Typically, the resistance of the jacket platform is assumed to be constant throughout its design life. However, the jacket platform's exposure to the marine environment and the occurrence of corrosion will certainly reduce the platform's resistance. The situation can lead to the failure of the jacket platform, as the environmental load will constantly act on the structure.
Therefore, the time-dependent reliability method is extremely necessary to evaluate the reliability of the jacket platform at different reference times. By performing the time-dependent reliability method, as a result of this study, the failure probability of the jacket platform is expected at different reference times. The probability of failure is obtained by analyzing the time-dependent probability model using the first-order reliability method (FORM).
The RSR results showed a downward trend, indicating that the resistance of the structure decreased over time. The result of FORM showed an upward trend and it proved that the probability of failure increased over time.
INTRODUCTION
- Background
- Problem Statement
- Objectives
- Scope of Study
The corrosion of the structural members causes the deterioration of global ultimate capacity of the structure. Reliability assessment method is used as a method to evaluate the safety of the offshore jacket platform. Meanwhile, jacket platform will continuously withstand the maximum load effect of the sea, even if the resistance of the structure is reduced.
Therefore, the resistance of the platform should not be assumed to be constant during its design life. Finite element software such as SACS will be used to perform the pushover analysis of the jacket platform. Probabilistic approach such as First Order Reliability Method (FORM) must be practiced to determine the probability of failure in the structure.
In this study, limit state design is applied to obtain the failure probability of the structure for different reference times, and finite element reliability using MATLAB (FERUM) will be used to solve the limit state equation. In this research, a corrosion model from the literature will be used to determine the thickness reduction of the construction parts.
LITERATURE REVIEW
Research Methodology
- Jacket Platform Characteristics
- General Flowchart
SACS is typically used to model and analyze the structural element of the jacket platform. However, due to the corrosion model in this study, the thickness and diameter of the platform in splash zone and immersion zone are reduced and the details will be postponed to section 3.1.3. Dead load and live load operate on the platform and were considered during the analysis.
In this study, extreme weather, such as a thunderstorm, can contribute to a large load on the platform. The corrosion model, based on previous literature, refers to the reduction of the thickness of the structural element. Accordingly, a static non-linear collapse analysis/push analysis is performed to obtain the basic shear capacity of the platform.
The reserve strength ratio (RSR) is expected as a result of the nonlinear static collapse analysis. As for the load model, the response surface is used to obtain the coefficients and the function of. The important part of the model conditions under resistance is the degradation of the shear capacity of the base due to the effect of corrosion.
The corrosion model in this project will cover the corrosion protection system (CPS) as the actual corrosion model. Therefore, the Weibull distribution is used to describe the growth of corrosion over time and there are four parameters that affect these Weibull equations, namely d, β, η and Tst. The value of d affects the final corrosion thickness of the structure since the value of d is directly proportional to the value of the final corroded thickness.
It can be observed that the greater the value of Tst, the later corrosion will occur. From the equation, the graph of corrosion growth versus time is plotted and shown below;. The corrosion zone can be classified into three different zones, namely the atmospheric zone, the splash zone and the immersion zone.
Corrosion Growth vs Time
- Static Non Linear Collapse Analysis/Pushover Analysis
- Reserve Strength Ratio (RSR)
- Limit State Function
- Project Key Milestones
- WEEK 5 - 12: Data Collection -Determine the relationship
- WEEK 14-20: Execution of Reliability Assessment
- Project Timelines
The original platform model is analyzed and the base shear capacity is obtained without changing the thickness of the structure. The original diameter and thickness of the member affected by corrosion can be referenced in appendix section. As the corrosion model is developed in different reference time, the corrosion growth of the member is determined.
The updated dimensions of the structure are then analyzed by thrust analysis to obtain the basic shear capacity. In conclusion, the base crash shear and base shear design time estimate will be obtained and the RSR value will be calculated. The red parts in Figure 3.6 show structural elements that have been affected by corrosion and the thickness of the elements has been reduced.
The reserve strength ratio is an important parameter in the limit state function, as it represented the resistance of the structure. RSR is the limit safety of a structure at which the basic failure shear is divided by the basic design shear. This limit state function is a key step in this study as it will indicate the failure of the structure.
Since some of the parameters in resistance and load models are uncertain, a large number of iterations are needed to calculate the limit state function. The resistance model is represented the capacity of the structure to withstand the load applied to the structure. Then the values of the scale and shape factors of the Weibull function are referenced from work of Mubarak, (2016).
The load combination of the load model used in this study is wave height of 100 year return period + current velocity of 100 year return period + wind speed of 100 year return period (1 min gust). The table of the values of resistance and load model parameters is displayed in the table below;. Finally, the final phase will be the time-dependent reliability assessment of the jacket platform which is the implementation of the First Order Reliability Method (FORM) of the limit state function.
RESULT AND DISCUSSION
- Chapter Overview
- Static Nonlinear Collapse Analysis
- Limit State Function
- Resistance Model
- Load Model
- Time Dependent Reliability Assessment
The basic shear capacity of a platform can be described as (1) basic failure shear and (2) basic design shear. In this case, option 1, in which the platform is collapsed, is considered to describe the failure of the platform. The thrust analysis of platform A gave the base shear capacity of the platform at each load step.
Subsequently, the result of the Reserve Strength Ratio (RSR) is shown in Table 4.1 below. Because the corrosion was applied to the structure at different reference times, these results ultimately proved that the resistance of the casing platform decreased over time. The RSR represents the marginal safety of the jacket platform and the data is presented in different loading directions.
This shows that the resistance of the structure to resist the load from direction 2 is higher at 20 years and decreased after 20 years. In the time-dependent reliability assessment, the jacket platform is evaluated and the probability of failure of the jacket platform is expected. As previously discussed, the RSR value decreases with time and proved that the jacket platform capacity has degraded with time.
Overall, based on Table 4.2, the probability of failure of the structure showed an upward trend, with the probability of failure increasing over time. This proved that the probability of the structure failing increases with time. These results proved that the member failure mechanism is random and likely to influence the RSR and probability of failure outcomes.
The load transfer to the members is random as the failure of the members is random. This assumption can lead to the unexpected variation of the RSR and probability of failure of load direction 2. The RSR is important value for resistance of the structure and the analysis in different reference time is necessary.
To conclude, the resistance or capacity of the platform has weakened and the probability of failure of the platform has increased over time. Therefore, the relationship between RSR and probability of failure of the platform was made as the probability of failure of the platform is inversely related to RSR.
Appendix A – Result from Pushover Analysis
Appendix B – Corrosion Growth
Appendix C – Original Dimension of Platform Structural Members
Appendix D – Matlab Algorithm
