An Integrated Approach to Pipeline Burial in the 21st Century

Mark Finch and Rob Fisher, Coflexip Stena Offshore, UK

Prof. Andrew Palmer and Alex Baumgard, University of Cambridge, UK

DEEP OFFSHORE TECHNOLOGY 2000

ABSTRACT The inter-relationship between trenching, backfilling and subsea pipeline system design is described and an integrated design approach developed. Risk areas for pipeline trenching and backfilling are discussed and practical advice on their mitigation given. Results from recent geotechnical research are presented to enable optimum design with respect to upheaval buckling and on-bottom stability. The benefits of an integrated design approach are discussed and the practical advantages illustrated. INTRODUCTION Subsea pipelines are trenched and buried to provide physical protection, on-bottom stability, thermal insulation or download to mitigate upheaval buckling. Any or all of these may be required for a given pipeline but all are inter-related and have a significant effect on the design of the pipeline system as a whole. The rapid increase in the number of small diameter pipelines (typically in-field tiebacks) transporting high pressure and high temperature hydrocarbons has significantly raised the importance of geotechnical engineering in the overall pipeline system design. The prediction of trenching and burial performance and the resulting geotechnical and geometric properties of the trench, pipeline and backfill have become critical. An integrated design approach is required. Over the past five years Coflexip Stena Offshore have sponsored a large number of geotechnical research programmes to investigate the issues related to pipeline burial and the resultant properties of trench and backfill. By combining the research with the practical experience of burying over 1000 kilometres of subsea flowlines, technical risks and issues have been identified and design guidelines developed to facilitate an integrated approach to pipeline burial design. The results are presented in this paper. This paper introduces the variables and risks that need to be considered for an integrated approach to pipeline burial. The paper illustrates how research programmes have allowed the inherent uncertainties of geotechnical design to be reduced and how these findings have been married to practical experience, resulting in a safe and efficient pipeline burial design.

TRENCHING AND BACKFILLING PERFORMANCE Selection of appropriate trenching and backfilling plant has been well documented (Fisher, 1998). The main drivers are: • Soil conditions • Pipeline size • Pipeline type • Trenching specification • Water depth • Cost • Availability There are three generic types of trenching method; ploughing, jetting and cutting. If the trenching specification is clearly defined and site specific soil conditions are known, selection presents few problems. However, in most cases the best trenching tool can only be defined in parallel with the pipeline design, thus there is a requirement for in integrated approach. Ploughing Ploughs are capable of trenching a wide range of soil conditions from very soft clay to fractured or weak rocks. The trenching and backfilling mechanisms of ploughs are have been described (Reese and Grinsted, 1986). Examples of a typical pipeline plough is shown in Figure 1. Traditional ploughing algorithms are widely used in the industry today. It should be noted, however, that the traditional algorithms introduce a significant level of uncertainty, particularly in sand soils. Recent consolidation of ploughing experience has reduced the level of uncertainty in the traditional ploughing equations. Ploughing in Clay Soils The mechanisms involved with cutting clay soils are relatively simple (Reese and Grinsted 1986) and are based on undrained shear failure. Rate effects are not significant. In general cutting clay soils does not present problems. For example Coflexip Stena Offshore have carried out trenching operations in very hard clay soils containing boulders at Gullfaks without incident at moderate speeds (150 m/hr to 400 m/hr) for relatively low tow forces (100 Te to 200 Te). The upper limit to ploughing in clay soils is defined by the capabilities of the trenching support vessel and stability of the plough system. Ploughing in Sand Soils Sand soils generally result in slow trenching speeds (< 250 m/hr) at high tow forces (> 200 Te). Rate effects are very significant (Palmer 1997). This is a function of the density and permeability, therefore the particle size distribution, of the soil. Models for the behaviour of ploughs in sand are available in the public domain (e.g. Reese and Grinsted, 1986):

( )( ) ( ) ( )vhkhkWT p3

23

1tan ++= δ (1) Where: T - Tow load (Te) Wp - Submerged weight of the plough + supported pipe (Te) δ - Soil / steel interface angle (degrees) k1 - Dimensionless factor derived from interface angle and

submerged weight of soil h - Trench depth (m) k2 - Dynamic factor dependent on grain size and density v - Plough speed (m/hr)

