Offshore Szte Znvestzgaaon and Foundatzon Behavzour '98 O SUT 1998

GEOTECHNICAL INVESTIGATION FOR PERFORMANCE PREDICTION OF SUBMARINE TRENCHING PLOUGHS

P G ALLAN SEtech Ltd. Broomhaugh House Riding Mdl Northumberland, UK

ABSTRACT

An extensive network of undersea pipehnes and cables exists. For the majonty of these some form of trench is required for protecuon from fishng acuvihes, stabihty and/or thermal insulahon. The predichon of the performance of the tool used to form the trench is based on geotechmcal data obmned as part of the seabed invesugahon. However, the geotechnical parameters determined may not be of direct relevance to the fadure modes imposed on the sod by the trenching tool. This paper descnbes the typical data provided and Qscusses its applicahon to prediction of trenchng tool performance with parhcular emphasis on submanne ploughs. Modifications and improvements to existing investigahon techmques are also discussed.

INTRODUCTION

Trenchng and bunal of offshore pipelines and cables is often required for reasons rangng from protection from fishmg gear and anchors to providmg thermal insulahon and upheaval buckling resistance. To achieve the reqwed trench, a wide variety of submarine trenchmg tools are avdable. A number of factors are important in the assessment and selechon of the correct trenching equipment for the proposed work including a good understandmg of the geotechnical properties of the sods and rocks along the route.

The cable industry uthses cable whch can run for considerable distances on the seabed and has previously adopted an approach based on inveshgating the seabed soils in a qualitahve manner by use of a scaled down cable plough. T h ~ s is in contrast to the oil industry whch uses investigation techniques having then origin in onshore geotechnics. Typical techniques include cone penetrahon teshng (CPT) and samphng by means of a

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dnven tube. Such techniques can provide a detaded vertical stratigraphy, but over a limted area

Thls paper summanses geotechnical invesugauon techmques commonly used for subsea pipehne and cable routes. Some of the problems encountered in interpretation of data are bscussed, with suggeshons made for some lrnprovements in investigauon techmques, based largely on exisung equipment.

TYPES OF TRENCHING TOOL

Emphasis in thls paper is placed on submarine ploughs, whch are one of the most widely used trenchmg techniques. They have the particular advantage of being essentially a passive process producing a well defined, stable trench. A minimal number of moving parts are requrred maxmsing reliabhty. The plough is normally pulled by a ship or barge mounted winch. For pipehnes, a V shaped trench may be cut before @re lay) or after (post lay) placing the pipeline. Cable ploughs cut a narrow slot through the seabed and the cable is placed within the plough share, whch provides temporary support to the sides. Ploughs work most effectively in sands and clays giving good performance rates ulth rmnimal maintenance m e . Their capabhty in rocks is highly dependant on fracture spacing and strength. Whlle there is httle expenence of ploughing in rock, on most projects on which rock has been encountered, the plough has generally exceeded the expectations.

Other widely used techmques include jet tools, mounted on a sled, seabed tractor or a free sw~rnmng ROV. Jemng was the earhest technology used for f o m n g submmne trenches. Excavabon is by a hgh pressure jet of water erobng the soil at the face of the trench. Advanced jet tools incorporate inductors to remove the soil more effecbvely from the trench. Operahon is hmted to sands and clays and performance rates tend to be relabvely slow. For clay, a simple calculation based on undrained shear strength and jet water pressure can be used to determine whether the tool has sufficient power to excavate the clay. For sands a smple analysis is not possible, however it is known that both relauve density and permeabhty are sigmlicant with a dense mpermeable sand being hardest to jet. In pracbce most performance prdcbons are based on previous expenence in similar soil types.

Cumng tools, comprising cham and wheel cutters are also available. Both remove sod or rock from the trench by cumng with a form of tooth or pick, and then transpomg the debris out of the trench. Cham cutters are generally considered appropriate for sods and weak, fractured rocks, while wheel cutters are more appropnate to stronger rocks. Performance is limited by the power available to cut the sod or rock in stronger materials. In softer matenals the depth of cut can be increased until the volume of matenal being excavated exceeds the ability of the cutter to transport the spoil out of the trench.

