SEABED SCOUR AND BURIED-PIPELINE DEFORMATION DUE TO ICE RIDGES

OBJECTIVE: The research described in this proposal seeks better understanding of the factors affecting soil and pipeline deformation below scouring ice ridges in the Arctic environment. Involved in the proposed study are:

• detailed characterization of moving ice-ridge morphology on the basis of available observation and measurement reports
• response simulations of ice-ridge-soil-pipeline systems by means of computational-fluid-dynamics representations and finite-element models that capture flow and deformation in porous media
• development of recommendations regarding the minimum required burial depth for pipeline protection from scouring ice ridges

It is expected that this investigation will contribute a substantial increase in confidence in our ability to engineer pipelines safely and economically in pristine settings where failure has been and remains intolerable.

BACKGROUND: Scour of the Arctic seabed has long been identified as a potentially catastrophic environmental hazard for marine pipelines. A moving ice ridge can scour the soil and destroy a pipeline in its path. In contrast to “upheaval buckling,” another phenomenon of grave concern, in which, the relevant (thermal) load is longitudinal, the effects of ice-ridge scouring are associated with lateral loads on pipelines. Also, while upheaval buckling can be prevented by means of heavy (rock or other material) cover placed on top of the pipelines, protection from scouring typically relies on sufficient pipeline-burial depth, or adequate trenching and trench-backfilling requirements.

There is general agreement that the ice-ridge scour depth along with the pipeline burial depth (both measured from the top of the soil) can be used in evaluating the outcome of ridge-soil-pipeline interaction. If the scour and burial depths are about the same, there is little doubt that pipeline integrity will be compromised. At the other extreme, if the burial depth is sufficiently greater than the scour depth, the pipeline is not expected to undergo any significant deformation. Survival of the pipeline can then be assumed. The minimum “sufficient” burial depth has not been firmly established, but values as low as three times the scour depth have appeared in the literature (Yang and Poorooshasb 1997). In the intermediate range (burial depth equal to a few scour depths), it appears reasonable to expect that the pipeline can be designed so that survival can be ensured, perhaps, with some permanent deformation. Such an outcome may be preferable to the costlier alternative of specifying a greater depth of burial. That these three ranges (Palmer 1997) are meaningful for design purposes has been demonstrated through laboratory and field observations (Woodworth-Lynas 1992, Paulin 1992, Lach et al. 1993, Yang et al. 1994, Clark et al. 1994) of substantial “sub-scour” soil deformation (at a few scour depths below the top of the soil). However, it is important to keep in mind that sub-scour soil deformation can only serve as an indicator of the loads likely to be experienced by the pipeline. Only through a study of the complete ridge-soil-pipeline system can the actual levels of stress and strain in the pipeline be ascertained. A complete system study is proposed herein.

Ice ridges move at speeds of about 0.1 m/s and scour the seabed at low “attack” angles, usually smaller than 30°, and various lengths and widths of the “keel.” At steady state, the scour depth is, in general, a function of the ridge geometry, speed, vertical stiffness (resulting from buoyancy) and the soil properties (near the surface of the seabed). The scoured soil is placed, partially, on berms on the sides of the ridge path. It is the rest of the scoured soil that is pressed downwards and sheared (and pushed) forwards. In turn, downward pressure and shear at the keel appear to produce upward soil movement ahead of the keel. It is the balance of soil placed on berms, pressed, sheared and displaced, along with the buoyancy and kinematics of the ice ridge that sets the stage for ice scouring of the seabed. In the presence of a pipeline buried within a few scour depths, soil deformation amounts to lateral load on the pipeline that can result in loss of integrity or permanent deformation. The three-dimensional nature of the phenomenon is quite clear and accounts for a relatively concentrated lateral load (over a few pipeline diameters) on the pipeline and the formation of berms. In fact, it is the absence of the latter features (berms) from “two-dimensional” centrifuge tests and corresponding computational simulations that limit the realism of studies to date. Nevertheless, two-dimensional investigations, experimental or computational, are useful in our quest for better understanding of the phenomenon. Effectively, they provide insight in seabed scour by an infinitely-wide ice keel.

Although conceivable, it appears very unlikely that “failure” of the scouring keel can occur as an ice ridge moves along the seabed, as the normal and shearing stresses are low. This is believed to be the case with multiyear and first-year ridges (Kovacs and Mellor 1974). For the same reason, it is reasonable to neglect the deformability of the ice ridge. Indeed, in experimental and computational studies to date, the ice ridge has been represented as a rigid body.