Coflexip Stena Offshore research and experience suggests that such models can only be used accurately within the limits presented in Figure 2. For particle size distributions with a significant proportion coarser than the right hand limit, rate effects diminish and speeds increase accordingly, due to the increase in the permeability of the soil mass. Where particle size distributions have a significant fines fraction, lying to the left of the limits shown in Figure 2, experience demonstrates that undrained failure mechanisms, and therefore faster trenching speeds, govern. It must be stressed, however, that ploughing in soils close to the left hand limits in Figure 2, incorporates a significant level of uncertainty as the failure behaviour of the soil mass is very sensitive. Ploughing in Layered Soils Predicting plough performance in layered soils is very difficult because the resistance across the ploughshare is non-uniform. Experience confirms that the differences in static and dynamic resistance (see Equation 1) can be significant. This can lead to surging of the plough, which contributes to increased as trenched Out-Of-Straightness (OOS) of the pipeline. Sand over soft clay trench profiles represent a high risk of OOS in this regard. Reduction of this risk is achieved by slow, controlled, ploughing. Performance predictions in layered soils are usually based on extrapolation of previous experience. Analytical models have been found to be unrepresentative. Further investigation is planned to develop accurate models for layered soil profiles. Experience has indicated that it is the soils at the share tip which govern plough speed. This is a function of these soils being more constrained. Jetting Jetting tools, predominantly ROV based, are significantly smaller than ploughs and many cutting machines with typical in-air weights less than 15 Te. Jetting tools are used for trenching both rigid and flexible lines in cohesionless and soft cohesive soils. Natural backfill or trench collapse is relied upon for backfill cover. Jetting has become the primary trenching method in deep water where soft soils are prevalent (Finch and Fihn, 1999). Jetting in Sand Sand soils are trenched by fluidisation, which reduces the submerged weight of the soil in the trenching zone significantly. This allows a denser product to settle to the base of the trench. This typically results in a depression in the seabed where the jetter has passed and the product buried some way below this depression. The effectiveness of any given jetting tool in sand soils is a function of: i. Jetting pressure ii. Input water volume iii. Layback of the product (i.e. touchdown length) In sands, pressure only has a significant influence when the soil is dense. In such cases it may be difficult to erode individual soil particles from the soil mass at the trench face and thus high pressure (i.e. increased energy) is beneficial. The influence of water volume introduced into the soil mass is significant. As the water volume increases the submerged density of the fluidised soil mass decreases for a given trenching speed. A fluidised soil mass will begin to reconsolidate beyond the influence of the jetting system. In most cases this is seen to be approximately 15 m behind the jetting tool. If the layback length of the pipeline is greater than this, the achievable depth of lowering will begin to reduce significantly, assuming a cable depressor is not employed. Flexible products, umbilicals and cables, which have relatively short layback lengths, can normally be trenched effectively in a single pass. Rigid pipelines, with longer layback lengths, often require multiple passes to achieve the specified depth of lowering.

Analytical models have been developed by Coflexip Stena Offshore to predict the degree of fluidisation, layback lengths and reconsolidation time. Jetting in Clay Jetting tools form a trench in cohesive soils by cutting and clearing the soil from the trench. Cutting is achieved by forward facing high pressure jets with clearing of the resultant debris facilitated by low pressure jetting or eduction, either to the rear or vertically through eduction tubes (Finch and Fihn, 1999). This typically forms an open, rectangular, trench profile. Consequently the layback length of the product does not significantly influence trenching performance, unless trench collapse occurs within this span. The risk of trench collapse within the time taken for the product to layback (i.e. the risk of increased as-trenched OOS) can be estimated by analysing the stability of the trench walls and the time taken for the product to reach the base of the trench. The cutting of clay soils has generally been considered to be a function of water jet pressure. Therefore, the majority of the large jet trenchers currently being commissioned have significant pressure potential. However, recent research on behalf of Coflexip Stena Offshore has indicated that increased pressure above certain thresholds may prove inefficient (Machin, 2000). Water jet cutting of a cohesive soil is strongly influenced by jet nozzle orientation, resulting in bearing or slope failure mechanisms. In order to carry out analytical performance predictions, both the efficiency of the cutting and eduction systems must be assessed. This was carried out in detail during the development of the Perry Tritech TXL 500 jetting system (Finch and Fihn, 1999). Cutting Cutting tools form a trench by using penetrating cutting teeth mounted on wheels or chains, which rotate to remove the spoil. Cutting tool performance is governed by: i. Trench depth ii. Soil conditions iii. Available mechanical power and rotation speed The interrelation of these parameters is complex. In general performance prediction is carried out based on previous experience. Allowances should be made for abrasion and replacement of worn parts that can be very significant where granular soils are anticipated. Cutting tools can also experience difficulty in very plastic clays that may clog the cutting mechanism and significantly reduced performance. AS-TRENCHED OUT OF STRAIGHTNESS As-trenched Out-Of-Straightness (OOS) describes the residual imperfections in a pipeline profile following a trenching operation. The as-trenched pipeline profile is primarily driven by the undisturbed seabed profile, method of trenching and in-situ soil conditions. Critically, OOS events are potential initiation points for upheaval buckling (Richards, 1990). For a given wavelength, the larger the imperfection is the more backfill or remedial rock dump is required to mitigate upheaval buckling. Therefore, it is important to trench in a manner to reduce OOS. The accurate prediction of OOS at an early stage in pipeline system design is critical.

