Pipeline Geotech

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    Frontiers in Offshore Geotechnics: ISFOG 2005 Gourvenec & Cassidy (eds) 2005 Taylor & Francis Group, London, ISBN 0 415 39063 X

    95

    1 INTRODUCTION

    Subsea pipelines are laid on the seabed and may ormay not be buried. Pipeline design needs to accountfor the ways in which the pipeline interacts with thesoil. This paper is about pipeline-soil interaction and, in

    particular, the geotechnical issues that must be facedby pipeline designers if soil-pipe interaction is to becaptured adequately in the design process. Pipelinegeotechnics is an emerging specialty that involvesapplications of geotechnical theory and practice uniqueto the construction of underwater pipelines.

    For this State-of-the-Art review, the material hasbeen organized broadly in the order in which the phe-nomena arise embedment and lateral friction mobil-isation during pipelay, stability against current andwave action when on the seabed, development of axialfriction as a result of thermal and pressure loading etc.Since embedment during pipeline affects axial and lat-

    eral friction this appears to be a logical progression.Some areas that are not covered include self-burial

    of pipelines as a result of wave/current action andsediment transport, liquefaction around pipelines, andthe whole area of ploughing and trenching (Cathie &Wintgens 2001) which would make the paper tooextensive.

    2 GEOTECHNICAL PARAMETERS

    If soil-pipeline interaction is to be understood andmodelled then it is clear that geotechnical data alongthe pipeline route is required. This basis point is notalways recognized and a group involved in the offshore

    geotechnical industry has put together a guidancedocument to assist non-specialists to understand the

    basic data required from a survey (OSIF 1999). Char-acterisation of the pipeline route with emphasis ontrenching issues is discussed in Cathie (2001).

    Pipeline design and engineering will require geo-technical data if major assumptions are to be avoided.For design, all soils require classification tests (par-ticle size distribution, index tests and basic shearstrength parameters). The in situ density of granularsoils and the undrained shear strength of cohesivesoils should be determined.

    If the soil is very soft it is not sufficient to collectdrop cores as the disturbance can lead to underestimat-ing the soil strength. This could affect axial and lateralfriction assessments, and trenching engineering issues.The box corer coupled with in situ vane testing (OSIF1999) is recommended for obtaining good quality datain very soft soils. A substantial quantity of soil can be

    collected using a box corer and this may be necessary ifriser-soil interaction needs to be investigated in specifictests, or if backfill properties need to be investigated.

    In granular material, the cone penetration test (CPT)remains the most attractive tool to determine the in situdensity (OSIF 1999). CPTs should always be com-

    bined with sampling techniques in order to determinethe physical properties of the sand, particularly thegrain size, which may be important for trenchability.

    3 PIPELINE EMBEDMENT

    Pipeline embedment begins during pipe lay and mayincrease with time due to hydrodynamic forces, pipe

    Pipeline geotechnics state-of-the-art

    D.N. Cathie & C. JaeckCathie Associates SA/NV, Brussels, Belgium

    J.-C. Ballard & J.-F. WintgensFugro Engineers SA/NV, Brussels, Belgium

    ABSTRACT: Pipeline geotechnics deals with soil-pipeline interaction. This covers installation issues (pipelinepenetration and short-term lateral stability), axial and lateral response to loads. It then encompasses pipelinetrenching, backfill engineering and pipeline stability when buried. This review provides an overview of all aspects

    of pipeline geotechnics except trenching. The focus of the paper has been on the mechanics of each problem,explaining the issues with a view to developing understanding, rather than providing ready made solutions. Theinterested reader can make use of the references for going deeper into particular aspects of the subject.

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    where z penetration, D pipeline diameter,su undrained shear strength, soil unit weight,Fz vertical load per unit length on soil.

    Verley & Sotberg (1992) propose equivalent equa-tions for penetration in sand but for cyclic loading.

    Murff et al. (1989) have presented upper and lowerbound plasticity solutions for rough and smooth pipes(full adhesion and no adhesion) in cohesive soils. Thelower bound results are shown on Figure 3 and com-

    pared with experimental data and the Verley & Lundapproaches. Upper bound solutions were only slightlyhigher.

