Fixed Offshore Platforms Research Articles

Methods for determining endurance limits for welded joints of marine stationary platforms

Today, oil, gas and other useful resources are being actively extracted at offshore fields using fixed offshore platforms. According to Federal Law No. 116 On Industrial Safety of Hazardous Production Facilities, offshore platforms are classified as hazardous production facilities, accidents at which are accompanied by significant damage. To prevent accidents at these facilities, an industrial safety examination is carried out, during which their resource is determined. The key parameter for determining the resource is the value of the endurance limit (or fatigue limit). Various theories are currently being applied to determine the fatigue limit. However, calculations on these theories give different results. The purpose of this article is to study various methods for determining endurance limits using welded joints of fixed offshore platforms as an example and choosing a method that allows you to obtain the most accurate value of this limit.

Section 1.01 Methodology for assessing local overloads of elements of fixed offshore platforms with corrosion defects based on data on temperature contrasts: results of physical modeling

Today, offshore oil and gas fields are being developed in the world using stationary offshore platforms. Offshore platforms are corrosive during operation. As a result of this effect, cavities or crack-like defects are formed in the platform elements, which are stress concentrators and can cause a critical overload of the platform elements. In order to prevent such a situation, it is necessary to identify dangerous corrosion defects in a timely manner. The author set up a physical experiment, which consisted in the creation of artificial corrosion defects of the V-shaped form, followed by the action of intense heat flows on them. On the basis of the formulas obtained by the author, a practical solution to the problem is given, which makes it possible to determine the size of corrosion damage and, from temperature contrasts, to calculate local overloads in the elements of the offshore platform.

Calibration of Load and Resistance Factors of API RP 2A-LRFD, Second Edition, for Fixed Offshore Platforms

AbstractFixed offshore platforms in the Gulf of Mexico (GoM) are traditionally designed following the working stress design (WSD) format, that is, the factor of safety approach. In 1993, the first ...

Estudo de viabilidade para a reutilização de plataformas offshore fixa como substrutura de torres eólica

Com o objetivo de dar um outro uso às plataformas offshore fixa com o tempo de vida útil vencidas, principalmente aquelas situadas em campos com condições ambientais favoráveis para instalação de turbinas eólicas offshore, o presente trabalho de pesquisa ressalta a importância de avaliar a possibilidade de reutilização de uma plataforma (jaqueta) submetida ao carregamento de uma torres de 10 MW, considerando sobre tudo as novas condições de operação. Para isso se tem em conta as análises dos elementos que conformam a estrutura (jaqueta) de modos a atinar as suas respetivas razões de tensões. Considerou-se o carregamento da torre eólica de maneira pontualmente distribuído sobre os 4 nós no topo das pernas principais da jaqueta. A estrutura investigada apresenta uma jaqueta de 16,85 m de altura, com a área da mesa inferior de 9,95 m x 9,95 m e 6,75 m x 6,75 m na mesa superior. O sistema estrutural foi desenvolvido com base à utilização do software OrcaFlex V10.3, que permite analisar as razões de tensões (“Unity Check”) incluído os efeitos de segunda ordem por meio da utilização do Método dos Elementos Finitos (MEF). Desta forma, os resultados obtidos ao longo das análises para avaliar o comportamento estrutural apontam para o fato de que a jaqueta investigada não atende aos critérios de projeto para dar suporte a torres eólicas de 10 MW.

Feasibility of reducing dynamic response of a fixed offshore platform using tuned liquid dampers

ABSTRACT Fixed offshore platforms are always subjected to dynamic loading. A potentially economical method to improve the dynamic behaviour of these platforms is using passive control devices. In this paper, the application of tuned liquid damper, TLD, in controlling the response of a fixed offshore platform located in the Persian Gulf against earthquake and wave loading was studied. Analysis of a 3D model of the structure equipped with TLDs was carried out using the finite element method, and the efficiency of TLDs was investigated. Also, the equivalent models of TLDs were constructed based on the lumped mass method, and linear wave theory and the results were compared to finite element modelling of TLD. Results showed good agreement between the finite element model of TLD and equivalent lumped mass model. Results also indicated that adding TLDs can mitigate jacket deck vibrations and their efficiency increases as wave height or ground motion increase.

