Hull Girder Research Articles

Numerical Assessment of FPSO Platform Behaviour in Ship Collision

Offshore platforms may potentially collide with vessels of various types, including visiting ships such as supply ships and passing ships. The most critical and relevant conditions, including the analysis and design approaches are introduced. Different ship types having different displacements and structural designs exert different vessel impact loads on impacted structures. This paper presents the findings of collision impact analyses of the side shell panel, bow and stern structures of Floating Production Storage Offloading (FPSO) platforms in case of impact, e.g. by a supply vessel or methanol tanker. As collision impact simulations continue to be conducted conservatively, the colliding positions of the striking vessel are presumed to be bow and stern only, with side force. In order to assess hull strength in collision events, non-linear FE simulations were performed by means of the MSC / DYTRAN tool, as these collision events result in more complex reactions. The degree of hull damage suffered by an FPSO vessel in different collision scenarios and at varying impact energy levels was determined in accordance with the NORSOK N-004 standard guidelines. Post-collision analyses were conducted to establish the structural integrity of the damaged hull after being exposed to environmental conditions for one year. The reduction of hull girder strength associated with the worst damage was evaluated and accounted for in the present study, providing no further damage occurs. Furthermore, the acceptance criteria for evaluation and corresponding consequences are calculated and discussed in detail. Finally, the findings from the present paper will help clarify the impact response of offshore structures and evaluation approaches and give valuable guidance for the design and operation of FPSO platforms.

A study on whipping related double bottom response as well as its statistical characteristics based on full-scale measurements

Recently, the great enlargement of container ships has brought broad concern on the effect of whipping response. Meanwhile, the accompanying wider width of double bottom may cause more flexible responses of the double bottom structures. The numerical simulation studies have shown that the longitudinal stresses of the hold central bottom of a container ship due to the double bottom bending superimpose on the longitudinal stresses due to hull girder bending in both wave response component and vibrational response component. In this research, the authors firstly extract the full scale measurements data whose sea states are closest to that of numerical simulation among existing time series, with the purpose of analyzing correlation between double bottom bending and hull girder bending in wave response and vibrational response. As a result, studies from full scale measurements well verified the phenomenon that double bottom of larger container ship responds to forced vibration due to whipping excitation as observed in theoretical studies. Then, the statistical characteristics of correlation factor between hull girder bending stress and double bottom bending stress were investigated, based on relatively large number of data with regard to wave height and relative wave direction, as well as the classification between low- and high-frequency-dominant response cases. All these studies may contribute to future optimized operation, maintenance and design of ship.

Hull Girder Ultimate Strength Assessment Considering Local Corrosion

The residual strength capacity of ship hull with full corrosion appearance in every structural member has been considered in a large number of research works; however, the influence of local corrosion on the ultimate strength and cross-section properties has not been taken into account and analyzed. Hence, this study aims to assess the effect of corrosion appearance in the flange section and web section on the ultimate vertical bending moment and several cross-section properties of a bulk carrier. To perform this task, a probabilistic corrosion rate estimation model and the common structural rule model are introduced and employed. The incremental-iterative method given by the International Association of Classification Societies-Common Structural Rules (IACS-CSR) is applied to determine the ultimate vertical bending moment, neutral axis position at the limit state, and other properties of the cross-section. The calculation results and discussions relative to the effect of corrosion on ship hull are presented.

The ultimate strength characteristics on double hull oil tanker

A double hull tanker is one of the specific ships where the payload is liquid. The payload is an internal load and gives pressure inside the cargo hold. Besides, the external load like wave act simultaneously on the ship during the voyage. Therefore, the ultimate strength of the ship caused by the internal and external loads must be investigated. The objective of the present study is to investigate the ultimate strength characteristic of double hull oil tankers under hogging and sagging conditions. The cross-section of the double hull tanker is modeled using the finite element method to analyze the ultimate strength. The master node is applied to both sides of the cross-section to know the characteristic of the hull girder under hogging and sagging conditions. One side of the master node is constrained, and the other one is given by rotational force. The analytical method is performed, and the result obtained by the numerical method is then compared, including their characteristic in terms of moment-curvature relationship and deformation.

