Harpoon Free Fall Cone Penetrometer Test Results: Hudson
Contents
1. Split FFCPT coupler Robust Need external safety wire attached to FFCPT housing in case core barrel breaks Split coupler fasteners Difficult to clean mud out of cap screws Portable high pressure washer for cleaning off mud Saturating hydraulic lines Enerpac hand pump gage needle seems prone to jamming Drill out pressure port on dial gage viscous mineral oil not allowing pressure to release Saturating filter ring Potential for oil to drain out of filter during deployment Use viscous silicone oil instead of mineral oil User manual Good but needs improvement Update as required develop Job Safety Awareness procedures for all activities Probe diameter Large enough to prevent mud from contacting core barrel friction on core barrel is minimized No action required Electronics Issues Battery pack Duration of battery life not known battery Bench test to check battery life 20 pack easily changed Sleep feature Assembly disassembly Serial sleep cable cannot be used inside core barrel if piston is in use probe has to be activated on bench assembled and deployed before Serial connection inside top end cap easily pulled apart during reassembly Install a manual switch on FFCPT probe to activate and put electronics to sleep battery life dies Solder wires to end cap connector pins Data transfer No communication2. Figure 3 GSCA large diameter piston corehead with 30 ft of core barrel and FFCPT Tests were conducted with as much as 50 ft of core barrel OAT aan A rt e Pra o o we A Figure 4 GSCA large diameter piston being rotated to a vertical position Figure 5 Trip arm attached to the wire and the piston corehead with 8 ft of free fall note loop in recovery wire The end of the recovery wire is attached to a sliding piston inside the core barrel which allows the system to fully penetrate the seabed Figure 6 3m gravity corer being attached to the end of the trip arm wire This allows for release of the piston corer into free fall at the correct height free fall distance above the seafloor A small sample is collected each time this corer is used Figure 7 FFCPT and 50 ft of core barrel being raised to the rail CCG Hudson can accommodate as much as 100 ft of core barrel using the dedicated monorail system see top of photo Figure 8 FFCPT after recovery showing mud up to the split coupling on the core barrel before being pressure washed STA NO OF NO BARRELS RIGGED m FIELD TESTING AND DATA INTERPRETATION Table 1 summarizes key data for the four FFCPT tests conducted during the GSCA Hudson 2004030 cruise to the Scotian Slope Test 1 was unsuccessful because the trigger thresholds for high speed data collection were set incorrectly The impact and penetration process onl
3. trigger events The data interpretation is done within custom FFCPT View software Signal conditioning is done at this time Each trigger event creates a set of datafiles stored in Flash memory within the FFCPT probe Upon recovery the datafiles are offloaded using Windows HyperTerminal software to the host computer for processing interpretation and hardcopy output Two methods of deployment were used For the first two stations 001 and 005 the system was lowered into the seabed on the winch at maximum payout speed However the data indicated that the dynamic strength profile of the sediment was not high enough to substantially decelerate the corer especially near the mudline The penetration ended when the corehead impacted the mudline indicating that more core barrels should have been mounted The last two tests stations 013 and 015 were carried out by tripping the corer from a free fall height of between 2 4 and 4 0 m using the pilot gravity corer as a trip weight The resulting test data were of much higher quality and can be put through the full FFCPT analysis A settling period of 45 seconds was allowed after impact to ensure that the datafiles had been written to the Flash card and the probe was rearmed Two pullout events were recorded stations 0005 and 013 giving useful data on the sediment suction behaviour Capturing pullout events is not an objective of the test however Figures 2a and 2b show photographs of the FFCPT rea
