Session: 04-03-03 Hydrodynamics

Paper Number: 107635

107635 - Identification of Interacting Free-Spans in a Subsea Pipeline 

Freespan in pipelines can occur intentionally either due to the seabed topography during laying, alternatively it can be caused by scouring of the seabed during operation. A freespan section can either be on its own called as a ‘single span’ or multiple spans can be in close vicinity and form a ‘multi-span’. The unsupported section (freespan) in the pipeline in the presence of currents has a potential of causing vortex induced vibration (VIV). In such a scenario the pipeline can vibrate along the flow direction (inline) or perpendicular to it (crossflow). This is detrimental to the pipeline especially to the welds, as this leads to accumulation of fatigue damage leading to a reduction in fatigue life.

The recommended practice DNV-RP-F105 ‘Free Spanning Pipelines’ lays out the methodology to assess fatigue in free-spanning pipelines. In a multi-span scenario the spans in close vicinity may interact with each other. The morphological classification plot (RP-F105 2006 Figure 1-2) in the RP provides means to decide whether the span is interacting based on relative lengths of the spans and intermediate support. In a recent update of the RP (2017), this classification plot was called out as inaccurate. Additional guidance was added describing an algorithm to assess whether a span is a single-span or an interacting multi-span.

In this work the additional guidance was used to classify the spans as isolated or interacting for an existing pipeline. The freespans were identified through a survey of the pipeline using multibeam echosounder and sidescan sonar. The multispans were identified from the freespan listing. For each multispan multiple finite element (FE) models were developed to compare the modal response of the multi-span against each span member. A comparison was also made with the morphological classification plot to check its validity.

Presenting Author: Rajil Saraswat ExxonMobil Services and Technology Private Limited

Presenting Author Biography: Rajil Saraswat has a PhD in Materials from Imperial College London. He has worked in area of structural integrity for more than 15 years. He works as a pipeline engineer at Exxon Mobil.

Authors:

Rajil Saraswat ExxonMobil Services and Technology Private Limited
Palash Patil ExxonMobil Services & Technology Private Limited
Edward Ciecko Esso Exploration Angola Limited
Parul Koul ExxonMobil PNG Limited