TWI has applied its direct assessment service to a network of 250km of buried, non-piggable gas and condensate pipelines in central asia. The project provided a complete service to plan and perform inspection and also assessed the inspection results in terms of the pipeline fitness for continued service. The approach combined three main elements:
Risk-based Inspection (RBI)
- Risk-based inspection (RBI)
- Inspection services
- Fitness for service assessment
With the recent release, development and adoption of direct assessment standards (eg NACE) for buried pipelines, TWI put together a service to maintain and prove the integrity of pipeline networks.
Direct assessment is a four step method to determine the condition of a pipeline. The methodology assesses the threat from external and internal corrosion and stress corrosion cracking for both liquid and gas carrying lines. The approach consists of the following:
- Phase 1 - Pre-assessment (risk based study - using TWI RISKWISE for Pipelines)
- Phase 2 - Indirect inspection (above ground inspection and flow modelling)
- Phase 3 - Detailed examination (excavation and inspection)
- Phase 4 - Post-assessment (fitness for service assessment)
Phase 1 was a desktop study using RISKWISE for pipelines and identified external and internal locations most susceptible to damage. The result was a plan for above ground inspection and flow analysis recommendations.
Phase 2 carried out the plan recommended by Phase 1, focusing resources to areas highlighted as at highest risk of failure. Above ground inspection techniques such as direct current voltage gradient (DCVG) and close interval survey (CIPS) were used to look for problems with the coating and cathodic protection system to screen for external damage areas. For internal damage, prediction flow modelling was used to highlight potential locations for build-up of water. The result of Phase 2 was a refined inspection plan for areas that required additional inspection by excavation.
Phase 3 was direct inspection by way of excavation. By this phase, the prior screening phases had filtered out areas where excavation would have been unnecessary, so that only the highest risk areas were excavated based on evidence that there was potentially a problem. TWI's long range ultrasonic system was used for inspection of key areas of the pipeline along with other inspection methods.
Phase 4 examined the inspection data and performed a fitness-for-service assessment to determine the safety of the pipelines and where necessary make run, repair, re-rate or replace recommendations.
The process allowed for an optimised condition assessment to be performed on the entire network of pipelines which could not be otherwise inspected without complete excavation.
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