ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V04AT00A001. doi:10.1115/OMAE2014-NS4A.

This online compilation of papers from the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering (OMAE2014) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Structures, Safety and Reliability

2014;():V04AT02A001. doi:10.1115/OMAE2014-23049.

It is shown that solutions to the governing equations for arbitrarily large static deflections of cables obeying any elastic law can be reduced to quadratures for four general types of distributed loadings. These solutions are given in closed form and ways of using them are discussed. It is observed that the final shape can be found without knowledge of the initial shape in three cases and without knowledge of the elastic law in two if the final length of the cable is regarded as known. These latter two cases considerably generalize the classical catenary and parabola solutions.

Topics: Cables , Deflection
Commentary by Dr. Valentin Fuster
2014;():V04AT02A002. doi:10.1115/OMAE2014-23051.

Free vibration analysis of plates with openings represents an important issue in naval architecture and ocean engineering applications. Namely, they are often primary design members of complex structures and knowledge about their dynamic behavior becomes a prerogative for the proper structural design. This paper deals with application of assumed mode method to free vibration analysis of rectangular plates with multiple rectangular openings at arbitrary defined locations. Developed method can be applied to both thin and thick plates as well as to classical and non-classical edge constraints. In the assumed mode method natural frequencies and mode shapes of a corresponding plate are determined by solving an eigenvalue problem of a multi-degree-of-freedom system matrix equation derived by using Lagrange’s equations of motion. The developed procedure actually represents an extension of a method for the natural vibration analysis of rectangular plates without openings, which has been recently presented in the relevant literature. The effect of an opening is taken into account in a simple and intuitive way, i.e. by subtracting its energy from the total plate energy without opening. Illustrative numerical examples include dynamic analysis of rectangular plates with single and multiple rectangular openings with various thicknesses and different combinations of boundary conditions. Also, the influence of the rectangular opening area on the plate dynamic response is analyzed. The comparisons of the results with those obtained using the finite element method (FEM) is also provided, and very good agreement is achieved. Finally, related conclusions are drawn and recommendations for future investigations are presented.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A003. doi:10.1115/OMAE2014-23057.

Steel Catenary Riser (SCR) offers an attractive solution to deepwater floating structure due to its economical effectiveness, large diameters, high resistance to internal and external pressure, simple and robust installation methods. SCR forms a prolongation of a subsea flowline attached to a fixed platform or a floating unit in a catenary shape. Due to the relatively large motion under waves and currents, SCR lines are sensitive to dynamic effects and vulnerable to damage in deep water. They are commonly subjected to high top tension and large bending moment due to platform or FPSO movements which may lead to fatigue damage. There are many uncertainties that can affect the safety and cost-effectiveness of the SCR. Offshore design codes typically adopt empirical safety factors to account for these uncertainties but this approach does not permit the prediction of failure probability of the riser system.

To address the above issue, this paper presents the coupling of the stochastic analysis concept to the deterministic computational model for the dynamic analysis of SCR. The finite element solution is developed for hydrodynamic and structural analysis accounting for nonlinear and dynamic coupling effects. Methods for reduction of dimensionality of uncertainties are investigated to help to make the analysis computationally feasible. Uncertainty and numerical realization of specific uncertainty parameters are modeled through riser dynamics software and uncertainty analysis software. Distributions of effective tension, bending moment, and API RP 2RD stress are illustrated for a specified SCR model. The correlation effects between structural responses and random variables are investigated. In addition, the failure probability of SCR API RP 2RD stress is investigated through Monte Carlo simulations. This will help to evaluate the behavior and reliability of SCR realistically, incorporating the environmental, geometry and operational uncertainties in engineering practice.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A004. doi:10.1115/OMAE2014-23062.

Corrosion in ship structures is influenced by a variety of factors that are varying in time and space. Existing corrosion models used in practice only partially address the spatial variability of the corrosion process. Typical estimations of corrosion parameters are based on averaging measurements over structural elements from different ships and operational conditions, without considering the variability among and within the elements. However, this variability is important when determining the necessary inspection coverage, and it may influence the reliability of the ship structure. We develop a probabilistic spatio-temporal corrosion model based on a hierarchical approach, which represents the spatial variability of the corrosion process. The model includes the hierarchical levels vessel – compartment – frame – structural element – plate element. At all levels, variables representing common influencing factors are introduced. Moreover, at the lowest level, which is the one of the plate element, the corrosion process is modeled as a spatial random field. For illustrative purposes, the model is trained through Bayesian analysis with measurement data from a group of tankers. In this application it is found that there is significant spatial dependence among corrosion processes in different parts of the ships, which the proposed hierarchical model can capture. Finally, it is demonstrated how this spatial dependence can be exploited when making inference on the future condition of the ships.

Topics: Corrosion , Ships
Commentary by Dr. Valentin Fuster
2014;():V04AT02A005. doi:10.1115/OMAE2014-23095.

15 failures of offshore mooring lines have occurred in the period 2010–2013 on the Norwegian Continental Shelf. The failures are cause by a mixture of overload, fatigue and mechanical damages.

Failure statistics from the period 2010–2013 are presented and compared it with earlier statistics. In the period 1996–2005 a high number of cases were observed. The industry reacted reasonably, and the number of incidents was clearly reduced for several years. In 2010 the number of cases started to increase again, rising questions on how to improve again. Some of the old failure modes have disappeared, as dragging of anchors and failures of chains above 20 years of age. But several other failure modes have appeared or reappeared. Differences between different types of units are found. The failure frequencies are highly uncertain, but the failure rate from 2010–2013 have been in the order of magnitude

• single line failures: 92*10−4 per line year and

• double line failures: 12*10−4 per line year.

No triple line failures have occurred in 2010–2013, but using data from 2000–2013, the failure rate have been in the order of magnitude 2*10−4 per line year.

The paper discusses possible changes in practice, regulations and standards, on quality of materials, maintenance, ALS-, FLS- and ULS-design.

Topics: Failure , Mooring
Commentary by Dr. Valentin Fuster
2014;():V04AT02A006. doi:10.1115/OMAE2014-23099.

Drilling derrick, acting as one of the most important structures in Jack-up Rig, is designed to accommodate drilling machines like top drive, utility winches and so on. According to the requirement of API Spec. 4F, the accuracy of the standard design ratings of each structure shall be tested by proof loading or a computer model such as Finite Element Analysis. Therefore, this thesis utilizes Finite Element Analysis software, ANSYS, to verify the design of derrick structure for CPOE-16 (No. 16 jack-up rig for China National Petroleum Offshore Engineering) jack-up based on API 4F [1], AISC Allowable Stress Design [2] and ABS MODU rules [3]. The permanent loads, live loads (variable functional loads), environmental loads (mainly wind loads) and dynamic loads will be included in the total derrick structure strength calculation with the purpose of verifying the security and reliability in all required conditions by rules and regulations. Furthermore, on the basis of analytical result, the authors focused on the adverse beams provide an optimization solution by improving the section of beams for reducing the distortion, and ultimately deal with the distorted problem.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A007. doi:10.1115/OMAE2014-23126.

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. Due to nonlinearity of the drag component of Morison’s wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the probability distribution of extreme responses are not available. To this end, the conventional Monte Carlo simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated extreme response values (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, the efficiency of an alternative technique in comparison with the conventional simulation technique is investigated.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A008. doi:10.1115/OMAE2014-23129.

In stress-based fatigue design and analysis of marine structures, a stress concentration factor (SCF) is used to compute the local stress in structural details when the hot-spot approach is used. For a typical steel material, the fatigue damage is computed as proportional to the power 3–5 of the stress values, defined by the chosen S-N curve. This means that a small change in the SCF value can lead to a large difference in the fatigue damage result and corresponding fatigue life prediction. Thus, the methodology used to compute the SCF should be clearly defined in classification society guidelines.

This study presents a review of different direct calculation procedures for how to obtain the SCF based on fatigue assessment guidelines. The effect of different element types and local stress extrapolation methods to the fatigue damage estimation is studied for both longitudinal load and bending load conditions in a container ship. A simple structural detail with cracks observed after only a few years in service is used for the case study. For this structural detail, two alternative methods to compute the local stress for fatigue assessments are compared. The difference in fatigue life prediction using the proposed approaches is compared; with at least 50% difference expected even within the guidelines from the same classification society. It is further discussed how to reduce the SCF with the objective to increase the fatigue life.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A009. doi:10.1115/OMAE2014-23142.

