scholarly journals Sensitivity and Uncertainty Studies for the Modelling of Marine Growth Effect on Offshore Structures Loading

2002 ◽  
Author(s):  
Franck Schoefs
Keyword(s):  
Extreme Events ◽  
Structural Integrity ◽  
Scale Effects ◽  
Structural Response ◽  
Fatigue Loading ◽  
Offshore Structures ◽  
Growth Effect ◽  
Sensitivity Analyses ◽  
Physical Analysis ◽  
Safety Level

Structural response to extreme events or fatigue loadings and structural integrity are major criteria to be quantified in a rational process of reassessment. It is now well established that the probabilistic mechanics approach gives an efficient means for measuring the relative changing in safety level compared to a predefined requirement. To this aim, effects of marine growth have been largely studied during the last two decades. This natural process of structural colonization is particularly hard to embrace because it leads to various consequences as over-loading effect coming from screen and drag effects, bio-chemical attacks of materials and mask effects for inspections methods. Only effects on loading are studied here. These effects are particularly hard to quantify because of the bio-variety of marine growth, season conditioning, natural cleaning or death of species, severe competition leading to replacement of some species and of course local hydrodynamic conditions. As in situ data collection through inspections is hard to practice and very expensive, lot of works propose experiments respecting scale effects and numerical modelling. Both are needed to perform uncertainty and sensitivity analyses. This paper proposes a numerical analysis of marine growth effects based on Response Surface Methodology. This method is here suggested to provide explicit approximations of load variables acting on offshore structures submitted to extreme events or fatigue loading as Jacket platforms. Then, from a sensitivity analysis, main factors conditioning load effects are pointed out. From a physical analysis of hydrodynamics parameters affecting these dominant variables, their probabilistic modelling is then suggested using available published experiments for several probabilistic characteristics.

Hull Structure Integrity Management in Floating Structures–FSO Puteri Dulang Case

2014 ◽  
Vol 69 (7) ◽  
Author(s):  
Ajith Kumar Thankappan ◽  
M. Fazli B. M. Yusof
Keyword(s):  
Fatigue Life ◽  
Structural Integrity ◽  
Structural Response ◽  
Offshore Structures ◽  
Design Phase ◽  
Floating Structures ◽  
Offshore Installations ◽  
Hull Structure ◽  
Integrity Management ◽  
New Buildings

This paper highlights the key differences in practices employed in managing hull structure integrity of permanently moored floating offshore structures as against sailing vessels which are subject to periodic dry docking. During the design phase, the structural integrity management over the life of a sailing vessel is primarily taken into account by means of Class prescribed Nominal Design Corrosion Values which are added to minimum scantling requirements calculated based on strength and fatigue criteria. In contrast, for permanently moored offshore installations like FPSOs, FSOs etc. the hull structure integrity over the entire design life of the asset is a key design consideration both for new buildings and conversions. Analytic methods and tools (primarily those developed by Class Societies) are available to evaluate the strength requirements (based on yielding, buckling and ultimate strength criteria) and fatigue life of the hull structure. Typically three levels of analysis with increasing degree of complexity and analysis time are used to predict the structural response and fatigue life of the Hull during design phase. The degree of detailed analysis required needs to be determined in light of the expected optimization in terms of savings in scantlings for new building or for steel renewal requirements in case of conversions.

Structural Integrity Control of Ageing Offshore Structures: Repairing and Strengthening With Grouted Connections

2013 ◽  
Author(s):  
S. M. S. M. K. Samarakoon ◽  
R. M. Chandima Ratnayake ◽  
S. A. S. C. Siriwardane
Keyword(s):  
Structural Integrity ◽  
Oil And Gas ◽  
Load Transfer ◽  
Fatigue Loading ◽  
Offshore Structures ◽  
Future Research ◽  
Offshore Platforms ◽  
Historical Evidence ◽  
Design Service Life ◽  
Pros And Cons

Structural integrity control (SIC) is an increasingly important element of offshore structures. Not only is it used in newly built and existing offshore structures (e.g. oil and gas (O&G) production & process facilities (P&PFs), wind turbine installations, etc.), but SIC is also essential for ageing offshore platforms which are subjected to an extension of their design service life. In these cases, SIC programs must be performed to assess the platforms. If any significant changes in structural integrity (SI) are discovered, then it is essential to implement an appropriate strengthening, modification and/or repair (SMR) plan. Currently, welded and grouted repairs are mostly used for SMR. Although a welded repair may typically restore a structure to its initial condition, if the damage is due to fatigue loading and welded repairs have been carried out, then historical evidence reveals that there is a high potential for the damage to reappear over time. On the other hand, mechanical connections are significantly heavier than grouted connections. Consequently, grouted repairs are widely used to provide additional strength, for instance, to handle situations such as preventing propagation of a dent or buckle, sleeved repairs, leg strengthening, clamped repair for load transfer, leak sealing and plugging, etc. This manuscript examines current developments in grouted connections and their comparative pros and cons in relation to welded or mechanical connections. It also provides recommendations for future research requirements to further develop SMR with grouted connections.

