Copyright 2000, Offshore Technology Conference

This paper was prepared for presentation at the 2000 Offshore Technology Conference held inHouston, Texas, 1–4 May 2000.

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AbstractThere is today no operational system able to performgeotechnical measurements with standard 36 mm CPT conesin deep sea with a depth of investigation of 20 to 30 metresbelow the sea-bed. The advantage of the new system is that itwill allow surveys in deep sea in a more cost-effective manner.GEOCEAN-SOLMARINE, a French Marine Contractor withdeep water geotechnical tools capabilities and IFREMER, aFrench Research Institute for exploitation of the sea, joined todesign and build a new seabed penetrometer which will beoperated by GEOCEAN-SOLMARINE for industrialoperations and by IFREMER for scientific applications. TheIFREMER's version is now under assembly.The first tests at sea of the penetrometer are expected for mid-2000.

IntroductionThe current trend for offshore oil developments is to move

into deeper water. There will therefore be a growing need forreliable soil design parameters in deep water, for the design ofstructure foundations, for the anchoring of floating productionand storage systems, for pipeline laying and for slope stabilityproblems.

Scientists are also interested in in-situ measurements fordifferent topics: sediment characterisation, slope stability,environmental studies, hazard mapping, stratification logging,indication of stress history and lithology deposits,...

The tool designed by GEOCEAN-SOLMARINE andIFREMER is a frame which is lifted over the seabed andwhich remains during the duration of the measurement. Thepenetrating system is based on a flexible rod which is pushedby friction into the soil. For scientific applications, the tool iscalled "Penfeld", the penetration force is limited to 40 kN

which allows a penetration of 20 metres into the soft soilsencountered in deep seas. Several locations can be investigatedduring the same deployment of the system. The industrialversion called "DS7000" will be built with higher capacities.

In a first step, the penetrometer will be equipped with aconventional cone including the measurements of pointresistance, shaft resistance and pore pressure. Further a newcone will be developed specially devoted to high pressureenvironment, including differential pore pressure andcomplementary sensors such as a gamma-densimeter and athermometer.

Need of sea bed measurements in deep seaThe offshore activities are now moving from the

continental shelf to deep seas. This change modifies the needsin soil surveys.

On the continental shelf, the main object of the surveys forfoundation design of platforms was to obtain information onthe soil for the design of the piles for the anchoring ofplatforms. The piles were 50 metres, 100 metres long, evenmore. A typical survey consisted generally in a cored boreholeover all the length of the pile, and an instrumented borehole,using a wire line Cone Penetrometer Test performed by lengthof 3 metres.

In order to perform such a survey, a geotechnical vesselwith facilities in drilling was needed.

These methods are operational only in water depths lessthan 200 metres. Even if some trials were performedsuccessfully in deeper water, they become difficult to operate:weight of the drillstring, hydrodynamic forces on thedrillstring,... and they are very expensive.

In deep water the subject of surveys moves. It does notconcern only the survey for the design of foundations ofstructures on the seabed, but also the survey of zones on whichgeological hazards may occur.

When in shallow water a survey can be limited only to thelocation of the foundation of the structure, in deep sea a surveymust be conducted at three different scales, then along theslope dangers can come from zones situated uphill.

The three scales of surveys are:The regional scale

It corresponds to an area of about 100x100 km2; the

OTC 12095

A New Seabed PenetrometerJ. Meunier, IFREMER, D. Sangouard and B. Lhuillier, GEOCEAN-SOLMARINE

2 J. MEUNIER, D. SANGOUARD OTC 12095

survey consists mainly in bathymetry and multibeam echosounder.The local scale

It corresponds to an area of about 10x10 km2; the surveyconsists mainly in sonar imaging and High Resolutionseismics.The site scale

It corresponds to the location of the foundation and thearea is about 1x1 km2; the survey consists in very highresolution seismics or in sediment penetrator and in in-situmeasurements.

Acoustic data can be calibrated thanks to samples collectedover the zones, but generally in-situ measurements areperformed on the site of the foundation or on a targetpresenting a particular risk.

In deep sea, the structures are founded on shallowfoundations or on suction caissons. Anchoring of lines arenecessary. The depth of investigation in the soils is smallerthan on the continental shelf and is limited to 20 to 30 metres.

Moreover the problems of slope stability require generallysurveys over 10 to 20 metres. The shallower sediment layersare concerned.

