Long-term probability distribution of fixed offshore structuralresponse using animproved version of finite memory nonlinear system procedure

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. Due to nonlinearity of the drag component of Morison’s wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian [1-2]; therefore, simple techniques for derivation of the probability distribution of extreme responses are not available. However, it has recently been shown that the short-term response of an offshore structure exposed to Morison wave loading can be approximated by the response of an equivalent finite-memory nonlinear system (FMNS) [3]. Previous investigation shows that the developed FMNS models reduce the computational effort but the predictions are not very good for low intensity sea states. Therefore, to overcome this deficiency, a modified version of FMNS models is referred to as MFMNS models is used to determine the extreme response values which improves the accuracy but is computationally less efficient than FMNS models. In this paper, the 100-year responses derived from the long-term probability distribution of the extreme responses from MFMNS and FMNS models are compared with corresponding distributions from the CTS method is investigated with the effect of current to establish their level of accuracy. The methodology for derivation of the long-term distribution of extreme responses (and the evaluation of 100-year responses) is discussed. The accuracy of the predictions of the 100- year responses from MFMNS and FMNS models will then be investigated

Comparison of the extreme responses from different methods of simulating wave kinematics

Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). Methods have been introduced to overcome this problem of high kinematics above the MWL consists of using linear wave theory (such as Wheeler, vertical stretching, effective node elevation and effective water depth methods) can be used to provide a more realistic representation of near- surface wave kinematics. There is promising as there is some evidence that the water particle kinematics from the Wheeler method are underestimated and that those from the vertical stretching method are somewhat exaggerated. In this paper, the comparisons of the probability distributions of extreme values from different methods of simulation wave kinematics are investigated by using Monte Carlo simulation procedure

The effect of wave in-deck in conventional pushover analysis

Subsidence is not a local settlement and one of the phenomena that may be experiencing by the offshore platform throughout the platform life. Compaction of the reservoir can cause it due to pressure reduction resulted to vertical movement of soils from the reservoir to mudline. The impact of subsidence on platforms will lead to a gradually reduces wave crest to deck air gap (insufficient air gap) and causing the Wave-in-Deck (WID) on platform deck. The WID load can cause a major consequence damage to the deck structures and potential to the collapse of the entire platform. The aim of this study is to investigate the impact of WID (with and without load) on structure response for fixed offshore structure. The usual run of pushover analysis only considering the base 100-years design crest height for the ultimate collapse. Thus, by calculating the wave height at collapse using a limit state equation for probabilistic model can give a significant result for WID. 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 in order to quantify the WID load effect on producing the new revised RSR. Finally, a parametric study on the probability of failure (POF) of the platform will be performed. As part of the analysis, the USFOS Software (Non-linear) and wave-in-deck calculation as suggested by ISO 19902 as practice in the industry are used in order to complete the study. It is expected that the new revised RSR with the inclusion of WID load will be lower hence increases the POF of the platform. The accuracy and effectiveness of this method will assist the industry, especially operators, for the purpose of decision-making and, ore specifically, for their outlining of action items as part of their business risk management

The accuracy and efficiency of the efficient time simulation procedure in derivation of the 100-year responses

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. To this end, the conventional (Monte Carlo) 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 extreme responses (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, three different versions of a more efficient time simulation technique (ETS) are compared by exposing a test structure to sea states of different intensity. The three different versions of the ETS technique take advantage of the good correlation between extreme responses and their corresponding surface elevation extreme values, or quasi-static and dynamic linear extreme responses. The accuracy and efficiency of an alternative technique in comparison with the conventional simulation technique is investigated