Offshore structures are exposed to random wave loading in the ocean environment, and hence the probability distribution of the extreme values of their response to wave loading is of great value in the design of these structures. Due to nonlinearity of the drag component of Morison’s wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the probability distribution of extreme responses are not available. Monte Carlo time simulation technique can be used to derive the probabilistic properties of offshore structural response, but the procedure is computationally demanding. Finite-memory nonlinear system (FMNS) modeling of the response of an offshore structure exposed to Morison’s wave loading has been introduced to reduce the computational effort, but the predictions are not very good for low intensity sea states. To overcome this deficiency, a modified version of the FMNS technique (referred to as MFMNS modeling) was proposed which improves the accuracy, but is computationally less efficient than the FMNS modeling. In this study, the accuracy of the 100-year responses derived from the long-term probability distribution of extreme responses from FMNS and MFMNS methods is investigated.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A010. doi:10.1115/OMAE2014-23145.

Numerous methodologies for estimating the reliability of composites have been published in the past few decades. This paper presents the experiences and findings by the authors regarding an assessment of different mechanical and probabilistic models for the calculation of reliability estimates of fibre-reinforced plastic laminates. The assessment was performed to determine the manner by which the models influence the reliability estimations. This knowledge is used to determine the most suitable combination of models for reliability-based design optimization of marine structures made out of fibre-reinforced plastics. The assessment consists of a brief overview of a number of probabilistic and mechanical models as well as the computation of reliability estimates for a number of fibre-reinforced laminates through Monte Carlo simulations. Among some of the findings, it was found that the definition of matrix cracking and the choice of mechanical model (description of damage initiation and development) influence significantly the reliability estimations of fibre-reinforced plastic laminates.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A011. doi:10.1115/OMAE2014-23153.

The risk of ship collision and grounding has increased significantly in recent years as a result of the growing size and number of ships at sea. The potentially costly consequences of collision and grounding in the form of fatalities, property, and cargo, as well as environmental pollution in the form of oil spills, etc., are the main motivations for research on collision and grounding. From a structural evaluation standpoint, there is a great deal of uncertainty related to the residual strength of damaged ships considering various influential parameters, such as damage size, geometry and location, internal structural arrangement, material property, loading case, and sea weather. Therefore, it is important to clarify the residual hull girder strength of damaged ships by collision or grounding in order to ensure their safety. The present study undertook a deliberate finite element analysis to investigate the residual ultimate strength of damaged ship hull, where two damage models were assumed and compared. One model simulated actual damage resulting from an accident in the form of hole with adjacent plastic deformation, while the other applied simplified damage, considering unavailable measurement of the damage by removing the damaged part from the original ship hull. The comparison showed that the assessment of residual ultimate strength of a damaged ship based on the simplified damage model could produce a sufficiently accurate result and stay slightly safer, provided that a reasonable criterion of simplification was defined first. The studies showed that it is possible to accurately estimate the residual ultimate strength of a damaged ship without detailed measurement of the damage, and consequently facilitate decision-making regarding the ship salvage under emergency.

Topics: Girders , Ships , Hull
Commentary by Dr. Valentin Fuster
2014;():V04AT02A012. doi:10.1115/OMAE2014-23155.

Structures in civil, naval and aeronautical engineering commonly use plates that experience shear loading. When subjected to loading, such as shear, axial compression, bending and lateral pressure, many panel designs use stiffeners to reinforce the structure and increase the buckling capacity. Particularly, in the oil industry, platform side shell structures are made up of stiffened panels that experience a considerable amount of shear force. When added to geometrical imperfection (e.g. fabrication or supply vessel collision), this compromises the ultimate strength capability. The aim of the present study is to develop a simplified model, using FEM, to predict the ultimate loading capability of stiffened panels under pure shear stress. Furthermore, an amplified parametric study varying the geometry and initial imperfection was considered to verify such influences. Comparison with several current and published studies yielded positive and conclusive results.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A013. doi:10.1115/OMAE2014-23156.

Ewans and Jonathan [2008] shows that characteristics of extreme storm severity in the northern North Sea vary with storm direction. Jonathan et al. [2008] demonstrates, when directional effects are present, that omnidirectional return values should be estimated using a directional extreme value model. Omnidirectional return values so calculated are different in general to those estimated using a model which incorrectly assumes stationarity with respect to direction. The extent of directional variability of extreme storm severity depends on a number of physical factors, including fetch variability. Our ability to assess directional variability of extreme value parameters and return values also improves with increasing sample size in general. In this work, we estimate directional extreme value models for samples of hindcast storm peak significant wave height from locations in ocean basins worldwide, for a range of physical environments, sample sizes and periods of observation. At each location, we compare distributions of omnidirectional 100-year return values estimated using a directional model, to those (incorrectly) estimated assuming stationarity. The directional model for peaks over threshold of storm peak significant wave height is estimated using a non-homogeneous point process model as outlined in Randell et al. [2013]. Directional models for extreme value threshold (using quantile regression), rate of occurrence of threshold exceedances (using a Poisson model), and size of exceedances (using a generalised Pareto model) are estimated. Model parameters are described as smooth functions of direction using periodic B-splines. Parameter estimation is performed using maximum likelihood estimation penalised for parameter roughness. A bootstrap re-sampling procedure, encompassing all inference steps, quantifies uncertainties in, and dependence structure of, parameter estimates and omnidirectional return values.

Topics: Storms
Commentary by Dr. Valentin Fuster
2014;():V04AT02A014. doi:10.1115/OMAE2014-23157.

Specification of realistic environmental design conditions for marine structures is of fundamental importance to their reliability over time. Design conditions for extreme waves and storm severities are typically estimated by extreme value analysis of time series of measured or hindcast significant wave height, HS. This analysis is complicated by two effects. Firstly, HS exhibits temporal dependence. Secondly, the characteristics of Display FormulaHSsp are non-stationary with respect to multiple covariates, particularly wave direction and season.

We develop directional-seasonal design values for storm peak significant wave height (Display FormulaHSsp) by estimation of, and simulation under a non-stationary extreme value model for Display FormulaHSsp. Design values for significant wave height (HS) are estimated by simulating storm trajectories of HS consistent with the simulated storm peak events. Design distributions for individual maximum wave height (Hmax) are estimated by marginalisation using the known conditional distribution for Hmax given HS. Particular attention is paid to the assessment of model bias and quantification of model parameter and design value uncertainty using bootstrap resampling. We also outline existing work on extension to estimation of maximum crest elevation and total extreme water level.

Topics: Design , Storms , North Sea
Commentary by Dr. Valentin Fuster
2014;():V04AT02A015. doi:10.1115/OMAE2014-23166.

This paper presents a comparison made of different fatigue calculation methods used in the maritime industry today, with the aim of having a higher control of a fatigue failure site. To provide an overview of the different fatigue calculation methods, a comparison study was performed, as well as a local weld parameter study for two typical fillet welded joints. The two methods used for this study were the structural hot spot and effective notch stress method. Two fillet welded joints were provided by Aker Solutions MMO AS, Bergen, Norway. The first joint is a rectangular hollow section from a davit, built as a truss. The second model is a part of a K-joint from a semi-submersible (Aker H3 design). Both joints were analysed using fine 3D finite element models.

The two different fatigue life calculation methods yielded different fatigue lives for the weld toe, with inconclusive results regarding their conservatism which is discussed in the paper. An increased weld toe radius gave a higher fatigue life for the weld toe, while the larger weld size increased the fatigue life in the weld root. Any weld size effect regarding fatigue life in the weld toe could not be established. Based on the effective notch stress method calculations, there was an indication of weld root failure for the K-joint of the drilling unit. Fatigue life improvement methods only increasing weld toe fatigue life are not recommended based on these results.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A016. doi:10.1115/OMAE2014-23175.

