The rehabilitation of pipelines is in many cases forced upon operators by the fact that leaks occur (severe pitting) or extended general corrosion has been recognised on the steel surface. Various inspection techniques are applied generate information about the condition of the steel surface, such as:
“The assessment, renovation and reclassification work necessary to establish a level of integrity based on known values, that enables an existing pipeline to continue in service, safely, and economically, for a determinate period.”
Suppose that on a pipeline in service, external corrosion has become unacceptable to the extent that production capabilities are affected or that the cathodic protection due to coating degradation has high operating costs and is no longer effective with respect of protecting the pipeline against corrosion.
In this situation the pipeline operator may consider to refurbish or rehabilitate the pipeline under consideration (or suspected pipeline sections). In most cases total replacement of a pipeline will not be necessary. Based on technical and financial considerations, weak sections can be identified and inspected in greater detail. Based on the design requirements and the current operational conditions it need to withstand, a decision can be taken to excavate and cut out specific sections and replace these sections with new parts. This becomes very relevant for those sections where Stress Corrosion Cracks (SCC) have been found. Whilst operating conditions can be modified to reduce the propensity to SCC, no method exists to repair SCC.
An alternative solution, whithout removing these sections, is to reinforce them by sleeving. In the case of SCC, its best to cut out the and replace it, than to rather risk the experience of uncontrolled pipeline failure by rupture that can occur with SCC.
In general, rehabilitation is interpreted to mean the refurbishment of the pipeline protective coating, the premier corrosion protection mechanism and the cathodic protection system, the supportive technique to ensure external corrosion mitigation systems are effective to prevent metal loss. Any existing areas of metal loss identified from metal loss surveys or found by visual inspections during coating repair, after interpretation to accepted Codes of Practice, must be repaired where necessary.
There are general tables available that include the mean current densities, that are acceptable as a function of the lifetime from the coating.
|Coating type||0 – 5 years
|5 – 15 years
|15 – 30 years
|Asphalt bitumen (AB)||0.004||0.10||0.20|
|Fusion bonded epoxy (FBE)||0.01||0.02||0.05|