Warping and shear behaviors involved in hull girder's torsional collapse process

Any research and development of the analysis methods, theoretical or numerical ones, for evaluating hull girder's ultimate torsional strength must be based on a thorough understanding of the torsional collapse behaviors of the hull girder, especially for the warping and shear characteristics involved in hull girder's torsional collapse process. In this paper, firstly, the torsional collapse behaviors of a single-compartment hull girder that could represent a scale model of the mid-ship compartment of a container ship are illustrated based on both experimental and numerical observations. Then, torsional collapse mechanisms of the global hull girder (i.e., the full-length model) are parametrically studied to figure out the major factors and the critical point that make the hull girder collapse in different failure mechanisms (i.e., warping or shear failure). Also, a formula is developed to quantitatively characterize the effect of hull girder's longitudinal dimension and transverse frameworks' rigidity on the ultimate torsional strength of the hull girder. Results show that the torsional collapse mechanisms between the single-segment hull girder and the global hull girder are very different due to warping effect. The longitudinal dimension and transverse frameworks of the global hull girder can have a considerable effect on hull girder's torsional collapse behaviors and ultimate torsional strength. The objective of this research is to make a detailed and comprehensive understanding of hull girder's torsional collapse process and behaviors, figuring out the major factors and the critical point that govern warping or shear failure mechanisms of the hull girder under torsion.

Statistical Approximation to Synthetic Midship Hull Girder Stress Response

Combined hydrodynamic and structural models are used to simulate structural responses on ship hulls in a seaway for design and risk assessment purposes. From a safety and inspection perspective, there is demand for continuously monitoring the ship hull conditions to estimate the structural utilization in real time. However, setting up the computer models and running the analysis are time consuming and costly, preventing such models from being used operationally. We developed a statistical model that approximates the wave bending moment output from the computer model, by linking the wave bending moment to ship design parameters and environmental information. This statistical model is computationally cheap and much faster to run than a hydrodynamic model, and may thus act as a virtual indicator sensor for structural condition monitoring. The approximated wave bending moment can also be used to compute fatigue damage for a ship as an indicator for crack risk. Although somewhat sensitive to the training dataset, a validation study reveals that the statistical model performs decently well. For fatigue rates, relative errors range from1.4% to 60% for out-of-sample results with weighted least squares, which is deemed acceptable for an indicator model used for screening a fleet of vessels.

Ultimate strength assessment of ship hull structures subjected to cyclic bending moments

The concept of “cyclic ultimate strength” of ship hull structures is proposed. In navigating in waves a ship is subjected to alternate bending moments in hogging and sagging conditions. The present study aims to demonstrate that the ship hull girder can experience a weaker ultimate strength under cyclic loading than under monotonically increasing bending moment as assumed in the conventional analysis. Nonlinear finite element method is employed, using the explicit LS-DYNA solver, to analyse the ultimate strength of a container ship hull structure subjected to monotonic and cyclic bending moments. The numerical analyses under cyclic loading consider the ratcheting effect and the Bauschinger effect in steel. The ship hull girder can break due to the plastic strain accumulation and alternating plasticity collapse when the cyclic external load is greater than the elastic limit but lower than the monotonic ultimate limit. The collapse modes of the hull girder under monotonic and cyclic loadings are compared to reveal their different failure principles.

Ultimate strength envelope of a 10,000TEU large container ship subjected to combined loads: From compartment model to global hull girder

The main objective of this research is to figure out the relationship of the ultimate strength characteristics between the compartment hull girder and the global hull girder, especially focusing on the ultimate strength envelope of the global hull girder of a 10,000TEU ultra large container ship subjected to combined loads, which is one of the most important indexes to quantify the safety margin of the container ship. To achieve these aims, both the compartment hull girder and the global hull girder are modeled based on true ship hull girder's configurations. A series of full-scale nonlinear finite element analyses are performed. In addition, to examine the reliability of the elastic-perfectly plastic material models employed in FEA, similar scale model collapse experiments of a compartment hull girder subjected to pure hogging and pure torsion are also presented. Finally, the ultimate strength envelope of the global hull girder of the 10,000TEU ultra large container ship subjected to combined vertical bending and torsion is quantitatively obtained, which can characterize the container ship hull girder's ultimate strength interactions between vertical bending and torsion and quantify the safety margin of the global hull girder of the container ship subjected to combined vertical bending and torsion.

