Effects of a floating wave barrier with square cross section on the wave-induced forces exerted to an offshore jacket structure

Offshore structures are exposed to cyclic loading and thereby at risk of fatigue failure, especially at the welded joints between trusses in offshore jacket foundations. In this paper, an optimization framework for welded joints considering fatigue damages is presented. The framework can be used to optimize the orientation and location of welds connecting the joint in a welded K-node in offshore jacket structures considering fatigue damage. It is known that longer welds are at higher risk of fatigue failure compared to shorter welds. To account for this, the statistical size effect is modelled using the Karhunen-Loéve expansion which can be used to take into account the effect of longer welds in the damage assessment. The approach is included in the optimization framework and it is shown that using this method it is possible to simulate the effect of having a good welding quality and a poor welding quality using the correlation length and coefficient of variation. The proposed optimization framework is validated on a simple plate structure and the effect of the statistical input parameters in the statistical size effect is examined. The results show that the proposed optimization approach is robust in predicting the expected tendencies for a simple plate structure. Furthermore, a full-scale welded K-node in an offshore jacket structure manufactured using cast steel is optimized with respect to mass. The optimization is performed considering a good quality weld and a poor-quality weld by including the statistical size effect. The results show that using high quality welds will result in lower mass of the K-node.

Fixed offshore jacket structures are constructed for facilitating oil/gas exploration and production. These structures and their foundations are designed to resist large vertical and lateral loads. Various factors including water depth, wave height and support conditions would affect the response of jacket structures. However, few studies have focused on understanding the response of offshore jacket structure due to variation in these factors. In this context, the present work evaluates response of typical X-braced, square base, 4-legged battered jacket structure using STAAD Pro. under combined vertical and lateral environmental loading. The variables included are water depth (60 m, 90 m and 120 m), wave height (5 m, 10 m and 15 m) and two foundation modelling approaches (viz., fixed at pile location and with defined pile stiffness). The connection between structure leg and pile location is modelled to simulate realistic connection. Deck loads, wind forces and current velocities are considered constant for this study. The wind and wave loads have been applied in parallel, perpendicular and diagonal directions with respect to jacket structure. The wave forces are calculated by Morrison’s equation. For obtaining pile stiffness, medium dense sand layer is considered as foundation soil. The increase in water depth and wave height results in corresponding linear increase in lateral deflection and support reactions, the effect of water depth being more prominent. Moreover, lateral and vertical deflection, shear force and bending moment in the legs, the axial forces in the lower tie beams and plan bracings, and support moments are observed to increase when pile locations are modelled with appropriate vertical, lateral and rotational stiffness instead of fixed support. The effect of water depth on member forces is higher as compared to wave height. The present work deliberates on the mechanisms/reasons to explain the observed results and contributes in direction of framing decision matrix for design optimization of jacket structures.

With industrial growth over the years, geologists have begun to search for offshore sources of oil and natural gas. Most offshore oil-production platforms are jacket type. Offshore jacket structures consist of welded steel space frame. Tubular pipes are opted as they reduce hydrodynamic loads making them highly durable structures. However due to natural phenomenon like cyclones, earthquakes, etc. these structures can get adversely damaged. Apart from that the structure also degrades due to corrosion and fatigue due to the severe conditions it is always exposed to. In many cases physical inspection of the structure for checking damages is not possible due to unfavourable onsite conditions or lack of trained professionals. Hence there is a need to develop alternatives solutions. In recent years much research has been done in the application of Artificial Neural Networks (ANN) in Civil Engineering. ANN are computation systems that are inspired by biological neural systems. This paper aims at creating an ANN to identify damage in an offshore jacket structure using the modal parameters. The training set for the ANN is obtained by using a finite element software. The ANN is then tested using a test set and then it will be used to predict the structural damage in an offshore jacket structure.

The primary purpose of the current study was to investigate the performance of artificial neural networks to predict the time series of the water surface level (WSL), base shear, and overturning moment using two types of ANN models: Nonlinear Autoregressive models with exogenous inputs (NARX) and Nonlinear Autoregressive models (NAR). After determining the suitable model, NARX, the possibility of predicting the time series of the base shear and the overturning moment data was investigated by considering the water surface level and time as the multivariable model inputs. A jacket model with a height of 4.55m was fabricated and tested in the 402m-long wave flume of the NIMALA marine laboratory. The jacket was tested at the water depth of 4m and subjected to random waves with a JONSWAP energy spectrum. Three input wave heights were chosen for the tests: 20cm, 23cm, and 28cm. The findings showed that using the NARX neural network is a convenient method to predict the base shear and overturning moment values based on the water surface level data as input values. Finally, after suitable neural network determination, using the NARX neural network, the correlation value (R) for calculating water surface level (WSL), Base Shear, and Overturning Moment were obtained as 0.994, 0.97, and 0.94, respectively.