Container ships are particularly susceptible to torsional loads. The distribution of torsion-induced warping stress in a container ship hull is more complicated and difficult to be expressed by beam theory formulas. In practice, finite element (FE) analysis is typically used to calculate the stress response to wave-loading conditions. However, it is time consuming to compute hull girder stresses for all relevant sea conditions through FE analyses. In this paper, an efficient and robust approach is proposed by combining beam theory and FE analyses in the determination of hull girder stresses. The parameters required by beam theory can be regressed through matching stress records from a FE analysis with the corresponding sectional and pressure loads from the hydrodynamic simulation. Stress records obtained using the proposed method are utilized in fatigue assessment of a case study container vessel. The results show that the accuracy of the regression approach is satisfactory compared with the full FE analyses.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A017. doi:10.1115/OMAE2014-23198.

This paper is based on the JIP study for the oceanographic hindcast and environmental load statistics of fixed steel structures at South China Sea under tropical cyclone conditions. It focuses on the sensitivity and comparison studies by exploring the degrees and reasons of variability that may occur in determination of design environmental conditions resulting from the selection of the design standards and approaches. The bias and efficiency in extreme values prediction are examined with respect to modeling uncertainty and statistical uncertainty. The long term distributions of maximum wave height as well as the associated wave period conditional on the design wave height are derived following the storm event based method. The approaches for combing wave, current and wind to define the design conditions and the associated biases on design load are investigated. Second order random and spreading wave theory is adopted to estimate the extreme wave crest height distributions. The extreme water level issue is addressed and recommendations are given for setting the deck elevation to achieve the explicit wave-in-deck probabilities.

The studies are carried out by applying a dataset of a grid point containing 182 typhoons spanning 40 continuous years to demonstrate the analytical procedures in an understandable fashion. The results of this paper should lead to improvements in prediction of the environmental conditions for design of new-built structures to attain their target safety levels, as well as for assessment of existing structures to demonstrate their fitness-for-purpose.

Topics: Design
Commentary by Dr. Valentin Fuster
2014;():V04AT02A018. doi:10.1115/OMAE2014-23210.

With the expectation of hull girder asymmetry and corresponding shift in elastic neutral axis resulting from collision damages and other forms of structural deteriorations, the interaction of vertical and horizontal hull girder capacities become quite significant in the assessment of ship structural safety. This paper therefore extends the application of a previously proposed interactive-numerical probabilistic based methodology for structural safety to assess the hull girder ultimate strength reliability of a damaged ship by means of a user-defined numerical framework. Hull girder capacity is calculated using the NS94D ultimate strength code, which is based on the Smith’s progressive collapse method. The resulting deterministic responses have been interactively linked to the NESSUS probabilistic framework so that the reliability of the damaged hull girder is predicted using an implicit limit state function defined based on a transformation of coordinates to appropriately account for any shift in the neutral axis. Random deviations of the constituent variables are directly applied to calculate the ultimate strength deterministic responses, thereby circumventing the need to characterize any correlated strength variable, which is at best subjective. The conventional approach of characterizing ultimate strength by an assumed coefficient of variation and distribution type was found to be conservative in predicting structural safety of ships relative to the proposed method. Application of the interactive-numerical technique for structural reliability is therefore considered significant for problems involving correlated random variables with unknown statistical characteristics. The method is being considered to predict the safety of cracked hull girders by accounting for the residual strength and further load bearing capabilities of deteriorated and adjacent elements.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A019. doi:10.1115/OMAE2014-23219.

The worldwide growing energy demand with the exploration of new gas fields has promoted the development of high toughness seamless pipeline steels which should sustain the increasing demands resulting from the complex loading situations. One of the most important prerequisites for safe installation and operation of long distance gas transmission pipelines is the detailed knowledge and characterization of their fracture performance for specific applications. However, recent industry experience has revealed concerns related to the limitations and reliability of current test methods for brittle-to-ductile transition evaluation. Regarding the transition temperature evaluation, the critical issues involve Drop-Weight Tear Testing (DWTT) and full-scale West-Jefferson (WJ) test applied to the smaller pipes with diameter less than 500mm. The DWTT leads frequently to invalid results in terms of abnormal fracture appearance and inverse fracture occurrence. It is still not clear if this behavior is only owed to a testing effect, which material characteristics cause it and how far it reflects the full-scale behavior. Similar observations were made for the West-Jefferson tests, which could not be assessed in the standard manner either. Again, the question was towards testing effects and the behavior of the pipeline transporting gaseous media remains unanswered. Therefore, this paper aims at identifying open questions on basis of a literature study and own experimental results and showing possible ways forward in demonstrating safety in design against propagating fracture.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A020. doi:10.1115/OMAE2014-23233.

Conductor and casing systems for subsea wells can be damaged due to excessive loads caused by vessel drift offs and snag loads. These types of accidental loads on the conductor and casing system are uncontrolled in nature and hence difficult to predict. Therefore, designing the system for these types of loads can be very challenging and economically not practical. Additionally, in the event of an incident subsea, a conductor and casing system is permanently installed and cannot readily be retrieved or inspected once the damage is done.

The decision to abandon or to produce from a well with a damaged conductor has serious financial and environmental consequences and should be fully understood. Therefore, to make a knowledgeable decision, it is necessary to understand extent of the damage and its implications for future operations by forensic analytical tools. This paper examines the methodology of a conductor and casing system damage assessment in order to support more informed decisions and provides a case example for a typical deepwater subsea well undergone excessive accidental loading. The objective of the assessment is (1) to verify wellhead system integrity and (2) evaluate the effect of any future operations on the integrity of the system. The paper outlines a process, suggested code checks, and analysis techniques which can be implemented to determine fitness for service of an excessively loaded wellhead system.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A021. doi:10.1115/OMAE2014-23238.

For the frequency-domain spectral fatigue analysis, the probability mass function of stress range is essential for the assessment of the fatigue damage. The probability distribution of the stress range in the narrow-band process is known to follow the Rayleigh distribution, however the one in the wide-band process is difficult to define with clarity. In this paper, in order to assess the fatigue damage of a structure under wide band excitation, the probability mass function of the wide band spectrum was derived based on the artificial neural network, which is one of the most powerful universal function approximation schemes. To achieve the goal, the multi-layer perceptron model with a single hidden layer was introduced and the network parameters are determined using the least square method where the error propagates backward up to the weight parameters between input and hidden layer. To train the network under supervision, the varieties of different wide-band spectrums are assumed and the probability mass function of the stress range was derived using the rainflow counting method, and these artificially generated data sets are used as the training data. It turned out that the network trained using the given data set could reproduce the probability mass function of arbitrary wide-band spectrum with success.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A022. doi:10.1115/OMAE2014-23252.

A new approach to estimating environmental contours has recently been proposed, where the contours are estimated in the original physical space by Monte Carlo simulations from the joint distribution directly rather than applying the Rosenblatt transformation. In this paper, the new and the traditional approach to estimating the contours are presented and the assumptions on which they are based are discussed. The different results given by these two methods are then compared in a number of case studies. Simultaneous probability density functions are fitted to the joint distribution of significant wave height and wave period for selected ocean locations and, for each area, environmental contours are estimated for both methods. The chosen locations are characterised by different wave climates. Thus, the practical consequences of the choice of approach are assessed. Particular attention is given to mixed sea systems, i.e. a combination of wind sea and swell. In these situations, the new approach for environmental contours may fail to identify realistic conditions along some parts of the contours while for other wave conditions the contours are quite similar. The paper also briefly discusses possible ways of amending the new approach to estimating the contours to obtain more realistic conditions all along the contour lines.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A023. doi:10.1115/OMAE2014-23254.

This paper focuses on the post-ultimate strength behavior of sandwich plates. With widely application of the laminate on the ship and offshore structures, the post-ultimate strength behavior is becoming more important for safety evaluation of structures. Since the post-ultimate strength behavior can reflect the collapse extent of sandwich plate when subjected to extreme loads. A sandwich plate was modeled by FEM, its load-displacement relationship was obtained and its collapse characteristics were analyzed. The load-displacement relationship indicates its post-ultimate strength behavior, which is shown as that the load carrying capacity has a rapidly reduction when the ultimate strength is exceeded, and that the failure modes of the sandwich plate are determined by the parameter of individual layer. The simulation results were validated against experimental results. Conclusions are drawn: the displacement of sandwich plate under axial compression increased slowly before reaching the ultimate strength, once the ultimate strength was exceeded, the loads exerted on the structures sharply decreased with slowly increased displacement until the plate cracked. The simulation results have a good agreement with the experimental results. The mainly failure modes of sandwich plates can be interpreted as delamination between skin & core and core compression fracture, which are typical failure modes in engineering. The stiffness of sandwich structures decreased due to the interlaminar cracking or skin fracture, further the load carrying capacity decreased, which is of significance for guiding the design of sandwich structures.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A024. doi:10.1115/OMAE2014-23255.

