Showing papers in "Transactions of the Society of Naval Architects and Marine Engineers in 1990"

Decision-based design - a contemporary paradigm for ship design

Abstract: For decades ships have been designed using the well known "basis ship approach" together with the equally well-known Evans-Buxton-Alexander spiral. The two principal limitations of the spiral are that the process of design is assumed to be sequential and the opportunity to include life cycle considerations is limited. In order to increase both the efficiency and effectiveness of the process of ship design a new paradigm for the process of design is needed. In this paper, recent developments in the field of design are reviewed and a contemporary paradigm is given. Decision-based design, for the design of ships, one that encompasses systems thinking and embodies the concept of concurrent engineering design for the life cycle.

Ultimate longitudinal strength of ships - a case study

Abstract: A rare opportunity to test the validity of theoretical predictions of the ultimate longitudinal bending strength of ship hulls arose when, in 1980, a VLCC broke its back during discharge of oil. The paper describes the accident and presents results of retrospective strength calculations in which the modelling of the buckling and post buckling behaviour of compressed parts of the structure has a major influence on the predicted ultimate moment of resistance of the hull. Sensitivity studies of the influence of uncertainties, such as initial deformations and corrosion rates are outlined and details of structural modelling and assumptions are given. Comparison of such results with the actual failure leads to conclusions concerning both the calculation and provision of longitudinal strength in excess of normal requirements for ships in both hogging and sagging conditions.

A numerical method for huge semisubmersible responses in waves

Abstract: A numerical procedure is described for predicting the wave exciting forces and motions of multiple 3-D floating bodies of arbitrary shape, and for predicting the motions and structural responses of huge offshore structures supported by a large number of the floating bodies in waves. The hydrodynamic interactions among the floating bodies are taken into account in their exact form within the context of linear potential theory. Numerical results are compared with the experimental or numerical ones. The method presented is applicable to huge structures for ocean space utilisation.

Hydrodynamics of remotely sensed surface ship wakes

Abstract: Recently, there has been much interest in the remote sensing of surface ship wakes using synthetic aperture radar (SAR). Ship wake signatures obtained using SAR have several distinct features such as a dark trailing centerline region, bright-line images aligned at some angle to the ship's path and sometimes the Kelvin wave pattern. The reasons for these features are not well understood. Many different theories are available to explain one or more of the features. These theories are often in conflict an no definitive answers are yet available. Much analytical and numerical research, and both laboratory and full-scale experiments are presently underway in order to develop proper explanations. This paper surveys the present research and the state of the art in the hydrodynamics of remotely sensed ship wakes.

An experimental investigation of ship hull ultimate strength

Abstract: This paper summarises results of an experimental investigation to determine the ultimate strength of ship hull girders. The 2-year experimental investigation was conducted at the University of California experimental facility. In the first phase a stiffened steel hull model of approximate dimensions 42 ft x 8 ft x 30 inches was tested. The model represented the middle section of a 75,600 dwt tanker and was subjected to a sagging moment in order to examine the failure behaviour of the deck under compression. The second model (phase 2) was designed to reflect several possible modes of failure of an open deck ship. It was tested with loads simulating a hogging bending moment along with lateral pressure on the bottom. A comparison between simple theoretical analyses and the experimental results for the two models are presented. Approximate methods for including the effects of residual stresses and initial distortions on the moment capacity of the models are also discussed. A number of possible reasons for the discrepancy between theoretical and experimental results are investigated in detail. Recommendations are also made for further research.

