Abstract
Hydrocarbon subsea pipelines are considered one of the most significant underwater assets. The importance of integrity assurance for such equipment comes from the huge impact on operation continuity, safety hazards and high repair cost. The scope of this paper is to provide a fitness for service assessment on a subsea pipeline with mechanical damage. The pipeline was displaced and dented during an anchor drag incident.
This research analyzes a case study of an oil subsea pipeline which has experienced an excessive displacement and localized dents due to accidental anchor dragging. Three- Dimensional (3-D) Nonlinear Finite Element (FE) model of the anchor dragging was developed. The FE model mimics the incident probable scenarios in order to reproduce the real deformation in the pipeline for further assessment. The results of the FE Model were compared to divers inspection reports to validate and confirm the deformation pattern of the pipeline, dent location and dent profile.
During a routine intelligent pigging of a subsea pipeline, an anomaly was noticed to the cross sectional profile of the line and was interpreted as a dent. Further diving inspection successfully confirmed and characterized the dent geometries and described a severe dislocation of the pipeline. The inspection reports revealed that the dent took place at filed joints girth weld. The incident took place at a crossing location with another oil line. The incident produced local dent in the pipeline, caused damage in many sleepers in the vicinity, displaced the line from its original location and caused local damages of the concrete coating in some locations. The study included a finite element analysis to assess the acceptance criteria of the dent. Excellent agreement between FE analysis results and actual failure was noticed. The FE stress and strain levels were analyzed and compared to the international design codes. Recommendations related to the required repair against de-rating of the line for future safe operation were provided.
