4_Mudmat[1]

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30 May 2008 Dr. S. NallayarasuDepartment of Ocean Engineering

Indian Institute of Technology Madras-36

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Mudmat Concepts and Design

OUTLINE FOR SESSION 10Mudmat

ConceptsStability RequirementsDesign

Special FoundationsBucket FoundationsGravity Foundations



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MudmatMudmats 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

Profilled 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 BearingAs 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 BearingThe Factor of Safety against bearing shall be

calculated as below.

. . u

a

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 e W HPA I

= +

Where WS is the total submerged weight of the jacket including ballast water on any compartments of legs, bouyancy 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 e W F hH 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 SlidingThe Factor of Safety against sliding shall be

calculated as below.

. . e

s

FF O SWµ

=

Where Fe is the total environmental loads applied and µ 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 OverturningThe Factor of Safety against Overturning shall be

calculated as below (for each edge).

. . e

s

F hF O SW x

=

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

The minimum FOS of 1.5 shall be required.



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Jacket SettlementMost 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 SettlementSettlement 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 SettlementElastic settlement of jacket on to the seabed can

be calculated as below.

2(1 )s

qB IE

δ ν= −

Where q is the uniform applied pressure, B is the width of the mudmat, E is the Modulus of the soil, ν 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

⎟⎠⎞

⎜⎝⎛ −

=41 γδ

QGR

uv ⎟⎠⎞

⎜⎝⎛ −

=41 υWhere

uv,un = vertical and horizontal displacement

Q, H = Vertical and horizontal loads

G = elastic shear modulus of the soil

υ = poisson’s ratio of the soil

R = radius of the base

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bhAm 4=

23

)2/2/(412

4 bBbhhbI yy −+=

yyxxm

sa I

xMI

yMAWP )()(

+−=

23

)2/2/(412

4 hHbhbhIxx −+=

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

Rectangular Mudmat system



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( )224

244

644 BDDI yy

ππ++=

( )224

244

644 HDDIxx

ππ+=

Circular Mudmat system

2

44 DAmπ

=

yyxxm

sa I

xMI

yMAWP )()(

+−=

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( )23

32

2236

4 bBbhbhI yy −++=

( )23

32

2236

4−+= HbhbhIxx

Triangular Mudmat system

24 bhAm =

yyxxm

sa I

xMI

yMAWP )()(

+−=



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( )23

22483 bBbhbhI yy −+=

( ) ( )223

31

32

232

2363 hHbhhHbhbhIxx −+−+=

Triangular Mudmat system

24 bhAm =

yyxxm

sa I

xMI

yMAWP )()(

+−=

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BEARING CAPACITY OF MUDMATS



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BEARING CAPACITY

The ultimate bearing capacity (qu) is defined as the least pressure which would cause shear failure of the supporting soil immediately below and adjacent to a formation.

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MODES OF FAILURE

a) General failureb) Local shearc) Punching failure

The mode of failure depends on the following - Foundation type and geometry- Soil compressibility



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MODES OF FAILURE

a) general shear b) local shear c) punching shear

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THEORY OF PLASTICITY

A suitable failure mechanism shall be found by either inspection, trial or limit theorems. Two bounds can be defined.

Lower BoundTrue failure load is large than the load corresponding to an equilibrium system

Upper BoundThe true failure load is smaller than the load corresponding to a mechanism if that load is determined using the virtual work principle



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EQUILIBRIUM SYSTEM

An equilibrium system, or a statically admissible field of stresses is a distribution of stresses that satisfies the following conditionsa) it satisfies the conditions of equilibrium in each point

of the bodyb) it satisfies the boundary conditions for the stresses

c) the yield condition is not exceeded in any point of the body.

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MechanismA mechanism, or a kinematically admissible field of displacement is a distribution of displacements and deformations that satisfies the following conditions.

a) the displacement field is compatible, i.e. no gaps or overlaps are produced in the body (sliding of one part along another part is allowed)b) it satisfies the boundary conditions for the displacementsc) wherever deformations occur the stresses satisfy the yield conditions



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IDEALIZED STRESS-STRAIN RELATIONSHIP

Shea

r str

ess

Shear strain

Y’

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STATE OF PLASTIC EQUILIBRIUM



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φφσφσ

φσσ

σσφ

cos2)sin1()sin1(

)cot2(21

)(21

sin

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31

31

c

c

−−=+∴

++

−=

⎟⎟⎠

⎞⎜⎜⎝

⎛+−

−⎟⎟⎠

⎞⎜⎜⎝

⎛+−

=∴

+−

−⎟⎟⎠

⎞⎜⎜⎝

⎛+−

=∴

φφ

φφσσ

φφ

φφσσ

sin1sin12

sin1sin1

sin1)sin1(2

sin1sin1

13

2

13

c

c

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LOWER BOUND SOLUTION



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( )

( ) ( )

cqqcccqq

cqcq

c

ult

ult

ult

4422

)1(2)1()1(2)1(

0for 12/45tan2/452

45tan22/45tan

1.31.1

1.31.2

2

231

=+=++=

+==+==

==+=+

⎟⎠⎞

⎜⎝⎛ +++=

−−

−

σσσσ

φφφ

φφσσ

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−

−

+=

=Χ

−Χ−Χ

qcq

BqBBBcBBq

ult

ult

π

π

2

022

UPPER BOUND SOLUTION



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Simplified bearing capacity for a ø – c soil

