Evaluation of the Behavior of the Jacket Type Offshore Platforms Piles using Incremental Dynamic Analysis B. Asgarian (Asgarian@kntu.ac.ir) K.N.Toosi University of Technology, Tehran, IRAN A. Fiuz Persian Gulf University, Bushehr, IRAN A. Shakeri Talarposhti & H. Rahman Shokrgozar K.N.Toosi University of Technology, Tehran, IRAN (ali_shakeri_talarposhti@yahoo.com)

ABSTRACT: Nonlinear response of piles is the most important source of potentially nonlinear behaviour of offshore platforms due to earthquake excitations. Soil-Pile-Structure Interaction (SPSI) has an important effect on the dynamic response of jacket type offshore platforms. Incremental Dynamic Analysis (IDA) is a computer-intensive procedure that offers thorough (demand and capacity) prediction capability using a series of nonlinear dynamic analyses under suitably multiply-scaled ground motion records. In this paper, jacket and soil-pile system have been modelled and the effect of soil-pile-structure interaction have been considered, the incremental dynamic analysis has been used to investigate nonlinear behaviour of offshore platforms. A practical BNWF model is used for estimating the lateral response of flexible piles embedded in layered soil deposits subjected to seismic loading. All the analyses performed with a 3D model of the platform in two directions. It is observed that piles have a low intensity measured level at Y-direction in comparison to X-direction. Keywords: nonlinear, offshore platform, incremental dynamic analysis, pile-soil-structure interaction 1. INTRODUCTION Incremental dynamic analysis (IDA) (Vamvatsikos and Cornell 2002) is an analysis method that has emerged as a promising tool for thoroughly evaluating the seismic performance of structures. It involves subjecting a structural model to a suite of ground motion records, each scaled to several intensities (as measured by the intensity measure, IM), and recording the responses (measured by engineering demand parameters, EDPs) at each level to form IDA curves of response versus intensity. The process in which the response of the soil influences the motion of the structure and the motion of the structure influences the response of the soil is termed as soil-structure interaction (SSI). The permanent deformation and failure of soil may further aggravate the seismic response of the structure. In this paper, nonlinear seismic response analysis of a sample newly designed offshore platform in Persian Gulf subjected to strong ground motions has been performed and the results in terms of lateral displacements of the platform pile versus its intensity measure have been presented. This model has been developed using Open System for Earthquake Engineering Simulation (OpenSees) software. In order to analyse the variations in soil layers response against earthquake, Nonlinear Earthquake site Response Analysis software (NERA) is used. In this software the nonlinear strain-stress behaviour has been modelled and the input accelerations in each sub-layer have been calculated. 2. PILE-SOIL INTERACTION ANALYSIS WITH BNWF MODELS BNWF (Beam on Nonlinear Winkler Foundation) models used to analyze the dynamic response of piles should allow for the variation of soil properties with depth, nonlinear soil behaviour, nonlinear behaviour 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. 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, Reese et al and O’Neil are recommended by the American Petroleum Institute and are widely used in both research and professional jobs (API-RP-2a). The soil stiffness is modelled employing the static p-y curves recommended by API. Also the damping component of the soil resistance is represented by a dashpot whose coefficient is established based on the Berger et al model (Eqn. 2.1):

4L sc B vρ= (2.1) Where B = pile diameter, vs = soil shear wave velocity and ρ= soil unit density. 3. 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 soil layer) are then used as the input excitation at support nodes of the model. In this paper, Iwan and Morz model is used on which the nonlinear and hysteretic stress-strain behaviour of soil is approximated by tangential shear modulus. A computer program NERA is used for free field ground motion analysis. The lowstrain shear modulus Gmax was calculated from the dimensionless form of the equations by Seed and Idriss (Eqn. 3.1 & 3.2): (3.1) (3.2) 4. MODEL AND GROUND MOTION RECORDS The structural model in this paper is a 3D model. The provided model is formed by an assembling of frame elements in the nodes in general coordination system. This selected jacket type offshore has 141.7 m height. The platform is a six-leg jacket type which installed in a water depth of 47.6 m. The platform has a three-stories topside with total mass of about 10,000 tons located in the center of each story and a four-story jacket with total mass about 2,000 tons modelled in main nodes of jacket. To accommodate the platform heavy topside installation using the float-over system, there is not any brace in the sea water level bay in the direction Y and a portal action is performed in this direction. The first natural period of platform is T1=3.03 sec. The members are modelled using a fiber type beam-column element. All analyses were performed using OpenSees.

