System Reliability-Based Assessment on Existing Jacket Platform

Objective: The jacket platform is made of various types of steel pipes and beams welded, mainly used in shallow water areas, such as Bohai oilfield in China. In winter, the sea area where the platform is located will freeze, and the influence of ice load on the platform can not be ignored. So the influence of ice load on the reliability of jacket platform is studied in this paper. Methods: In this research, the finite element model of the jacket platform was established in the form of beam element, and the loads were applied from different directions to find the load direction with the greatest influence on the platform state. When calculating the ultimate load of the platform structure, it is necessary to carry out nonlinear analysis. Selecting the ice load condition, increasing the ice load step by step on the platform structure. As the applied load gradually increases, the platform will appear nonlinear, then the ultimate load of the jacket platform will be obtain. The pile radius and pile thickness are uncertain components in the reliability study of the jacket platform due to manufacture, processing, and other factors, therefore the values of the pile's parameters are all random. Consider the deviation of geometric parameters with the extreme value type I distribution of ice load, the limit state equation for the strength failure of the platform under ice loads is obtained using Response Surface Method, then the reliability of the platform is calculated by the Monte Carlo method and First Order Second Moment Reliability method. Results: By nonlinear analysis, the ultimate bearing capacity of the platform studied in this paper is about 8000kN. The reliability probability of the platform calculated by Monte Carlo method is 0.9999993, and the reliability index is 4.813. The reliability probability of the platform calculated by the first order second moment method is 0.9999994, and the reliability index is 4.852. Conclusion: The results obtained by the two methods are almost the same, and the reliability index is very large, indicating that the platform can operate safely and reliably under the ice load.

In addition to the usual loads, fixed jacket offshore platforms can be exposed to accidental loads from ship collisions. Indentation of tubular components is a significant defect that occurs when a supply vessel collides with a jacket platform, which can affect the ultimate strength of the offshore platform. This paper performs a nonlinear dynamic analysis using ABAQUS software to evaluate the ultimate strength of a wellhead jacket platform and to investigate its structural response to two consecutive impacts from a 2700-ton ship. A total of 16 collision scenarios were simulated and analyzed. The research incorporates time-varying zonal corrosion for jacket platforms, as discussed by Yang et al., and employs beam and shell elements for the numerical modeling of the jacket platform, using a rigid model for the offshore supply vessel. The results indicate that the key factors affecting structural damage are the corrosion rate, consecutive impacts, and the shape of the impacting structure. Findings reveal that local indentation and local displacement are directly proportional to increases in the corrosion rate. However, the bending deformation parameter shows a different pattern: it has an inverse relationship with the corrosion rate and a direct relationship with the number of collisions. In sideway and stern collisions, the primary impact in each corrosion scenario has almost the same effect on the ultimate strength of the structure as the secondary impact in the previous corrosion scenario. However, in forecastle impacts, the effect of the secondary impact is greater than that of the primary impact in the subsequent corrosion scenario, equivalent to a 10-year increase in corrosion.

As the world is progressing towards deeper water technology, jacket platform still remains as an important production structure for oil and gas in Malaysian shallow water regions. Besides the discovery of new oil and gas, enhanced oil recovery (EOR) allows more oil to be extracted from the old fields. As a result, service life of existing fixed jacket platforms need to be extended. The overall reliability of aged platform has to be reevaluated in order to monitor the integrity of the structure, and to prevent unexpected failures. Reserve Strength Ratio (RSR) is a widely adopted measure to quantify the ultimate strength of a structure. However, structural strength alone is inadequate to express a structure's reliability. Reliability is best defined as the probability that the extreme load will not exceed the ultimate strength of a structure. It can be expressed either in probability of failure or reliability index, which is the inverse normalized form of probability of failure. This paper presents the study of existing jacket platforms in Malaysia's three oil and gas operating regions. Relationship between probability of failure, reliability index and RSR has been formed to provide a simple reliability assessment model of jacket structure in Malaysia water regions.

