jack-up geotechnical hazards and their mitigation

DNV GL © 2014 OFFSHORE JACK UP MIDDLE EAST - 13TH OCTOBER 2014 SAFER, SMARTER, GREENER DNV GL © 2014 JACK-UP & GEOTECHNICAL TEAM, LONDON

OFFSHORE JACK UP MIDDLE EAST - 13TH OCTOBER 2014

Dr. David Edwards - Principal Geotechnical Engineer

OIL & GAS

Jack-Up Geotechnical Hazards and their Mitigation

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DNV GL © 2014 OFFSHORE JACK UP MIDDLE EAST - 13TH OCTOBER 2014

Introduction to Jack-up & Geotechnical Team

We are:

Part of Noble Denton Marine Assurance & Advisory

Based in London with ‘satellite’ geotechnical team in Singapore

25 engineers (9 geotechnical +51!) and expanding!

Combined >110 years of experience

Primarily focused on

– Providing engineering basis/support for Jack-up Location Approvals

– Jack-up site-specific assessments

– Advanced engineering and geotechnical consultancy for jack-up installations

and operations

– Geotechnical consultancy for Oil & Gas and renewables projects

We use our engineering ingenuity to provide a positive outcome wherever

possible and smart solutions.

We work closely with our marine colleagues for holistic approach

We also provide jack-up and geotechnical training courses to clients

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Jack-up Geotechnical Hazards

Types of Geotechnical Hazards:

Quality of Geotechnical Assessment

Quality of Data

Specific seabed hazards

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Quality of Geotechnical Assessment

Before placing a rig on location you need a Certificate of Approval

From a reputable Warranty Surveyor

Must include comprehensive geotechnical assessment of foundation conditions

Assessment of all geotechnical risks to the jack-up

Practical recommendations for dealing with those risks

Spudcan penetration analysis and foundation capacity check

Jack-up Geotechnical assessments require :

– Geotechnical knowledge and expertise

– Experience of soil conditions in that locality

– Experience to know what risks do exist / could exist

– Understanding of jack-up operations – jack-up site assessments are complex!

– Ingenuity to deliver smarter, safer mitigations to hazards

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Quality of Data

The Warranty Surveyor will need sufficient seabed information to

develop the necessary confidence in their understanding of the

soil conditions at the proposed the jack-up location

Sufficient, good quality seabed data:

Allows an informed, high quality assessment

Avoids nasty surprises and expensive delays during installation

Minimises the need for conservative assumptions and limitations

Minimises the need for cautious recommendations and further assessments

Minimises potential for time and money consuming activities

Allows jack-up to be assessed to its optimum capability

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Quality of Data

Seabed information should comprise:

Recent bathymetry and debris survey

Shallow seismic survey Geophysical site survey

Geotechnical borehole to an adequate depth (and Cone Penetrometer Tests?)

Number of boreholes depends on potential for lateral soil variability

Loads, penetrations and locations of previous rig installations

What constitutes good quality data?

Please refer to our Guideline 0016/ND – Revision 7

“Seabed and Sub-seabed Data Required for Approvals of Mobile Offshore Units (MOU)”

Free to download from:

http://www.dnv.com/industry/oil_gas/rules_standards/noble_denton_rules_guidelines.asp

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http://www.dnv.com/industry/oil_gas/rules_standards/noble_denton_rules_guidelines.asp


Specific Seabed Hazards

A brief list of some of the potential seabed hazards

related to jack-up operations:

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Punch-through Boulders

Scour Unexploded Ordnance

Squeezing of clay layers Buried channels

Lateral variability of soil layers Uneven rock

Spudcan footprints Pipelines / cables

Seabed slopes / unevenness Spudcan-pile interaction

Shallow gas Leg extraction problems

Rack phase differences (RPDs) Limited penetration / fixity

Insufficient bearing capacity Debris

Deep spudcan penetrations (insufficient leg length)

Insufficient sliding capacity


Geotechnical Hazards

PUNCH-THROUGH

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Punch-through

Occurs during preloading

An event where significant vertical

footing displacements are observed

over a relatively short period of time

Caused by spudcan pushing on a

strong layer over weaker layer

Can cause serious damage and even

loss of jack-up unit

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http://www.anzlf.com/viewtopic.php?p=26564




Punch-through risk – Information needed

Risk is due to presence of a stronger layer overlying a

weaker layer

Data required to accurately assess punch-through risk :

Spudcan geometry

Required preload bearing pressure

Soil stratigraphy and properties (borehole)

Understanding of any lateral variability in soil layer

thickness (geophysical site survey)

Spudcan penetration analysis:

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Vertical foundation load during preloading (kips)

Sp

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Vertical foundation load during preloading (tonnes)

Lower Bound

Average

Upper Bound

Spudcan p

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Spudcan s

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Expected spudcan tip penetration= 1.8m (6ft).Range = 1.7m (6ft) to 8.3m (27ft)

Soil Profile

Risk of rapid spudcan

penetrations


Punch-through risk – Spudcan penetration prediction

Upper, average and lower

bound predicted curves

due to scatter and

uncertainty in soil data.

Better data reduces range

in predicted responses

Here there is a risk of

rapid penetrations if the

actual soil strength

approaches the lower

bound inferred profile.

Typically recommend

cautious approach to

preloading.

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Soil Profile


penetrations


Punch-through risk – Mitigation – Reduced preload

Is it necessary to apply full preload?

Perform site-specific assessment of unit for ‘hang-up’ condition

Compare storm footing reactions against foundation bearing capacity envelope

Here preload can be reduced to 4100t and satisfy bearing capacity check

Avoids punch-through and saves time

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Spudcan p

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Spudcan s

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Soil Profile

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Horizontal load capacity (kips)

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Horizontal load capacity (tonnes)

Maximum Hull Weight

Minimum Hull Weight

Factored sliding capacity

Unfactored V-H capacity

Factored V-H capacity

Unfactored sliding capacity

Stillw ater spudcan reaction


Punch-through risk – Mitigation – Reduced preload

Have other rigs been

installed at this location

previously?

