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A Fluid-pipe-soil Approach to Stability Design of Submarine Pipelines Jay Ryan, Dean Campbell, Atteris Pty Ltd, David White, University of Western Australia, Eric Jas, Atteris Pty Ltd.

Copyright 2011, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 2–5 May 2011. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract The conventional approach to submarine pipeline stability design considers interactions between water and pipeline (fluid-pipe) and pipeline and seabed (pipe-soil). The seabed is typically assumed hydrodynamically stable in this approach. Interactions between the water and the seabed (fluid-soil) are generally considered only as an afterthought. A new approach for assessing the stability of submarine pipelines is under development and is aimed at including seabed stability (or mobility) as a key aspect of the design analysis. An overview of this approach is presented in this paper. A practical method for utilising this design approach has also been developed, and is based on a combination of numerical analysis and physical model testing. Background On-bottom Stability

On-bottom stability design of submarine pipelines is based on assessing the effects of the environment, namely the ocean and the seabed, on the pipeline. In short, a 'stable' pipeline does not displace (or displaces only by a small and allowable distance) when subjected to any environmental loading that may occur – in particular steady-state and oscillatory (wave-induced) on-bottom currents. This approach is known as 'absolute stability' design. As this method has evolved, the criteria for defining pipeline stability have loosened, and now extend to allowing the pipeline to displace a significant predefined distance laterally – up to tens of pipe diameters – under a given loading condition. Whether the distance is arbitrary, based on the operational constraints or the mechanical strength of the pipeline depends on the design approach and the code of practice utilised. This approach is known as 'dynamic stability' design, reflecting that a full dynamic analysis of the structural response is required to predict the displacement of the pipeline during a design storm event.

There are three main interactions that affect the stability of a submarine pipeline. They are the interactions between the water and the pipeline (fluid-pipe); the interactions between the pipeline and the seabed (pipe-soil); and the interactions between the water and the seabed (fluid-soil). Fluid-pipe interactions result in hydrodynamic loading of the pipeline. Pipe-soil interactions result in the mobilization of soil resistance – which is often treated as two independent components, arising respectively from ‘friction’ between the pipeline and the seabed, and passive resistance to pipeline movement provided by the soil that is ahead of the embedded part of the pipe. Strictly these two components are not separate mechanisms, but it is common practice, and a reasonable simplification, to consider them in this way. Fluid-soil interactions result in seabed instabilities such as scour, fluctuations and potential build-up of excess soil pore pressure, and potentially liquefaction of the seabed soil. Pipe-soil interactions such as pipeline displacement may also lead to excess pore pressure generation.

Each of the interactions outlined above are dependent on the other interactions and their effect on certain parameters. For example, the degree of pipeline embedment is affected by scour and liquefaction (fluid-soil), and in turn affects the hydrodynamic loads acting on the pipeline (fluid-pipe), as well as the passive resistance provided by the soil (pipe-soil). Figure 1 summarises these interactions that affect subsea pipeline stability.

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Figure 1 - Interaction between the Pipeline, Seabed and Water (White & Cathie 2010)

Current Design Approach The current approach to stability design is based on modelling of the fluid-pipe and pipe-soil interactions interdependently.

The latest recommended practice from Det Norske Veritas titled DNV RP F109 On-bottom Stability Design of Marine Pipelines (DNV, 2007) was released in 2007 and outlines the most recent design approaches recommended to industry.

DNV RP F109 provides a guideline for three design approaches comprising the following:

• Absolute Lateral Static Stability Method • Generalised Lateral Stability Method • Dynamic Lateral Stability Method

Absolute Lateral Static Stability Method The absolute stability method is a simplistic design approach that considers the static force balance between the

hydrodynamic loads on the pipeline and the resistance provided by the soil. Small lateral displacements of the pipeline are typically allowable using this approach, but are limited to the point where the pipeline will break out of its embedment in the seabed.

