Derivation of the Probability Distribution of Extreme Values of Offshore Structural Response by Efficient Time Simulation Method

Husain, M. K. Abu; Zaki, N. I. Mohd; Najafian, G.

doi:10.2174/1874149501307010261

Cited by 5 publications

(3 citation statements)

References 17 publications

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“…The load estimates increased as the crest height and horizontal particle velocities increased. The loading from a wave impact event can be significant, and numerical prediction is quite challenging [33][34][35]. Wave impacts can be critical for local structural details as well as global structural integrity [32][33][34][35][36][37].…”

Section: Introductionmentioning

confidence: 99%

Structural Integrity of Fixed Offshore Platforms by Incorporating Wave-in-Deck

Azman¹,

Husain²,

Zaki³

et al. 2021

JMSE

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The structural integrity of offshore platforms is affected by degradation issues such as subsidence. Subsidence involves large settlement areas, and it is one of the phenomena that may be experienced by offshore platforms throughout their lives. Compaction of the reservoir is caused by pressure reduction, which results in vertical movement of soils from the reservoir to the mud line. The impact of subsidence on platforms will lead to a gradually reduced wave crest to deck air gap (insufficient air gap) and cause wave-in-deck. The wave-in-deck load can cause significant damage to deck structures, and it may cause the collapse of the entire platform. This study aims to investigate the impact of wave-in-deck load on structure response for fixed offshore structure. The conventional run of pushover analysis only considers the 100-year design crest height for the ultimate collapse. The wave height at collapse is calculated using a limit state equation for the probabilistic model that may give a different result. It is crucial to ensure that the reserve strength ratio (RSR) is not overly estimated, hence giving a false impression of the value. This study is performed to quantify the wave-in-deck load effects based on the revised RSR. As part of the analysis, the Ultimate Strength for Offshore Structures (USFOS) software and wave-in-deck calculation recommended by the International Organization for Standardization (ISO) as practised in the industry is adopted to complete the study. As expected, the new revised RSR with the inclusion of wave-in-deck load is lower and, hence, increases the probability of failure (POF) of the platform. The accuracy and effectiveness of this method will assist the industry, especially operators, for decision making and, more specifically, in outlining the action items as part of their business risk management.

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Section: Introductionmentioning

confidence: 99%

Structural Integrity of Fixed Offshore Platforms by Incorporating Wave-in-Deck

Azman¹,

Husain²,

Zaki³

et al. 2021

JMSE

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“…According to this approach, appropriate transfer functions can be used to determine wave kinematics at different nodes of an offshore structure from a simulated surface elevation record. However, this method has shown unacceptable results near free surface, especially for unrealistically high-frequency wave components [8][9][10]. To cope with the offshore industrial practice, reasonable results of wave kinematics near-surface zone are considered essential.…”

Section: Introductionmentioning

confidence: 99%

Various methods of simulating wave kinematics on the structural members of 100-year responses

Husain¹,

Zaki²,

Najafian³

2017

J. MECH. ENG. SCI.

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The main force acting on an offshore structure is usually due to wind-generated random waves. According to the Morison equation, the wave force on a cylindrical member of an offshore structure depends on wave kinematics at the centre of the element. It is therefore essential to accurately estimate the magnitude of wave-induced water particle kinematics at all points in a random wave field. Linear random wave theory (LRWT) is the mostfrequently used theory to simulate water particle kinematics at different nodes of an offshore structure. Several empirical techniques have been suggested to provide a more realistic representation of the near-surface wave kinematics. The empirical techniques popular in the offshore industry include Wheeler stretching and vertical stretching. Most recently, two new effective methods (effective node elevation and the effective water depth) have been recently introduced. The problem is that these modified methods differ from one another in their predictions. Hence, the aim of this study is to investigate the effects of predicting the 100-year responses from various methods of simulating wave kinematics accounting for the current effect. In this paper, four versions of the wave kinematics procedure have been tested by comparing the short-term probability distributions of extreme responses. For all current cases, the highest vertical ratios for zero, positive and negative current cases are 1.414, 1.175 and 1.831, respectively. It is observed that even for positive-current cases, the difference between Wheeler and vertical stretching predictions is quite high and cannot be neglected. Thus, further investigation is necessary to resolve this problem and the outcomes in providing useful design information for the oil and gas industry.

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“…The probability distribution of response extreme values for each group is then calculated individually based on a relatively small number of simulations, and then the total probability theorem is used to derive the probability distribution of response extreme values. In a recently published paper (Abu Husain and Najafian 2010;Abu Husain et al 2013), the ETS procedure was validated against results from the CTS procedure by comparing the probability distribution of extreme values of base shear and overturning moment from the two methods. The ETS procedure was found to be many times more efficient than the CTS method.…”

Section: Introductionmentioning

confidence: 99%

Short-term probability distribution of the extreme values of offshore structural response by an efficient time simulation technique

Husain

Zaki

Mallahzadeh

et al. 2014

Ships and Offshore Structures

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Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. Wave loading on slender members of bottom-supported jacket structures is frequently calculated by Morison's equation. Due to nonlinearity of the drag component of Morison wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of their extreme response probability distributions are not available. To this end, the conventional time simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated response extreme values (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, a more efficient version of the time simulation technique (ETS) is introduced to derive the probability distribution of response extreme values from a much smaller sample of simulated extreme values. The ETS procedure is found to be many times more efficient than the CTS method.

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Derivation of the Probability Distribution of Extreme Values of Offshore Structural Response by Efficient Time Simulation Method

Cited by 5 publications

References 17 publications

Structural Integrity of Fixed Offshore Platforms by Incorporating Wave-in-Deck

Structural Integrity of Fixed Offshore Platforms by Incorporating Wave-in-Deck

Various methods of simulating wave kinematics on the structural members of 100-year responses

Short-term probability distribution of the extreme values of offshore structural response by an efficient time simulation technique

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