UKOPA Dent Management Strategy 1 Background Pipelines are thin shell structures which are susceptible to geometric distortions and dents during handling, construction, and operation. These dents and distortions, particularly when associated with other forms of damage such as gouges, or which are associated with welds, can lead to failure of the pipeline. Consequently, rigorous assessment is required. The identification of the damage mechanism which has caused the dent, the severity of the dent and an assessment of the potential for failure is essential. The severity of the dent and the assessment of the impact on pipeline integrity is dependent upon its location, the size and shape of the dent, the line pipe mechanical properties, and the applied static and cyclic stresses. The majority of pipeline in-line inspections (ILI) are carried out using the magnetic flux leakage (MFL) inspection tools. These tools are capable of identifying and locating dents in the pipeline, but most cannot currently size the dent. As a result, large numbers of dent features are reported by ILI companies for further consideration by the operator with little or no information for identifying any critical features which require investigation. Geometric ILI tools are capable of sizing the dent. The combination of data from MFL and geometric ILI can provide the data required for quantitative dent assessment using a range of published criteria. However, where dents have been identified by MFL ILI only, there may be a time delay before geometric inspection can be scheduled. A Dent Management Strategy is therefore required which uses all available information and highlights critical actions. 2 Requirements for a Dent Assessment Methodology The UKOPA Dent Management Strategy is being developed by the UKOPA RAWG using data, direction and advice from members who have supplied inspection data and investigation and assessment information. The purpose of the Dent Management Strategy is to: i) Allow prioritisation of dent features detected by ILI for investigation. ii) iii) Take account of dent location, depth and strain, association with other damage and association with welds. Assess dents under static and cyclic loading. 1
iv) Consider whether the dent features were present at commissioning, and the operational service to date and future operating requirements of the pipeline. The work to develop a UKOPA Dent Management Strategy is being progressed in a short, medium term and long term work programme. This document summarises the results of the short term work programme, which is complete. 3 Technical Basis The technical basis supporting the development of the UKOPA Dent Management Strategy is drawn from the references listed. 4 Prioritisation of Dent Features Reported by ILI for Investigation A methodology for the prioritisation of dent features detected by ILI in the form of a list giving the numerical order for investigation, ie a dig list, is required for use by the pipeline operator. Algorithms for prioritising dent features detected by ILI and generating a dig list for investigation have been developed as follows:- i) Dent Prioritisation Algorithm 1 MFL/UT Inspection Figure 1 This algorithm allows screening of MFL or UT ILI results to identify the number of features to be investigated. The algorithm may be used to identify dent features for immediate investigation and repair and to justify the need for geometric inspection. ii) Dent Prioritisation Algorithm 2 MFL + Geometric Inspection Figure 2 This algorithm allows quantified assessment and screening of dent features for investigation where both MFL and geometric inspection data is available. The algorithms have been subject to challenge and review by UKOPA members who have experience of the management of pipelines with identified dent features. The algorithms are currently based on either a depth limit or a strain limit being satisfied. Generally, the depth limit will be applied as high resolution caliper data required to determine strain may not be available in all cases. It is therefore recommended that a strain assessment is conducted where possible, where this is not possible then the assessment should be based on dent depth. This recommendation is under review as part of a continuing programme of work to determine the validity and practicability of the Dent Management Strategy. 2