4. lowered into the seabed at a rate of 97 m min using the ship s foredeck winch Note the cyclic acceleration response which is due to vessel heave effects transmitted down the deployment wire The system came to rest at about 8 sec Chri Sit pupa lt BOT o G Ocean GSC ATLANTIC HUDSON 2004030 STATION 005 MOHICAN CHANNEL SCOTIAN SLOPE FFCPT 005 DAY 196 TIME 2009 WATER DEPTH 894 m 1200 m MUDLINE DETECTOR 800 600 OPTICAL OUTPUT counts 2g ACCELEROMETER VELOCITY m s ACCELERATION m s PRESSURE HEAD m 25 F DIFF go llo rr rr br br bas 0 1 2 3 4 5 6 7 8 TIME sec ERENTIAL GAGE 14 Figure 10 Summary interpreted engineering log for FFCPT 005 location As the system was not deployed in free fall no useful acceleration data was obtained note NA comments on Qa Bg and Qn logs GSC ATLANTIC Chrislian Situ Saas HUDSON 2004030 STATION 005 MOHICAN CHANNEL SCOTIAN SLOPE BOT T ees FFCPT 005 DAY 196 TIME 2009 WATER DEPTH 894 m oa DYNAMIC DYNAMIC UNDRAINED DYNAMIC NORMALIZED PENETRATION PORE SHEAR PORE DYNAMIC DESCENT RESISTANCE PRESSURE STRENGTH PRESSURE PENETRATION VELOCITY Qg U S PARAMETER RESISTANCE kPa m kPa Ba a V m s O 100 200 3000 10 20 300 10 20 30 1 6 0 8 0 0 0 8 1 6 0 7 7 7 E m 200 mr 400 rooke Ocean y echnolog Ltd amp ACCEL OUTPU
5. 99 1955 1 200 TRIPPED 4 0 STA APPARENT VISUAL SEDIMENT DESCRIPTION FROM MUD ON OBSERVATIONS NO PENETRATION OUTSIDE OF CORE BARRELS Data Interpretation After the datafiles were transferred to the host computer they were processed and displayed within FFCPT View software Version 3 1 This custom interface was written for analyzing FFCPT data collected with previous FFCPT models and is of limited usefulness in reviewing and displaying data collected with the piston coring FFCPT equipment due to system configuration variances However flat ascii files were created after signal conditioning was completed then the geotechnical analysis and sediment behaviour interpretation was completed using spreadsheet methods The interpretation of FFCPT data involves determining the descent velocity immediately prior to impact then selecting the portion of the dataset containing the FFCPT penetration into the sediment thereafter one of the acceleration channels is chosen and for forward integration against time which yields the time velocity curve after impact The integration process typically results ina non zero residual velocity value which is then used to adjust the impact velocity so that the at rest velocity integration residual is reduced to zero The velocity time curve is then itself integrated yielding the distance versus time curve The depth at impact as determined from the ship s echosounder defines the zero baseline for the T
6. FFCPT 015 somewhat masking the sediment induced excess porewater pressure response This effect is due to water hammer surging on the negative side of the differential tip pressure transducer diaphragm which occurs during free fall penetration The dynamic pressure response is much more linear when the system is lowered into the sediment on the winch rather than allowing it to free fall into the sediment This is more typical of cone penetration test results wherein soundings have consistently shown a gradual increase in shear strength with depth The dynamic porewater pressure response is directly related to the undrained shear strength in clays according to extensive testing over 25 years of CPT testing practice It may be possible to significantly reduce the water hammer effect during free fall penetration by installing a water surge chamber on the negative side of the differential tip pressure transducer This would hopefully result in datasets that more accurately reflect the in situ shear strength profile Replacement of the differential gage with an absolute gage would not be feasible as the reduction in sensor sensitivity would have a degrading effect on the pressure measurement data Figures 9 through 16 show interpreted results for the FFCPT tests summarized in Table 1 in graphical format The raw data time series are shown along with interpreted geotechnical engineering logs and sediment classification charts Figures 9 and 10
7. Harpoon Free Fall Cone Penetrometer Test Results Hudson 2004030 Field Verification Project Prepared for Petroleum Research Atlantic Canada by Christian Situ Geosciences Inc 1204 1414 Barclay Street Vancouver BC V6G 1J4 and Brooke Ocean Technology Ltd 50 Thornhill Drive Unit 11 Dartmouth NS B3B 1S1 October 06 2004 Chrislian Situ Ea eS INTRODUCTION This report summarizes field testing and geotechnical engineering interpretations for data collected with the Harpoon Free Fall Cone Penetrometer Test FFCPT This apparatus was developed with financial support from the Petroleum Research Association of Canada PRAC by Brooke Ocean Technology Ltd and Christian Situ Geoscience Inc Shiptime was provided by the Geological Survey of Canada Atlantic GSCA for testing purposes The tests conducted during this cruise fulfilled a PRAC requirement for field verification of the technology in preparation for commercial application BACKGROUND The FFCPT method involves dropping an instrumented penetrometer into the seafloor using its gravitational mass as a means of developing sufficient downward force to cause shear failure of the sediments The FFCPT sensors are designed to record the decceleration and the porewater pressure response of the sediment during the penetration process Geotechnical engineering data can be obtained from these measurements useful in characterizing the sediment layering composition and shea
