On Bottom Stability

Onbottom Stability of Jackets

5/24/2014 Dr. S. NallayarasuDepartment of Ocean Engineering

Indian Institute of Technology Madras-36

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ONBOTTOM STABILITY OF JACKETS




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OUTLINE Onbottom Stability Piling Sequence Mudmat Concepts Stability

Requirements Design




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ONBOTTOM STABILITY What is Onbottom Stability ?When the jacket is floated and upended from horizontal floating position, it shall stand vertically on the seabed. The stability of the same shall be maintained until its is fixed on to the seabed by piles. This temporary phase is called Unpiled Stability or Onbottom Stability.

The jacket with pile segment and hammer should be able to stand without, sliding, settling and overturning due to external forces.




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PILING SEQUENCE Planning a piling sequence can reduce the offshore construction and installation time to considerable extent.

Since the piles or pile segment and hammer is temporarily supported on to the jacket, the weight of the same needs to be taken in to consideration during the onbottom stability.

Further, during this period, external environmental forces from wave, current and wind also needs to be considered.




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A1 A2

B1B2

Preferred Piling SequenceFor example, if four corner piles (A1, A2, B1 and B2) needs to installed on to a jacket, following sequence can be adopted. extent.

Each time the crane lifts the pile including rigging and de-rigging, the handling time approximately 3 to 6 hours. This is due to manual handling of rigging for the pile and hammer.

To avoid, multiple rigging and de-rigging activities, one would consider the piling sequence 2 (refer to table)




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Preferred Piling SequenceID Piling sequence 1 Piling sequence 2 ID

1A Place pile at corner A1 and release the crane hook.

Place pile at corner A1 and release the crane hook.

1A

1B Lift hammer place on top of pile at corner A1 and drive to target penetration

Place pile at corner B2 and release the crane hook.

2A

2A Place pile at corner B2 and release the crane hook.

Place pile at corner A2 and release the crane hook.

3A

2B Lift hammer place on top of pile at corner B2 and drive to target penetration

Place pile at corner B1 and release the crane hook.

4A

3A Place pile at corner A2 and release the crane hook.

Lift hammer place on top of pile at corner A1 and drive to target penetration.

1B

3B Lift hammer place on top of pile at corner A2 and drive to target penetration

Lift hammer place on top of pile at corner B2 and drive to target penetration

2B

4A Place pile at corner B1 and release the crane hook.

Lift hammer place on top of pile at corner A2 and drive to target penetration

3B

4B Lift hammer place on top of pile at corner B1 and drive to target penetration

Lift hammer place on top of pile at corner B1 and drive to target penetration

4B




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PILING SEQUENCE

1A 2A 3A 4A




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9




10




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Case Set 100Maximum Jacket weight Installation Wave and current (8 directions)

Case Set 200 Maximum Jacket weight Installation Wave and current (8 directions)Main pile section P1 inserted into B2 Leg

Case Set 300 Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 LegMain pile section P1 inserted into A1 Leg

Case Set 400Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 LegMain pile section P1 inserted into A1 LegMain pile section P1 inserted into B1 Leg

Case Set 500Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 LegMain pile section P1 inserted into A1 LegMain pile section P1 inserted into B1 LegMain pile section P1 inserted into A2 Leg

Case Set 600 Maximum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into B2 LegMain pile section P1 inserted into A1 LegMain pile section P1 inserted into B1 LegMain pile section P1 inserted into A2 Leg Pile section P2 stabbed and welded on B2 leg

LOAD COMBINATIONS FOR BEARING CHECKFor bearing pressure check and mudmat design maximum possible gravity loads shall be considered. The dead loads of the jacket shall be considered including contingency applied at that stage.




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Case Set 700

Minimum Jacket weight Installation Wave and current (8 directions)

Case Set 800

Minimum Jacket weight Installation Wave and current (8 directions)Main pile section P1 inserted into B2 Leg.

Case Set 900

Minimum Jacket weight Installation Wave and current (8 directions) Main pile section P1 inserted into A1, B1 and A2 Leg Pile section P1+P2 stabbed and welded on B2 leg

LOAD COMBINATIONS FOR STABILITY CHECKFor sliding and overturning stability checks minimum gravity loads applicable shall be considered. The dead loads of the jacket shall be considered without any contingency.




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Ultimate Bearing Capacity

5 1 0.2 1 0.2 eu ue e

D Bq SB L

Where Su Undrained shear strength at 0.5Be

from the bottom of the MudmatBe Effective Mudmat widthLe Effective Mudmat lengthD Depth of Embedment of the

Mudmat below seabed

Undrained shear strength at depth 0.5Be below the Mudmat bottom shall be evaluated using the linear interpolation of the shear strength of layers.




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Mudmat Mudmats are temporary floor support for the jacket immediately after the jacket has been upended from floating horizontal position prior to supported by piles.

Need to designed with adequate surface area and sufficient strength strength to avoid excessive settlement of the jacket.

Usually made of steel plate and reinforced by steel beams. However, alternate materials like Timber and FRP has been used to reduce weight and cost




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Advantages of

FRP and Timber Mudmat

FRP and Timber mudmats are used when lift weight is a concern. They will reduce the weight considerably.

