G. Najafian

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

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 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. A more efficient method (ETS) was recently introduced which takes advantage of the correlation between the extreme values of surface elevation and their corresponding response extreme values. The method has proved to be very efficient for high-intensity sea states; however, the correlation and hence the efficiency and accuracy of the technique reduces for sea states of lower intensity. In this paper, a more efficient version of the ETS technique is introduced which takes advantage of the correlation between the extreme values of the nonlinear response and their corresponding linear response values.

Short-Term Distribution of the Extreme Values of Offshore Structural Response by Modified Finite-Memory Nonlinear System Modeling

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 of great value in the design of these structures. 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; 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). Previous investigation shows that the developed FMNS models perform better for high Hs values and their performance for low Hs value is not particularly good. In this paper, the modified version of FMNS models is referred to as MFMNS models is discussed. The improvement of MFMNS model is simply by dividing the structure into two zones (Zones 1 and 2) so that the horizontal distance between the nodes in each zone is relatively small compared to the wavelength. The modified version of MFMNS is used to determine the short-term probability distribution of the response extreme values with great efficiency.Copyright

Comparison of Extreme Offshore Structural Response from Two Alterna- tive Stretching Techniques

Linear random wave theory (LRWT) has successfully explained most properties of real sea waves with the ex- ception of some nonlinear effects for surface elevation and water particle kinematics. Due to its simplicity, it is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record; however, predicted water particle kinematics from LRWT suffer from unrealistically large high-frequency compo- nents in the vicinity of mean water level (MWL). To overcome this deficiency, a common industry practice for evaluation of wave kinematics in the free surface zone consists of using linear random wave theory in conjunction with empirical techniques (such as Wheeler and vertical stretching methods) to provide a more realistic representation of near-surface wave kinematics. It is well known that the predicted kinematics from these methods are different; however, no systematic study has been conducted to investigate the effect of this on the magnitude of extreme responses of an offshore structure. In this paper, probability distributions of extreme responses of an offshore structure from Wheeler and vertical stretching methods are compared. It is shown that the difference is significant; consequently, further research is required to deter- mine which method is more reliable.

Efficient Estimation of Offshore Structural Response Based on Threshold Upcrossing Rates

Reliable estimation of offshore structural response due to random wave loading is essential for ensuring safe and economical designs. However, the conventional Monte Carlo time simulation method requires the simulation of an extremely large number of response records in order to derive extreme response probability distributions with acceptably low sampling variability. The Efficient Threshold Upcrossing (ETU) method, presented in this paper, enables rapid calculation of these probability distributions by using information about threshold upcrossing rates in conjunction with an Efficient Time Simulation (ETS) technique. Extreme response probability distributions from this novel technique are compared with those from the conventional and the ETS methods using a simple structural model exposed to Morison wave loading. It is shown that the method allows a very efficient calculation of response statistics.

Efficient derivation of extreme offshore structural response exposed to random wave loads

ABSTRACT The reliability of offshore structure is dominantly affected by the wind-generated random waves load. Hence, an appropriate technique is required in predicting the extreme response due to the dominant load. Monte Carlo (MC) time simulation is said to be the most accurate technique. However, such analysis leads to a large number of response records which is computationally demanding. Current finding shows that a modified finite-memory nonlinear system offered more efficient technique. Still, the accuracy is getting severe once negative current is considered. Hence, improved version of MFMNS technique is required by modelling the residue between the extreme values from MC and the approximate MFMNS techniques; known as the eMFMNS technique. In advanced, a comprehensive study on eMFMNS technique will be carried out involving a wide range of environmental conditions. From the investigation, it is proven that eMFMNS technique improved the accuracy in predicting extreme values compare to MFMNS technique.

Extreme structural responses by nonlinear system identification for fixed offshore platforms

ABSTRACT Offshore structures are exposed to random wave loading in the ocean environment. The response is commonly non-Gaussian due to the nonlinearity of the drag component of Morisons wave loading and also the load intermittency on members in the splash zone. The previous study shows that the developed finite-memory nonlinear system (FMNS) model can efficiently simulate the response of an offshore structure to random Morison wave loading. However, the FMNS models minimise the computational effort, and the estimates are not particularly good for lower significant wave height value. To overcome this deficiency, a modified version of FMNS models (MFMNS) is used to compute the extreme response values which increase the accuracy but is computationally less efficient than the current FMNS models. In this study, the probability distributions of extreme responses from both models are compared with corresponding distributions from the conventional time simulation technique to establish their level of accuracy.