    These solutions are of most importance in verysoft clays where embedment may be significant. Muchof the experimental data shown in Figure 3 is from

    Wagner et al. (1987) and is applicable to remouldedclay with an undrained shear strength around 1 kPa.Some of the scatter may arise from the difficulty ofmeasuring strength and its variation with depth.

    The plasticity solutions do not account for buoy-ancy effects, soil heave, or any increase in strengthwith depth. Murff et al. have shown the potential impactof heave and increase in strength. The effect of buoy-ancy would typically be about 510% depending onthe soil strength but would be more important in fluidmuds. Soil heave is potentially even more important.Soil heave around the pipe increases the bearing area

    by about 20% at z/D 0.2 and increases the contactarea at the same depth by 35%. At a penetration ofabout z/D 0.25 almost the full diameter of the pipeis bearing on the soil when heave is accounted for. Itis difficult to see how laboratory and field data can be

    properly interpreted without taking account of the

    geometric changes that occur with penetration. Thereis scope for further theoretical work in this respect.

    Since pipe penetration involves remoulding thesoil locally to the pipe wall, and since repeated load-ing due to hydrodynamic effects would only accentu-ate this effect, it seems reasonable to assume that

    penetration assessment should consider a zone ofremoulded soil below the pipeline during laying. Thiswould suggest that using a low soil-pipe adhesion forembedment assessment would be appropriate.

    In the view of the authors, the issue of initial pene-tration resistance is relatively well understood and

    established. Focus should now be on the nature andmagnitude of the loading applied by a pipeline duringlaying, and the possible magnitude of the cyclic effects.

    3.3 Penetration due to repeated loads

    As discussed above, as a pipe is laid on the seabed themovement of the pipe during the laydown processwill increase the penetration.

    Cyclic vertical loading has been investigated byDunlap et al. (1990), Fontaine et al. (2004) and in the

    STRIDE and CARISIMA joint industry projectsaimed at understanding riser-soil interaction (Bridgeet al. 2004). The latter authors provide a clear explan-ation of repeated penetration and pullout, particularlyfocusing on the suction that develops during pulloutin soft cohesive soils. Dunlap et al. indicate thatlimited cyclic loading without break out resulted inlittle additional burial while larger cyclic loads which

    broke suction and pulled the pipe free resulted in fur-ther penetration. This may be due to the additionalremoulding experienced in the full breakout case.

    Repeated lateral loading induced by current load-ing and dynamic response of the suspended section of

    pipeline can result in a considerable increase in pene-tration (Morris et al. 1988), particularly if the pipe is

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    heave

    z

    a)

    b)

    Figure 2. Pipeline embedment (a) initial penetration, (b)lateral movement.

    0

    1

    2

    3

    4

    5

    6

    0.0 0.2 0.4 0.6 0.8 1.0

    Normalised penetration (z/r)

    Normalisedresistance(F/SuD)

    Murff et al - rough Murff et al - smooth

    Verley and LundVerley linear

    Data (Murff et al)

    Figure 3. Lower bound plasticity solutions and empiricalapproaches for pipeline embedment for cohesive soil.

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    not overpenetrated. Consider a pipe penetrated with acertain vertical load and let us assume that the load iskept constant. Any horizontal load will then result in

    both vertical and horizontal movements since the pipeis on the failure envelope. This is discussed further inthe next section.

    Morris et al. (1988) carried out laboratory tests toassess the embedment due to horizontal cyclic load-ing in very soft clay. The additional penetration waslargely dependant on the magnitude of the force, ordisplacement, and the duration during for which itwas applied. Although the rate of penetration wasfound to decrease over the duration of each test, thisrate did not appear to reach a constant value or the

    pipeline to reach a limiting burial depth. As the pipecontinued to sink, the mounds grew in size, attainingheights of up to 0.8D. Morris et al. also consideredthe accumulated burial effect of variable cyclic load-

    ing and found that the effect could be modelled withsets of uniform cycles.

    Verley & Lund (1995) proposed the empiricalequation

    (2)

    where a is the amplitude of horizontal movement, toassess the maximum penetration that can be achievedfor a given amplitude of motion and to assess the

    development of the penetration as a function of thework done by the pipe on the soil. The authors suggestthat the applicability should be limited to z/D of 0.3due to range investigated in their study.

    Lund (20