Generalised Lateral Stability Method The generalised lateral stability method is based on utilising a series of dimensionless parameters known to be of primary

significance to pipeline stability. These include parameters such as the significant weight parameter, relating pipeline weight to hydrodynamic loading; the Keulagan Carpenter Number, Steady to Oscillatory Current Ratio and Spectral Acceleration Factor, all describing flow kinematics; the relevant Soil Parameter – related to either the strength or density of the soil – and the number of on-bottom current oscillations during the design seastate (Lambrakos et al, 1987). A series of design curves based on dynamic stability analyses are provided in F109, derived using an updated version of the methology set out by Lambrakos er al. (1987). These curves relate the dimensionless parameters discussed above to the allowable displacement criteria selected by the designer. The design curves do not provide information on the bending and axial deformation of the pipeline. Instead, the displacement of the pipeline is limited to ten pipeline diameters, and it is assumed that these movements will not lead to the allowable structural capacity of the pipeline beign exceeded (noting that this only applies well away from points of fixity such as pipeline terminations). In order to allow for larger displacements, the design must use the dynamic lateral stability method.

Dynamic Lateral Stability Method The dynamic lateral stability method is based on simulating the response of a length of pipeline subjected to hydrodynamic

loads during a design seastate. The approach analyses the pipeline response to the applied hydrodynamic loads, and simulates the resistance to displacement provided by the seabed throughout a time series realisation of the design seastate.

Several force models have been developed that simulate hydrodynamic loading on a pipeline. In particular, considerations have been made to wake effects on the lift force, the effects of the superposition of steady-state currents over oscillatory currents, and the effects of irregular waves on the hydrodynamic loads. Much work has been performed focusing on these issues and force models have been presented by Jacobsen et al (1988), Jacobsen et al (1989), Verley et al. (1989), and Fyfe et al (1987) amongst others. It is the responsibility of the designer to utilise the most appropriate force model applicable to the pipeline under design.

The soil resistance models utilised in dynamic stability analyses almost always include Coulomb friction between the pipeline and seabed, and sometimes additionally model the soil passive resistance provided by pipeline embedment. The

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Coulomb friction component utilises a pre-defined friction coefficient. Some design codes specifiy particular values of friction coefficient for a given soil type, which overlooks the influence of the pipeline coating roughness. Better practice is to measure an appropriate friction coefficient for the pipeline coating and soil unit under consideration. The soil resistance model described in DNV RP F109 for utilisation in dynamic stability analyses superposes two soil resistance components. Firstly, a Coulomb friction component, and secondly an additional component based on a particular form of lateral force-displacement relationship which attempts to capture the soil passive resistance. In particular, this model attempts to capture the variation in the passive resistance with pipeline embedment, and the variation of that embedment as the pipeline undergoes small-amplitude lateral cycles and then breaks out. The relationship provides empirical expressions for (i) the relative increase in embedment as the pipeline displaces back and forth laterally, (ii) the decrease in embedment as the pipeline breaks-out, followed by a constant penetration, and (iii) the layeral soil resistance that corresponds to each of these stages. The relationship provided in DNV RP F109 for sandy soils was originally set out by Verley & Sotberg (1992). The monotonic response of this model is displayed graphically in Figure 2.

Figure 2 - Soil Passive Resistance Force-Displacement Relationship – as Idealized in DNV RP F109

Several Finite Element Analysis (FEA) models have been developed by the pipeline engineering industry in order to

analyse the dynamic stability of pipelines in 3-dimensions. These software packages typically model the response of the entire, or partial sections of the pipeline subjected to the design storm event using irregular waves, and resting on a seabed with given properties. The results of these modelling campaigns are then used to assess the pipeline against the relevant limit state design criteria, typically based on allowable displacement, and allowable stress and strain. At the present time, the models available in the industry allow the designer to account for the influence of changes in the embedment profile of the pipeline. Any changes in embedment that are calculated within the soil model can be used to update the hydrodynamic loading, and soil resistance itself. Within the models, the embedment profile can change due to self burial caused by small oscillatory displacements of the pipeline, and also due to the build-up of soil berms on either side of the pipeline as it displaces.

The recent trend within Australia has been towards designing pipelines using dynamic stability analyses at least along critical sections of the route.

Limitations in the Current Design Approach

There are significant limitations to the current design approach utilised by the industry. These limitations are predominantly related to the assumption that the seabed profile and properties are not affected during the design seastate. There are two key areas where these limitations apply:

1. Seabed instability caused by on-bottom currents (fluid-soil) 2. Seabed changes caused by pipeline displacement (pipe-soil)

Seabed Instability Caused by On-bottom Currents It has been suggested that for many seabed conditions, the seabed will become unstable before the pipeline becomes

unstable under hydrodynamic loading caused by on-bottom currents during a design seastate (Palmer, 1996). As well as loading the pipeline, hydrodynamic action also induces fluctuating normal and shear stresses on the seabed. These leads to the onset and development of seabed scour around the pipeline, and may also lead to excess pore pressure generation within the soil.

