Axially-Orientated Gouges or Similar Metal Loss Defects 1 7 .1 Basic Equations

External interference during operation, or damage during construction, can cause gouges or scratches on the pipe’s surface, Figure 18. These metal loss defects may be accompanied by local plastic deformation. If this deformation caused a dent, then the gouge must be assessed using sophisticated fracture mechanics methods (see later).

Figure 18. Damage on a Pipeline Failure in Louisiana, USA (Image courtesy of National Transportation Safety Board, USA)

17 See Section 4.4.9.2 for methods for assessing the fatigue life of these type of defects.

Image from National Transportation Safety Board website: www.ntsb.gov

Comment: delete

In ductile linepipe, the failure stress of an axially-orientated gouge subject to internal pressure loading is described by Equation (7):

2c

R

d t

Defect Dimensions

It has been recommended that [Cosham and Hopkins, 2001]:

2

for use in Equation 7, and D = outside diameter to be used in the equations. Note that these equations are only validated for pipewall thicknesses up to 22mm and toughnesses of 21J (2/3 Charpy) [Cosham and Hopkins, 2001].

Figure 19^{1 8}, shows the accuracy of using Equations 7, 9 and 10 to assess gouge or gouge-like defects in linepipe.

4.5.3.2 Note on Structural Assessment of Gouges

If a gouge is detected in the field, an engineer needs to check:

- FOR SURFACE CRACKING - There may be some crack-like indications (spalling) caused by the damaging object. If the cracking is deep, it may be indicative of a gouge that has cracked due to denting (the denting may not be visible, as it may have been ‘pushed out’ [Hopkins et al, 1989; Hopkins et al, 1992]. This is severe, and requires repair.

- FOR EVIDENCE OF DENTING - The impact may have also dented the pipe.

Residual denting around a gouge is severe – see later.

Gouges can be assessed using the above equations, providing your pipeline has a toughness >20J [Cosham and Kirkwood, 2000; Cosham and Hopkins, 2002]. Note that a gouge needs to be checked for possible fatigue crack growth in some pipelines (e.g. some liquid lines).

18 See Hopkins and Cosham, 2001 for data used in Figure 19.

Allowance (e.g. adding 0.5mm to defect depth) for the hard layer or sub-surface cracking is advisable, if they are to be left in the pipeline, but there should be no risk of environmental cracking, and no residual denting, and no problems from cyclic loading.

Figure 19. Predicted Failure Stresses of Full Scale Burst Tests on Vessels containing Gouges or Similar Defects.

Finally, an engineer should always think carefully of the consequences of ‘getting things wrong’. If damage is in a pipeline in a ‘high consequence’ area, the damage should be inspected closely before assessment, and appropriate safety factors included in the assessment.

4.5.4 Dents

4.5.4.1 Burst Strength of Plain Dents

Dents in pipelines are assessed using data derived from full scale tests, and large dents can be tolerated, although their behaviour under cyclic loads, or when they coincide with seam welds, remain a problem [ Hopkins et al, 1989; Hopkins et al, 1992; Fowler et al, 1994; Hope et al, 1995; Kiefner et al, 1996; Kiefner & Alexander, 1997;

Bjornoy et al, 2000; Rosenfield, 1998; Roovers et al, 2000].

The effect of a ‘plain’ dent (i.e. one with no associated loss of wall thickness defect, and of smooth shape) is to introduce high localised stresses and cause yielding in the pipe material. The high stresses and strains caused by the dent are accommodated by the ductility of the pipe. Full scale test results have confirmed this by showing that plain dents do not generally affect the burst strength of the pipeline [Hopkins et al, 1989; Kiefner et al, 1996; Kiefner & Alexander, 1997]. On pressurisation the dent

0 20 40 60 80 100 120 140 160

Failure Stress/Yield Strength, percent

0 20 40 60 80 100 120 140 160

Predicted Failure Stress/Yield Strength, percent

CONSERVATIVE UNCONSERVATIVE See original reference

for test details

attempts to move outward, allowing the pipe to regain its original circular shape.

Provided that nothing restricts the movement or acts as a stress concentration (e.g. a gouge or a kink), then the dent will not reduce the burst strength of the pipe.

Empirical limits for plain dents under static internal pressure loading have been derived from extensive full scale testing. It should be noted that all of the dent depths (usually measured as % pipe diameter) in the full scale tests were measured at zero pipeline pressure. Based on these full scale tests [Hopkins et al, 1989; Hopkins et al, 1992; Fowler et al, 1994; Hope et al, 1995; Kiefner et al, 1996; Kiefner and Alexander, 1997; Rosenfield, 1998; Roovers et al, 2000; Bjornoy et al, 2000], a variety of dent sizes have been quoted as ‘acceptable’ – dents of depth¹⁹ up to 10%

pipe diameter have little effect on the burst strength of pipe. Additionally, there is an API publication [Kiefner and Alexander, 1997; Anon 1997] specifically on the assessment of dents caused by rocks in pipelines. The reader is directed to these references for more detailed information.