OOS imperfections are a result of: i. Original seabed profile, which may already contain imperfections ii. Trench collapse before pipeline touchdown

iii. Debris falling under the pipeline during trenching

iv. Surging and change in pitch of trenching tool Virtually no OOS data is available in the public domain, however, Coflexip Stena Offshore have developed an extensive database based on high resolution survey data from over 150 km of ploughed rigid pipelines. Analysis of this database shows that for ploughing, the frequency of OOS is a function of local soil conditions. Figure 3 shows the typical trend for OOS frequency per kilometre for a large number rigid pipelines trenched using a pipeline plough. It should be noted that seabed features, such as sandwaves, have not been removed from these data. In order to assess upheaval buckling and predict download requirements (i.e. required backfill and/or rock volumes) at an early stage, both the frequency and the wavelength of any imperfection are required. Traditional predictive upheaval buckling assessments adopt the natural, or theoretical, wavelengths of the pipeline. This is very conservative. Review of in-house data clearly demonstrates that the wavelengthsof as-trenched OOS imperfections are greater than the corresponding natural wavelengths for a given pipeline. The data allows more accurate prediction of OOS at an early stage in a tender and therefore allows the early quantification of mitigation requirements and price. It must be stressed, however, that such analysis is predictive and the as-built mitigation can vary from those predicted at tender stage. BACKFILLING Backfill is placed over pipelines to provide download to resist upheaval buckling, to enhance a lines thermal insulation properties or to provide additional protection from physical impact. Backfilling Methods Burial of a trenched pipeline is achieved by either mechanical or natural backfill. Backfill over a trenched pipeline can be achieved in a number of ways. After ploughing a separate mechanical backfill plough is used to return deposited spoil heaps back into the trench. Cutting tools either create a vertical slot which is left to collapse naturally or open 'V' shaped trenches above which a second pass is carried using augers to screw deposited spoil back into the trench. Jetting tools normally rely on natural backfill. However, with the increasing need for thermal insulation, particularly for deepwater applications, tools have been developed to initiate trench collapse of vertical sided trenches in clay soils. For example, an undereaming method was developed for the TXL 500 jetting system (Finch and Fihn, 1999) and was successfully used for the Shell Angus pipeline project in the Gulf Of Mexico.

Backfilling Issues There are two phenomena that must be considered before any backfilling operation: i. Uplift i. Floatation These issues have often been treated as a single problem called �pipeline floatation� (e.g. Cathie et.al. 1996). Recent research and backfilling experience has shown that the two phenomena must be considered separately. Uplift Uplift can occur wherever trenched spoil is mechanically returned or allowed to collapse into an open trench with an exposed product in the base. Several examples of uplift have occurred world-wide. The majority of incidents have been associated with mechanical backfill ploughs. Research programmes sponsored by Coflexip Stena Offshore have identified two force vectors acting on a pipeline during backfilling: i. Transverse flow and wedging

The mechanical backfill process induces transverse and longitudinal soil flow into the trench resulting in uplift forces on the underside of the pipeline. Entrapment of water in any voids under the pipeline also contributes to the uplift force.

ii. Hydrodynamic effects Turbulence ahead of the backfill plough induces a net upward movement of water trapped in the backfill spoil which results in uplift forces on the pipeline. Low permeability soils in the base of the trench compound this issue by directing pressure relief upwards.

The testing of these mechanisms was carried out using models at the University of Leuven, Belguim and the University of Cambridge, England (Finch 1999, Finch and Machin 1998 and Cathie et. al. 1998). The interrelation of the forces causing uplift and the dynamics involved is complex and will probably require detailed finite element analysis. Research results and experience in the field has demonstrated that four factors are required to initiate and propagate uplift during backfilling: i. Low as-backfilled pipeline specific gravity and flexural rigidity

Pipelines of low specific gravity provide little inherent resistance to uplift forces.

ii. Sensitive soils Sensitive soils are defined as those prone to loss of strength and/or density due to the trenching and backfilling processes. Sensitive soils are typically very soft clays of low plasticity and very silty fine sands. Such soils, when liquefied, tend to provide very little download to prevent uplift.

iii. High backfilling speed

The higher the speed of mechanical backfill or trench collapse the greater the energy, and thus the uplift force, that is imparted to the pipeline.

iv. As-trenched out-of-straightness

Imperfections create voids under the pipeline allowing backfill soil to flow underneath and generate uplift forces.

