Integrity Assessment of Non-Piggable Offshore Oil Pipeline

 

Integrity Assessment of Non-Piggable Offshore Oil Pipelines   ABSTRACT

  ABSTRACT

At the request of the asset managers integrated, an integrity assessment of the existing offshore bulk-lines was conducted to determine whether:

 

• to replace all the bulk-lines valued at 350 million US$
• to sweat the assets
• or a combination thereof

The field consists of seventy-one, six inches diameter oil bulk lines arranged in seventeen clusters criss-crossing the Estuary field in Southern Nigeria. The lines totaling 131 Km were laid at 30 meters water depth for wet crude transmission, between 1971 and 1978. They have an inner coal tar coating overlaid with an outer concrete layer. Replacement of rises for all lines was conducted in 1989/90.

The lines were first grouped into sister lines based on internal corrosion.  This resulted in sister groups for lines radiating from the same cluster because the lines feed from common rings at the cluster platform and hence share same fluid characteristics. The integrity status was then based on two criteria:

• Modeled corrosion rates. 
• Leak history due to corrosion

Preliminary Results

 

• Replacement: 56% of  lines were recommended for replacement as the modeled corrosion rate was high and there was direct evidence of leaks due to corrosion.  It can thus be stated that the integrity status of these lines is poor with a high degree of confidence.  The primary reason for the poor integrity state is the prolonged unavailability of CP.
• Repair: 27% of the lines have evidence of poor integrity on the riser due to leaks.  Inspect/Maintain:  17% of the lines have been judged to have 5 years life but this is solely based on modeled corrosion rates.  Lines should be inspected in a similar manner as articulated above.