Installation and Instrumentation of Offshore Piles

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FUGRO B.V. CONSULTING GEOTECHNICAL ENGINEERS

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The Netherlands,P.O. Box 63, 2260 AB Leidschendam.

Phone: (0)70 - 209250. Telex: 31010.

FUGRO B.V.is affiliated with FCI (Fugro Consultants International) B.V., The Netherlands,

with offices throughout the world.

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INSTALLATION AND INSTRUMENTATION OF OFFSHORE PILES

by

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FUGRO B.V. CONSULTINGGEOTECHNICAL ENGINEERS

P.O.BOX 63,2260 AB LEIDSCHENDAM· THE NETHERLANDS.PHONE070-209250-TELEX 31010

~GAD ~

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F.L. Beringen

(Managing Director Fugro B.V.)

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Post-Academical Course ~Civil-Technical

Offshore Technology", Delft, 1984

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CON TEN T S 1.0 INTRODUCTION 2.0 DRIVABILITY ANALYSES

2.1 Wave Equation for Driving Problems

2.2 Determination of Soil Resistance During Driving (SRD)

2.3 Selection Optimum Pile-Hammer Combination

3.0 PILE AND HAMMER INSTRUMENTATION

3.1 Hammer Monitoring

3.2 Pile Monitoring

3.2.1 Above Water Monitoring

3.2.2 Underwater Montoring

3.3 Data Processing and Analysis

3.4 Final Remarks

4.0 LIST OF REFERENCES

Appendix A Illustrations

Appendix B Dynamic Pile Testing; an aid in analyzing driving

behaviour (ref. 3)

Appendix C Dynamic Pile Testing; an advance in offshore

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INSTALLATION AND INSTRUMENTATION OF OFFSHORE PILES

F.L. Beringen

Leidschendam, December 11, 1984

1.0 INTRODUCTION

Typically offshore piles consist of open ended steel pipes (see fig. 1) which are instalIed by driving procedures using large steam and/or hydraulic hammers. Often drivability analyses are performed beforehand to establish the optimum combination of pile and hammer for given soil conditions and required pile

penetrations.

To further monitor and guide actual pile installations, pile and hammer instrumentations are increasingly employed.

In this paper drivability analyses and instrumentation methods are briefly explained, as weIl as the interpretation of

instrumentation data.

For sake of completeness some of the referenced publications are appended enabling further analyses.

2.0 DRIVABILITY ANALYSES

Until some years ago the installation phase was practically

disregarded in the design stage. Consequently, problems were often

encountered during installation, usually a result of piles meeting refusal prior to reaching target penetration. With the

introduction of pile driving programs such as the wave equation

(ref. 1,2), it became possible to analyse the most significant

parameters in the driving process. It soon appeared that in particular the stiffness of the pile plays an important role.

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Nowadays this is reflected in most piled platform designs with the

l~sult that pile diameters and wall thicknesses have increased significantly. Until some years ago 1.22 x 0.025 (48 x 1 in.) piles featured in several North Sea designs. At present wall thicknesses smaller than 0.051 m (2 in.) have practically

disappeared, only due to drivability criteria. In that respect it is remarkable that nowadays even in the hardest North Sea soil conditions piles are and can be installed by driving only, while up to some years ago piles encountered often premature refusal requiring auxiliary measures such as drilling out of plugs, jetting, redriving, insert piling, etc., causing tremendous delays.

This section describes briefly the most important tools in the drivability analysis. Methods are.indicated to predict drivability behaviour, while some typical design criteria are qiven, allowing proper selection of pile and hammer sizes required.

An extensive description of the wave equation as a pile driving aid can be found in ref. 1 and 2. Hereunder only some basic aspects of the method are discussed.

In the most common wave equation computer programs the pile, soil and hammer system is modeled as a series of masses, springs and dashpots (fig. 2). Often the ram is considered as one infinitely stiff element. However, it is possible to divide the ram also into segments, thus introducing stiffness properties for the hammer. Due to the impact of the ram a force wave starts travelling through the pile at high speed (about 5,000 mis). The wave equation computer program calculates for all elements in the

system per time increment velocities, displacements and forces as generated by the impact. Computations are continued until the permanent set of the pile tip is achieved, which provides the expected blowcount for a certain combination of soil resistance,

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pile and hammer. The time interva~ selected should be such that

the traveling distance of the force wave per time step is

significantly less than the length of the segments of the system.

Hence for typical segment lengths of 1.5 m a time step is

recommended of about 0.55~o6·5

=

0.00015 sec. or 0.15 ms.

The soil model in the computer program is elasto-plastici moreover damping factors are introduced to compute inst~nteneous values of the soil resistance during driving:

(SRD)i

=

SRD (1 + JV)

where

(SRD)i

=

instantaneous soil resistance during driving

SRD

=

input soil resistance during driving (see 2.2)

J

=

damping factor of soil

V

=

velocity of pile segment relative to the soil

Significant input parameters are listed in fig. 3. Typical values

for elastic soil deformation (quake) and soil damping factors are included, as weIl as a summary of the hammers presently most often used in the North Sea.

The analysis results in curves of expected blowcount versus soil resistance during driving for given pile and hammer combinations.

Some typical curves for a North Sea soil profile and 1.52 (60 in.)