The design requirement for Cathodic Protection will also be reduced




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Large Timber Mudmat




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FRP Mudmat




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MUDMAT CONCEPTS




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Jacket with Rectangular Mudmat




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Triangular Mudmat




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Rectangular Mudmat




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Circular Mudmat




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Triangular Mudmat




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Mudmat Panels

Mudmat panels can be any one of the following. Flate Plate (Steel) Corrugated Plate (Steel) Timber Plank Profiled Panel (FRP)

These panels will be appropriately supported by steel structural members attached to the jacket structure




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Flat Steel plate




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Timber Plank




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Corrugated Steel plate




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FRP PANEL




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Design Requirements

When the jacket is resting on seabed, it shall satisfy following requirements

Stability against bearing Stability against sliding Stability against overturning Structural members shall have adequate strength




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Design Loads

Dead loads Bouyancy Loads Wave and Current Loads Wind Loads Loads from Pile stabbing sequence




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Design Requirements When the jacket is resting on seabed, it shall satisfy following requirements (API RP 2A)

Stability against bearing Stability against sliding Stability against overturning

Sometimes it is also called Unpiled Stability since this is prior to the piling of the jacket after which the jacket is firmly fixed to the seabed by piles




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Stability Against Bearing As explained earlier, stability against bearing is to have adequate bearing area to avoid excessive settlement of jacket / failure of mudmat. This has two parts.

Geotechnical Requirement Structural Requirement




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Factor of Safety against Bearing The Factor of Safety against bearing shall be calculated as below.

. . ua

QF O SP

The minimum Factor of Safety shall be 2.0 for loads arising from dead weight of the jacket only and 1.5 for dead weight + environmental loads.

Where Qu is the ultimate bearing capacity of soil and Pa is the applied pressure




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Applied Mudmat Pressure (Dead Load) The applied mudmat pressure can be calculated for dead loads alone very easily.

2S x S

a

M yy

W eW HPA I

Where WS is the total submerged weight of the jacket including ballast water on any compartments of legs, buoyancy tanks and AM is the total mudmat area

If the Jacket is not symmetrical and has self weight acting at an eccentricity of ex, and not at the geometric centre of mudmat, then the effect shall be included as moment component.




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Applied Mudmat Pressure

(Dead Load + Environment Load) The applied mudmat pressure can be calculated for dead loads alone very easily.

2 2S x S e

a

M yy yy

W eW FhH HPA I I

Where Fe is the total environmental loads from wave, current and wind and h is the height from seabed at which the environmental loads are applied and Iyy is the moment of inertia of the mudmat system about YY axis.




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Factor of Safety against Sliding The Factor of Safety against sliding shall be calculated as below.

. . se

WF O SF

Where Fe is the total environmental loads applied and is the friction coefficient between the soil and mudmat system.

The minimum FOS of 1.5 shall be required.




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Factor of Safety against Overturning The Factor of Safety against Overturning shall be calculated as below (for each edge).

. . se

W xF O SFh

Where x is the distance between the vertical load (jacket submerged weight) and the geometric centre of mudmat system at mudline.





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SLIDING STABILITY IN UNDRAINED CONDITION

API RP 2A requires the mudmat sliding stability to be considered using undrained methodology.

. . u me

C AF O SF

Where Fe is the total environmental loads applied and Cu is the undrained shear strength and Am is the total area of the mudmat.





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COMBINED HORIZONTAL AND VERTICAL STABILITYThe revised APRI RP 2GEO requires consideration of combined effect of horizontal and vertical loading on the stability of jackets.

The FOS against this combined loading for drained and undrained conditions are shown in figure below.




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Jacket Settlement Most of Settlement will take place immediately after the jacket has been placed on seabed.

Hence the only immediate settlement using elastic theory will suffice.




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W Fe




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Jacket Settlement Settlement of jacket is an important criteria in designing the mudmat system as excessive settlement woill lead submergence of bottom framing in to the soil. This will lead following issues.

The mudline framing will be subjected to constant upward force on the members

The conductor guide if any will be submerged in to mud thus driving conductors will become difficult

Boulder if present at shallow depth may damage structural braces

The jacket cut-off level will get affected




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Jacket Settlement Elastic settlement of jacket on to the seabed can be calculated as below.

2(1 )s

qB IE

Where q is the uniform applied pressure, B is the width of the mudmat, E is the Modulus of the soil, is the poissons ratio and Is is the influence coefficient and shall be calculated depending on the shape of the mudmat.




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Settlement of Circular Footing

Vertical settlement of circular footing is given by

QGR

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Where

= Vertical displacementQ = Vertical load

G = elastic shear modulus of the soil

= poissons ratio of the soilR = radius of the base




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bhAm 4

23

)2/2/(4124 bBbhhbI yy

yyxxm

sa I

xMIyM

AWP )()(

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)2/2/(4124 hHbhbhIxx

Where x and y are co-ordinates of points at which the mudmat pressure is required

Rectangular Mudmat system




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2 244 4 264 4yy D BI D 224 244644 HDDIxx

Circular Mudmat system

2

44 DAm

yyxxm

sa I

xMIyM

AWP )()(




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3 24 12 2 336yy bh BI bh b 3 24 12 2 336xx bh HI bh b

Triangular Mudmat system

24 bhAm

yyxxm

sa I

xMIyM

AWP )()(




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3 23 2 2 248yy bh bBI bh 223 31322322363 hHbhhHbhbhIxx

Triangular Mudmat system

24 bhAm

yyxxm

sa I

xMIyM

AWP )()(




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Determine the factor of safety against bearing, sliding and over turning for the jacket shown in the sketch subjected to environmental forces as shown in the table. Estimate the immediate settlement of jacket.

W = 12000 kNX COG = -6 mY COG = 1.0 m

Direction Force Centre of forceF1 2000 kN 52 mF2 2800 kN 45 mF3 2500 kN 48 m




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On Bottom Stability

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