Jacket-type Offshore Structures Research Articles

Damage localization and quantification in offshore jacket structures using signal processing and intelligent system

Offshore platforms operate in harsh environmental conditions, and a significant portion of their structure is submerged underwater. Consequently, inspection and repair operations are costly and complex, making it crucial to have a precise damage detection system to increase safety and reduce maintenance expenses. This paper presents an intelligent signal processing-based system for damage detection in offshore jacket platforms. First, two damage indicators based on the signal's entropy and power are introduced to localize damage in jacket-type offshore structures. Then, an intelligent system that combines an artificial neural network and several signal processing techniques is used to quantify damage in various parts of these structures. To this end, various signal processing techniques extract the response signal features. Next, the proposed neural network model is trained using a data set, including the extracted feature vectors and the severity of the known damage scenarios. As a case study, an offshore jacket platform installed in the Persian Gulf was examined to evaluate the proposed method's effectiveness. The results reveal that the provided method detects the damage locations and severities with high processing speed and accuracy. Further, selecting suitable features and performing initial processing on training feature vectors can increase the accuracy of detecting damage intensity.

Seismic response analysis of fixed jacket-type offshore structures based on power spectrum density Driven input

ABSTRACT Offshore platforms in seismically active areas should be properly designed to survive within the face of intense earthquakes without a global structural collapse. This paper scrutinizes the seismic performance of a newly designed and established jacket type offshore platform based on the API-RP2A normalized response spectra during seismic events. A finite element model is developed from a typical jacket type offshore platform taking into the effect of the interaction among structure, pile, and soil components. The seismic responses of jacket type offshore platforms subjected to random earthquake excitations are evaluated by means of the power spectral density (PSD) analysis. Dynamic characteristics, the response function, output PSD and transfer functions for various elements of the platform are discussed. The spectrum compatible PSD is directly used to estimate the peak structural responses and determine the dynamic response of offshore jacket platforms that meet the required level of engineering practice for preliminary design.

Offshore petroleum rigs/platforms: An overview of analysis, design, construction and installation

Jacket platforms are one of the most important and regularly used types of offshore structures for oil and gas extraction that have a big impact on the economy of the countries. In this paper, all aspects including design, analysis, construction and installing of the jacket type offshore structure, are summarized and classified. This type of structure is one of the specified platforms for shallow water, and for long term service, it also has the ability to carry large deck loads. This paper aims to present general guidance about the planning, design and construction of offshore jacket (template) platforms. Jacket platforms are fixed type platforms which are attached to the seabed using piles which provide stability against the wind, wave and current loads. Also, this type of offshore platform has a high initial and maintenance cost because of its exposure to corrosion, and cannot be reused after the end of its service period. Jacket platforms are most suitable for shallow water having no better alternative while it has the cost disadvantage for deep water.

Fatigue Assessments of a Jacket-Type Offshore Structure Based on Static and Dynamic Analyses

AbstractOffshore structures are mainly associated with the oil and gas industry given that globally, nearly one-third of the oil and gas extracted worldwide comes from offshore sources. This situat...

Structural model updating of the Gageocho Ocean Research Station using mass reallocation method

To study oceanic and meteorological problems related to climate change, Korea has been operating several ocean research stations (ORSs). In 2011, the Gageocho ORS was attacked by Typhoon Muifa, and its structural members and several observation devices were severely damaged. After this event, the Gageocho ORS was rehabilitated with 5 m height to account for 100-yr extreme wave height, and the vibration measurement system was equipped to monitor the structural vibrational characteristics including natural frequencies and modal damping ratios. In this study, a mass reallocation method is presented for structural model updating of the Gageocho ORS based on the experimentally identified natural frequencies. A preliminary finite element (FE) model was constructed based on design drawings, and several of the candidate baseline FE models were manually built, taking into account the different structural conditions such as corroded thickness. Among these candidate baseline FE models, the most reasonable baseline FE model was selected by comparing the differences between the identified and calculated natural frequencies; the most suitable baseline FE model was updated based on the identified modal properties, and by using the pattern search method, which is one of direct search optimization methods. The mass reallocation method is newly proposed as a means to determine the equivalent mass quantities along the height and in a floor. It was found that the natural frequencies calculated based on the updated FE model was very close to the identified natural frequencies. In conclusion, it is expected that these results, which were obtained by updating a baseline FE model, can be useful for establishing the reference database for jacket-type offshore structures, and assessing the structural integrity of the Gageocho ORS.