Scaling characteristics of hull girder's ultimate strength and failure behaviors: An empirically modified scaling criterion

Scale models are normally designed as alternatives to true ship structures in ultimate strength experiments. In this paper, a series of true ship hull girders of a 10,000 TEU container ship under different scale combinations of geometric length and plate thickness are modeled and analyzed to investigate the scaling characteristics of hull girders’ ultimate strength and collapse behaviors, aiming at developing a more precise scaling criterion of the ultimate strength between scale model and true ship. In addition, the reliability and applicability of the proposed scaling criterion has been validated by a series of simplified ship hull girder models, of which the numerical results have been validated by scale model experiments. Results show that, under different scale combinations of geometric length and plate thickness, the first order scaling law which is derived based on dimension analysis has significant errors in quantifying the converting process of the ultimate strength between scale model and true ship. Correspondingly, an empirically modified scaling criterion is proposed in this paper to improve the accuracy of the converting process of the ultimate strength between scale model and true ship.

Post-accidental structural reliability of double-hull oil tanker with near realistic collision damage shapes

ABSTRACT The consequence of the near realistic modelling of collision damage shape on the post-accidental ultimate hull girder strength of double-hull oil tanker is investigated. For known striking ship’s geometry of the bulbous bow, damaged elements are removed from the finite element model of the midship section of the struck ship. Progressive finite element collapse analysis is then performed to determine residual strength index (RSI) of damaged ship. The analysis is performed for set of 50 collision scenarios, obtained from the historical ship collision database. In addition, probabilistic analysis of RSI is performed by approximating damage shape as ‘rectangular box’, based on IMO and GOLADS project. Structural reliability analysis is then employed to assess the consequences of various modelling assumptions used in RSI computations. Results of the analysis show that near realistic modelling of damage shape results in much lower failure probability of damaged ship compared to the ‘rectangular box’ models.

A parametric study on the dynamic ultimate strength of a stiffened panel subjected to wave- and whipping-induced stresses

ABSTRACT Although it is well known that the slamming-induced whipping can significantly increase the structural loading of ships, its consequence on the hull girder's collapse is still unclear. Moreover, the whipping-induced stresses are having a higher frequency than the wave-induced stresses. Hence some doubts are cast on the probability that the dynamic effects may provide additional strength reserves. This paper aims to investigate in a parametric way the dynamic ultimate strength of a stiffened panel, subjected to axial compression and lateral loads. The load amplitude curves for the axial stress are defined analytically by superimposing different components associated with several dominant load parameters, such as wave and whipping periods. In order to derive realistic loading scenarios, a broad range for each dominant load parameter is determined by analysing the hydro-elastic response of multiple container ships. The dynamic load factors are derived as the ratio between the dynamic capacity and the quasi-static one. Furthermore, it is shown that the dynamic effects are already existent in the wave loading scenarios, and this fact is attributable to some limitations of the Cowper–Symonds model. Thus the last section of this paper is dedicated to developing a new strain rate sensitivity model, which describes more accurately the strain rate sensitivity at low strain rates.

Conversion of an oil tanker into FPSO in Gulf of Mexico: strength and fatigue assessment

ABSTRACT The paper presents the conversion of an oil tanker into FPSO to be operated in Gulf of Mexico (GoM) considering the strength and fatigue analyses. First, hydrodynamic analysis is performed for a typical full load condition and a typical ballast condition in order to determine hydrodynamic loads for ultimate strength and fatigue for operating as an FPSO in GoM. The section scantlings are then generated to assess the hull girder ultimate strength based on still water and site-specific wave loads. Fatigue analysis is carried out to estimate of used and remaining fatigue life, which is based on DNVGL Classification Note 30.7: ‘Fatigue Assessment of Ship Structures’. The analysis comprises a check of the longitudinal material amidships, and is based on the approved drawings applying ‘as measured’ scantlings taken from UTM measurements. The actual trade pattern of the vessel, and other assumptions, has a large effect on the fatigue life.