This paper concerns the mechanical behavior of an integrated superstructure with opening. Integrated superstructure with different openings was constructed by using ABAQUS code based on DDG 1000 Zumwalt Class multimission destroyer, and simulation calculations with eight cases of the superstructure under slamming load and inertial load were carried out. The influential effects of the opening positions, opening dimensions and opening types on the strength and stiffness of superstructure were explicated. Conclusions are drawn: The strength and stiffness increase with the increase of the opening dimension in the design region in the initial stage, and decrease in the later stage; With the increase of distance between the small opening and the large opening, the strength and stiffness of local structure near the large opening become weaker; the maximum Mises stress and the maximum displacement are larger when the opening is in transverse direction than in longitudinal direction. The maximum Mises stress and maximum displacement occurred at the transition surface and the borders between top and broadside, the value of maximum Mises stress is about 70–90MPa, and the maximum displacement is approximately 5mm.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A025. doi:10.1115/OMAE2014-23267.

Failure Mode Effect and Analysis (FMEA) is one of the most powerful techniques for performing risk analysis for marine machinery systems, with risk being quantified through evaluating Risk Priority Numbers (RPNs) for all failure modes of the systems. The RPN is traditionally evaluated as the product of three risk criteria; occurrence (O), severity (S) and Detection (D). FMEA has several limitations such as the challenge of aggregating experts’ risk criteria rating that may be imprecise or incomplete.

In this paper some of the limitations in the conventional FMEA technique are addressed using two approaches; AVeraging technique integrated with conventional Risk Priority Number (AVRPN) and AVeraging technique integrated with TOPSIS (AVTOPSIS). Both proposed techniques use a novel approach simple average in aggregating imprecise experts’ risk criteria ratings. A case study illustrates the suitability of both techniques for use in risk prioritisation jointly or independently as the results generated by both techniques are very similar. Furthermore, the AVRPN technique has been applied to an example from the literature and it has been demonstrated to be both computationally simple and capable of producing results which almost completely match those generated by modified Dempster-Shafer evidence theory techniques.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A026. doi:10.1115/OMAE2014-23270.

The behavior of web girders is of crucial importance during ship collision and grounding accidents. A new theoretical deformation model for ship web girders subjected to in-plane localized force is proposed in this paper. It is based on a summary of the existing theoretical models and progressive deformation process of the web girder in the numerical simulation, which is a reproduction of a previous experiment. From the analysis of the deformation process of the web girder in the numerical simulation it is found that there are some important features which have not been considered by any of the existing models. Based on these new features, plastic analytical method is employed, and special emphasis is placed on the folding mechanism establishing and major energy dissipation pattern identifying. Thus, a new theoretical deformation model is proposed.

The proposed model is verified by two previous experiments, one is small-scale and the other is large-scale. From the force-indentation curves in comparisons, it can be found that the results of the proposed method compare well with those of the experiments. Therefore, the proposed method can be a useful part in the quick and reliable assessment of the performance of the ship structures in the accidental collision and grounding events.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A027. doi:10.1115/OMAE2014-23273.

In this paper an examination of methods for vibration analysis of moderately thick rectangular plates has been presented. First, the state-of-the art in thick plate vibration theories and analysis methods is described and it is followed with basic equations of the original Mindlin theory, which represents a starting point for the development of all other mathematical models. Then, the problem of analytical solving of equilibrium equations is considered based on the modified Mindlin theory of thick plate vibrations, which has been published in the literature recently. Further, energy methods that can be applied to arbitrary boundary conditions are discussed and outline of the assumed mode method is presented. Finally, in the context of numerical methods a new quadrilateral finite element, based on the above mentioned advanced thick plate theory, is included. It should be emphasized that it doesn’t suffers of shear locking problem associated with finite elements, due to natural relation among bending and shear polynomials, and moreover, it gives very accurate results. Application of the presented methods is illustrated by several numerical examples which include natural vibration analyses of rectangular plates with various thicknesses and different combinations of boundary conditions (simply supported, clamped, free and elastically restrained). Comparisons of natural frequencies and mode shapes with results available in the relevant literature and with those obtained by the commercial finite element software are also provided.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A028. doi:10.1115/OMAE2014-23288.

Recently the research on freak waves has focused on the formation mechanism as well as the experimental and numerical simulation, however the study of freak waves’ action on marine structures which is often confined to numerical methods is still not much. As beams are often studied as the simplified model of plates for structural safety assessment, in this paper, the response of a beam which is hit by a 2-D freak wave is studied. The freak wave is generated in a numerical wave tank (NWT) which solves the 2-D incompressible Navier-Stokes equations. The freak wave is based on the data of real sea condition in the Sea of Japan. An efficient wave absorbing method which satisfies the mass conservation is applied in the numerical wave tank. The influence of the beam’s motion on the freak wave fluid field is also considered in this paper, as well as different boundary conditions of the beam. It is found that the natural frequency has a great impact on the response of the beam.

Topics: Waves
Commentary by Dr. Valentin Fuster
2014;():V04AT02A029. doi:10.1115/OMAE2014-23308.

The present paper presents a fatigue life prediction method for chains subjected to tension and Out-of-Plane-Bending (OPB). The investigation was carried out on a high strength mooring chain segment with a diameter of 165 mm and a steel quality R5. Such chains are used in mooring systems for large Floating Production Storage and Offloading (FPSO) units in harsh weather condition. Due to the fact that the mooring chains are pre-tensioned the wave induced displacements will introduce secondary bending effects about the weak axis in the chain links, particularly in the bilge hang-off area.

In such chains both conventional tension fatigue and fatigue damage accumulation due to Out-of-Plane-Bending (OPB) have to be analyzed. Results from full scale behavior tests, Finite Element Analyses and a case study with simulation of in-service loading are included in the present study. Finally, fatigue life predictions and an operational strategy are presented for a case study for a floater in the Barents Sea. With a target service life of 30 years, the endurance with respect to fatigue damage is a major design criterion for the mooring chain in this case.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A030. doi:10.1115/OMAE2014-23333.

For the last few decades, necessity of direct non-linear FE analysis has been increasing for the accidental events at the vessel/offshore structures. One of major areas for the accidental design, dropped object analysis using non-linear analysis is indispensable for the verification of structural safety at the design process. This paper is concerned with the methodology, conditions, and design consideration of dropped object analysis using dynamic FE analysis. By comparing the results from direct FE analyses to those from simplified energy method described in DNV-RP-C204, necessities and advantages of direct non-linear analysis can be verified.

In this paper, the effect of analysis condition is investigated using parametric study. The results are influenced by the application of failure criteria according to the rule requirements, application of material properties, dropping position, condition of the object, and so on. This study can suggest appropriate determination of the methodology and condition for the dropped object analysis using direct FE analysis.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A031. doi:10.1115/OMAE2014-23356.

Waves in finite-depth and shallow water represent an important and challenging research topic, both from practical and academic perspective. Most of practical applications related to the ocean are conducted in coastal areas, but most of research attention has been dedicated to the surface waves in deep water. Shallow areas are effectively a different physical environment, where wave kinematics change, waves become steeper and consequently more nonlinear but less or non-dispersive, and as a result their nonlinear behaviors change dramatically, waves directly interact with the bottom, in a number of different ways, and most importantly release their energy and momentum through intensive and extensive breaking.

In the paper, modulational instability in finite depths will be discussed. This mechanism is regarded responsible for rogue waves, and hence extreme wave heights in deep water. Extreme wave heights are a key design criterion for the ocean industry, including shallow environments too. It is believed, however, that wave breaking rather than wave instabilities is the primary mechanism that controls the maximum possible wave height in depth-limited environments. Here, data of the LoWish JIP will be used in order to identify the criteria for transition from modulational instability being active in the deep-water environments to being suppressed in the shallow waters.

Topics: Waves
Commentary by Dr. Valentin Fuster
2014;():V04AT02A032. doi:10.1115/OMAE2014-23370.

