Session: 04-04-04: Rigid Pipelines IV

Submission Number: 157399

Probabilistic Approach for Predicting Pipeline End Loads in Short HPHT Deep-Water Flowlines 

The lateral buckling design of HPHT deep-water pipelines is challenging due to uncertainties in the parameters that govern the pipeline lateral buckling response. Traditionally, pipeline lateral buckling design is based on a deterministic approach, but this may result in a conservative design or lead to a non-feasible solution. The alternative is to remove conservatism by adopting a probabilistic approach, resulting in a more cost-effective and feasible solution. Probabilistic lateral buckling design ensures the integrity of the pipeline given probability distributions of uncertain parameters. The focus of lateral buckling probabilistic assessment is the integrity of the pipe; however, in the case of pipelines with extremities restrained against pipeline expansion (fixed ends), the probabilistic approach could also be applied for a robust estimation of the pipeline end loads, as the pipeline extremities develop reaction loads due to thermal cycles, and the loads are also affected by uncertainties in the parameters that govern the pipeline response. These end loads must not exceed the maximum structural and foundational capacities of the equipment; otherwise, the integrity of the pipeline system may be compromised. 

This paper presents a case study for a deep-water, HPHT, short flowline evaluated using a probabilistic lateral buckling design approach that aims to predict the end forces. The lateral buckling assessment of short flowlines with fixed ends and lengths in the range of 500m to 1500m, may show an acceptable lateral buckling result, not requiring a controlled lateral buckling strategy, i.e., no engineered triggers, to ensure the pipe integrity.  Although the integrity of the pipeline is ensured, it is recommended to introduce at least one engineered trigger in the pipeline to reduce the end loads by creating a lateral buckle to release the compressive axial force in the pipeline. The end loads, affected by uncertain parameters, are evaluated based on a probabilistic approach to have a robust and demonstrably efficient design. It is expected that even short flowlines have small, lateral out-of-straightness features (which are inherent to the pipeline installation process). The uncertainty in the out-of-straightness features along the line can influence the critical buckling force and therefore the end loads due to cyclic thermal loads. 

The study is based on a global finite-element model with a single-sleeper for mitigation for a pipeline on flat seabed. The pipe cross-section and operating parameters are known and considered deterministic variables, while other inputs such as axial/lateral pipe-soil interaction parameters, pipe/sleeper friction factor, sleeper height and pipeline horizontal out-of-straightness are considered stochastic variables. Based on the previous experience, the pipe walking for a short flowline becomes stable after few cyclic loads to reach a pipeline end load.  Finite element analyses are based on a load-case matrix selected by the ‘Design of Experiments’ approach. A lateral buckling response surface is created considering variations of stochastic parameters and cyclic thermal loads. Monte Carlo simulations based on the response surface are performed to obtain the distribution of lateral buckling responses under cyclic thermal loads, including limit-states utilizations and end loads. The probability distributions are compared with the target failure probabilities corresponding to safety classes defined in DNV-ST-F101. The maximum end load estimated based on a target failure probability is reduced compared to a case of extremely conservative deterministic estimation.  Therefore, the pipeline end structures/foundation could be designed with lower capacity. 

The proposed approach is useful for predicting the pipeline end loads for the end structures design at early or detailed phase in the presence of uncertain parameters. This probabilistic approach requires more simulation time, compared with the deterministic approach, but it helps to predict a most robust range for the lateral buckling response, which may result in project cost savings. 

Presenting Author: Carlos Ingar Valer TechnipFMC Brazil

Presenting Author Biography: • Technical Authority for Rigid Pipelines Engineering in TechnipFMC.
• Senior Technical Expert in TechnipFMC.
• PhD in Mechanical Engineering.
• Thirty years of experience as engineer, from which more than seventeen years with subsea engineering pipelines projects, including developments in deep waters (Brazil) and shallow waters (Norway).