The scour process will cause the embedment profile of the pipeline to change. This can lead to self-burial of the pipeline as scour holes form beneath the pipeline (piping) and propagate along its length. The pipeline then deflects into the locally lowered seabed triggering a process of sediment deposition into the scour hole, and as a consequence causing the pipeline to be further embedded into the seabed. While scour may result in an increase in embedment in certain circumstances, it may also

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result in a decrease in pipeline embedment for a part of a storm and pipeline freespans may be left from a partially-complete self-burial process.

Based on the points outlined above, it is clear that scour, if it occurs, will cause changes to both the hydrodynamic loading of the pipeline, and the resistance provided by the seabed, throughout a storm representative of the design seastate. Both the hydrodynamic loads and the soil resistance are dependent on pipeline embedment level. As a consequence, from a fundamental standpoint the effects of scour around the pipeline should be considered in stability design.

Depending on the loading history of the seabed, normal and shear stresses on the bed may cause excess pore pressure generation in the soil. In circumstances where the time period of oscillation is short relative to the drainage rate of the soil, there may be a build-up of excess pore pressure within the soil. The period of oscillation of the on-bottom current is significant in this process because there may be a scenario where the excess pore pressure dissipation rate of the soil does not allow full dissipation before a significant shear stress is applied in the opposite direction. This scenario will generally cause a build-up of excess pore pressure. A build-up of excess pore pressure will lead to weakening of the soil and as a consequence may reduce the lateral resistance provided by the seabed.

In the special circumstance where the excess pore pressure approaches the initial effective stress level within the soil, liquefaction may occur. Liquefaction may cause the pipeline to float up through the seabed, reducing embedment; or sink further into the seabed, increasing embedment. This is dependent on the specific gravity of the pipeline and the density of the liquefied soil respectively. (Teh et al, 2003, Bonjean et al. 2008)

Based on the points outlined above, it is argued that excess pore pressure generation may also cause changes to the resistance provided by the seabed. This is caused by its effects on soil strength and pipeline embedment. As a consequence, the process of excess pore pressure build-up caused by on-bottom currents should also be considered in stability design.

Seabed Changes Caused by Pipeline Displacement As discussed in the previous sections, the impact of pipeline lateral displacement on the seabed is taken into account by

considering the build-up of a soil berm beside the pipeline as it displaces, effectively increasing the embedment depth. However, small cyclic displacements of the pipeline will also impose cyclic shear stresses on the seabed, which can lead to excess pore pressure generation.

Along with the case of cyclic shear stresses caused by direct on-bottom current loading, this process can lead to a weakening of the soil, reducing soil resistance, and ultimately liquefaction of the soil, albeit restricted locally around the pipeline.

Based on the point outlined above, it is argued that displacement of the pipeline may reduce the resistance provided by the soil due to the build-up of excess pore pressure. As a consequence, the effects of pipeline displacement on excess pore pressure and soil strength should also be considered in stability design, along with its effect on soil berm build-up which is already considered in design.

As outlined in this section, the current design approaches consider only fluid-pipe and pipe-soil interactions interdependently, and most consider the fluid-soil interactions, or seabed instabilities, as an afterthought or a separate analysis.

The reality is that the fluid-pipe-soil interdependency must be modelled if the fluid-soil interactions play a role in the overall behavior in the conditions being considered, otherwise accurate results cannot be obtained. This becomes particularly important when considering fully dynamic models that attempt to predict the displacement of the pipeline over time. Currently, these models do not consider the fluid-soil interactions that take place, nor do they consider the effects of pipe-soil interactions on the properties and stability of the seabed. These interactions can alter the hydrodynamic loading and resistance provided by the soil over time, placing a question mark on the accuracy of a fully dynamic model that does not consider these effects.

Fluid-Pipe-Soil Design Approach Overview

A design approach to pipeline on-bottom stability that considers the interdependency between the fluid-pipe-soil interactions is required. The concept behind a fluid-pipe-soil model for pipeline stability is displayed in Figures 3 and 4. Figure 3 represents the current approach to stability design utilised by the industry. It depicts that the key processes affecting stability are only partially considered. Figure 4 represents a fluid-pipe-soil approach to stability design.