Liquefied Petroleum Gas (LPG) ships carry a liquefied gas either pressurized, refrigerated or both at the same time. Due to that, among the other cargo containment solutions on LPG ships, type C tanks may be distinguished: they are independent tanks built as conventional pressure vessels. A single hull design of such ships is common and a question of their crashworthiness arises. In particular, the safety of the cargo may be of concern in the case of high energy collision event. In a previous study, crashworthiness of both common and novel sandwich hull design of an LPG ship was studied. In both cases, cargo tank was damaged. It is the aim of this work to study this damage in detail. An LPG ship and ferry collision event was modeled and studied by the means of nonlinear FEM. Collision parameters were altered to study their influence on the resulting collision damage on both ships. Bi-lobe cargo tank crashworthiness is analyzed and the possibility of cargo spill examined.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A033. doi:10.1115/OMAE2014-23375.

Ship weather routing is routinely employed to assist ship-masters in avoiding storms. During these storms, however, both material and hydrodynamic nonlinearities grow with the significant wave height (Hs). Hence, to fully understand the importance of weather routing, it may be necessary to go beyond a conventional spectral-based fatigue analysis which incorporates a linear damage hypothesis (i.e., the Palmgren-Miner rule).

To this end, the present paper examines macroscopic fatigue crack growth in which the nonlinear phenomena omitted in current design practices are included. We start by considering time-dependent ship structural loading sequences which include non-linear wave-induced bending and whipping responses taken from time-domain seakeeping simulations. These stresses are then analyzed using a fatigue crack growth model previously developed by the authors [1] in which material hysteresis is included using a mechanistic rather than phenomenological approach based on numerical simulations requiring only experimentally measured fatigue crack growth rates under constant amplitude cyclic loading (e.g., ASTM E647-13) and a full material constitutive model defined through experimental push-pull tests for the same material. Using this approach, we quantify the importance of weather routing by systematically substituting storms above a certain threshold with more moderate sea conditions.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A034. doi:10.1115/OMAE2014-23377.

Mooring systems for wave energy converters (WEC) have to be designed to survive the cyclic loads and motions they are subjected to as a result of the wave load-WEC interaction and the motions of the WEC in the random elevation of the sea surface. The current study compares four simulation procedures for the analysis of fatigue of WEC moorings. The objective is to recommend the type of simulation procedure that can be used to make reliable fatigue design of WEC mooring systems at a reasonable computational effort. A cylindrical floating WEC with four spread mooring lines is chosen for case study. The mooring dynamics of the WEC is simulated using coupled and de-coupled approaches in the time domain. In total, four types of simulation procedures are compared using the commercial simulation software DNV DeepC and an in-house code MOODY. A parameter sensitivity analysis of environmental conditions, model and numerical parameters is presented. The results are compared with respect to fatigue damage calculated using a stress-based approach and the rainflow counting method. It is found that a de-coupled approach, using DNV DeepC to simulate the buoy’s motions and cable response, is recommended since (i) it gives reliable results in terms of motion and stress responses of the buoy, mooring lines and accumulated fatigue damage, (ii) it requires the least model preparation by the engineer and the computation time is reasonable.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A035. doi:10.1115/OMAE2014-23397.

It is a normal practice nowadays in structural engineering, including ships and offshore industry, to perform non-linear finite element analysis to assess the structure’s capacity for design or evaluation purposes. However, experience has shown that the quality and accuracy of the non-linear FE analysis results are highly dependent on the skill of the person performing the analysis and the analysis procedure used. The difference between results obtained by different people can be significant. In some cases, the results can be misleading. It is considered that a unified procedure is necessary. This paper is moving a step further and trying to develop a standard procedure which can provide a guideline for structural collapse analysis of stiffened panels under any load combinations. The paper provides the technical background on the analysis procedure and the key steps such as model extent, mesh density, initial imperfections, and boundary conditions. Analysis examples are provided in the paper for reference and discussions.

Topics: Collapse
Commentary by Dr. Valentin Fuster
2014;():V04AT02A036. doi:10.1115/OMAE2014-23410.

The implementation of new technology introduces uncertainties. Due to the consequence of failures, these uncertainties generate risks for the technology developers, manufacturers, vendors, operators and end-users. In this context, the application of proven technology in a new environment is also considered as new technology. In essence, a system as a whole is considered to be new technology when it has been assembled in a novel way consisting of individual components built using proven technology. The concerns in deploying new technology are significant in offshore oil and gas (O&G) operations, as they involve hazardous procedures. Moreover, technology is considered new when applied to offshore O&G operations due to the variability of technical challenges from field to field and the complexity of systems with limited space and experience in a harsh and sensitive environment. When new technology is integrated into a large system, it is necessary to evaluate the effect on the total system’s reliability in order to increase the level of confidence via a technology qualification procedure. Hence, it is vital to interpret how the risks are managed by the provision of evidence to reduce uncertainties. This manuscript presents an illustrative case of failure mode, effect and criticality analysis (FMECA) in qualifying new technology when it is implemented in an unfamiliar environment.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A037. doi:10.1115/OMAE2014-23411.

Concerns have been raised to navigational safety worldwide because of the increasing throughput and the passing ships during the past decades while maritime accidents such as collisions, groundings, overturns, oil-spills and fires have occurred, causing serious consequences. Formal Safety Assessment (FSA) has been acknowledged to be a framework widely used in maritime risk assessment. Under this framework, this paper discusses certain existing challenges when an effective safety assessment is carried out under a variety of uncertainties. Some theories and methodologies are proposed to overcome the present challenges, e.g., Fault/Event Tree Analysis (FTA/ETA), Evidential Reasoning (ER), Bayesian Belief Network (BBN) and Belief Rule Base (BRB). Subsequently, three typical case studies that have been carried out in the Yangtze River are introduced to illustrate the general application of those approaches. These examples aim to demonstrate how advanced methodologies can facilitate navigational risk assessment under high uncertainties.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A038. doi:10.1115/OMAE2014-23444.

The run-up of long, strongly nonlinear waves impinging on a vertical barrier can result in remarkable amplification of the far-field amplitude of incoming waves [1,2]. Such an extreme run-up is the result of an evolution process in which long waves experience strong amplification under the action of nonlinear steepening followed by the formation of undular bores, consisting of nonlinearly dispersive wave trains.

Rather than addressing the genesis of such extreme run-ups in any detail, this article describes a specific aspect of the problem; the associated pressure fluctuations at the wall. Numerical computations of the near-wall pressure field show that non-hydrostatic effects can strongly affect the dynamic loads exerted on the wall, and consequently, the high-frequency component of the pressure loads results is significantly enhanced with respect to that of the wave spectrum itself. This observation suggests that also long oceanic waves, at least in some conditions, can be a source of seismic noise.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A039. doi:10.1115/OMAE2014-23446.

A probabilistic metocean model for hurricane conditions is briefly described. The model is based on site-specific, hindcast data and defines the time variation of the metocean conditions during the realisation of a hurricane at the site. The annual extreme value distribution of mooring line tension for a large, semi-submersible, mobile drilling unit is computed. Time domain analysis is applied to obtain the short-term, extreme value distribution of line tension, conditional on stationary metocean conditions. A large number of different conditions are considered. A response surface is used to interpolate on the short-term distribution parameters in order to describe the tension response during the varying conditions associated with the passage of a hurricane. The hurricane duration is split into a sequence of 15-minute intervals such that the conditions can be assumed stationary during each such short interval. The tension distribution, conditional on the realisation of a hurricane, is accumulated across the sequence of short intervals. The distribution of hurricanes is taken into account to obtain the tension distribution in a random hurricane. Finally, the frequency of hurricanes is taken into account to give the annual extreme distribution of line tension. The characteristic tension computed using 10-year return conditions and the ISO 19901-7 design standard is found to correspond to a return period of 29 years in the test case. The effects of various assumptions in the design analysis are investigated. Sensitivities to simplifications of the metocean model are considered. The effects of uncertainties in the response calculation and in the distribution of peak significant wave height during hurricanes are quantified and included in the response analysis.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A040. doi:10.1115/OMAE2014-23460.