piles using the heaviest hammer presently available are shown in fig. 4a.

~~~

__Q~~~E~!~~~!~~_~f_§~!!_~~~!~~~~~~_~~E!~~_QE!~!~9_1§~Ql

Routinely, soil investigations provide sufficient data to produce ultimate bearing capacities for any pile type. In particular when

cone penetration tests are included arealistic capacity can be

determined. For several years those static capacity results have been used as reference data in drivability analyses. Hence all

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back-figured SRD's from observed blowcounts were related to the statie bearing capacities yielding an empirical relation figure that could be used for predictionsof SRD's at new locations. Plug measurements immediately after or even during driving mostly

indicated that fixed plugs during driving did not occur. Accordingly it was feIt justified to assume the pile to be unplugged during driving which resulted in the folowing design method to predict SRD's:

where

SRD

=

soil resistance during driving FDi

=

inner friction during driving FDo

=

outer friction during driving WD = wall end bearing during driving

€

=

empirical relation figure correlating hindcasted FD's to computed statie outer skin friction

Ap

=

cross sectional area of pile point (annulus) qc

=

cone resistance below pile tip.

A typical average € versus depth result is given in fig. 4b. This figure was obtained combining driving results from several North Sea locations, generally including overconsolidated clays and sands. As only an average figure is given, one needs to estimate as yet a possible upper and lower bound. Such a range is taken often in the order of ± 10%, also being based on back-figured data.

An alternative procedure to compute SRD's is to use directly the soil strength data from soil investigations. As driving disturbs the soil, it is acceptable to consider in such a procedure

remolded friction values in sands and clays, while for pile tip resistances the undisturbed values still apply. Such analysis provides directly the values for FDi and FDo of above given equation.

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For the profile and 1.52 m (60 in.) pile considered in this paper, the computed SRD's are presented in fig. 4c, using the average E-value and an estimated remoulded friction profile. For this example the former method provides the lower boundary, the latter the upper boundary. This is merely due to the assumptions. Many cases are known where the reversed situation occurs.

If piIe and hammer sizes are known, above described analysis can be completed, resulting in expected blowcounts versus depth. Fig. 4d presents such results for the Menck 12500 and a 1.52 x 0.051 m

(60 x 2 in.) pile. For an assumed final penetration of 40 m, the range of final blowcounts is 50 to 120 bI/ft.

In the earlier stages of the platform design, pile dimensions are often not yet known. At most some preferred pile diameters are available. In that case drivability studies require flexibility to finally establish the pile dimensions in a stage as late as

possible.

A typical set of curves of such a flexible driving analysis is shown in fig. Sa and b, giving SRD versus pile wall thickness and rated hammer energy for certain blowcount levels.

Accepting a blowcount limit of for instanee 150 bI/ft, these results can even be combined in one plate (fig. Sc), enabling an optimum pile-hammer selection.

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3.0 PILE AND HAMMER INSTRUMENTATION

From preceding chapters and earlier lectures during this course, it will be clear that quite some empiricism is involved, both with pile design methods and with pile installation analyses. Bearing in mind the large offshore scales in dimensions and loads methods of direct calibration (e.g. static load tests) are practically

impossible.

Consequently methods have been developed to calibrate design and driving criteria by means of "indirect" methods, such as

instrumentation.

By measuring stresses and displacements or accelerations during driving very valuable information is gathered on the following

topics:

1) Accurate figure on hammer performance or driving energy delivered by means of hammer înstrumentation.

2) Accurate figure of the driving energy entering into the pile by means of pile instrumentation (enthru energy).

3) Estimate of dynamic soil resistance experienced during driving, and eventually a further estimate of the static soil resistance during driving.

The last topic is of prime importance, but is also the most

difficult one to establish. In fact this problem is hardly solved

yeti however, experience gathered so far is promising and certainly in a global sense, pretty strong interpretative

statements can be made from instrumentation data today in terms of static soil resistances, provided geotechnical expertise is

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~~l__

~~~~~~_~~~!~~~!~g

Hammer monitoring is an objective way of evaluating the driving performance. Monitoring is especially importftntif premature

refusal is anticipated. Should premature refusal occur, then it is essential to know whether this is caused by soil resistance or by malfunctioning of the hammer.

When steamhammers are applicable, the dropheight and/or impact velocity can be measured by one or more small magnets mounted onto the control bar of the hammer (Plate 6). A hall sensor detects the magnet(s) when passing, which allows assessment of the required parameter(s). The hall sensor is provided with a short length of cable with connector. A cable with the other half of the connector ends at the data acsuisition unit. If a drive has to be monitored, asingle connection has thus to be made only.

In case of hydraulic hammers, which have become very popular

because of underwater pile driving, alternative systems have been developed.

In cooperation with Bomag-Menck GmbH, manufacturer of the most commonly used hydraulic hammer, the MHU-1700, Fugro have developed a monitoring system (fig. 7).

Inside the MHU-1700 there is provision to measure the oil pressure during driving, the dropheight and the impact velocity of the

ram. If monitoring is requested sensors are built in/connected and the signals are transmitted using cores of the hammer's umbilical.

The signals are fed into a system that has an oil pressure meter as weIl as digital displays, for dropheight, impact velocity and blownumber.

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~~~_

_

~~!~_~2~~~2~~~9

3.2.1 Above water monitoring

Above water monitoring has become a widely accepted and standard

technique in the last five to ten years. Tt is most used in areas

with no/relatively little "piling-history" or areas where

optimisation of pile-design still seems feasable, because a lot of

experience with above water offshore piling exists already. The

DPT-system as employed by Fugro consists of a series of components (Plate 8 thru 12):

- Two re-usable strain transducers, positioned on opposite sides of the pile to account for the effects of possible eccentric driving. The transducers are mounted to the pile using bolts.

- Two re-usable piezo electric accelerometers, mounted next to the

strain transducers. These transducers consist of a measuring

element and a mounting block that has to be bolted t~ the pile.

- A signal conditioning unit, specifically designed for these

tests to provide the power supply for the transducers. Moreover

the signals fed into this system are amplified, multiplied by calibration factors and averaged. Tntegration of the

accelerations is also possible.

- A multitrack FM-magnetic tape recorder to store the data gathered during pile monitoring. On site raw field data are

recorded during driving; blows that have to be analysed are

selected afterwards.

- A digital storage oscilloscope to monitor functioning of the system and data gathered during driving. During the analyses it permits convers ion of analogue signals to a digital form

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- A data acquisition system. The specifications of this system depend on the clients requirements and may vary from desk top mini computer (HP85) to PDP11 computer system including floppy disc/magnetic tape drive, graphic terminal with printer and plotter.

The above equipment needs to be placed in a dry area, preferably in the sight of pile driving, with a single phase 220/240V A.C. power supply and at least three square metres desk space. It is essential that no frequencies other than 50 or 60 Herz are

supplied, in order to prevent signal distortion. In (sub)tropic areas the measuring cabin has to be air-conditioned.

3.2.2 Underwater monitoring

When underwater pile driving systems are used, pile and/or at least hammer monitoring are considered to be essential, as a direct visual inspection of the driving process is simply

impossible.

The underwater system components - the underwater transducers, a means to transmit the signals and a signal conditioning unit above water - have all been developed within Fugro (Plates 13 thru 19). The novel components in this unique development consist of:

- strain- and acceleration transducers mounted in watertight housings, guaranteed to resist water pressures of a least 30 bars.