Of the above, the only parameter that can be accurately controlled is the pipeline weight. In sensitive soils no pipeline should be backfilled with a specific gravity less than 1.8 without preventative measures being considered. It is preferable to maintain the as-backfilled pipeline specific gravity above 2.0 where soil conditions are very sensitive to liquefaction and thus the risk of uplift is high. Examples of uplift are shown in Figure 4. Linear uplift, which occurs in homogenous soils, sinusoidal uplift, occurring when the download provided by the backfill soil changes due to variations in the soil conditions, and uplift initiation associated with as-trenched out-of-straightness events are shown. In deepwater locations, where backfill is created by induced trench collapse, it is recommended that pipeline uplift is carefully considered. To date no instances of uplift have been recorded, however, it must be stressed that the pipelines backfilled in this manner over recent years, have been relatively heavy (i.e. As backfilled specific gravity > 2.5). Floatation Floatation is a different phenomenon to uplift and must be considered separately. The following sections consider floatation induced by the backfilling process only. It should be noted that post-installation floatation is also possible if an external force (e.g. earthquake or wave induced loading) is applied to a buried pipeline with sufficient magnitude to liquefy the in-situ soil. Floatation can occur if: i. The submerged weight of the pipeline is lighter than the soil which

surrounds it

ii. The backfill soil above the pipeline has little or no strength Unlike where instantaneous liquefaction can be sufficient to allow uplift, floatation requires sustained liquefaction and relatively slow backfill re-consolidation. Many soils, mainly those that contain significant sand fractions will re-consolidate quickly and therefore pose low risk of floatation. Very soft clays, particularly those with low plasticity are particularly prone to liquefaction and may take considerable periods to reconsolidate (Been and Sills, 1981). These issues are of particular importance for gas pipelines that may be trenched and backfilled in a flooded (i.e. heavy) condition and then dewatered before start up. In such circumstances the definition of consolidation rates and soil strengths at start-up are critical. Numerical analysis models for predicting floatation potential have been developed (Ghazzaly , 1975). Figure 5 shows a example of a 14" pipeline in very soft clay. The critical parameter in this analysis is the undrained shear strength - moisture content relationship. Site specific relationships should be developed wherever possible. If this is not possible empirical formula can be used, although it must be recognised that this increases the inherent uncertainty. In the example, a pipeline submerged weight of 14 kN/m3 is likely to result in floatation. However, if this value were increased to 16 kN/m3 the risk will be mitigated. Significant uncertainty exists with respect to the behaviour of clay backfill soils that have been derived from water jet induced trench collapse. As the moisture content, either in-situ or artificial due to the effects of jetting, increases the undrained shear strength reduces correspondingly. Additional research is planned to define the as-jetted properties of marine soils.

Backfill Improvement Coflexip Stena Offshore have carried out extensive research and model testing into potential backfill improvement. The aim of the programmes was to investigate ways of reducing the magnitude of rock dump required to mitigate upheaval buckling in very weak backfill soils by increasing their strength during, or immediately after, backfilling. The options investigated included: i. Compaction

e.g. Mechanical compaction and vibrofloataion

ii. Consolidation

e.g. Electro-osmosis

Ground Improvement Grouting, soil mixing and reinforced soil

iii. Mechanical restraint The main drawback with any ground improvement technique employed subsea is the significant complexity and therefore cost of operations. The use of electro-osmosis, particularly for very soft clay backfill, has been identified as having potential benefits and will be the focus of future research efforts in this challenging area. THERMAL INSULATION The thermal insulation provided by a soil is defined by its thermal conductivity (k). Definition of thermal conductivity for backfill soils is required for: i. Flow assurance analyses ii. Pipeline upheaval buckling and expansion analyses Thermal conductivity can be related to moisture content for both cohesionless and cohesive soils (Kersten, 1949). By combining empirical formulae, relationships between thermal conductivity, moisture content, undrained shear strength and density have been developed on behalf of Coflexip Stena Offshore. The results of this work are presented in Figure 6. The results in Figure 6 are specific to single soil types only. Engineering judgement, and preferably laboratory tests (Brandon and Mitchell, 1989), are required to define design thermal conductivity values in layered soils. Upper bound values should be adopted for thermal insulation design, as high thermal conductivity represents high heat loss. Conversely, lower bound values are applicable to upheaval buckling assessments where heat retained in the pipeline will tend to increase the uplift forces experienced by a buried pipeline. The changes in moisture content occurring after jet trenching are not well understood and introduce significant uncertainty when considering the thermal properties of backfilling after jetting operations. There will be a point at which the moisture content will become so great that thermal conductivity is replaced by convection. This is a significant, and uncertain, issue for very soft clay soils. Research is planned to develop a better understanding of the properties of jetted marine soils. AXIAL FRICTION The magnitude of axial friction between a pipeline and the seabed is a critical factor in any pipeline design. Extensive axial friction testing programmes have been sponsored at Oxford University (Finch, 1999) and the University of Newcastle. The selection of appropriate pipeline/soil response conditions is dependent on four factors:

