Subsea Pipeline Integrity and Risk Management - Qiang Bai, Yong Bai

Subsea Pipeline Integrity and Risk Management (eBook)

Qiang Bai, Yong Bai (Autoren)

eBook Download: PDF | EPUB

2014 | 1. Auflage
428 Seiten
Elsevier Science (Verlag)
978-0-12-394648-5 (ISBN)

Subsea repairs and inspection are costly for petroleum and pipeline engineers and proper training is needed to focus on ensuring system strength and integrity. Subsea Pipeline Integrity and Risk Management is the perfect companion for new engineers who need to be aware of the state-of-the-art techniques. This handbook offers a 'hands-on' problem-solving approach to integrity management, leak detection, and reliability applications such as risk analysis.

Wide-ranging and easy-to-use, the book is packed with data tables, illustrations, and calculations, with a focus on pipeline corrosion, flexible pipes, and subsea repair. Reliability-based models also provide a decision making tool for day-to-day use. Subsea Pipeline Integrity and Risk Management gives the engineer the power and knowledge to protect offshore pipeline investments safely and effectively.

Includes material selection for linepipe, especially selection of standard carbon steel linepipe
Covers assessment of various types of corrosion processes and definition of anti-corrosion design against internal as well as external corrosion
Gives process and flow assurance for pipeline systems including pipeline integrity management

Dr. Yong Bai obtained a Ph.D. in Offshore Structures at Hiroshima University, Japan in 1989. He is currently President of Offshore Pipelines and Risers (OPR Inc., a design/consulting firm in the field of subsea pipelines, risers and floating systems. In the 1990's, he had been a technical leader for several Asgard Transport pipeline and flowline projects at JP Kenny as Manager of the advanced engienering department. Yong was previously a lead riser engineer at Shell and assisted in offshore rules development at the American Bureau of Shipping (ABS) as Manager of the offshore technology department. While a professor, he wrote several books and served as a course leader on the design of subsea pipelines and irsers as well as design of floating systems. He also serves at Zhejiang University in China as professor.

Subsea repairs and inspection are costly for petroleum and pipeline engineers and proper training is needed to focus on ensuring system strength and integrity. Subsea Pipeline Integrity and Risk Management is the perfect companion for new engineers who need to be aware of the state-of-the-art techniques. This handbook offers a "e;hands-on"e; problem-solving approach to integrity management, leak detection, and reliability applications such as risk analysis. Wide-ranging and easy-to-use, the book is packed with data tables, illustrations, and calculations, with a focus on pipeline corrosion, flexible pipes, and subsea repair. Reliability-based models also provide a decision making tool for day-to-day use. Subsea Pipeline Integrity and Risk Management gives the engineer the power and knowledge to protect offshore pipeline investments safely and effectively. Includes material selection for linepipe, especially selection of standard carbon steel linepipe Covers assessment of various types of corrosion processes and definition of anti-corrosion design against internal as well as external corrosion Gives process and flow assurance for pipeline systems including pipeline integrity management

Chapter 2

Buckling and Collapse of Corroded Pipes

Abstract

In this chapter, an analytical solution is given for the calculation of moment capacity for corroded pipes subjected to internal pressure, bending and axial force. The corrosion defect is assumed to be symmetrical to the plane of bending, which represents the worst case. The derived analytical capacities of corroded pipes are compared with the results from finite element analysis. The work by Timoshenko and Gere is extended to account for the effect of a corrosion defect on the collapse capacity of pipes and modification to Timoshenko’s equations is conducted; The analytical solution is in agreement with Miller’s moment capacity equations for non-pressurised corroded pipes. For external overpressure well below the collapse pressure for the corroded pipe, the equations presented in this paper will be in good agreement with finite element results.

Keywords

Corroded PipesMoment CapacityInternal PressureBending and Axial ForceCombined LoadsAnalytical Analysis

Contents

1. Introduction

2. Moment Capacity of Pipe Under Combined Loads

General

Case 1: Corroded Area in Compression

The Fully Plastic Neutral Axis

Bending Moment Capacicity

Corrosion in Compression

Discussion of the Equations

3. Collapse Due to External Pressure

4. Modification to Timoshenko and Gere’s Equations

5. Interaction of Bending and Pressure

Analytical versus Finite Element Results

Guidelines for Bending Strength Calculations

Maximum Allowable Bending Moment

Limit Bending Moment for External Overpressure Cases

Collapse Pressure for External Overpressure Cases

Limit Bending Moment for Internal Overpressure Cases

6. Conclusions

References

Introduction

The purpose of this chapter is to derive analytical equations for the capacity of corroded pipes under combined loads. The derived capacity equations are compared with the results from finite element analysis. The derived analytical capacity equations may be used to extend applicability of the existing pipeline rules/guidelines.