- a junction box, being an amplification unit that has to be

mounted in between the two sets of transducers. The unit accepts all signals at the input side and powers and amplifies them such that cross-talk and electro-magnetic noises do not interfere with the signals. To the output side one multicore cable is

connected. This cable can be short and provided with a connector that can be mated underwater by a diver. Alternatively it can be made long, without a connector, to be connected above water.

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Dependent on the operators and pile installation contractors requirements the signal transmission system and eventual back-up systems can be defined. Diving facilities and installation

procedures govern the selection. The short length of cable has to be connected to a longer cable provided with the other half of the connector. This longer cable can either be run from the

installation barge by diver/bell or be pre-installed along the jacket leq(s). The latter minimizes on site delays in the

installation but requires some preparation in the yard during jacket-fabrication.

Wireless signal transmission seems ideal, especially for

under-water monitoring. However, so far (December 1984) no system has been designed with sufficient resolution and bandwidth for this type of measurement. If possible the underwater system has to be pre-installed because otherwise it would take divers a couple of hours to do it. Pre-installation in almost any case means that the system has to be mounted to the inner piIe wall because of

tolerances between pile guide and pile and the risk of damage. All

components are bolted to special mounting facilities. Except for the signal amplification and power supply, the data

recording/acquisition is the same as for above water monitoring.

The tapes containing all field data are played back at the begin-ning of the analysis to have a close look at every series of

blows. Representative blows (Plate 20) are then selected.

The scope of the analysis typically requires:

decisions whether changes in blowcount are due to changes in

soil resistance or hammer performance (Plate 21).

- assessment of soil resistances and/or dynamic soil parameters.

- analysis of the redrive data to increase the confidence in the

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Dependent on the clients requirements any of the following

techniques can be adopted: - DPT- or impedance methode

The Dynamic Pile Test-method determines the soil resistance during driving from strain and velocity at pile head using an

impedance technique (ref. 3). The strain is measured directly,

the velocity is obtained through integration of the measured acceleration-signals.

The DPT-method allows a division of the total resistance into the skin friction and the toe resistance. The method gives trends rather than quantitative results, especially for the

Lor.qer offshore piles. It is often used on site for quick

reference because it is a relatively simple technique - Wave equation back-analysis method (see chapter 2.0).

The wave equation is a finite difference method that computes, amongst others, the set of the pile toe per blow as a result of a driving force that overcomes the soil resistance.

A hammer model or, even better, measured strain-time curves can be used to provide the driving force of the system. The latter

type of input from DPT's is preferred because problems

associated with modelling the driving system and selecting the proper hammer parameters are then by-passed. Having eliminated one set of assumptions this way, actual and computed blowcounts can be matched by varying the components of the soil

resistances. The dynamic soil parameters are varied only to assess the sensitivity of the solution to eventual changes of these parameters.

To be able to cover a wider range of possibilities, the observed and computed strain-time curves can be matched as weIl. This will yield the stiffness of cap and cushion and the coefficient of restitution that applies. The driving performance at differ-ent hammer-energy levels can then be computed more accurately.

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This method permits refinements of wave equation-type analyses to increase the level of confidence in the result. The technique can be applied on site using a mini comput~r.

- CAPWAP-method.

A more sophisticated analysis of DPT-results is possible with a computer programme called CAPWAP (Case Pile Wave Analysis Pro-gramme). This programme was developed in the seventies by G.G. Goble and F. Rausche (ref. 5).

The CAPWAP-technique uses the pile-soil model as proposed by E.A.L. Smith, i.e. a system of lumped masses, springs and linear dashpots to simulate soil and pile response during driving. Un-like standard wave equation programrnes,CAPWAP uses measurements at pile head as driving force for the pile instead of a model of masses and springs to simulate the hammer and cap system.

An analysis is started by selection of pile- and soil parameters and an estimate of the soil resistance. Using a velocity versus time record (obtained through integration of measured acceler-ation versus time) as boundary condition, a wave equacceler-ation run is performed resulting in a computed force versus time at piIe head and a computed set per blow. Both data are compared with the ob-served data and if they do not coïncide, parameters andjor re-sistances are changed until acceptable agreement is obtained

(Plate 23).

The difference with the wave equation back-analysis method is that all input parameters and data can be varied, so that both dynamic soil parameters and distribution of soil resistance are optimised. If required, this analysis can be done on site. Like any other analysis technique this procedure requires

geotechnical judgement to decide whether a certain set of results is sensible or not.

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If only a limited number of piles is to be monitored, corre-lations between hammer monitoring results and enthru energies computed from DPT-data are very useful. The performance of the uninstrumented piles can be evaluated if hammer monitoring data and this correlation are available.

3.4 Final Remarks

For sake of completeness and for those further interested, two papers have been appended. One paper giving more theoretical backgrounds on dynamic pile measurementsi a second paper

summarising some recent instrumentation projects, sucessfully completed throughout the offshore world.

These papers once more emphasise, that although quite a distance has to be gone yet, instrumentation as such has already proven its power and seems one of the strong ~nstruments for the coming years to further develop and interprete understanding on offshore

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4.0 LIST OF REFERENCES

1. Smith, E.A.L.: "pile Driving Analysis by the Wave Equation",

Transactions ASCE, Vol. 127, Part I, 1962.

2. Lowery, L.L. et al: "Pile Driving Analysis State-of-the-Art", Texas A&M University, 1969.

3. Beringen, F.L., Hooydonk, W.R. van, and Schaap, L.H.J. (1981):

"Dynamic Pile Testing: An Aid in Analysing Driving Behaviour",

in Bredenberg, H., editor, Application of Stress-Wave Theory on Piles, International Seminar on the Application of Stress-Wave Theory on Piles, Stockholm. Rotterdam, A.A. Balkema, pp. 77-97.

4. Krause, E.R., Pluimgraaff, D.J.M.H., Richards, A.F. (1984): "Dynamic Pile Testing: An Advance in Offshore Installation Practice", ECOR International Conference and First Argentine Ocean Engineering Congress, Vol. I, Buenos Aires.