i. Soil conditions ii. Pipeline roughness iii. Rate of pipeline movement iv. Degree of burial For soils with particle sizes greater than sixty three micrometres (i.e. the interface between sand and silt soils) the response is considered to be drained as the soil is sufficiently permeable to allow dissipation of any excess pore pressure around the pipeline soil interface induced during loading. For finer soils (i.e. silts and clays) behaviour can be drained or undrained. Engineering judgement is required to define the most appropriate model. Drained Conditions For drained conditions where the pipeline is on the seabed the soil / pipeline interface resistance is derived as:

pd wR µ= (2) Where: Rd - Drained axial resistance (kN) µ - Resistance coefficient = 'tanφ×rf wp - Submerged weight of the pipeline (kN) fr - Resistance factor (-) φ' - Angle of internal friction (degrees) In the case of a buried pipeline (Schaminee, 1990):

'µσ=dR (3) Where:

���

�

���

����

�

�++�

�

�

� ++=Dw

HDHKH pa '

2'2'25.0' γγγσ (4)

Where: σ' - Average stress on the sides of the pipeline (kPa) γ' - Submerged unit weight of backfill soil (kN/m3) H - Backfill cover height (m) D - Pipeline outside diameter (m) Ka - Coefficient of active earth pressure (-) wp - Submerged weight of the pipeline (kN) The testing programme has shown that the resistance factor (fr), in granular cohesionless soils, can be defined by comparing the average grain size (D50) and the coating roughness, where: D50 < pipeline roughness fr = 1.0 D50 > pipeline roughness fr = 0.75 to 0.90

In fine grained cohesive soils the resistance factor (fr) can be defined as: D50 < pipeline roughness fr = 1.0 D50 > pipeline roughness clay fr = 0.6 silt fr = 0.4 Where the angle of internal friction is not available from laboratory tests it can be derived from empirical relationships. Undrained Conditions Undrained conditions can be applicable to cohesive soils. For undrained conditions the axial resistance is a function of the contact area with the seabed. The contact area is dependent on the degree of penetration (Verely and Lund, 1995). Coflexip Stena Offshore consider that the most realistic undrained resistance is derived by using direct simple shear strengths. The strength anisotropy relationship between undrained simple shear and undrained compressive shear is typically 0.8. Depending on the type and criticality of pipeline movement under consideration the selection of peak or residual, based on remoulded shear strength, resistance requires engineering judgement. It is industry practice to use a drained approach for buried pipelines in fine grained, cohesive, sediments. However, undrained conditions should not be overlooked, particularly where rapid pipeline movements are possible. Design Considerations The axial friction values indicated above are appropriate for correlations with laboratory tests and short sections of pipeline and represent an increase in the understanding of pipeline soil interface behaviour. The values quoted above are appropriate for on-bottom stability (i.e. non-trenched) and expansion analyses. In practice the longitudinal profile of a pipeline is not the level surface represented in laboratory tests due to seabed features and OOS. This has the effect of increasing the apparent frictional resistance of pipelines (Palmer and Ling, 1981). Care is required in selecting axial friction values for analyses that include the entire pipeline profile, e.g. upheaval buckling analyses. An axial resistance coefficient (µ) of at least 1.0 is recommended in such cases. It should be noted that for deepwater applications, where very soft cohesive soils are predominant, the resistance provided by an undrained approach may result in equivalent resistance coefficients greater than 1.0. UPLIFT RESISTANCE Between 1996 and 2000 Coflexip Stena Offshore have sponsored several research programmes to investigate the uplift resistance provided by a range of soil conditions. Both monotonic uplift and cyclic behaviour have been modelled using centrifuge apparatus and a full scale test tank constructed at the University of Cambridge (Finch, 1999). Pore pressure transducers placed within the models enabled the definition of test conditions, i.e. drained, partially drained or undrained. Full scale testing confirmed that the centrifuge tended to overestimate the uplift resistance due to scaling effects. Monotonic Uplift Monotonic uplift tests were carried out on all soils to a point where the backfill cover height was reduced sufficiently to cause a significant loss in download.