Extreme dynamic response and fatigue damage assessment for self-elevating drilling units in deep water

Abstract: The behaviour of jackup mobile drilling units is known to be dominated by the effect of drag rather than inertia in terms of wave excitation especially in deep water applications. In this paper the effects of nonlinearity and structural resonance upon various important parameters governing the behaviour of seabed supported offshore structures exposed to such conditions are first examined. The investigation begins with an overview of the basic mechanics of structural resonance on the premise of wave excitation characterised by Morison's equation. The response behaviour is analysed by way of both a deterministic and a Monte Carlo formulation simulating what is known as the "design wave" concept and that of random wave excitation, respectively. The random process is examined by way of the time domain random response resulting from the random simulations. Statistical parameters derived reveals certain important aspects in both the extreme value of the response and the cumulative fatigue damage sustained by structural members. Finally, some practical aspects of analysis applicable to day-to-day engineering practice are proposed.

A Method for Resistance and Flow Prediction in Ship Design

Abstract: The purpose of the paper is to introduce a new numerical method for predicting the flow and resistance components of ships and marine structures. The theory is based on an approach where the flow is divided into three zones: the potential flow with a free surface, the boundary layer and the Navier-Stokes flow. A separate method is used for each zone, thus enabling a high resolution without excessive computer efforts. The necessary pre and post processors are also included. A brief account of the theory is given in the paper and the operation of the system of computer programs is explained. The usefulness and accuracy of the system is demonstrated through a number of computed examples, for which comparison is made with measurements. Predicted quantities include resistance components, wakes and other local flow quantities, also in the presence of a working propeller. The method should be a useful tool in ship design, particularly at an early design stage and may be considered a complement to the traditional model testing.

Design of apl c-10 propeller with full-scale measurements and observations under service conditioning

Abstract: This paper describes the results of a team effort in the design of the propeller for the APL C-10 container ships. Previous experiences of the owner with regard to C-9 propeller failures due to fatigue and excessive backing stresses helped mould the design program. The propeller design was subject to considerations of: powering, cavitation, hull pressure fluctuations and vibration, blade stresses and fatigue. In the process of the design, various methods of prediction were compared with each other, with model test results and with previously obtained full scale results including a database from the APL C-9 class ships. The design was confirmed during the sea trials on the lead ship which were conducted under the most optimal conditions for verifying the design: calm sea, negligible wind and design draft and trim. Trial measurements included standard powering runs, hull vibration, pressure fluctuations and propeller viewing. Following sea trials, an adjustment was made in the scaled nominal wake that later shed light on the model - full scale comparisons of fluctuating hull pressures.

The design and construction of the u.s. coast guard's 47 ft self-righting, heavy weather rescue craft

Abstract: The 44 ft motor lifeboat (44MLB) is the U.S. Coast Guard's primary rescue craft for inshore, heavy weather Search and Rescue. The 44MLB was introduced in 1963 and over 100 were built. They are rapidly approaching the end of their useful service life and suffer from increasing repair and logistical support problems. In 1984 the Coast Guard initiated a multi-phase program to develop a replacement for the 44 MLB which would be faster, more stable and offer improved crew habitability and safety for extended operations. This paper reviews the requirements for the replacement craft and describes the design development and construction of the 47 ft prototype craft. The paper highlights unique design features required for heavy weather rescue craft including self-righting, damage stability, structure and machinery. Note: The views expressed in the paper are the opinions of the authors and are not necessarily the official views of the U.S. Coast Guard.

Hydro-numeric design : performance prediction and impact on hull design

Abstract: Today ship design systems are being developed which can perform a complete cycle of design modification, hydrodynamic analysis and performance assessment in a single day. Examples of seakeeping assessments, wave resistance predictions and submarine hydrodynamic assessments are presented to indicate the range of current capability in hull design. The current design assessment capability has evolved due to remarkable advances in hydronumeric techniques, computer hardware, software, graphics, networking and databases. This dramatically improving capability will place unprecedented demands on incorporating the wealth of new technology into design practice. The new design codes will be based on new theoretical approximations, employ unfamiliar numerical techniques, run on a variety of super computer architectures and produce a large amount of detail. Acquiring, transitioning and managing new technology will become major issues for ship design. An investment strategy for new technology in ship design is presented which acknowledges the necessary long term national commitment through the creation of a permanent activity in hydronumeric ship design.