5. Rausche, F., Goble, G.G., and Moses, F. (1971): nA new Testing Procedure for Axial Pile Strength" , Offshore Technology

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APPENDIX A

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~GAD

~---~---~

OPEN ENDED PIPE PllE ~ IF sAs ~ .,_:...~_;_~--- SOll PLUG ---~.;+-+

1

DRAAGVERMOGEN OPEN BUISPAAL ( Op) :

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ACTUAL

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(----',

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l'__J" STROKE ... ;.:.~.:.:.:.:-:.: ~~- PILE .. ',

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W=MASS K =SPRING R=SOIL RESISTANCE K2

lF'

_{RST PILE SEGMENT} K3 R4 K4 R5 K5 R6 K6 R7 K7 Ra Ka R9 K9 R10 R11 R12 R12 AS REPRESENTED

HEIPAAL IN DE WAVE EQUATION

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PILE uATA CHASER LENGTH PILE LENGTH

TYPE OF CONNECTORS

PILE AND CHASER DIMENSIONS PILE PENETRATION

~ 3~

HAMMER DATA: WEIGHT OF RAM RATED ENERGY EFFICIENCY WEIGHT OF CAP CAP STIFFNESS

RESTITUTION COEFFICIENT

SOIL DATA· POINT QUAKE AND SIDE QUAKE POINT DAMPING AND SIDE DAMPING SKIN FRICTION

POINT RESISTANCE

ALONG SIDE OF PILE BELOW PILE POINT

SOIL TYPE QUAKE DAMPING QUAKE DAMPING

(mm) (slm) (mm) (slm)

SAND 2.54 0.164 2.54 0.492

CLAY 2.54 0.656 2.54 0.033

TYPE OF HAMMER RAM WEIGHT_(MN) RATED ENERGY(MNm)

MENCK 2500 0.25 0.31 3000 0.30 0.45 7000 0.70 0.87 8000 0.80 1.20 12500 1.25 2.18 MHU 900 0.50 0.90 1700 0.94 1.70 VULCAN 060 0.27 0.24 560 0.27 0.41 5100 0.43 0.68 HBM 3000A 0.69 1.10 4000 0.93 1.60 S 800 0.40 0.80 S 1600 0.80 1.60

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50 _...,

-»: /' /

/

EMPIRICAl CORRElATION COEFFICIENT E~Fdyn/Fst.t

z ! 40 e Z s ~ 35 I-__:"_-/ ... o '" :I 30 j: l-C '" ~ 25 C IÄ ili 20 a: ... "' ö 15 45 10

-o 0.2 0.4 0.6 0.8 1.01.2 1·.4 1.6 1.8 2.0 2.2

__

.--- o~~-+~--~~~--+-~~--~~-5 10 5 15 J\'2.)fa~ HAMMER:MENCK 12500 PENETRATION:37.5m - - - 1.52 x 0.064 m PllES 1.52 x 0.051mPllES - •- 1.52 x 0.038 m PllES 20 25 !30

t

35 ~ 40 ~ 45 50 (bI 55 60

TYPICAl AVERAGE RElATION DYNAMIC TD STATIC SKIN FRICTION FROM HINOCASTEDDRIVING DATA

o o 20 40 60 80 100 120 140 160 180 200220 240 260 280300 BLOW COUNT (bl/ftl -30 40 50 70 ANTICIPATED BlOWCOUNT(bl/ftl -20 40 60 80 lo.l 120 140 160 180 200 220 240 260 280 300 o 20

SOll RESISTANCEAT TIME OFDRIVING(MNI

-o

Ê 10

70

UNDER 80UND FROM EMPIRICAl CORRElATION COEFFICIENT 10 60 Ê 10 UPPERBOUND FROM REMOlDED FRICTIONMETHOD

(cl '" ... ;;: (dl 70 HAMMER:MENCK 12500 PllE; 1.52 •0.051m 80

KARAKTERISTIEK VOORBEELD HEI ANALYSE

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z

! Cl50 Z

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i0

...

0 lil ::E

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j:~40 lil u Z c(

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Cl! in lil a:: .J, Ö

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Cl! 30

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50 (b] Z ::E Cl Z 40 > a:: o

...

o 150 aL/FT lil ::E j: 200 aL/FT 100 aL/FT ~ ~ 30 Z c( I-Cl! in lil 50 aL/FT a:: .J, Ö Cl! 20 (,...

.t

g

_...

~ u Z lil ::E 10 0.8 1.0 PILE: 1.52x0.051m 0 0 lil N ~ ~ u Z lil ::E 2.0 2.2 (al 1.52 m O.O. 1.2 1.4 1.6 1.8 HAMMER ENERGY (MNml 38 (1.6"1 51 (2.0"1 64 mm (2.5"1 WALL THICKNESS (mml z ! Cl 60 Z

s

a:: 0 ₅₀

...

0 lil ::E 40 j: l-c( 30 lil U Z c( I- ₂₀ ti) in lil a:: .J, ₁₀ Ö Cl! 0 0 0.5 1.0 1.5 2.0 HAMMER ENERGY (MNI PILES: 1.52 x 0.064 m 1.52 x 0.051 m 1.52 x 0.038 m

SELECTIE OPTIMUM PAAL - HAMER - SRD COMBINATIE

5

200 aL/F 150 aL/F 100 aL/F 50 aL/FT

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] alternative1 I sensors rol bar Magnet

:J

alternative 2 Hall sensors

STEAM HAMMER MONITORING SYSTEM

OFFSHORE PI LE AND HAMMER MONITORING

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1

.

-

:

~i~PIY--+--,::::r Oil pressure transducer

.-Hammer control room 5 signals t t t t t I I I I I Fugro Hammer monitoring equipment

r

I t t t t t t

Pile driving Magnetic tape recorder recorder t t t HP 85 I- Oscilloscope Micro computer ~""'öiii.i;;;~_Electrical umbilical Extension ----1--pipe Stroke switches Steering unit --+-+ ""...___Velocity switches Measuring rod Anvil

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.

Pile 7

MHU 1700 MONITORING SYSTEM (STANDARD)

OFFSHORE PILE AND HAMMER MONITORING t

UV recorder

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Strains and __. SignaI

Accelerations P" _{conditioning unit}

"'I~ .Magnetic tape "'Ir recorder ""

,.

Digitalstorage oscilloscope ""

,.

_""~

~

Floppy disc drive ....

Minc- HP-85

and/ or

computer computer

magnetic tape drive ...

,

"",.

