2TL Free Span Assessments

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 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.  A key recommendation was to perform further work to redefine the allowable span lengths in a number of locations where ongoing scour is resulting in significant spanning. Further detailed analysis was performed in three key locations 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.

Benefits

Atteris developed revised screening allowable span lengths for key zones along the pipeline where scour has resulted in significant spanning. The revised allowable span lengths are expected to reduce the number of reported anomalous spans significantly in future surveys.