8. T SBT 0 5 100 5001000 A D counts P 17kN m E S P 0 3 F NA DEPTH BELOW SEABED m o N C Line 11 E NA L L COREHEAD ARRESTED L PENETRATION L PAST THIS DEPTH VERY SOFT TO SOFT SILTY CLAY TO CLAYEY SILT 2 L MAXIMUM DEPTH 10 4 m 0 10 A m s 20 15 Figure 11 Raw data time series for FFCPT 013 location The tool was deployed in free fall using the piston coring trip arm with a free fall distance of about 2 4m The accelerometer data show the deceleration response as the system penetrates the seabed coming to rest at about 3 sec Chrisfian Situ Brooke Ocean a A i lt BOT Bo0k oc Ltd GSC ATLANTIC HUDSON 2004030 STATION 013 TORBROOK BLOCK SCOTIAN SLOPE FFCPT 013 DAY 198 TIME 1907 WATER DEPTH 1581 m ok De o o 1000 E MUDLINE DETECTOR 800 600 400 200 OPTICAL OUTPUT counts 0 H 1 1 j 1 1 1 1 1 1 1 1 1 0 1 2 3 50 r 10 50g ACCELEROMETER a CALCULATED VELOCITY 20 VELOCITY m s ACCELERATION m s DIFFERENTIAL GAGE CALCULATED DEPTH 40 E 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 1 2 3 TIME sec DIFFERENTIAL PRESSURE m 16 Figure 12 Summary interpreted engineering log for FFCPT 013 location Note the large excess porewater pressure response from a
9. UN AE ESTA AT EEL T E P E CEMENTED j LA al FIRM SILTY CLAY DEPTH BELOW SEABED m N 12 P 17kN m F S P 03 L NC Line b y VERY SOFT TO SOFT SILTY CLAY TO CLAYEY SILT 3 L MAXIMUM DEPTH 11 75 m 19 DISCUSSION The key findings of the field testing are summarized below in Table 2 Overall the equipment functioned as designed was easily deployed and recovered and produced useful geotechnical datasets Some further work is required to improve the quality of the data and to compare it to shear strength test results measured in high quality core samples still in progress Table 2 Summary of key testing objectives and findings along with suggested actions to remedy perceived problems either with the mechanical and electronic equipment or with the method of data interpretation Mechanical Issues System handling Mounting split coupler on core barrel very difficult in rough seas significant pinch hazard to fingers Install centering ring on top end cap to guide FFCPT onto core barrel fabricate custom web sling for chain falls to support FFCPT at rail Corer rigging To obtain free fall it was necessary to attach recovery to top half of split piston inside core barrel Fabricate custom open centre piston for FFCPT work which allows more water flow and fits core barrel with no core liner
10. ail Pressure channel The tip differential pressure is already compensated for hydrostatic pressure since its back side is open to the water column inside the core barrel so it is a direct high resolution measurement of the excess dynamic porewater pressure response to FFCPT penetration Typically in very soft to soft clay the dynamic porewater pressure response is in excess of hydrostatic due to the low permeability of sediment surrounding the probe The undrained shear strength is calculated from the dynamic penetration resistance which is obtained from Newton s Law F mA A second fully independent estimate of undrained shear strength is calculated from the dynamic porewater pressure Both of these strength calculations utilize published empirical relationships developed from numerous analyses of static cone penetration test data One long term objective is to develop greater understanding of the relationship between the CPT cone and pore pressure factors used in evaluating undrained shear strength as it applies to tests conducted at high rates of penetration as is the case with the FFCPT To date there seems to be very close agreement between the various cone penetrometers especially with respect to the porewater pressure prediction of undrained shear strength It is also possible to calculate a continuous profile of sediment type by plotting FFCPT analysis parameters excess pore pressure ratio and normalized dynamic penetratio
11. bout 7 to 11m due to the water hammer effect A thin stiffer layer was encountered from 0 to 2m below seabed which was also observed in adjacent piston core samples Chrislian Situ GSC ATLANTIC A as HUDSON 2004030 STATION 013 TORBROOK BLOCK SCOTIAN SLOPE 4 BOT Brooke Ocean PA FFCPT 013 DAY 198 TIME 1907 WATER DEPTH 1581 m DYNAMIC DYNAMIC UNDRAINED DYNAMIC NORMALIZED PENETRATION PORE SHEAR PORE DYNAMIC DESCENT RESISTANCE PRESSURE STRENGTH PRESSURE PENETRATION VELOCITY OBS Qa Uj S PARAMETER RESISTANCE amp ACCEL OUTPUT SBT kPa m kPa B Qn V m s A D counts 1000 0 10000 0 20 40 60 1 6 0 8 0 0 0 8 1 60 50 1000 5 100 5001000 _ SaaS E E CEMENTED 1E E L E H FIRM r E C n E ij T SILTY CLAY 21 h F f E t RI p 17kN m f F 3 ri s P 03 E F E E K F al E C E E a E E i 5E t E E F q E F N et s E L E E TO SOFT g p E 5 N SILTY CLAY O 7t C L L E GUNEY SILT L L H L ee f zs 8p y K s y E F w YE f y y n x a F 10 F F y F T 11 E L L Li L E 12 E E C o U N C Line 3 L MAXIMUM DEPTH 12 3 m 17 Figure 13 Raw data time series for FFCPT 015 location The tool was deployed in free fall using the piston coring trip arm with a free fall distance of about 4 0m The accelerometer data show the deceleration re
