Suction Anchors in Clay

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Suction Anchors The best of all moorings!

© T.T. Bakker, M.A. de Heer, A.E Heerema,P. Smeets; Offshore Engineering; TU Delft, 2006 The use of the information of this document is at the own risk of the user/reader. The persons mentioned above assume no responsibility for the accuracy of the information provided in this document.

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L

t

D

z

h

Water

Soil

COG

zCOG

Theory of a vertically loaded Suction Pile in CLAY

Mainly based on the Det Norske Veritas DNV-RP-E303

1. Convention

Figure 1 – Suction Pile Figure 2 – Suction pile with main parameters

Possible forces: Main parameters:

• Total resistance of suction pile (Rd) - Water depth (h) • Vertical load force (Td) - Pile or caisson length (L) • (Submerged) Self weight caisson (W’) - Pile or caisson diameter (D) • Effective overburden pressure (P0’) - Pile thickness (t) • Penetration resistance (Qtot) - Penetration depth (z) • Side shear along the side wall (Qside) - Center of gravity (CoG) • The bearing capacity at the skirt tip (Qtip) • Necessary underpressure (∆un)

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2. Soil conditions For the calculation of the holding capacity or resistance R the most important variable is the DSS (Direct Simple Shear test) cyclic shear strength ,

Df cyτ . With this the shear strength

between the pile and the soil can be determined. The cyclic shear strength is associated by the ‘characteristic storm’ and the equivalent number of cycles it produces Neq. This can be done using an empirically determined graph.

With the graph the value of ,f cy

uS

τcan be determined with the normalized shear stress

,

a

u DS

τ ,

consisting of the average shear stress τa divided by the static undrained shear stress Su.

Figure 3 – Cyclic DSS shear strength of clay for various Neqv The average shear stress can be calculated by:

When having determined ,f cy

uS

τ, from this the cyclic DSS strength can be determined.

0

z

u

apile

S dz

zτ

−

=∫

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In short, for the suction pile calculation in offshore clay the following variables are needed with their ranges: Cyclic DSS strength of the intact clay, ,

Df cyτ 0 – 100 kPa

Plasticity index, Ip 20% - 100% Static undrained shear stress, Su 5 – 100 kPa Soil sensitivity, St 1 - 8 Effective unit weight of soil, γ’ 9 – 11 kN/m3

Average shear stress, τa

The soil sensitivity St is a factor to show the strength of the soil after it’s been ‘remoulded’, determined from laboratory tests; The Plasticity index Ip is also determined in by laboratory tests and is determined by the difference between the liquid limit and the plastic limit of the soil.

3. Design

Limit state design For the general design criterion is the following,

Where R is the design value of the resistance of the pile and T is the tension in the mooring line, both at the depth of the pad-eye; the connection point between the suction pile and the mooring line. The tension will be determined as follows:

Where

,

ut

u remoulded

SS

S=

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In the following figure the case is described that, with horizontal loading, the tension in at the pad-eye TD(zD) differs from the tension at the dip-down point TD(zP).

Figure 4 - Change of tension and uplift angle between dip-down point and padeye depth. The resistance is determined as follows:

, where

In this resistance the submerged weight of the suction pile is included and determined by the cyclic shear strength. The γ depends on the limit state; the ultimate limit state ULS or the accidental damage limit state. These are put in line in this table:

Table 1 – the various factors depending on the limit state

The limit state design is a general requirement for calculating the eventual required resistance. As the process will be done by iteration, it is recommended to start with the preliminary design by means of rules of thumb.

Preliminary design For the preliminary design, there are two rules of thumb for determining the dimensions suction pile in clay:

L/d = 3 to 6 d/t = 100 to 250

Where L is the total length of the pile, d the diameter and t the wall thickness. The first rule of thumb is determined by the behavior of the suction piles with general soil properties of clay and the second by the elasticity and buckling ranges of the caisson.

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After assuming the variables, the bearing capacity can be calculated, knowing the soil conditions, and be compared to the maximum load on the suction piles. This is a process of iteration to achieve the desired results.