Graphic terminal

with printer /plotter 8

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DATA RECORDFOR PILE MONING AND ACQUITORINGISITION SYSTEM

OFFSHORE PILE AND HAMMER MONITORING

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2

1) POP 11/23 minicomputer (mine)

2) Floppy disc drive

3) Printer/plotter

4) Video display

5) Fugro conditioner

6) Magnetic tape recorder

7) Transient oscilloscope

DATA RECORDING/ACQUISITION EQUIPMENT

OFFSHORE PILE AND HAMMER MONITORING

~GAD

~

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ABOVE WATER PILE MONITORING INSTRUMENTATION

OFFSHORE PILE AND HAMMER MONITORING

~GRD ~ 10 1 Protection bar 2 Accelerometer 3 Strain transducer 4 Connectors

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Strain transducer

A

/

_/ Mounting

A

:

Protection bars A

INSTALLATION OF ABOVE WATER PILE MONITORING INSTRUMENTATION

OFFSHORE PILE AND HAMMER MONITORING

~GAa ~

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12

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.Offshore Hammer Gage with cushion

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Transducers

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TRANSDUCER LOCATIONS BELOW HAMMER r.AGE

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Fugro-underwater strain

-and acceleration transducers

~ ~ ~1)

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Fugro - underwater junctionbox

( amplification unit)

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~ -r ~1)

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Short length of underwater

cable with connector

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~ -r ~1) ~ ~ ~2) 4)3) 3)

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Pre-installedcablewith connector

Long underwater cable with connector

( optional )

~ ~ __, 5)

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Data recording-acquisition unit

on board installation -barge

~ ~ 5)

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Î

1) Pre-instalied in piles

2) Pre-instalied on the jacket

3) Diver to make the connections

4) Diver to bring cable from surface

5) On board barge

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UNDERWATER PILE MONITORING SYSTEM

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1 Strain transducer 2 Accelerometer 3 Junction box 4 Cable connector 5 Short cable coil

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UNDERWATER DPT - SET READY FOR MOUNTING

---- - - --- ---- ---- ---

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Underwater strain transducer

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Underwater junction box

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UNDERWATER STRAIN TR.A.NSDUCER AND JUNCTION BOX

OFFSHORE PILE AND HAMMER MONITORING

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~GAD

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•

16 2 2

1 Underwater strain transducer 3 Junction box

2 Accelerometer

UNOERWATER OPT - SET MOUNTEO INSIOE 72 INCH PIPE PILE

Stabbing tooi -

--tt----I

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Strain 1 Strain 2

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Accel.1

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Accel.2

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During stabbing Af ter stabbing

,070mm hole

r

Support hook

~.-+-- / Connector on

short cable

----1-- ~~I- ready on hook

above .070mm hole.

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(ûO:n

Short

--t--\. ~ cable coil I Stress relief

ft i Ft I

(

)

~~__~

.

""~

::

ç!

Junction box () ~..-__- __-._-..~;~ ,;..." ..,.;...~, -, \,,, lnstrurnentation '~, {"I J,' -- - f'I h-~,rl

U

t""~~' level U\~~~,' L-....' ~;'

POSITION OF UNDERWATER DPT - SET INSIDE PILE

OFFSHORE PILE AND HAMMER MONITORING

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Long underwater cable with connector

--==-

-'--=._

To data recordinq/acquisttionunit on board installation barge

Pile sleeve

'MHU 1700 hammer

.----Short underwater cable

with connector Pile---- .... Jacket sleeve Strain transducers on mounting bloeks Junction box on mounting plate Accelerometers on mounting plate

UNDERWATER DPT-SET MOUNTED INSIDE PI LE

OFFSHORE PILE AND HAMMER MONITORING

~GAD

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-Sealevel

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Seabed

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Todata acquisition unit

,... Alternative back-up

: cable

,1--__ I_Umbilical tor

underwater-: hammer

,

I I I ---f--Extension pipe I I I I

,

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~===C:CD=======~

r---*--:--Pre- instalied cable

I I I

,

,

I Hydraulic

underwater-

~--hammer (Menck MHU 1700)

--Instrumentation level 72" Pipe pile

UNDERWATER PILE MONITORING SYSTEM WITH PRE-INSTALLED CABLE ALONG JACKET

OFFSHORE PILE AND HAMMER MONITORING

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t

c 0

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'

...~ \-Q) Q) o

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<t:o

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t

I

_... >-'0 0

I

~ Time

--I

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c

t

..._y _~

I

z::

\-... Cf)

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Time

--I

TYPICAL D.P.T. RECORDS

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OFFSHORE PllE AND HAMMER MONITORING

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Blow count --"0 "0a> a> .0 .0 co co a> a> Cf) Cf) ~ ~ 0 a> a> .0 .0 .c

...

s:

...

Cl. Cl. _a> a> Cl Cl

l

l

Note: Oecrease in blowcount due to increase of

enthru energy rather than decrease of soil resistance

(.) EnthruEnergy-

(")Rdyn-•

.. já

..

•

_~

..

..

.

I!

..

•

...

..

~ ~.. VI.

•

~ . • y.

,

COMPARISON BLOWCOUNT / ENTHRU ENERGY / ROYN

OFFSHORE PILE AND HAMMER MONITORING

~GRD

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_t

a.

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:J I ...-Cl> Cf)

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Time-~'

-

_--

Á~----

---

-

---___

--

~ __ J

~--

_I--

-

-___

-

~

-

-

-."....,.-__:

-~

---

-~ Legend:

W Skin friction from capwap analyses

P Toe resistance from capwap analyses

SET-UP VE RSUS TI ME

OFFSHORE PILE AND HAMMER MONITORING

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t

Initial trial " ,.... ,.,

..

, \'~

--",~----~-"",,,-,,.,,--'-'

o

Time (ms)

-t

Time (ms) -Intermediate trial Cl> o '-~

o

t

Time (ms) -Final trial Cl> o '-~

o

Measured - - - Computed

EXAMPLE CAPWAP - RESUL TS

OFFSHORE PILE AND HAMMER MONITORING

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APPENDIX

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DYNAMIC PILE TESTING

an aid in analyzing

driving

behaviour

By

F.L. Beringen, W.R.van Hooydonk and L.H.J. Schaap

Paper presented at

Seminar on the application of stress - wave theory

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Contents Abstract 1.0 Introduction 2.0 Theoretical background 2.1 General

2.2 Application for soil resistance during driving

2.3 Application for driving energy determination 2 2 5 7

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3.0 Selection and development of equipment ₇

7 8 10 12 12

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3.1 General 3.2 Transducers 3.3 DPT conditioning unit 3.4 Recording equipment 3.5 Testing procedure 4.0 Typical applications 4.1 Pile capacity 4.2 Drivability 4.3 Driving -:model 4.4 Soil

-

model 4.5 Pile damage 5.0 Case history 6.0 References 13 13 14 15 16 17 18 20

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ABSTRACT

Dynamic Pile Testing (DPT) is facing a rapidly increasing popularity. Within the author's company the possibilities and problems involved in applying the procedures and electronic equipment available to date, have been evaluated.