The offshore jacket structure has the advantages of suitable stiffness, convenient construction, anti-collision, and strong fatigue resistance, and it is the main structural form of offshore converter station. By constructing a numerical wave tank for the hydrodynamic response analysis of the offshore jacket structure, the wave field distribution around and the wave slamming load on the offshore jacket structure for the converter station under the action of waves are analyzed based on the Star CCM+ software 2206. In addition, the effects of wave height and wave period on its hydrodynamic loads are discussed. The results indicated that: (1) A thin jet layer can be formed on the wave-facing side of the square box when the waves attack the box, and the height of the jet is not the maximum when the horizontal load generated by the jet at the front of the box reaches the maximum value. (2) The pressure distribution on the wave-facing side of the square box for the converter station is relatively discrete, with the pressure in the middle part being slightly larger than that on both sides. At the bottom of the box, the pressure in the middle and back part is significantly larger than that in the front part. (3) When the waves attack the box for the converter station, it caused significant energy dissipation, and the horizontal load on the offshore jacket is less than that when no wave slamming occurs.

An efficient nonlinear dynamic approach for calculating wave induced fatigue damage of offshore structures and its industrial applications for lifetime extension

The dynamic behavior of offshore structures becomes complex due to the effect of the combination of marine environmental and operational conditions. There are some damages and failures that need to be detected early to establish a suitable maintenance strategy. Therefore, the online structural health monitoring (SHM) system has been investigated and developed continuously to ensure safety performance by timely warning. The SHM requires a suitable real-time identification of dynamic characteristics of offshore jacket structure. This article presents a discussion of the advantages, disadvantages, and development trends of existing real-time identification systems and techniques commonly applied to offshore jacket structures. The main identifications in the real-time domain methods including the Short-Time Fourier Transform (STFT), Wavelet Transform (WT) and Hilbert Huang transform (HHT) techniques in predicting dynamic characteristics were also discussed and evaluated. Meanwhile, HHT is the most suitable identification method for real-time identification methods in SHM of offshore jacket structures.

The integrity of offshore structures subjected to degradation depends on the structures’ system capacity and the applied loading through time. Inspections are used to establish the current condition of degrading components. Inspection plans can be developed through application of risk-based inspection methodologies using, for example, the guidance given in ISO 19901-9 [1]. Inspections give information on the condition of the members inspected. The detail of the information about weld condition depends on the inspection method: Flooded member detection gives information on the presence of through-thickness cracks; detailed non-destructive inspection such as ultrasonic inspection identifies defects based on their size; and structural monitoring detects brace severance. As ageing structures are life-extended, semi-quantitative Risk Based Inspection (RBI) methods can result in onerous and sometimes impractical levels of weld fatigue Non-destructive Testing (NDT) inspections. There is a need to refine the inspection plan using a quantitative method which can directly calculate the impact of inspection plans in terms of their influence on the probability of platform collapse. The present study outlines a Monte Carlo Simulation (MCS) based probabilistic assessment methodology for inspection planning of offshore jacket structures. The method calculates the probability of collapse of a jacket structure when subjected to the combined effects of the time-dependent fatigue degradation of structural elements, characterized by member severance, leading to reduced jacket strength, and collapse when the degraded jacket is subjected to extreme metocean loading. The outcomes of in-service weld inspections and online structural monitoring are incorporated into the probabilistic assessment by applying Bayesian updating based on no-find inspection results and probability of detection relationships. The dates and scope of future inspections may be planned by calculating the probability of collapse at future time conditional on no-find inspection and monitoring outcomes. The inspections and monitoring plans are developed to ensure the probability of structural collapse or unacceptable levels of widespread fatigue damage is maintained below acceptable quantified risk-based targets. The method generates sequences of fatigue failures using Monte-Carlo simulation and weld fatigue life probability distributions. Allowance is made for the redistribution of fatigue loading throughout the structure during the member fatigue failure sequences. The method takes advantage of the redundancy afforded by the structural system and the method is currently applicable to offshore jacket-type structures. However, it could be extended to ship-shaped and other floating type structures. Given the limited redundancy of offshore wind turbine structures this methodology is not applicable. A case study example is presented describing the motivation for the use of this method as well as the updates to the inspection plans justified by the results. The approach outlined is a useful method for quantitatively planning inspections of late-life offshore jacket structures that can be applied by practicing engineers responsible for structural integrity management and/or life extension.

