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STEWART TECHNOLOGY ASSOCIATES
1
Mat-Supported Jack-Up Foundation On Soft Clay – Overturning Storm Stability
Mat-Supported Jack-Up Foundation On Soft Clay – Overturning Storm Stability
STEWART TECHNOLOGY ASSOCIATES
2
Relatively Mild Storm EnvironmentWind 90 knotsWave 9 meterCurrent 2 knots
Very soft clay soil
Very Active Seismic Area
Relatively Mild Storm EnvironmentWind 90 knotsWave 9 meterCurrent 2 knots
Very soft clay soil
Very Active Seismic Area
STEWART TECHNOLOGY ASSOCIATES
3
Mat Supported Jack-UpBuilt in Late 1970’s
Converted for Gas ProductionMadura - Indonesia
Fixed Platform Design Requirements
ABS Class Rules
Mat Supported Jack-UpBuilt in Late 1970’s
Converted for Gas ProductionMadura - Indonesia
Fixed Platform Design Requirements
ABS Class Rules
STEWART TECHNOLOGY ASSOCIATES
4
Initial Sea Bed Mat Penetration Larger than Predicted
Soil Strengths Questioned
Method of Predicting Overturning Resistance Questioned
Initial Sea Bed Mat Penetration Larger than Predicted
Soil Strengths Questioned
Method of Predicting Overturning Resistance Questioned
STEWART TECHNOLOGY ASSOCIATES
5
Very Detailed Site Investigation of Mat-Effected Zone
In Situ Soil Strength MeasurmetsCPT + T-Bar + Vane
Method of Predicting Overturning Resistance Re-Visited
Very Detailed Site Investigation of Mat-Effected Zone
In Situ Soil Strength MeasurmetsCPT + T-Bar + Vane
Method of Predicting Overturning Resistance Re-Visited
STEWART TECHNOLOGY ASSOCIATES
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Don Murff, Consultant to GEMS
Alan Young, GEMSDan Spikula, GEMS
Jean Audibert, Quest Geo-Technics
Jack Templeton, Sage USA
Vladimir Rapoport, Consultant
Don Murff, Consultant to GEMS
Alan Young, GEMSDan Spikula, GEMS
Jean Audibert, Quest Geo-Technics
Jack Templeton, Sage USA
Vladimir Rapoport, Consultant
COL1
COL3
COL2
1
BH1
BH2 BH3
BH7
BH8
BH9
BH10
BH14
BH15
BH4
56ft.
40ft.
25ft.
9ft.
22ft.
34ft.
A
A
59.00ft.
STBD AFT CORNER
PORT FWD CORNER
60ft.
20ft. 34ft.
65ft.
BH6
BH16
BH17
-5 ft
0 ft
5 ft
10 ft
15 ft
20 ft
25 ft
30 ft
35 ft
40 ft
0 psf 50 psf 100 psf 150 psf 200 psf 250 psf 300 psf 350 psf 400 psf 450 psf 500 psf 550 psf
BH1, BH3 and Consensus Su, From Roson (T-bar Nkt=10.5) Relative to BH3 Mat Top Plate - PORT FWD CORNER.
Dep
th b
elo
w T
OP
PL
AT
E O
F M
AT
Soil Start Record BH10
Skirt Tip Elevation
BH1 T-bar Nkt = 10.5
BH3 T-bar Nkt = 10.5
Consensus
MAT and SKIRTS(Top Plate of Mat is Zero Depth Reference on this chart)
3.40 ftDifference Between BH10 Zero Elevation and BH3 Mat Top Plate = Elevation match made with Su spike at 38.4' below mat top plate.
Elevation match made with Su spike at 38.4' below mat top plate seen in BH3. No spike seen in BH4.
Possible Mat Penetration Stages and Failure Zones with Borehole Locations Indicated and Displaced Soil Partly Derived from SI ResultsFE Solution is for Initial Two Feet of Penetration (Skirt tips penetrated four feet)
Final Estimate of Penetration at Port Fwd Corner of Mat = 6.4 feet (WPS)
32.0ft.
6.40ft.
3.60ft.
1.9ft.
10.00ft.
40ft below mat top plate
BH10
BH9 BH15 BH14BH8 BH7
BH2BH3
BH4
Initia
l pos
sible
Prand
tl She
ar P
lanes
Final p
ossib
le Pra
ndtl S
hear
Plan
es
Initial mat position
Final mat position
Soil Heave Levels Not Known in This Area
Soil area (volume) displaced as inferred from borehole records = 125 sqft.
6.4 feet penetration would displace 6.4 x 32/2 = 102 sqft on each side.
The factor of safety against overturning, or OTSF, is defined as:OTSF = (SRmoment – Wmoment)/OTmomentWhere:SRmoment = soil ultimate capacity resisting momentWmoment = (weight – buoyancy) momentOTmoment = overturning moment from environmental forcesIt must be noted that the above definition of the OTSF is more accurately a definition of first yield in typical normally consolidated soft clays, where the increasing strength with depth may result in increasing resistance following first yield as deeper penetration occurs.