Existing DPT-systems were often feIt to be either too compact, too sophisticatèd or too.~omplex in installation and control, to be uSed as standard measuring methode Especially the possi-bilities to condition and interface transducer signals with recording instruments were feIt to be open for improvement. Based on input from geotechnical as weIl as electrotechnical side a conditioner could be designed and built that facilitates the testing procedure considerably.

The combination of in-house geotechnica~ and equipment expertise appears to be of decisive importance in evaluating the data as weIl. The quality of DPT results depends on equipment perfor-mance of course, but often even more on geotechnical judgement and interpretation. The latter may easily be underrated, there-fore "black box" type procedures are not recommended.

In this paper the referred conditioner.will be described as weIl as the selection of other components of our DPT-system. Moreover, a theoretical review on the pile driving phenomenon, some typical DPT-applications and a case history are presented.

DYNAMIC PILE TESTING: an aid in analyzing driving behaviour

F.L. Beringen, W.R. van Hooydonk and L.H.J. Schaap *)

1.0 Introduction

From the very first moment that piles are being driven, efforts

have been generated to interpret and evaluate the pile driving

behaviour to acquire a better understanding of the driving

phenome-non. Thus numerous driving formulae have been derived, based upon

considerations of conservation of energy. Usually these formulae

(if practical at all) appear to satisfy very local conditions only.

Since about 20 years computerized solutions of the pile driving

behaviour have been developed, based on the fundamental concept of

. **)

the wave equation (1,2) . Those analyses opened more general and

better possibilities to study the pile problem, than conventional

methods by then available. Mathematically this problem seems to be

solved nowadays, physically many questions are still to be

answered.

In addition to the theoretical approach of the pile driving

problem, an experimental approach has rapidly gained ground.

Especially the development of electronic instrumentation

simu-lated these procedures, causing a boom in the last 5 to 10 years.

Quality and reliability of straingauges, accelerometers,

displace-ment measurements and electronic recording equipment have proven

to be adequate under most severe conditions. As a consequence

modern piling involves more and more instrumentation. This

opti-malization of information on driving behaviour of ten results in

.)

Manager Geot. Eng. Dept., Manager Instrumentation and Pile

Testing Group and Manager Electronical Dept., respectively,

all of Fugro B.V., Leidschendam, The Netherlands •

..

)

Figures between brackets refer to references.

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considerable savings in installation costs.

This paper contains an oLtline of the developments on Dynamic Pile Testing (DPT) as rnaterializedin recent years within the authors cornpany.The extensive in-house know-how, both on soil engineering and on design and construction of instrumentation systems for field rneasurements, of which the widely known and

applied Fugro cone penetrometer (3) is probably the best exarnple, appeared to be a significant advantage. Following to a theoretical review on the pile driving phenomenon this paper gives detailed inforrnationon the type of equipment as selected and utilized to monitor and analyse pile driving behaviour. Finally some

charac-teristic applications of pile dynarnicsare described, emphasizing the pile driving guidancei typical project exarnplesillustrate those application possibilities.

2.0 Theoretical background

2.1 General

---As several references are available (4,5), which thoroughly des-cribe the theoretical background of the pile driving theory, this chapter only presents same basic concepts, ensuring that the con-tents of this paper can be understood without the requirernentto call for those references.

A pile element with length (dx), cross section (Ar and specific mass (p) obeys Newton's law of motion when subjected to an

exter-nal force (F) (fig.I): dx

F = m.a or

r

'I

~Fd

~x x = pAd.x. a ••••••••••.••• 2.1

Fig.l - Newtonls force balance

The acceleration (a) can be rewritten as the second derivative to time (t) of the displacement (u); while the force F satisfies

du

Hookls law: F

=

EAdx' where E stands for the modulus of elasticity

-of the pile material.

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Substitution in equation 2.1 yields:

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or

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2. 2

Equation 2.2 is the wellknown wave equation (2). The constant c equals c

= ~

in units of length per time, being the wave

propa-p

gation speed.

The solution of this second order partial differential equation

I

is: u (x,t)

=

<p (x-ct) + lJ! (x+ct) ...•...•••...•...••.. 2.3

Ix- ct)is consL

A physical interpretation of this

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Fig.2 - Graphical display of <p (x-ct)

solution can be visualized easily

I

(fig.2), by obliging (x-ct) and/or

(x+ct) to be constant for increasing

11

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values of time (t). When both

<P(x-ct) and lJ!(x+ct)remain constant, the function values propagate

them-I

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selves undisturbed at a wave velocity c.

A convenient manner of notation has been initiated by Voitus van Hamme et al (5), to indicate positive and downward waves by means of arrows:

I

·

u(x,t)

=

<P(x-ct) + lJ!(x+ct). u+ + ut •••.•.•.••••.•.••••• 2.4 Further elaboration of this function (differentiation to time and

'

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place) yields the up- and downward waves of velocities and forces:

.

tSu

·

_EAtSu

V _tSt

=

V-4-+ vt and F

·

₌

F-4-+ Ft

.

...

...

...

2.5

tSx where V-4-

₌

-cu'-4- and F-4-

₌

-EAu'-4-Vt

₌

cu' t and Ft

=

-EAu' t

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I

and accordingly it can be shown that:

F-4-

=

EA

-

_c cu' -4- ZV-4- and

=

Ft

=

-zvt

...

2.6 F

₌

F-4-+ Ft

₌

Z(V-4-- Vt) and V

₌

!.(F-4-

-

Ft)

...

2.7 Z

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The quantity Z

=

EA is defined as the impedance of the pile.

c

SUbstitution of c

= ~

yields Z

=

A/Ep. This impedance plays an

p

important role in analyses of piling instrumentation data. When piles are instrumented, one can obviously measure only total forces and total velocities. On the other hand it will be

clear that upward waves (resulting from reflections) bear all

in-formation on external phenomena, causing those reflections. Hence a further modification of above equations is required, to obtain explicitly the upward or downward force waves:

Ft

=

F - ZV

2 and F

I

=

F + ZV

T 2 2.8

For interpretation of the upward Ft, it is required to evaluate the influence of several boundary conditions, such as pile tip resist-ance and skin friction. Fig. 3 gives typical boundary conditions and their effect on the propagation of waves.

