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The real-time dynamic deformation monitoring of offshore platforms under environmental excitation is crucial to their safe operation. Although Global Navigation Satellite System-Precise Point Positioning (GNSS-PPP) has been considered for this purpose, its monitoring accuracy is relatively low. Moreover, the influence of background noise on the dynamic monitoring accuracy of GNSS-PPP remains unclear. Hence, it is imperative to further validate the feasibility of GNSS-PPP for deformation monitoring of offshore platforms. To address these concerns, vibration table tests with different amplitudes and frequencies are conducted. The results demonstrate that GNSS-PPP can effectively monitor horizontal vibration displacement as low as ±30 mm, which is consistent with GNSS-RTK. Furthermore, the spectral characteristic of background noise in GNSS-PPP is similar to that of GNSS-RTK(Real Time Kinematic). Building on this observation, an improved Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN) has been proposed to de-noise the data and enhance the dynamic monitoring accuracy of GNSS-PPP. Field monitoring application research is also undertaken, successfully extracting and analyzing the dynamic deformation of an offshore platform structure under environmental excitation using GNSS-PPP monitoring in conjunction with improved CEEMDAN de-noising. By comparing the de-noised dynamic deformation trajectories of the offshore platform during different periods, it is observed that the platform exhibits reversible alternating vibration responses under environmental excitation, with more pronounced displacement deformation in the direction of load action. The research results confirm the feasibility and potential of GNSS-PPP for dynamic deformation monitoring of offshore platforms. [ABSTRACT FROM AUTHOR]

The efficiency of a tuned liquid damper (TLD) in controlling the dynamic responses of offshore monopile platforms under seismic excitation has been investigated in this paper. Damping is performed by applying a type of reservoir inside a tower, which is designed optimally via seawater and a monopile body. Hydrodynamic forces due to water surface oscillation in the reservoir act as resistant forces against structure vibration and displacement. Using ANSYS finite element (FE) software, a monopile structure with the same dimensions as the samples in the Persian Gulf climate was modeled and then analyzed in this research using the transient time history analysis related to the records of El-Centro, Kobe, and Tabas earthquakes for seismic investigation. The dynamic responses of the monopile platform with and without TLD were compared after the completion of FE results. Findings show that using the mentioned TLDs reduced structure displacement by more than 50% based on the earthquake frequency content. [ABSTRACT FROM AUTHOR]

DC microgrids present a very effective solution that enables the power systems of offshore platforms to achieve increased integration of renewable sources. Since the areas of offshore platforms are limited, the associated DC microgrids have lower line impedances, and short-circuit faults cause fault currents to rise rapidly. Thus, fault detection is a challenging issue due to the strict time limits for interruption imposed by these rapid rising fault currents. According to the fault characteristics and the ring structure of DC microgrids, this paper proposes a rapid detection scheme based on the differential current and current derivative without de-energizing the entire DC microgrid. It achieves rapid and selective fault detection and ensures an uninterruptible load power. The synchronization issues of the current differential and a rapid processing method for fault currents are investigated. The tripping threshold settings are discussed. Implementation of the fault detection scheme is also presented in detail. The proposed scheme is verified on a physical experimental platform. It is shown that here are some advantages such as good selectivity, low cost, and rapid fault detection. The scheme provides a strong guarantee for the uninterruptible operation of important equipment on offshore platforms. [ABSTRACT FROM AUTHOR]

The aim of this work is to study the stress distributions and the location of hot spots stress in the vicinity of the intersection lines of the tubular elements of the tubular TY-joints. Using the finite element models, we analyze the effects of geometrical parameters on the stress concentration factor in the case of in-plane bending and out-of-plane bending loads, around the weld toe of the tubular joints. Our results reveal the location of the maximum stress concentration factor at the heel or toe in the case of in-plane bending loads and at the saddle point in the case of out-of-plane bending loads. Six parametric equations are established and used to calculate the stress concentration factor at critical locations using the non-linear regression method. The results obtained from the finite element analysis are close to the results of the parametric equations and the experimental data from the previous work. [ABSTRACT FROM AUTHOR]