Fatigue life of tubular joints in offshore structures is significantly influenced by the degree of bending (DoB). The DoB exhibits considerable scatter calling for greater emphasis in accurate determination of its governing probability distribution which is a key input for the fatigue reliability analysis of a tubular joint. Although the tubular X-joints are commonly found in offshore jacket structures, as far as the authors are aware, no comprehensive research has been carried out on the probability distribution of the DoB in tubular X-joints. In the present paper, results of parametric equations available for the calculation of the DoB have been used to develop probability distribution models for the DoB in the chord member of tubular X-joints subjected to four types of bending loads. Based on a parametric study, a set of samples was prepared and density histograms were generated for these samples using Freedman-Diaconis method. Twelve different probability density functions (PDFs) were fitted to these histograms. In each case, Kolmogorov-Smirnov test was used to evaluate the goodness of fit. Finally, after substituting the values of estimated parameters for each distribution, a set of fully defined PDFs have been proposed for the DoB in tubular X-joints subjected to bending loads.

Degree of bending (DoB) in tubular K-joints of offshore structures subjected to in-plane bending (IPB) loads: Study of geometrical effects and parametric formulation

A nonlinear fatigue damage model: Comparison with experimental damage evolution of S355 (SAE 1020) structural steel and application to offshore jacket structures

According to the British Standard BS7448, the article presents a method to determine crack tip opening displacement (CTOD) and has evaluated the toughness of welded joints in offshore jacket structures (wall thickness is 90 mm) by this method. The results show that (1) it is excellent to evaluate the toughness of welded joint with the CTOD test, (2) the CTOD test can be taken as the acceptable test of welding craft, and (3) the particular offshore jacket structures tested no longer need heat treatment.

Simple analytical methods are shown for stochastic nonlinear dynamic analysis of offshore jacket and jackup structures. Base shear forces are first modelled, and then imposed on a linear 1DOF structural model to predict responses such as deck sway. The force model retains the effects of nonlinear wave kinematics and Morison drag on base shear moments, extremes, and spectral densities. Analytical models are also given for response moments and extremes Good agreement with simulation is found for a sample North Sea jackup. The effects of variations in environmental and structural properties are also studied.

A discussion is given on an alternative approach, using a knowledge-based expert system, to the design of offshore structures. The implemented expert system can assist engineers in the preliminary design of shallow-water offshore jacket structures. The system is capable of configuring and automatically generating the basic structural model of a fixed template-type steel jacket structure for analysis and design. In addition, the system allows the user to alter any of the inferred recommendations. The system is called IPDOJS (interactive preliminary design of offshore jacket structures). >

Among different types of loads on jacket structures, wave loads, especially breaking wave loads, are the most likely to threat the stability of the structure; therefore, the correct estimation of the wave loading of an offshore jacket structure is crucial for the design of these structures. Despite the importance of the correct estimation of breaking wave induced forces on jacket structures, so far, no slamming formulae to predict these forces are available in the design standards and guidelines or in other publications. Moreover, the implications of such extreme wave load events and the associated uncertainties for the dynamic response of the entire jacket structure, including the response of the foundation piles, are still not fully clarified. This PhD study attempts to improve the understanding of processes associated with the interaction of waves and jacket structures and to develop reliable wave slamming formulae for the prediction of breaking wave-induced loads on jacket structures. First, the present knowledge is analysed to identify the processes involved in the interaction as well as the related knowledge gaps. Second, the data available from previous tests performed on a truss structure under breaking and non-breaking waves in the Large Wave Flume (GWK tests) in Hannover are analysed to identify the most relevant influencing parameters and to provide reliable slamming formulae for breaking waves on legs and braces of jacket structures. Third, using a CFD model set-up for the waves generated in the large wave flume GWK and a CSD model for the truss structure tested in GWK, the laboratory tests are reproduced and a methodology is proposed to predict total forces induced by near-breaking and breaking waves on jacket structures. Finally, the proposed methodological approach including the slamming force formulae developed in this study is implemented to calculate total forces by breaking waves on a full-scale jacket structure (OC4 jacket). The Finite Element FE model of the OC4 jacket is extended by pile foundation model and the structural performance of the entire structure was examined by comparing the results by those of the numerical models developed for the same jacket structure. Finally, the dynamic response of the jacket structure with pile foundation to breaking waves is systematically analysed to achieve a substantially improved understanding of the processes involved in the wave-jacket-pile foundation interaction.