Time-and Frequency-Domain Analysis of Ice-Induced Displacement Responses Offshore Fixed Platforms

On the basis of ice- induced forced vibration model, ice- induced displacement responses of offshore fixed platforms are investigated in both time domain and frequency domain. The relationships of ice-induced displacement responses with ice breaking modes, ice acting directions and platform structures are analyzed and determined. The results lead to an important conclusion obtained for the first time that ice breaking frequency and the natural frequency of the first mode of the platform are the two main factors that dominate the degree of vibration. The present work provides a firm basis for both design and operation of fixed platforms against ice loading.

Seismic Response Reduction for Fixed Offshore Platform by Tuned Liquid Damper

An approach to seismic response reduction for offshore platforms by the use of the tuned liquid damper is presented in this paper. First of all, the effects of the tuned frequency ratio and excited frequency ratio on the seismic response reduction of the platform structure are investigated. Based on the results, a mechanical model and equation of motion for the TLD-platform system are established. And then effectiveness of the appraoch is verified by numerical calculation.

Dynamic Ice-Induced Responses of Offshore Fixed Platforms

The dynamic responses of offshore fixed platforms under ice loads are analysed in this paper. The responses calculated include the displacement and the hot spot stresses on the basis of a dynamic equation of ice-induced forced vibration for offshore structures. The results obtained can be cited and used for the design and operation of offshore fixed platforms against ice loading.

Semi-Active TLCD Control of Fixed Offshore Platforms Using Artifical Neural Networks

In this paper, the control method for fixed offshore platforms using semi-active tuned liquid column damper (TLCD) is presented. The equation of motion for the platform-TLCD control system is given and the semi-active control strategy is established. A back propagation artificial neural network (ANN) is used to adjust the orifice opening of TLCD because of the nonlinear motion of liquid in TLCD. The effectiveness of the control method is verified by numerical examples.

Long-Term Effect of TMD on Vibration Control of An MDOF Offshore Fixed Platform

A three-dimensional fixed offshore platform in deep water modeled by the finite element method is studied in this paper. Analysis of the dynamic response of the MDOF structure is realized taking the non-linearity of the wave drag force and the wave-structure interaction into account. The structural response statistics, which have Gaussian distributions, are used to evaluate the vibration effect of the structure without TMD and with TMD. And an optimal method to design TMD controlling the first mode of the multi-mode structure is proposed. Moreover, the probabilities of occurrence of sea states at the platform site are considered for prediction of the long-term effect of a TMD. Simulation results demonstrate that the long-term effect of a well-designed TMD is good and the practical use is possible due to the good stability of its optimal parameters under different sea states.

Inelastic Nonlinear Pushover Analysis of Fixed Jacket-Type Offshore Platform with Different Bracing Systems Considering Soil-Structure Interaction

In this study, inelastic nonlinear pushover analysis is performed on a 3-D model of a jacket-type offshore platform for the North Sea conditions. The structure＇ is modelled, analyzed and designed using finite element software SACS （structural analysis computer system）. The behavior of jackets with different bracing systems under pushover analysis is examined. Further, by varying the leg batter values of the platform, weight optimization is carried-out. Soil-structure interaction effect is considered in the analyses and the results are compared with the hypothetical fixed-support end condition. Static and dynamic pushover analyses are performed by using wave and seismic loads respectively. From the analyses, it is found that the optimum leg batter varies between 15 to 16 and 2% of weight saving is achieved. Moreover, it has been observed that the type of bracing does not play a major role in the seismic design of jacket platform considering the soil-structure interaction.

Investigation of Allowable Stresses for Fatigue Design of Fixed Offshore Platforms

Fatigue design method is usually used for estimating whether a platform structure meets the requirements of fatigue resistance. It is described in API RP-2A and ZC rules for fixed offshore platforms, in which the allowable stresses for fatigue design are defined. In this paper discussed are the allowable stresses related factors, such as wave climate, structural response, fatigue behaviour of structural members.

A preliminary study on seismic design criteria of offshore platforms in Bohai Sea of China

This paper analyzes the seismicity in Bohai Sea,introducing a shape factor K to characterize the seismic risk distribution in sub-regions of the sea. Based on the seismic design ground motions for 46 platforms located in the Bohai Sea,a statistical analysis was performed for different peak ground acceleration (PGA) ratios at two different probability levels. In accordance with the two-stage design method,a scheme of two seismic design levels is proposed,and two seismic design objectives are established respectively for the strength level earthquake and the ductility level earthquake. By analogy with and comparison to the Chinese seismic design code for buildings,it is proposed that the probability level for the strength level earthquake and ductility level earthquake have a return period of 200 and 1000 - 2500 years,respectively. The validity of these proposed values is discussed. Finally,the PGAs corresponding to these two probability levels are calculated for different sub-regions of the Bohai Sea.

Seismic response of a typical fixed platform foundation under earthquake loading

Offshore platforms in seismically active areas should be designed to service severe earthquake excitations with no global structural failure. This paper summarizes the dynamic analysis of a typical fixed platform under the earthquake loading in the seismically active area. The dynamic analysis includes interpretation of dynamic design parameters based on the available site-specific data,together with foundation design recommendations for earthquake loading conditions,which include free-field site response analyses,liquefaction analyses and soil-pile interaction analyses.

A comparison of the short-term effects of fixed platform versus rotating high-fiexion prosthesis for the patients with osteoarthrities of knee

Objective To compare the preliminary clinical outcome between fixed platform and rotating high-flexion prosthesis following total knee replacement(TKR).Methods Form January 2007 to December 2009,68 patients with osteoarthritis of