In this paper, structural characteristics are evaluated by displacement and frequency indicators that indicate the real-time health status of offshore platforms. This paper uses an accelerometer to collect the dynamic response of the platform in the event of a ship collision. The main contributions of this research are reflected in three aspects. Firstly, based on Empirical Mode Decomposition (EMD) multiscale decomposition, the noise range is determined according to the scale and the average value of the standardized accumulation mode, and the original acceleration sequence is denoised. Secondly, two impact tests were carried out to understand the platform's structural characteristics under an external load. Combined with the FFT algorithm and Hilbert Huang transform, the three-dimensional information of the time, frequency, and energy is analyzed. Finally, a method of high-frequency dynamic displacement reconstruction is proposed. According to the extracted vibration frequency information, the parameters for the filter are reasonably set, and the denoised acceleration time sequence is processed with bandpass filtering and quadratic integration to obtain the high-frequency dynamic displacement of the structure. The results show that the high-frequency dynamic displacement of the accelerometer reconstruction is 1.5 mm. Two collision event frequencies, 1.477 Hz and 1.483 Hz, were successfully extracted from the north direction. [ABSTRACT FROM AUTHOR]

In-place analysis for offshore platforms is essentially required to make proper design for new structures and true assessment for existing structures. The structural integrity of platform components under the maximum and minimum operating loads of environmental conditions is required for risk assessment and inspection plan development. In-place analyses have been executed to check that the structural member with all appurtenances has robustness and capability to support the applied loads in either storm condition or operating condition. A nonlinear finite element analysis is adopted for the platform structure above the seabed and the soil–pile–jacket interaction to estimate the in-place behavior of a typical fixed offshore platform. The analysis includes an interpretation of the dynamic design parameters based on the available site-specific data, together with foundation design recommendations for in-place loading conditions. The SACS software is utilized to calculate the natural frequencies, dynamic amplification factors, and the stresses at selected members, as well as their nodal displacements. The directions of environmental loads and water depth variations have important effects on the results of the in-place analysis behavior. The incidence angle of seismic waves is an essential parameter, where, for some crucial angles, the internal forces and the maximum deformations can be doubled or more in comparison with the ones corresponding to other, less crucial angles. Therefore, the incidence angle considerably affects the response quantities. The results confirm that the in-place analysis is quite essential for the reliable design of new offshore platforms and the assessment of existing offshore structures. [ABSTRACT FROM AUTHOR]

Coupled dynamic analyses of a deep-water Semi-submersible platform, in the South China Sea region, is carried out under postulated damage of the restraining system for both 10 and 100-years return period events. Under the combined action of wind, wave, and current loads, motion responses of Semi-submersible at 1500 and 2000 m water depths are analyzed in time-domain. Dynamic tension variations in the mooring lines are investigated for a fatigue failure using the S–N curve approach. Inclusion of a submerged buoy in the mooring system resulted in a marginal increase of the response due to a reduction in the restoration force of the mooring lines; submerged buoy also resulted in additional damping. The results of numerical studies showed an increase in tension in the mooring lines, which are adjacent to the damaged ones, causing reduced fatigue life. With the inclusion of submerged buoy in the mooring system, there is a considerable decrease in tension variation in mooring lines, increasing fatigue life. Failure of a mooring line causes an increase in tension of the adjacent mooring line, but not valid under all circumstances. It is seen from the studies that despite the postulated failure induced in a mooring, the adjoining line remains unaffected due to a steady coupling motion of the platform. [ABSTRACT FROM AUTHOR]

The identification of variations in the dynamic behavior of structures is an important subject in structural integrity assessment. Improvement and servicing of offshore platforms in the marine environment with constant changing, requires understanding the real behavior of these structures to prevent possible failure. In this work, empirical and numerical models of jacket structure are investigated. A test on experimental modal analysis is accomplished to acquire the response of structure and a mathematical model of the jacket structure is also performed. Then, based on the control theory using developed reduction system, the matrices of the platform model is calibrated and updated. The current methodology can be applied to prepare the finite element model to be more adaptable to the empirical model. Calibrated results with the proposed approach in this paper are very close to those of the actual model and also this technique leads to a reduction in the amount of calculations and expenses. The research clearly confirms that the dynamic behavior of fixed marine structures should be designed and assessed considering the calibrated analytical models for the safety of these structures. [ABSTRACT FROM AUTHOR]