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Figure 3 - Current Design Approach to Stability Figure 4 - Fluid-Pipe-Soil Design Approach

Figure 4 outlines the key processes the designer should consider in adopting a fluid-pipe-soil design approach. However, the mechanisms that result in the interdependency between these processes are equally important. The interacting effects caused by one process on the others must be considered. The interdependency of the key physical processes and mechanisms affecting pipeline stability is outlined in Table 1.

Table 1 Interdependency between Physical Processes

Item Dependent Process Hydrodynamic Loading Pipeline Displacement, Soil Resistance Soil Resistance Pipeline Displacement Bed Shear Stress Excess Pore Pressure Build-up, Scour Scour Pipeline Embedment Profile, Pipeline Vertical Displacement (piping/self burial) Excess Pore Pressure Build-up Soil Resistance, Liquefaction, Scour Liquefaction Pipeline Displacement (floatation / sinking) Pipeline Displacement Bed Shear Stress, Pipeline Embedment Profile (berm build-up / self burial) Pipeline Embedment Hydrodynamic Loading, Soil Resistance

It is clear from Table 1 that given the large number of interdependent processes taking place within the pipeline stability

problem, a fully dynamic stability analysis must be performed to model the processes with a high level of accuracy. However, performing a dynamic stability analysis that correctly models the full fluid-pipe-soil interactions is a difficult task to perform reliably Robust theoretica models for all these effects do not exist, and where models do exist there is no established procedure for calibrating their parameters for a particular set of pipeline, metocean and geotechnical conditions. As a consequence, it would be useful for the industry to develop an approach that considers all of the key processes, and which is practical for use in the immediate timeframe.

Benefits of a New Approach

The key benefits of developing a new approach for pipeline on-bottom stability design include the following:

• Uncertainty in design is reduced, as a consequence minimising over-conservatism compared to the current design approach

• Potential under-conservatism in the current design approach is removed A large degree of uncertainty in stability design of submarine pipeline is caused by the non-inclusion of seabed instability

in the current design approach, along with other uncertainties such as metocean conditions and geotechnical conditions along the pipeline route. These uncertainties typically result in the introduction of safety factors into the analysis, aimed at ensuring conservatism in design. These safety factors are not generally expressed as an explicit function of the degree of uncertainty, but rather have been developed over time and experience within the industry in different locations worldwide.

Including the effects of seabed instability into the design approach will reduce the uncertainty in design. This will help to ensure that pipelines are not over-designed in an industry where both cost efficiency and safety in design are paramount.

Scour and liquefaction may lead to an increase in pipeline embedment over time and as a consequence increase the stability of the pipeline. For the case where scour is the primary cause of the increase in embedment, the process is typically referred to as pipeline self-burial.

There are also some areas of potential under-conservatism caused by ignoring the fluid-soil interactions. These items include:

• A reduction of soil resistance caused by a build-up of excess pore pressure • Potential pipeline floatation caused by free-field and/or local liquefaction • Points of fixity occurring in a displacement-allowed pipeline caused by fluctuations in embedment along the route

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As discussed in the previous sections, bed shear stresses caused by cyclic on-bottom currents or cyclic pipeline displacements can ultimately cause a reduction in the resistance provided by the seabed. As a consequence, the magnitude of resistance provided by the soil may be over-estimated using the current design approach.

It is possible that seabed instability will result in regions of a pipeline becoming partially buried, leaving the remainder of the pipeline resting on the seabed. While this can be considered a stabilising effect, in the case where the pipeline is allowed to displace under design conditions, it may actually create a less stable environment for the pipeline. A point of effective fixity caused by the seabed adjacent to a displacing pipeline will increase the bending stresses in the pipeline at the fixed (more buried) location. As a consequence, the bending stress in the pipeline may be under-estimated under these conditions unless the effects of seabed instability are included in the dynamic analysis.

A Practical Method for Design

A practical method for incorporating the key fluid-pipe-soil processes into stability design in the immediate timeframe can be utilised by performing physical model testing in combination with standard stability analysis techniques such as FEA. The concept is displayed in Figure 5.

