Offshore Structural Systems Research Articles

A novel computational approach for assessing system reliability and damage detection delay: Application to fatigue deterioration in offshore structures

Fatigue cracks in welded components can cause loss of structural integrity and failures of structural systems. To ensure the safety and reliability of deteriorating structural systems, system damage should be detected as early as possible through inspection, and timely maintenance or repair is required. Delays in damage detection lead to maintenance or repair delays, which adversely affect structural integrity and functionality. In this study, a probabilistic approach to quantify the reliability of deteriorating structural systems with explicit damage detection delay computations was developed for the first time and applied to an offshore structural system. To obtain the system damage detection delay, the system damage occurrence and detection times were rationally defined. Subsequently, the effects of different inspection strategies on system damage detection delay were studied. Finally, the relationship between system reliability and system damage detection delay was investigated. The results show that the system damage detection delay decreases as the inspection quality, number of inspections, or number of components inspected simultaneously increases. It is interesting to determine the law of diminishing marginal system damage detection delay. In addition, there is an approximately negative linear relationship between the system reliability index and system damage detection delay.

A Markov resilience assessment framework for tension leg platform under mooring failure

During the service life of Tension Leg Platform (TLP), it may be exposed to various risk, with mooring failure being one of the most catastrophic events. Resilience, as an integrated assessment philosophy, can evaluate the overall post-event response performance and further improve system's operation safety. In this paper, a Markov resilience assessment framework for TLP under mooring failure is firstly proposed. The failure process and recovery process are mathematically described by Markov process and continuous-time Markov process, respectively. The internal and external effect has been taken into account, including extreme environment, structural degradation, recovery process schedule and structural strength etc. The resilience assessment framework is developed by 2 aspects, including robustness and recovery resilience. Besides, an illustrative example is developed as a walk-through of proposed methodology. The applications here demonstrate the versatility of the Markov framework towards handling resilience problems with varying levels of complexity, especially the offshore structure systems.

Introducing multi-round clustering on capturing the multidirectional extreme current profile for deep-water ROV operation

ROVs (Remotely Operated Vehicles) are playing an important role in installation, construction, and maintenance of subsea systems. During operation, ROVs are usually connected to surface facilities through umbilical lines where the entire umbilical-ROV system is affected by vessel motions, wave, and current induced loads. During the design process, it is commonly assumed that the current profile that leads to the extreme response of an offshore structural system has a typical profile with a uniform direction. However, in deep water, the variation of current direction along water depth becomes more apparent. Hence, the effect of direction variation cannot be neglected when establishing the extreme current profile for deep-water ROV operation.The study focuses on how to efficiently acquire the multidirectional extreme current profiles through a new multi-round clustering method. Firstly, static analysis of a typical deep-water umbilical-ROV system was performed under thousands of multidirectional current profiles extracted from a database of one-year continuously recorded ocean current. Different responses of the system such as ROV offset, umbilical lateral displacement, and maximum tension in the umbilical were investigated and set as a benchmark for further process. Then, a multi-round clustering procedure using a modified maximum dissimilarity algorithm for ROV operation was introduced and applied to extract the extreme multidirectional current profile more efficiently.It is shown that the proposed multi-round clustering methodology can be implemented efficiently and accurately in capturing the extreme vertical current profiles from a current database for ROV operation in different working depths. The study also showed that the effect of bottom current on deep-water ROV operation is more significant than the surface current. Therefore, for ROV operation in deep waters, more precautions shall be made on the bottom current profiles.

Spectral identification of nonlinear multi-degree-of-freedom structural systems with fractional derivative terms based on incomplete non-stationary data

A novel spectral identification technique is developed for determining the parameters of nonlinear and time-variant multi-degree-of-freedom (MDOF) structural systems based on available input-output (excitation-response) realizations. A significant advantage of the technique relates to the fact that it can readily account for the presence of fractional derivative terms in the system governing equations, as well as for the cases of non-stationary, incomplete and/or noise corrupted data. Specifically, the technique relies on recasting the governing equations as a set of multiple-input/multiple-output systems in the wavelet domain. Next, an l1-norm minimization procedure based on compressive sampling theory is employed for determining the wavelet coefficients of the available incomplete non-stationary input-output data. Finally, these wavelet coefficients are utilized to reconstruct the non-stationary incomplete signals, and consequently, to determine system related time- and frequency-dependent wavelet-based frequency response functions and associated parameters. Two illustrative MDOF systems are considered in the numerical examples for demonstrating the reliability of technique. The first refers to a nonlinear time-variant system with fractional derivative terms, while the second addresses a nonlinear offshore structural system subjected to flow-induced forces. It is worth noting that for the offshore system, a novel recently proposed evolutionary version of the widely used JONSWAP spectrum is employed for modeling the non-stationary free-surface elevation in cases of time-dependent sea states.

