Probabilistic Model Application in the Integrated Stability Analysis of Offshore On-Bottom Pipeline

Abstract

Offshore pipelines provide the main link between offshore oil and gas fields and hydrocarbon development onshore. Due to their economical installation, untrenched pipelines laid “on-bottom” are finding increased popularity over other types of offshore pipelines. However, the stability of untrenched pipeline design remains the subject of criticism. In many cases around the world, severe loading conditions, such as those during hurricanes, result in severe pipeline damage and disruption of oil and gas supply. In on-bottom pipeline stability analysis, hydrodynamic loads are applied to the pipe structure. The pipe passes these loads onto the supporting soil along its length. A variety of parameters need to be defined to model this loading scenario and to reflect the complicated interaction between the hydrodynamic load, pipe structure and the supporting soil. Moreover, there are uncertainties regarding the input values of these parameters, as any difference in these parameter values will result in a considerable difference in the final pipeline stability result (though the sensitivity to any differences is not well studied). In this study of the offshore pipeline stability, the hydrodynamic loads are estimated using Fourier analysis, which is currently the best practice in hydrodynamic modeling. The pipe-soil interaction is simulated with a force-resultant model, which is derived from a plasticity framework and is based on the results of centrifuge test calibration. The pipeline is modeled using an integrated numerical modeling tool developed by implementing the hydrodynamic load model and force-resultant model codes in the finite element package ABAQUS. Use of the integrated modeling tool allows for the coupling effect of the hydrodynamic-pipe-soil interaction to be accounted for, with the added ability to modify the applied hydrodynamic loads due to pipe movements during the analysis. The main aims of this paper are to demonstrate methods to estimate the probability of exceeding pipeline stability and quantify the importance of the on-bottom pipeline statistical analysis and the sensitivity of the parameters included in the pipeline stability design. After first describing the integrated model and providing an illustrative example of its use these aims are achieved by i) performing probabilistic analysis for a typical pipeline case and investigating the probability of exceeding pipeline stability under different maximum pipeline displacement values; and ii) developing a sensitivity analysis of the input parameters included in the on-bottom pipeline stability and ranking these parameters according to their sensitivity to the pipeline stability design.

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“…Seabed stability analysis may be carried out in accordance with AGA Level 3 or F109 dynamic stability method. Time-domain solvers have been developed by DNV through the PILS JIP (Vedeld et al, 2018) and by industry (Zeitoun et al, 2009;Youssef et al, 2011;Abdolmaleki and Gregory, 2018). These predict the 1D (lateral), 2D (lateral and vertical) and 3D (lateral, vertical and longitudinal) solutions for pipe displacement as a function of time resulting from a simulated near-bed velocity storm timeseries.…”