The rationale used in the algorithms is outlined below, together with additional guidance agreed as part of the challenge and review process. 4.1 Rationale Dents which cannot be assessed and sentenced using published criteria must be investigated and repaired. Dents which can be assessed and sentenced using published quantified criteria are prioritised according to i) association with welds or metal loss and ii) circumferential location. Definitions applied are detailed below. The numerical ranking to be applied to the prioritisation for investigation of dent features detected using MFL/UT ILI is given in Table 1. The numerical ranking to be applied to the prioritisation for investigation and assessment of dent features detected using MFL/UT and geometric ILI is given in Table 2. The dent assessment criteria for gas and liquid pipelines are given in Table 3, and the dent assessment procedure using these criteria is given in Figure 3. It is recommended that in applying the dent prioritisation ranking, assessment criteria and assessment procedure, pipeline operators should record any additional pipeline specific data taken into account and document the justification for any additional decisions made. 4.2 Definitions Dent - Plain (or smooth) dent - a depression which produces a gross disturbance in the curvature of the pipe wall, caused by contact with a foreign body, resulting in plastic deformation of the pipe wall. a dent which causes a smooth change in the curvature of the pipe wall, causing depression on external surface, with no metal loss, no change of curvature at any adjacent seam or girth weld Dent associated with weld - a dent changes curvature at seam or girth weld Kinked dent - a dent which causes an abrupt change in the curvature of the pipe wall. An abrupt change in curvature is defined as one where the 3
Unconstrained dent Constrained dent Spring back Rerounding Position radius of curvature (in any direction) of the sharpest part of the dent is less than or equal to five times the wall thickness 1. a dent that is free to rebound elastically (spring back) when the indenter is removed, and is free to reround as the internal pressure varies. a dent that is not free to rebound or reround, because the indenter is not removed. A rock dent is an example of a constrained dent. (also referred to as rebounding) the reduction in dent depth due to the elastic unloading that occurs when the indenter is removed from the pipe. the change in dent depth under internal pressure. Top of line (TOL) 1-8 o clock to 4 o clock Bottom of line (BOL) 2-4 o clock to 8 o clock Notes:- 1 -TOL dents are usually assumed to be unconstrained 2 -BOL dents are usually assumed to be constrained Table 1 Prioritisation Rationale MFL/UT Inspection Description Dent associated with metal loss and weld, TOL Dent associated metal loss which is not corrosion, TOL toughness < 27J, TOL toughness 27J, TOL Plain dent associated with coating damage, TOL Dent associated with metal loss and weld, BOL Priority for Investigation 1 2 3 4 5 6 Additional guidance Excavate, NDT, repair or geometric inspection Excavate, NDT, repair or geometric inspection Excavate, NDT, repair or geometric inspection Excavate, NDT, repair or geometric inspection Excavate, NDT, repair or geometric inspection Check CIPS and DCVG data. Check for any evidence of local ground movement, backfill 1 This definition is based on the guidance in the EPRG recommendations for the assessment of mechanical damage [Error! Reference source not found.-error! Reference source not found.]. 4
Description Priority for Investigation Dent associated metal loss which is not corrosion, BOL 7 toughness < 27J, BOL toughness 27J, BOL Plain dent associated with coating damage, BOL Plain Dent, no evidence of coating damage, TOL Plain Dent, no evidence of coating damage, BOL 8 9 10 11 12 Additional guidance disturbance, and washout. If yes, reprioritise as 1. Check CIPS and DCVG data. Check for any evidence of local ground movement, backfill disturbance, washout. If yes, reprioritise as 2. Check CIPS and DCVG data. Check for any evidence of local ground movement, backfill disturbance, and washout. If yes, reprioritise as 3. Check CIPS and DCVG data. Check for any evidence of local ground movement, backfill disturbance, and washout. If yes, reprioritise as 4. Excavate, NDT, repair or geometric inspection Excavate, NDT, repair or geometric inspection Excavate, NDT, repair or geometric inspection Table 2 Prioritisation Rationale MFL/UT + Geometric Inspection Description Priority for Investigation Additional guidance Dent associated with metal 1 loss and weld, TOL Dent associated metal loss 2 which is not corrosion, TOL 3 toughness < 27J, TOL 4 toughness 27J, strain > 4%, depth >2% OD, TOL Plain dent, strain > 6%, depth 5 5
Description Priority for Additional guidance Investigation > 7% OD, TOL Plain dent, strain 6%, depth 7% OD, fatigue life expired, TOL 6 Dent associated with metal 7 loss and weld, BOL Dent associated metal loss 8 which is not corrosion, BOL 9 toughness <27J, BOL 10 toughness 27J, strain > 4%, depth > 2% OD, BOL Plain dent, strain > 6%, depth 11 > 7% OD, BOL Plain dent, strain 6%, depth 7% OD, fatigue life expired, BOL 12 Plain dent, strain 6%, depth 7% OD, fatigue life OK No further action Check CIPS and DCVG data. Check for any evidence of local ground movement, backfill disturbance, and washout. If yes, reprioritise as 6. Table 3 Criteria applied to Assessment of Dents in Gas and Liquid Pipelines Criterion Reference Source Plain dent maximum depth 1 7% OD EPRG, PDAM 6