Modified incremental-iterative method for bulk carriers under alternate hold loading conditions

ABSTRACT A modified incremental-iterative method for bulk carriers under alternate hold loading condition is outlined, focusing on the assessment of the local stress field, due to double bottom bending by the orthotropic plate theory, and the development of a modified strain field of double bottom structures, after first failure of bottom stiffeners occurs. Particularly, the ISSC bulk carrier is assumed as reference ship and a comparative analysis with the FE results available in literature is performed to investigate the effectiveness of the proposed method. Subsequently, the influence of bottom strengthening and corrosion wastage on hull girder hogging capacity, under pure bending and alternate hold loading conditions, is investigated. Current results, obtained by a dedicated code developed in Matlab, are fully discussed to prove the effectiveness of the proposed method for practical design purposes.

Progressive collapse analysis of ship hull girders subjected to extreme cyclic bending

This paper introduces a novel analytical method to predict the buckling collapse behaviour of a ship hull girder subjected to several cycles of extreme load. This follows the general principles of the established simplified progressive collapse method with an extended capability to re-formulate the load-shortening curve of structural components to account for cyclic degradation. The method provides a framework for assessing residual hull girder strength following a complex series of unusually extreme load events where the wave induced bending moment rises close to, or even surpasses, the monotonic ultimate strength. These load events may be sequential, such as might be caused by a series of storm waves, or they may occur as a collection of discrete events occurring over a longer period. The extreme cyclic bending amplifies the distortion and residual stress initially induced by fabrication in the flanges of the girder, which results in a deterioration of the residual ultimate strength. Validation is firstly completed through a comparison with previously published experimental work and secondly via comparison with numerical simulation on four ship-type box girders using the nonlinear finite element method.

Statistical Analysis of Vertical and Torsional Whipping Response Based on Full-Scale Measurement of a Large Container Ship

In this study, as a preliminary attempt to reveal the whipping response of large container ships in actual seaways, the stress monitoring data of an 8600 TEU large container ship were analyzed. The measurement lasted approximately five years, and using a large amount of data, we investigated how the sea state and operational conditions affected the whipping response. In addition, the midship longitudinal stresses were decomposed into hull girder vertical bending, horizontal bending, and torsional and axial components. Thereafter, we found that the whipping magnitude on the torsional and horizontal bending components is much smaller than that on the vertical bending component. Future research would include the analysis of a larger amount of data, analysis of other sensor data, and effects of various patterns of vibrational response on the ultimate strength and fatigue strength. The obtained results will benefit the future design and operation of large container ships for safer navigation.

Strain prediction for critical positions of FPSO under different loading of stored oil using GAIFOA-BP neural network

FPSO (floating, production, storage and offloading) units are widely used in the offshore oil and gas industry. Generally, FPSOs have excellent oil storage capacity owing to their huge oil cargo holds. The volume and distribution of stored oil in the cargo holds influence the strain level of hull girder, especially at critical positions of FPSO. However, strain prediction using structural analysis tools is computationally expensive and time consuming. In this study, a prediction tool based on back-propagation (BP) neural network called GAIFOA-BP is proposed to predict the strain values of concerned positions of an FPSO model under different oil storage conditions. The GAIFOA-BP combines BP model and GAIFOA which is a combination of genetic algorithm (GA) and an improved fruit fly optimization algorithm (IFOA). Results from three benchmark tests show that the GAIFOA-BP model has a remarkable performance. Subsequently, a total of 81 sets of training data and 25 sets of testing data are obtained from experiment using fiber Bragg grating (FBG) sensors installed on the surface of an FPSO model. The numerical results show that the GAIFOA-BP is capable of predicting the strain values with higher accuracy as compared with other BP models. Finally, the reserved GAIFOA-BP model is utilized to predict the strain values under the inputs of a 10-day time series of volume and distribution of stored oil. The predicted strain results are further used to calculate the fatigue consumption of measurement points.