In full scale tests on plain dents on welds very low burst pressures have been recorded. Therefore, the burst strength (and the fatigue strength) of dents containing welds cannot be reliably predicted, and caution is recommended with this type of damage.

It should be noted that a dent of depth 10% the pipe diameter might be associated with surface damage, which makes it a severe defect. Also, this deep dent may restrict both product flow, and the passage of pigs in the pipeline. Finally, this depth may be

‘acceptable’ in some pipelines under static pressure loading, but it will be reduced significantly if the pipeline is subjected to cyclic loading (see next).

4.5.4.2. Fatigue Life of Plain Dents

Large cyclic stresses and strains are localised in a dent under cyclic pressure loading.

The depth of a dent changes with internal pressure, meaning that the magnitude of the stress concentration changes as dents can ‘reround’ under cyclic internal pressure loading.

Full scale fatigue tests [Eiber et al, 1981; Wang and Smith, 1988; Hopkins et al, 1989;

Hopkins et al, 1992; Fowler et al, 1994; Hope et al, 1995; Kiefner and Alexander, 1997; Roovers et al, 2000] on plain dents indicate that they reduce the fatigue life compared to plain circular pipe. The greater the dent depth the shorter the fatigue life.

No fatigue failures occurred in those tests where the pipe was hydrotested prior to fatigue cycling, because the dent was permanently pushed out (rerounded), reducing the stress concentration.

A number of semi-empirical or empirical methods for predicting the fatigue life of a plain dent subject to cyclic pressure loading have been developed [ Fowler et al, 1994;

Hope et al, 1995; Kiefner and Alexander, 1997; Rosenfield, 1998; Roovers et al, 2000]. One of the relationships, developed by SES in Houston [Fowler et al, 1994;

Kiefner and Alexander, 1997], is:

19 The literature indicated that the key dent parameter is the depth, with length and width secondary.

74

N number of cycles to failure



σ stress intensification factor (obtained from original references)

∆p cyclic pressure (psi)

This fatigue model is based on an S-N curve, modified for the stress concentration due to the dent. The ‘stress intensification factor’ was derived from non-linear elastic- plastic finite element analyses to account for the stress concentration due to the dent. The reader is directed towards the original references if they wish to apply the various fatigue methods.

4.5.4.3 Plain Dent Containing a Defect 4.5.4.3.1 Burst Strength

The failure behaviour of a dent containing a gouge is complex. A dent and gouge is a geometrically unstable structure. Outward movement of the dent promotes initiation and growth of cracking in the base of the gouge, changing the compliance of the dent.

The failure of a dent and gouge defect involves high plastic strains, wall thinning, movement of the dent, crack initiation, ductile tearing and plastic flow [Leis et al, 2000].

Empirical relationships for predicting the burst strength of a smooth dent (of depth H) containing a gouge have been proposed by British Gas [Hopkins et al, 1989], the EPRG [Roovers et al, 2000], and Battelle [Mayfield et al, 1979; Maxey, 1986].

The Battelle model is:

( )

A semi-empirical fracture model for assessing the burst strength of a dent-gouge defect has been developed by British Gas [Hopkins, 1992], and has subsequently been included in the EPRG recommendations for the assessment of mechanical damage [Bood et al, 1999].

The fracture model is based on tests in which the damage was introduced into unpressurised pipe; therefore, the dent depth measured at zero pressure must be used

H

R

D

t d

or corrected for any internal pressure [Hopkins et al, 1992; Roovers et al, 2000;

Rosenfield, 1998].

The fracture model gives more accurate and reliable predictions that the above empirical relationship of Battelle. The model is defined as follows (in SI units):



The flow stress (Equation (15)) assumed in the dent- gouge fracture model is not appropriate for higher grade steels (greater than X65), due to the increasing yield to tensile ratio with line pipe grade.

Ho dent depth measured at zero pressure (mm) Hr dent depth measured at pressure (mm) K1 non-linear regression parameter K2 non-linear regression parameter

This failure criterion for a dent containing a metal loss defect does not give a lower bound failure stress. It is a mean predictive model. Additionally, the model is semi-empirical and therefore limited by the bounds of the original test data, and is prone to high scatter [Cosham & Hopkins, 2001; Cosham & Hopkins, 2002].

