52).

The statistical variation of diameters in (11) at the stage of manufacturing, construction, and operation of the linear part of trunk pipelines is small (0.99 ≤ Dв/Dm ≤ 1.01) and can be neglected in deterministic and statistical strength calculations according to (1). However, if during operation there are significant reductions in wall thickness δ, then a significant local increase in diameter (by 5–10 due to plastic deformations with the formation of shape defects) is possible. Similar processes of loss of shape and increase in are possible with nonstandard bending of pipelines with loss of stability and formation of corrugations.

The change in the average values and variation of the design characteristics of strength (σв,σТ) according to (50) is associated with the instability of technological processes for the production of pipe steels, rolling and heat treatment of sheets, pipe manufacturing, construction of pipeline systems, as well as temporary processes of aging and degradation.

Figures 11 and 12 show histograms and distribution functions of the mechanical properties of a long-term (up to 50 years) operated 17G1S tubular steel. This information is used in the implementation of calculations for paragraphs 2.1–2.3.

Figure 11.

Strength tensile histograms (total number of tests n = 160).

Figure 12.

Distribution function of the ultimate strength of pipe steels 17G1S.

In the process of development (in time) of pipeline transportation of hydrocarbons in Russia and abroad, three trends remain dominant using deterministic (D), statistical (C), and probabilistic (P) methods (Table 1 and Figure 13).

  • Increase in diameters of pipelines D (from 250 to 300 mm to 1200 to 1400 mm) and pressures p (from 2.0 to 2.5 MPa to 14.0 to 16.0 MPa)

  • Increase of mechanical properties of pipe hoists (yield strengths σТ) from 200 to 250 МПа to 600 to 800 MPa and strengthfrom 400 to 450 MPa to 700 to 900 MPa

  • Reduction of strength margins nТ(от 1.8–3,2 до 1.2–1,5) and(от 2.4–3,5 до 1.6–1,8) in expression (3) and the estimated coefficients in expression (5)

No.DescriptionSymbolsValue
1.Coefficient of working conditionsm0.6–0,9
2.Load reliability factorK11.1–1,5
3.Material reliability factorK21.34–1,55
4.Reliability factor to destinationKн1.0–1,05

Table 1.

Calculated standard values of coefficients.

Figure 13.

The main regularities of changes in the estimated parameters of pipelines.

4. Conclusion

The above data (pp. 1–3) allow you to build a comprehensive strength analysis system using deterministic, statistical, and probabilistic methods for various components, taking into account design, technological, and operational factors.

Deterministic strength calculations are used as part of regulatory national and international approaches for design calculations. They apply to the majority of functioning oil pipelines systems.

Statistical calculations become relevant in cases where during operational process the diagnostic studies of the condition of pipelines are carried out or during construction routine tests and during operation defects of technological and operational origin are detected. Probabilistic calculations are necessary on the most dangerous sections of the pipeline (in case of crossing water barriers, transport infrastructures, and laying offshore pipelines).

In all cases, safety margins are linked to the normative standard documentation (deterministic approach), the results of diagnostics and defect identification and measurement (statistical approach), and taking into account the most dangerous operating conditions (probabilistic approach).

Download

chapter PDF

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

How to cite and reference

Link to this chapter Copy to clipboard

Cite this chapter Copy to clipboard

Dmitry Neganov and Nikolay Makhutov (November 14th 2019). Combined Calculated, Experimental and Determinated and Probable Justifications for Strength of Trunk Crude Oil Pipelines [Online First], IntechOpen, DOI: 10.5772/intechopen.89036. Available from:

chapter statistics

19total chapter downloads

More statistics for editors and authors

Login to your personal dashboard for more detailed statistics on your publications.

Access personal reporting

We are IntechOpen, the world's leading publisher of Open Access books. Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. We share our knowledge and peer-reveiwed research papers with libraries, scientific and engineering societies, and also work with corporate R&D departments and government entities.

More About Us