-<::;

---c:

TIl ~ _~

CD

I TI

m

t

Ft

t

Ft

-

F

Ft

~F,

F,t

~ _{lW'i ".l.W} ~

t

2 2 ~

t

~

t

~

t

~~

~t

,

t

0

t::>-I . Free pile end

ll. Fixed pile end

m.

Finite tip resistance P nl. Skin friction W

~ig. 3 Typical boundary conditions

The following applies to the above figures:

for a free pile end (case I), F

=

O.

Hence Ft

= -

F~ and V

=

2V~ .•••...••••. 2.9

-4-- for a fixed p;le end (case 11), V

=

O.

Hence vt = -V+ and F = 2F+ ••••• 2.10

- for a finite tip resistance (case 111), F = P. 2F+ - P

Hence Ft = P-F+ and V =

z

- skin friction can be derived from conditions

2.11

of equilibrium (Fl - W - F2 = 0) and

compatibility (VI = V2). The following equations for the reflection wave F1t and the refraction wave F2+ satisfy these conditions:

FIt = F2t + ~W and F2+ = F1+ - ~W 2.12

Impedance changes in the pile are not considered herein, but can be analysed as the skin friction problem (5).

For a simplified pile model which assumes a concentrated, ideal plastic coulomb friction force halfway the pile and similarly classified tip resistance, four crucial stages can be distinguished. These phases are indicated in fig. 4. Obviously the model-friction W can only be detected at time t = i when the reflection wave reaches the

instru-c

mentation level. For an impulse starting at t=O, the generalized solution for Wult reads:

Wult = Ft=2t/c - ZVt = 2i/C .•••

. .

.

.

.

. .

.

2.13

If the tip reflection reaches the measuring level at 2i

t2

=

t1 +

C-'

the end resistance P can be computed from (5):

P

=

~(Ft=t2 + Ft=t - ZVt=t + ZVt=t ) - Wult 121

2.14

The way a certain wave front propaga~es and deforms is

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illustrated in fig. 4 I II

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level 1 1 31 - - -0 Time- 2C C 2C 21 2"d reflection (further neg!ected) I to IV =Significant stages + =Compression wave - =Tension wave F = Stress wave

W= ldealized concentrated friction P = End resistance

Fig. 4 - Significant stages during one complete passage of a

stress wave

The equations 2.13 and 2.14 are in fact the key formulae for dy-namic pile testing. They explicitely show that the skin friction and the tip resistanc~ can be determined by recording the total force and velocity at pile head level during the passage of one complete energy wave.

Of course the model assumed above is very simple. Further re-finement of this model is possible and is even felt to be required, especially when DPT-results are to be translated into ultimate

statie pile capacities. In these cases it might be necessary to perform a statie load test as calibration of the DPT data. For drivability purposes however, DPT's can usually be employed in an absolute sense.

-6-Above formul-2 can be combined in numerous ways, representing as many application possibilities. One typical application however,

is worthwhile to emphasize, being the determination of the driving energy EoPT: t EOPT

= ~

t F~V~dt

=

i

f F2~dt o

.

....

.

.

....

...

...

..

.

..

.

.

..

2.15 Of course second reflections or reflected waves should be elimi-nated from the integrand. This can be achieved by compensating the second order reflection (being a tension force wave) with the mag-nitude Ft. Thus camputed energy EOPT can be seen as the effective input and might be compared with the rated or impact energy,

resulting in a so-called "enthru" figure (6).

3.0 Selection and development of equipment

3.1 Géneral

Existing dynamic pile testing systems that have been observed were often felt to be either too compact or too sophisticated. Most of the instrumentation projects experienced appeared to be

complex in installation and control and therefore not suitable to be adapted to a standard measuring methode Moreover there are no transducer systems which suit all the pile testing applications. For each project the best transducer should be chosen and this may not even be restricted to one type within a project. Consequently, it was decided to aim at a systernwhich satisfies the follow-ing requirements:

- the equipment should be simple, reliable and portable

- installation of instruments and transducers should take little time

- various types of transducers should be adoptable - redundancy should be built in the system

7

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- direct evaluation and interpretation of the data in the field by geotechnical engineers should be possible.

The general philosophy behind the selection and development of equipment was that it should not result in a so-called black box-type testing procedure.

In an early stadium a system to gather velocity-data had to be selected. An evaluation of the merits and the practicality of differentiation of displacements versus integration of

accelera-tions went in favour of the latter. Not considered at the time, yet promising, is the recently developed strain difference method

(7) •

Subsequent chapters describe the most important features of Fugro's DPT-capabilitiesj especially conditioner design and

selection of transducers and recording equipment will be discussed.

3.2 Transducers

---The transducers to convert strain and acceleration into an

electric_alsignal are very important in the process of dynamic pile testing. As mounting of the transducers interferes with the pile installation process it should be areliabIe and quick procedure without time consuming preparations, tests and drop outs.

Strain transducers to measure forces in structures are available in many forms.As straingauge transducers are the most practical, these types are being used exclusively. The straingauge is an extremely sensitive device with an electrical resistance that varies in direct proportion to changes in strain.

A type that can be mounted and replaced easily is the so-called bolt-on transducer (fig.5)j for the type shown two holes have to be bored and tapped into the pile wall. The latter is often d LffL>

cult in the field in view of the tolerances involvedj special tools or accessories are necessary then.

-Fig. 5 - Bolt-on strain

transducer (right)

and accelerometer

(left)

Z

!20I--+++I-~II"---t---+- -+--+--i----.---,----; ~

of 30I--+-+II--~f---<,__,,~=.'...Range of results of

I:

:

=

~

:

~

-

-l-+-

_-td

"

"flTI

T

I

o 20 40 60 BO 100 120 Time (mHc. )-Fig.6 - Comparison different types of strain transducers ;.v,t--r.l---- Man

Plezo .Ieetrle diK'

~~It""--:::;:~ Output terminals Bu.