Estimation of efficiency of the thermal diagnostic method for detecting corrosion defects of fixed offshore platforms

Currently, the world is actively developing offshore fields. This production is carried out using fixed offshore platforms. During operation, offshore platforms are exposed to various influences, including corrosion. As a result of this effect, caverns are formed on the surface of the structural elements of the platform, which are stress concentrators and capable of causing critical overvoltage of the platform elements. To prevent this situation, it is necessary to identify hazardous corrosion defects in a timely manner. The article describes a comparative analysis of modern non-destructive testing methods, as well as the results of a method developed by the author on the basis of experimental studies for assessing the depth of corrosion defects.

Determination of the endurance limit of structural elements of fixed offshore platforms with accumulated damage based on experimental studies

Russia has a large number of fixed offshore platforms, which were built during the Soviet era and are in operation for more than 20 years. In connection with such a long life platform for a long time was exposed to various loads that cause stresses and variables as a consequence of fatigue damage. In connection with this very urgent becomes the problem of estimation of residual resource of elements of the platforms with consideration of cumulative damage. Obviously, the residual resource it is long maintained elements of platforms subjected to intense impact, will differ from that caused by changes in the metal structure and the accumulation in it of fatigue damage. In this article the solution to the problem of determining the fatigue limit for the elements of the platforms with accumulated damage based on the experiments is proposed.

Comparative analysis of the fatigue characteristics of repaired compounds of the “T” and “K” types of fixed offshore platforms

The study of the fatigue characteristics of repaired welded joints is an important task to determine their remaining lifetime. At present, there is no methodology that allows calculating the resource of reconditioned welded joints; therefore, industrial safety experts use the diagrams and fatigue equations given in the normative documentation for new joints when extending the resource. It is also not established whether there is a difference in fatigue characteristics depending on the type of pipe joint “T” or “K”. In this regard, the authors conducted research during which a new welded joint was brought to primary failure, and then repaired and brought to repeated failure. Based on these experiments, graphs were constructed that describe the dependence of the number of cycles on the values of the amplitudes of the alternating stresses and a mathematical equation was obtained that describes the fatigue parameters for both “T” and “K” types of welded joints.

Numerical Simulation and Analysis of Phase-Focused Breaking and Non-Breaking Wave Impact on a Fixed Offshore Platform Deck

AbstractExtreme wave impact due to tsunamis and storm surge create large wave heights that cause destruction to coastal and offshore structures. Focused waves have been used to represent such extreme waves, and in the present study, its impact on offshore deck has been studied numerically. Numerical modeling has been carried out using open-source software reef3d, with the level set method to capture the air–water interface. Focused waves are generated by phase focusing a group of waves at a particular position and time. The nonlinearity of focused waves and its effect on the vertical impact force has been quantified for different airgaps and increasing wave heights. The wave steepness was increased to initiate phase-focused breaking in the numerical wave tank, which was then validated with the experimental results. This breaking-focused wave impact on offshore deck is then studied at different breaking locations. The results for different positionings of the deck with respect to breaker location show that the maximum horizontal impact force on the deck occurs when the plunging crest hits the deck side.

Stress condition of the elements of support blocks of fixed offshore platforms and resource evaluation of them

The article analyzes the factors causing the stress state of the support blocks of fixed offshore platforms (FOPs). The data on the wind-wave regime of the Subbotinsky field of the Black Sea are analyzed, the parameters are determined and the cycles of wind-wave loads acting on the platform are systematized. Using the StructureCAD software package, the values of the effective equivalent stresses were determined from the weight of the structural elements of the FOP, the installed technological equipment, the floating forces of the aquatic environment and wind-wave loads under various conditions of the orientation of the wave beam with respect to the support block. Various design options for FOPs are analyzed and the problem of finding “zero elements” is solved. The proposed solutions will allow developing a comprehensive methodology for assessing the stress state of the support block of the FOP. Besides, for the purpose of determining the resource of the elements, the values of the equivalent stresses of the columns, horizontal belts and braces of the support block of the FOP are determined. The example of calculating the resource of platform elements is given.

Modeling the influence of variable temperature fields on the stressed state of offshore facilities

The article considers the influence of temperature fields on the stress state of offshore facilities using the example of fixed offshore platforms for oil and gas production. It is noted that there is no theory for offshore oil and gas facilities that would allow a numerical description of the nature and mechanism of action of both conventionally stationary and variable temperature fields. An analysis of various literature sources showed that currently there are solutions for the dynamics of propagation and stress state for an axisymmetric temperature field, for a non-axisymmetric case, characteristic of the operating conditions of the support blocks of offshore platforms, there is no such solution. The author notes that a change in the thermal state of the elements of offshore facilities as a result of various factors leads to a change in their stress state. In some cases, in the presence of significant defects, these defects act as stress concentrators, thereby increasing the nominal strain. A numerical-analytical modeling of the effects of both variable and conditionally stationary fields was carried out, which made it possible to identify a number of patterns.

