Session: 04-04-04 Lateral Buckling and Reeling

Paper Number: 126874

126874 - Probabilistic Approach of Lateral Buckling Analyses for Hpht Pipelines With Buckle Mitigators 

Lateral buckling mitigation devices are generally used in the design of High Pressure and High Temperature (HPHT) flowline systems to ensure planned lateral buckle reliability and to decrease the stresses/strains and fatigue damage. In many cases, due to the uncertainties associated with the input parameters, acceptable lateral buckling behavior cannot be demonstrated. Design based on a traditional deterministic approach can be extremely conservative. Since mitigation measures are expensive, it is important to remove unnecessary conservatism by adopting a probabilistic approach for the pipeline design, which also leads to a more cost-effective solution.

The probabilistic analysis of buckle formation along the pipeline route to optimize the mitigation has been applied from the early lateral buckling design for HPHT subsea flowlines. The SAFEBUCK JIP and the recommended practice DNV-RP-F110, based on the concept of Virtual Anchor Spacing (VAS), have included a methodology for probabilistic assessments, in addition to the deterministic approach. The software BUCKFAST from the SAFEBUCK JIP has been developed to perform the reliability analysis of buckle formation, but it is applicable for a flat seabed only and with limited loading conditions. Meanwhile, in-house software presents an alternative approach to modify these limitations by accounting for the bathymetry and cyclic operation profiles. Both BUCKFAST and the in-house tool are based on analytical methods and simplified FE analysis results. However, more detailed FE lateral buckling analysis for the flowline system with mitigators is required in the detailed pipeline design to account for the uncertainties in the non-linear axial and lateral pipe-soil interactions (PSI) and pipeline vertical and horizontal plane out-of-straightness (OOS). Several probabilistic approaches for FE VAS analysis associated with Monte Carlo method have been developed for the lateral buckling design, but these analyses do not account for the seabed imperfections and buckle interactions in the models.

Horizontal and vertical OOS of the as-laid pipeline due to pipelay and seabed features can have serious implications related to unplanned (rogue) and planned buckle formations in the HPHT flowlines, especially for the line on a significantly undulating seabed. In this case, free spans affect the effective axial force distributions, and this then impacts buckle formation along the flowline, buckle interactions, and even pipeline walking.  

Recently, probabilistic methods based on finite element techniques have become more feasible, due to the availability and progress in computing technology. This paper presents a probabilistic approach for lateral buckling analysis based on a global FE model via a case study of 5 km long subsea flowline with sleeper mitigators on undulating seabed. In the case study, axial/lateral PSI, pipe/sleeper friction factors, and horizontal OOS at flowline and sleepers are defined as stochastic parameters, while other parameters, including flowline as-laid vertical OOS, pipe section and operating parameters, are defined as deterministic variables in the probabilistic analyses. First, the response surfaces for lateral buckling response of the whole flowline, due to the variations of stochastic parameters, are created through FE analyses based on a load case matrix selected by the Design of Experiments approach. Then, Monte Carlo simulations based on the response surfaces are performed to obtain the distributions of lateral buckling responses, including parameters such as the limit-states for post-buckle, buckle amplitudes, walking speed or end pile loads, rogue and engineered buckle locations. At last, the probability distributions of the limit-state ratios are compared with the target failure probabilities defined by DNV-ST-F101 for different safety classes to determine whether the design satisfies the DNV criteria.

This probabilistic approach will require more simulation time, compared with the deterministic approach, but it can provide more detailed probability ranges for the lateral buckling responses, and save the project on total cost. This approach can be used for assessing the reliability of lateral buckling responses during the detailed design phase of subsea pipelines susceptible to lateral buckling.

Presenting Author: Qiang Bai TechnipFMC

Presenting Author Biography: Qiang Bai, PHD, PE., Sr. Expert Engineer, TechnipFMC

Authors:

Qiang Bai TechnipFMC
Endryws Moura TechnipFMC
Rodrigo Primieri TechnipFMC
Mark Brunner TechnipFMC
Lijuan Wang TechnipFMC