A nonlinear analysis of laying a floating pipeline on the seabed

Offshore pipelines are increasingly being employed to transport offshore hydrocarbons to onshore processing facilities. Pipelines laid directly on the seabed are subject to a considerable hydrodynamic loading from waves and currents and must be accurately designed for on-bottom stability. Confidence in the stability of pipelines requires appropriate models for their assessment and, in this paper, particular emphasis is placed on achieving an integrated and balanced approach in considering the nonlinearities and uncertainties in the pipe structure, the reaction of the restraining soil, and the hydrodynamic loading applied. A statistical approach is followed by developing a response surface model for the pipeline maximum horizontal displacement within a storm, while including variability in parameters. The Monte Carlo simulation method is used in combination with the developed response surface model to calculate the extreme response statistics. The benefit of this approach is demonstrated and also used to investigate the sensitivity of the on-bottom pipeline simulation for a variety of model input parameters. These results provide guidance to engineers as to what uncertainties are worth reducing, if possible, before a pipe is designed.

Application of Statistical Analysis Techniques to Pipeline On-Bottom Stability Analysis

This paper addresses numerical simulations of the lifting operation of an offshore wind turbine monopile foundation considering both shielding effects from the vessel and the spreading of the waves. A numerical model of the coupled monopile-vessel system is established. The disturbed wave field near the vessel is investigated and observed to be affected by the diffraction and radiation of the vessel. The shielding effects of the vessel during the lifting operation are accounted for in this study by interpolating fluid kinematics between pre-defined wave points near the vessel using SIMO software and an external Dynamic Link Library (DLL). The effects of short-crested waves on the wave field and on responses of the system are investigated by implementing the directional spreading function in the wave spectrum. Based on the time-domain simulations, the critical responses of the lifting system in various conditions are studied. The results indicate that the effects of the wave spreading are considerable in both incident and disturbed waves. The shielding effects are less significant in short-crested waves than in long-crested waves.

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Analysis of Lifting Operation of a Monopile Considering Vessel Shielding Effects in Short-crested Waves

The design of bottom-laid deep-ocean pipelines requires estimates of the dynamic stresses and motions that result from a random wave environment. A 3-D finite element algorithm is presented that estimates the large displacements of pipelines sliding on irregularly contoured marine sediments and subjected to episodic hydrodynamic loads. Nonlinear elastic-plastic springs simulate transverse and axial bottom resistances for both cohesionless and cohesive sediments. Inclusion of a hydrostatic pressure differential across the pipe wall modifies axial stresses and influences the flexural stiffness of the finite element model through the geometric stiffness matrix. Hydrodynamic inertia and nonlinear viscous drag forces are represented by a relative-motion form of the Morison equation. The equations of motion during each episode of extreme wave activity are integrated using the Newmark method of implicit integration. Within each time step, a Newton-Raphson scheme of iteration is used to satisfy equilibrium while accurately predicting the nonlinear path-dependent response. An updated Lagrangian formulation shifts the reference configuration of the pipeline and separates rigid-body movements from pipeline deformations. This provides a large-deflection, small-strain, transient analysis of the pipeline motion, which is capable of computing finite-amplitude motions that have been accumulated over the random wave episodes.

Dynamic Behavior of Deep‐Ocean Pipeline

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. Copyright © 2011 by ASME.

K.F. Lambrakos

Papers

Marine pipeline dynamic response to waves from directional wave spectra