2-trench effect on the fatigue life of a scr_y.t.kim.pptx

7/28/2019 2-Trench effect on the Fatigue life of a SCR_Y.T.Kim.pptx

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Trench effect on the Fatigue life of a SCR

Prepared by Y.T.Kim

Offshore Engineering Lab. Seminar No. 2


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Current developments of SCR technology focus on improving fatigue life

in the touchdown region and better understanding the interaction between

the SCR and the seabed.


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1. Review

The pipe is initially in contact with a virgin soil

The pipe penetrates into the soil, plastically deforming

it. The pipe and soil interaction curve tracks on the

backbone curve.

The pipe moves upward and the soil responds

elastically. The pipe/soil interaction curve breaks away

from the backbone curve ,the force reduces over a

small displacement.

The pipe again penetrates the soil, deforming it

elastically. The pipe/soil interaction curve follows an

elastic lading curve similar to the previous elastic

unloading curve of step 3.

The pipe again penetrates into the soil, plastically

deforming it. The pipe/soil interaction curve rejoins and

follows the backbone curve.

(Steel catenary riser touchdown point vertical interaction models-Bridge, C., Laver, K.,

Clukey, Ed., and Evans, T.)

igure 1-Illustration of Pipe/Soil Interaction


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1. Review

1. Penetration-the pipe penetrates into th

soil to a depth where the soil force equal

the penetration force following backbon

curve. The soil deforms plastically.

2. Unloading-the penetration force reduce

to 0 N allowing the soil to swell as the pip

moves upwards.

3. Soil suction-as the pipe continues to mov

upwards the adhesion btw the soil and th

pipe causes a tensile force that resists th

pipes motion. The adhesion force quickl

increases to a maximum then reduces to

N as the pipe moves vertically upwards and

out of trench.

Backbone Curve

Penetration

Unloading

Soil Suction

Re-penetration after breakout

Figure 2. Re-penetration pipe/soil interaction curves

4. Re-penetration-the pipe penetrates into the existing trench that was created during the initial penetration. The re-penetration force/displacement curve has zero force when the pipe re-enters the trench, only increasing the interaction

force when the pipe re-contacts the soil. The pipe/soil interaction force then increases until it rejoins the backbone curve

at a lower depth than the previous penetration. Any further penetration follows the backbone curve.

(Steel catenary riser touchdown point vertical interaction models-Bridge, C., Laver, K., Clukey, Ed., and Evans, T.)


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2. Soil Stiffness

In order to analyze any kind of structure, or any kind of solid or fluid continuum, it is

necessary to have relation ships btw stresses and strains.

A

Figure 3. A typical stress-strain curve for soil Figure 4. Tangent and secant stiffness moduli

Siffness- the gradient of the stress-strain line. If this is linear the gradient is easy to

determine but, if it is curved, the stiffness at a point such as A in fig. 4 may be quoted as atangent or as a secant.

(The mechanics of soils and foundations, John Atkinson)


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2. Soil Stiffness

The two tests commonly used in soil mechanics are the triaxial test and the shear test

illustrated in Fig.5

A

Figure 5. Common soil tests



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2. Strength, Stiffness and Rigidity

The strength of a material is the maximum shear stress which it can sustain and its

stiffness is the ratio of changes of stress to the resulting strain.

A material ay be relatively strong or relatively weak: it may be relatively stiff or relatively

soft. Concrete and rubber have similar strengths but concrete is much stiffer than rubber.


Rigidity R-the ratio of stiffness to strength, commonly defined as

=

-Youngs modulus (stiffness)

-strength expressed as the diameter of the Mohr circle at failure


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2. Strength, Stiffness and Rigidity

Table 1. Typical values of stiffness, strength and

rigidity of some common materials



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2. Trench effect on the Fatigue life of a SCR

Non-Degradating and Degradating Models

Non-Degradating-neglecting degradation effect of cyclic loading, that is, it states

that a riser follows the same P-y curve repeatedly if cyclic loading remains the

same.

Degradating-In reality, cyclic loading degrades the soil condition. After many

cycles of pounding the pipe into the soil, a plastic deformation is formed, which

explains why the observed penetration is much greater than computed

penetration governed by the backbone curve using a non-degradation P-y curve.

Ali Nakhaee & Jun Zhang Effects of the interaction with the seafloor on the fatigue life of a SCR. ISOPE 2008


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The degradation mechanism involves soil remolding in each downward and upward

motion that may reduce the stiffness and strength of the soil

Figure 6. Stiffness degradation due to

cyclic loading

Degradation of the soil or plastic deformation of

the seafloor can significantly affect the curvature

of a riser near its TDZ, thus the bending moment

and the variation in bending moment.

Cluckey et al. (2005) concluded that as the pipe

moves back toward the soil , the water

underneath the pipe is pushed downward. The

jetting action by the water can lead to soil-water

mixing and trench erosion which can also reduce

the strength and stiffness of the soil.



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Figure 7. Typical Degradading P-y Curves

Yaguang Jiao Non-linear load-deflection models for seafloor interaction with steel catenary risers A thesis for master of science


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After hundreds of loading cycles thetrench development on the seafloor

make the riser penetrate into the soil.Figure 8. Trench development

(heave amp=1m, heave period=12s, medium soil)

ff f f SC


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Figure 9. Maximum movement variation along the riser

Maximum moment variation near the TDZ afunction of time after they started

experiencing continuous cyclic loading.

It shows maximum variation of bending

moment near the TDZ gradually reduces due

to the development of trenching.

The reduction in the maximum variation in

bending moment reduction of dynamicstress of a riser prolongs the fatigue life ofthe riser


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Figure 10. Maximum moment variation in Riser

for different types of soils

A deeper trenching is developed on the

seafloor of soft soil greater reduction in themaximum variation of bending momentgreater increase in fatigue life of the riser


3 F t Pl


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How to derive the fatigue life from the TDP result?

Fatigue analysis based on DNV

3. Future Plan

2-trench effect on the fatigue life of a scr_y.t.kim.pptx

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