Did they have similar sized

spudcans?

Did they apply a higher

bearing pressure?

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Soil Profile


penetrations

Rig X: • Similar spudcans • Preload = 6200t • Penetrations = 2.0m


Punch-through – cautious installation

For location with potential risk,

cautious preloading procedures are

practiced to minimise potential for rig

damage

Preloading procedures such as:

– Leg-by-leg preloading

– Hull at positive draught

– Do not elevate to positive airgap

until all preloading completed

Do not apply simultaneous all leg

preloading at airgap

Further assessments of potential for

damage can be performed

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Punch-through – Survival assessment

If punch-through occurs during

preloading:

– spudcan penetrates rapidly

– Rig leans towards lower leg

– Load redistributes TOWARDS affected

leg

– Increases load on punch-through leg!

– No reduction in leg load until hull

buoyancy arrests movement.

Need to check hull inclination

Bending in legs

Loads in legs

Hull deflection (if next to platform)

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Punch-through

Punch-through survivability

analysis

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Punch-through – seabed modification

Perforation Drilling

Weakens the strong layer

Has been used successfully

Cannot be used for sand over clay

Uncertainty in performance

Expensive if the jack-up is used to drill

the holes (and requires multiple rig

soft pinning positions)

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WEAK CLAY

WEAK CLAY

STIFF CLAY


Punch-through

Excavation

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Punch-through – seabed modification

Original Soil Profile 2m Gravel Pad

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Soil Profile


penetrations

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Soil Profile



SPUDCAN FOOTPRINTS

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Spudcan footprints

Can occur in sand or clay

May be visible on surface

Need recent data

May have been infilled – check for

previous rig history

Infill may indicate a scour location

Spudcan-footprint interaction has been

the cause of serious incidents:

– Rig sliding

– Bent leg braces

– Legs jamming in guides

Monitor leg rack phase differences for

early signs of eccentric support

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SAND

CLAY


Spudcan footprints - examples

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Spudcan Footprints - Mitigation

Avoid!

– Simplest solution, not always

practical

– offset of 1.5 times the diameter of

the spudcan that caused the

footprint is maintained between the

centre of the footprint and centre of

the spudcan to negate any

interaction effects

Seabed Excavations / Dredging

– Create a crater for spudcan to

position within, or

– Even out the seabed

– May compromise integrity of

adjacent structures (platform

foundations, pipelines, etc.)

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SAND SAND


SAND

Spudcan Footprints Mitigation

In-Filling footprints

– Placement material is usually sand or

gravel

– Gravel can cause ‘hard spot’

Gravel Pads

– Expensive

– Infilled footprint can be a ‘hard spot’

– May create Punch-Through risk

Reaming

– Incremental vertical downward and

upward movements of the leg, to

penetrate the spudcan at the target

position overlapping a footprint (max

stroke length of approx. 1m)

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SAND


Spudcan Footprints Mitigation

Stomping

– Objective is to remove asymmetry

– Often most effective method

– Full preload or enough to achieve

75% of predicted penetration depth

– Needs careful planning and

sequencing of stomps

– Requires good weather window to

allow frequent rig re-positioning

National University of Singapore

Spudcan-footprint JIP just completed.

Still in confidentiality period.

– Complex lab model tests and Finite

Element Analyses

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CLAY



SCOUR

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Scour

Partial removal of granular soil below the spudcan

(local) and around the footing (global)

Can occur relatively rapidly (hours/days)

Typically problematic for:

– Shallow water

– Cohesionless seabed soils

– Shallow penetrations

Scour potential is a function of current and wave

velocities, water depth, seabed particle size and

relative density, and spudcan shape

Perform quantitative scour assessment

Recent OSCAR JIP into spudcan scour by Deltares

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Scour can undermine spudcans causing:

Reduced ultimate bearing capacity

Reduced foundation fixities

Additional settlement

Chord and brace overutilisations

Punch-through or rapid penetration

Eccentric loading RPD & Leg sliding

Scour

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SCOUR DEVELOPING


Scour – mitigation measures

Skirted spudcan

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Scour - Mitigation

MINIMUM THICKNESS 0.3 - 0.5m (1 – 1.6 ft)

Gravel Dumping

Gravel Bags / Sand Bags

Prefabricated Mattresses

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Scour - Mitigation

INITIAL DEPOSITION

AFTER 10 DAYS

AFTER 20 DAYS

Frond mats

Can be very effective

Careful installation /

anchoring required

Careful with ROVs !!

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Scour – Automatic monitoring systems

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Scour Monitoring System

Uses sonar to measure distance to seabed


Summary

All jack-up installations require careful geotechnical assessment

Geotechnical assessments must be performed properly and carefully

Needs extensive experience of jack-up operations and soil conditions

Supplying the assessor with sufficient, good quality data is smart and safe – it

reduces the risk to the people, the jack-up and the project

Numerous geotechnical risks exist that can damage jack-up units

For most geotechnical risks there is a range of solutions and mitigation measures

Not every solution is appropriate for every site

However ‘office-based’ solutions to a risk can avoid the need for expensive

mitigation measures in the field.

e.g. avoiding a punch-through risk by applying a reduced preload based on a site-

specific storm assessment.

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SAFER, SMARTER, GREENER

www.dnvgl.com

Thank you for your attention

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David Edwards – Jack-up & Geotechnical Team

[email protected]

+44 207 812 8792

mailto:[email protected]

jack-up geotechnical hazards and their mitigation

Documents

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