Primary structural components of a spar hull are designed to resist lateral hydrodynamic and hydrostatic pressure and global loads. The scantlings of each primary component are usually determined based on the largest pressure it may encounter in various phases such as wet-tow, upending, and in-place operational conditions. The effect of global bending moment and shear force on the spar hull is often evaluated much later via laborious finite element analysis. This paper proposes a simple analytical tool for quickly assessing global hull strength of classic spars in the in-place condition. A spar platform undergoes steady, low-frequency, and wave-frequency motion of comparable magnitude at the same time in a storm event. The present approach separates the wave-frequency component from the steady and low-frequency response. A closed-form solution is developed for wave-induced motion and loads by taking advantage of the simple cylindrical shape of classic spar hulls. The theoretical solution is verified by comparing to numerical WAMIT results. The low-frequency response is approximated as one part of the steady response, since its dynamic effect is weak. The steady structural response is mainly a function of the heel angle. It is demonstrated that local effect of wind pressure and current load is not significant. The total response, as a summation of the wave-induced loads and the steady solution, is represented by global bending moment and shear force envelopes along the spar hull for a given sea state. Global bending and shear stresses of primary structural components can be further calculated and checked against code requirements. This procedure has been implemented through Matlab scripts. A comparison with global finite element analysis for a classic spar is made showing very good agreement. The present simple procedure allows us to evaluate primary structural components of a spar hull without resorting to expensive finite element modeling. It can help the scantling design by providing the global load. The approach can also be used to identify critical environmental conditions and structural components for detailed finite element analysis.

Topics: Spar platforms , Hull
Commentary by Dr. Valentin Fuster
2014;():V04AT02A041. doi:10.1115/OMAE2014-23466.

The assessment of the residual strength of a damaged ship is a key element of ABS’ Rapid Response Damage Assessment (RRDA) program. When determining the residual strength, it is important to understand how the initial structural damage can spread in response to sea wave dynamic loads and can lead to a gradual reduction of the ship’s residual strength. This progressive, time-dependent structural failure caused by cracks emanating from the damaged area could eventually lead to total hull girder collapse. This is why it is important to quantify the progressive structural failure over time when assessing the residual strength of the damaged ship. Until now, progressive structural failure analysis has been conducted numerically using the Finite Element (FE) modeling approach. While this approach is accurate, it is extremely time-consuming, which makes it inappropriate for incident response, where time for decision-making is very limited. In order to overcome this limitation, an alternative analytical modeling approach for assessing the progressive structural failure of a damaged ship is proposed.

This paper presents a new comprehensive procedure for analytical prediction of crack propagation under sea wave loading using spectral fatigue analysis, beam theory, fracture mechanics and an equivalent stress intensity factor (SIF) range concept. The SIF range obtained analytically is validated by FE modeling of a damaged ship subjected to sea wave dynamic loading. The procedure for analytical prediction of the crack propagation is demonstrated for a typical, modern 170,000 DWT bulk carrier in a full load condition. The results of this research can be used to support informed decision-making when analyzing a vessel’s residual strength for the transit voyage from the accident location to a repair facility.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A042. doi:10.1115/OMAE2014-23498.

During normal operations, ship and offshore structures, are subjected to combined lateral pressure and in-plane stresses. The effect of the lateral pressure is often ignored in hull girder ultimate assessments. This paper investigates the influence of the lateral pressures on the nonlinear collapse behavior of stiffened panels subjected to in-plane longitudinal stress. In this study, nonlinear finite element analyses were first conducted for the desired pressure alone; the longitudinal stress was then applied up to and beyond the collapse of the structures. Four representative stiffened panels taken from the bottoms of different double hull oil tankers were considered. The nonlinear analyses were performed using LR’s in-house finite element program VAST and following the procedure for nonlinear collapse analysis developed by LR. The numerical results indicated that the application of the initial pressure loads not only reduced the ultimate load carrying capacity of the panels significantly, but also changed the failure modes of the structures. The sensitivity of the ultimate strength to lateral pressure was dependent upon the panel geometry and whether the pressure was applied on the plate or the stiffener side. The numerical results and findings from this study are presented in this paper.

Topics: Pressure , Collapse
Commentary by Dr. Valentin Fuster
2014;():V04AT02A043. doi:10.1115/OMAE2014-23535.

The motivation of this research is that the ship deck is subjected to uni-axial compression under sagging condition, when in the severe condition, the deck is dynamically loaded in nature, with impact type loads, which is the reason of deck collapse under severe condition. In addition, imperfection probably substantially influences the dynamic response of the impacted deck. Based on nonlinear explicit finite element method, the paper aims at studying the influence of three types of initial geometric imperfection on dynamic response of cross-stiffened deck subjected to in-plane impact. Three types of imperfection are local imperfection on deck plate between stiffeners, imperfection of overall positive deflection of the deck, and imperfection of overall negative deflection of the deck. Impact function is a half sine wave function with two parameters, impact duration and amplitude of the impact load. First order natural vibration period of the deck is selected as impact duration. Amplitude of the impact load is selected according to the value of ultimate strength of the deck, and varied by times of the ultimate strength. Strain rate effect and Strain hardening effect of the material are accounted in the analysis. Axial residual displacement of the end of the deck after impact is selected as dynamic response of the deck. The result shows that dynamic response of deck is sensitive to local imperfection, while not sensitive to overall imperfection under low or moderate level of imperfection magnitude. When imperfection is severe, residual axial displacement of the end of the deck is large, especially amplitude of impact load is somewhat larger than ultimate strength. Among 3 kinds of imperfection, dynamic response of overall negative deflection imperfection is the largest, followed by that of local imperfection, while that of overall positive deflection imperfection is the least.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A044. doi:10.1115/OMAE2014-23540.

This paper presents the prediction of residual ultimate strength of a very large crude oil carrier considering damage extents due to collision and grounding accidents. In order to determine extents of damage, two types of probabilistic approaches are employed: deterministic approach based on regulations based on ABS [1], DNV [2], and MARPOL [3] and probabilistic approach based on IMO probability density functions (PDFs) (IMO guidelines [4]). Hull girder ultimate strength is calculated using Smith method which is dependent on how much average compressive strength of stiffened panel is accurate. For this reason, this paper uses two different methods to predict average compressive strength of stiffened panel composing hull girder section: CSR formulas and nonlinear FEA. Calculated average compressive strength curves using CSR formulas (IACS [5, 6]) and nonlinear FEA are imported by an in-house software UMADS. Residual ultimate moment capacities are presented for various heeling angles from 0° (sagging) to 180° (hogging) by 15° increments considering possible flooding scenarios. Three regulations and IMO guidelines yield minimum of reduction ratios of hull girder moment capacity (minimum of damage indices) approximately at heeling angles 90° (angle of horizontal moment) and 180° (angle of hogging moment), respectively, because damage area is located farthest from neutral axis.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A045. doi:10.1115/OMAE2014-23576.

There is a wide range of fracture criteria available in the literature to simulate the ductile fracture in large structures. Almost all criteria depend in some form on the mesh size and some criteria also account the effect of the stress state on the fracture ductility. Furthermore, a material model employed could considerably influence the analysis results. Therefore, in this study, four different fracture criteria, three different mesh densities and two different material models are used to simulate ship collision with a rigid bulb. Thereby, plastic dissipation energy, force-displacement curves and structural failure mechanism is compared between different fracture criteria. Advantages and disadvantages of each criterion are discussed.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A046. doi:10.1115/OMAE2014-23597.

Offshore tension leg platform (TLP) is a compliant type offshore structure where the tendons are deployed under initial pretension to counteract the excessive buoyancy. TLPs show large amplitude response under environmental loads due to their compliancy, which poses threat under extreme loads. Use of passive dampers like Tuned Mass Damper (TMD) is common to control such large amplitude motion, however their deployment in offshore structures is relatively new. Response control of a scaled model of TLP is attempted using tuned mass damper of pendulum type under regular waves. Based on the experimental studies carried out, it is seen that there is a significant reduction in the surge response under the folded pendulum type damper. Results also show that there is a reduction in the heave response due to the control envisaged in the surge motion. The discussed method of response control is one of the effective methods of retrofitting offshore platforms whose operability at rough sea states is a serious concern.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A047. doi:10.1115/OMAE2014-23603.

A method based on the vibration transmissibility is presented for subsea pipeline in this paper. Fist, the vibration transmissibility for a pipeline structure was presented theoretically. An outlier analysis algorithm was used to detect the structural damage by using of the transmissibility function calculated with the measured vibration signals. Next, the experimental investigations were carried out to verify the feasibility of the proposed method. The results demonstrate that the local damage in the pipeline can be effectively detected. It is indicated that the transmissibility based approach can be used to monitor the damage or structural deterioration for subsea pipelines.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A048. doi:10.1115/OMAE2014-23610.

