A SNS pipeline operator found a significant number of freespans (>1000) during a subsea survey of their 20” gas export pipeline. The client came to Jee for support with initial trending and screening of the freespans to identify which spans were most critical and warranted further assessment / remediation.
Because excessive span growth can lead to increased bending stresses, vortex-induced vibrations (VIV), and ultimately structural fatigue, it is essential that any spans that are unacceptable are identified. This ensures that they remain within the pipeline’s maximum allowable span limits and is a critical part of maintaining long-term integrity.
Firstly, a span screening assessment was conducted to determine which of the >1000 spans were most at risk. Part of this included Jee creating a tool that was used to trend the spans across previous surveys, detailing if a span was buried, exposed, spanning with VIV or spanning without VIV. This would also show which of the spans remained static and which were transient (static being worse). A short section of the results of this are shown in Figure 1-1.
Figure 1‑1: Jee’s tool to identify spans and their condition
Once this was completed, a total of 11 spans were defined as requiring a more refined assessment. Finite element analysis (FEA) was used to accurately model the individual spans and any interacting spans on the pipeline route. This allowed the natural frequencies of the spans and greater defined bending forces to be extracted which would more accurately determine the mechanical behaviour of the span once subject to VIV. DNV-RP-F105 calculations was then utilised to determine if the spans were acceptable.
For any spans still deemed unacceptable, a rock berm design study was conducted to determine the rock support configuration required to mitigate them. The benefit of this process to the client is that out of the >1000 spans identified, Jee were able to screen down the number of spans requiring immediate mitigation so that the client knew where to focus their efforts, saving both money and mitigating the key spanning threats to the pipeline.
For more information, visit www.jee.co.uk/integrity-management
To contact Jee’s Technical Authority, Graham Wilson, email Graham.Wilson@jee.co.uk.