Quite extensive centrifuge tests of ridge-soil systems have already been conducted (Winsor and Parsons 1997, Hurley and Philips 1999, Schoonbeek and Allersma 2006), many with support from the Minerals Management Service. The results are available for interpretation and understanding of various facets of the phenomenon such as sub-scour deformation, berms and multiple (or repeated) scouring, the influence of soil behavior (clay vs. sand), layered seabed as well as the effects of parameters such as keel width, length and attack angle. Also, the test results can be used toward calibration and verification of computational models.

As mentioned earlier, experiments and computations to date have focused largely on sub-scour deformation as an indicator of pipeline vulnerability. Essentially, they have explored behavioral features of ridge-soil systems. This proposal is aimed at widening the present computational horizons by means of development and application of ridge-soil-pipeline models, thus enabling direct (explicit) estimation of strain and stress levels in the pipeline (not on the basis of levels of sub-scour deformation reached in the absence of the pipeline). Such models are particularly significant in the intermediate range of burial depths where pipeline integrity can be maintained, albeit with tolerable permanent deformation.

APPROACH: Toward simulations of ridge-soil-pipeline systems, a novel procedure that combines computational-fluid-dynamics (CFD) and finite-element (FE) modeling will be synthesized. Recent experience at the Offshore Technology Research Center with CFD analysis of torpedo-pile installation in clayey soils (Raie and Tassoulas 2006) has shown excellent promise. As long as the material (soil) does not undergo any significant reversals of deformation during the time window of interest (seabed penetration by torpedo pile, ridge passage over pipeline), such computations are successful and much more straightforward than large-deformation, solid or porous solid FE treatments based on Arbitrary Lagrangian-Eulerian (ALE) formulations with frictional-contact constraints. The pressure and shear distribution on the pipeline, resulting from CFD simulations, can then be used in FE computations of pipeline stress and deformation for integrity-evaluation purposes. Elastoplastic beam or tube finite elements along with a simple representation of the surrounding soil, for example, using p-y and t-z curves, can lead to accurate estimates of extreme pipeline response. Worth exploring in future work, is integration of an elastoplastic FE model of the pipeline in CFD analysis of the system. This is certainly a possibility and should be pursued in further development of CFD software. Current CFD capabilities can provide estimates of pressure and shear on a rigid pipeline. FE computations with these loads on a deformable pipeline will then complete the system simulation.

The simulations to be carried out will require specifications of several model parameters for each of the ridge-soil-pipeline system components. It is reasonable to idealize ice ridges as rigid bodies. The ridge parameters to be specified are the width, length and initial depth of its keel, the attack angle, the buoyancy stiffness and the horizontal speed. Sandy and clayey soils will be considered. In CFD analysis, viscous models will be adopted for representation of soil undrained behavior, for example, “Power-Law,” “Bingham Plastic” and “Herschel-Bulkley” models (see the report by Raie and Tassoulas 2006). These are simple models with parameters that can be extracted easily from the most common soil tests (direct shear, triaxial compression). The pipeline burial depth is a parameter that will be varied in a range to be selected by trial and error so that experience can be gained and extensive enough to permit conclusions to be drawn and recommendations to be justified. In pipeline FE computations, an elastoplastic (elastic-plastic-hardening) model of metal behavior will be used while the p-y and t-z curves for the soil springs will be the ones consistent with the sandy or clayey soils of the seabed.

Both two-dimensional (2D) and three-dimensional (3D) CFD simulations will be necessary in the course of this project. The primary purpose of 2D computations will be calibration of the CFD models with available data from 2D centrifuge tests. Also, such simulations will help build confidence in the computational procedure and, by comparison with 3D simulations, enhance our understanding of the phenomenon, especially, the berms formed on the sides of the ridge path and the distribution of load (pressure and shear) on the pipeline. Although computationally more demanding, it is expected that 3D CFD models will be the ones that will be relied on in this study. Of course, the possibility of proceeding on the basis of 2D CFD simulations will be considered and, if found adequate, will be pursued. FLUENT is the CFD software that will be used in the proposed computations. Licenses for use of the software in this project are already available.

The results of CFD simulations, in the form of pressure and shear distributions on the pipeline, will be imported into conventional FE models. ABAQUS is the software that will be used for FE analysis. Beam, tube and shell finite elements will be combined with (lateral) p-y and (longitudinal) t-z springs so that the effects of (soil-pipeline) interaction on stress and deformation in the pipeline can be taken into account. Such springs are better known as components of lateral and axial models of pile foundations but can be used with pipelines and, in general, tubular structures embedded in (clayey or sandy) soils. By means of a series of FE computations, it will be possible to determine the minimum thickness that will ensure pipeline integrity, with all other parameters kept fixed. It is important to note that both cross-section deformation (and stability) and overall bending of the pipeline are to be examined. Indeed, the pipeline is subjected to combined bending and external pressure. Licenses for use of ABAQUS in this project are already available.