Structural behavior and design of high-strength steel welded tubular connections under extreme loading

The present work is motivated by the increasing need for cost-efficient solutions in offshore structural systems for wind energy production and for improvement of their structural performance. The structural behavior and design of high-strength steel welded tubular connections (yield strength higher than 700 MPa) subjected to monotonic and strong cyclic loading is investigated. In the first part of the paper, an experimental investigation is presented on high-strength steel tubular X-joints subjected to monotonic and cyclic loading far beyond the elastic limit of the material, leading to weld fracture. Two grades of weld metal material are employed in the welding process of the specimens. The experimental results indicate that the weld material grade has a significant influence on the deformation capacity of the welded connection under monotonic loading conditions, and its low-cycle fatigue life. The experimental procedure is simulated using advanced finite element models, elucidating several features of joint behavior and complementing the experimental results. Overall, a good agreement is found between numerical simulations and experimental results, in terms of both global response and local strains at the vicinity of the welds. Furthermore, the structural performance of the welded tubular joints under consideration is assessed using available design methodologies in terms of both ultimate strength and low-cycle fatigue resistance, in an attempt to validate an efficient design methodology for low-cycle fatigue. The results from this research effort are aimed at developing the necessary background for the possible use of high-strength steel in tubular steel lattice structures, particularly in offshore platforms for renewable energy production. They can also be used as a basis for the possible amendment of relevant design specifications and recommendations for including special provisions for high-strength steel structural systems.

Dynamic Behaviour of a Mono-pile with Skirted Template Structure for Offshore Wind-Power System

The purpose of this research is to develop an offshore structural system that may sustain a stronger wave and have better stability. The object of the research is a hybrid type of offshore mono-pile-template structural system, which is more flexible and relatively easier to construct compared to the heavy mass type offshore structural system. It is expected that this hybrid type of supporting structure for an offshore wind-power generation platform will have the flexibility as presented by a typical template structural and yet, consistent properties for supporting function of the structure. Two types of structure are designed for the analytical study, namely, the mono-pile supporting structural system and the hybrid type combined with the mono-pile and the template structure as an outskirt frame system. Both annual regular waves and extreme storm waves of a 100-year return period are applied to the structural system as exerting forces.

A new approach to system reliability analysis of offshore structures using dominant failure modes identified by selective searching technique

The reliability analysis of offshore structures, as conceived by current design practice, is typically carried out at the component level with an implicit assumption that the structural system will be safe as long as all its members are safe according to the corresponding limit state equations. However, it is widely accepted that a more general and rigorous approach to the risk assessment of offshore structures based on system reliability analysis is needed, in which the reliability of a structure is estimated with respect to the system-level failure caused by potential failure modes consisting of component failures with their statistical dependence considered. To this end, the merits of a new risk assessment framework, originally developed for truss and frame structures, are investigated in this paper in view of an extensive application of this method to the offshore structural systems. The main advantage of the proposed method is that the identification process of dominant failure modes is decoupled from the evaluation process of the probabilities of failure modes and the system failure event. The identification phase consists of a multi-point parallel search employing a genetic algorithm, and it is followed by the evaluation phase, which performs a multi-scale matrix-based system reliability analysis where the statistical dependence among both components and failure modes is fully considered. In order to demonstrate the applicability of the proposed method to the offshore field, the problem of a jacket platform under an extreme sea state is considered, in which the uncertainties are assumed both in the wave and hydrodynamic models and in the material properties of the structural members. The computational efficiency and accuracy of the proposed approach are successfully demonstrated through comparison with Monte Carlo simulations.

