Research On Collapse Mechanism And Dynamic Catastrophe Simulation For Jacket Platforms Under Super Typhoon

With the high demand for oil and gas resources, the Chinese Government is stepping up implementation of the South China Sea oil and gas resources exploration strategy, while typhoons trend to more frequent, powerful and strong in recent years, which pose a serious safety hazard to the offshore oil and gas facilities. So it’s necessary and essential to research the collapse mechanism and dynamic catastrophe simulation of the offshore platform under extreme typhoon, to provide technical support for the project of large jacket platform’s typhoon-resistant design and safe operation. The dissertation is supported by the National Natural Science Foundation Program(No. 50679083) of “Dynamic Catastrophe Simulation and Damage Control of large Offshore Jackets under Super Typhoon”. The theory and methods of collapse analysis and simulation has been studied systematically. The research covers some hot issues of wind load accurate assessment, wave-in-deck calculation, structural ultimate limit state analysis, dynamic collapse determination, collapse simulation and robustness assessment for progressive collapse. The main work is summarized as follows:1. Wind load analysis and numerical simulation of jacket platform topsidesThe scale model of large jacket platform topside is built, using for wind tunnel test. Wind pressure distribution and load are effective measured by Miniature Pressure Scanner and High Frequency Base Force Balance respectively. Wind field of the topside is simulated based on CFD. Research on the rules of pressure, shape coefficient and wind load, and contrastive analysis among experiment, simulation and standard are carried out. The results show good agreement between the experimental and simulation results, but more conservative of standard recommended practice. Further, the wind load and exertion method of the topside are performed under super typhoon named “Haiyan” using the partitioned loading method, based on the wind load calculator result. The research of this chapter can provide valuable reference for the safety assessment of topside structure in typhoon condition.2. Research on extreme wave loads of jacket platforms under super typhoonRegular and irregular wave theory are analysed, and calculation programs of surface elevation and water particles velocity are developed using MATLAB. Considering on the spectrum, random and nonlinear characteristic of extreme wave, the short-time random wave process is simulated based constrained New Wave theory and P-M wave spectrum. In view of the issue of wave-in-deck(WID) loading caused by typhoon on fixed offshore platform, the ‘detailed’ method and ‘silhouette’ approach of WID are investigated and summarized. The theoretical formulas of WID histories are derived and WID processes are simulated based on Ariy wave, Stokes 5th wave and New Wave theory. Equivalent drag coefficient called Ceq is introduced in order to compare the differences and similarities of different WID models which provide reference in the choice of WID methods. Based on simplified three-line hydrodynamic load history of WID, the calculation method and procedure of the WID are established according to different wave theory, and finally the simple, providing support to the performances assessment of jacket platforms under WID load.3. Evaluation of collapse performance for typhoon resistant of jacket platformMulti-dimensional nonlinear finite element model of large jacket platform is built. The anti-typhoon ultimate bearing capacity(UBC) of deepwater jacket platforms is researched using Pushover method. The UBC curves and plastic strain distribution(PSD) of the design loads applied in three directions(end-on, broadside, and diagonal) are determined by considering two boundary constraints, i.e., clamped support and pile-soil nonlinear springs; further, the inherent relationship between the UBC and failure mode(FM) of the platform is investigated. Additionally, the WID loads at different heights are taken into account for pushover analysis to reveal the collapse mechanism in typhoon condition. A novel approach called Load Sequence Incremental Analysis(LSIA) is established to estimate the anti-collapse performances of jacket platforms against typhoon environmental. Based on the equivalent bearing capacity curve obtained by LSIA, an appropriate parameter called collapse return period(NCP) was introduced, which make the evaluation indicator of platform performance more visualized, and provided direct information for platform integrity management and risk warning. Environmental parameters are determined using 3-parameter Weibull distribution. On this basis, two different configurations of the platform are used for anti-collapse assessment utilizing LSIA. In this way, the advantage of LSIA is illustrated.4. Nonlinear dynamic analysis of jacket platform under extreme conditionsBased on the theories of structural nonlinear dynamic analysis and HHT-α numerical integration method, dynamic performances of the platform were studied for different inundation levels of WID. The dynamic amplification factor(DAF) is discussed in elastic and plastic response stages. The influence of dynamic amplification effect on structure ultimate bearing capacity as well as failure modes effect is also investigated. Two novel approach called Dynamic Pushover Method(DPA) and Load Sequence Incremental Dynamic Analysis(LSIDA) are proposed, and the dynamic collapse criterion and assessment indicators are provided, which promote the development and improvement of the methods for collapse analysis and assessment of deepwater jacket platform. The comparative analysis between DPA and Pushover is conducted from three aspects, which are capacity, ductility and failure mode, as well as between LSIDA and LSIA, to illustrate the advantages of dynamic collapse analysis. Further, the dynamic collapse behavior is analysed under irregular wave by applied New Wave theory to DPA. The dynamic response of the platform is investigated under random wave load, and statistical parameters of displacement are acquired using Monte Carlo method based on constrainted New Wave.5. Dynamic catastrophe simulation for entire collapse process of jacket platformsThe simulation method of the entire process for progressive collapse due to typhoon is established combine with the advantages of implicit and explicit algorithms. The equivalent transformation program of implicit and explicit finite element model is developed using APDL and secondary development technology to overcome the difficulties of highly-efficient calculation and exact exertion of dynamic load caused by typhoon. Component’s failure criteria are proposed, and the nonlinear factors such as contact and collision for post-buckling and progress collapsed are considered. On this basis, the entire catastrophe collapse process of one steel jacket platform caused by strong typhoon was achieved using LD-DYNA, and the force redistribution and sequence of components failures are clearly displayed, as well as the system collapse mechanism is unveiled. This study can provide valuable reference in the aspect of jacket platform anti-typhoon design and theory.6. Assessment of robustness for progressive collapse of jacket platformsIn foundation of ultimate limit states analysis, the influence of damage to the platforms is investigated using Alternate Path(AP) method. Deterministic indexes named Residual Strength Reserve(DSR), Residual Influence Factor(RIF) and Strength Redundancy Factor(SRF) are used for robustness assessment of jacket platform; In view of environment load impact on UBC of platforms, ‘Safety Assessment Figure’ is proposed combine Multi Strips Analysis(MSA) and LSIA, to evaluate collapse probability effectively. Quantitative assessment of probabilistic robustness is established. Further, risk analysis is introduced on the assessment of robustness of structure, and conditional robustness index and analysis procedure of platform performances is investigated systematically. This study provides valuable reference to the collapse-resistant design of fixed platforms.