12. cable inside core barrel when piston in use Carry probe into lab for data transfer fabricate longer deck cable for connecting to probe at rail Data Interpretation Issues Mudline detector High frequency noise on Mudline Detector signal Evaluate electronics on bench hardware modification as required Accelerometers Terminal velocity Differential pressure Movement of piston inside core barrel produces high frequency noise on accelerometers Not achievable with 4 m of free fall due to hydrodynamic drag effect Dominated by water hammer when system deployed in free fall Apply high frequency filter to acceleration data Increase free fall to 10m streamline corehead Install air surge chamber on negative side of differential pressure gage FFCPT Interpretation Software Not properly configured for Harpoon system Modified FFCPT View software to produce special Harpoon View version REFERENCES Robertson P K 1990 Soil classification using the cone penetration test Canadian Geotechnical Journal 27 1 pp 151 158 21 APPENDIX A HUDSON 2004030 FFCPT SUMMARY LOG STATION FFCPT 001 Datafile HUDO100 b01 Day 193 Time 1006 GMT Site St Pierre Slope The FFCPT was configured to trigger on the Mudline Detector at a level of 1 600 mV and rearm on the Mudline Detector at a level of 1 300 mV The equipment was winched into the bottom at a speed of 97 m s and pen
13. correspond to a test where the coring system was winched lowered into the sediment column The subsequent figures 11 through 16 show results from the following two tests which were done by free falling the coring system into the seafloor The system operated well and had no electronic or mechanical problems It was concluded that the best trigger method was to use the tip differential pore pressure channel set to a trigger threshold of 5 m in head Rearming was best done using the Tail Pressure water depth channel set to a value much greater than the maximum water depth which ensured that the system rearmed itself after all trigger events The maximum operating range of the equipment is 3 400 m however the maximum water depth encountered was only 1 581 m In general the sites tested comprised soft normally consolidated silty clays to clays in some cases there was a stiff layer present at the seafloor extending to several metres in thickness The differential porewater pressure measurements were used to evaluate the undrained shear strength profile The dynamic penetration resistance profile was calculated from the vertical acceleration data 12 and was also used to predict the undrained strength profile Sediment Behaviour Type SBT was predicted from CPT classification charts based on the combined dynamic penetration resistance and dynamic pore pressure response 13 Figure 9 Raw data time series for FFCPT 005 location The tool was
14. died for deployment Figure 2a FFCPT Mounted on the end of the GSCA large diameter piston corer at the rail in preparation for lowering Figure 2b FFCPT on core barrel ready for lowering Figures 3 shows a photograph of the piston corehead Figure 4 shows the coring system being rotated to a vertical position and Figures 5 and 6 show the trip arm and gravity corer mechanisms being fastened to the wire at the corehead Once deployed and allowed to impact the seafloor the winch is used to recover the system to the ship Once at the rail the trip arm is removed from the wire and the corehead is lifted into its cradle Then the entire piston coring system is rotated back up to the horizontal position for cleaning and removal of the FFCPT tool Figures 7 and 8 At present the FFCPT must be taken into the lab after each recovery to download the data In future data downloading will be possible at the rail The entire operation is highly efficient and takes less time than is needed for taking a physical core sample Since there are no core samples inside the core barrel all that needs to be done is to download the data each time There is sufficient capacity built into the FFCPT datalogger for conducting over 100 tests before recovery is actually required This makes collection of in situ geotechnical data highly cost effective especially at deep water locations where lowering and raising operations can consume hours of shiptime
