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2TL Free Span Assessments

2TL Free Span Assessments

Benefits
Atteris developed revised screening allowable span lengths for key zones along the pipeline where scour has resulted in significant spanning, to reduce the number of reported anomalous spans significantly in future surveys.
Background
The Woodside 2TL pipeline connects NRA to the onshore LNG plant. During the design phase allowable span limits were developed based on generic data for various pipeline sections. During the operating life to date, ongoing scour processes have resulted in spans developing in discrete sections of the pipeline that are close to, or exceed the allowable span limits set in the design phase. Woodside engaged Atteris to assess all reported spans, and to re-evaluate the allowable span lengths for 3 discrete sections along the pipeline route where scour induced spanning is an ongoing issue.
Work Performed by Atteris

Atteris performed assessments on anomalous spans resulting from a 2014 ROV survey of the 2TL pipeline. This included a number of Level 2 and Level 3 assessments to accurately determine the requirement for any intervention work, including interacting spans. The work concluded that all of the reported spans were acceptable without any requirement for intervention.

Further detailed analysis was performed in three key locations where ongoing scour is resulting in significant spanning to redefine the allowable span lengths considering:

  • Effects of interacting spans
  • Means of calculating ULS allowable span lengths, particularly taking into consideration effects of direct wave action
  • Improved knowledge on the characteristics of soliton currents.

The work utilised Atteris in-house version of Fatfree to perform multiple fatigue simulations, as well as Corus 3D, Atteris dynamic stability software, to assess the pipeline stresses due to hydrodynamic loading, resulting in justification to significantly increase the allowable span lengths in 2 of the 3 discrete sections assessed.

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