The other geological hazards met in deep water slope, needto be recognised in order to be avoided or in order to estimatethe risk they implies on the oil field installations. It is the casefor diapirs, mud volcanoes, gas expulsions, outcrops ofhydrates,...

The needs expressed above, show that in deep sea anoptimal penetrating depth for geotechnical in-situmeasurements is 20 to 30 metres.

Existing toolsThe Tethered Seafloor Platform (TSP) prototype was

developed and tested for the purpose of obtaining in situproperty measurements in water depths as great as 3000metres. The concept was to design and built a subsea modulethat could continuously measure geotechnical properties fromthe seafloor down to approximately 70 metres. The fullinjection force is 180 kN. The work was performed by RappMarine in Norway, Fugro-McClelland in Houston for Mobil.[Humphrey and al, 1995]

The Seascout seabed frame deploys a mini-cone, and isassembled on a tripod weighing less than 10 kN. Thepenetration is 2.5 metres or by extension 5.0 metres. It can beoperated by an umbilical or by a ROV. [Power and al, 1992]

The wheeldrive Seacalf has been developed by Fugro forcarrying out seabed CPTs. Since it utilises a hydraulicumbilical, it has a limitation in water depth of 800 m.

The Geotechnical Module has been developed byIFREMER. It can support a penetrometer, a vane and a pushsampler. It is weighing 20 kN in air. The penetration of thepenetrometer is limited to 2 meters deep and data are recordedevery 2 cm. The penetration of the vane is 0.6 metres. The

Geotechnical Module is powered by batteries and thesequences are pre-established before deployment. During eachlaunching, one core and several measurements can beperformed by moving the module from a location to an otherone. It can be operated down to 6000 metres. Special coneswere developed with high sensibility, and a gamma-densimeter cone is available. [Balzer and al, 1994]

Description of the systemGEOCEAN-SOLMARINE and IFREMER have joined in

order to realise a sea-bed penetrometer deploying a rod downto 20 to 30 meters deep inside the sediment.

The final objectives being slightly different between thecontractor and the research institute, it has been decided tobuilt two systems based on the same principle.

The main differences are as follow:The IFREMER system must be operated on oceanographic

vessels equipped with U-frame at the rear of the vessel.The IFREMER system has to be operated down to 6000

meters deep regarding scientific topics, while the GEOCEAN-SOLMARINE system is limited to 2 000 metres, where are themost of offshore activities.

For industrial purposes, it is compulsory to collect the datain real time.

When the system is operated in deeper sea, the soils aresofter, and the reaction required to penetrate the tube isweaker.

Taking account of these remarks, the main characteristicsof the two systems are given in table 1.

GEOCEAN-SOLMARINEDS7000

IFREMERPENFELD

Depth ofpenetration

30 metres 20 to 30 metres

Force ofpenetration

60 to 100 kN 40 kN

Soil reaction Suction foundation Shallow foundation withskirts

Energy supply Electric by umbilical from thesurface

Batteries

Data transfer By umbilical Recording on board thesystem or acoustic link

Command By operator Automatic by programmeTotal weight 60 kN 40 to 60 kN

Table 1 : Comparison between DS7000 and Penfeld

At the beginning of the year 2000 the IFREMER apparatusis in assembly. Trials of each element are performedsuccessively. Tests of the whole system are expected in April2000 and the first sea trials are planned in June 2000 on R/VLe Suroit in the Mediterranean sea.

A description of each element of the Penfeld (Fig. 1) isgiven below.

OTC 12095 A NEW SEABED PENETROMETER 3

Rod

Injector

Hydraulicpower

unit

Joint

Straightnerandbendingsystem

Hydraulicmotors

Batteries

Drum

Fig. 1 : Scheme of the Penfeld penetrometer

The frameThe function of the frame is to ensure the mechanical link

between every parts of the system, to transmit the efforts ofthe injector during the penetration or the retrieval of the tubeand to spread the efforts over the soil.

It includes three parts: the higher part which protects themechanical elements against the handling shocks, the middlepart which holds the mechanical elements and the lower partwhich holds the batteries and which is used as foundation. Ajoint between the lower frame and the other parts allows theintroduction of the rod horizontally during the setting of thestraightner module.

The system of penetrationThe system of penetration called "injector" is a system

based on two chains including slippers. Jaws applied by fourspring jacks compress the rod between the slippers. Thecontact between the jaws and the tube is steel. Two hydraulicmotors including a reduction gear set the chains in motion.