Varieties and kinetics of fatigue cracking of fixed offshore platforms for oil and gas production

The article investigates the different types of cracks and kinetics of propagation of cracks in main structural components and welds of the support blocks of fixed offshore platforms. Describes the process of metal fatigue for offshore installations. The article provides methods for calculating of the stress intensity factor and describes the methods of calculation for some cases. Based on the analysis of scientific studies, the author provides methods allowing to calculate two parameters describing the kinetics of crack propagation in relation to the elements and welded joints of offshore fixed platforms, namely the methods of calculation of threshold and critical stress intensity factor. On the basis of the conducted research the author constructed the kinetic chart of the development of fatigue cracks in relation to the elements and welded joints of the supporting blocks of fixed offshore platforms. In addition, the author gives the equation in which it is possible to determine the growth rate of fatigue cracks in various areas of the kinetic diagram.

Assessment of jacket-type platform stress state in corrosive environment: Case study

The structural tubular elements of the offshore platform substructure work in combined conditions of bending, tension, compression and torsion loading. Evaluation of the platform stress state is an important task. Effective solution of which can be received by a CAD modelling. The real stress state of the structure depends on the characteristics of corrosion processes. Therefore, corrosion defects should be taking into account for numerical and analytical modelling to determine/predict actual state of stress of bottom fixed offshore platforms. The stress concentration factor is the real characteristic of stress value for the structural elements of the platform in the zone of corrosion. The paper deals with the methods of the actual stress state calculation for the structural elements and welded joints of stationary platform under corrosion. The features of corrosion defects are also a subject of analysis. The proposed a calculation method of stress concentration factors for the primary tubular members of platform substructure in condition of corrosion.

Effects of sample size of ground motions on seismic fragility analysis of offshore jacket platforms using Genetic Algorithm

Intense seismic excitations might be considered as one of the most important loading cases imposed on offshore jacket platforms. To perform seismic assessments for these structures, appropriate suites of ground motions can be selected by different methodologies. Since time history analyses of fixed offshore platforms are truly time consuming, number of selected records can considerably change the computational expenses. This paper quantitatively investigates the effects of sample size of records on seismic performance of these structures. Thus, several record suites with different sample sizes are selected from a target population with 40 records so that the differences between the logarithmic mean and standard deviation of their response spectra become minimum using Genetic Algorithm. The results show that the fragility curves based on the suites with larger sample sizes are closer to the target fragility. Nevertheless, according to the results of the investigated case study, some suites with smaller sample sizes (e.g. 20 or 25) could lead to relatively acceptable accuracy at almost half the computational expenses. Also, the correlation coefficients between the collapse fragilities of the selected record sets and the target one are very close to unity, indicating that these collapse fragilities are linearly correlated with the target one.

Flow and Scour Analysis Around Monopole of Fixed Offshore Platform Using Method that Couples Computational Fluid Dynamics and Discrete Element Method

When an offshore foundation is exposed to waves and currents, local scour could develop around a pile and even lead to structural failure. Therefore, understanding and predicting the scour due to sediment transport around foundations are important in the engineering design. In this study, the flow and scour around a monopole foundation exposed to a current were investigated using a method that coupled the computational fluid dynamics (CFD) and discrete element method (DEM). The open source computation fluid dynamics library OpenFOAM and a sediment transport library were coupled in the OpenFOAM platform. The incipient motion of the particle was validated. The flow fields and sediment transport around the monopole were simulated. The scour depth development was simulated and compared with existing experimental data. For the upstream scour hole, the equilibrium scour depth could be reproduced qualitatively, and it was underestimated by about 23%.

Seismic performance assessment of fixed offshore structures by endurance time method

ABSTRACTThe aim of this paper is to consider the ability of endurance time (ET) method to estimate the seismic response of fixed offshore platforms at various excitation levels. In this way, various sources of nonlinear behaviour and challenging issues in the ET approach are included in the three-dimensional model of a real jacket platform utilising the finite element method. In addition to common-designed, over-designed and under-designed cases of this platform have been investigated to examine the effects of the structural characteristics on the ET response. To consider the accuracy and reliability of this approach, results of the ET method are compared with results of nonlinear time history analysis (THA) and response spectrum analysis (RSA) methods at different excitation levels. It is shown that the ET method can predict the seismic performance of the offshore structures with both small computational efforts and reasonable accuracy.