Ultimate strength characteristics of as-built ultra-large containership hull structures under combined vertical bending and torsion

ABSTRACT The aim of the present paper is to investigate the ultimate strength characteristics of as-built ultra-large containership hull structures under combined vertical bending and torsional moments with varying the ship size. The intelligent supersize finite element method (ISFEM) is employed for the ultimate hull girder strength analyses. A total of three as-built containerships carrying 9,300 TEU, 13,000 TEU, and 22,000 TEU are studied. Based on the computations, ultimate strength interaction relationships of containership hull girders under combined vertical bending and torsional moments are also formulated by a curve fitting. Insights and structural design recommendations obtained from the study are summarised, which will be useful to not only enhance the ultimate limit state capacity of containership hull structures in existing sizes but also achieve the robust structural design of ultra-larger containerships which have never been built before.

Reliability analysis of hull girder ultimate strength for large container ships under whipping loads

The two incident reports for MSC (Mediterranean Shipping Company) Napoli in 2008 and MOL (Mitsui O.S.K. Lines) Comfort in 2015 both pointed out that whipping response was one main reason for inducing the catastrophes. Subsequently, International Association of Classification Societies (IACS) proposed a Unified Requirement (UR) S11A for ultimate strength evaluation of container ships, and main classification societies also updated their guidelines for whipping response. However, there are big differences in these requirements, and the safety level of ultimate strength under whipping loads is still unclear. This article is to find the effect of the whipping loads on hull girder reliability. Two large container ships 14500TEU and 19000TEU designed by China Classification Society (CCS) Rules are selected as case studies. Hull girder ultimate capacity in mid-ship section under hogging conditions is adopted as the dangerous failure mode, which is predicted by the increment-iterative method in IACS Common Structural Rules (CSR). The whipping effect on hull girder reliability is accounted for by a factor attaching on wave-induced moment. Then, a new evaluation criterion is proposed to evaluate hull girder ultimate strength for container ships under whipping loads, and sensitivity analysis of whipping effect is carried out. The conclusions in this article can provide a guidance for revising rule requirements in classification societies and optimising container ship structural design.

Ultimate strength of container ships subjected to combined hogging moment and bottom local loads, Part 2: An extension of Smith's method

This paper is the second of two companion papers concerning the ultimate hull girder strength of container ships subjected to combined hogging moment and bottom local loads. The nonlinear finite element analysis in Part 1 has shown that local bending deformation of a double bottom due to bottom lateral loads significantly decreases the ultimate hogging strength of container ships. In this Part 2, extending Smith's method for pure bending collapse analysis of a ship's hull girder, a simplified method of progressive collapse analysis of ultimate hogging strength of container ships considering bottom local loads is developed. The double bottom is idealized as a plane grillage and the rest part of the cross section as a prismatic beam. An average stress-average strain relationship of plate/stiffened plate elements employed in Smith's method is transformed into an average stress-average plastic strain relationship, and implemented in the conventional beam finite element as a pseudo strain hardening/softening behaviors. The extended Smith's method is validated through a comparison with nonlinear finite element analysis.

Ultimate Limit State Function and Its Fitting Method of Damaged Ship under Combined Loads

The ultimate limit state function is one of the premises for the assessment of structure strength and the safety of ships under severe conditions. In order to study the residual strength of damaged ships under the combined load of vertical and horizontal bending moments acting on the hull girder, the ultimate limit state function of a damaged ship under combined load, and its fitting methods are investigated in this paper. An improved Smith Method is adopted to calculate the residual load carrying capacity of damage ships, where the rotation and translation of the neutral axis of the damaged cross-section are obtained using a particle swarm optimisation method. Because the distribution curve of the residual load carrying capacity of a damaged ship under combined load is asymmetric, the application of traditional explicit polynomial fitting methods results in poor accuracy. In this study, a piecewise weighted least square fitting method is adopted so as to guarantee the continuity in the transitions, and a method is proposed for fitting the ultimate limit state function of a damaged ship under combined load. Calculations of the residual strength show that the improved Smith Method is more accurate than the original Smith Method for the accurate position of the neutral axis. The error analysis of the fitting methods shows that the ultimate limit state function that is fitted using a piecewise weight least square method is more accurate.