The development of heavy oil with high efficiency is a worldwide difficulty for offshore oil field. The technology of rod pumping provides a possible effective way for offshore heavy oil thermal recovery, but the safety of working platform is the prerequisite for the implementation of this new technology. In this paper, the mechanical model of LD27-2 WHPB platform is established, and the safety performance of the platform under hydraulic pumping unit (HPU) load is evaluated. The distribution of the combined HPU load accords with the classical probability model. When the HPUs are all synchronous, the combined load reaches its maximum. The finite element-based platform safety analysis under the extreme condition is carried out. Under the combined action of wave current, wind load and the extreme HPU load, the maximum stress of the jacket is 83.2 MPa, and the safety coefficient is 4.33, indicating the overall strength of LD27-2 WHPB platform meets the safety requirement. [ABSTRACT FROM AUTHOR]

This paper investigates the possibility of utilizing response from natural ice loading for modal parameter identification of real offshore platforms. The test platform is the JZ20-2MUQ jacket platform located in the Liaodong Bay, China. A field experiment is carried out in winter season, as the platform is excited by floating ices. The feasibility is demonstrated by the acceleration response of two different segments. By the SSI-data method, the modal frequencies and damping ratios of four structural modes can be successfully identified from both segments. The estimated information from both segments is almost identical, which demonstrates that the modal identification is trustworthy. Furthermore, by taking the Jacket platform as a benchmark, the numerical performance of five popular time-domain EMA methods is systematically compared from different viewpoints. The comparisons are categorized as: (1) stochastic methods versus deterministic methods; (2) high-order methods versus low-order methods; (3) data-driven versus covariance-driven stochastic subspace identification methods. [ABSTRACT FROM AUTHOR]

The structural damage identification, which entails finding a best approximate solution to an over-determined system of linear equations, is actually one of the practical problems encountered in structural health monitoring. A major challenge to structural damage identification is the ill-conditioned problem caused by noise contamination and spatially incomplete measurements. A novel Tikhonov regularization iterative method (TRIM) is proposed to solve the ill-conditioned system of linear equations. This method iteratively reconstructs the regularization matrix by gradually updating the regularized solution. A merit is that the false-positive indicators of damage are greatly reduced; and as a result this approach would be able to detect smaller damages that could not be detected by using traditional approaches. Another development embedded in TRIM is that a procedure, called singular value dichotomy, is developed to determine the regularization parameters. Several problems, such as the tremendous prior trials, due to the application of L-curve, are avoided. A numerical study is conducted on an offshore platform structure to demonstrate the effectiveness of the proposed method. The result shows that the new method outperforms the traditional Tikhonov regularization method in damage identification. [ABSTRACT FROM AUTHOR]

A modified multiple generalized regression neural network (GRNN) is proposed to predict the noise level of various compartments onboard of the offshore platform. With limited samples available during the initial design stage, GRNN can cause errors when it maps the available inputs to sound pressure level for the entire offshore platform. To obtain more relevant group for GRNNs training, fuzzy C-mean (FCM) is used. However, outliers in some group may interfere the prediction accuracy. The problem of selecting suitable inputs parameters (in each cluster) is often impeded by lack of accurate information. Principal component analysis (PCA) is used to ensure high relevance input variables in each cluster. By fusing multiple GRNNs by an optimal spread parameter, the proposed modeling scheme becomes quite effective for modeling multiple frequency-dependent data set (ranging from 125 to 8000 Hz) with different input parameters. The performance of FCM-PCA-GRNNs has improved significantly as the results show a 25% improvement on the spatial sound pressure level (SPL) and 85% improvement on the spatial average SPL than just GRNNs alone. By comparing with data obtained from real engine room on a jack-up rig, the FCM-PCA-GRNNs noise model performs better with around 16% less error than the empirical-based acoustic models. Additionally, the results show comparable performance to statistical energy analysis that requires more time and resources to solve during the early stage of the offshore platform design. [ABSTRACT FROM AUTHOR]

The high-efficient development of heavy oil is difficult for offshore oil field. Based on the mature technology of onshore heavy oil thermal recovery, a new rod pumping technology of “heavy oil steam stimulation along with conventional sucker rod pumping system” is proposed. According to the structure space of aimed offshore platform, we designed one new kind of miniature hydraulic pumping unit with long-stroke, low pumping speed and compact structure. The paper also studies the offshore heavy oil thermal recovery well safety technology, establishes a leakage model of wellhead sealing and a mechanical model of sealed rubber sleeve on downhole nonlinear large deformation. The study shows that a series of equipment for offshore rod pumping oil recovery and the key technology of well safety lay a solid foundation for carrying out rod pumping of offshore heavy oil thermal recovery. [ABSTRACT FROM AUTHOR]

