Ying Min Low

A method for predicting the flexural response of tubular members with non-linear stress–strain characteristics

Abstract The paper presents an analytical method for calculating the moment–curvature relationship of CHS members with non-linear stress–strain behaviour, described by the Ramberg–Osgood model. The proposed method is subsequently incorporated in a further analytical procedure for predicting the load–deflection response of statically determinate members in bending. The results obtained from the proposed method are compared with some recently performed tests on stainless steel CHS beams under two-point bending. Moreover, deflection estimates are also obtained using other available methods, in order to undertake an overall assessment of the alternative approaches for estimating deflections in CHS members with non-linear stress–strain characteristics.

Multi-dimensional rheology-based two-phase model for sediment transport and applications to sheet flow and pipeline scour

Sediment transport is fundamentally a two-phase phenomenon involving fluid and sediments; however, many existing numerical models are one-phase approaches, which are unable to capture the complex fluid-particle and inter-particle interactions. In the last decade, two-phase models have gained traction; however, there are still many limitations in these models. For example, several existing two-phase models are confined to one-dimensional problems; in addition, the existing two-dimensional models simulate only the region outside the sand bed. This paper develops a new three-dimensional two-phase model for simulating sediment transport in the sheet flow condition, incorporating recently published rheological characteristics of sediments. The enduring-contact, inertial, and fluid viscosity effects are considered in determining sediment pressure and stresses, enabling the model to be applicable to a wide range of particle Reynolds number. A k − e turbulence model is adopted to compute the Reynolds stresses. In addition, a novel numerical scheme is proposed, thus avoiding numerical instability caused by high sediment concentration and allowing the sediment dynamics to be computed both within and outside the sand bed. The present model is applied to two classical problems, namely, sheet flow and scour under a pipeline with favorable results. For sheet flow, the computed velocity is consistent with measured data reported in the literature. For pipeline scour, the computed scour rate beneath the pipeline agrees with previous experimental observations. However, the present model is unable to capture vortex shedding; consequently, the sediment deposition behind the pipeline is overestimated. Sensitivity analyses reveal that model parameters associated with turbulence have strong influence on the computed results.

Understanding the Dynamic Coupling Effects in Deep Water Floating Structures Using a Simplified Model

The global dynamic response of a deep water floating production system needs to be predicted with coupled analysis methods to ensure accuracy and reliability. Two types of coupling can be identified: one is between the floating platform and the mooring lines/ risers, while the other is between the mean offset, the wave frequency, and the low frequency motions of the system. At present, it is unfeasible to employ fully coupled time domain analysis on a routine basis due to the prohibitive computational time. This has spurred the development of more efficient methods, including frequency domain approaches. A good understanding of the intricate coupling mechanisms is paramount for making appropriate approximations in an efficient method. To this end, a simplified two degree-of-freedom system representing the surge motion of a vessel and the fundamental vibration mode of the lines is studied for physical insight. Within this framework, the frequency domain equations are rigorously formulated, and the nonlinearities in the restoring forces and drag are statistically linearized. The model allows key coupling effects to be understood; among other things, the equations demonstrate how the wave frequency dynamics of the mooring lines are coupled to the low frequency motions of the vessel. Subsequently, the effects of making certain simplifications are investigated through a series of frequency domain analyses, and comparisons are made to simulations in the time domain. The work highlights the effect of some common approximations, and recommendations are made regarding the development of efficient modeling techniques.

Influence of low-frequency vessel motions on the fatigue response of steel catenary risers at the touchdown point

Fatigue design of a steel catenary riser (SCR) at the touchdown point (TDP) is a challenging problem. Many previous studies on this topic considered only the effect of wave-frequency (WF) vessel motions; the low-frequency (LF) motions are often neglected due to excessive computational costs. The LF vessel motion shifts the TDP, thus spreading the damage along the riser, but it can also increase the fatigue damage due to the bimodal stress response. Moreover, it may affect the trench depth and profile, and influence the WF dynamics. This paper investigates the above phenomenon to provide a better understanding. To avoid the high cost associated with simulating the combined WF and LF responses, this paper proposes a simple and efficient strategy for incorporating the LF motions. The approach is found to be satisfactory in accuracy, based on case studies that consider linear/non-linear soil models, as well as flat seabed and trench profiles.

Experimental Investigation on Scour under a Vibrating Catenary Riser

This paper investigates the impact of a vibrating steel catenary riser on scour in a steady current, motivated by the fact that pipeline scour studies are traditionally based on a horizontal stationary pipe. Laboratory experiments were conducted using a truncated riser subjected to a vertical sinusoidal motion of varying amplitude and frequency. By including a stationary test case for comparison, the results show that the scouring profiles for a stationary and a vibrating riser are fundamentally different. Using an acoustic Doppler velocimeter profiler to measure the flow velocities, the surrounding flow field was shown to be markedly modified by the riser vibration. The velocities fluctuated considerably at the vibration frequency, thereby promoting scour. The maximum scour depth increases significantly with both vibration amplitude and frequency, but frequency has a more profound influence. An explanation for this phenomenon is proposed based on various observed and measured results.