Validation of Hydro-Structure Analysis Using Partial Structural Model

2016 ◽  
Author(s):  
George Jagite ◽  
Xiang-Dong Xu ◽  
Xiao-Bo Chen ◽  
Sime Malenica
Keyword(s):  
Structure Analysis ◽  
Structural Model ◽  
Structural Integrity ◽  
Structural Response ◽  
Offshore Structures ◽  
3D Fem ◽  
Fem Model ◽  
Ship Model ◽  
Partial Model ◽  
Analysis Methodology

Nowadays direct Finite Element Method (FEM) calculation using partial or full length model is necessary for checking the structural integrity of ship and offshore structures under given environmental conditions. The main advantage of using hydro-structure analysis on partial model is to obtain better accuracy than usual computation based on rule loads and also a consistent decrease of the time necessary to build a complete ship model. The comparison of different three cargo hold models with the complete ship model and the improvement of our partial FEM models are the main objectives of the work. Unlike the classical partial FEM models approach, our hydro-structure analysis is based on creating an equivalent full FEM model from the partial model. The equivalent full FEM model is built by adding to the partial model two concentrated masses in the center of gravity of missing aft and fore parts. The mass and inertia properties of the equivalent full FEM model are the same as full ship FEM model. By using an equivalent full FEM model the problem of balancing the partial model transforms into the same problem for the corresponding full model. Instead of using the traditional method for interpolating the pressure from hydrodynamic mesh to structural mesh, the pressure components are recalculated over structural mesh. The inertial loads are then determined by motion equations integrating all pressure loads. In this way, the structural model is fully balanced. The balancing of the 3D FEM structural models represents one important issue to avoid unphysical structural response induced by an unbalanced structural model. This paper is focused on the validation of hydro-structure analysis methodology by comparing the results on a FSO unit using an equivalent full FEM model and a complete ship model.

Possibilistic Analysis of Structural Systems Subjected to Blast Loads

2003 ◽  
Author(s):  
Hari B. Kanegaonkar
Keyword(s):  
Pulse Duration ◽  
Rise Time ◽  
Peak Pressure ◽  
Pressure Rise ◽  
Structural Response ◽  
Offshore Structures ◽  
Blast Loads ◽  
Safety Level ◽  
Ignition Point ◽  
Blast Pulse

The accidental release of the hydrocarbons and the possibility of resulting explosion have to be taken into account while designing the topside systems of the offshore structures. Determination of design explosion loads for the topside structures is a complex task since it involves several sources of uncertainty. Dimensioning of blast loads is important in achieving the desired safety level against the structural failure and related consequences. The design loads must incorporate uncertainties due to variability in the ignition point location, the type of ignition source, the volume of the gas released and the characteristics of the gas cloud etc. These uncertainties which are not statistical in nature may not be categorised as random or probabilistic but are cognitive and fuzzy in nature. The probabilistic framework for structural analysis subjected to blast loads could be quite cumbersome due to high number of uncertain variables and complex interdependency. The uncertainty in the load and corresponding uncertainty in the structural response can either be predicted from variations in the uncertain load parameters — a sensitivity evaluation or through a compact “possibilistic analysis”. The blast loads are usually defined as a triangular pulse through peak pressure, rise time and the blast pulse duration as the parameters. In the present investigation, the parameters in the triangular blast load description are assumed fuzzy. The peak pressure, rise time and blast pulse duration are defined using triangular fuzzy numbers. The possibilistic dynamic response of simple structural system — beam — used in the blast wall is obtained using single-degree of freedom approximation. It is shown that the possibilistic response provides rational decision making tool to arrive at desired safety level.

