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Engineering Critical Assessment for Offshore Pipeline

Introduction

An engineering critical assessment (ECA) – also known as fracture analysis – is a fitness-for-service procedure that uses fracture mechanics principles to determine the defect tolerance of safety critical items. When evaluating the integrity of structures such as pipelines and pressure vessels, platforms, rigs and wind turbines, an ECA enables the user to make informed and confident decisions on the most appropriate remedial measures to take.

An ECA is used to decide whether a given flaw is safe from brittle fracture, plastic collapse, fatigue crack growth or creep crack growth under specified loading conditions. It can therefore be used:

• During design, to assist in the choice of welding procedure and/or inspection techniques.
• During fabrication, to assess:
• a) the significance of known defects which are unacceptable to a given fabrication code; or 
• b) the maximum critical flaw size, minimum fracture toughness or maximum operating stresses.
• During operation, to assess flaws found in service and to make decisions as to whether they can safely remain, or whether down-rating, repair or replacement are necessary.

There are standardised methods for ECA and these are now used routinely by the oil and gas, nuclear, aerospace, petrochemical and power industries to determine the safety of their structures. TWI has been deeply involved in the development of methods for conducting ECAs for more than 40 years, and can offer this service to its members on a consultancy basis. Our broad range of expertise and facilities, including advanced NDT, fatigue and fracture testing allows us to gather in-depth data as the basis for our ECAs.

The integrity of pipelines, plant, equipment and structures is vital to ensure a continued, safe and economic operation. Flaws such as cracks, welding defects and corrosion damage can occur during manufacture or service life. For safety critical items like pipelines and pressure vessels, platforms, mooring chains and risers, rigs and wind turbines the failure of a single component due to the presence of a flaw can threaten human life, as well as presenting severe economic and environmental consequences.

Other flaws may be harmless, as they will not lead to failure during the lifetime of the component. Replacement or repair of such 'insignificant' flaws is unnecessary and economically wasteful. ECA enables us to make this assessment of whether a known flaw is 'critical'.

ECA of Subsea Pipeline Girth Welds for Reeling Installation

ECA is based on fracture mechanics and has the objective to generate the allowable cracks size in the girth welds. ECA can be conducted as described in several standards such as BS7910 (Guide to methods for assessing the acceptability of flaws in metallic structures), DNV-OS-F101 Appendix A
(Structural Integrity of Girth Welds in Offshore Pipelines) and DNV-RP-F108 (Fracture Control for Pipeline Installation Methods Introducing Cyclic Plastic Strain).

BS7910 is a common industry practice for flaw assessment procedures. However, BS7910 is not developed for pipeline condition with large plastic strain. The recommended practice DNV-RP-F108 is therefore established to provide guidance for defect assessment of pipeline subjected to cyclic plastic strain e.g. reeling installation method. CRACKWISE is one of the software that can be used for the flaw assessment of pipeline girth welds during reeling installation. In order to reduce the conservatism of existing failure assessment methods, SINTEF recently have developed a new failure assessment approach which depends on finite element calculations of pipeline model.
LINKpipe is based on four-node ANDES shell elements and a non-linear line-spring element (Olsø et al., 2008). The software established an efficient and adequately accurate model even for large level of strain, thus it has potential as an alternative ECA tool for pipelines subjected to plastic strains.