15. etrated to the corehead 10 4 m There was no damage The datalogger recorded one datafile HUDO100 b01 which was created with the system still at the rail The OBS signal levels in the datafile never fell below 1 343 mV so the probe did not rearm after being triggered at the rail In future the rearm level should be set as high as possible so that it will always rearm Testing on the bench showed that the response of the Mudline sensor is dependent on the reflectivity of the material near the OBS port White paper and orange plastic were found to induce triggering ie signals exceeded the trigger threshold however dark green rubber did not The signal response of the detector has a peak in a certain wavelength of light as indicated by previous datasets sometimes showing signals falling off to the baseline with penetration into darker coloured sediment It was concluded that the Mudline Detector could not be relied upon to trigger the probe as the seafloor sediment colour is often quite dark Bench testing in the shop should be done to determine the range of sensitivity of the mudline detector in various colours of sediment The system penetrated soft clay and came to rest with the corehead at the mudline so the penetration process was incomplete STATION FFCPT 005 Datafile HUD0504 b01 Day 196 Time 1935 GMT Site Mohican Channel The FFCPT was configured to trigger on the Tip Differential Pressure at a level of 3 m and rearm o
16. in the raw data times series depicted in Figures X and X wherein the acceleration at the time of impact was about 50 less than 10 m s2 or 1g This indicates that the downward gravitational forces are severely limited by hydrodynamic form drag on the corehead A quick calculation indicates that the drag coefficient for the corehead is at best 0 5 at the time of impact Taking into account an acceleration baseline at impact that is less than 1g a slight modification to the data analysis was required The algorithm for integrating the area beneath the acceleration time curve used a straight line varying baseline drawn between the measured acceleration at impact and that when the system had come to rest by definition 1g This adjustment in the analysis consistently yielded an accurate calculation of the total depth of penetration which was compared to measured mud marks on the outside of the piston core barrel The calculation of dynamic penetration resistance was likewise modified to shift the starting acceleration to be equal to 1g as otherwise negative forces result from the data processing An equivalent interpretation would also work wherein all acceleration data are referenced to Og instead of 1g For the purpose of this testing the former adjustment was made 11 Dynamic Porewater Pressure Response The dynamic tip pressure response measured just behind the probe tip shows a deceleration effect in tests at locations FFCPT 013 and
17. n resistance on a CPT classification chart commonly known as a Bq 10 Qn chart Data from each test are plotted and each data point is assigned a numeric code according to which region or Sediment Behaviour Type it falls into This chart has been developed based on thousands of CPT soundings and sampled boreholes for which the soil classification was available More experience is needed with FFCPT data before it can be concluded that the chart is also valid at high penetration rates To date there appears to be close agreement between FFCPT SBT predictions and actual sediment type based on grain size testing Dynamic Penetration Resistance The FFCPT probe was mounted on the end of the piston corer and allowed to free fall into the seabed to test the acceleration based data interpretation method Normally the FFCPT falls a considerable distance through the water column before impacting the seafloor and thereby achieves a constant terminal velocity at which time by definition vertical accelerations are zero This zero acceleration is equivalent to a free fall condition However with the Harpoon FFCPT configuration there are large hydrodynamic drag forces that act on the piston coring head which is not very streamlined In fact it appears that at the time of impact the gravitational forces transmitted into the sediment were about half what they would have been had there been no hydrodynamic drag on the system This is illustrated