4.5.4.3.2 Fatigue Life

The fatigue life of a dent containing a gouge is difficult to predict. Full scale tests indicate that the fatigue life of a combined dent and gouge can be of the order of between ten and one hundred times less than the fatigue life of an equivalent plain dent [Hopkins et al, 1989]. In some cases even shorter fatigue lives have been observed during testing.

4.5.4.4 Note on the Assessment of ‘Mechanical Damage’

Dents and/or gouges in a pipeline are indicative of impact damage. When a dent or a gouge is suspected in a pipeline, they should be carefully investigated to determine if they are co-incidental, as the combined dent and gouge is a very severe defect, and usually requires rapid repair.

4.5.5 Corrosion²⁰

There are several approaches that have been used to characterise the behaviour of both through and part wall corrosion defects. The first two methods (approved by ASME) described below are the oldest and most proven. The most modern and most accurate (DNV RP 101) is covered last.

4.5.5.1 ANSI/ASME B31G

The most popular document for the assessment of the remaining strength of pipelines with smooth corrosion has been ANSI/ASME B31.G [Anon., 1984; Anon., 1991].

This supplement to B31 was developed over 20 years ago, based on work in the early 1970s [Kiefner and Duffy, 1973], although it has since been updated [Kiefner and Vieth, 1989; Anon., 1991].

It is based on an empirical fit to an extensive series of full scale tests on vessels with narrow machined slots. The basis of the equation used in B31G is relatively simple and involves:

• assuming the maximum pipe hoop stress is equal to the pipe material's yield strength, and,

• characterising the corrosion geometry by a projected parabolic shape for relatively short corrosion, and a rectangular shape for long corrosion.

The equation used in B31G is a derivative of Equation (7):

σf σ

where A=cross sectional area of defect in pipewall (for a rectangular, flat bottomed defect this is 2c.d),

and Ao=pipe wall area occupied by defect (for a rectangular flat bottomed defect this is 2c.t).

In the B31G code, a simplified equation is provided which represents the defect as a parabolic shape:

20 Usually, the most difficult data to obtain when assessing corrosion, is the expected corrosion growth rate. This is important, because most assessments of corrosion are based on intelligent pig data, where the defect must be assessed over its ‘whole life’, and its size at the end of the pipeline’s design life needs to be used in the calculations.

The flow strength (σ) is defined by 1.1xSMYS. The parabolic shape of the projected area is used as an approximation to the actual defect, and the Folias (bulging) factor is: It is stated in the B31G code that the above equations should only be applied to

corrosion defects, which have a maximum depth greater than 10% of the nominal wall thickness, and less than 80% of the nominal wall thickness. Furthermore, the relative longitudinal extent should satisfy the following equation:

M c

The above equation limits the use of the parabolic shape formulation because when M is greater than 4.0 (i.e. long corrosion), the approximation of a parabolic shape is no longer adequate. Instead a rectangular shape is used. Accordingly, the failure equation is replaced by the following equation:

σ_f σ d

The B31G criterion has been used successfully in the pipelines industry for many years. The method has been proven, in general, to be conservative and as a result an

‘improved’ method was developed which modified the existing B31G guidance. The modified B31.G method [Kiefner and Vieth, 1989; Anon., 1991] has recently been adopted as the preferred method for the fitness for purpose assessment of corrosion defects in the ANSI/ASME B31 Code.

The hoop stress at failure is given by:

 Equation (23) shows that the representation of the area of metal loss was revised (A=0.85d(2c)). A simple, arbitrary, geometric idealisation was proposed for hand

calculations (a factor of 0.85 rather than the 0.67 in the original ASME B31G was recommended). Additionally, an effective area method using the measured profile of the corroded area (A), was also developed to give more accurate predictions. The effective area method is most commonly known as RSTRENG (‘Remaining STRENgth’), as this is the name of the software that does the assessment of irregular-shaped corrosion defects, using the modified equations above.

4.5.5.3 New Methods

New assessment methods are becoming available. These include both in-house methods from leading research departments [Stewart et al, 1994], and the results of a large group sponsored projects [Kirkwood et al, 1996; Bjornoy et al, 1997; Bjornoy et al, 1999; Fu and Batte, 1999; Bjornoy et al, 2000]. The latest corrosion model given in DNV RP 101 [Anon., 1999b] is:

This is the same form as ASME B31G and the modified ASME B31G, but note that the flow stress is related to ultimate tensile strength (σ ), which can be estimated _U from the materials specified minimum tensile strength (SMTS). The DNV RP 101 methods are considered ‘best practice’ for corrosion assessment in modern linepipe steels [Cosham and Kirkwood, 1999; Cosham and Hopkins, 2000].

4.5.5.4 Comparison of Corrosion Assessment Methods

The above methods for assessing corrosion can be compared, Table 3.

METHOD

‘Original’ Battelle (19)