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Investigation of the Pile Aging Effect of a Fixed Offshore Platform Located in Persian Gulf using Nonlinear Soil-Pile Interactions

Article History: Received: 27 May. 2017 Accepted: 21 Sep. 2017 The study about the jacket platforms in the past has revealed that the most of the collapse failures occur due to the lack of strength of the pile foundation. However, when the jacket platforms which have been collapsed due to extreme condition were looked into, it was found that most of them had their foundations intact. These contrasting facts can be explained with the help of the phenomenon called “aging of piles”. Aging effect of piles has been proven by experiments indicating gradual increase in pile capacity which is due to the thixotropic characteristics of clayey soils, leading to gradual improvement of soil clamping property. But due to lack of proper understanding and suitable techniques to incorporate them, these aging effects have been ignored during the pushover analysis. In this study, a simple technique of stepping up of the soil curves in order to accommodate the increase in capacity of pile foundation due to aging is utilized, and then the pushover analysis is performed using commercial software SACS. SPD19C (South Pars gas field Development, phase 19) is a new constructed jacket platform in Persian Gulf which is used as the case study in this paper. The platform is considered to be under the storm condition in 180o direction which is the worst condition. This study shows that the incorporation of pile aging effect results in the improvement the piles performance and Reserve Strength Ratio (RSR) about 10 percent. This research also has provided a deeper knowledge into the behavior of aged offshore jacket platforms.


Introduction
Offshore jacket platforms are normally designed for around 25 years design life. During the service life of the platform, it may be required to take more provisions than its design capacity due to the change in environmental loading, modification in the use of the platform and work over demands. Due to the reliability assessments of the aged platforms, PCSB (Petronas Carigali Sdn Bhd) have found out that the factor of safety for pile foundation capacity is very low [1]. Such cases necessitate the evaluation the integrity of the structure. Nonlinear pushover analysis is the most common technique for the purpose of the investigating the pile aging [2]. Study about the reasons of the failure of the jacket platforms in the past, has shown that the failure of a platform happens mostly due to the foundation failure. But the actual case scenarios of platforms which were acted upon by extreme environmental condition like storm and hurricanes have shown that the foundation remained intact when the platform failed [3]. These contrasting facts can be explained with the help of the phenomenon called aging of piles. Aging effect of piles have been experimentally proven to improve the pile capacity with time, but due to lack of proper understanding and suitable techniques to incorporate them, these aging effects have been ignored during the relevant analyses such as pushover analysis [4]. This effect can be incorporated into the pushover analysis using a simple technique of stepping up of pile soil interaction curves to model the improvement in pile capacity.
The main objective of this study is to investigate the aging effect on the RSR (Reserve Strength Ratio) of the platform and also to provide a better understanding of the behavior of aged offshore jacket platforms.

Aging Effect of Piles
The capacity of piles is known to be changing with time. The pile capacity calculated from the past equation does not consider the effect of time on the pile capacity. After longtime there will be a good bond between the pile and the surrounding soil, hence this additional adhesion is not considered in the calculation of pile capacity. In this regard, some studies were performed to define the behavior of the axial capacity in clay soil with time [5]. Clarke (1993) and Bogard and Matlock (1990) conducted field measurements studies and concluded that the time required for driven piles to reach ultimate capacity in a cohesive soil can be relatively long, as much as 2_3 years [6], [7]. The gain in capacity of piles is known as setup. The gain in capacity form the end of driving of pile to the end of consolidation phase is known as short term effects. Short term effects are mainly due to equalization of excess pore water pressure built up during driving (also known as consolidation) [8]. The gain in capacity after end of consolidation is known as long term effect or aging effects and it can be the result of a combination of mechanisms such as [9]:  Increase in the earth pressure against of the pile surface on the long term, due to creep of the soil structure.  Long-term build-up of new diegetic bonds between soil particles, after the complete destruction of the soil the structure due to the severe displacement and disturbance resulting from the driving of the pile into the ground.  Chemical bonding due to the interaction the steel pile surface and the soil minerals (cation exchange)  Effects of sustained load on the piles, gradually causing a more stable the soil structure and increase strength.  Effects of previous loading and unloading cycles of the piles which may have similar effects as sustained loading. The most popular time effects for capacity of piles was presented by Skov and Denver (1988), which models aging as linear with respect to the log of time. They proposed a semi-logarithmic relationship to describe aging as [10][2]: Qt = Axial capacity at time t after consolidation, Qo = Axial capacity at the reference time to, ∆ 10 = Setup factor, a constant depending on soil type o = the reference time at which Qo is measured The average setup factors for offshore clayey soil conditions were found to be 0.215 for a reference time of 100 days [4]. This setup factor value has been used in Eq. (1) to find the ratio of the pile capacity after design life (25 years) to the pile capacity at the reference time (100 days) as 1.42.
SACS is the commercial structural analysis software which is used in this study. In SACS, the pile soil modelling is done in a module known as PSI (pile structure Interaction). In PSI, the soil is defined in terms of soil curves namely side shear curve (t-z), end bearing curve (Q-z) and lateral strength curve (p-y) [11]. The soil curves should be modified to accommodate the pile capacity improvement due to aging effect. Q and t are the factors in the Q-z and the t-z curves that should be stepped up by a factor for the same axial displacement to modify the axial capacity of piles. In another words, t and Q values are identically and manually increased by the author in every axial displacement and 25-year bearing capacity is obtained via interpolation in order to improve Q0 to Qt. By studying the variation of capacity of piles with the application of random t factor and Q factors, it was observed that t factor has more influence on capacity improvement. Also the lateral capacity improvement due to aging effect is included by stepping up of the py curve.

