100-year Response Research Articles

Application and improvement of iterative methodology for response based analysis of an FPSO mooring system in tropical cyclones

Response based analysis (RBA) is an advanced alternative to traditional design approach for mooring systems of floating structures, which reduces uncertainty associated with extreme responses. In the traditional approach, mooring analysis is performed for specified sets of extreme metocean conditions, representing typically a return period of 100 years. In RBA, the dynamic analysis is carried out over a long-term database of metocean conditions and statistics of the responses are used to identify responses associated with a return period of 100 years. Whilst the RBA has a more rigorous probabilistic basis, it is also computationally demanding. An “Iterative methodology” has been developed recently, comprising a combination of time domain (TD), frequency domain (FD) and probabilistic analyses of the structural system into a single iterative process, to minimise the number of time-consuming simulations through distinguishing between “mild” and “severe” response events. In the study, the “iterative methodology” is applied for RBA of the mooring system of a turret moored FPSO in the tropical cyclone environment of the North West Shelf of Australia. In the first stage, the time-efficient FD analysis is utilised to identify the contribution of all the storms to the 100-year response. In the second stage, an iteration process is set up, in which only the most contributing response events (tropical cyclones) and sea states are analysed by TD simulations. The iterations proceed by selecting the next most contributing storm until the convergence of the 100-year response is achieved. Application of the iterative methodology leads to significant reduction in computational time and makes RBA feasible as a part of the usual design process by using fully coupled TD analysis. Further optimisation can be achieved through a new method of applying thresholds to metocean parameters, which can reduce the analysis effort further. In the case study considered, it is found that 61% of metocean time history could be excluded from any dynamic analysis by applying a variable storm-dependent threshold. Among the remaining intervals, only 5.2% need to be re-analysed by TD simulations. Several methods for assessing errors in the convergence process are compared, and a sensitivity analysis on the number of storms that should be selected at each iteration step is performed.

Comparison of Various Spectral Models for the Prediction of the 100-Year Design Wave Height

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. In most cases, the dominant load on offshore structures is due to wind-generated random waves where the ocean surface elevation is defined using appropriate ocean wave energy spectra. Several spectral models have been proposed to describe a particular sea state that is used in the design of offshore structures. These models are derived from analysis of observed ocean waves and are thus empirical in nature. The spectral models popular in the offshore industry include Pierson-Moskowitz spectrum and JONSWAP spectrum. While the offshore industry recognizes that different methods of simulating ocean surface elevation lead to different estimation of design wave height, no systematic investigation has been conducted. Hence, the aim of this study is to investigate the effects of predicting the 100-year responses from various wave spectrum models. In this paper, the Monte Carlo time simulation (MCTS) procedure has been used to compare the magnitude of the 100-year extreme responses derived from different spectral models. Additionally, the linear random wave theory (LRWT) was implemented to simulate the offshore structural responses due to random wave loading. The models have been tested for three different environmental conditions represented by Hs = 15m, 10m and 5m respectively. The accuracy of the predictions of the 100-year responses from Pierson-Moskowitz and JONSWAP spectrums will then be investigated.

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

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 most-frequently 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.

Analysis of tether anchored floating suspension bridge subjected to extreme environmental loads

The Norwegian public road administration wants to replace eight ferry crossings on the main road of western Norway with fixed straight crossings. Many of these crossings are too long, and too deep for conventional bridges, requiring novel technologies and new methods. One of the challenges these bridges face is environmental loading.This paper studies the response of a tether anchored floating three span suspension bridge over Bjørnafjorden subjected to environmental loads with a return period of 100 years. The crossing has a length of over 4000 m and depths in excess of 500 m. With eigenperiods spanning from 100 s and downwards, the bridge is susceptible to severe dynamic amplification from many types of environmental loads. The two largest load contributions are due to wind and waves. Wind loads are included with a stochastic wind field over the middle span of the bridge. This includes both temporally - and spatially varying wind speeds in all three directions. Second order slowly varying wave drift forces on the floaters are also taken into account.The analysis is performed by time domain dynamic simulation using the computer program USFOS. The bridge girder, pylons cables and hangers are modelled with beam elements. The key response quantities studied and reported in the paper are horizontal displacement of both floaters and the midspan, accelerations of the floaters, both at the bridge girder and at the top of the superstructures and at the midspan, and the tension force both in the main cables and in the tethers. The simulations are based on the contour line method, which is commonly adopted by the offshore industry. Thirty one-hour time domain simulations were conducted to estimate the 100-year response of the bridge to environmental loads.

Extreme dynamic, non-linear response of fixed platforms using a complete long-term approach

The general theoretical basis for application of the direct long-term approach to offshore structures with non-Gaussian response is outlined. The key to the method is the determination of the short-term response distribution. Special considerations should be taken, depending on the application, and the Weibull distribution with a special fitting procedure is suggested as a short-term probability model for fixed offshore platforms. The efficiency of the method is improved by introducing smoothing functions for the variation of the Weibull parameters with the seastate. The method is applied to a jacket and jackup platform. The results show that smoothing functions increase the efficiency considerably. The design storm method compares well with the long-term results for the selected structures, but only the long-term approach yields realistic estimates of the design extreme response when both extreme and resonant seastates contribution to the 100-year response.