Atteris assessed the 102 anomalous freespans. A review of the ROV and MBES survey data was performed. The Level 1 and Level 2 freespan assessment was carried out in accordance with DNV RP F105.
The freespans were caused by scour along the pipeline and a review of the survey data revealed that the bottom roughness analyses were misleading.
Re-assessment of the screening (Level 1) span was conducted and revised allowable span lengths were determined, capturing updated metocean condition, new shut-in pressure of 1.5MPa and applying Finite Element Analyses to obtain a more accurate natural frequency. The revised allowable span lengths were determined to be longer than the operating allowable span length. A freespan interaction check was also conducted.
A Level 2 freespan assessment was performed on the freespans that exceeded the revised allowable span criteria. This fatigue assessment indicated that there is sufficient remaining fatigue life up to the planned cleaning and flushing operations, when the pipeline will be made free of hydrocarbon.
Finally, Atteris also performed a check on the proposed pigging operation with a back pressure of 7MPa and determined that this pigging operation would not have an adverse effect on the fatigue life.