The aim of this research is to develop an algorithm and application that can perform real-time monitoring of the safety operation of offshore platforms and subsea gas pipelines as well as determine the need for ship inspection using data obtained from automatic identification system (AIS). The research also focuses on the integration of shipping database, AIS data, and others to develop a prototype for designing a real-time monitoring system of offshore platforms and pipelines. A simple concept is used in the development of this prototype, which is achieved by using an overlaying map that outlines the coordinates of the offshore platform and subsea gas pipeline with the ship’s coordinates (longitude/latitude) as detected by AIS. Using such information, we can then build an early warning system (EWS) relayed through short message service (SMS), email, or other means when the ship enters the restricted and exclusion zone of platforms and pipelines. The ship inspection system is developed by combining several attributes. Then, decision analysis software is employed to prioritize the vessel’s four attributes, including ship age, ship type, classification, and flag state. Results show that the EWS can increase the safety level of offshore platforms and pipelines, as well as the efficient use of patrol boats in monitoring the safety of the facilities. Meanwhile, ship inspection enables the port to prioritize the ship to be inspected in accordance with the priority ranking inspection score. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation of a float-over installation for an offshore platform is presented to verify the feasibility of the actual installation. The hydrodynamic performance of a T-barge is investigated in the frequency domain, and the coupled motions are analyzed in the time domain. We then compare with those of the model test and determine that the response amplitude operator and the time series agree quite well. The barge exhibits favorable hydrodynamic behavior in the considered sea state, and the equipment loads are allowable. The mooring system and sway fender forces are within the permissible range. Based on these results, we can verify that the actual installation of the offshore platform is feasible. We accurately simulated many important factors and effectively reduced the risk associated with the offshore installation, which is of great importance. As such, we demonstrate that the numerical simulation of the float-over installation for offshore platforms has practical engineering significance. [ABSTRACT FROM AUTHOR]

A new technology of offshore oil rod pumping production is developed for offshore heavy oil recovery. A new type of miniature hydraulic pumping unit with long-stroke, low pumping speed and compact structure is designed based on the spatial characteristics of offshore platforms. By combining the strengths of sinusoidal velocity curve and trapezoidal velocity curve, a kinematical model of the acceleration, the velocity and displacement of the pumping unit's hanging point is established. The results show that the pumping unit has good kinematic characteristics of smooth motion and small dynamic load. The multi-degree-of-freedom dynamic model of the single-well pumping unit is established. The first and second order natural frequencies of the sucker rod string subsystem and the pumping unit subsystem are studied. The results show that the first and the second order natural frequencies among the pumping rod string, pumping unit-platform subsystem and the dynamic excitation have differences over 5 times from each other, indicating that resonance phenomenon will not appear during the operation and the dynamic requirements for field use are met in the system. [ABSTRACT FROM AUTHOR]

This paper presents a robust mixed $$H_2 /H_\infty $$ control method for wave-excited offshore jacket platforms. Its objective was to design a controller that minimizes the upper bound of the $$H_2 $$ performance measure on platform dynamics satisfying some $$H_\infty $$ norm bound constraint simultaneously. Based on mixed $$H_2 /H_\infty $$ control theory and linear matrix inequality techniques, a novel approach to stabilize offshore platform vibration with constrained $$H_2 /H_\infty $$ performances is proposed. Uncertainties of the wave excitation are considered in dynamic performance analysis of offshore platforms. A reduced mode offshore platform structure under wave excitation is analyzed, and simulations are used to verify the effectiveness of the proposed approach. Compared with existing $$H_\infty $$ control methods, the proposed approach makes a significant improvement for dynamic performances of offshore platforms under random wave excitation. [ABSTRACT FROM AUTHOR]

This paper was devoted to presenting a systematic approach based on finite element method to numerical analysis of gear surface hardening with the consideration of the contact problem. After the contact finite element method theory of gear and rack was introduced, the numerical analysis of gear surface hardening was discussed in detail. The element selection of gear-rack hardening layer based on finite element method was proposed, the algorithm characteristics of gear-rack elastic-plastic contact problems were also put forward, and the material properties and constitutive relations of gear-rack hardening layer were then discussed in detail. The gear-rack elastic-plastic contact analysis results were discussed, and the experimental results with the laser treatment were analyzed with the microstructure changes of the laser hardening layer to measure the performance of the hardening layer. The wear simulating test of gear results in improving the wear resistance greatly, and this surface hardening treatment has been officially put into engineering application. [ABSTRACT FROM AUTHOR]