It is expected that the 3D CFD simulations will be carried out once for each candidate pipeline size (outside diameter), once for each type of soil and once for each combination of parameters describing the morphology, including the (initial) depth of keel embedment in the soil, and motion (magnitude and direction) of the ice ridge. For each of these simulations a series of (much simpler) FE computations will lead to the minimum required pipeline thickness. It will then be possible to arrive at recommendations regarding the minimum required burial depth, given the pipeline size (diameter and thickness) and the rest of the parameters.

As expressed unambiguously by Palmer (2000), “the petroleum industry can operate safely in the Arctic environment, but on a zero-defect basis: one error will not be forgiven.” An Arctic oil spill would bring not only environmental, but wildlife, human, social and cultural devastation (Ahmaogak 2000). Management, containment and clean-up of oil spills are not convincing options in Arctic waters, despite considerable progress. Pipelines engineered primarily for safety and, to the extent that current technical knowledge allows, for economy, constitute the only viable approach at this time. Work to date, experimental and computational, has focused rather exclusively on levels of sub-scour deformation (in the absence of a pipeline) as pointers to the required pipeline burial depth. Further progress will come from studies of complete ridge-soil-pipeline systems. This proposal is a contribution in this direction.

DEPLOYMENT OF RESULTS: The results obtained in this study will be communicated to Arctic pipeline designers, owners and regulatory agencies through conference presentations and publications, and through dissemination of project reports and student theses through normal OTRC and MMS channels. It is expected that conclusions and recommendations will be reached that may lead to better estimates of the minimum required burial depth for safe and economical pipeline design.

PROJECT PLAN FOR 2007 - 2008: The Project will be configured in 5 Tasks as described below.

Task 1: Ice-Ridge Morphology and Motion
A "multiyear ice ridge" (MY ridge) will be useful not only for the purposes of the study described in this proposal but, more generally, as a specification for Arctic pipeline design. Ice ridges are known to be in isostatic equilibrium, as verified by extensive field measurements, many available in the public domain. In the proposed study, a similarity shape for a MY ridge will be developed and scaled by volume. Initially, this will be a 2D model of an ice ridge, since most field data pertain to cross-sectional shapes of ridges. The similarity shape can then be scaled by cross-sectional area. A comparable 3D model will be developed during year 2 of the Project.

In addition to the shape and size of the MY ridge, available data on the motion (speed and direction) will be studied in order to establish the range of each of the associated parameters for the CFD Models in task 2.

Task 2: CFD Modeling and Simulations
For all combinations of parameters describing ridge-soil-pipeline systems of interest, CFD models will be assembled and simulations will be carried out to determine the loads (pressure and shear distributions) on the pipeline. These loads will then be applied on FE models of the pipeline (several cases corresponding to different values of the pipeline wall thickness) in order to arrive at stress and deformation estimates. In the CFD models, the pipeline will be treated as a rigid cylinder of specified diameter, embedded in the soil. Two different soil types (sandy and clayey soils) will be examined with properties varied according to test results from earlier studies. The seabed slope will be considered as well. Available sea bottom surveys show that the slope is small. The ridge morphology, initial position and motion (speed and direction) will be taken in accordance with findings of task 1.

The commercial code FLUENT, based on a Reynolds-Averaged Navier-Stokes (RANS) procedure, will be used in order to perform the CFD simulations. The Department of Civil, Architectural, and Environmental Engineering (CAEE) at The University of Texas at Austin (UT Austin) owns an academic license for FLUENT that permits use of the software in research projects. In addition, a 16-node (32-CPU) cluster, available at the Computational Hydrodynamics Laboratory at UT Austin, will be used. Parallel processing capabilities such as those of the available cluster will be important in cases with high concentration of computational cells (including 3D simulations).

The approach will be similar to the one taken in current research at OTRC on torpedo piles (Raie and Tassoulas 2006) that involves calculation of the pressure and shear distributions on an axisymmetric projectile that is released from within the water, penetrates the water-soil interface, and embeds itself in the soil. FLUENT has been used in axisymmetric mode with two media, water and soil, their interface being captured by the volume-of-fluid (VOF) technique. The procedure has been very successful in tracking the trajectory of the projectile (velocity-depth profile), in very good agreement with laboratory and field tests. The numerical characteristics of the method are being studied by means of grid-dependence studies and verification of the flow and pressure distributions when the soil is replaced with water.