18. n the Tail Pressure at a level of 2 000 m The equipment was winched into the bottom at a speed of 97 m s and penetrated to the corehead 10 4 m There was no damage 22 The datalogger recorded 24 datafiles some of which were created by false trigger events during lowering and raising through the water column Several data records were obtained recording the impact event HUD0504 b01 the system at rest in the seabed HUDO505 b01 and HUD0506 b01 and the pullout event HUD0507 b01 The rearming worked well and ensured the probe recovered into a ready state after each false trigger event In future the trigger level could be increased to 5 m to prevent false triggering The data buffer split of 50 50 was also too biased toward the pre trigger data in future deployments using the winch in method a 20 80 split should be used especially if more core barrels are mounted The pore pressure response was very good however there was some surging due to vessel heave that was transmitted down the wire This surging was also observed on the accelerometer signals which did not vary much from 1 g until the corehead impacted the mudline The weight of the system was carried on the wire up until that time indicating that longer penetration was possible had more core barrels been in place The system penetrated soft clay and came to rest due to friction buildup on the core barrels The accelerometer signal will likely not be very useful in determi
19. n the bench in the forward lab onboard CCG Hudson during the July 2004 GSCA cruise to the Scotian Slope Figure 1b Mounting the FFCPT on the end of the GSCA large diameter piston corer at the rail in preparation for deployment The probe houses electronics and hydraulic systems for sensing water pressure around the tip during penetration dynamic porewater pressure hydrostatic pressure at the top of the instrument open to the water column optical backscatter at a location just behind the conical point mudline detector as well as acceleration over 3 ranges 2g 5g 50g Data is captured into two files after each trigger event The system is activated to capture data initially into a low speed 25 Hz buffer file This file is later used to check calibration constants for the Tail Pressure gage Once the system senses that the preset trigger condition has been met it also begins logging data into a high speed 2 kHz buffer After the buffer is full maximum duration of 20 925 sec the system switches back to low speed and checks to see if it should rearm itself for another high speed data capture Once the rearm condition is met it will go into a standby mode awaiting another trigger event Triggering can be done off any of the channels at any signal level Similarly rearming can be done from any channel at any signal level Ideally the system should be set so that it always remains armed for data capture following any
20. ning the depth of penetration when the system is lowered on the winch in this manner It may be advantageous to free fall the system with a longer core barrel to generate higher decelerations for integration However the tip pore pressure data are acceptable STATION FFCPT 013 Datafile HUDO1332 b01 Day 198 Time 1907 GMT Site Torbrook Block The FFCPT was configured to trigger on the Tip Differential Pressure at a level of 3 m and rearm on the Tail Pressure at a level of 3 000 m The equipment was rigged like a piston corer using the upper half of the split piston A free fall distance of 2 4 m was used The safety wire was removed No water hose was used inside the core barrel The system penetrated to 13 2 m There was no damage The datalogger recorded 41 datafiles most of which were created by false trigger events during lowering and raising through the water column Several data records were obtained recording the impact event HUDO1332 b01 the system at rest in the seabed HUD01333 b01 HUDO1334 b01 and the pullout event HUDO1335 b01 HUDO1336 b01 HUDO1337 b01 The system was triggered at 5 m on the differential gage and rearmed at 3000 m on Tail Pressure A 50 50 pre trigger post trigger data buffer split exactly the impact event The pore pressure response was very good with no vessel heave effect during penetration into the seabed There was excessive hydrodynamic drag created by 23 the bullnose shape of the c
21. orehead which prevented it from reaching terminal velocity before impact Accelerations were slowly increasing toward 0 5g during the free fall period The entire penetration through the sediment lasted about 3 sec The system came to rest in soft clay largely due to friction buildup on the core barrels A stiff layer several metres in thickness was noted at the mudline STATION FFCPT 015 Datafile HUDO1503 b01 Day 199 Time 1955 GMT Site Torbrook Block The FFCPT was configured to trigger on the Tip Differential Pressure at a level of 3 m and rearm on the Tail Pressure at a level of 3 000 m The equipment was rigged like a piston corer using the upper half of the split piston A free fall distance of 2 4 m was used The safety wire was not used No water hose was used inside the core barrel The system penetrated to 12 0 m There was no damage The datalogger recorded 14 datafiles some of which were created by false trigger events during lowering and raising through the water column Several data records were obtained recording the impact event HUDO1503 b01 and while the system was at rest in the seabed HUDO1504 b01 but none during pullout The system was triggered at 5 m on the differential gage and rearmed at 3000 m on Tail Pressure A 50 50 pre trigger post trigger data buffer split exactly the impact event The pore pressure response was very good with no vessel heave effect during penetration into the seabed Excessive hyd