PERFORM PHYSICAL MODEL TESTING

PERFORM NUMERICAL STABILITY ANALYSIS

(utilising results from physical model testing)

Figure 5 - Concept for Practical Method of Fluid-Pipe-Soil Design This method is based on performing physical model testing in order to model processes that may be considered difficult or

impractical to analyse using desktop techniques at a project level. The physical model testing program can be developed in order to model conditions specific to the pipeline under investigation. It is proposed that the following processes should be permitted to occur as part of the physical model testing program:

• Local scour • Free-field excess pore pressure generation/dissipation (fluid-soil) • Local excess pore pressure generation/dissipation (pipe-soil) • Soil breakout resistance properties after changes in embedment profile and soil strength • Hydrodynamic loading with assymetrical embedment profiles induced by scour

The potential for these processes to occur should be investigated by performing a simulation involving random time series

realisations consisting of irregular waves (or corresponding on-bottom currents) for the design seastate/s. This approach is consistent with the current approach to dynamic stability analysis adopted by the industry.

Upon completion of a well developed physical model testing program, the designer should have an understanding of the processes and the effect that each process has on the overall pipeline and seabed stability problem. The designer should have sufficient information to be able to adapt the more readily modelled processes such as hydrodynamic loading, soil resistance and ultimately pipeline response, to include the relative effects of seabed instabilities on the entire system. This would typically include a dynamic stability analysis using FEA. However, the designer may even choose to take a simpler, force balance approach to the final stability analysis.

For example, provided the development of the local scour profile over time is understood, the changes in hydrodynamic loading due to scour can be analysed. It may even be practical to utilise the hydrodynamic loads measured during the physical model testing program in the final stability analysis. Also, if a significant build-up of excess pore pressure is observed during the tests, and the breakout resistance of the soil can be measured, it is possible for the designer to gain an understanding of any decrease in soil resistance caused by seabed instability. The designer can then adapt the soil spring characteristics that are utilised in the final stability analysis. Measurement of the breakout characteristics of the soil may also prove particularly useful if the prototype seabed contains carbonate soil, as is generally the case offshore Australia, given that the current soil models typically adopted are based on experience from siliceous soils.

It is noted here that it will likely be impractical to assess the stability of an entire pipeline using this method. The variances in seabed, metocean, and pipeline conditions along a pipeline route that could be in excess of several hundred kilometres would make the testing program very labour and cost intensive. However, this method can be used for specific regions along a pipeline route such as the following:

• Regions where the seabed is known to, or likely to experience instability under design conditions • Regions where excessive, expensive secondary stabilisation methods are required to ensure stability

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The designer should isolate these regions of the pipeline route and may choose to adopt this method for stability design. For the remainder of the pipeline, it may be argued that full fluid-pipe-soil stability analyses is not required and the current design approach is sufficient. Regions where this argument is strengthened include the following:

• Regions where absolute stability is required such as the shore crossing • Regions where mechanical protection, as well as secondary stabilisation, is required such as the shore approach • Regions where the seabed is unlikely to experience instability • Regions where primary stability in the form of pipeline self-weight is sufficient for stability

Physical Model Testing Testing Facilities

Wave Flumes The traditional type of facility used for physical model testing of hydrodynamic loading and sediment transport are wave

flumes. An example of a wave flume facility is presented in Figures 6 - 8. Wave kinematics and on-bottom currents can be modelled to a good level of accuracy in wave flumes. However, wave

flumes result in the use of significantly small length scales in the model relative to the prototype conditions. This is caused by the requirement to model water depth appropriately in order to achieve the desired on-bottom currents. The scaling of length by these levels often causes severe distortion of the results from the model compared to the prototype conditions. The majority of wave flumes do not have the capacity to model steady state currents, which is often an important contributor to the hydrodynamic loading of the pipeline. Figures 5 and 6 display an example of a wave flume facility in use. In these figures, the wave flume is being utilised to examine the stability of subsea rock berm particles.

Figure 6 - Wave Flume Schematic

Figure 7 - Side View of Wave Flume Figure 8 - Front View of Wave Flume

U-Tube Facilities Another type of facility, known as a “U-tube”, has been used successfully for modelling the behaviour of pipelines and the

surrounding seabed soil. A schematic of a U-Tube facility is presented in Figure 9.