Offshore Srte Investrgatron and Foundatron Behavrour '98 O SUT 1998

While the above tools are used to form trenches, there may also be a requuement to backfi a trench, usually for enhanced upheaval buckhng resistance and thermal insulation. This is normally only required for pipelines, and dedicated backfill ploughs have been budt to return spod to a trench. They are normally designed to work in conjunction with a trenchmg plough, but have been used to backfill trenches formed with a jet tool on occasions

LNVESTIGATION FOR OIL AND GAS PIPELINES

Early offshore installabons were typically large fixed structures imposing large loads over small areas. Borehole sampling and tesbng, complimented by CPT's, are ideal for such situations. With the development of subsea pipelines, the techniques used for fixed structures were adapted to suit pipehnes. Invesbgabon normally compnses samphng by vibroconng and in situ testing by CPT to shallow depth (normally between 3m and 6m). Such tech~llques have many advantages as they idenhfy the strata types and strengths over the depth range of the proposed trench and provide samples sufficiently large for most standard laboratory tesbng. However the typical spacing of lOO0m cannot give an accurate indicabon of all sod types which may be encountered along the route.

Both the in situ CPT and standard laboratory tests performed on samples were developed primarily for the purposes of stabc, vertically loaded foundations. While many of the properties deterrmned, may be used to assist performance predicbons of trenching tools and backfii, it is important to appreciate the effect that rate of sod failure and Qsturbance can have on the properties of the soil. Guidance on the selecbon of suitable tests has been given by the Offshore Site Invesbgation Forum (1996).

Where trenches are to be backfilled, the properbes of the soil, excavated and returned to the trench, may be completely different to those of the in situ soil. For example Bruton et a1 (1998) has suggested that a soft clay backfill may be considered as a fncbonal matenal at low stress levels. Useful guidance on the assessment of soil as backfill has been provided by Cathie et a1 (1998).

I

INVESTIGATION TECHNIQUES USED FOR CABLE ROUTES

Cables have been l ad on the sea floor for many years (the first transatlanbc cable was laid in 1854) and where orignally surface laid. Dunng the last twenty to thirty years, the value of bunal for protection from fishmg gear and small anchors has become widely appreciated and now cables are routinely buned to water depths between of 500m and 1000m.

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The geotechnical engneenng associated with a cable route is relahvely small compared to a pipeline and h s is often reflected in the scope of the geotechcal investigation. At worst a few grab samples or drop cores will be obtained whlle at best CPT's will be performed at regular intervals with widely spaced samphng and no laboratory teshng.

fistoncally one ophon for inveshgahon of a cable route has been a bunal assessment survey (BAS). The BAS tool is essentially a scaled down and simplified cable plough. As such it provides an assessment of the whole route, gving good indicahon of the tow force and other operahonal parameters such as sinkage and stability which may be of concern on a very soft seabed. As no convenhonal geotechnical parameters are obmned, all interpretahon must be based on scahng from the BAS tool with soil types tentatively interpreted from the tow tension trace. Noad (1993) has discussed in greater detad the use of BAS tools.

Use of BAS tools has declined in recent years, as they are normally deployed from a cable ship and relahvely expensive to use. As an alternahve the cable industry is increasingly using CPT's, penetrabng to a depth of 2m, achievable with a relatively hght, simple frame. Sampling may also be performed, but is often h t e d to drop cores, or pushed samphng, incorporated into the CPT frame. Such samphng techmques may be adequate in very soft soils, where 2m of core may be recovered, however in sands and s m clays penetrahon may be limited to a few centimetres.

A further replacement of the BAS tool is C-BASS, which has been developed by Cable and Wlreless recently (Lewis and McGinnis, 1997). C-BASS is a towed sledge like vehicle incorporabng both geotechnical and geophysical techniques to gve an assessment of the geology and ploughabihty along the whole route Geophysical testing comprises a resistivity array, gving an indication of the porosity of the seabed and a low frequency acoustic profiler intended give an indication of the grain size and strength of the sedment. Ground truthing IS provided by a mini cone penetrometer mounted on the frame. Th~s equipment has been commercially available for less than 2 years. At the time of wnhng plough data is becoming avadable and back analyses are expected to be performed.

USE AND INTERPRETATION OF GEOTECHNICAL PARAMETERS

The complex nature of the soil mechanics associated with rapid shearing of soils make mathematically ngorous performance predichons for trenclung tools difficult. As a result the basis of most performance prdchons is previous experience in sumla. soil types and empirical models (eg Reece and Grinsted, 1986) Attempts have been made to develop mathemabcal models (eg Palmer, 1998) however the complexity of the model and uncertainty associated with many of the parameters required, make such models difficult to use.

Offshore Site Investigation and Foundation Behavlour '98 O SUT 1998

Tow Force

Plough Speed Figure 1 : Plough speed/tow force relabonshps in vmous sol1 types.