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yqcult

pppp

pp

ult

pult

pppp

H

O

H

Op

yBNNqcNq

KKyBKK

qKK

cq

PcAHByBq

KcHKHqKyHP

dzcqyzdzP

++=

⎥⎥⎦

⎤

⎢⎢⎣

⎡−++⎥

⎦

⎤⎢⎣

⎡+=

=−−+Χ

++=

⎭⎬⎫⎟⎠⎞

⎜⎝⎛ ++⎟

⎠⎞

⎜⎝⎛ +

⎩⎨⎧ +==

−

−

−

−

∫∫

φφφ

φρρ

φφσ

cos4coscos2

0cossin

cos2

.22

.2..2

245tan2

245tan)()(

2

2

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FAILURE UNDER A STRIP FOOTING

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FOOTING AT DEPTH D BELOW THE SURFACE



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Susceptible to long-term consolidation settlement

300 – 600150 – 30075 – 150<75–

Very stiff boulder clays and hard claysStiff claysFirm claysSoft clays and siltsVery soft clays and silts

Width of foundation (B) not less than 1 m. Water table at least B below base of foundation

>600200 – 600

<200>300100 – 300<100

Dense gravel or dense sand and gravelMedium dense gravel or medium dense

sand and gravelLoose gravel or loose sand and gravelCompact sandMedium dense sandLoose sand

Remarks Bearing value(kN/m²)

Soil type

PERSUMED BEARING VALUS (BS 8004: 1986)

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factorscapacity bearing and ,Depth

Breadth capacity bearing ultimate the

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====

++=

qc

u

qcu

NNND Bq

DNcNBNq

γ

γ γγ



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factorscapacity Bearing ,

)4.1tan()1(

tan)1(80.1N

factorscapacity Bearing ,

cot1

/2) 45( tan) tan ( exp 2

=

−=

−=

=

−=

+=

γ

γ

γ

φ

φ

φφπ

NN

NN

N

NN

φ) (NN

N

q

q

q

cq

qc

oq

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qcf DNcNBNq γγ γ ++= 2.14.0

Length Breadth

capacity bearing ultimate The

2.13.0

==

=

++=

LB

q

DNcNBNq

f

qcf γγ γ

Circular footing

Square footing



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Skempton’s values of Ncfor øu = 0 (Reproduced from A.W.Skempton (1951) Proceedings of the Building Research Congress, Division 1, p.181, by permission of the Building Research Establishment, ©Crown copyright)



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Mudmat Concepts and DesignRECOMMENDED BEARING CAPACITY FACTORS

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ECCENTRICALLY-LOADED FACTORS



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AREA REDUCTION FACTORS

ECCENTRICALLY-LOADED FACTORS

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42 – 58 85 – 100 Very dense> 50

25 – 42 65 – 85 Dense30 – 50

8 – 25 35 – 65Medium dense10 – 30

3 – 8 15 – 35 Loose4 – 10

0 – 3 0 – 15 Very loose 0 – 4

(NI)60Id (%)ClassificationN Value

DENSITY INDEX OF SANDS



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Bearing capacity calculations by Davis and Booker

The bearing capacity can be calculated when the soil profile is varying linearly with depth

)1(4 ccuoru SBNCFq +⎟⎠⎞

⎜⎝⎛ +=

ρfactor Shape==

LB

NN

Sc

cγ

NC= 5.14 for strip footing

Nγ = 1 for footing at top of soil

Fr = shear strength factor depends on the variation of the soil profile

ρ= rate of increase of shear strength

B = width of footing

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F r - Shear Strength F acto r

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000

Rho



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SPECIAL FOUNDATIONS

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Special Foundations

Suction Anchor (Bucket Foundation)Gravity Foundation



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Suction Anchors (Piles)

A suction anchor is an inverted top capped hollow cylinder of fairly large diameter with a length to diameter ratio (L/D) of 1.0 to 2.0 that is embedded into the sea bed. Self-weight and differential water pressure can facilitate easy installation of this type of anchor into the sea bed. This differential water pressure (active suction) can be created by pumping out the water from the interior of the anchor.

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Mudmat Concepts and DesignThe main pile advantages of this anchor over tension piles are due to the weight of the soil plug inside and the freely available high ambient water pressure which offers two advantages; easy installation of the anchor with its active suction arrangement and mobilization of passive suction force at the anchor bottom during uplift. Further, the large-diameter sealed top provides a substantial space for additional ballast, which can increase thebreakout resistance



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Suction Breakout FactorsFrom the equilibrium considerations (referring to figure 1) the uplift pullout capacity of the suction anchor is given by

Pu = Wa + Fext + Ws + Wb + Rb

WhereWa = is the weight of the anchorFext = is the shear resistance along the external wallWs = is the weight of the soil plugWb = is the weight of the ballast (if any) at the topRb = is the suction-induced reversed end bearing

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Pu = Wa + Ws + Fext + Rb

Rb1 = Pu – (Wa + Ws + Fext)

From consideration of rupture in clay under tensile loading (Vesic, 1971) the bottom breakwater resistance is expressed in a non-dimensional form as

Fext = αCu Ase

From the plug equilibrium (refer to figure 13) equations can be written as:

Rb2 + Ws - Ps + Fint

Rb2 = Ps + Fint - Ws

Rb2 = Nb2 Cu AbWhere

Nb1 and Nb2 are bottom breakout factors from overall and plug equilibrium, respectively, ps is suction pressure measured at the top of soil plug, Ab is the base area of the anchor, α is an adhensionfactor, Fint is internal skin friction, Asi is the area of internal skin friction and Ase is the area of external skin friction

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4_Mudmat[1]