This paper presents numerical results of structural post- fire bahaviour of I-shaped beam-to-chord joints in offshore platforms topside. Considering the high risk of fire events in offshore oil/gas platforms, this study focuses on the structural behaviour of these connections at post- fire condition. A highly detailed three-dimensional (3-D) finite element (FE) model of this connection has been created using the ABAQUS software. Steel members and connection components are considered to behave nonlinearly. The results of finite element and experimental tests conducted on I shape beam-to-tubular column connections in furnace fire conditions are compared, and the obtained failure modes and moment-rotation-temperature characteristics are in good agreement with those associated with experimental tests. The validated model has been used to conduct numerical parametric studies to generate theoretical data to help develop detailed understanding of the joint behaviour in post-fire condition. [ABSTRACT FROM AUTHOR]

The interface behavior of a concrete grouted connection is studied in a monopile wind turbine offshore structure. The grouted connection between transition piece and monopile is fixed by filling it with grout materials made of high-strength concrete. We observe a nonlinear equilibrium path from the grouted connection in experiments. For comparison with the experimental results, nonlinear finite element analysis of the grouted connection is performed, taking the effects of both material and geometrical nonlinearities into consideration. A finite element model of the grouted connection between the steel and concrete materials is realistically simulated using an interface element. The finite element results compares well with the experimental results. Thus, the finite element analysis showed that the numerical model based on the interface element is a good method for investigating the behavior of grouted connections of monopile offshore structures. [ABSTRACT FROM AUTHOR]

Modal strain energy based methods for damage detection have received much attention. However, most of published articles use numerical methods and some studies conduct modal tests with simple 1D or 2D structures to verify the damage detection algorithms. Only a few studies utilize modal testing data from 3D frame structures. Few studies conduct performance comparisons between two different modal strain energy based methods. The objective of this paper is to investigate and compare the effectiveness of a traditional modal strain energy method (Stubbs index) and a recently developed modal strain energy decomposition (MSED) method for damage localization, for such a purpose both simulated and measured data from an offshore platform model being used. Particularly, the mode shapes used in the damage localization are identified and synthesized from only two measurements of one damage scenario because of the limited number of sensors. The two methods were first briefly reviewed. Next, using a 3D offshore platform model, the damage detection algorithms were implemented with different levels of damage severities for both single damage and multiple damage cases. Finally, a physical model of an offshore steel platform was constructed for modal testing and for validating the applicability. Results indicate that the MSED method outperforms the Stubbs index method for structural damage detection. [ABSTRACT FROM AUTHOR]

In this study, the effectiveness of a tuned liquid column-gas damper, TLCGD, on the suppression of seismicinduced vibrations of steel jacket platforms is evaluated. TLCGD is an interesting choice in the case of jacket platforms because it is possible to use the structural elements as the horizontal column of the TLCGD. The objective here is to find the optimum geometric parameters, namely orientation and configuration of vertical columns, length ratio, and area ratio of the TLCGD, considering nonlinear damping of the TLCGD and water-structure interaction between the jacket platform and sea water. The effects of different characteristics of ground motion such as PGA and frequency content on the optimum geometry are also investigated and it is observed that these features have some influence on the optimum area ratio. Finally it is observed that pulse arrangement of ground acceleration is one of the most important parameters affecting the efficiency of a TLCGD. In other words, it is found that the TLCGD's capability to reduce the RMS responses depends only on the frequency content of the ground acceleration, but its capability to reduce the maximum responses depends on both the frequency content and the pulse arrangement of the ground acceleration. [ABSTRACT FROM AUTHOR]

The dynamic equations of motion of asymmetric offshore platforms under three different environmental conditions: seismic action, wave action and their combination are established in this paper. In establishing these motion equations, three typical eccentricity types including mass eccentricity, rigidity eccentricity and their combination were considered, as are eccentricities that occur uni-directionally and bi-directionally. The effects of the eccentricity type, the dynamic characteristics and the environmental conditions on the torsional coupling response of platforms are investigated and compared. An effort has also been made to analyze the influence of accidental eccentricity on asymmetric platforms with different eccentricity in two horizontally orthogonal directions. The results are given in terms of non-dimensional parameters, accounting for the uncoupled torsional to lateral frequency ratio. Numerical results reveal that the eccentricity type has a great influence on the torsionally coupled response under different environmental conditions. Therefore, it is necessary to consider the combination of earthquake and wave action in the seismic response analysis of some offshore platforms. [ABSTRACT FROM AUTHOR]

