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3/15


4/15

NIGERIA

Akpo


5/15

Umbilicals and flowlines

69 km of 10 production flow lines in 4 loops

32 km of 10 water injection flow lines (4 line

8 km of 8 gas injection flow line

150 km 16 gas-export pipelinefrom AKPO to AMENAM

8 main umbilicals to subsea wellsand manifolds

Subsea production system44 wells, with vertical Christmas Trees

22 oil producers20 water injectors

2 gas injectors

9 offline production manifolds

1 offline gas injection manifold

In the field's Miocene reservoir the fluid is

in critical conditions, i.e. the gas and

liquid hydrocarbons are in a single

phase, at high pressure and tempera-

ture.

The produced gas is partly re-injected

and partly exported onshore to the

BONNY liquefaction plant (NLNG) via the

AMENAM field facilities. This hybrid

injection/export gas scheme optimises

hydrocarbon recovery: gas is injected

only in reservoirs which can benefit from

this type of pressure support.

On the same OML 130 block as AKPO,

three oil discoveries (EGINA, EGINA-

SOUTH and PREOWEI) now form the

basis for an oil development with a newFPSO located in the EGINA zone. Both

AKPO and EGINA, with their ability to

handle a variety of fluids, will be ideal

hubs for developing future hydrocarbon

discoveries in the area.

AKPO Partners

A deep offshorecondensate & gas

field in Nigeria

The AKPO field was discovered in 2000 by

Total Upstream Nigeria Ltd (TUPNI).

Located 150 kilometres off the Niger Delta at

a water depth of 1,400 metres, AKPO is the

first deep-offshore development involving

light oil with a high gas content.


6/15

ffloading buoy

Dual 16 offloading lines

Wells

Intelligent completions

Extended-reach deviated and horizonta

Sand control with stand-alone screens,expandable screens and Frac-Packs

PSO | Floating Production, Storagend Offloading Unit

Hull dimensions: 310 m x 61 m x 31 m

Oil storage: 2 million barrels

Oil processing: 185,000 barrels/day

Water injection:420,000 barrels/da

Produced water treatment:150,000 barrels/day

Gas processing:15 million standard cubic metres per day

Gas export:9.6 million standard cubic metres per day

Gas injection:6.1 million standard cubic metres per day

Living quarters sleeping: 220

Umbilicals and flowlines69 km of 10 production flow lines in 4 loops

32 km of 10 water injection flow lines (4 lines)

8 km of 8 gas injection flow line

150 km 16 gas-export pipelinefrom AKPO to AMENAM

8 main umbilicals to subsea wellsand manifolds

Subsea production system44 wells, with vertical Christmas Trees

22 oil producers20 water injectors2 gas injectors

9 offline production manifolds

1 offline gas injection manifold


7/15


8/15

The 14th

World Conference on Earthquake Engineering

October 12-17, 2008, Beijing, China

EFFECT OF SOIL-PILE-STRUCTURE INTERACTION ON NONLINEAR

RESPONSE OF JACKET TYPE OFFSHORE PLATFORMS THROUGH

INCREMENTAL DYNAMIC ANALYSIS

Behrouz Asgarian

1

, Alireza Fiouz

2

and Ali Shakeri Talarposhti

3

1

Assistant Professor, K.N.Toosi University of Technology, Tehran, Iran2

Assistant Professor,Persian Gulf University, Bushehr, IRAN

3Msc in Structural Engineering, K.N.Toosi University of Technology, Tehran, Iran

Email: [email protected],[email protected],[email protected]

ABSTRACT :

The response of a fixed offshore tower is greatly affected by nonlinear behavior of the supporting piles.

Pile-Soil-Structure Interaction (PSSI) can significantly affect the seismic performance of structures. The

pile-soil interaction during earthquake loading is one of the most important sources of nonlinearity of offshoreplatforms.

Incremental Dynamic Analysis (IDA) is an emerging analysis method that offers thorough seismic demand andcapacity prediction capability. This involves performing a series of nonlinear time history analyses under a suite

of ground motion records by equally scaling both components of each record to several levels of intensity and

recording the structural response.

This paper presents an efficient method to specify the effect of Seismic Soil-Pile-Structure Interaction (SSPSI)on structure through IDA method and shows suitable length to model offshore with equivalent dummy piles for

more accuracy. Three-dimensional finite element model of offshore, jacket with both equivalent dummy piles(pile stub) and true piles considering soil-pile-structure interaction are subjected to Incremental Dynamic

Analysis and the results of both are compared in terms of IDA curves.