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Figure 9 - U-Tube Schematic

U-Tube facilities can generate on-bottom oscillatory currents accurately, and some can also introduce a steady state current

component to the model. The length scales in a U-Tube facility are not affected by water depth given that only the on-bottom currents are modelled, not the surface waves themselves. The main limitation affecting modelling scales is the blockage of flow. The model pipeline must not be so large relative to the test section dimensions that it causes blockage, and other inaccuracy effects on the water flow between the pipeline and the roof of the test section. In addition, the period of the oscillatory (or wave induced) currents are limited by the resonant frequency of the system. Modelling of sheet flow sediment transport is also difficult given the limited supply of sediment upstream of the model pipeline.

A significant body of research associated with pipeline stability has been performed using U-Tube facilities. An example includes the physical model testing reported by Gao et al. (2002).

The O-Tube Recently, a new type of facility has been developed at the University of Western Australia. This facility is known as the

“O-Tube”. An overview schematic and views of the facility test section are displayed in Figures 10 - 12.

Figure 10 - O-Tube Schematic (Plan View)

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Figure 11 - Test Section with Model Pipe (Partially Filled) Figure 12 - Test Section with Model Pipe (Partially Filled)

The facility consists of a closed circuit of 1.2 m diameter piping linking into a test section 17.4 m in length, 1 m in width

and 1.4 m in total height. The total height of the test section includes a reservoir 0.4 m in depth for the installation of model seabed sediments. The flow of water is generated via an axial flow pump which is inline within the circuit of fluid. The pump is powered by a diesel generator. The facility has the capacity to generate oscillatory flows in excess of 1.0 m/s with periods of 5 s, oscillatory flows of 2.5 m/s with a period of 13 s, and steady state flows in excess of 3.0 m/s. The motor has a rated power capacity of 580 kW.

The facility is also equipped with a model pipe. The model pipe is is connected to a set of actuator arms that enter the test section from its roof. The model pipe is connected to the actuator arms via a set of load cells. The actuator system consists of an integrated control system along with a feedback loop between the load cells and its control mechanisms. The pipe is subjected to three sources of load (or resistance) – (i) hydrodynamic load, from the moving water, (ii) soil resistance, when it is in contact with the model seabed and (iii) direct loading from the actuator, which can be targeted to be zero through the feedback control system.

The O-Tube has the capacity to measure key pieces of data related to pipeline stability design during each physical model test. These include the following:

• Hydrodynamic pressure (and resultant loading) on the pipeline • Local scour profile • Free-field soil pore water pressure • Local soil pore water pressure around the pipeline • Pipeline displacement • Mobilised soil resistance to pipeline displacement

The O-Tube can measure hydrodynamic loading on the pipeline using two different methods. For a test where the pipeline

is kept fixed in position, not in contact with the seabed, the drag and lift loads can be measured using the load cells that connect the model pipe to the actuator arms. For a test where the pipeline is in contact with the seabed, the hydrodynamic loads can be measured using a series of 16 pore pressure transducers (PPT) installed around the diameter of the model pipe.

The local pore pressure around the pipeline can be measured using the PPTs located on the model pipe that are in a buried condition (below the seabed surface). The pore pressure can also be measured directly beneath the pipeline using a series of PPTs that can be installed into the model seabed. Additional series of PPTs can be installed into the model seabed at locations further upstream and downstream of the model pipe in order to measure the soil pore water pressure in the free-field. The model pipe and a model seabed PPT tower are displayed in Figures 13 and 14.

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Figure 13 - Model Pipe with PPTs Figure 14 - PPT Tower Prior to Installation into Model Seabed

The local scour profile around the pipeline can be measured throughout the development of the seastate conditions. The profile is measured using an acoustic echo sounder located above the test section within the frame that also houses the actuator system. The echo sounder has the capacity to scan the seabed profile along the test section at pre-defined scanning and travelling rates. This allows for detailed data on the development rate and profile of local scour around the pipeline to be measured.

Pipeline displacement during a test can be measured using the actuator. The position of the actuator arms is known at all times using the length of arm extension and the angle of arm rotation.

The actuator can also be programmed to perform a pipeline breakout test. A controlled lateral and/or vertical load is applied to the model pipe by the actuator in order to force the model pipe to breakout of its embedment. The force, displacement and velocity of the model pipe is measured, indicating the break-out resistance characteristics of the model seabed.

The actuator frame, including the model pipe and echo sounder is displayed in Figures 15 and 16.