A control algorithm for nonlinear offshore structural dynamical systems

A reasonable knowledge about the response of nonlinear offshore structural systems under environmental loads is necessary but challenging. This is due to the coupling of internal forces along with external excitations. In this paper, a mathematical model of nonlinear offshore systems is studied with the intention of keeping the response close to the desired one. This is achieved using a novel sub-optimal control mechanism derived from nonlinear quadratic regulator (NQR) theory. Herein, two linearized functions of nonlinear motions—displacement and velocity—are introduced such that the parametrization of the state-dependent system matrices is obtained. By doing so, the system becomes conditioned only on the present state and therefore one needs to solve only an algebraic state-dependent Riccati problem. This results in a control law which may either be partial or full rank for the dynamical system depending on measurable states. The performance of the controller is compared with conventional NQR. The performance of the proposed control strategy is illustrated through a range of models of nonlinear offshore problems. The motions (generalized displacements and velocities) show that the proposed controller was not only able to restrict the undesirable behaviour but also provide means of shaping the transient performance.

Structural health monitoring of offshore wind turbines using automated operational modal analysis

This article will present and discuss the approach and the first results of a long-term dynamic monitoring campaign on an offshore wind turbine in the Belgian North Sea. It focuses on the vibration levels and modal parameters of the fundamental modes of the support structure. These parameters are crucial to minimize the operation and maintenance costs and to extend the lifetime of offshore wind turbine structure and mechanical systems. In order to perform a proper continuous monitoring during operation, a fast and reliable solution, applicable on an industrial scale, has been developed. It will be shown that the use of appropriate vibration measurement equipment together with state-of-the art operational modal analysis techniques can provide accurate estimates of natural frequencies, damping ratios, and mode shapes of offshore wind turbines. The identification methods have been automated and their reliability has been improved, so that the system can track small changes in the dynamic behavior of offshore wind turbines. The advanced modal analysis tools used in this application include the poly-reference least squares complex frequency-domain estimator, commercially known as PolyMAX, and the covariance-driven stochastic subspace identification method. The implemented processing strategy will be demonstrated on data continuously collected during 2 weeks, while the wind turbine was idling or parked.

Risk-identification-based hybrid method for estimating the system reliability of existing jacket platforms under fire

This paper proposes a risk-identification-based hybrid method for estimating the system reliability of steel jacket structures under fire. The proposed method starts with risk identification; according to the results of hazard identification and Dow’s fire and explosion index (F&EI) methodology, the most dangerous hazard sources are determined. In term of each equipment layout in steel jacket structures, fire load is imposed and elasto-plastic analysis is performed. According to the deformed state of steel jacket structures, the weakest failure mode of steel jacket structures is identified. In order to know the effect on ultimate bearing capacity of the offshore structural system, a series of elasto-plastic analyses are performed in which single failure element contained in the weakest failure mode is removed from the whole offshore platform structural system. Finally, the failure function of the steel jacket structure is generated and the failure probability of the steel jacket structure system is estimated under fire by genetic algorithm via MATLAB program.

The deformation offshore of Mount Etna as imaged by multichannel seismic reflection profiles

Despite the clear evidence of active flank dynamics that is affecting the eastern side of Mount Etna, the contribution of tectonic processes has not been yet understood. So far, the various models proposed to explain the observed flank deformation have been based on onshore structural data, coming from the volcanic edifice. The Ionian offshore of Mount Etna has been only recently investigated using multichannel seismic profiles, and offers the opportunity to image the structural features of the substrate of the unstable flank of the volcano. This contribution aims at describing the deformation located offshore Mount Etna using multichannel seismic profiles recently acquired during three seismic surveys. The onshore flank deformation of Mount Etna appears to be laterally confined by two tectonic guidelines, trending roughly E–W, located to the north and south of the deforming flank; the northern guideline, in particular, takes the surface expression of a sharp fault (Pernicana Fault). Though often assumed that these boundary structures continue offshore as linear features, connected to a frontal thrust ramp, the occurrence of this simple offshore structural system has not been imaged. In fact, seismic data show a remarkable degree of structural complexity offshore Mount Etna. The Pernicana Fault, for instance, is not continuing offshore as a sharp feature; rather, the deformation is expressed as ENE–WSW folds located very close to the coastline. It is possible that these tectonic structures might have affected the offshore of Mount Etna before the Pernicana Fault system was developed, less than 15ka ago. The southern guideline of the collapsing eastern flank of the volcano is poorly expressed onshore, and does not show up offshore; in fact, seismic data indicate that the Catania canyon, a remarkable E–W-trending feature, does not reflect a tectonic control. Seismic interpretation also shows the occurrence of a structural high located just offshore the edifice of Mount Etna. Whereas a complex deformation affects the boundary of this offshore bulge, it shows only limited internal deformation. Part of the topography of the offshore bulge pre-existed the constructional phase of Mount Etna, being an extension of the Hyblean Plateau. Only in the northern part, the bulge is a recent tectonic feature, being composed by Plio-Quaternary strata that were folded before and during the building of Mount Etna. The offshore bulge is bounded by a thrust fault that can be related to the intrusion of the large-scale magmatic body below Mount Etna.