The friction coefficient taken into account is at least 0.4and has been verified during the preliminary tests.

The tubeAs it is impossible to operate a 30 metres straight tube in

one piece, it is necessary to bend it. The technique used is the"coiled tubing" which consists in bending and straighteningthe tube plastically. The bended tube is winded on a drum.

The material chosen is stainless steel. Tests have proventhat the tube could resist over 100 cycles.

The tube has a normalised section of 10 cm2 (35.8 mm indiameter). The tube is able to let a cable through, between thecone and the head of the tube.

Procedures have been established in order to weld thedifferent elements of the tube.

The straightner and bending moduleThe straightner and bending module includes five wheels

in the same plane. The same system is used downwards instraightening the tube and upwards in bending the tube. Thesystem is powered only by the injector. Once the tube isplasticized, the straightness is ensured. The wheels must bepositioned very accurately, but they have not to be modified aslong as the same tube is used. The tube is not subjected todeformation in the plane perpendicular to the wheels, andremains always in the same plane. The radius of curvature ofthe tube is constant.

The system is placed above the injector (Fig. 2).

The drumThe bended tube is maintained on a drum. The tube goes

on the side into the drum with the assistance of a wheel. Thediameter is constant and is 2.2 metres large. The turns arejoined. The drum does not need to be powered, only theweight of the tube has to be hold up. The centre of the drum isat the same level as the bending system.

4 J. MEUNIER, D. SANGOUARD OTC 12095

Fig.2 : Injector and drum during tests

The batteriesThe energy is provided by batteries. IFREMER which

operates unmanned vehicles as well as manned vehicles has agreat experience in batteries immersed in equipression.

For cost reasons, lead batteries have been chosen. Theweight does not penalise, as the system must be weighing inorder to balance the reaction during the penetration.

The electric motorThe hydraulic energy is provided by an asynchronous

electric motor compatible with the two energy sources, dcbatteries or ac electric power from umbilical from the surface.

The hydraulic systemThe oil balanced hydraulic system supplies the two injector

motors and the jacks of the accessories. Its contains two parts.The hydraulic power unit has a two constant delivery radialpiston pomp, one flow for the motors of the injector, the otherone for the jacks. The hydraulic distribution devices areplugged on a multi flange subplate in an oil balancedcontainer.

The power electronics containerIt contains the electric conversion chain and associated

safety devices necessary to drive a 5 kW ac motor from a dcpower source, as well as dc/dc converters for auxiliary needs.The main components are :

Power reverser, over current and over temperatureprotection devices, internal air flow control, and powermanagement electronics.

The mission control and measurement unitThis computer is in charge of the high level machine

sequencing along a deployment, and the data acquisition fromthe instrumented cone. Penetration sequences are programmedon this computer before launch. However, it is planned toadapt an acoustic data link between this unit and the surface

vessel which will allow modifications of the penetrationprogram while a deployment (Fig. 3).

Fig. 3 : Scheme of the command unit

The machine control unitThis is a slave unit of the mission control and measurement

computer, that drives the hydraulics and power electronics ofthe penetrometer. It is linked to a set of sensors (machineinclination, penetration force, hydraulic pressure...) in order toensure the machine integrity throughout the deployment. Incase of major failure, this unit can decide to interrupt apenetration sequence and retrieve the tube in emergency.

The mission control and measurement unit and themachine control unit are located in a same pressuretightcontainer.

Servitude sensorsIn addition to the instrumented cone some sensors give

important information on the progress and the security of theoperations of the system: total force of penetration of theinjector, horizontality of the platform, length of the tube intothe soil, speed of penetration, tension on the bearing cable,hydraulic pressure, electric capacity of batteries, waterpressure, water temperature.

The instrumented coneIFREMER has already developed for the Geotechnical

Module an instrumented cone which can be operated in deepsea. But the maximum point resistance is very low, as thepenetration is limited to 2 metres.

Today there is not any cone available which corresponds tothe requirement of the new Penetrometer. GEOCEAN-SOLMARINE and IFREMER are developing a new conewhich will be operated down to 6000 metres deep.

The cone will classically measure the followingparameters: point resistance, lateral friction, differential porepressure, tilt angle in two directions regarding the vertical line.