Typical tow force speed relationships for sands and clays have been hscussed by Reece and Gnnsted (1986) and Allan (1997) (Figure 1). The tow force required is the sum of the following forces:-

1. Fncbonladhesion between the seabed and the plough, 2. Passive resistance of the soil over the cross secbonal area of the trench 3. A rate effect dependant on the speed of ploughing

In a soft clay the tow force may be preQcted relabvely easily. If no layer of sand is present, the adhesion may be estmated as:-

Total adhesion = a . s, . Ab Where a = adhesion factor

s, = undrained shear strength Ab = Bemng area of plough

Passive resistance may be calculated fiom classical soil mechanics. Alteinatively Palmer (1998) has suggested that the passive resistance will approxlmatelto:-

Total passive resistance = 5 . s, . A, Where A, = Trench cross secbon area

Offshore Srte Investrgatron and Foundatron Behavrour '98 O SUT 1998

Shear rate effects in very soft clays are not normally simcant when compared to other forces Shear rate effects in cone penetrahon teshng have been discussed, for example, by Meigh (1987). Such data may be extrapolated for the case of ploughs. For typical plouglung speeds, the increase in strength is typically less than 1.25 hmes measured undraned shear strength at standard teshng rates.

An example of a CPT plot and measured plough tow force is given as Figure 2. The geology compnsed a veneer of sand, overlying soft clay with an estunated undraned shear strength of 25kPa Thus tow force may be eshmated as follows and found to compare favourably with the measured tow force:-

Passive resistance - Trench area x s, x 5 - - 250kN Fnchon - Skids on sand, 450 x tan(35x213) = 195kN

Share on clay, 10 x s, x a - - 175kN Estunated tow force - - 62OkN

Note: Plough submerged weight = 90Te Beanng area of shares = 10m2

Figure 2 : CPT and plough performance data in soft clay.

Medlurn dense SAND

Soft CLAY wth occasional thm sand lenses

Plough Performance - 1 4m trench depth 600kN tow force 6 O m / m speed

Tow forces in soft clay are usually low and therefore not a prime concern. However very low forces, may present thelr own problems, for example surgng on the plough due to stored energy in the tow line catenary, partrcularly in deep water. More significant may be the need to accurately determine the undrained shear strength in very soft clays. Most ploughs able to operate on seabed clays with strengths greater than

OfJshore Site lnvestigatzon and Foundation Behavlour '98 O SUT 1998

between 3kPa and 10kPa. If the seabed is unable to support the plough, damage to a pipehne, or overbunal of a cable may occur.

Slmilar relahonshps are also suitable for fm and stiff clays However particular attenhon should be pad to the adhesiodfnchon. Eshmates based on undraned shear strength are hkely to be upperbound and an estimate based on fnchon is hkely to be more accurate. High quality logging of samples is highly desirable to give an indicahon of the macrofabnc of the clay as any fissures which may be present can sigmficantly reduce the required trenching force

The vanous components associated with predichon of trenchng forces in sands may be estimated from the fnchon between the plough and the seabed, the passive resistance of the plough and the speed effect. Reece and Gmsted (1986) have suggested the following re1ahonshp:-

Tow force = K1 W + K2. z3 + K3 . z3 . v . (AVIk)

Where K1 = Coefficient of friction W = Submerged weight of plough K2 = Coefficient relahng to passive resistance z = Depth of trench K3 = Coefficient related to geometry of plough v = Speed AVIk = Volumetric dllation/permeabllityty

Fnchon between sand and steel may be estimated from pile formula. These range from 2/3.tan0 to tan(+5). The result is not particularly sensitive to changing 0 with frichon coefficients in the range 0.4 to 0.5 calculated for typical 0 values. S d a r l y the passive resistance may be calculated for a range of 0 values and found to be relahvely small.

Whlle the two stahc components of the tow force may be approximated relatively easily, the most sigmficant is the dynarmc component. l k s is associated with the dilahon (volume increase) which occurs in any granular matenal dense of it's cntical state dunng shearing. As the volume increases, the void space also increases. At slow rates of shearing, the water can flow through the sand mass and occupy the increased void space. However at hgher speeds water may not be able to flow in sufficiently quickly and pore 'suchons' (or pore pressure below hydrostahc head) are generated. At high speeds there is potenhal for cavitation to occur and the head of water above the sand surface will effechvely act as a surcharge sigmficantly increasing the effective stress in the sand. The contribution of the vanous components (friction, passive resistance and effect of cavitation in water depths of 10m and 20rn) are shown as Figure 3 for a typical pipehne plough trenchmg at 1.4m depth. The friction and passive resistance are relatively small in comparison to the potenhal effect of cavitation. Clearly in deeper

Offshore Site Investigation and Foundation Behaviour '98 O SUT 1998

water, tow forces of the magnitude imphed cannot be achieved and water must flow into the sod mass to at least partially relieve pore suctions.