Stochastic Modeling of Forces on Jacket-Type Offshore Structures Colonized by Marine Growth

The present paper deals with the stochastic modeling of bio-colonization for the computation of stochastic hydrodynamic loading on jacket-type offshore structures. It relies on a multidisciplinary study gathering biological and physical research fields that accounts for uncertainties at all the levels. Indeed, bio-colonization of offshore structures is a complex phenomenon with two major but distinct domains: (i) marine biology, whose processes are modeled with biomathematics methods, and (ii) hydrodynamic processes. This paper aims to connect these two domains. It proposes a stochastic model for the marine organism’s growth and then continues with transfers for the assessment of drag coefficient and forces probability density functions that account for marine growth evolution. A case study relies on the characteristics (growth and shape) of the blue mussel (Mytilus edulis) in the northeastern Atlantic.

Optimized SMA Dampers in Vibration Control of Jacket-type Offshore Structures (Regular Waves)

Optimized SMA Dampers in Vibration Control of Jacket-type Offshore Structures (Regular Waves)

고정식 자켓형 해양구조물의 지반 물성치에 따른 구조 응답에 관한 연구

For a fixed jacket type offshore structure directly supported by the seabed, the structural behavior of offshore structure depends on the soil properties. Soil properties affect on the stiffness of the piles and the boundary condition in the structural analysis. The structural analysis is performed using PSI (Pile-Soil Interaction) suggested in the code and design rule. PSI analysis of the jacket structure is carried out after various soil types are selected according to the soil properties like internal friction angle, undrained shear strength, unit weight and so on. Three types of soil are selected by varying strength for a clay and sand, respectively. The structural analysis of the jacket structure is performed using these soils. The results about axial and lateral reaction force and the stress and displacement on the structure are compared. As a results, the structural response is smaller as the soil becomes more stiff. In conclusion, it is confirmed that the structural response of fixed jacket type offshore platform supported by seabed is sensitive to the change of soil properties.

Residual compressive strength of inclined steel tubular members with local corrosion

In jacket-type offshore structures, corrosion damage affects the structural performance under compressive loading, which is created by the working and design loads of the main system. In this study, the effects of corrosion damage on the compressive structural behavior and strength of steel tubular members were investigated. Artificial corrosion damage was applied to the tubular specimens via mechanical processing and hand drilling to replicate the inclined nature of jacket-type offshore structures. The damage was applied to either half or all of the circumference of the specimens. The compressive failure modes of the artificially corroded tubular members were affected by the corrosion conditions. The compressive strengths were also affected by the level of corrosion. From the results of this study, the residual compressive strengths of corroded tubular steel members can be estimated based on the condition of the damaged sectional areas.

Laboratory tests on local damage detection for jacket-type offshore structures using optical FBG sensors based on statistical approaches

In this study, a local damage detection based on statistical approach for jacket-type offshore structures by principal component analysis (PCA) and linear adaptive filter (LAF) techniques using strain response data measured by FBG sensors was proposed while dynamic responses are being popularly utilized for damage detection of civil infrastructures including jacket-type offshore structures. In addition, environmental effects due to variations in temperature and external loading were intensively investigated and an efficient remedy was proposed using the nonparametric PCA and LAF models. Unlike many existing statistical damage detection methods, the mean of residual values eliminating the environmental effects was utilized as damage index for rational for enhancing the normality based on the central limit theorem and the normality was first checked before damage estimation using the mean of residual values. Laboratory tests for a scaled tidal current power plant structure, one of many jacket-type offshore structures, were conducted to investigate the technical feasibility of the proposed method for damage detection and localization. It was found that the PCA technique could more efficiently eliminate undesired environmental effects contained in the measurement data from FBG sensors without any additional information on the environmental changes, resulting in more damage-sensitive features under various environmental changes.