The aim of the present work is to analyze numerically the progressive collapse behavior of stiffened plates subjected to extreme and/or cyclic loads. In this paper, a series of elasto-plastic large deflection analyses is performed on stiffened plates subjected to cyclic uniaxial in-plane loading by non-linear Finite Element Method. The target stiffened plates with two different dimensions are analyzed by two kinds of models. In the analyses, forced displacements are applied, and all the cyclic loading starts from compression to clarify the effect of cyclic loading on the buckling/plastic collapse behavior of stiffened plates. There are totally 28 model cases under discussion to investigate the behavior of a stiffened plate under cyclic loadings. From the numerical simulation, some of the behavior of stiffened plates, such as axial rigidity, ultimate compressive strength and the hysteresis mean stress vs. mean strain curve can be found out and the primary conclusions are drawn.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A049. doi:10.1115/OMAE2014-23618.

Wave-current interaction, a common phenomenon in the areas of estuary and gulf, is also notable in deep water sea area. When the current is strong enough, wave “blocking” accompanied by wave breaking takes place and leads to hazard sea conditions. On the other hand, wave countering current is also thought to be one of the main causes of occurrence of rogue waves in the ocean, which imperils offshore structures and is of much importance for research. Knowing this, viscous flow of wave generation on current is simulated by using Reynolds-Averaged-Navier-Stokes (RANS) equation for better understanding of its mechanism and flow field. In this study, two-dimensional waves propagating without current, along and against pre-existing large-scale current have been investigated. The validation of calculation has been demonstrated through comparing with preliminary experiments in Circulating Water Channel (CWC). When propagating in counter-current, wave is steepened and wave height increases dramatically, thus wave breaking occurs occasionally. Therefore, the variation of wave height and steepness with current velocity has been analyzed quantitatively. It has been confirmed that wave “blocking” occurs when current speed is comparable with group velocity of waves. And this is analyzed in detail in terms of kinematics, energy transfer and others. Accordingly, wave breaking is evaluated by the well-known classical wave breaking criterion. Furthermore, the proposed viscous model is compared with “simple bore dissipation model” to evaluate energy dissipation of wave breaking and improvement of numerical model for wave blocking is introduced. It has been shown that the present numerical model is applicable to reproduce the wave blocking and breaking phenomenon and it will be useful for future engineering purpose on predicting real sea conditions.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A050. doi:10.1115/OMAE2014-23701.

Liquefied natural gas (LNG), a cleaner energy resource compared to heavy fuel oil (HFO), has been utilized as an energy source by vessels of various types, e.g., ferries, cargo vessels and platform supply vessels (PSV), notably after the release of International Maritime Organization (IMO) interim guideline MSC. 285(86) which officially authorized the natural gas as a marine fuel for merchant vessels in June 2009. LNG fuel is expected to have a promising prospect in green shipping industry with advantages in decreasing the emissions of NOX, SOX, and particulate material. However, as an inflammable and explosive energy source, safety issues of LNG should also be taken into account, especially under the circumstances of fuel leakage during a long voyage. In this paper, failure mode, effects and criticality analysis (FMECA) is conducted for the study on leakage failure modes of LNG fueled vessels. The criticalities of LNG leakage modes are calculated and ranked by taking failure rate, causes and effects (consequence probability and associated severity) of each failure mode into consideration. Event tree analysis (ETA) approach is utilized to identify possible failure consequences and estimate associated probabilities of occurrence, while computational fluid dynamics (CFD) modeling and simulation are applied for the consequence analysis of each failure mode. A typical Chinese LNG powered cargo ship in the Yangtze River is studied for critical leakage modes identification and risk control options (RCOs) provision so as to provide recommendations on the daily operations and safety managements of LNG fueled vessels.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A051. doi:10.1115/OMAE2014-23726.

Due to the influences of the excitation and measurement noise, the misjudgments are inevitable when the coefficients of an autoregressive (AR) model in the time series combined with neural networks are used to identify the damage. So, in this study, the pseudofree response data are extracted from the acceleration responses data with the random decrement (RD) technique, the AR model is employed to fit the pseudofree response data, the Akaike information criterion (AIC) is used to estimate the order of AR model, the coefficients of the AR model will be changed if the structure is damaged, so we can determine if the structure is damaged according to the changes of the coefficients of the AR model. If the structure is damaged, the differences of the first 4 order AR coefficients pre and post damage are extracted and composed as damage sensitive vector which is put into back-propagation (BP) neural network to identify the damage location. The numerical model of a four-floor offshore jacket platform excited with white noise is used to testify the proposed damage identification method, the different amplitudes of the white noise excitation and the different level of the measurement noise are also considered. The simulation results show the proposed method is almost not affected by the changes of excitation amplitude, and when the noise level is no more than 5 percent, the damage location can be identified by the method correctly. The proposed method uses the acceleration responses to identify the damage directly, which is not dependent on the modal parameters (frequency, mode shape, damping), therefore, it is suitable for the on-line damage identification.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A052. doi:10.1115/OMAE2014-23737.

The substructure for offshore wind turbines is strongly influenced by the effect of wave forces as the size of substructure increases. Therefore, it is very important to reduce the wave force acting on substructures. In the present study the hybrid substructure, which is composed of a multi-cylinder having different radius near free surface and a gravity substructure at the bottom of multi-cylinder, is suggested to reduce the wave forces. The fluid domain is divided into two regions to calculate the wave forces acting on the hybrid substructure with multi-cylinder and the scattering wave in each fluid region is expressed by an Eigen-function expansion method. The comparison between the mono pile and the hybrid substructure is made for wave forces. Using the wave forces obtained from this study, the structural analysis of hybrid substructure is carried out through ANSYS mechanical. In order to investigate the resonance between the wind turbine and the hybrid substructure, the modal analysis is also carried out.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A053. doi:10.1115/OMAE2014-23750.

The top-of-line corrosion (TLC) is corrosion observed at the upper section of a pipeline, as measured from the circumference of the pipe. The magnitude of TLC is mostly determined using the mechanistic condensation models. The spatial prediction of TLC, however, has not been of the interest among researchers, so as to know at what o’clock orientations (with respect to pipeline cross section) corrosion may accumulate the most. TLC spatial prediction is directly related to the release of corrosion inhibitor (CI) in the pipeline. The probability of retaining CI at the upper part of the pipeline is always a challenge due to the inconsistency of the operational flow parameters coupled by acceleration due to gravity. Thus, there is the need to properly understand the development of TLC with regard to its space. This paper proposes the analysis on TLC spatial prediction to be carried out by means of statistical approaches called the Exploratory Data Analysis (EDA) due to the nature of corrosion that are random. EDA is a simple tool that is able to summarize the main characteristics of TLC data using visual methods. The TLC data was taken from a gas pipeline operating in Malaysian offshore region. A median polish model (of EDA) for the TLC was later generated. A prediction table was also developed to guide users on the estimate of TLC in gas pipelines with regards to the o’clock orientations of the pipe circumference.

Topics: Corrosion , Pipelines
Commentary by Dr. Valentin Fuster
2014;():V04AT02A054. doi:10.1115/OMAE2014-23762.

The application of hybrid panels composed of polyurethane foam cores and steel faceplates to hull structures was studied.

First, the strength and collapse behavior of the panels were examined using three-point bending tests for hybrid panels with different core densities. Based on the experimental results, low-density core materials, such as those having a density of 300 kg/m3, are recommended for hybrid panels with respect to strength and weight. The structural strength of hybrid panels was investigated through a series of calculations utilizing FEA for beams clamped at both ends under uniform pressure by changing the thickness of the faceplates and core, and the length of the clamped span.

Taking account of the structural advantages of hybrid panel structures, two kinds of applications for hybrid panels are proposed.

One is in the inner bottom structure of bulk carriers. There is the possibility of improving the residual strength of hybrid panels after they are subjected to severe impacts. The dynamic characteristics of hybrid panels were investigated. It was confirmed that the residual deformation of hybrid structures after impact loading is less than that of ordinary stiffened structures of the same weight.