In this paper, a computer program for Nonlinear Earthquake site Response Analyses of layered soil deposits

(NERA) is used for nonlinear response of soil layers. Modeling of structure of offshore with its pile isperformed with a FEM program (OpenSees) considering the effects of pile-soil-structure interaction using p-y

curves.

KEYWORDS:Jacket Platform, Incremental Dynamic Analysis, Pile-Soil-Structure Interaction,

Euivalent Dummy Piles.


9/15

The 14th



1. INTRODUCTION

In recent years experimental and analytical investigations have been directed toward evaluating inelastic

behavior of jacket type offshore structures subjected to strong ground motions. [1] Earthquake design ofoffshore platforms in seismic active areas is one of the most important parts in offshore platforms design.

Dynamic response of piles in offshore platforms is a function of the characteristics of the loading, dynamic

pile-soil interaction behavior and dynamic characteristics of the piles structural system. The SSPSI (SeismicSoil-Pile-Structure Interaction) analysis is the main step in evaluation of seismic behavior of pile supported

offshore platforms. The pile-soil interaction problem during earthquake loading is one of the most important

sources of nonlinear dynamic response analysis of offshore platforms. [2] incremental dynamic analysis (IDA)is a promising method that has recently risen which involves performing nonlinear dynamic analyses of the

structural model under a suite of ground motion records, each scaled to several intensity levels designed to

force the structure all the way from elasticity to final global dynamic instability [3]. Kimiaei.M et al. [2] hasanalyzed nonlinear response of offshore piles under seismic loads. They used BNWF model for the modeling of

pile-soil interaction and finite element method for the modeling of jacket members in nonlinear range of

deformation. Asgarian.B & Ajami.A [4] have surveyed dynamic behavior of jacket type offshore platforms

through incremental dynamic analysis.

In this paper, the effect of considering Seismic Soil-Pile-Structure Interaction (SSPSI) on structure nonlinearseismic response was investigated by comparing with equivalent dummy piles model. For this purpose analysis

of an existing sample offshore platform in Persian Gulf with Soil-Pile-structure interaction and equivalentdummy piles subjected to strong ground motions has been performed and the results in terms of peak interstory

drift ratio of platform in IDA curves have been presented. This model has been developed using OPEN Systemfor Earthquake Engineering Simulation (OPENSEES) [5] software. In order to analyze the variations in soil

layers response against earthquake, "NERA" software [6] is used. In this software the nonlinear strain-stress

behavior has been modeled and the relative displacements (or accelerations) in each sublayer have been

calculated. [7]

2. INCREMENTAL DYNAMIC ANALYSIS

The Incremental Dynamic Analysis (IDA) [8], is a computer intensive procedure that offers thorough (demand

and capacity) prediction capability by using a series of nonlinear dynamic analyses under suitablymultiply-scaled ground motion records. It can estimate accurately the seismic performance of structures.

Applying IDA to determine the performance of a structure requires several steps. First, a proper nonlinear

structural model needs to be formed, and a suite of records must be compiled. Then, for each record, the scaling

levels must be selected, the dynamic analyses run and the results post processed. Thus, IDA curves of thestructural response can be generated, as measured by a Damage Measure (DM, e.g., peak roof drift ratio roofor

max), versus the ground motion intensity level, measured by an Intensity Measure (IM, e.g., peak groundacceleration, PGA, or the 5%-damped first-mode spectral acceleration Sa(T1,5%)). In turn these are interpolated

for each record and summarized over all records to estimate the distribution of demand DM given intensity IM.

3. PILE-SOIL INTERACTION ANALYSIS USING BNWF

BNWF models used to analyze the dynamic response of piles should allow for the variation of soil propertieswith depth, nonlinear soil behavior, nonlinear behavior of pile-soil interfaces and energy dissipation through

radiation and hysteretic damping. Special attention must be given to the evaluation of the free-field excitation.