EXPERIMENTAL TESTS ON LOCAL DAMAGE DETECTION USING OPTICAL FBG SENSORS

In this study, the fiber Bragg grating (FBG)-sensor based local damage detection method is proposed under circumstances with temperature and external loading variations. To compensate the environmental effects, principal component analysis (PCA) is utilized and also the performance of PCA is compared with that of the conventional linear adaptive filter (LF) model. Laboratory tests with a 1/20 scale model of a jacket-type offshore structure with six jacket-legs and a heavy super structure have been carried out for investigating the performance of the proposed damage detection method. From the experimental tests, it is observed that the local damage feature is mostly hidden and difficult to identify due to the environmental effects. By utilizing the conventional LF and PCA models, the effects of the undesirable environmental effects can be efficiently eliminated, and it is also found that the performances of the LF and PCA models are very similar and competitive to each other. However PCA model does not require the information on the temperature and external load variations, hence it can be concluded that the PCA-based local damage detection can be more efficiently applied for FBG-based local damage detection under temperature and external loading variations.

DYNAMIC ANALYSIS OF JACKET TYPE OFFSHORE STRUCTURE UNDER IMPACT OF WAVE AND WIND USING STOKE’S SECOND ORDER WAVE THEORY

This paper presents finite element algorithms and dynamic analysis of the jacket type offshore structure under water wave and wind impact by using stoke’s second order wave theory. The result of the paper is the scientific basis for the calculation, design and selection of the appropriate parameters, the study contributes to the optimization of fixed offshore structures such as buildings DKI, serving defense, security and contributing to improving the capacity to defend Vietnam’s sovereignty over seas and islands.

Electromechanical impedance-based long-term SHM for jacket-type tidal current power plant structure

Jacket-type offshore structures are always exposed to severe environmental conditions such as salt, high speed of current, wave, and wind compared with other onshore structures. In spite of the importance of maintaining the structural integrity for an offshore structure, there are few cases to apply a structural health monitoring (SHM) system in practice. The impedance-based SHM is a kind of local SHM techniques and to date, numerous techniques and algorithms have been proposed for local SHM of real-scale structures. However, it still requires a significant challenge for practical applications to compensate unknown environmental effects and to extract only damage features from impedance signals. In this study, the impedance-based SHM was carried out on a 1/20-scaled model of an Uldolmok current power plant structure in Korea under changes in temperature and transverse loadings. Principal component analysis (PCA)-based approach was applied with a conventional damage index to eliminate environmental changes by removing principal components sensitive to them. Experimental results showed that the proposed approach is an effective tool for long-term SHM under significant environmental changes.

Approximate Design of Optimal Disturbance Rejection for Discrete-Time Systems with Multiple Delayed Inputs: Application to a Jacket-Type Offshore Structure

The study is concerned with problem of optimal disturbance rejection for a class of discrete-time systems with multiple delayed inputs. In order to avoid the two-point boundary value (TPBV) problem with items of time-delay and time-advance caused by multiple delayed inputs, the discrete-time system with multiple delayed inputs is transformed into a delay-free system by introducing a variable transformation, and the original performance index is reformulated as a corresponding form without the explicit appearance of time-delay items. Then, the approximate optimal disturbance rejection controller (AODRC) is derived from Riccati equation and Stein equation based on the reduced system and reformulated performance index, which is combined with feedback item of system state, feedforward item of disturbances, and items of delayed inputs. Also, the existence and uniqueness of AODRC are proved, and the stability of the closed-loop system is analysed. Finally, numerical examples of disturbance rejection for jacket-type offshore structure and pure mathematical model are illustrated to validate the feasibility and effectiveness of the proposed approach.

Corrosion effects on structural behaviour of jacket type offshore structures

Corrosion effects on structural behaviour of jacket type offshore structures

EFFECT OF SOME FACTORS ON REACTION DYNAMICS OF THE OFFSHORE STRUCTURE UNDER WAVE

The jacket type offshore structures are widely applied for marine states, it has an important role in security, defense and marine economic development. In Vietnam, research and development of marine structures is an urgent task at present. This paper presents finite element algorithm and a calculating method of nonlinear dynamic interaction between jacket type offshore structures and coral foundation under wave and wind loading. The authors have solved the nonlinear dynamic equations by Newmark and Newton-Raphson method and built the calculation program on Matlab software. On the basis of the algorithm and the calculation program has been developed, the authors have investigated the problem with many different parameters to see the influence of these factors: weight, geometry, material to the dynamic response of the marine structures. The results can be applied in calculation, design and construction of the marine structures on the coral foundation under wave and wind loading, contribute to the development of future marine structures.

