1.0 OBJECTIVE
1.1 To support ongoing work by SNAME Ad Hoc Panel #6 (Structural Design and Response in Collision and Grounding).
1.2 To assess and integrate existing simplified collision-damage models and mechanisms to predict probabilistic collision damage extents given a probabilistic description of collision scenarios. This requires that sub-model physics be sufficiently simple to support overall computational efficiency in probabilistic applications where thousands of runs are required.
1.3 To help identify and apply probabilistic collision scenarios and assess their relative impact on collision damage extents.
1.4 To validate the final selection of sub-models in the context of a collision simulation using real and finite element model data.
1.5 To achieve international acceptance of this validation by publishing results, and making all data and aspects of the research open for discussion and collaboration through SNAME and the Ship Structure Committee.
1.6 To identify important ship global and structural characteristics that impact collision damage extents, and quantify their relative impact through sensitivity analysis.
1.7 To provide the basis for further work in which a parametric analysis of probabilistic results would be incorporated in IMO oil outflow and damage stability regulations. It is generally agreed that structural design has a major influence on tanker oil outflow and damaged stability in grounding and collision, but crashworthiness is not considered in present regulation because it is not sufficiently understood.
2.0 BACKGROUND
2.1 Current International Maritime Organization (IMO) probabilistic methods for calculating oil outflow and damaged stability use generic damage extent descriptions (probability density functions or pdfs) independent of structural design or crashworthiness. Recent work by Crake, Rawson and Brown [1] provides a probabilistic methodology for calculating damage extent in grounding and collision that considers structural design. This approach has great potential value for improving current regulations.
2.2 The collision model developed in [1] calculates both the longitudinal and transverse extent of damage for the struck ship as a function of collision scenario parameters. A modified Minorsky method is used [2]. In order to support a probabilistic treatment of the collision process which includes a range of scenarios, the Minorsky method is generalized to allow three degrees of freedom (motion in the x-y plane, and yaw) for both the striking ship and the struck ship. A time-domain simulation is used with this generalization because the final state of the ship velocities and relative positions cannot be determined in a closed-form solution.
During the collision, forces are developed through two separate mechanisms. In high energy collisions, the most significant of these is the force associated with plastic deformation of the decks and inner bottom. This is the force described by Minorsky, and further developed by Hutchison [3] and Reardon and Sprung [4]. By assuming that the impinging structure is rigid and of triangular shape, the depth of penetration at each time step can be related to both a decrement of kinetic energy, and a resultant force. This force is considered to act in a direction opposite that of the relative motion between the ships.
The second force is from the resistance of the shell of the struck ship to puncture and tearing. In 1979, the Ship Structure Committee concluded that the most promising collision analysis alternative was to extend Minorsky’s original analysis of high-energy collisions by including consideration of shell-membrane energy absorption [5]. Jones developed an approach to estimate this resistance, which was later extended by Van Mater. This approach treats the shell plate as a broad thin column, pinned at both ends, and provides a closed form solution to calculate the energy stored in the membrane for a given deflection, as well as a method to estimate the deflection at which fracture should be expected. In [1], this membrane force is calculated for each time step until rupture occurs, and is considered to be zero thereafter.
2.3 Although a good beginning, improvements to the model are required. The model provides an excellent global framework for testing other sub-models and collision damage mechanisms. Improvements may include: bow structure energy absorption; different bow shapes and relative ship drafts; additional effects of transverse frames, webs and bulkheads; data for relative distribution of striking ship characteristics compared to struck ship; alternative plastic deformation models; and revised collision scenarios.
2.4 SNAME Ad Hoc Panel #6 is addressing a number of issues related to this proposal including: model assessment, data, model validation, and accident probabilistic scenarios. This Ship Structure Committee Project would directly support and benefit from the additional work of the Ad Hoc Panel (2-1 leveraging of Ship Structure Committee Funding). This Ship Structure Committee Project represents new research that goes beyond the scope of voluntary work to be completed by the Ad Hoc Panel #6.
3.0 REQUIREMENTS
3.1 Scope.
3.1.1 The Contractor shall form a student working-group at Virginia Tech on collision, and educate and identify student(s) to work on specific tasks and sub-tasks of this proposal and other Ad Hoc Panel #6 work.
3.1.2 The Contractor shall conduct a thorough literature search on existing collision models.
3.2 Tasks.
3.2.1 Task 1 - The Contractor shall re-code (C++ or Fortran 90) the Crake collision model [1].
3.2.2 Task 2 - The Contractor shall refine the model to consider:
3.2.2.1 Bow structure energy absorption;
3.2.2.2 Different bow shapes and relative ship drafts;
3.2.2.3 Effects of transverse frames, webs and bulkheads;
3.2.2.4 Data for relative distribution of striking ship characteristics compared to struck ship;
3.2.2.5 Alternative plastic deformation models recommended by the Project Technical Committee; and
3.2.2.6 Revised collision scenarios as recommended by the Project Technical Committee.
3.2.3 Task 3 The Contractor shall develop a validation matrix and validate the model(s) based on data (actual and FEM), and evaluate alternative sub-models and damage mechanisms.
3.2.4 Task 4 The Contractor shall develop a matrix of balanced tanker designs for sensitivity analysis that spans a range of ship global and structural design parameters. The parameters may include:
3.2.4.1 Displacement;
3.2.4.2 Length;
3.2.4.3 Beam;
3.2.4.4 Draft;
3.2.4.5 Major subdivision;
3.2.4.6 Side, bottom and deck plating thickness;
3.2.4.7 Frame and stiffener spacing; and
3.2.4.8 Frame and stiffener moment of inertia.
The initial matrix will use relatively large increments of these parameters. It will require the extensive use of SAFEHULL or other concept design tools to insure feasibility and class compliance for a given combination of parameters in the matrix.
3.2.5 Task 5 The Contractor shall apply the validated collision model to calculate damage for the matrix of ship and structural design parameters developed in Task 4.
3.2.6 Task 6 The Contractor shall based on the Task 5 sensitivity analysis, select the most important ship and structural design parameters to use in subsequent analyses.
3.2.7 Task 7 The Contractor shall report findings to the Project Technical Committee and contribute to the Ad Hoc Panel #6 paper for the 1999 SNAME Annual Meeting.
3.2.8 Task 8 The Contractor shall oversee the completion of two student theses.
3.2.9 Task 9 The Contractor shall write a joint SNAME / Ship Structure Committee paper for the Journal of Ship Research.
3.2.10 Task 10 The Contractor shall write a Ship Structure Committee (SSC) Report for publication by the SSC.
4.0 DELIVERY REQUIREMENTS
4.1 The Contractor shall provide quarterly progress reports to the Project Technical Committee, the Ship Structure Committee Executive Director, and the Contract Specialist.
4.2 The Contractor shall participate in project review meetings to be scheduled at the discretion of the Project Technical Committee (at least 1 review meeting every 6 months).
4.3 The Contractor shall validate and demonstrate the refined collision model as defined in Task 2 for the Project Technical Committee.
4.4 The Contractor shall provide for review by the Project Technical Committee the parameters and results of the sensitivity analysis defined in Task 5.
4.5 The Contractor shall provide a listing of the most important ship and structural design parameters to be used in subsequent analyses for review by the Project Technical Committee, and make recommendations for what subsequent analyses (follow on projects) should entail.
4.6 The Contractor shall provide a joint SNAME / Ship Structure Committee paper suitable for publication in the Journal of Ship Research.
4.7 The Contractor shall provide a print-ready master final report (paper copy and on 3.5" diskette in MS Word Format) including the above deliverables formatted in accordance with the Final Report Style Manual (Enclosure 2).