General HydroStatics
Ship Stability Software |
Command of the Week
(New or interesting aspects of GHS that you may not know about) |
ROLL IMO & ROLL PERIOD
(Requires GHS version 17.00 or later) |
John Bonn read last week's COW and promptly called our support line. He reminded us that the ROLL command has long been a part of GHS and that it can be used to calculate the roll period, so the SEAKEEPING command is not necessary. OK, John, you've obviously been reading the manual lately...
John is right: a form of the ROLL command does compute an uncoupled natural roll period, but it does so using the IMO Severe Wind and Roll formula--an approximation based on a host of assumptions that are valid for the specific needs of the IMO criterion (find the formula in HELP ROLL). At its core, the formula computes a so-called "roll constant" which simply draws a proportionality between roll period and the ratio of the beam and square root of the GM. That might sound esoteric, but it has physical significance: the natural period of a spring-mass system follows a similar form, and the linear roll stiffness of a ship is proportional to GM. So how would we use this command to compute an approximate natural roll period according to the IMO formula? It's rather easy: |
`... establish GF and loading condition
ROLL IMO ROLL |
Using ROLL alone as shown above will display the following breakdown of the terms in the IMO formula: |
Notice that the roll constant "C" is given as 0.480 and the roll period "T" is given as 4.1 seconds. For this example, we used the exact same geometry and loading condition used in the seakeeping example in COW176, which returned a natural period of 4.263 seconds. Not bad for a regression formula! But we are also running SEAKEEPING in the precise condition that the IMO formula assumes (beam seas, zero speed), so agreement is expected. Change the heading, speed, or condition, and the IMO roll period is less accurate.
But in truth, the ROLL and SEAKEEPING commands live in harmony. You see, the IMO criteria allows for the input of a better roll period, if you have one, and GHS allows you to input this period using the recently added /T parameter: |
ROLL IMO /T:4.263 |
Using SEAKEEPING, it is very easy to get condition and geometry specific roll periods for input. Another form of the ROLL command, ROLL PERIOD, allows you to input a known roll period separate from the IMO form of the command. This period is then converted into a roll constant which may be used to compute the roll period for any GM in places like Load Editor, GLM, and the like. This is convenient when you want to establish a good baseline natural period from a known condition using a single analysis, and then "scale" this period based on GM for a whole bunch of conditions. This is efficient, especially when there are many loading conditions to evaluate and you don't want to run SEAKEEPING for every single condition. If you're looking for more details on the implementation of IMO Severe Wind and Roll in GHS, you can find an example run file here and a bulletin about the subject here. For more information on the theoretical background behind the approximate IMO formula, see IMO MSC.1/Circ.1281 Explanatory Notes to the IS Code. |
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