Engineering Critical Assessment (ECA) and workmanship criteria are two acceptance levels for welding flaws. The workmanship acceptance levels for welding flaws in pipeline girth welds can be found in several guidelines such as BS 4515-1, API 1104 and DNV-OS-F101.
These acceptance levels are not fitness-for purpose defect limits, but it clarifies what a “good welder” should be able to accomplish. Furthermore, ECA applies the fracture mechanics in order to ensure the weld integrity on a rational basis.
Mostly the ECA procedures were not applied in the older onshore and offshore pipelines. Lately, the ECA has been conducted widely since the latest pipeline designs are introduced higher complexities such as high-temperatures and pressures, plastic strain during installation, deep water installation, and aggressive internal conditions. Other reason is the use of transition technology application from the radiography to the Automatic Ultrasonic Testing (AUT). This is used as the main inspection method during construction and it produces the flaw sizing and information of location in 2-dimension (Macdonald and Cheaitani, 2010).
In the present industry practice, ECA is carried out along the subsea pipeline design work in order to analyze the acceptable flaws size in the girth weld. The ECA is carried out through all the phases of pipeline’s life cycle from the installation until the end of the design life. Furthermore, the fracture mechanics based ECA is also used to evaluate the acceptable flaw sizes in structures i.e. “to demonstrate fitness-for-purpose”.
Usually defects exist initially in the girth welds during the pipeline fabrication. The main purpose of applying ECA in the reeled rigid pipeline is to determine the largest bounding envelope of initial defect sizes (for depth and length of defect) that could be accepted for the given loading history in pipeline design life. The basic procedure is to assume the existence of certain defect size in the girth welds and to carry out the ECA in order to ensure these defects are acceptable without resulting in fracture during the loading history of pipeline.
As the basis of ECA, fracture mechanics provides criticality predictions of structures with existing crack like defects, given:
1. Geometry (size, orientation and location of cracks, geometry of structure, etc.);
2. Material properties (tensile yield and strength, stress strain curve, weld metal mismatch, fracture toughness, tearing resistance, etc.);
3. Total loading history (from initial spooling onto vessel to end of design life conditions).
 

Steel structures that have a particular minimum ductility, such as rigid pipelines with existing defects in the girth welds, could fail by fracture during reeling installat ion. The failure during reeling installation can be induced by many mechanisms:
1. Extreme tearing during single action of high axial load (spooling/reeling);
2. Cyclic tearing, or so-called ‘tear-fatigue’, during the repeated actions of high axial load (spooling/reeling/straightening cycles) in plastic range;
3. High cycle fatigue or cyclic growth of cracks during higher frequency smaller amplitude cyclic loading (installation hold periods on vessel) in the elastic range.
 

In case of seamless rigid pipelines, preventing possible failure due to fracture is mainly concentrated in the girth welds. As was mentioned in the work from Subsea7 (2011), there need to be considered several features such as:

1. The basic geometry and material data

2. Misalignment at the girth welds

3. Effect of weld residual stress

4. Evolution of stress-strain curve of parent material under reeling cycles

5. The effect of internal pressure.

Engineering Critical Assessment (ECA) Codes

The code, BS 7910 outlines procedures in detail regarding how to carry out the Engineering Critical Assessment. The procedures are mainly stress based and the codes could not directly be applied to the strain-based situations. As a general standard, BS7910 is also supplemented by additional guidance in pipeline design codes and standards.
 

The design code, DNV-RP-F108 was established to provide guidelines for ECAs of girth welds subjected to cyclic plastic strains during installation. It introduced the constraint matched Single Edge Notch Tension (SENT) fracture mechanics specimen design. SENT specimen developed for pipeline girth welds assessment.
 

The code, DNV-OS-F101 provides additional guidelines for operation and installation methods, involving plastic strain in the pipeline, such as reeling which introduce several cycles of tensile and compressive plastic deformation.

In accordance with DNV-OS-F101, Section 5 D1100 (Fracture and supplementary requirement P), it is stated that pipeline systems shall have adequate resistance to unstable fracture. Table 2.2 summarizes the requirements of unstable fracture against the safety as described in Table 5-10 from Section 5 D1100, DNV-OS-F101.

Supplementary requirement (P) refers to line pipe for plastic deformation (Section 7 I300, DNV-OS-F101). The main objective of supplementary requirement (P) is to ensure that the material has sufficient properties after being subject to plastic deformation, and that the material has sufficient ductility.
 

Section 10E from DNV-OS-F101 (check) gives additional requirements for pipeline installation methods that involve plastic deformation (e.g. reeling) (Macdonald and Cheaitani, 2010).

Sources:

http://www.twi-global.com/capabilities/integrity-management/engineering-critical-assessment/

Permana, Indra.2013."A Study on Engineering Critical Assessment (ECA) of Subsea Pipeline Girth Welds for Reeling Installation".Master's Thesis Faculty of Science and Technology, University of Stavanger.