The other application is in the structural members of pressurized LPG (Liquefied Petroleum Gas) tanks. In utilizing hybrid panels in LPG tanks, the difference in rigidity between the core and skin produces a high peak bending stress in the vicinity of the clamped ends. In this paper, measures for the mitigation of this phenomenon are proposed. Also, the difference in rigidity between core and skin also produces weak points in hybrid panels such as the interface between the core and skin-plate. The applicability of hybrid panels in tank systems was examined by numerical methods utilizing the experimental results and it was confirmed that the shear and peel strength of the interface is more than sufficient.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A055. doi:10.1115/OMAE2014-23768.

This paper proposes a simulation-based method to estimate collision risk for a ship operating in a two-lane canal. According to rule 9 of the Colreg-72 navigation rules, in a narrow canal, a vessel shall keep as near to the wall that lies on its starboard side. However, a busy harbor entered through a narrow canal still presents impact hazards. Certain conditions in a two-lane canal, such as a head-on situation in the straight part of the canal during an overtaking maneuver and large curvature of a turning maneuver in the bend part of the canal, could lead to accidents. In the first condition, the ship alters its own course to the port side to overtake another ship in the same lane but the course altered is too large and hits the wall of the canal. In the second condition, the target ship may take an excessively large turn on the bend part of the canal, causing collision with the ship on the opposite lane. Collision risk is represented as the risk of damage to the ship structure and includes the probability of impact accident and severity of structural damage. Predictions of collision probabilities in a two-lane canal have been developed based on a simulation of ship maneuvering using a mathematical maneuvering group (MMG) model and automatic identification system (AIS) data. First, the propeller revolution and rudder angle of the subject ship are simulated to determine safe trajectories in both parts of the canal. Second, impact accidents are simulated for both conditions. The ship’s speed, and current and wind velocity are randomly simulated based on the distribution of the AIS and environment data for the research area. The structural consequences of the impact accident are measured as collision energy losses, based on the external dynamics of ship collision. The research area of the two-lane canal is located at the Madura Strait between the Java and Madura islands in East Java of Indonesia, as shown by the red line in Figure 1. A project for developing a new port and dredging a new two-lane canal to facilitate an increase in the number of ship calls is currently underway in the research area. Figure 1 shows the ships’ trajectories plotted using the AIS data as on January 1, 2011. The trajectories are mostly seen to be coming out of the canal, confirming that it is shallow and needs to be dredged.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A056. doi:10.1115/OMAE2014-23785.

The response of a platform subjected to a strong ground motion is highly affected by the method of modelling, and special attention should be paid to such advanced analysis.

While the potential of dynamic inelastic analysis and performance based design of offshore platforms is recognised in earthquake design and assessment, they have inherent shortcomings. This paper discusses the critical issues and their effects on the obtained results in the application of inelastic dynamic analysis. Areas of possible developments that would render the method more applicable to the prediction of dynamic response, and new developments towards a fully adaptive performance based design are explored. These developments lead to dynamic analysis results that are closer than ever to the true inelastic time-history behavior of the structures.

It is concluded that there is great scope for improvements of this powerful technique that would increase confidence in its employment, and encourage its use during design as the primary tool for reliability assessment under seismic loads. The results from a sample project indicated that inelastic time history seismic analysis provides a more realistic assessment of structural behavior and strongly recommended to be implemented for the design and reliability assessment of the fixed offshore structure under extreme seismic event.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A057. doi:10.1115/OMAE2014-23790.

This study deals with failure strain prediction according to stress triaxiality of a typical polar class steel EH36. Choung et al. [1, 2] have formulated failure strain curve of EH36 steel by carrying out a number of tensile tests for round and flat specimens with various notches. Extra tests for notched flat specimens which machined in transverse direction are conducted to formulate the failure strain curve. In order to verify the validity of failure strain curve, we develop user-subroutine (VUSDFLD) of a commercial finite element code, Abaqus/Explicit. Fracture simulation with user-subroutine shows that two results are closely coincident by comparison of force-elongation curves.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A058. doi:10.1115/OMAE2014-23795.

Since there is an evident tendency of development of large scale ships, the interaction between the propulsion shaft and ship hull becomes severe due to the tremendously increased ship size. As a result the reliability of the vessels has been put in an important position by the companies and the governments all over the world. The excited forces caused by severe sea waves have considerable effects on the hull deformation which could have further impact on the shaft propulsion system. This paper aims to investigate the coupling dynamics between the large ship propulsion system and hull subjected by sea wave in 2-dimensional circumstance. To look into the coupling mechanism between the ship propulsion shaft, hull and sea waves, a 2-dimensional novel model of large ship propulsion-hull coupling system is presented in this work to analyze the dynamic interactions of the ship propulsion system and hull. According to the dynamic equations of the coupling model, the dynamical responses of the ship shaft and hull are obtained under different stiffness of the support bearings. The analysis indicates that choosing the suitable stiffness of bearings have an important effect on the coupled system.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A059. doi:10.1115/OMAE2014-23803.

Aboard ships windows are exposed to static as well as dynamic loads, e.g. impact loads. Failure can lead to serious consequences. Therefore two research projects were initiated in order to analyze the load carrying behavior of windows. In addition to quasi-static ultimate load tests and drop tests with water filled rubber bags special attention is paid to the Finite Element (FE) modeling. In particular the response — stresses and deformations — to quasi-static lateral loads can be calculated with good agreement to test results. Hence FE calculations can be useful to determine and compare failure mechanisms of different window designs. An ultimate load range can be estimated by taking into account the breaking strength range of glass. A comparison between FE calculations and results of the impact tests showed that these are sensitive to conditions which could hardly be measured during the test, e.g. the shape of the approaching water-filled rubber bag. Varying of parameters eventually yielded that window response to impact loads can also be calculated sufficiently, at least, to evaluate different window designs. Further investigations on this topic are in progress.

Topics: Stress , Ships
Commentary by Dr. Valentin Fuster
2014;():V04AT02A060. doi:10.1115/OMAE2014-23807.

Reliability is one of the most important features of the wind turbine gearbox, especially in offshore wind turbines (OWTs). This paper describes a general way to perform gear contact fatigue reliability analysis for wind turbines considering inspection and repair. A simplified predictive surface pitting model for estimating gear fatigue lives is applied to establish the ‘so-called’ limit stated functions. The National Renewable Energy Laboratory (NREL)’s 750kW land-based wind turbine is used to perform time domain simulations considering different wind speeds that the turbine will experience, whose occurrence frequencies are described by a generalized gamma distribution. The time series of the torques in the main shaft are obtained from the global dynamic response analysis of the wind turbine. The time series of the gear contact forces are obtained from the dynamic analysis of the gearbox through multi-body simulation. The 2-parameter Weibull distribution is used to fit the long-term probability distribution of the gear tooth contact pressures. The reliability analysis is based on fracture mechanics (FM) analysis of crack growth. Finally the sensitivity of the reliability index and failure probability on initial crack size, critical crack size, detectable crack size, crack size after repair, material property and environmental loads is estimated considering the effect of inspection.

Commentary by Dr. Valentin Fuster
2014;():V04AT02A061. doi:10.1115/OMAE2014-23811.

The purpose of the present study is to investigate residual Ultimate Longitudinal Strength (ULS) of bulk carriers after ship-ship collision. A series of a large-scale explicit finite element analysis (FEA) as well as simplified analysis (SA) are carried out using a cape size bulk carrier. In order to accurately investigate collapse mechanism of “damaged ships” under vertical bending moment nonlinear FEA are carried out where two steps analysis is adopted. First step is ship-ship collision analysis; Second step is ULS analysis of the damaged ship. Ship-ship collision analysis is carried out assuming the right angle collision at the midship region of the struck ship, and damage extent of the struck ship is estimated with varying collision speed of 3kt, 6kt, 9kt and 12kt. In the second step of analysis, residual ULS analysis is carried out taking into account residual stress and deformation of the struck ship caused by ship-ship collision. Collapse mechanism of the bulk carrier in damaged condition due to sagging moment as well as combination of longitudinal and horizontal bending moment is investigated and discussed in detail. ULS of hull girder of the bulk carrier in intact condition is also estimated and compared with that in damaged condition. The effect of damaged condition on the reduction of ULS is discussed in detail. Finally some of numerical methodologies are summarized in assessing residual ULS of hull girder after collision.

Commentary by Dr. Valentin Fuster

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