High voltage securitiesPower inverter

Converters

Batteries

acMotor

Hydraulics

180 V dc

24 V dc

Motor control

Electro-valvescontrol

Batteriesvoltage & current

Power blockTemperature

Mission control and

measurement

Machinecommand

& control

Oil pressure

Servitude sensors

Command, control, power & hydraulics block diagram

Instr. cone

Surface unit

Acoustic link

OTC 12095 A NEW SEABED PENETROMETER 5

Additional measurement will be added: gamma-densimetry, temperature, heath transfer.

Differential pore pressureOn classical cones, the pore pressure is measured by an

absolute sensor. In deep sea application this technique cannotbe applied, then the variations of the pressure induced by porepressure are very low compared to hydrostatic pressure and themeasurement wouldn't be accurate enough.

So, it is necessary to measure the differential pressurebetween the pore pressure and the hydrostatic pressure at thesame level. To ensure this function, the sensor must be linkedby a small pipe to the water in the sediment in contact with thecone through the porous stone (as usually) and in contact withthe water inside the hole of the tube which is in relation withthe free sea water over the seabed. The problem to solve is tolet a free room in the electronic tank for this small pipe in atotal diameter of 36 mm.

Tilt angleThe two angles with respect to the vertical line must be

measured in order to be able to integrate during the penetrationthe deviation of the cone. It is possible to calculate exactly theposition of every point of the tube. If the deviation is too high,the measurements are considered as being of bad quality andcan be stopped.

Gamma-densimetryThe density measurement principle relies on the fact that

the interaction of gamma rays by Campton scattering effect isdependant only on the number density of scattering electrons.

The direct parameter obtained with a gamma-densimetrydevice is the electron density. The sediment density isdetermined using the fact that the atomic weight isapproximately twice the atomic number in most rock-formingelements.

The use of radioactive source imposes safety precautionson the personnel handling the probe. A container will protectpeople from radiation when the cone is on the deck of thevessel. It will also be used during the transportation of thecone and during the storage in laboratory.

ThermometerThe measurement of the temperature as well as the thermal

conductivity is of high interest. The values of the geotechnicalparameters can be influenced by the temperature. Temperaturecan also be an indicator for movement of fluids inside thesediment.

Further developmentIt is expected to adapt a seismic cone in order to perform

Vertical Seismic Profiling in addition to the geotechnicalmeasurements. VSPs help to correlate the seismic profiles withthe in-situ measurements. [Nauroy and al, 1994]

Deployment of the system

The IFREMER system is powered by batteries. A timesequential programme is established. It is loaded just beforethe deployment into the water. The system is lowered and laidover the seafloor. The first penetration can begin at the righttime. The tube is pushed and retrieved. Then the system isshifted to the second location and so on, until the programmeis over. A maximum of 9 to 10 locations can be measured bydeployment taking account of the energy stored in thebatteries.

An acoustic link gives information to the operator in thevessel about the progress of the operations In a first time, theacoustic link is not used to send orders to the sea-bed system,because the link must be very reliable. The reliability of thesystem has not yet been tested enough.

Conclusion

The new penetrometer will allow the acquisition of in-situgeotechnical data using a CPT cone in deep sea. A new conewill be developed including new sensors, in particular agamma-densimeter. This new tool will offer new possibilitiesin geotechnical surveys in deep sea in a cost effective manner.

Acknowledgement

The authors like to thank the staff of GEOCEAN-SOLMARINE and IFREMER who works on the project.Thanks to their co-operation the penetrometer will be soonoperational.

References

1. G.D. Humphrey, J.B. Adams, TSP - New Solution for geotechnicalStudies in Deep Water Environments, OTC 7673, Houston, May1995.

2. P.T. Power and J.M. Geise, Offshore Soil Investigation Techniquesand Equipment for the next Century, BOSS '94, Boston.

3. Baltzer A., Cochonat P., Piper D.J.W. In situ geotechnicalCharacterisation of Sediments on the Scotian Slope, EasternCanadian Continental Margin. Marine Geology, 1994, 120 p.291-308.

4. J.F. Nauroy, J.C. Dubois, J. Meunier, A. Puech, J.L. Colliat, D.Poulet, F. Lapierre, The Use of VSP Technique in GeotechnicalBoreholes: First Tests in Offshore Monaco, BOSS '94, Boston.

5. J.F. Nauroy, J.C. Dubois, J. Meunier, B. Marsset, A. Puech, F.Lapierre, J.P. Kervadec, H. Kuhn, Tests in offshore Monaco ofnew Techniques for a better Integration of Geotechnical andSeismic Data, OTC 7375, Houston , May 1994.