A simple formulation for predicting the ultimate strength of ships

The aim of this study is to derive a simple analytical formula for predicting the ultimate collapse strength of a single- and double-hull ship under a vertical bending moment, and also to characterize the accuracy and applicability for earlier approximate formulations. It is known that a ship hull will reach the overall collapse state if both collapse of the compression flange and yielding of the tension flange occur. Side shells in the vicinity of the compression and the tension flanges will often fail also, but the material around the final neutral axis will remain in the elastic state. Based on this observation, a credible distribution of longitudinal stresses around the hull section at the overall collapse state is assumed, and an explicit analytical equation for calculating the hull ultimate strength is obtained. A comparison between the derived formula and existing expressions is made for largescale box girder models, a one-third-scale frigate hull model, and full-scale ship hulls.List of symbols A _B total sectional area of outer bottom - A _B total sectional area of inner bottom - A _D total sectional area of deck - A _S half-sectional area of all sides (including longitudinal bulkheads and inner sides) - a _s sectional area of a longitudinal stiffener with effective plating - b breadth of plate between longitudinal stiffeners - D hull depth - D _B height of double bottom - E Young's modulus - g neutral axis position above the base line in the sagging condition or below the deck in the hogging condition - H depth of hull section in linear elastic state - I _s moment of inertia of a longitudinal stiffener with effective plating - l length of a longitudinal stiffener between transverse beams - M _E elastic bending moment - M _p fully plastic bending moment of hull section - M _u ultimate bending moment capacity of hull section - M _uh ,M _us ultimate bending moment in hogging or sagging conditions - r radius of gyration of a longitudinal stiffener with effective plating [=(I _s /a _s )^1/2] - t plate thickness - Z elastic section modulus at the compression flange - Z _B ,Z _D elastic section modulus at bottom or deck - slenderness ratio of plate between stiffeners [= (b/t)(_y/E)^1/2] - slenderness ratio of a longitudinal stiffener with effective plating [=(l/r)(_y/E)^1/2] - _y yield strength of the material - _yB , _yB , _yD yield strength of outer bottom, inner bottom - _yS deck, or side - _u ultimate buckling strength of the compression flange - _uB , _uB , _uD ultimate buckling strength of outer bottom - _uS inner bottom, deck, or side     

Time-variant ultimate longitudinal strength of corroded bulk carriers

Many bulk carrier losses have been reported of late, and one of the possible causes of such casualties is thought to be the structural failure of aging hulls in rough weather. In aging ships, corrosion and fatigue cracks are the two most important factors affecting structural safety and integrity. This paper uses a set of the time-dependent corrosion wastage models for 23 different member locations/categories of bulk carriers previously developed by the authors, based on the available corrosion measurements for existing large bulk carrier structures. Differences due to the location and corrosion severity of every member type are taken into account. The nominal design corrosion values for the primary members are suggested based on the annualized corrosion rates obtained in the present study. The effect of time-variant corrosion wastage on the ultimate hull girder strength as well as the section moduli is studied. The criteria for repair and maintenance of heavily corroded structural members so as to keep the ultimate longitudinal strength at an acceptable level are discussed. Important insights and conclusions developed are summarized.     

Quantitative assessment of hydrocarbon explosion and fire risks in offshore installations

A risk-based design framework should involve both risk assessment and risk management. This article introduces and describes a number of procedures for the quantitative assessment and management of fire and gas explosion risks in offshore installations. These procedures were developed in a joint industry project on the explosion and fire engineering of floating, production, storage and off-loading units (the EFEF JIP), which was led by the authors. The present article reports partial results, focussing on defining the frequency of fires and explosions in offshore installations. Examples of the aforementioned procedures’ application to a hypothetical floating, production, storage, and off-loading unit (FPSO) are presented. A framework for the quantitative risk assessment of fires and explosions requires the definition of both the frequency and consequences of such events. These procedures can be efficiently applied in offshore development projects, and the application includes the assessment of design explosion and fire loads as well as the quantification of effects of risk control options (RCO) such as platform layout, location and number of gas detectors, isolation of ignition sources etc.     