PurposeDuring its operation, vibrations in jacket platform due to high operational and environmental loads could reduce its productivity and endanger its safety. Tuned mass damper (TMD) is one of the vibration control devices commonly used in buildings to reduce their response. Basically, TMD is a device attached to a structure as a mass on properly tuned spring and damping elements. The purpose of this paper is to study the utilization of TMD to reduce the wave-induced vibration of platforms.Design/methodology/approachFirst, the study investigates the optimum TMD parameters to reduce the response of the platform. The effectiveness of the optimized TMD in reducing platform response due to wave load is then analyzed. Finally, the reduced response of the platform is attributed to an extension of platform service life. Cyclic load in the form of vibration is related to cyclic stress on the tubular connections on a jacket platform, which causes fatigue failure. Fatigue performance of a platform is quantified by its fatigue life, which should be at least twice the intended service life.FindingsIt is found that the optimized TMD cannot be achieved by simply adjusting the TMD damping as high as possible. The results show that the response of controlled platforms by properly tuned TMD could be reduced by about 20 percent from the uncontrolled case.Originality/valueAlthough vibration of a structure can be reduced by increasing its stiffness, this approach is considered not economical since it increases the cost for material construction. With the falling of oil prices, an optimization of jacket platform structure is considered necessary to reduce its construction cost. TMD can be used as an alternate solution, even though the only recorded utilization of TMD on a jacket platform is known in Sakhalin Platform in Russia. This fact gives a motivation to carry out more studies about TMD application on jacket platform, as this device is already used in numerous of inland structures.

API RP2A WSD is a design code in practice for design of jacket platforms in the Persian Gulf but is based on the Gulf of Mexico environmental condition. So for the sake of using this code for the Persian Gulf, it is better to perform a calibration based on this specific region. Analysis and design of jacket platforms based on API code are performed in a static manner and dynamic analysis is not recommended for such structures. Regarding the fact that the real behavior of the offshore jacket platforms is a dynamic behavior, so in this research, dynamic analysis for an offshore jacket platform in the Persian Gulf under extreme environmental condition is performed using random time domain method. Therefore, a new constructed offshore jacket platform in the Persian Gulf is selected and analyzed. Fifteen, 1-h storm, simulations for the water surface elevation is produced to capture the statistical properties of extreme sea condition. Time series of base shear and overturning moment are derived from both dynamic and static responses. By calculating the maximum dynamic amplification factor (DAF) from each simulation and fitting the collected data to Weibull distribution, the most probable maximum extreme (MPME) value for the DAF is achieved. Results show that a realistic value for DAF for this specific platform is 1.06, which is a notable value and is recommended to take into practice in design of fixed jacket platform in the Persian Gulf.

The Jacket platform codes such as API LRFD and ISO 19902 are based on probabilistic design of component and joint reliability. They consider overall structural integrity, redundancy and multiple failure paths only indirectly by using structural integrity assessment methods. In this paper, probability of failure is determined as per design requirement of 100 year extreme conditions using Monte Carlo simulation. To get information on maximum strength, maximum wave height was increased till the reserve strength ratio reached 1, using SACS pushover analysis. Stokes's 5th order theory and Morrison Equation were used for finding the environmental loads. Regression analysis was used for the load model using surface fit tool of Matlab. The wave which gave an RSR value of 1 is considered as the maximum wave, the jacket can withstand with the available resistance of material. This theory has already been applied on land based structures, such as proof loading used against existing structures to gauge the strength of structure. This new maximum wave height has been used to find the updated failure probability and compare it with failure probability of design wave. The study covers one platform, and recommendation is made whether the platform is suitable for the extension of life or not. This study can further lead to updating based on new information on material resistance of platforms.

