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General HydroStatics
Ship Stability Software |
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Command of the Week
(New or interesting aspects of GHS that you may not know about) |
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DYNSTAB Parametric Roll (PR)
(Requires GHS with AF, DS, and SK) |
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The IMO Second Generation Intact Stability guidelines are a set of criteria to evaluate the vulnerability of a vessel to dynamic stability failures. They are a first of their kind, and we've featured the SGIS criteria in previous COWs 190 and 241, as well as in a paper with the American Bureau of Shipping (ABS), which explains the implementation in full detail. In GHS, all level 1 and level 2 IMO SGIS criteria can be conveniently invoked through the aptly-named DYNSTAB command.
This week, we want to share an example of how the DYNSTAB command can be used to evaluate vulnerability to parametric roll (PR) failure. A full-fledged PR analysis is achieved rather simply by using the following set of commands. |
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roll imo /Ak:64 /T:25.1 /S /K
dynstab PR /speed:24 |
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In the above commands, DYNSTAB will start with a level 1 analysis, and move to level 2 if the vessel is found to fail level 1. Note that a level 1 evaluation is intended to be efficient and conservative. In this example, we are analyzing the KCS containership hull, which is widely known to be susceptible to parametric roll. Unsurprisingly, the vessel fails level 1, as shown below: |
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| In the parametric roll failure mode, level 2 is divided into two metrics: C1 and C2. The first is a measure of vulnerability based on quasi-static GM variation in a range of wave conditions and forward speed. Sixteen different wave conditions are investigated. The results from this evaluation are given below: |
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| Unfortunately, the KCS doesn't pass the first of the level 2 checks, which means we need to call out the big guns. The second metric, C2, requires a nonlinear, time-domain simulation of the vessel at increasing speed and in head and following waves of different heights. DYNSTAB only invokes this analysis if C1 fails, or if the user requests it via the /LEVEL:2t parameter. The simulation is time consuming, so it is best not to invoke this analysis unless necessary. The result of the simulation is a table of roll angles, and a statistical analysis of the response in a range of wave conditions. Below is an image of one of the time domain simulations. |
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| The vessel still fails the second metric, with a C2 of 0.116. The final results of the level 2 analysis are shown below: |
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The conclusion is that the vessel, in the given condition, is vulnerable to parametric roll.
Files for this example may be found here. |
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Questions, comments, or requests?
Contact Creative Systems, Inc. USA phone: 360-385-6212 Fax: 360-385-6213 Office hours: 7:00 am - 4:00 pm Pacific Time, Monday - Friday Mailing address: PO Box 1910 Port Townsend, WA 98368 USA Click here for an index to this and previous COWs |