20m

Passive component \ Fnctl-

3 0 3 2 34 3 6 3 8 40 Angle of friction (")

Figure 3 : Tow force components for a typical pipeline plough trenching sand at hgh speed in water depths of lorn and 20m.

The mass permeabihty of the sod governs the rate at whch water can flow into the sod mass whde the volumetnc dilation determnes the total volume required. Unfortunately neither of these values are determined in a typical geotechnical investigabon. An approxunation of dllation may be made based on relative density, determined by CPT, however, permeabhty, which may vary be several orders of magnitude within granular soils cannot be esbmated other than very approximately from particle size dlstnbubon curves. For h s reason, empirical correlations associated with the speed component are normally based on the paxhcle size &stribubon of the sod

The sigmficance of the soil permeabihty may be demonstrated by experience on a recent project. Two CPT's were performed adjacent to the pipeline route, approximately lkrn apart and are reproduced as Figure 4. CPT A encountered a very dense sand, whde CPT B encountered a loose silty sand. Confirmation of the sllty nature of the sand at CPT B was confirmed by a particle size dlstnbution analysis from an adjacent vibrocore.

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Analysis ignonng dilahon and permeabhty would indicate sand at CPT A being sigruficantly harder to plough. In practice slmilar tow tensions were required to acheve slrmlar speeds. The low permeability of the loose sdty sand at CPT B offset the reduced volumetric dilation on sheanng and lower passive resistance Reliable predichon of tow forces in intermedate sods can only be acheved when samples are obtamed and subjected to appropnate laboratory tests.

Figure 4 : Comparison of CPT and Plough Data in Sand

Loose sllty SAND, becomng very dense with depth (PSD 100% fine sand)

Plough Performance - 1 4m trench depth 1800kN tow force 1 Om/m speed

CPT B

Soft sandy CLAY

Loose sllty SAND (PSD 10% clay

17% sllt 73% sand)

Plough Performance - 1.3m 'trench depth 1800kN tow force 1 2 m l m speed

Offshore Site Investigation and Foundation Behaviour '98 O SUT 1998

In sands, loose of cnbcal state, no speed effect is observed as no dilahon occurs and hence no pore sucbons are developed However in practice such sands are rare and it is normal for some &labon to occur dunng ploughing.

RECOMMENDATIONS FOR GEOTECHNICAL INVESTIGATION

The above &scussion has idenhfied the man parameters which are desirable for the predicbon of plough performance. The suitability of standard techniques to detemne relevant sod parameters more accurately is descnbed in Table 1. Some of the hmtahons in problemahc soils are discussed below with suggesbons for mprovement.

Suitabhty of tests A : Good, B : Intermdate C : Poor

Sampling and standard

laboratory tesbng

Notes 1) F m and stiff clays 2) Dependant on specification 3) Dependant on tests performed

Table 1 : Suitability of vanous tests methods for detemnahon of soil properties for estlrnabon of plough performance.

Undrained shear strength

SoiVsteel fncbon Soil as backfill

Soft Clays

The accurate measurement of the strength of very soft clays (

* > - .

Offshore Szte Investzgation and Foun$atzon Behavzour '98 0 SUT 1998

Offshore Szte Znvestrgatron and Foundatzon Behavrour '98 O SUT 1998

known to the author of ploughmg rates of 5Omlhr being acheved in soil whch would appear to be looselvery soft The plough may be noted to pitch aft in a manner consistent with low beanng capacity of the underlying sod, confirming its looselsoft nature. The consequences of h s in terms of shp programme and cost can be significant. Sampling and laboratory testing, including parhcle size analysis is helpful but cannot provide the complete answer.

