At-sea towing of a Mega-Float unit

A huge floating offshore platform (359m long, 60m wide, and 3m deep) was towed into the Pacific Ocean for a validation experiment for a floating airport. Full-scale measurements of towline tension and the bending strain on the upper-deck were made during towing. The measured bending moment agreed well with numerical calculation without taking the draught and towing speed into consideration.     

Stress intensity factor measurement of cracks using piezoelectric element

A stress intensity factor (SIF) measurement method for cracks using a piezoelectric element and an electrostatic voltmeter is presented. In this method, an isotropic piezoelectric element is first attached near the tip of the crack. Then surface electrodes are attached to three different positions on the piezoelectric element. The electric potentials of the surface electrodes, which are proportional to the sum of the stress ( _x + _y ) on the structural member, are measured by an electrostatic voltmeter during load cycling. The mode I and mode II SIFs of the crack are estimated using the relationship between the SIF and ( _x + _y ). The applicability of the proposed method is examined through experiments and numerical analysis.     

Digital exchange of design models between marine equipment libraries using hybrid neutral formats

The selection of marine equipment is a time-consuming process for a ship outfitting designer. The inefficiency originates from the paper-based exchange of design information between shipyards and equipment manufacturers. We propose a method to exchange equipment models for electronic commerce. Shipyard practices were investigated to find out how the outfitting CAD library is being used. We also surveyed international standards for the exchange of digital equipment models, and analyzed the macroparametric methodology. We propose a hybrid method to represent the geometric information using STEP AP203 or the macroparametric method. The nongeometric information is defined according to the Parts Library (PLIB) standard. We implement a pilot system and test it with a ship design data set. The equipment model proposed can be used in the design process with heterogeneous shipbuilding CAD systems. The equipment model can improve the design and purchase process in shipyards.     

Evaluation of added resistance in regular incident waves by computational fluid dynamics motion simulation using an overlapping grid system

A computational fluid dynamics simulation method called WISDAM-X was developed to evaluate the added resistance of ships in waves. The Reynolds-averaged Navier–Stokes (RANS) equation was solved by the finite-volume method and a MAC-type solution algorithm. An overlapping grid system was employed to implement rigorous wave generation, the interactions of ships with incident waves, and the resultant ship motions. The motion of the ship is simultaneously solved by combining the solution of the motion of the ship with the solution of the flow about the ship. The free surface is captured by treatment by the density-function method. The accuracy of WISDAM-X is examined by a comparison with experimental data from a container carrier hull form, and shows a fairly good agreement with respect to ship motion and added resistance. Simulations were also conducted for a bow-form series of a medium-speed tanker to examine the effectiveness of the WISDAM-X method as a design tool for a hull form with a smaller resistance in waves. It was confirmed that the WISDAM-X method can evaluate the added resistance with sufficient relative accuracy and can be used as a design tool for ships.     

PIV velocity field measurements of flow around a KRISO 3600TEU container ship model

The main purpose of this investigation was to demonstrate a useful application of the particle image velocimetry (PIV) method to analyze the complex flow characteristics around a ship. For a sample illustration, the KRISO 3600TEU container ship model was chosen. The flow structure in the stern and near-wake region of the model has been investigated experimentally in a circulating water channel. Instantaneous velocity fields measured by the PIV velocity field measurement technique have been ensemble-averaged to give details of flow structures such as the spatial distributions of the local mean velocity, vorticity, and turbulent kinetic energy. The free-stream velocity was fixed at U _o = 0.6m/s, and the corresponding Reynolds number based on the length between perpendiculars was about 9.0 × 10⁵. The container ship model shows a complicated three-dimensional flow structure in the stern and near-wake regions. The PIV results clearly revealed the formation of large-scale bilge vortices in the stern region and their effect on the flow in the near-wake. The results shown here provide valuable information for hull form design and the validation of viscous ship flow codes and of turbulence models.     