The computed ground motion at different levels within the soil is then applied to the nodal boundary supports

representing the support motions [2]. Figure 1 shows the general view of a BNWF model and its main

components in dynamic nonlinear response analysis of piles. [9]

In the present study, the soil stiffness is established using the p-y curve (lateral soil resistance versus lateral soil

deflection) approach. The procedures for generating p-y curves proposed by Matlock et al [10], Reese et al [11]and ONeil [12] are recommended by the American Petroleum Institute and are widely used in both research


10/15

The 14th



and professional jobs (API-RP- 2a) [13]. Therefore in this study, the soil stiffness is modeled employing thestatic p-y curves recommended by API.

Fig. 1. Characteristics of Nonlinear p-y Element:(a) Components; (b) Behavior of Component

Also the damping component of the soil resistance is represented by a dashpot whose coefficient is established

based on the Berger et al [14] model, i.e.,

sL BC 4= (1)WhereB= pile diameter, vs= soil shear wave velocity and =soil unit density.

4. FREE FIELD EXCITATIONS

Free field ground motion time histories are usually computed using common site response analysis techniques.

In site response analysis, the ground motion of the soil layer is calculated due to earthquake excitations applied

at bedrock. The results of such free field analysis (acceleration or displacement time history at different soillayer) are then used as the input excitation at support nodes of the BNWF-Fiber Element model. [15]

In the present study the nonlinear stress-strain response of soil layers approximated by a nonlinear approach. In

the analyses, Iwan [16] and Morz [17] model is used on which the nonlinear and hysteretic stress-strain

behavior of soil is approximated by tangential shear modulus. A computer program NERA (NonlinearEarthquake site Response Analysis) developed by Bardet et al [6] is used for free field ground motion analysis.

The lowstrain shear modulus Gmaxwas calculated from the dimensionless form of the equations by Seed andIdriss [18]:

SandforP

K8.21P

G

atm

mmax,2

atm

max =

(2)

3/)21(65 0max,2 vcm KK +== pressurecatmospheriP,6.0K atm0 ==

Clayfor380c

G

u

max=

(3)


11/15

The 14th



5. MODEL AND GROUND MOTION RECORDS

Two structural models in this paper are 3D models and similar but one of them is with SSPSI and other is with

equivalent dummy piles (without SSPSI). The provided model is formed by an assembling of frame elements inthe nodes in general coordination system. This selected jacket type offshore has 141.7 m height. The platform is

a six-leg jacket type which is installed in a water depth of 47.6 m. The jacket is located between -47.6 m and

+7.25 m relative to L.A.T and the top side is located between +9 m and +24 m with three stories. In plan, theacket is rectangular, 36m by 36m that is shown in figure 2. The elevations of jacket are shown in figure 3.

The platform has a three-stories topside with total mass of about 10000 tons located in center of each story anda four story jacket with total mass about 2000 tons located in main nodes of jacket. The platforms has different

geometries in x and y directions. To accommodate platform heavy topside installation using float-over system,

there are not any braces in sea water level bay in direction y and a portal action is formed in this direction.

The first natural period of platform is T1=3.03 sec. The members are modeled using a beam-column element.All analyses were performed using OPENSEES.

Figure 2. The jacket in plan Figure 3. Elevation of offshore

For the modeling of SSPSI model, some nodes are introduced on pile with the same coordinate of main layer

and sub-layer nodes. These two points have the same coordinates on the general coordinates. Based on the

conditions of the cave-in and break in interaction between soil and pile, the interactive elements are introduced

in the model. In this model, the relative movements of the nodes between pile and soil would be possible. Inclay soil a gap is formed in tension stress situation. So the interactive element in the model should separate the

node between pile and soil. For non-sticky soil, when the loading process is completed, the gap which is formed

due to the non-sticky material would be filled. In BNWF nonlinear model, apart from modeling the pile indynamic forces, the gapping and cave-in are modeled. For the modeling of structural steel, a bilinear

elasto-plastic model with kinematic and isotropic hardening materials is suitable. The selected model for thisstudy is an elasto-plastic model with strain hardening of 5 %.

For modeling of jacket, deck and pile members, the fiber elements have been used. By using this model, thebuckling behavior and post buckling behavior of the tubular braces can be controlled by adding geometric

stiffness [5]. As accuracy in P-delta analysis is suitable for the application in Earthquake engineering, it is used

for applying the effects of decreasing the stiffness and strength.