There are similarities and differences between the two applications of the CFD approach to projectiles and ice ridges. The case of 2D ice ridges is similar to that of axisymmetric projectiles, since both involve only two dimensions in space. In 2D, the motion of the ice ridge has three degrees of freedom (2 translations and one rotation), while that of the projectile has only one degree of freedom. The projectile has a significantly longer trajectory of interest than the ice ridge and this means that in the latter case a better grid resolution (thus accuracy) can be achieved, since a significantly smaller calculation domain will be involved. It should be noted that very good resolution of the ice-ridge/water/soil interface would be required so that the “gouging” phenomenon could be predicted with satisfactory accuracy. The speed of the projectile is significantly greater than that of the ice-ridge. Thus, the flow in ridge-soil-pipeline simulations is expected to be at smaller Reynolds numbers and more nearly laminar. In turn, this means that better resolution of the boundary layers can be achieved in cases of ridge-soil-pipeline systems for fixed total number of cells. Finally, with projectiles, there is significant separated flow in the wake, while the flow around the tip of the ice ridge should not exhibit any flow separation. In conclusion, using the same methodology, it should be possible to handle the case of a 2D ridge-soil-pipeline system with the same level of accuracy (if not better) than that in the case of projectiles.

Application of the method in 3D would certainly be more time-consuming in terms of geometry description, grid preparation, and CPU time required for the calculations. As a first step, a prismatic ice ridge, of finite width, traveling in a direction within its sectional plane, will be considered toward evaluation of the effects of berm formation from its edges. The 2D study will help determine the required number of cells for acceptable accuracy at each sectional plane. The effect of the number/distribution of computation cells near the edges will be studied using the 3D simulations. In addition, the effect of the keel width on the load distribution over the buried pipeline will be evaluated and the results will be compared to those from 2D CFD modeling. Furthermore, the ice-ridge geometry will be allowed to change in the third direction and motions with out-of-plane components will be considered.

Task 3: Finite-Element (FE) Modeling
As results of CFD simulations become available (task 2), in the form of pressure and shear distributions on the pipeline, they will be imported into conventional FE models for pipeline stress and deformation computations. ABAQUS, a computer code for finite-element analysis, will be used for this purpose. The CAEE Department at UT Austin owns an academic license of the software that permits use in research projects. Springs comparable to the ones used in axial and lateral pile modeling, known as “p-y” and “t-z” curves, will be used for representation of soil-pipeline interaction, in combination with beam, tube and shell finite elements for the pipeline. For each CFD simulation, a series of FE computations will be conducted in order to determine the minimum wall thickness that will ensure pipeline integrity, all other parameters kept fixed. Both cross-section deformation (and stability) and overall bending and axial deformation (and stability) of the pipeline are to be examined.

Task 4: Burial Depth Recommendations
On the basis of results obtained from FE and CFD simulations (tasks 2 and 3), it will be possible to determine the required minimum pipeline thickness for each ridge-soil-pipeline configuration analyzed. By examination of configurations with different pipeline embedment, design recommendations regarding minimum burial depth for several values of wall thickness will be developed. During year 1, it is expected that these recommendations will be based largely on results of 2D CFD simulations.

Task 5: Report Preparation
A report will be prepared that documents all assumptions, methods and procedures used, results of 2D and 3D CFD simulations as well as pipeline FE computations and conclusions on the minimum wall thickness for each ridge-soil-pipeline system considered.

PROJECT PLAN FOR 2008 - 2009: The Project will be configured in 5 Tasks comparable to the ones of year 1 of the project as indicated below.

Task 1: Ice-Ridge Morphology and Motion
The "multiyear ice ridge" (MY ridge) developed during year 1 will be extended to three dimensions

Task 2: CFD Modeling and Simulations
The CFD modeling and simulations initiated during year 1 will be continued. It is expected that, during year 2, attention will be focused on 3D modeling and simulations.

Task 3: Finite-Element (FE) Modeling
As in year 1, the results of CFD simulations (task 2) in the form of pressure and shear distributions will be imported into conventional FE models for pipeline stress and deformation computations. Again, for each CFD simulation, a series of FE computations will be conducted in order to determine the minimum wall thickness that will ensure pipeline integrity.

Task 4: Burial Depth Recommendations
Results obtained from FE and CFD simulations (tasks 2 and 3) will lead to the required minimum pipeline thickness for each ridge-soil-pipeline configuration analyzed. Conversely, examination of configurations with different pipeline embedment, will point to design recommendations regarding minimum burial depth for several values of wall thickness. During year 2, it is expected that these recommendations will be based on results of 3D CFD simulations.

Task 5: Report Preparation
A report will be prepared that documents all assumptions, methods and procedures used, results of 2D and 3D CFD simulations as well as pipeline FE computations and conclusions on the minimum wall thickness for each ridge-soil-pipeline system considered.

PRINCIPAL INVESTIGATOR(S) & OTHERS INVOLVED IN PROJECT:

Principal Investigators: Drs. Spyros A. Kinnas, and John L. Tassoulas, University of Texas, Austin, Richard S. Mercier, TAMU.

Support Staff: One graduate student to be determined.