Integrity Management of Offshore Structures With Emphasis on Design for Structural Damage Tolerance

2020 ◽  
Vol 142 (3) ◽  
Author(s):  
Torgeir Moan
Keyword(s):  
Structural Damage ◽  
Structural Integrity ◽  
Oil And Gas ◽  
Damage Tolerance ◽  
Progressive Failure ◽  
Offshore Structures ◽  
Limit States ◽  
Safety Level ◽  
Integrity Management ◽  
Conservative Values

Abstract Based on relevant accident experiences with oil and gas platforms, structural integrity management of offshore structures is briefly outlined, including adequate design criteria, fabrication and operational procedures, as well as life cycle quality assurance and control. The focus is on developing an operational design standard for accidental collapse limit states to ensure robustness or damage tolerance. The focus is to ensure an acceptable safety level against progressive failure leading to total loss in view of initial damage caused by accidental actions due to operational errors and abnormal structural damage due to fabrication errors and abnormal deterioration during operation as well as the actions on the damaged structure and inherent uncertainties. Moreover, the damage tolerance required for achieving safety by inspection, monitoring and repair strategies, is briefly addressed. While the basic damage tolerance requirement refers to the survival of the structure in certain damage conditions, wider aspects of robustness in terms of the structure’s sensitivity to the deviation of action effects and resistances from normal conditions are also briefly addressed. In particular, it is suggested to provide robustness in cases where the structural performance is sensitive to uncertain parameters, by choosing conservative values of these parameters.

State of Art in Life Extension of Existing Offshore Structures

2012 ◽  
Author(s):  
Abe Nezamian ◽  
Robert J. Nicolson ◽  
Dorel Iosif
Keyword(s):  
Structural Integrity ◽  
Oil And Gas ◽  
Offshore Structures ◽  
Life Extension ◽  
Operational Environment ◽  
Original Design ◽  
Safety Level ◽  
Operational Conditions ◽  
Offshore Installations ◽  
State Of Art

A large number of the old oil and gas facilities have reached or exceeded their initial design life. With a continued requirement to produce oil or gas, either from the original fields or as a base for neighbouring subsea completions, many of these respective offshore installations are likely to remain operational for a period of time in the foreseeable future. The ageing offshore infrastructure presents a constant and growing challenge. Ageing is characterised by deterioration, change in operational conditions or accidental damages which, in the severe operational environment offshore, can be significant with serious consequences for installation integrity if not managed adequately and efficiently. In order to ensure technical and operational integrity of these ageing facilities, the fitness for service of these offshore structures should be maintained. The maintenance of structural integrity is a significant consideration in the safety management and life extension of offshore installations. Detailed integrity assessments are needed to demonstrate that there is sufficient technical, operational and organisational integrity to continue safe operation throughout a life extension. Information on history, characteristic data, condition data and inspection results are required to assess the current state and to predict the future state of the facility and the possible life extension. This paper presents state of art practices in life extension of existing offshore structures and an overview of various aspects of ageing related to offshore facilities, represented risk to the integrity of a facility and the required procedures and re assessment criteria for deciding on life extension. This paper also provides an overall view in the structural requirements, justifications and calibrations of the original design for the life extension to maintain the safety level by means of a maintenance and inspection programs balancing the ageing mechanisms and improving the reliability of assessment results.

Neural Network Optimization for Lifetime Structural Adaptation to Evolving Natural Events

2021 ◽  
Author(s):  
Xiaowei Wang ◽  
YeongAe Heo
Keyword(s):  
Neural Network ◽  
Structural Engineering ◽  
Structural Response ◽  
Offshore Structures ◽  
Regression Modeling ◽  
Sensitivity Analyses ◽  
Structural Adaptation ◽  
Structural Problems ◽  
Learning Rates ◽  
Structural Responses

Abstract Recent advances in data analytics, numerical modeling, and structural health monitoring (SHM) boost the application of machine learning methods in the field of structural engineering. Among them, the multi-layer neural network (MLNN) is one of the most popular ones. However, mathematical details of MLNN have yet to be well understood for structural problems. This study aims to identify optimal MLNN parameters for regression modeling of structural response estimates. SHM data-validated finite element models considering stochastic uncertainties in the natural events and structural properties are used to prepare a large dataset for regression modeling. The efficacy and accuracy of regression modeling are optimized by extensive sensitivity analyses for key MLNN parameters (e.g., numbers of hidden layers and neurons, activation functions and learning rates) via a k-fold cross-validation process. The optimized regression modeling is incorporated into a conceptual smart framework for lifetime structural performance assessment adapting to evolving natural events. The presented optimization process and smart framework is applicable to marine and offshore structures by characterizing the offshore hazards and structural responses.

