Cost efficient offshore field development often involves tiebacks to existing field infrastructure. Efficient field development requires life extension of existing production facilities and pipelines to accommodate the new field resources over their life expectation. For fields near the tail end of their production the pipelines may be close to the end of their design life, and it must be shown that they have potential for extended life beyond the original design life until the end of the period of operation of the new field.
Offshore pipelines are designed and constructed to recognized standards, such as the widely applied DNV OS-F101 2013 Submarine Pipelines Systems and earlier versions. The latest edition of the code was recently issued as a standard with some major updates and a modified code number i.e. DNVGL ST-F101 .
As pipelines age, they will inevitably be exposed to various types of degradation and an Operator must be able to both assess the significance of this damage and the pipeline remaining life to ensure that the pipelines do not fail as they age before the end of their design lives. Currently, many pipelines are operated far beyond the original design life and as mentioned above for cost efficient field development the pipeline operator often needs to demonstrate that the pipeline’s useful life can be extended another 10 or in some cases up to 30 years. For some pipelines, new operating conditions will be introduced by tie-in of new fields and this will impact the future rate of degradation. Hence, it cannot be assumed that the future degradation will be similar or less severe than experienced since commissioning of the pipeline.
Extension of the life of the pipeline can be demonstrated by adopting methods of analysis that show the line is safe for an extended life under the future expected operating condition.
This paper describes the risk based approach applied for pipeline remaining life and life extension analyses based on DNV GL codes and other relevant recommended practices. For illustration of the methodology a typical case of remaining life assessment of and life extension of a gas export pipeline is presented in the Case Study.