Plain dent strain maximum strain 1 6% ASME B31.8 2 Dent associated with ductile 3 weld 2% OD ASME B31.8 2 maximum depth 1 Dent associated with ductile 3 weld maximum strain 1 4% ASME B31.8 2 Dent associated with metal loss due to corrosion If corrosion 20% wt, treat as plain dent UKOPA Member procedures 4 Notes:- 1 Where both dent depth and depth strain are available, dent depth should be used to prioritise dents. 2 ASME B31.8 addresses gas pipelines, integrity criteria are assumed to apply to all products. 2 Ductile weld Charpy energy 27J (full size) 3 Corrosion limit is proposed based on that applied by a number of UKOPA member companies and is being checked as part of the future work to develop the UKOPA Dent Management Strategy. The accepted code limit is 12% wt depth. 5 Simple Dent Assessment Rules The following rules apply to the assessment of dents in liquid and gas pipelines:- Kinked dents (eg wrinkles) - repair Static assessment of dents - depth/strain limits:- Plain dents :- depth 7% OD and/or strain 6% 1 Dents associated with welds: - depth 2% OD and/or strain 4% 1, providing weld toughness and quality are acceptable. If pipeline is pressure cycled - assess dent location for associated damage: TOL coating damage (DCVG/Pearson) BOL disturbance, washout etc BOL dents in rocky locations should be monitored not excavated Dents associated with welds depth 2% OD and/or strain 4% 1 acceptable if weld toughness & quality are acceptable 7
In all cases the dent assessment should take account of the pipeline pressure at the time of the inspection and the sizing accuracy of the inspection tool. The minimum acceptable weld toughness is full size Charpy energy 27J. Acceptable weld quality must meet the requirements of a recognised welding standard (eg BS 4515-1). If there is no indication of associated damage, assess dent fatigue life using recommended dent fatigue SN curve 2. New dent features identified between MFL/UT inspections should be investigated. Notes :- 1 Where dent depth and dent strain measurements are available, the assessment should be based on dent depth. Where measured dent depths up to the specified depth criteria result in strains which exceed the strain criteria, a 25% increase in strain may be applied to identify dents for investigation, NDT should be carried out on such dents and the strain level reduced if crack indications are detected. 2 A dent fatigue SN curve based on the EPRG approach recommended in PDAM is being developed. 6 Fatigue Assessment of Plain Dents The fatigue assessment of plain dents is carried out using the EPRG formula as follows:- Where:- N c = Predicted number of cycles to failure σ U = UTS 2σ A = equivalent cyclic stress at R = 0, N/mm 2 t = wall thickness K s = stress concentration factor K s is given by the equation:- 8
Where:- H o = dent depth at zero pressure H r = dent depth at pressure NOTE:- PDAM recommends that a factor of 13.3 is applied to the calculated fatigue life to ensure a 95% probability of lower bound prediction of test data. An example of a pipeline dent fatigue SN curve is given in Figure 4. The SN curve is based on the mean (rather than the lower bound) EPRG formula, and is shown for dents of depth 2.5%, 5%, 7.5% and 10% dent depths at varying wall thicknesses assuming a base stress cycle of 125N/mm 2. An approximate indication of the impact on fatigue life of plain dents is given in Table 4. This table may be used to obtain a simple screening assessment of the impact of plain dents on pipeline design fatigue life. Table 4 Simple Assessment of Fatigue of Dented Pipelines Plain dent depth % dia 2.5 5 7.5 10 Fatigue life assessment EPRG Mean EPRG Lower Bound Similar to design life of 10% of design undented pipeline life for undented pipeline Reduced to 20% Reduced to 2% Reduced to 10% Reduced to 1% Reduced to 5% Reduced to 0.5% It is normal practice to use the lower bound fatigue prediction. The effect of dents on pipeline fatigue predictions indicated in Table 4 show that in most cases, this will result in very low fatigue lives. Where the above screening assessment indicates the fatigue life is exceeded, a more detailed dent fatigue assessment should be carried out or the dent feature should be investigated. 9
Note - This approach is being considered using real pipeline data so that practical recommendations can be made. The above fatigue prediction methodology has been developed as an excel calculator (DENT FATIGUE CALCULATION NOV 09) for use by UKOPA members. 7 References 1 Integrity Assessment of Construction Dents Subject to Fatigue Loading Report Prepared for UKOPA. Dr J M Race, Newcastle University. May 2007 2 High Level Dent Assessment from ILI. M Dafea, T Swankie. Advantica 1 Report 8498 November 2008. 3 The Pipeline Defect Assessment Manual Report to the PDAM Joint Industry Project May 2003. A Cosham, P Hopkins. Penspen Integrity. 4 Dent Assessment Based On In-Line Inspection Results. J M Race. Proceedings of the 5th International Conference on Pipeline Rehabilitation and Maintenance, Manama, Bahrain : GICC, 2002 5 DENT FATIGUE CALCULATION NOV 09 Excel file for UKOPA. R A McConnell. Note:- Advantica is now Germanisher Lloyd Nobel Denton (GLND). 10