A local model for the simulation of two–dimensional spilling breaking waves

A numerical model for the simulation of two dimensional spilling breaking waves is described. It is derived from a previous model which, in turn, takes its underlying ideas from the Cointe and Tulin theory of steady breakers. With respect to the former model, the present one is local, i.e., the inception, extension, and geometry of the breakers are determined through the local shape of the water surface. The model has been implemented in a RANSE code, which was developed for the simulation of ship flows, through a modification in the boundary conditions. This yields a simple and effective way to reproduce the breakers influence on the underlying flow. The resulting code has been used to simulate the flow past a submerged hydrofoil. The numerical results are compared with those of the previous model and with the experimental data obtained by Duncan.     

Stability assessment for intact ships in the light of model experiments

A systematic method for assessing intact ship stability with a free-running model in a seakeeping and maneuvering basin is proposed in this paper. Model experiments were carried out in extremely steep regular waves for a model drifting, running in head seas, and quartering seas. This method was applied to two purse seiners, and efficiently identified thresholds in metacentric heights for capsizing of these ships. These capsizing thresholds are compared with requirements of the IMO Code on Intact Stability. This series of model experiments also confirms that capsizing at the threshold occurs only in quartering seas, and shows that capsizing is caused by broaching, loss of stability on a wave crest, or bow diving. Received for publication on Jan. 20, 1999; accepted on July 6, 1999     

Safety culture on board six Swedish passenger ships

This paper presents the results of a Swedish study on maritime safety culture. The study aims to increase knowledge about the characteristics of safety culture in the maritime setting and about the relationship between safety culture and organizational climate. Such knowledge can be vital in the process of developing and improving safety culture. The paper reports the results of safety culture assessments on board six Swedish passenger ships in international traffic, using observations, questionnaires, and interviews focussing on 508 employees who completed the questionnaires. The results of the assessments revealed a generally good existing safety culture. Differences in individuals' safety culture perceptions were found between ships, vessel type (high-speed craft versus Ropax), and hierarchical working position. Important relationships between organizational climate and safety culture were also found. A comparison with other transport sectors where the same safety culture assessment had been applied yielded that passenger shipping has an average safety culture score level that falls between air traffic control and airport ground handling. The method of assessment, which can be used by shipping companies and vessel crews, identified specific areas of concern in the safety culture that could support continuous improvements of safety and safety culture.     

Active control for installation of underwater structures

The trend towards deepwater development requires a new approach to underwater installation of offshore structures. The present method using crane vessels has some drawbacks in operations at more than 2000 m depth. The natural period of the coupled system of the rigged structure and the crane vessel becomes longer, so that it is no longer possible to manipulate the cranes to achieve the desired positioning accuracy. This paper examines the application of an active control technique for underwater installations as one of the solutions to the present problems. An active control technique also has the advantage that it can deal with the structural flexibility which allows the structure to be large and light-weight. This structural flexibility imposes problems of suppressing the elastic responses and securing the stability of the control system. In this paper, anH _∞ controller combined with a low authority control/high authority control (LAC/HAC) feedback controller is designed for cases where structural flexibility cannot be ignored. A robust model-following controller is examined for cases where the structure can be treated as rigid. In order to confirm the control algorithm and verify the possibility of the active control installation method, basin tests are executed using two types of neutrally buoyant flexible models with ultrasound ranging systems and thrusters.     

A consideration of the structural design of a large-scale floating structure

The land area of Japan, especially flat land, is very small compared with its economic size. Large-scale floating structures are one solution to satisfy the demand for space by utilizing the ocean. This paper presents a general view of the dynamic response characteristics of large-scale floating structures, pontoons and semi-submersibles. For example, it is shown that the natural frequencies of eigenmodes are higher than the natural frequency of heaving motion. The response of the outer framework of a structure is shown to be generally larger than the response of the central part. Within the limits of our present understanding of the general dynamic response characteristics of such structures, the design and analysis of semi-submersible, large-scale floating structures is discussed. For a pontoon-type large-scale floating structure, a type whose perimeter structure has been modified to become lighter and more rigid is proposed. With this modification, the dynamic response of the whole structure is imporved.