max2,max

2,max 0

0

max

21.8

65; (1 2 ) / 3;0.6; atmospheric pressure

380

m

atm atm

m vc

atm

u

GK for S A N D

P PK K

K P

Gfor CLA Y

c

σ

σ σ

⎧ ′=⎪

⎪⎪ ′ ′= = +⎪⎪ = =⎨⎪⎪⎪

=⎪⎪⎩

Figure 4.1. The jacket in plan Figure 4.2. Elevations of offshore

The pile and surrounding soil are subdivided into a number of discrete layers. Pile response is traced independently at nodal points of the pile segments within each layer. The dynamic characteristics (i.e. stiffness, damping and mass) of the pile segments are established at these nodes. The soil reaction to pile movement during transient seismic loading comprises stiffness and damping components. In the present study, the soil stiffness is established using the p-y curve (lateral soil resistance versus lateral soil deflection) approach. Fig. 4.3 shows the general view of a BNWF model and its main components in dynamic nonlinear response analysis of piles. Table 4.1. Soil of platform at the field Table 4.2. The suite of twenty ground motion records

For the material nonlinear behaviour, Fiber Beam Column Element of OpenSees is used. For the geometric nonlinearity, p-delta stiffness is used which is accurate enough for such an application. The mass used in the dynamic analysis consist of the mass of the platform associated with gravity loading, the mass of the entrapped fluids in main legs, and the hydrodynamic added mass. The added mass is estimated as the mass of the displaced water for motion transverse to the longitudinal axis of the individual structural members and appurtenances. In computing the dynamic characteristics of braced, pile supported steel structures, viscous damping ratio of 5% is used for the analysis.

Figure 4.3. General view of BNWF models of offshore and its characterizes For transferring acceleration from bedrock to soil layers, its characteristics, layers and selected record are introduced in NERA software. The time history of relative displacement at a selected sub layer 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 analysed by a nonlinear dynamic analysis. A set of twenty ground motion records is selected as listed in table 4.2, 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. 5. PERFORMING THE ANALYSIS AND IDA CURVES Once the model has been formed and the ground motion records have been selected, nonlinear dynamic analyses are required to be performed for IDA results. This entails appropriately scaling each record to cover the entire range of structural response, from elasticity, to yielding, and finally global dynamic instability. Figs. 5.1 & 5.2 show the deflected shape of the structure in height at different levels of record #6 (table 4.2) spectral acceleration in X and Y directions respectively. Figs. 5.3 & 5.4 show IDA curves for different levels of the jacket and pile under record #6 in X and Y directions respectively. Figs. 5.5 & 5.7 show all IDA curves for all twenty earthquake time histories. In this figures, first mode spectral acceleration with respect to maximum displacement as in platform X and Y directions is shown respectively. The IDA curves display a wide range of behaviour, showing large record-to-record variability, thus making it essential to summarize such data and quantify the randomness introduced by the records. The IDA curves can be easily summarized into some central value and a measure of dispersion. Consequently, to calculate the 16%, 50% and 84% fractiles value of DM and IM capacity is chosen, as shown in Figs. 5.6 and 5.8.

6. SUMMARY AND CONCLUSION In this paper, a model of offshore platform including jacket, topside and piles are developed using OpenSees software and the seismic behaviour of the structure and pile are studied through incremental dynamic analyses method. In this model, the effect of Soil-Pile-Structure Interaction (SPSI) has been considered using BNWF model that used for estimating the lateral response of flexible piles embedded in layered soil. The results of incremental dynamic analysis are extracted as IDA curves for piles and structure in both direction. Piles are shown the different behaviour in two directions. In the Y-direction due to removing the brace at top level of jacket for installation operation of topside, (Float Over Deck installation system) the large displacement and drift is occurred in the pile at small intensity measured compared to the X-direction in same intensity. The flat line and the dynamic instability of piles are happened in low level of intensity measure at Y-direction compared to X-direction measured. REFERENCES 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.