22. r strength profile Such data are required for design of deepwater suction pile foundations for tethered leg platforms and FPSO s The FFCPT yields geotechnical measurements that are directly comparable to conventional cone penetration test CPT methods used in current site investigation practice by industry Traditionally these data have only been obtainable from specialized deepwater site investigation vessels The long term objective of this project is to develop a commercially viable alternative method of geotechnical site investigation that is deployable from a vessel of opportunity EQUIPMENT AND METHODS The configuration in this project employed the GSCA large diameter piston coring system as a delivery device The FFCPT probe was mounted on the end of the core barrel in place of the cutting shoe The FFCPT probe operates fully autonomously in this configuration wherein system operational parameters such as trigger channel trigger threshold rearming channel rearm threshold and data buffer characteristics are set at the time of probe activation The penetrometer is mounted on the end of the piston core barrel after the internal computer has been given a set of preset trigger thresholds and test parameters Figure 1 shows two photographs of the instrument a on the bench being configured for testing and b being mounted on the core barrel which is held horizontally at the ship s rail Figure 1a Setting up the FFCPT o
23. rodynamic drag created by the bullnose shape of the corehead again prevented the system from reaching terminal velocity before impact Accelerations were slowly increasing toward 0 5g during the free fall period The entire penetration through the sediment lasted about 3 sec The system came to rest in soft clay largely due to friction buildup on the core barrels A stiff layer several metres in thickness was noted at the mudline 24
24. sponse as the system penetrates the seabed coming to rest at about 3 sec OPTICAL OUTPUT counts ACCELERATION m s DIFFERENTIAL PRESSURE m Chrisfian Situ alo BOT cean GSC ATLANTIC HUDSON 2004030 STATION 015 TORBROOK BLOCK SCOTIAN SLOPE FFCPT 015 DAY 199 TIME 1955 WATER DEPTH 1200 m S Q o 1000 E 800 600 400 200 f 50 p 40 30 f MUDLINE DETECTOR 20 L 50 g ACCELEROMETER CALCULATED VELOCITY 35 E 40 DIFFERENTIAL GAGE CALCULATED DEPTH 1 2 TIME sec VELOCITY m s 18 Figure 14 Summary interpreted engineering log for FFCPT 015 location Note the large excess porewater pressure from about 5 to 10m due to the water hammer effect A stiffer layer was encountered from 0 to about 1 5m which was also observed in an adjacent piston core sample Chrislian Situ GSC ATLANTIC i als HUDSON 2004030 STATION 015 TORBROOK BLOCK SCOTIAN SLOPE BOT Brooke Ocean __ FFCPT 015 DAY 199 TIME 1955 WATER DEPTH 1200 m a DYNAMIC DYNAMIC UNDRAINED DYNAMIC NORMALIZED PENETRATION PORE SHEAR PORE DYNAMIC DESCENT RESISTANCE PRESSURE STRENGTH PRESSURE PENETRATION VELOCITY OBS Qa Uj S PARAMETER RESISTANCE amp ACCEL OUTPUT SBT kPa m kPa B Q V m s A D counts 1000 0 20 40 60 1 6 0 8 0 0 0 8 160 50 1000 5 100 5001000 pr T ETTTUTT TT ri oe
25. y takes about 3 seconds with 50 ft of core barrel 12 to 13 m of actual penetration Test 2 successfully demonstrated that the system can be lowered into the seafloor using the ship s winch and can achieve almost as the same amount of penetration as when the system is deployed in free fall by tripping the piston corer tests 3 and 4 The depth of penetration for test 1 was limited by the length of core barrels installed on the system three 10 ft long barrels The data record showed the corehead impacting the seafloor which prevented deeper penetration It is likely that had 5 barrels been installed the ultimate depth of penetration would have been very similar to tests 3 and 4 wherein the system was deployed in free fall Table 1 Summary of FFCPT test data for field trials onboard CCG Hudson during GSCA 2004030 cruise PINGER CORER LENGTH PINGER HT OFF BOT IMPACT TRIGGER CHANNEL TRIGGER LEVEL REARM CHANNEL TO CORER LATITUDE LONGITUDE GEOGRAPHIC LOCATION JULIAN WATER WINCH NO AT IMPACT AT IMPACT REGION NAME DAY TIME DEPTH SPEED FALL m m min m 001 44 41 8696N 54 27 2130W Grand Banks St Pierre Slope 193 1006 1 013 97 NA 005 42 50 0880N 62 03 1020W Scotian Slope Mohican Channel 196 2009 894 97 NA 42 33 0707N 62 28 6169W Scotian Slope Torbrook Block 198 1907 1 581 TRIPPED 2 4 42 44 2752N 62 05 0831W Scotian Slope Torbrook Block 1
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