Freespan Analysis for Subsea Pipeline Integrity Management Strategy

Over a rough seabed or on seabed subject to scour, freespans can occur when contact between a subsea pipeline and the seabed is lost over an acceptable distance. When this exceeds the allowable freespan length, design stresses can be exceeded, and a vortex induced vibration (VIV) response can be initiated, resulting in the risk of fatigue failure. If this is not predicted and controlled properly, it will affect pipeline integrity, leading to expensive rectification and intervention work. Freespan analysis consisted primarily of a screening check in which the as-found freespans from Remotely Operated Vehicle (ROV) or multibeam Side Scan Sonar (SSS) inspection survey were compared against the design allowable lengths and determine the expected fatigue life of a freespan that may be experiencing Vortex Induced Vibration (VIV). Freespans are considered acceptable if the calculated fatigue life exceeds the design life criteria. This paper describes the freespan analysis that has been developed to perform detailed freespan engineering assessments, incorporating the latest survey and as-laid conditions. This analysis follows a methodology in standard code DNVGL RP F105 that has been accepted and used by operators to produce more accurate and less conservative freespan analysis results, leading to a subsea pipeline integrity management strategy with fewer unnecessary interventions and greater cost benefits.