The following relationship (Pedersen, 1988), is recommended. Correlation with the full-scale test results was significant:

2

' 211.01 �

�

���

� +��

���

�+��

���

�+=HD

DHf

HD

HDP

γ (5)

Where: P - Uplift resistance (kN) γ' - Submerged unit weight of backfill soil (kN/m3) H - Backfill cover height (m) D - Pipeline outside diameter (m) f - Uplift factor (-) A backfilled pipeline experiences a static download at zero vertical displacement due to the weight of the backfill above it. The first term on the right hand side of the equation above (i.e. γ�HD) is recommended as an approximation to the static download at zero displacement. Any given soil has a specific uplift factor (f) and associated mobilisation displacement (δmob) behaviour. As with axial friction, this is dependent on loading rate and particle size distribution. Soils experiencing uplift can behave fully drained, partially drained or undrained, dependant on the rate of the applied loading (i.e. upheaval buckling). The Cambridge testing programme has facilitated a significant increase in the understanding of pipeline uplift resistance. Traditionally accepted values are potentially too conservative. When all other variables are constant, an increase in the uplift factor and/or a decrease in the associated the mobilisation displacement decreases the amount of backfill or rock dump required to mitigate upheaval buckling. Uplift Resistance of Silt and Clay Silt was used to allow testing of a cohesive material under drained, partially drained and undrained conditions with short consolidation times. The behaviour of clay soils has been extrapolated from these tests along with other data in the public domain. Figure 7 clearly shows the decrease in uplift resistance achieved as the loading rate increases, i.e. moving from fully drained to undrained conditions. It is considered that a transition in the behaviour of clay and silt backfill (i.e. from drained to undrained behaviour) occurs at an undrained shear strength of approximately 10 kPa. This transition is confirmed by observations of model ploughing and backfilling tests performed at the University of Cambridge. Above 10 kPa uplift behaviour is observed to be drained due to the backfill acting as a mass of discrete cohesive blocks. Figure 8 shows the increase in uplift resistance from a homogenous mass tested in a loose state to the same material removed and replaced as a blocky mass. Typical blocks are also shown in Figure 8. In practice Coflexip Stena Offshore has observed clay spoil deposited by ploughing which contains blocks of significant size. For drained behaviour in silt and clay a range of uplift factors between 0.3 and 0.5 has been adopted. This is based on results from the Cambridge programmes and publicly available data. The selection of an appropriate uplift factor is governed by the degree to which a silt or clay backfill will form intact blocks of soil, the strength of the blocks and, in the case of layered soils, the amount of sand mixed into the matrix by the trenching and backfilling process. For cohesive soils with undrained shear strengths less than 10 kPa care is required in defining the uplift mechanism. Research and field observation has shown that for such clays a relatively homogeneous backfill cohesive is formed. The resulting backfill material will have a remoulded shear strength based on the sensitivity of the soil and the backfilling process employed. This results in undrained behaviour and appropriate models can be used (e.g. Schaminee, 1990).

Uplift Resistance of Sand A number of full scale uplift tests were performed on clean (i.e. < 5% fines content) sand (Finch, 1999). Identical uplift tests were also performed on the same sand with a defined silt content (5% and 15%). Typical monotonic test results in sand soils are presented in Figure 9. It should be noted that the two tests carried out containing silt exhibited partially drained behaviour. As such the uplift resistance is considered to represent a lower bound. A fully drained silt test, which demonstrates greater uplift resistance, is shown for comparison. CSO research has demonstrated that uplift factors of between 0.1 and 0.5 are applicable depending on the silt content of the sand. Further, it is been shown that all sand soils will behave drained under typical upheaval buckling loading rates. Uplift Resistance of Rock Dump Figure 10 shows a range of monotonic uplift tests in rock dump material. Rock dump behaves drained under all loading scenarios. Results of the research programmes has shown that a peak uplift resistance factor of 0.8 is applicable for the typical rock dump specification tested. Cyclic Ratcheting Cyclic ratcheting has previously been postulated and failures have been observed in the field (Nielsen et al, 1990). Cyclic ratcheting was confirmed by Coflexip Stena Offshore for clean sand soils in 1998 (Finch, 1999). Cyclic ratcheting is an issue for clean cohesionless soils only, i.e. clean sand and rock dump. It has been observed that these soils readily collapse under the pipeline once uplift displacement occurs. In order to mitigate progressive creep (i.e. ratcheting), limits must be placed on the maximum displacement experience by the pipeline during uplift. In soils with definable fines content, i.e. those with greater than 5% fines, there is sufficient cohesion and/or suction locally at the base of the pipeline as uplift occurs to prevent soil collapsing under the pipeline. In practice the time between cycles may be significant, allowing the dissipation of suction pressures, therefore limits have been placed on the maximum allowable uplift to prevent significant collapse over time under the pipeline (see table below).