Risk-based structural integrity management for offshore jacket platforms

This paper presents a method for risk assessment and inspection plan development as part of the risk-based structural integrity management of offshore jacket platforms. The method provides a global risk assessment for the whole platform's structure and local risk assessment for the platform's structural components (e.g. tubular joints). The global risk assessment uses semi-quantitative or quantitative approaches depending on the available platform's data. The semi-quantitative method is used either at a high level to perform relative risk ranking of platforms in a fleet in order to identify the platforms most at risk and which require more inspection focus or a detailed risk analysis; or at the unit level to define inspection interval and general inspection requirements. The quantitative method involves either structural analysis results (e.g. reserve strength ratio) with a dedicated metocean hazard curve or structural reliability method; and it is used at the unit level only to define inspection interval and inspection requirements. The local risk assessment uses also semi-quantitative or quantitative approach. The semi-quantitative method is used to provide local risk ranking of the structural components of a platform, which allows, if required, local inspections' scope to be defined. The quantitative method involves a probabilistic fatigue method to define inspection plans for selected tubular joints subjected to fatigue and the failures of which are critical for the overall platform structure. The inspection strategy and program, developed by the method presented in this paper, are focused on the routine underwater inspections and are based on the recommended practice of the American Petroleum Institute for the structural integrity management of fixed offshore platforms. An example of application of the method is set out by showing the results of a project carried out by Bureau Veritas.

Extreme structural responses by nonlinear system identification for fixed offshore platforms

ABSTRACTOffshore structures are exposed to random wave loading in the ocean environment. The response is commonly non-Gaussian due to the nonlinearity of the drag component of Morison's wave loading and also the load intermittency on members in the splash zone. The previous study shows that the developed finite-memory nonlinear system (FMNS) model can efficiently simulate the response of an offshore structure to random Morison wave loading. However, the FMNS models minimise the computational effort, and the estimates are not particularly good for lower significant wave height value. To overcome this deficiency, a modified version of FMNS models (MFMNS) is used to compute the extreme response values which increase the accuracy but is computationally less efficient than the current FMNS models. In this study, the probability distributions of extreme responses from both models are compared with corresponding distributions from the conventional time simulation technique to establish their level of accuracy.

Evaluation of Failure Capacity of Offshore Riser Protectors Under Vessel Impact

Conventional riser protectors, also known as riser-guards, are installed on fixed offshore platforms to protect risers from vessel impact. These space frame structures made of hollow mild steel tubular sections are mainly designed using the approach of boat fenders which may result in over- or under-protection provided to risers. Lack of in-depth study on the dynamic behavior and capacity of conventional riser-guards is considered to be the primary challenge for creative and efficient design of conventional riser protectors. This paper presents a detailed numerical investigation on the dynamic response, damage and failure mechanics of conventional steel riser-guards during accidental vessel collision using nonlinear finite element analysis. Collision forces equivalent to vessel collision with riser-guards for different spans were estimated to provide information for load-based design. Variation in damage patterns for broadside and bow impacts is presented for riser protectors with different spans. The actual capacity of a typical riser-guard in terms of maximum impact energy sustainable prior to failure was also determined from dynamic pushover analysis. The structural response and damage parameters presented in this study can be used for better understanding of damage mechanism and failure capacity of riser protectors which can act as a baseline for further design optimization, as well as the development of other alternative riser protection systems.

Development of Risk-Reliability Based Underwater Inspection for Fixed Offshore Platforms in Indonesia

In RBUI method, platform with higher risk level will need inspection done more intensively than those with lower risk level. However, the probability of failure (PoF) evaluation in RBUI method is usually carried out in semi quantitative way by comparing failure parameters associated with the same damage mechanism between a group of platforms located in the same area. Therefore, RBUI will not be effective for platforms spread in distant areas where failure parameter associated with the same damage mechanism may not be the same. The existing standard, American Petroleum Institute, Recommended Practice for Structural Integrity Management of Fixed Offshore Structures (API RP-2SIM), is limited on the general instructions in determining the risk value of a platform, yet it does not provide a detail instruction on how determining the Probability of Failure (PoF) of platform. In this paper, the PoF is determined quantitatively by calculating structural reliability index based on structural collapse failure mode, thus the method in determining the inspection schedule is called Risk-Reliability Based Underwater Inspection (RReBUI). Models of 3-legs jacket fixed offshore platform in Java Sea and 4-legs jacket fixed offshore platform in Natuna Sea are used to study the implementation of RReBUI.