Structural Response of Ice-Going Ships Using a Probabilistic Design Load Method

2015 ◽  
Author(s):  
Boris Erceg ◽  
Freeman Ralph ◽  
Sören Ehlers ◽  
Ian Jordaan
Keyword(s):  
Scale Effects ◽  
Probabilistic Approach ◽  
Structural Response ◽  
Probabilistic Methods ◽  
Stiffened Panel ◽  
Safety Level ◽  
Design Load ◽  
Northern Sea Route ◽  
Ice Conditions ◽  
Probabilistic Nature

Ships operating in ice-covered waters experience intense loads from ice features, particularly multiyear ice. Therefore, their structures have to be able to withstand these loads, making structural design paramount. Current formulations of ice class rules do not fully account for the probabilistic nature of ice loads, i.e. scale effects for local ice pressures captured in full-scale measurements. Furthermore, ice class rules do not consider route-specific ice conditions when calculating the design load, i.e. the exposure of the vessel to ice crushing determined by the number and duration of rams. An approach to arctic ship design based on probabilistic methods was developed by Jordaan and co-workers in 1993 and is described in this paper. The approach is used to estimate extreme design loads based on the annual interaction events and the design strategy (target exceedence criteria). The objective of this paper is to select an appropriate ice class for a vessel navigating along the northern sea route, and to compare the design requirements with those determined using the probabilistic approach based on measured data and expected exposure. Local hull pressures have been measured using the USCGC Polar Sea for a range of ice conditions including first year and multi-year ice. Impact conditions similar to those expected along the Northern Sea route were selected and corresponding pressurearea parameters used for input into the probabilistic approach discussed above. This paper will compare the design and response of an exemplary stiffened panel using the described approach to requirements given in Finnish Swedish Ice Class Rules. A case study structure will be analyzed using Finite Element Method for a chosen exposure scenario and target safety level.

Structural Analysis of Various Stool Configurations on FPSO Topsides Modules

2014 ◽  
Author(s):  
Amy Marie Zahray ◽  
David Sandor Smith
Keyword(s):  
Finite Element ◽  
Structural Response ◽  
Fatigue Loading ◽  
Offshore Structures ◽  
Strength Analysis ◽  
Fe Model ◽  
Element Analysis ◽  
Modern Approach ◽  
Modeling And Analysis ◽  
Hull Girder

This thesis investigates some of the structural issues associated with the conversion of an oil tanker or a very large crude carrier (VLCC) into a floating production, storage, and offloading unit (FPSO). Specifically, a series of calculations were completed, including Finite Element Analysis (FEA), to evaluate the structural response of the stood interface of the topside module, resulting from its interaction with the FPSO’s hull girder in waves. The interfaces between topside modules and the hulls of converted tankers experience high fatigue loading. This loading, which is caused primarily by hull girder bending elongation in addition to inertia loading on the topside modules, creates a structural design challenge. A modern approach to solving a problem of such complexity requires the generation of a finite element (FE)model of the topside module, the stool interface, and the structure located immediately below the interface. The objective of this thesis was to determine a stool arrangement that performs the best in fatigue, while also meeting all class requirements for maximum allowable stress. The modeling and analysis of the so-called deck sub-model was carried out using the FEA program Sesam GeniE. GeniE is a program developed by Det Norske Veritas (DNV)Software that sees wide use in the industry for engineering and strength analysis of ships and offshore structures. The loading of the model represents the dynamic loads experienced by an actual FPSO concept or design. The FPSO concept was provided by an industry professional at Viking Systems, Lars Henriksen. A total of six different stool configurations were investigated in this thesis. Variables of consideration were: flexibility of connection points, sliding and welded connections, and number and placement of stools. In addition, the producibility challenges related to the stool design selection and integration, which is expected to impact the conversion cost.

Nonlinear Finite Element Analysis of Pipe Bends Subjected to Inplane Bending

2016 ◽  
Author(s):  
Venkata M. K. Akula ◽  
David W. Martin
Keyword(s):  
Finite Element ◽  
Structural Integrity ◽  
Three Dimensional ◽  
Structural Response ◽  
Sensitivity Analyses ◽  
Model Parameters ◽  
Element Analysis ◽  
Modeling And Analysis ◽  
Shell Elements ◽  
Length Changes

Pipe bends are structural components that provide flexibility to accommodate length changes in pipelines while allowing fluid flow. Estimating the collapse load is critical to ensuring the structural integrity of the pipeline. This research discusses modeling and analysis of pipe bends utilizing the finite element method. Three-dimensional models utilizing elbow elements, shell elements, and brick elements are generated to predict the collapse moment of pipe bends subjected to in-plane loading. All simulation was performed using Abaqus. To obtain a more physically-consistent response, material, geometric, and boundary nonlinearities are all included. A MPC user subroutine is utilized to capture the end behavior of the pipe bends correctly when utilizing shell and brick elements. Experimental data from two sources, available in literature, was used to evaluate the effect of the different element types on the predicted structural response. Finally, utilizing the shell, brick, and the elbow elements, parameter sensitivity analyses are performed to identify the key parameters influencing the response of pipe bends. Multiple parameters are varied independently of each other to fully understand and capture their influence on the response. SIMULIA’s Isight software was used to automate the workflow and vary the model parameters about their respective baseline values.

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