Design Uplift Factors and Mobilisation Displacement Criteria The following table presents the design peak uplift factors and mobilisation displacements derived from CSO full scale research since 1996. Soil Type Peak Uplift

Factor 70% Peak Uplift

Resistance A

(mm)

100% Peak Uplift Resistance

B (mm)

100% Peak Uplift Resistance

C (mm)

Rock Dump 0.8 - 5.0 20.0 Sand Clean SAND: f = 0.5 and Mobilisation Displacement C = 20 mm Very silty SAND: f = 0.1

0.1 to 0.5 1.0 5.0 30.0

Silt 0.3 to 0.5 0.5 - 30.0 Blocky clay (i.e. drained behaviour ) 0.3 to 0.5 - - 60.0 Clay (i.e. undrained behaviour) Undrained shear strength, Su <10 kPa

Based on Su - - 60.0

It is recommended that the behaviour of very soft clays, particularly those with undrained shear strengths less than 10 kPa are assessed with care. The mobilisation displacements shown in column C above are to be used as the limiting values to mitigate the risk of cyclic ratcheting. Any assessment that involves jetting or cutting tools may result in significant variations in uplift resistance and mobilisation displacement parameters. Detailed consideration is recommended. The research programmes sponsored by Coflexip Stena Offshore have resulted in significant increases in the understanding of monotonic and cyclic pipeline uplift behaviour of all soils. Consequently uplift parameters have been optimised and the requirement for additional download to mitigate upheaval buckling, either from backfill soils or remedial rock dump, has been reduced. The parameters quoted in the table above have already been adopted for projects and accepted by certifying authorities, resulting in significant cost savings and risk reduction for all parties. CONCLUSION The science and art of pipeline burial has changed little for a significant period of time. In the same period the associated geotechnical risks have increased dramatically, as very heavily insulated (i.e. lighter) flowlines are required to transport hotter product over increasing distances. In order to identify and minimise these risks Coflexip Stena Offshore have sponsored an extensive geotechnical research programme in parallel with the consolidation of over 1000 kilometres of practical trenching experience. The result is an integrated approach to pipeline trenching and burial design that facilitates an optimum design for the whole pipeline system. This integrated approach takes pipeline burial design into the 21st century. It allows risks to be reduced by intelligently using the results of extensive research programmes performed over the past four years. The results of the research and the adoption of the integrated approach represent a significant advance in the current state-of-the-art with respect to trenched and backfilled pipelines.

REFERENCES Been, K. and Sills, G.C., "Self Weight Consolidation of Soft Soils: An Experimental and Theoretical Study", Geotechnique 31, No. 4, 1981. Brandon, T.L., and Mitchell, J.K., "Factors Influencing Thermal Resistivity of Sand", Journal of Geotechnical Engineering, Vol. 115, No.12, 1989. Cathie, D., Machin, J.B., and Overy, R.F., "Engineering Appraisal of Pipeline Floatation During Backfilling", OTC 8136, 1996. Cathie, D., Barras, S. and Machin, J., �Backfilling Pipelines : State of the Art�, Proc. OPT Conf., 1998. Finch, M. and Machin, J., "Pipeline Foundation Considerations", Proc. Offshore Site Investigation and Foundation Behaviour Conf., 1998. Finch, M., "Upheaval Buckling and Floatation of Rigid Pipelines: The Influence of Recent Geotechnical Research on the Current State of the Art", OTC 10713, 1999. Finch, M., and Fihn, "Deepwater Pipeline Burial", Proc. Deep Offshore Technology Conference, 1999. Fisher, R., "The Right Tool for the Job � Selection of Appropriate Trenching and Backfilling Equipment", Proc. Subsea Geotechnics Conf., Aberdeen, 1998. Ghazzaly, O.I., and Lim, S.J., "Experimental Investigation of Pipeline Stability in Very Soft Clay", OTC 2277, 1975. Kersten, M.S., "Thermal Properties of Soil", Bulletin 28, Engineering Experiment Station, University of Minnesota, 1949. Machin, J., �Recent Research on Cable Jet Burial�, Ocean News and Technology, pp. 34-35, May/June 2000. Nielsen, N-J. R., Lyngberg, B. and Pedersen, P. T., �Upheaval Buckling Failures of Insulated Buried Pipelines : A Case Story�, OTC 6488, 1990. Palmer, A., "Speed Effects In Cutting and Ploughing", Geotechnique 49, No. 3, 1999. Palmer, A., and Ling, M.T.S., "Movements of Submarine Pipelines Close to Platforms", OTC 4067, 1981. Pedersen, P., and Jensen, J., "Upheaval Creep of Buried Heated Pipelines with Infill Imperfections", Dept. of Ocean Engineering, Technical University of Denmark, 1988. Reese, A.R., and Grinsted, T.W., "Soil Mechanics of Submarine Ploughs", OTC 5341, 1986. Richards, D.M., "The Effect of Imperfection Shape on Upheaval Buckling Behaviour", Proc. Society for Underwater Technology Conf., Aberdeen, 1990. Schaminee, P.E.L., Zorn, N.F., and Schotman, G.J.M., "Soil Response for Pipeline Upheaval Buckling Analyses: Full-Scale Laboratory Tests and Modelling", OTC 6486, 1990. Verley, and Lund, "A Soil Resistance Model for Pipelines Placed on Clay Soils", OMAE Vol. 1, Pipeline Technology ASME, 1995.