Calculation of the resistance The resistance can be calculated as the sum of the shear force along the outside of the pile and the inside of the pile. For suction piles in clay there are two optional cases for which the shear strength and thus the total resistance can be calculated, depending on the type of installation; the shear strength for the suction pile installed by penetration by self weight and by penetration by underpressure. Firstly the set-up factor for both methods of installation has to be determined.

Set-up factor The ratio between the shear strength at the interface between clay and outside skirt wall and the original shear strength is referred to as the shear strength factor or set-up factor:

With this factor, the determining shear strength for the bearing capacity calculation of the suction pile can be calculated. The set-up factor depends on the type of installation, as in penetration by self weight or by underpressure. For self weight penetration, the factor is determined as follows: The set-up factor in this case can be calculated by the following formulae, with a plasticity index of the clay of Ip>20%:

Where P0’ effective overburden pressure Su static undrained shear stress The effective overburden pressure is the overburden pressure (=ρgz) minus the pore pressure at the depth.

The self weight penetration depth will be determined later.

0 , ,' grain wet water pile grain dry pileP gz gz gzρ ρ ρ= − =

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For penetration by underpressure, the set-up factor will be calculated using the method described below. Here, the set-up factor needs to be determined by an empirically determined table, depending on the plasticity index Ip and the soil sensitivity St of the clay:

Table 2 – the lower bound set-up factor α (after consolidation)

Resistance along skirts penetrated by self weight With the earlier calculated set-up factor for self weight installation and the known original cyclic DSS strength of the clay the shear strength Su,rr between the skirt and soil can be calculated. If this is integrated over the surface Askirt of the skirt, the holding capacity or resistance R can be calculated:

The surface of the skirt is on the outside of the skirt over the self weight penetration depth. The set up factor that is used is the one calculated earlier for the self weight penetration.

Shear strength along skirts penetrated by underpressure Keeping in mind the earlier determined set-up factor for penetration by underpressure, the formula for the resistance can be applied again:

The surface of the skirt Askirt is on the outside of the skirt over the penetration depth.

4. Installation For installation there are two types or phases; self weight penetration and installation by underpressure. In general, a suction pile will be installed with two methods combined. The pile will penetrate first into the soil by its’ own submerged weight where after the pumping installation will penetrate. In this case the bearing capacity will be calculated by the method of the underpressure.

, ,D

u rr skirt f cy skirtR S A Aα τ= ⋅ = ⋅ ⋅

, ,D

u rr skirt f cy skirtR S A Aα τ= ⋅ = ⋅ ⋅

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Self weight penetration depth The submerged self weight (W’) penetration depth can be calculated by assuming the weight to be equal to the penetration resistance Qtot. This is the total penetration resistance resulting of the side shear along the side wall Qside plus the bearing capacity at the skirt tip Qtip.

Where: Awall skirt wall area (sum of inside and outside) Atip skirt tip area α shear strength factor or set-up factor (normally assumed equal to the inverse of

the sensitivity; if the skirt wall is painted or treated in other ways, this must be taken into account in the α-factor)

,avu DS average DSS shear strength over penetration depth

,avu tipS average undrained shear strength at skirt tip level (average of triaxial

compression, triaxial extension and DSS shear strengths) γ' effective unit weight of soil Nc bearing capacity factor, plane strain conditions z skirt penetration depth The bearing capacity factor Nc varies from 6.2 to 9, calculated by the following formula:

The average DSS shear strength over penetration depth ,avu DS and the average undrained

shear strength at skirt tip level can be calculated using: and

,

,

Df cyav

u Dswpd

dzS

z

τ= ∫ , , ,

, 3

D C Ef cy f cy f cyav

u tipSτ τ τ+ +

=

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Installation by underpressure For installation by underpressure, the most important variable is the required underpressure. The necessary underpressure ∆un, needed for the finalization of the installation or penetration, depends on the self weight penetration depth and the total resistance for the penetration Qtot:

Where: W’ submerged weight during installation Ain plan view inside area of the pile where underpressure is applied

Suction Anchors in Clay

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