소성해석법의 선체 GRILLAGE 설계에의 응용

A plastic analysis method is commonly used in ship and offshore structural system to utilize the ultimate strength. In this paper, the basic principle of plastic analysis method is applied to ship grillages such as transverse oil-tight bulkheads. The main emphasis is placed on the optimum arrangement of grillage system to give minimum weight. Additional parametric study is carried out to find the effect of various arrangement of grillage system. The above methods are applied to oil-tight bulkhead design, and results are compared with the existing one.

Crustal structure of the southeastern Brazilian margin, Campos Basin, from aeromagnetic data: New kinematic constraints

The continental and adjacent marginal features along southeast Brazil were investigated, focusing on the basement structural relationships between onshore and offshore provinces. Lateral and vertical variations in the magnetic anomalies provided a good correlation with the regional tectonic features. The sin-rift dykes and faults are associated with the magnetic lineaments and lie sub parallel to the Precambrian N45E–S45W basement structure of the Ribeira Belt, but orthogonally to the Cabo Frio Tectonic Domain (CFTD) basement, implying that: (1) the upper portion of the continental crust was widely affected by Mesozoic extensional deformation; and (2) tectonic features related to the process of break up of the Gondwana at the CFTD were form regardless of the preexisting structural basement orientation being controlled by the stress orientation during the rift phase. The deep crustal structure (5 km depth) is characterized by NE–SW magnetic “provinces” related to the Ribeira Belt tectonic units, while deep suture zones are defined by magnetic lows. The offshore Campos structural framework is N30E–S30W oriented and resulted from a main WNW–ESE direction of extension in Early Cretaceous. Transfer zones are represented by NW–SE and E–W oriented discontinuities. A slight difference in orientation between onshore (N45E) and offshore (N30E) structural systems seems to reflect a re-orientation of stress during rifting. We proposed a kinematical model to explain the structural evolution of this portion of the margin, characterized by polyphase rifting, associated with the rotation of the South American plate. The Campos Magnetic High (CMH), an important tectonic feature of the Campos Basin corresponds to a wide area of high crustal magnetization. The CMH wass interpreted as a magmatic feature, mafic to ultramafic in composition that extends down to 14 km depth and constitutes an evidence of intense crustal extension at 60 km from the coast.

Polynomial Approximations of Wave Loading and Superharmonic Responses of Fixed Structures

Besides the commonly considered drag force, inundation due to variable water surface is another important nonlinear effect of wave loading. Quadratic and quartic approximations of the inundation drag force are derived using the least squares method. Other nonlinear effects, including the second-order wave kinematics and nonlinear inertia wave forces, are also considered. Superharmonic forces and the corresponding structural responses due to different nonlinear effects are compared using a single mode representation of the fixed offshore structural system. The appropriate expressions that can serve as the basis for the Volterra series representation of the nonlinear wave forces are presented.

Risk Assessment and Management of Offshore Structures

During the past 30 years, reliability based risk assessment & management processes have been developed and applied in assessing and maintaining the integrity of a worldwide infrastructure of offshore structure systems. These systems are characterized as a combination of structures, hardware / equipment, operating personnel, organizations, environments and the interfaces between these components. Background on current and future trends in development of comprehensive programs to help improve the quality and reliability of offshore structure systems are outlined. The importance and characteristics of human and organizational factors are discussed. A combination of proactive, reactive, and interactive approaches have been developed that employ three strategies to achieve desirable quality and reliability: reduce likelihoods of malfunctions, increase detection and correction of malfunctions, and decrease the effects and consequences of malfunctions. System robustness and fail-safe design are key elements of the third strategy. Experience is now showing that implementation of these advanced approaches is more often marked with failure than success. Creation of high reliability organizations is not quick, easy, or free and experience with implementation in seven international oil and gas production organizations is summarized.