A Comparison of Methods for the Coupled Analysis of Floating Structures

The dynamic analysis of a deepwater floating platform and the associated mooring/riser system should ideally be fully coupled to ensure a reliable response prediction. It is generally held that a time domain analysis is the only means of capturing the various coupling and nonlinear effects accurately. However, in recent work it has been found that for an ultra-deepwater floating system (2000m water depth), the highly efficient frequency domain approach can provide highly accurate response predictions. One reason for this is the accuracy of the drag linearization procedure over both first and second order motions, another reason is the minimal geometric nonlinearity displayed by the mooring lines in deepwater. In this paper, the aim is to develop an efficient analysis method for intermediate water depths, where both mooring/vessel coupling and geometric nonlinearity are of importance. It is found that the standard frequency domain approach is not so accurate for this case and two alternative methods are investigated. In the first, an enhanced frequency domain approach is adopted, in which line nonlinearities are linearized in a systematic way. In the second, a hybrid approach is adopted in which the low frequency motion is solved in the time domain while the high frequency motion is solved in the frequency domain; the two analyses are coupled by the fact that (i) the low frequency motion affects the mooring line geometry for the high frequency motion, and (ii) the high frequency motion affects the drag forces which damp the low frequency motion. The accuracy and efficiency of each of the methods are systematically compared.Copyright

Extreme Response Analysis of Floating Structures Using Coupled Frequency Domain Analysis

In the dynamic analysis of a floating structure, coupled analysis refers to a procedure in which the vessel, moorings and risers are modeled as a whole system, thus allowing for the interactions between the various system components. Because coupled analysis in the time domain is impractical owing to prohibitive computational costs, a highly efficient frequency domain approach was developed in a previous work, wherein the drag forces are linearized. The study showed that provided the geometric nonlinearity of the moorings/risers is insignificant, which often holds for ultra-deepwater systems, the mean-squared responses yielded by the time and frequency domain methods are in close agreement. Practical design is concerned with the extreme response, for which the mean upcrossing rate is a key parameter. Crossing rate analysis based on statistical techniques is complicated as the total response occurs at two timescales, with the low frequency contribution being notably non-Gaussian. Many studies have been devoted to this problem, mainly relying on a technique originating from Kac and Siegert; however, these studies have mostly been confined to a single-degree-of-freedom system. The aim of this work is to apply statistical techniques in conjunction with frequency domain analysis to predict the extreme responses of the coupled system, in particular the modes with a prominent low frequency component. It is found that the crossing rates for surge, sway and yaw thus obtained agree well with those extracted from time domain simulation, whereas the result for roll is less favorable, and the reasons are discussed.Copyright

Importance Sampling Technique for Simulating Time Histories for Efficient Rainflow Fatigue Analysis

AbstractFrequency domain analysis of a dynamical system yields the response spectral density. There are many spectral fatigue methods for evaluating the mean fatigue damage from a stress spectrum, but they are only approximate. The only accurate method is to simulate the time history from the spectrum, succeeded by rainflow counting. However, this procedure is time-consuming because of the need for numerous realizations to achieve statistical convergence, making it incompatible with the high efficiency of frequency domain analysis. To overcome the slow convergence rate of conventional Monte Carlo simulation (MCS), this paper proposes an efficient simulation approach, which to date, appears to be the first of its kind for such an application. The proposed approach reduces the variance of the fatigue damage samples by invoking the technique of importance sampling, and entails only the coefficient of variation (CoV) of the damage to construct the importance sampling density. The CoV can be estimated initiall...

Predicting the Probability of Riser Collision Under Stochastic Excitation and Multiple Uncertainties

As the offshore industry moves to deeper waters, riser collision becomes a more crucial concern. Riser interference assessments need to rely on time domain simulations due to nonlinearities such as hydrodynamic interferences, however, one difficulty is that riser collision is an extreme event. In a recent work, the authors proposed an efficient procedure for predicting the probability of riser collision, based on extrapolating the dynamic response characteristics; thus obviating the need to capture actual collisions during simulation. However, the prior work considers randomness only from the irregular waves. This paper extends the prior work by developing a method to account for multiple uncertainties. The random variables considered herein are the current, drag coefficient, vessel motions, and riser mass. The proposed method is computationally efficient; the additional simulations necessary to incorporate four random variables are only slightly more than the original simulation case. Using a top-tensioned riser system as a case study, the likelihood of collision predicted by the proposed method is found to compare well with the Monte Carlo simulation. Moreover, it is shown that the random variables can increase the probability by an order of magnitude and all of the considered variables meaningfully contribute to this increase.

Sensitivity study of wake oscillator parameters influencing the fatigue damage of a riser undergoing VIV

A great deal of attention has recently been paid to the semiempirical modelling, nonlinear response prediction and estimation of stress and fatigue damage of marine risers undergoing vortex-induced vibrations (VIV). This is because when as the offshore industry move into deeper waters, the impact of VIV caused by ocean currents becomes detrimental being one of great concerns for offshore operators and riser engineers. For computational efficiency in time domain, the van der Pol wake oscillator, coupled with the structural equation of motion, has been an attractive approach. However, wake oscillator models rely on empirical coefficients, which have uncertainties whose effect on the fatigue reliability is not well understood. This study focusses on the wake oscillator proposed by Skop and Balasubramanian (1997). Using a toptensioned riser model, a sensitivity study is performed on several random variables to determine their influence on the uncertainty of the fatigue damage. These variables include the maximum vibration amplitude, frequency ratio, damping ratio and lift coefficient. In addition, Monte Carlo simulations are conducted to predict the probability of failure.