Mountlng broek

Fig. 7 - Basic construction of a piezo-electric accelerometer

I

For steel piles weldable

strain-gauges can be used as weIl; advan-

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tages of this gauge are its small

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dimensions and the high level of accuracy that can be achieved by applying a full bridge of gauges. A

third type that can be used is a

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bonded and protected gauge on a small aluminium plate which is attached to the pile wall with a quick curing adhesive. These gauges

appear to be very suitable for con-

I

crete piles; correct bonding

howevèr, is sometimes difficult

I

,

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especially in cold, salty and/or wet environrnents(8).

As far as the results are con-cerned there is no preference for a

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certain type. A number of tests has been performed to compare the dif-ferent transducers under operational

conditions. A very goed correlation

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appeared to exist (fig.6).

There are also several types of transducers on the market; since

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small dimensions and acceleration ranges up to 2000 gare required,

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the piezo-electric accelerometers are considered to be the most suita-bIe. The bolt-on transducers (fig.5)

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consist of a mass resting on

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electric discs, mounted on an aluminium block (9, fig.7). When the mass is subjected to an acceleration a force is exerted to the discs, which generate a voltage proportional to that force. Accele-rometers with and without a very small built-in amplifier are

available; a built-in amplifier has the advantage that simple cables can be used over long lengths, whereas the ether types re-quire special, typically fragile low noise cables. The latter ac-celerometers are less expensive, reliable and have excellent lin-èarity and overload specifications: a disadvantage is that the cable length and type effect the calibration of the accelerometer unless a pre-amplifier in the vicinity of the transducer is used. In all cases.a lot of attention has to be paid to the mounting of the accelerometers since improper mounting rnayinfluence the frequency response.

~~~

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Q~!_~Q~~!~!Q~!~9_Y~!~

None of the conditioning units observed satisfied the require-ments listed in 3.1. Af ter having tested sornesystems for some

time the decision was taken to design and build one single instru-ment to condition and interface transducer signals with recording

instruments. Based on input from geotechnical as weIl as electro-technical side the first conditioner was built and tested in 1979i at the moment several conditioners are operational and have worked at great satisfaction en various projects.

The unit consists of a standard 19 in. eurocard rack in which several printed boards are built. The boards.are designed to

pro-vide electronic amplification and conversion of the transducer signais. The conditioner .(fig. 8) consists of two'strain channels, two ac'celerorneterchannels and two analog integrators. As each transducer has a typical calibration value which is given by the

-tc..Iibf.tionmput 0"""OIIintllgrltGf

0p.... _ter

Fig. 8 - Fugro DPT-conditioner

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rnanufactureror obtained by workshop

calibration, the transducer signals

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can be adjusted to a normalised output which is chosen at 1.0 Volt for 1000

g

il

or 1000 micro-strain. The adjustment

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of all signa~s can be achieved by di-gital potentiometers on the frontpanelll of "the conditioner.

For reasons of fast interpretation

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all output signals are visualized on small panelmeters showing alterna-tively zero level and minimum or maximum peak value. The proper

func-I

tioning of the transducer can con-tinuously be checked by these

panel-I

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As stated before several types of transducers may be used within meters which is of vital importance.

vide this input flexibility.

one series of measurementsi internal switches are built in to

Two analog integrators have been built into the conditioner

pro-I

the accelerometers.

viding a velocity signal based on the normalised output values of

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As noise may interfere with strain signals in particular, the possibility to switch the bridge supply voltage off is built in. Checks on the presence of noise inductions, for example caused by

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the hammer, can be performed during testing.

Routinely all original transducer and velocity signals are

recorded continuously to obtain an authentic data file as weIl as data redundancy. Moreover average va lues of strain, acceleration

desired.

and velocity can be obtained directly from the conditioner if

- 11

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During the selection procedure of recording equipment several

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instruments were tested under field conditions. This piece of

equipment has to be reliable, simple in operation, easily tran

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portable and relatively stalwart.

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of one particular blow or for the statistical analysis of a seriesThe data have to be stored either for a detailed investigation

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of blows. An eight-track tape recorder has been selected to store

the data on magnetic tape. In general the original signals of two

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strain transducers and two accelerometers are recorded leaving

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four channels for optional information such as a time signal,

velocity signaIs, mean values or comments on a voice channel.

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During the fieldwork inspection of a number of blows is required for a preliminary interpretation as weIl as chedk on functioning of t~e en-tire system. A two channel digital storage oscilloscope serves these purposesi the transients can thus be disp'layedon a screen, moreover hard copies of the signals can be ob-tained from an analog chart recorder if necessary. The oscilloscope can also provide a digital output for storage on cassette or paper punch tape to be used in a computer.

The standard set-up is presented

x-TPLOTTER

1

AElD- 'TRAIN .,.. TIME

J

ACCElERATION .. TIME

IlI'TA _{VELOCITY .,.. TIME}

t

.1 SKIN FRICTION end

TEST- END RESIST"NeE

RESlA.T DURING ORIVING

bi DRIVING ENERGY.CAP

-Md CUSHION PARAMETERS cl 'ILE INTEORITY

Fig.9 ~ Dynamie pile

testing set-up

in fig.9 •

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The DPT-procedure highly depends on client- and job requiremen

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ts.

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In general two strain transducers and two accelerometers are

mounted onto the pile aL the same elevation.

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Blows given by any type of hammer can be used for the analyses,

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both during and after the pile installation. Testing after

instal-I

lation applies to cast in-situ piles in particulari for these tests a truck-mounted, fully self supporting hammer has been

developed.

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In principle the data are being elaborated and interpreted in the office afterwards. Preliminary information can be obtained in

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the field from chart recorder plots or if necessary desk computer calculations.

4.0 Typical applications

In principleDPT can assist in gathering information on pile

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capacity and, if applicable, pile drivabilitYi basically the test-

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ing procedure is the same in both cases.

In this chapter some general aspects of the first topic will be describedi more emphasis is given to drivability-related items.

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,,

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,(.

.

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~

-:

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l?

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iT";0hom,.tiDJ

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;.:!. /o~ ;/_

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/. / (-0 t '00 Fig;10 - Comparison DPT

and statie load test results

600

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An interesting and profitable

application of the DPT-method is the

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fact that an estimate of the bearing capacity of a pile can be given on the basis of DPT-results and

eorre-I

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lations between dynamie and statie

soil parameters (10, fig.IO).'

Sitni-larly, relations are being developed

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to correlate eone resistances, measured with the electric

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