Pushover Analysis
The pushover analysis is conducted for the SPD19C jacket platform, using the actual and aged soil curves. This jacket exists in Persian Gulf (South Pars gas field Development, phase 19) and it is a four-legged platform, as shown in the Figure-1. The self-weight of the jacket platform, buoyancy, installed equipment and live load were applied on the platform in the first phase of the pushover analysis with load factor of 1.0. The second phase of the pushover analysis is performed by the environmental load on the platform with increasing the load factor until the platform collapsed. Pushover analysis is carried out separately for eight selected loading direction namely; N(0º), NE(45º), E(90º), SE(135º),S(180º), SW(225º), W(270º), and NW(315º). The worst loading condition which causes the minimum RSR is the storm condition in S (180º) direction. So this direction is selected to investigate the aging effect on the piles. Reserve Strength Ratio (RSR) is a measure of structure's ability to withstand loads in excess of those determined from platform design and this can be obtained using the ultimate strength of the platform through pushover analysis. This reserve strength can be used to maintain the platform in service beyond their intended service life. Knowledge from this analysis can be used to determine the criticality of components within the structural system for prioritizing the inspection and repair schemes [12]. = BS collapse = the ultimate base shear capacity of the jacket prior to Collapse BS design = the design base shear loading on the jacket The design base shear can be identified when the environmental load factor = 1.0, while collapse base shear is the maximum base shear prior to collapse.

Platform Data
The jacket platform is four-legged drilling jacket with grouted steel piles for the purpose of supporting 2700 tones maximum operation weight located in the south pars gas field which is approximately located 210 km south east of port of Bushehr in a water depth of around 65.25 m. The total height of the jacket is 93.85 m and the jacket footprint at sea floor is 32.16m×23.04m and leg spacing at working point is 24 m x 13.716 m. A perspective plot of the model is shown in Figure 1.

Foundation
The foundation is modelled using uncoupled non-linear soil springs acting along the piles length. The loaddisplacement characteristics of these springs are defined by p-y, q-z and t-z curves based on geotechnical report. Based on pile makeup drawing the piles are modelled to penetration of 88.47m and 94.48m below mud-line. Pile outer diameter is 1524 mm. The scour readings by survey report ranged from 400mm to 900mm, so the final scour for modelling the platform was assumed equal to 1m on all pile locations.

Deck
The topside has three deck levels and includes accommodations and different equipment. The model includes all the deck primary and secondary beams, truss chords, bracing and columns. Deck plates have been included as quadrilateral isotropic plate element for the in-plane stiffness of the deck.

Material
As per API RP 2SIM 2014 material specifications and properties of an existing structure are defined based on data from original design.  The maximum water depth considered in the analysis is 68.05m and the minimum water depth is 65.25m. Max. Water Depth = Water Depth + HAT + Storm Surge + Subsidence

Wind
The wind loads are calculated based on the API RP 2A, using following-directional wind speeds for extreme storm conditions. Shape coefficients for perpendicular wind approach angles with respect to each projected area should be considered as follows (API RP2A-WSD-2014): Beams 1.5 Sides of buildings 1.5 Cylindrical sections 0.5 Overall projected area of platform 1.0

Wave and Current
Directional waves are used for the pushover analysis. Wave height with associated period for extreme storm conditions are as follows: The following currents are considered for the design of the platform.

Hydrodynamic Coefficients
Basic drag and inertia coefficients used to evaluate wave forces on cylindrical members are as follows:  [10]. Presents the marine growth thickness measured by underwater survey. The specific weight of marine growth in air considered equal to 1.4 kN/m 3 .

Soil Condition
The analysis includes the effect of the non-linear soil stiffness through the soil-structure interaction software named SACS PSI. The soil model is subdivided into seven layers. The design soil parameters are presented in Table 10.  Where: T-1 = Clay T-2 = calcarenite = soil-pile friction angle Cu = undrained shear strength Nq = bearing capacity factor flim = limit unit skin friction (kPa) qlim = limit unit end bearing pressure (MPa)

Results and discussion
Pushover analysis results in with and without incorporating the aging effects of piles in clayey soil in 180º direction which are given in table 11. As mentioned before, this direction causes the minimum RSR (Reserve Strength Ratio) for the platform so it is the critical condition. The improved RSR for the aged case proves the hypothesis of aging effect. The improvement of 10% is observed in pushover analysis results.

Conclusion
The increase in RSR (Reserve Strength Ratio) proves that an aged platform contains higher capacity than the designed capacity. However, the reduction in the RSR does not convey the exact opposite sense. Even when the structure has collapsed, the foundation has more capacity which should be utilized. Also for most of the cases where the RSR was improved, the foundation capacity was not completely used. Therefore, the performance of the jacket platforms can be further improved with careful planning and execution of the maintenance and strengthening of the critical member in the jacket which contributed to the structure collapse.
The certain conclusions of this research are listed below:  Disagreement between the simulation and actual cases in terms of their collapse strength is due to the aging effects of piles.  The incorporation of aging effect of pile into the pushover analysis of offshore jacket platforms can produce improvements in the RSR (Reserve Strength Ratio) of the structure. In this special research, this effect resulted in 10 percent improvement in the RSR.  The aged foundation capacity is not utilized in the most of the rehabilitation scenarios but this can be averted if a target based maintenance and strengthening of the jacket is performed.