This paper presents how non-linear waves and wave loading induce non-Gaussian dynamic response of fixed offshore structures, and the effects this may have to fatigue life and extreme dynamic response of such platforms. Furthermore, the validity of different theoretical wave and wave load models is discussed. The basis for the paper is full-scale measurements of waves and dynamic response recorded on a jacket platform in the North Sea and time domain computer simulations of the response of this platform using a finite element structural model. INTRODUCTION Fixed deep water platforms will because of the large water depths and high deck masses behave highly dynamically. Such platforms must therefore be designed against irregular environmental load processes with the energy distributed over a wide frequency range. This demands the use of stochastic methods for dynamic response analysis both for prediction of fatigue life and extreme load effects. The practical procedures for such analysis which are available today are based on Gaussian environmental excitation processes, linearized load models, linear structural models, and, consequently, on Gaussian dynamic response. Full-scale measurements from existing platforms have, however, indicated that the dynamic response of fixed structures generally is non-Gaussian [1, 2, 3]. In this paper it is, based on the results of systematic analysis of full-scale measurements of waves and dynamic response, discussed which are the sources of non-Gaussian dynamic response and what are the consequences of this to fatigue life and extreme response. Furthermore, the capabilities of existing methods for non-linear dynamic response analysis is outlined through comparison of measured and simulated dynamic response. The effects of the observed type of non-linear load effects to fatigue life and extreme response of deepwater platforms are discussed in [4, 7]. FULL-SCALE MEASUREMENTS The Instrumented Platform The applied data are recorded on the Val hall quarters platform. This platform is four legged and x-braced double corner piles. The water depth is 70 m. Typical dimensions for wave loaded structural elements are 1.5 m for the legs and 0.7 m for the bracing. The platform is connected with a bridge to two neighboring platforms. The platform is instrumented for measuring of wind speed and direction, wave heights, current speed and direction, strain in one of the braces of an x-joint and platform accelerations at cellar deck and mudline levels. The platform is shown in Figure 1 together with the two connected platforms. In Figure 2 a more schematic picture of the platform is given which also indicates the location of the different sensors. The Applied Data The data involved in the study are the wave data, current data and strain data in the location indicated in Figure 3. Key parameters for the different data types are given in Table 1. The standard length of each recording period is 20 minutes. The study included 28 recording periods, all representing sea states where the mean wave direction was within the 30 degrees sector indicated in Figure 3. The distribution of the recording periods with respect to sea state severity is given in Table 2

Nonlinear seismic soil-structure interaction analysis of nuclear reactor building considering the effect of earthquake frequency content

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Many offshore jacket platforms worldwide have approached or exceeded their original design life but are still in use and productive. According to the international codes, standards, and industry best practices, structural assessments of ageing fixed offshore jacket platforms shall be conducted against relevant target values to assess whether it is fit for purpose or risk reduction measures should be considered for continuing its operation. This research examines the collapse behaviour of an ageing offshore jacket platform under extreme storm conditions. Nonlinear collapse analysis has been performed to assess fixed offshore jacket platforms' structural integrity and reliability in shallow water under extreme storm conditions. Two tripods and 4-legged jacket platforms at water depths between 30 to 80 meters, located in the Mahakam Delta, Kalimantan, Indonesia, have been selected in this research as wellhead platform models commonly installed in shallow water. Sensitivity studies examine the effects of pile-soil interaction, variations in pile depth, topside load adjustments, marine growth removal, and jacket strengthening on structural performance. From the structural integrity and reliability perspective, the findings highlight that strengthening the jacket by adding soldier piles is the most effective approach for extending the platform's lifespan, especially for a wave-dominated platform. Additionally, a cost feasibility analysis is advised for future evaluation to determine whether jacket strengthening is viable or if alternative risk reduction strategies should be further explored for the ageing offshore platform.