Figures 1. Ploughing equipment

2. Limiting criteria for plough performance in sand soils

3. As-trenched out-of-straightness frequency

4. Examples of pipeline uplift

5. Pipeline floatation potential (after Ghazzaly, 1975)

6. Thermal conductivity vs. moisture content relationships for sand and clay soils

7. Uplift resistance behaviour of silt for varying loading rates

8. Blocky and loose homogeneous uplift behaviour in silt

9. Uplift resistance behaviour of sand

10. Uplift resistance behaviour of rock dump

Recovery of CSO Multi-Pass Plough Onboard TSV Normand Pioneer

Figure 1 � Ploughing Equipment

0

10

20

30

40

50

60

70

80

90

100

0.001 0.01 0.1 1 10 100

Particle Size (mm)

Perc

enta

ge P

assi

ng (%

)

Figure 2 � Limiting Criteria for Plough Performance Prediction in Sand Soils

OOS (m)

OO

S F r

eque

ncy

Figure 3 � As-Trenched Out-Of-Straightness Frequency

0

0.25

0.5

0.75

1

1.25

1.5

1.75

KP (km)

Depth (m)

As-Trenched DOL Depth of Cover As-Backfilled DOL

Linear Uplift

0

0.25

0.5

0.75

1

1.25

1.5

1.75

KP ( km)

D ep t h ( m)

As-Trenched DOL As-Backf illed DOL Depth of Cover

Sinusoidal Uplift

0

0.25

0.5

0.75

1

1.25

1.5

1.75

KP ( km)

D ep t h ( m)

As-Trenched DOL As-Backfilled DOL Depth of Cover

Uplift Initiation Associated With Imperfections

Figure 4 � Examples of Pipeline Uplift

5.0

7.0

9.0

11.0

13.0

15.0

17.0

19.0

21.0

23.0

25.0

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Moisture Content (%)

Pipe

Uni

t Wei

ght (

kN/m

3 )

0.10

1.00

10.00

100.00

1000.00

Clay sat unit weight Pipe floatation, cover = 0.1 m Pipe floatation, cover = 0.3 mPipe floatation, cover = 0.5 m Unit weight of pipe plus contents Pipe Settlement, cover = 0.1 mPipe Settlement, cover = 0.3 m Pipe Settlement, cover = 0.5 m Su

Figure 5 � Pipeline Floatation Potential (after Ghazzaly, 1975)

THERMAL CONDUCTIVITYMOISTURE CONTENT RELATIONSHIPS (SANDS)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

50 10 20 30 40 50 60 70

Moisture Content, w (%)

Ther

mal

Con

duct

ivity

, K (W

/Km

)

VD

MD

D

L

Legend

VL VERY LOOSEL LOOSEMD MEDIUM DENSED DENSEVD VERY DENSE

VL

WATER : 0.6W/Km

Sand Soils

THERMAL CONDUCTIVITYMOISTURE CONTENT RELATIONSHIPS (CLAY/SILTS)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

50 10 20 30 40 50 60 70

Moisture Content, w (%)

Ther

mal

Con

duct

ivity

, K (W

/Km

)

VERY HARD

HARD

VERY STIFF

STIFF

SOFT

FIRM

VERY SOFT K + 25%

K - 25%

WATER : 0.6W/Km

Clay Soils

Figure 6 � Thermal Conductivity vs. Moisture Content Relationships

Displacement (mm)

Upl

ift R

esis

tanc

e (N

)

Undrained SILT

Partially Drained SILT

Undrained SILT

Figure 7 � Uplift Resistance Behaviour of Silt for Varying Loading Rates

Displacement (mm)

Upl

ift R

esis

tanc

e (N

)

LOOSE SILT

BLOCKY SILT

Uplift Resistance vs. Displacement

Blocky Material Removed from the Tests Tank

Figure 8 � Blocky and Loose Homogeneous Uplift Behaviour in Silt

Displacement (mm)

Upl

ift R

esis

tanc

e (N

)

DENSE SAND

LOOSE SAND

Fully Drained SILT

5% SILT, 95% SAND

15% SILT, 85% SAND

Figure 9 � Uplift Resistance Behaviour of Sand

Displacement (mm)

Upl

ift R

esis

tanc

e (N

)

H/D = 3.1

H/D = 2.63

H/D = 2.3

H/D = 1.67

Figure 10 - Uplift Resistance Behaviour of Rock Dump