By John French
The scope
Our client operates a 15 km long pipeline bundle in the North Sea. The bundle was installed in the 90s and the original design documentation defined allowable span lengths but gave very little information about how these limits were defined. This led to a lack of confidence in the acceptability of existing spans and poor understanding of how future operating conditions might affect the remediation requirements of unsupported spans.
Jee’s scope was to define new allowable span limits that take advantage of the latest code requirements and account for future operating conditions in the bundle.
The solution
Pipeline spans need to be evaluated to make sure that they can withstand ultimate limit state and fatigue limit state conditions. Fatigue limit state conditions need to account for pressure and temperature cycling, direct wave action and vortex induced vibrations (VIV).
VIV is the most challenging condition to assess and the approach is well defined within codes such as DNV-RP-F105 for pipeline spans but no clear method is defined for the assessment of bundled pipelines where the various internal components and carrier pipe interact to form a complex system. A common way to assess this type of system is to derive characteristics for an “equivalent pipe” that represents the overall stiffness and weight of the bundle.
However, this approach is overly simplistic and does not allow for accurate assessment of the stress in the individual internal components. Jee developed a solution based on FE modelling techniques to derive the natural frequency response of the system that accurately accounted for the interaction between the internal pipes within the bundle including load transfer through the spacers and intermediate bulkheads. The FE approach to derive frequency response and associated stress was combined with DNV recommended practices to assess vortex shedding and amplitude of vibration.
Jee’s method allowed for removal of conservatism and generated allowable span lengths that minimised the number of spans needing rectification. Our approach provided a greater understanding of the influence of temperature and pressure from each of the component pipes. The analysis showed that the limiting component within the bundle changed between the production line, methanol line and water injection line depending on the mode of operation.
This allowed our client to modify operating conditions if needed to maximise the allowable freespan length and potentially saves our client thousands in future repair costs.
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