Modal identification of a jacket-type offshore structure using dynamic tilt responses and investigation of tidal effects on modal properties

Dynamic responses including acceleration, dynamic tilt, and strain were measured using a long-term monitoring system for the Uldolmok tidal current power plant, which is a jacket-type offshore structure. Modal properties such as natural frequencies and modal damping ratios for the lowest three vibration modes were identified using the proposed least square-based frequency domain decomposition method based on measured dynamic tilt responses. Long-term trends of the identified modal properties were investigated in the time and frequency domains, and it was found that the identified modal properties of the Uldolmok tidal current power plant periodically fluctuated approximately twice a day and also twice a month, and also the identified modal properties were highly correlated with the tidal components M2 and M4. Finally, nonparametric regression models were considered as a means of modeling the relationship between tidal changes and modal properties and the model with tidal height, tidal current speed and the multiplication of tidal height and speed was found to be the most appropriate one.

Feedforward and feedback optimal control with memory for offshore platforms under irregular wave forces

This paper studies the vibration suppression of actively controlled jacket-type offshore platforms with fixed delay in the control. Based on the wave theory and Morison equation, an exosystem is designed to describe the irregular wave forces. Through a particular transformation, the original delay system is reduced into a non-delay system. Based on the reduced system, the paper develops a feedforward and feedback optimal control law with memory (FFOCLM). The memory terms in FFOCLM compensate the time-delay in control input. The feedforward term of the controller includes the information of the irregular wave forces. The feedback loop incorporates the displacement and velocity of structure into the control law. The FFOCLM is proved to be existent and unique, and able to stabilize the time-delay system. The feasibility and effectiveness of the presented control law is demonstrated by a numerical example of a jacket-type offshore structure.

Nonlinear Effects on Fatigue Analysis for Fixed Offshore Structures

Fatigue analysis for fixed offshore structures is an important practical issue. These structures are often drag dominated, which makes the deck response a non-Gaussian process when it is assumed that the irregular waves are Gaussian. Incorporating nonlinear and non-Gaussian modeling in the fatigue analysis can be a complicated issue, cf. work of Madhavan Pillai and Meher Prasad [2000, “Fatigue Reliability Analysis in Time Domain for Inspection Strategy of Fixed Offshore Structures,” Ocean Eng., 27(2), pp. 167–186]. The goal of this paper is to provide evidence that for drag dominated offshore structures it is, in fact, sufficient to perform linearization in order to obtain accurate estimates of fatigue damage. The latter fact brings fatigue analysis back into the Gaussian domain, which facilitates the problem solution. Beyond straightforward linearization of the exciting wave forces, this paper employs two different approaches accounting for nonlinear effects in fatigue analysis. One is an application of the quadratic approximation approach described in the work of Naess and co-workers [1997, “Frequency Domain Analysis of Dynamic Response of Drag Dominated Offshore Structures,” Appl. Ocean. Res., 19(3), pp. 251–262;1996, “Stochastic Response of Offshore Structures Excited by Drag Forces,” J. Eng. Mech., ASCE, 122, pp. 155–160]. to the stochastic fatigue estimation of jacket type offshore structures. An alternative method proposed is based on a spectral approximation, and this approximation turns out to be accurate and computationally simple. The stress cycles causing structural fatigue are considered to be directly related to the horizontal excursions of the fixed offshore structure in random seas. Besides inertia forces, it is important to study the effect of the nonlinear Morison type drag forces. Since no direct method for dynamic analysis with Morison type forces is available, it is a goal to find an accurate approximation, allowing efficient dynamic analysis. This has implications for long term fatigue analysis, which is an important issue for design of offshore structures.

Nonlinear Analysis of Jacket-Type Offshore Platforms Using Fiber Elements

A nonlinear fiber element for analysis of jacket type offshore structures is formulated and implemented in the nonlinear program DRAIN-3DX. This element can be used for modeling the nonlinear behavior of both strut and portal members. The element predicts buckling load and post buckling behavior of strut members accurately. It also produces fairly accurate results for yield load and post yield behavior of portal members. This element is verified using the experimental data for individual strut and portal members subjected to cyclic displacements. The element is then used to predict nonlinear behavior of two tested X-braced jackets made of tubular members under cyclic lateral displacement. The results are in good agreement with experiments and the results of other analytical models in terms of frame hysteretic behavior, energy dissipation, buckling load, load-deformation curve, strength and stiffness degradation.