Figure 15 - Actuator System including Model Pipe and Echo Sounder Figure 16 - Actuator Frame and Model Pipe

The O-Tube is an extension of a U-Tube facility given that both generate on-bottom currents within the test section through

a direct drive pump. However, the O-Tube has the inherent capacity to model both steady state, and oscillatory currents combined. In addition, an approximate equilibrium between the upstream supply and downstream loss of sediment can be achieved through the spread of sediments around the periphery of the entire system.

The main limitation associated with scaling in an O-Tube facility is the relative size of the model pipeline to the test section, as discussed regarding the U-Tube type facilities. The period of oscillatory currents is only limited by the pump properties and the power available to drive the pump.

The O-Tube is currently being utilised to perform physical model testing related to the stability of an existing pipeline in the North West Shelf of Australia.

Provided the designer develops the physical model testing program with careful consideration, a facility such as the O-Tube can provide data to support a more accurate understanding of the key processes related to seabed instability for the specific pipeline in question.

Additonal details on the O-Tube facility have been presented by Cheng et al (2010).

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Testing Considerations There are many considerations that the designer must address in the development of a robust physical model testing

program related to pipeline stability. The key testing considerations include the following:

• Modelling to suit specific pipeline conditions • Model scaling • Limitations of test results

Specific Pipeline Conditions It is of paramount importance that the setup of the physical model tests be consistant with the prototype conditions in the

region of pipeline under examination. The pipeline parameters may require adjustment from the prototype conditions in order for the physical processes to be

correctly replicated at the model scale. Having established the relevant scaling regime for the test/s, the designer can then specify the key parameters for the model pipe, which include:

• Outside diameter of pipeline • Specific density of pipeline • Initial embedment level

The seabed must also be modelled correctly for accuracy of results. It is critical for the designer to ensure the model soil

used in the tests is representative of the prototype soil conditions, albeit using the adopted scaling regime. The key parameters to consider include the following:

• Predominant soil type [sand (carbonate/silica) or clay] • Applicable depth range of soil conditions • Particle size and grading curve • Carbonate content • Particle density and bulk density • Degree of consolidation, strength and relative density parameters • In situ cementation or fabric, erosion potential

The correct scale modelling of these parameters is critical to the accuracy and applicability of the results of each test. The

designer should carefully consider these parameters as they exist in the prototype conditions prior to developing the tesing set-up and program. In addition to the basic strength and density of the soil, it is important to consider the susceptibility of the soil to scour and liquefaction, and it may be difficult to replicate in situ features of natural seabed sediment such as fabric, light cementation. It is also difficult to reconstitute large homogenous samples of natural soils that are well-graded, particularly if there is a significant fines content.

Model Scaling Applying the most accurate scaling regime to each test is of primary importance. The selection of scaling regime should be

based on the following:

• The prototype pipeline, seabed and metocean conditions • The processes that the designer wishes to model, or permit to occur, for each test (this may vary test to test) • The scaling parameters that are constrained by the testing facility and apparatus

All of the above mentioned items will affect the scaling regime the designer selects and the details of the testing program.

It is typical for certain parameters to be fixed at the prototype scale such as the fluid density and viscosity, and gravitational acceleration. The length scale is also likely to be constrained by the dimensions of the testing facility. These scaling parameters will subsequentely affect the scaling of other parameters.

The prototype conditions must also be considered prior to selection of a scaling regime. For example, whether the flow around the pipeline is typically sub-critical or super-critical should be considered in any tests related to hydrodynamic loading. The importance of modelling the wave, and on-bottom current kinematics should also be considered in this case.

In tests related to scour, the appropriate selection of the model soil particles is key. In order to achieve the "best model" scaling approach identified by Hughes for physical model testing of scour processes at model scales, the soil particle size typically must be scaled down according to the length scale (Hughes, 1993) i.e. the same scale that the model pipe has been scaled to. However, this may not always be practical, and may distort processes that are critical to the behaviour, and in such cases the designer must develop the testing program so that any distortions caused by scaling can be understood and addressed.

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This may include modelling of scour processes over several tests for the same design conditions, but performed using different scaling regimes.

A thorough understanding of the processes that the designer wishes to model is also of importance, particularly when it comes to selection and procurement of model soil. For example, while it is possible in certain circumstances for hydrodynamic loading and local scour to be modelled in the same test with an acceptable level of scaling distortions, this would typically require the soil particle size to be scaled down the same as the model pipe. However, this may change the particle shape and packing arrangement of soil, and as a consequence introduce significant scaling distortions in the behaviour of excess pore pressure generation and dissipation. A possible solution for this issue is to perform separate tests for modelling excess pore pressure behaviour.