It is probable that with mn i cones (being increasingly used for subsea invesbgations) the dranage path is shortened and finer grained soils are hkely to behave in a h n e d manner. This may negate some expenence obtaned with standard size cones. An obvious method for assessing the speed effect is by varylng the speed of the cone to assist deterrmnabon of the dilatancylspeed effect. Thls was done by Grinsted (1985) as part of research into sheanng of submerged sands. Results are shown in Figure 5 for a very silty fine sand (permeabhty = 4 x 10-~m/sec), known to be relahvely hard to plough when submerged. A measurable change in cone resistance was recorded in the silty fine sand, however a large change in speed was used and the results are wthin the normal vanabon whch might be expected w i h n a single geological unit. S d a r tests were also performed in a medlum sand (permeability (4 x 10-~m/sec). The recorded vanation in cone resistance was within the scatter of the data. It is concluded that thls is consistent wth full h n a g e occumng dunng the test. Unfortunately a piezocone was not used for these tests, further work with a piezocone could be beneficial.

Cone Resistance, qc (MPa) 0 1 2 3 4 5

Figure 5 : Effect of varylng cone penetrabon speed in a sdty fine sand (permeability = 4 x 10-~m/sec)

1 ' - >

Osshore Slte Investzgatzon and Foundaaon Behavzour '98 O SUT 1998 r- r

In prachce, varyng speed would require a mnimum of two profiles at each test location and confidence that both tests were being performed in slrmlar soil. While further inveshgahon is required, it is probable that the inherent varia~on in test results would not sigmficantly Improve plough performance prehctions.

CONCLUSIONS

A large number of geotechnical investgations are performed for subsea cable and pipehne routes. These are often done to a standarhsed 'formula' which may not give the informahon desired. There is scope for adaptmg standard techniques to improve the data obtaned, and in particular for very soft clays and fine sllty sands.

ACKNOWLEDGEMENTS

The author wishes to thank DSND Oceantech Ltd and PGS Offshore Technology Ltd for permssion to publish plough and soil investigation data Thanks are also extended to Chns h m a x for proof reading the manuscript and Tim Gnnsted for permission to pubhsh data from hls PhD thesis.

REFERENCES

Allan, P.G. (1997) Ploughmg forward, Ground Engineering, 29, August 1997, 26-27.

Bolton, M.D. (1986) The strength and dllatancy of sands. Gbotechnique, 36, No. 1,65- 78.

Bruton, D.A.S., Bolton, M.D. and Nicolson, C.T. (1998) Posiedon Project - Pipehne design for weak clay. 21St Annual Offshore Pipeline Technology Conference, Oslo 1998.

Bugno, W.T. and McNeilan, T.W (1984) Cone penetration test results in offshore Cahfornia silts. Strength testing of manne sediments: Laboratory and in situ measurements. ASTM special techcal pubhcation no. 883,55-71.

Catlue, D., Banas, S. and M a c h , J. (1998) Backffing pipelines : State of the art. 21" Annual Offshore Pipehne Technology Conference, Oslo 1998.

Lee, I.K., m t e , W. and Ingles, O.G. (1983) Geotechnical Engineering, Pitman (pg 154).

Offshore Srte Znvestrgatron and Foundatron Behavrour '98 O SUT 1998

Lewis, S. and McGinnis, T (1997) C-BASS: A cable bunal assessment survey system Sub-Optic '97, Los Angeles.

Lunne, T., Powell, J. and Robertson, P. (1996) Use of piezocone tests in non-textbook matenals. Int. Conf. on advances in site inveshgat~on practice, Institution of Civil Engineers, 438 - 45 1.

Machin, J.B. (1998) Pipeline Foundahon Considerations Society for Underwater Technology, Offshore Site Investigahon And Foundahon Behaviour "New Frontiers".

Meigh, A.C. (1987) Cone Penetration Teshng Methods and Interpretetahon. Construction Industry Reseach Association and Butterworths

Noad, J. (1993) Successful Cable Burial - Its dependence on the correct use of plough assessment and geophysical surveys. SUT Conf. Offshore Site Investigahon and Foundahon Behaviour, 39 - 56.

Offshore Site Inveshgation Forum, Pipehnes Worlung Group (1996) Gmdance notes on geotechnical invesbgahon for mmne pipehnes

Palmer, A. (1998) Speed effects in cumng and plouglung. Submtted to Gkotechnrque.

Reece, A.R. and Gnnsted, T.W. (1986) Sol1 Mechmcs of Submarine Ploughs. lgth Annual Offshore Technology Conference, Houston, Texas, 453 - 46 1.

Stewart, D.P. and Randolph, M.F. (1994) T-Bar penetration teshng in soft clay. ASCE Journal of Geotechnrcal Engineering, 120, No. 12,2230 - 2235.