Characteristics of wireless sensor network for full-scale ship application

In this study, basic experiments regarding the wireless sensor network were conducted on a 3,000-ton-class training ship as the first step in applying the ubiquitous technology to a real ship. Various application fields of the technology in terms of the provision of safety and convenience on a ship were identified through these experiments. To efficiently adopt the ubiquitous technology for ship application, it is necessary to identify the state-of-the-art ubiquitous technology and to prepare countermeasures against the harsh environment of a ship. The characteristics of the wireless sensor network were investigated on a test bed ashore as well as on a real ship before full-scale ship application. In particular, experimental results concerning communication depth, data transmission ratio, and battery consumption in a sensor node are described in detail.     

Investigation on the vortex structure of propeller wake influenced by loading on the blade

The vortex structure of the wake behind a marine propeller was investigated in terms of loading variation by using particle image velocimetry. One hundred and fifty instantaneous velocity fields were ensemble averaged to study the spatial evolution of the wake and the behavior of the tip vortices in the region ranging from the trailing edge to one propeller diameter downstream. The trailing vorticity was found to be related to the radial velocity jump, and the viscous wake was affected by the boundary layers developed on the blade surfaces. A vortex identification method using the swirling strength was employed to extract the location of the tip vortex. The loading on the blade made a clear difference to the contraction angles. Slipstream contraction occurred in the very near wake region, and unstable oscillation occurred because of reduced interaction between the tip vortex and the wake sheet behind the maximum contraction point for each loading condition. The maximum tangential velocity around the tip vortex center revealed the average radius of its core, which was used for calculating the vortex strength. Additionally, variation of the average radius of tip vortices with the change of blade loading was related to vortex tube stretching in the wake region. The nearly constant vortex strength continued up to one diameter downstream for light loading and design loading conditions.     

URANS analysis of a broaching event in irregular quartering seas

Ship motions in a high sea state can have adverse effects on controllability, cause loss of stability, and ultimately compromise the survivability of the ship. In a broaching event, the ship losses control, naturally turning broadside to the waves, causing a dangerous situation and possibly capsizing. Classical approaches to study broaching rely on costly experimental programs and/or time-domain potential or system-based simulation codes. In this paper the ability of Reynolds averaged Navier–Stokes (RANS) to simulate a broaching event in irregular waves is demonstrated, and the extensive information available is used to analyze the broaching process. The demonstration nature of this paper is stressed, as opposed to a validated study. Unsteady RANS (URANS) provides a model based on first principles to capture phenomena such as coupling between sway, yaw, and roll, roll damping, effects of complex waves on righting arm, rudders partially out of the water, etc. The computational fluid dynamics (CFD) method uses a single-phase level-set approach to model the free surface, and dynamic overset grids to resolve large-amplitude motions. Before evaluating irregular seas two regular wave cases are demonstrated, one causing broaching and one causing stable surf riding. A sea state 8 is imposed following an irregular Bretschneider spectrum, and an autopilot was implemented to control heading and speed with two different gains for the heading controller. It is concluded that the autopilot causes the ship to be in an adverse dynamic condition at the beginning of the broaching process, and thus is partially responsible for the occurrence of the broaching event.     

Comparative measurements on the flow structure of a marine propeller wake between an open free surface and closed surface flows

In order to investigate the effects of a free surface on the wake behind a rotating propeller, experiments were carried out in a circulating water channel for two cases: one with an open free surface and one with a closed free surface covered by a rigid plate. Four hundred instantaneous velocity fields were measured using a two-frame particle image velocimetry (PIV) technique at four different blade phases. These were ensemble-averaged to investigate the time-averaged flow structure in the near-wake region. For a surface ship, the flow behind the propeller is influenced by the hull wake and the free surface. The phase-averaged mean velocity fields show the potential wake and the viscous wake formed by the boundary layers developed on the blade surfaces. The interaction between the bilge vortices and the incoming flow along the ship’s hull deforms the wake structure. Tip vortices are generated periodically, and the slipstream contraction occurs in the near-wake region. The free surface was found to affect the axial velocity component and vortex structure behind the propeller. As the flow goes downstream, the tip and trailing vortices dissipate due to turbulent diffusion and active mixing with adjacent vortices.