Pipe being wound onto reel at the onshore spoolbase. Courtesy Coflexip Stena
Pipe reeling is a fast and efficient method of laying offshore pipelines. Girth welds between pipe lengths are fabricated and inspected at an onshore spool base before being wound onto a spool mounted on a reeling vessel. The vesselsails to site, the spool is unwound and the pipe laid. The ramp on the vessel allows the pipe to be laid at angles between S-lay and J-lay.
However, the reeling process imposes repeated high plastic strains on the pipe. To avoid failure during reeling or subsequent service, it is important that the pipe properties, dimensional tolerances and fabrication flaws both inthe weld and parent pipe, are controlled.
To define acceptable flaw sizes and material properties and to ensure fitness for service, full-scale tests are possible but expensive and time consuming and only applicable for the materials and conditions examines.
Fracture mechanics-based assessment procedures, used successfully by TWI, are less expensive and have general application. However, they need further development and validation for high strain situations especially where weld metalstrength undermatching is possible.
To address these issues and to develop generally applicable fitness-for-service assessment procedures for reeled pipelines TWI and Det Norske Veritas, have launched a joint industry project - Fracture control for installation methods introducing cyclic plastic strains. One of the project aims is to update and improve guidance on the use and assessment of reeled pipe currently given in the DNV Rules for submarine pipeline systems of December 1996. Sponsors for this project are being sought.
For further information, please call Henryk Pisarski at TWI. E-mail: henryk.pisarski@twi.co.uk