Moment Capacity of Pipe Under Combined Loads

General

In this section, an analytical solution is given for the calculation of the moment capacity for corroded pipes subjected to internal pressure, bending, and axial force. The corrosion defect is assumed to be symmetrical to the plane of bending, which represents the worst case. For simplicity, initial out of roundness is not included in the solution. The rationality of this is that the influence of initial out of roundness is small (for thick-walled pipes with practical out of roundness). The moment capacity is defined as the moment at which the entire pipe cross section yields.

FIGURE 2.1Four Discussed Combination of Defect and Bending.

The solution presented in this section takes the following configurations into account: corroded area in compression (case 1), in compression and some in tension (case 2), in tension (case 3), in tension and some in compression (case 4). The four cases are shown in Figure 2.1. Only case 1 is fully discussed here, but the final solutions for cases 2–4 are given in the guideline at the end of the chapter.

Case 1: Corroded Area in Compression

To keep the complexity of the equations on a reasonable level, the following assumptions have been made:

• Diameter/wall-thickness (D/t) ratio 15–45.

• No initial out of roundness and no diameter expansion.

• Cross sections remain circular throughout deformation.

• Entire cross section in yield due to applied loads.

• Material model is elastic and perfectly plastic.

• Defect region is symmetric around plane of bending.

• Corrosion defect is of infinite length and does not cause local stress concentrations.

In general, the Von Mises yield criterion can be expressed as

[2.1]

where σl is the longitudinal stress, σθ is the circumferential/hoop stress, and σY is the material yield stress. Solving the second-degree equation for the longitudinal stress σl gives

[2.2]

If compression is defined as negative and σcomp as the longitudinal compressive stress that causes the pipe material to yield, then σcomp is equal to σl as just determine with a negative sign in front of the square root. In the same way, σtens is equal to σl with a positive sign in front of the square root:

[2.3]

[2.4]

The hoop stress in the pipe at a given pressure may be found based on the following equation, Kiefner and Vieth [1]:

[2.5]

Here, D is the average diameter, t is the wall thickness, d is the defect depth, L is the defect length, and p is the resulting pressure acting on the pipe. As may be noted, the defect width is not included in the equation. This is mainly because, for all practical applications, the width has only a minor influence on the pressure capacity. For small defect widths in particular, it may though be favorable to replace Eq. [2.5] with hoop stress based on, say, finite element analysis. For further details on the influence of corrosion width on pressure containment, see Mok et al. [2].

The Fully Plastic Neutral Axis

For case 1, the true longitudinal force, F, can be expressed as

[2.6]

[2.7]

[2.8]

[2.9]

where Acomp1 is the compressed part of the nondefect cross section, Acomp2 is the compressed part of the defect cross section, and Atens is the part of the cross section in tension, see Figure 2.2. Here, rav is the average radius, β is half the defect width, and Ψ is the angle from the plane of bending to the plastic neutral axis.

Inserting Eqs. [2.7]–[2.9] into Eq. p2.6 and solving for Ψ gives

[2.10]

[2.11]

FIGURE 2.2Pipe Cross Section and Idealized Stress Diagram for Fully Plasticized Cross Section.

Now, inserting Eq. [2.3] for σtens and Eq. [2.4] for σcomp in Eq. [2.10] gives

[2.12]

where FY: is the plastic axial force for a corroded pipe:

[2.13]

Bending Moment Capacicity

Corrosion in Compression

The bending moment capacity, MC, of the pipe can now be calculated as

[2.14]

where Acomp1, Acomp2, and Atens are as defined previously and is the perpendicular distance from the bending axis to the mass center of each area, see Figure 2.2:

[2.15]

[2.16]

[2.17]

Inserting Eqs. [2.15]–[2.17] into Eq. [2.14], one gets the following expression for the bending moment capacity:

[2.18]

[2.19]

Substituting the expressions for the tensile and compressive stress, Eqs. [2.3] and [2.4], into Eq. [2.18] gives the final expression for the bending moment capacity for case 1:

[2.20]

where the angle to the plastic neutral axes is given by Eq. [2.12], the plastic yield force for a corroded pipe by Eq. [2.13], and the constants k1 and k2 by Eqs. [2.11] and [2.19], respectively.

Based on the limitation that the expression under the squarer root must be positive and that the angle to the plastic neutral axes between 0° and 180°, the moment equation is mathematically valid for the following range of hoop stress and axial force:

[2.21]

[2.22]

Discussion of the Equations

To demonstrate the consistency of the equations and the influence of different input parameters, some examples are shown in Figures 2.3 to 2.6. For all the examples, the D/t ratio is 25, d/t is equal to 30%, and σY is equal to 450 MPa. It is common to the four figures that the areas where each of the four cases given in Figure 2.1 is governing is plotted with a different line type. The line types are marked Case 1 to 4,...

Erscheint lt. Verlag	21.2.2014
Sprache	englisch
Themenwelt	Naturwissenschaften ► Physik / Astronomie
	Technik ► Elektrotechnik / Energietechnik
	Wirtschaft ► Betriebswirtschaft / Management ► Unternehmensführung / Management
ISBN-10	0-12-394648-4 / 0123946484
ISBN-13	978-0-12-394648-5 / 9780123946485

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