Low‐temperature hygrothermal degradation of ambient cured E‐glass/vinylester composites

AbstractFreeze and freeze‐thaw durability characteristics of fiber reinforced polymer (FRP) composites, especially in the presence of moisture, need to be investigated prior to the widespread implementation of these materials in civil, polar, and offshore structural components and systems. The hygrothermal degradation characteristics of an ambient cure E‐glass/vinylester system due to exposure to −10°C conditions and conditions of freeze‐thaw, including in the presence of water and seawater, was investigated. Changes in mechanical characteristics such as strength and modulus, and thermo‐mechanical dynamic characteristics such as storage and loss moduli, and glass‐transition temperature were measured, and short‐term effects of environmental exposure were assessed. It is seen that the presence of moisture/solution has a significant effect; both in terms of physical and chemical aging, and in terms of microcracking and fiber–matrix debond initiation. Results indicate the critical importance of cure characteristics and diffusion related phenomena. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2255–2260, 2002

Non-Gaussian Model for Ringing Phenomena in Offshore Structures

Significant interest has been shown in identifying the nonlinear mechanisms that induce a ringing type response in offshore structural systems. This high frequency transient type response has been observed in offshore systems, particularly in tension leg platforms (TLPs). Given the implications of this behavior on the fatigue life of TLP tendons, it is essential that ringing be considered in the overall response evaluation. This study presents two non-Gaussian probabilistic models of nonlinear viscous hydrodynamic wave forces that induce ringing. The response of a single-degree-of-freedom system exposed to these non-Gaussian wave force models is then evaluated using analytical and numerical studies based on the Ito⁁ differentiation rule and the Monte Carlo simulation procedure, respectively. The results demonstrate that the proposed models induce ringing type response in a simplified structure. This study provides a probabilistic framework for modeling ringing type phenomenon which will serve as a building block for more refined hydrodynamic load models.

Human and organizational factors in reliability assessment and management of offshore structures.

Today, there is a worldwide infrastructure of offshore structure systems that include fixed, floating, and mobile platforms, pipelines, and ships. Background on current and future trends in development of comprehensive programs to help improve the quality and reliability of offshore structure systems are discussed. A combination of proactive, reactive, and interactive risk assessment and management approaches have been developed and applied. Two risk assessment and management instruments are detailed in this article: a qualitative Quality Management Assessment System (QMAS), and a quantitative System Risk Analysis System (SYRAS). Application of QMAS to produce human and organizational performance shaping factors that are used as input to SYRAS is discussed.

Risk assessment and management of offshore structures

AbstractDuring the past 30 years, reliability‐based risk assessment and management processes have been developed and applied in assessing and maintaining the integrity of a worldwide infrastructure of offshore structure systems. These systems are characterized as a combination of structures, hardware/equipment, operating personnel, organizations, environments and the interfaces between these components. Background on current and future trends in development of comprehensive programs to help improve the quality and reliability of offshore structure systems are outlined. The importance and characteristics of human and organizational factors are discussed. A combination of proactive, reactive and interactive approaches have been developed that employ three strategies to achieve desirable quality and reliability: reduce likelihood of malfunctions, increase detection and correction of malfunctions, and decrease the effects and consequences of malfunctions. System robustness and fail‐safe design are key elements of the third strategy.

Simulation of Ringing in Offshore Systems under Viscous Loads

In recent years, significant interest has been shown in identifying the nonlinear mechanisms that induce ringing in complex offshore structural systems. This high-frequency transient-type response has been observed in offshore systems, particularly in tension leg platforms (TLPs). Given the implications of this behavior on the fatigue life of TLP tendons, it is essential that ringing be considered in the overall response evaluation. This study uses a simplified structural model exposed to viscous-type loading to demonstrate several of the salient features observed in higher-order potential–type wave–induced loads. Ringing response in pitching mode caused by viscous-type loading is simulated on a cylinder piercing the water surface, and significant mechanisms reponsible for inducing ringing are delineated. Results clearly indicate that, under certain conditions, ringing-type response characteristics are captured under viscous-type loading. The cylinder characteristics are varied to ameliorate the system pe...