In order to validate the feasibility of applying wireless sensing technique to structural monitoring of offshore platform, the experiment of wireless sensor network on offshore platform is presented in this paper. First, wireless sensor network and its topology structure is put forward, and the design of sensor nodes, base station, communication protocol is discussed according to self-developed wireless sensor network. Second, true offshore platform and its experimental model are introduced. Finally, wireless sensing experiment for offshore platform structure is completed and the analysis of the experimental result is given. The research shows that wireless sensor network applied to offshore platform can reflect the vibration of the structure; the sensor nodes are fixed and removed expediently, which saves the cost of signal line as well as installation time. [ABSTRACT FROM AUTHOR]

Jacket platform was simulated by non-uniform cantilever beam subjected to axial loading. Based on the Hamilton theory, the equation of bending motion was developed and solved by the classical Ritz method combined with the pseudo-excitation method (PEM) for non-stationary random response with non-classical damping. Usually, random response of this continuous structure is obtained by orthogonality of modes and some normal modes of the structure are needed, causing inconvenience in the analysis of the non-uniform beam whose normal modes are not easy to be obtained. However, if the PEM is extended to calculate random response by combining it with the classical Ritz method, the responses of non-uniform beam, such as auto-power spectral density (PSD) function, cross-PSD and higher spectral moments can be solved directly avoiding the calculation of normal modes. The numerical results show that the present method is effective and useful in aseismic design of platforms. [ABSTRACT FROM AUTHOR]

A new method that uses time-domain response data under random loading is proposed for detecting damage to the structural elements of offshore platforms. In our study, a time series model with a fitting order was first constructed using the time-domain of noise data. A sensitivity matrix consisting of the first differential of the autoregressive coefficients of the time series models with respect to the stiffness of structural elements was then obtained based on time-domain response data. Locations and severity of damage may then be estimated by solving the damage vector whose components express the degrees of damage to the structural elements. A unique aspect of this detection method is that it requires acceleration history data from only one or a few sensors. This makes it feasible for a limited array of sensors to obtain sufficient data. The efficiency and reliability of the proposed method was demonstrated by applying it to a simplified offshore platform with damage to one element. Numerical simulations show that the use of a few sensors’ acceleration history data, when compared with recorded levels of noise, is capable of detecting damage efficiently. An increase in the number of sensors helps improve the diagnosis success rate. [ABSTRACT FROM AUTHOR]

Structural cracks can change the frequency response function (FRF) of an offshore platform. Thus, FRF shifts can be used to detect cracks. When a crack at a specific location and magnitude occurs in an offshore structure, changes in the FRF can be measured. In this way, shifts in FRF can be used to detect cracks. An experimental model was constructed to verify the FRF method. The relationship between FRF and cracks was found to be non-linear. The effect of multiple cracks on FRF was analyzed, and the shift due to multiple cracks was found to be much more than the summation of FRF shifts due to each of the cracks. Then the effects of noise and changes in the mass of the jacket on FRF were evaluated. The results show that significant damage to a beam can be detected by dramatic changes in the FRF, even when 10% random noise exists. FRF can also be used to approximately locate the breakage, but it can neither be efficiently used to predict the location of breakage nor the existence of small hairline cracks. The FRF shift caused by a 7% mass change is much less than the FRF shift caused by the breakage of any beam, but is larger than that caused by any early cracks. [ABSTRACT FROM AUTHOR]

API code is introduced at first, and then a comparison of seismic character and seismic hazard between Bohai and Southern California is carried out. The analysis indicates both the seismic frequency and intensity and the seismic hazard of Bohai are much weaker than that of Southern California. API code states the strength level and deformation level of permanent structures in Southern California takes 200 a and several hundred to a few thousand years respectively. But in the reference codes in China, the seismic levels take 500 a and 10 000 a for strength design and deformation design, and it seems too conservative. In China, the deformation level of class A structure takes 2%–3% probabilities of exceedance in reference period 100 a, and that of class B and C often takes 2%–3% in 50 a. Now that offshore platforms may cause server subsequent risk, it is safe to take 1% in 30 a as its deformation design level. On the basis of the above analyses and social economic level and the consistency with present codes, the strength design level and deformation design level of Chinese offshore platforms is suggested to take 200 a and 3 000 a respectively. [ABSTRACT FROM AUTHOR]