Session: 04-04-05: Rigid Pipelines V

Submission Number: 156162

Residual Curvature Zone As-Laid Roll and Out-of-Straightness 

Residual curvature zones have been formed in reel-laid pipelines to trigger lateral buckles and control their formation since 2012.  Buckle initiation with residual curvature depends on factors that include the as-laid rotation and horizontal out-of-straightness.

In this article, a finite element method to predict the as-laid residual curvature configuration is described.  With the method, a catenary is formed from the reel-lay vessel to the touch-down point.  The model includes a representative length of laid pipe on the seabed.  A single residual curvature zone, or train of such zones, transits from the vessel the catenary touch-down point.  Hence, the analysis is a static and stationary simulation of a dynamic and kinematic process.

The as-laid roll of residual curvature zones has many influencing factors.  Project teams can want a target roll value to be attained or to have no roll, for critical locations.  A roll onset graph would have a lower shelf, where pipe bends about the horizontal axis as it lands, and an upper shelf where there is a lateral-torsional roll response.  There is an instability zone between the two shelves that can be passed through suddenly.  The finite element method is used to get an envelope of the likely roll and out-of-straightness values.

The potential for residual curvature zones to roll out of the vertical plane comes about because of a small imperfection that is seeded into the numerical simulation in consort with other characteristics.  The magnitude of the imperfection may be small but may cause roll, if the imperfection has a strong enough influence and if the characteristics predispose the pipe to roll.

If there were no imperfection then the pipe would not roll, in both the numerical analysis and reality.  No imperfection could also be the result of the different random factors cancelling each other out.  Conversely, if influencing factors are relatively strong there can be more roll. 

The finite element method is computationally efficient and is flexible enough to cover single and pipe-in-pipe systems and multiple residual curvature zones in shallow, medium and deep water; to incorporate pipeline end termination structures and in-line tees; to include residual curvature of notionally straight pipe; to give an indication of highly stressed zones; and to permit application of vessel motions.  Examples are given in the article.

Presenting Author: Yasin Kassim Subsea7

Presenting Author Biography: Yasin Kassim has in doctorate from Imperial College in London on the subject of shell elements in finite element analysis. Yasin has worked with detailed installation engineering for global subsea projects since 2006. He has worked for Subsea7, a Tier 1 offshore installation contractor, for around fifteen years. Initially he focused on OrcaFlex analysis for installation activities but more recently with Abaqus for reeling and other non-linear assessments relating to offshore rigid pipelines. Yasin has been involved with residual curvature method for shallow and ultra-deep water pipeline systems, both single pipe and pipe-in-pipe.