The key point is that the scaling regime and the design of the testing program is likely to vary from project to project depending on the specific conditions prevalent to the project. As a consequence, it is important that the designer performs the appropriate scaling analysis prior to development of the testing program, and during the process consider the specific requirements of the project.

Limitations of Test Results While physical model testing is a practical and accurate way of modelling the complex processes related to pipeline

stability, there are significant limitations that must be addressed in the interpretation of the results. The majority of the limitations relate to the limitations of physical modelling facilities. These typically include:

• The modelling restricts a 3-dimensional process to 2-dimensions (or at least a short 3rd dimension) • End effects disrupt the flow and scour processes at the walls in the test section • At model scales, it is typically impractical to model all processes together accurately as they occur in the

prototype conditions • Distortions in results occur due to scaling

The limitations outlined above represent the key issues the designer is likely to have to address during the interpretation of

physical model testing results. There are several important processes that are not typically modelled correctly in physical model tests including, but not limited to, the following:

• Scour hole propagation along the length of the pipeline • Transmission of hydrodynamic loads along the pipeline through pipeline stiffness • The 3-dimensionality of waves • Pipeline displacements, causing local excess pore pressure generation, may not represent the response of the

pipeline in 3-dimensional prototype conditions Given these limitations it is important that the designer analyses the results from each test considering the limitations

appropriately prior to utilising the data in further analyses. It is also important to note that the physical model testing program should not typically be considered as an assessment of pipeline stability. The reality is that physical model testing will only give the designer an understanding of the processes affecting the stability of a pipeline. A proper stability assessment that incorporates the results from the testing program must still be performed. This should typically include a dynamic stability analysis using FEA.

Post-testing Analysis

It is possible to adapt existing dynamic stability modelling techniques using FEA to include the effects of seabed instability. The current numerical and FEA models in use by the industry consist of a technique for applying hydrodynamic loads to the pipeline, and a pipe-soil model that includes modeling of the lateral resistance by combining a resistance caused by friction between the pipeline and seabed, with passive resistance calculated from the embedment of the pipeline into the seabed. A well developed physical model testing program should provide the designer with an additional understanding of the following key parameters:

• Changes in embedment over time • Changes in soil strength (and soil resistance) over time • Changes in hydrodynamic loading on the pipeline as the embedment changes

Given this knowledge, it becomes possible to update the pipe-soil model, typically denoted as a soil spring, throughout the

time series realisation of the seastate conditions. This will effectively model the relative effect of scour, excess pore pressure generation, and liquefaction on the overall response of the pipeline. The same can be adapted to hydrodynamic loading. Updating the existing dynamic stability models to include the results from physical model testing will allow the designer to include seabed instability appropriately in the final stability assessment.

OTC 21368 13

Conclusions The following conclusions are made based on the discussion in this paper:

1. The current approach to pipeline stability design is incomplete given that it does not consider several significant

processes primarily related to seabed instability 2. This approach may be over-conservative or unconservative depending on the circumstances 3. A practical method of utilising a fluid-pipe-soil approach to stability includes performing physical model testing

to understand the processes that may be difficult or impractical to model analytically 4. The designer must consider the specific pipeline conditions, the appropriate scaling regime, and the limitations of

the facility and test results in the development of the physical model testing program 5. Physical model testing does not represent the final stability assessment and must be accompanied by interpretation

of the test results, using some model for the behavior, and subsequent post-testing analysis using this model. This interpretation serves two purposes – (i) it allows any differences in scale between the tests and the field conditions to be accounted for, and (ii) it allows the test outcomes to be applied to specific conditions that may differ from the precise scenarios simulated in the physical tests.

Acknowledgements

The O-tube facilities at University of Western Australia (UWA) have been developed with support from the Australian

Research Council (ARC) (Linkage Grant LP0989936), Woodside Energy Ltd and Chevron Australia Pty Ltd, as well as the support from the University of Western Australia. These projects are led by Professor Liang Cheng, of the University of Western Australia. His assistance, and that of Dr Hongwei An and Mr Tuarn Brown (both of UWA) is acknowledged.

The 3rd author is supported by an ARC Future Fellowship (FT0991816)

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