According to API (RP-2A) [13], in this essay, the mass used in the dynamic analysis consist of the mass of the

platform associated with gravity loading, the mass of the fluids enclosed with the structure and the


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appurtenances, and the hydrodynamic added mass. The added mass may be estimated as the mass of thedisplaced water for motion transverse to the longitudinal axis of the individual structural framing and

appurtenances. In computing the dynamic characteristics of braced, pile supported steel structures, viscous

damping ratios of 5% are used for an elastic analysis.

In the SSPSI model, for soil dynamic analysis, the soil characteristics, layers and selected record are introducedin NERA. Then by using NERA, the time history of relative displacement at a selected sublayer is attained.

After the formation of model, the time history of relative displacement of soil (in NERA) in pile nodes is

applied and later the structure is analyzed by a nonlinear dynamic analysis [8].

The second model of jacket was created with above mentions and eliminating soil-pile-structure and modeling

pile with equivalent dummy piles (pile stub). The length of pile stub was considered 15 times of pile diameter.

A set of twenty ground motion records is selected as listed in Table 1, that belong to a bin of relatively large

magnitudes of 6.5 - 6.9 and moderate distances, all recorded on firm soil and bearing no marks of directivity.

Table 1. The suite of twenty ground motion records used.

6. PERFORMING THE ANALYSIS AND IDA CURVES

Once the model has been formed and the ground motion records have been selected, a way to perform theactual nonlinear dynamic analyses required for IDA is needed. This entails appropriately scaling each record to

cover the entire range of structural response, from elasticity, to yielding, and finally global dynamic instability.

[3] To use a stepping [8] algorithm to trace the IDA curves of platform is chose. Analyses are performed atincreasing levels of IM at constant steps, until numerical non-convergence is encountered [4].

An IDA Curve set is a collection of IDA curves of the same structural model under different accelerograms thatare all parameterized on the same IMs and DM [8]. Figures 4 to 5 shows all twenty IDA curves in x and y

direction for two models.By generating the IDA curve for each record a large amount of data can be gathered, only part. There, the IDA

curves display a wide range of behavior, showing large record-to-record variability, thus making it essential to

summarize such data and quantify the randomness introduced by the records. [4] They can be easily

summarized into some central value (e.g., the mean or the median) and a measure of dispersion (e.g., thestandard deviation, or the difference between two fractiles). Consequently, to calculate the 16%, 50% and 84%

fractile values of DM and IM capacity is chosen, as shown in Figures 6 and 7.


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Figure 4. All twenty IDA curves in X direction of Jacket platform

(a) With considering Soil-Pile-Structure Interaction (b) without considering Soil-Pile-Structure Interaction

Figure 5. All twenty IDA curves in Y direction of Jacket platform(a) With considering Soil-Pile-Structure Interaction (b) without considering Soil-Pile-Structure Interaction.

Figure 6. The summery of IDA curves in X direction of Jacket platform

(a) With considering Soil-Pile-Structure Interaction (b) without considering Soil-Pile-Structure Interaction.

0

1

2

3

4

5

6

7

8

0 0.02 0.04 0.06 0.08 0.1 0.12

Maximum interstory drift ratio

FirstmodespectralaccelerationSa(T1,

%5)g

0

1

2

3

4

5

6

7

8

0 0.02 0.04 0.06 0.08 0.1 0.12



%5)g(a) (b)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.02 0.04 0.06 0.08 0.1



%5)g

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1



%5)g

(a) (b)

0

1

2

3

4

5

6

7

8

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Max. interstory drift ratio

"first-mode"spectralaccelerationSa(T1,5

%)

16% IDA

84% IDA

50% IDA

(a)

0

1

2

3

4

5

6

7

8

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14


"first-mo

de"spectralaccelerationSa(T1,5

%)

16% IDA

84% IDA

50% IDA

(b)


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Figure 7. The summery of IDA curves in Y direction of Jacket platform(a)

With considering Soil-Pile-Structure Interaction (b) without considering Soil-Pile-Structure Interaction.

7.CONCLUSION

SSPSI is a fundamental subject in evaluation of offshore platforms behavior. In order to the figures in each

direction before 0.06 drift ratio, behavior of the each models in both direction is almost similar. But after that

depended to frames and interstory stiffness, behavior of the models in each direction is different. As in Xdirection that frames have more stiffness, flatline in model without SSPISI is upper than SSSPSI model. In this

direction the model without SSPSI has almost linear behavior.In Y direction because of conditions of deck installation with float-over method and less frames stiffness,

flatline in model without SSPSI is lower than SSPSI model.