It is important to consider geometric parameters on Stress Concentration Factor (SCF) at the tubular joint, especially multi-planar joint, when designing a jacket platform. Thus, a jacket platform capable to withstand the extreme environmental loads. Many researchers use the Finite Element (FE) method to analyze stress at joints and estimate the SCF with the variation of geometric parameters. That studies obtained the result that geometric parameters have significant effect on decreasing or increasing the SCF. Therefore, this study will delve the effect of geometric parameters (β, τ, and γ) on maximum SCF caused by axial loading, in-plane bending moment, and out-of-plane bending moment at DKT tubular joint. The increase of β results in the decrease in maximum SCF at brace-side and chord-side weld toe, respectively. The increase of τ and γ results in the increase in maximum SCF at brace-side and chord-side weld toe, respectively.

Effect on non-linear soil-structure interaction due to base slab uplift on the seismic response of a high-temperature gas-cooled reactor (HTGR)

To develop a population model describing temocillin pharmacokinetics (PK) in patients undergoing haemodialysis and investigate how pharmacokinetic/pharmacodynamic (PD) targets can be met with different dosage regimens. Sixteen patients received the currently licenced dosing of 1, 2 or 3 g of temocillin (total of 61 doses) corresponding to an inter-dialytic period of 20, 44 or 68 h, respectively, and a dialysis period of 4 h. A non-linear mixed-effects model was developed jointly for total and unbound temocillin serum concentrations. The performance of clinically feasible dosing regimens was evaluated using a 5000-subject Monte Carlo (MC) simulation for determining the highest MIC for which the PK/PD target of 40%ƒT>MIC would be reached in 90% of patients [probability of target attainment (PTA)]. This PK study was registered at ClinicalTrials.gov (NCT02285075). Temocillin unbound and total serum concentrations (429 samples) were used to fit an open two-compartment model with non-linear albumin binding and first-order elimination. In addition to total body clearance, dialysis clearance was modelled using the Michaels function. The currently licenced dosing achieved a 90% PTA for an MIC up to 8 mg/L. A new temocillin dosage regimen was designed that would achieve a 90% PTA for an MIC of 16 mg/L (MIC90 of target organisms) adjusted to patient weight and inter-dialytic period. Currently licensed dosage regimen is suboptimal for MICs >8 mg/L (frequently found in clinical isolates). Model-based simulations allowed suggestion of a new dosage regimen with improved probability of microbiological success, applicability in routine clinical practice and more appropriate for empirical therapy.

Hundreds of cascading trip faults have occurred in recent years in Chinese wind farms, leading to challenges in the secure operation of the power system. To prevent cascading trip faults, the concept of a voltage security region (VSR) has been discussed in previous studies. However, the VSR proposed in previous work cannot provide decoupled voltage control ranges for each wind farm, which would be a more promising and applicable solution for wind farms in China. In addition, with increasing wind farm active power penetration, the VSR of a wind power integration area may not exist. In this situation, the control centre must curtail some wind power to guarantee security. Thus, this study proposes an iterative VSR model to guarantee secure operation and obtain a larger VSR in wind farms considering wind power curtailment. In each wind farm, the VSR is designed to guarantee normal operation of each wind turbine generator, while in the control centre, the region is designed to guarantee normal operation both under normal conditions and N–1 contingencies considering wind power curtailment. The proposed VSR demonstrated good performance in a simple test case and in a real system in Northern China using a Monte Carlo method.

This paper discusses the influence of asymmetrical anchors on guyed mast behavior under wind loads. Anchor asymmetry is due to variations on anchor levels in guyed mast placed at the top of hills. Six FEM models were developed with different guy anchor asymmetries. The mast was modeled as a three dimensional structure formed by pinned members (braced and horizontal members) and continuous members (columns). Cables were modeled with catenary formulation. Non-linear analysis under extreme wind loads was performed to calculate internal force on the guyed mast components. Comparison between members force on each model was made to assess structural behavior. Relative increments in internal forces in asymmetric models in relation to symmetric models were observed in all members. Results of this investigation show that asymmetry on anchor levels of guyed mast can produce a significant increase in internal forces of masts members, hence disregard of anchors asymmetry at guyed mast design can conduce to important errors.