Asgarian A, Aghakouchack AA. (2004). Nonlinear Dynamic Analysis of Jacket Type Offshore Structures Subjected to Earthquake Using Fiber Elements. 13th World Conference on Earthquake Engineering. Paper No. 1726.

Asgarian B., Raziei A. (2007). Comparison of Incremental Dynamic and Pushover analysis of Jacket Type Offshore Platforms. 26th International Conference on Offshore Mechanics and Arctic Engineering. San Diego, California. OMAE2007-29469.

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 Offshore Mechanics and Arctic Engineering. San Diego, California. OMAE2007-29470.

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. (http://gees.usc.edu/GEES/Software/Default.htm)

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.

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.

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.

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.

Matlok, H. (1970). Correlations for design of laterally loaded piles in soft clay . Proceeding of the 2nd Offshore Technology Conference, Houston, Texas. Vol. 1: 577-588.

Mazzoni S, McKenna F, Fenves GL. (2006). OpenSees Command Language Manual. (http://opensees.berkeley.edu).

Mroz, Z. (1967). On the description of anistropic work hardening. Journal of Mechanics and Physics of Solids. Vol 15: 163-175..

Mylonakis, G. , Gazetas, G. (2000). Seismic soil structure interaction: Beneficial or Detrimental? Journal of Earthquake Engineering. Vol 4:3, 277-301.

O’Neill, M. and Murchison, J. (1983). An evaluation of p-y relationships in sands. Report GTDF02-83. Department of Civil Engineering, University of Houston.

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.

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, California.

Shome N, Cornell CA. (1999). Probabilistic seismic demand analysis of nonlinear structures.. RMS Program, Stanford University, Stanford. Report No. RMS-35.

Vamvatsikos D, Cornell CA. (2002). Incremental dynamic analysis. Earthquake Engineering and Structural Dynamics. 31:3,491-514.

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Figure 5.1. The variations of the maximum displacement with variation of the spectral acceleration for each level of the jacket and pile in X direction under record 6.

Figure 5.2. The variations of the maximum displacement with variation of the spectral acceleration for each level of the jacket and pile in Y direction under record #6.

Figure 5.3. The variations of the IDA curves for each level of the jacket and pile in X direction under record 6.

Figure 5.4. The variations of the IDA curves for each level of the jacket and pile in Y direction under record 6.

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8 9

Maximum Displacement (m)

"Firs

t mod

e" s

pect

ral a

ccel

erta

tion,

Sa(

T1,%

5) g

Imperial Vally0.139

loma-0.638

Imperial Vally-0.117

loma-0.179

Imperial Vally-0.11

Imperial Vally-0.074

loma-0.159

super-hills-0.181

super-hills-0.207

Imperial Vally-0.042

Imperial Vally-0.309

Imperial Vally 0.057

loma-0.207

loma-0.209

loma-0.269

loma-0.279

loma-0.370

loma-0.371

Imperial Vally-0.254

loma-0.244

Figure. 5.5. All twenty IDA curves in X direction for the cap of pile

Figure 5.6. The summary of the IDA curves into their 16%, 50% and 84% fractiles in X direction.

0

0.2

0.4

0.6

0.8

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1.2

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1.6

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Maximum Displacement (m)

"Firs

t mod

e" s

pect

ral a

ccel

erta

tion,

Sa(

T1,%

5) g

Imperial Vally0.139

loma-0.638

Imperial Vally 0.117

loma 0.179

Imperial Vally-0.042

Imperial Vally 0.11

Imperial Vally-0.074

loma-0.159

super hills-0.181

super hills-0.207

Imperial Vally-0.309

Imperial Vally 0.057

loma-0.207

loma-0.209

loma-0.269

loma-0.279

loma-0.370

loma-0.371

Imperial Vally-0.254

loma-0.244

Figure. 5.7. All twenty IDA curves in Y direction for the cap of pile

Figure 5.8. The summary of the IDA curves into their 16%, 50% and 84% fractiles in Y direction.