Difference of the model behavior with SSPSI and without SSPSI is depended to equivalent pile stiffness

(length), frames stiffness and interstories stiffness. So nonlinear behavior of pile and surrounding soil plays animportant role in actual behavior of a jacket in nonlinear range of deformation.

REFERENCES

[1] Asgarian A, Aghakouchack AA. (2004) Nonlinear Dynamic Analysis of Jacket Type Offshore StructuresSubjected to Earthquake Using Fiber Elements. 13th World Conference on Earthquake Engineering. Paper No.

1726.

[2] Kimiaei M, Shayanfar MA, El Naggar MH, Aghakouchak AA. (2004) Nonlinear Response Analysis of

Offshore Piles Under Seismic Loads. 13th World Conference on Earthquake Engineering. Paper No. 3056.

[3] Vamvatsikos D, Cornell CA. (2003) Applied incremental dynamic analysis. Earthquake Spectra.[4] Asgarian B, Ajami A. (2006) Nonlinear Dynamic Behavior of Offshore Structures, Using Incremental

Dynamic Analysis", 8th U.S. National Conference on Earthquake Engineering, San Francisco, USA.

[5] Mazzoni S, McKenna F, Fenves GL. (2006) OpenSees Command Language Manual.[6] Bardet, JP, Tobita T. (2001) "NERA- a computer program for Nonlinear Earthquake site Response Analysis

of Layered Soil Deposits." Department of Civil Engineering, University of Southern California..[7] Asgarian B., Raziei A. (2007) Comparison of Incremental Dynamic and Pushover analysis of Jacket TypeOffshore Platforms. 26th International Conference on Offshore Mechanics and Arctic Engineering.

OMAE2007-29469.San Diego, California, USA.

[8] Vamvatsikos D, Cornell CA. (2002) Incremental dynamic analysis. Earthquake Engineering andStructural Dynamics; 31(3):491_514.

[9] Boulanger RW, Curras CJ, Kutter BL, Wilson DW, Abghari A. (1999) "Seismic soil pile structure

interaction experiments and analysis." Journal of Geotechnical and Geoenvironmental Engineering, ASCE,125(9), 750-759.

[10] Matlok, H. (1970) Correlations for design of laterally loaded piles in soft clay. Proceeding of the 2nd

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14


"first-mode"spectralaccelerationS

a(T1,5

%)

16% IDA

84% IDA

50% IDA

(a)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14


"first-mode"spectralaccelerationS

a(T1,5

%)

16% IDA

84% IDA

50% IDA

(b)


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Offshore Technology Conference, Houston, Tx., Vol. 1, pp. 577- 588.[11] Reese, L.C., and Welch, R.C. (1975) Lateral loading of deep foundations in stiff clay. Journal of the

Geotechnical Engineering Division, ASCE, 101(GT7), 633-649.

[12] ONeill, M. and Murchison, J. (1983) An evaluation of py relationships in sands. Report GTDF02-83,Department of Civil Engineering, University of Houston, May.

[13] American Petroleum Institute. (2000) Recommended practice for planning, designing and constructing

fixed offshore platforms. API Recommended Practice 2A (RP-2A). 21st ed. American Petroleum Institute,Washington, D.C.

[14] Berger, E., Mahin, S.A., and Pyke R. (1977) Simplified method for evaluating soil-pile-structure

interaction effects. Proceedings of the 9th offshore Technology Conference, OTC paper 2954, Huston, Texas,589-598.

[15] Asgarian B., Roshandel Tavana MA. (2007) Bedrock Depth Effect Investigation in Seismic Response of

Offshore Platforms Considering Soil- Pile- Structure Interaction. 26th International Conference on OffshoreMechanics and Arctic Engineering. OMAE2007-29470. San Diego, California, USA.

[16] Iwan, W.D. (1967) On a class of models for the yielding behavior of continuous and composite systems.

Journal of Applied Mechanics, ASME, 34: 612-617.

[17] Mroz, Z. (1967) On the description of anistropic work hardening. Journal of Mechanics and Physics of

Solids, Vol. 15, pp. 163-175.[18] Seed, H. B., and Idriss, I. M. (1970) Soil moduli and damping factors for dynamic response analysis.

Report No. UCB/EERC-70/10, Earthquake Engineering Res. Ctr., University of California, Berkeley, Calif.

toshiba computer structure

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