|
|
General HydroStatics
Ship Stability Software |
|
Command of the Week
(New or interesting aspects of GHS that you may not know about) |
|
DYNSTAB Surf/Broach (SB)
(Requires GHS version 17 or later with DS, SK and AF) |
|
Recently, we published a paper with colleagues at the American Bureau of Shipping documenting our collaborative efforts to develop and verify a practical implementation of the IMO Second Generation Intact Stability criteria. The methods discussed therein are available in GHS via the DYNSTAB command, which leverages the Dynamic Stability, SeaKeeping, and Advanced Features modules to make it all happen. While the paper itself is a good read for those seeking a more in-depth discussion of the guidelines, we thought it would be a good time to share another example of DYNSTAB in action. In this COW, we will evaluate the Surf-riding/Broaching (SB) failure mode on a fishing vessel.
Surf-riding and Broaching is a stability failure that affects small vessels moving at higher speeds. It manifests itself as a violent uncontrollable turn that often follows surf-riding and may cause stability failure. It occurs when the wave length is between one and three times the ship length, the waves are steep, and the ship speed is close to the wave speed. To evaluate SB failure in a run file, you need some information you aren't typically used to entering into GHS: resistance and propulsor data. For the propulsor, we use the PROPULSOR command to store information such as the propeller diameter (2.6 meters) and hub location (9.75a, 0, 0.8), wake fraction (via /WF), and thrust deduction fraction (via /TD). We also give the thrust curve regression coefficients (via /THRUST). |
|
`... set up geometry, condition
vari thr="0.2244,-0.2283,-0.1373" prop 2.6,9.75a,0,0.8 /thrust:{thr} /wf:0.156 /td:0.142 |
|
You'll notice we store the thrust coefficients in the variable thr and then passed this variable to /THRUST. This technique just keeps things looking a little more organized but isn't necessary. HELP PROP gives more information on these parameters for those who are curious about the defaults. To input the resistance information, we again use regression coefficients to define the curve. These coefficients have strange units, as the polynomial they represent gives force as a function of speed. HELP DYNSTAB has more about units, but the force unit will always respect the current unit in GHS. In this case, we set the UNIT to kiloNewtons before passing the resistance coefficients to the /RES parameter on the DYNSTAB command. |
|
unit kn
vari res="-0.43563,0.76362,-0.27198,0.041611,-0.0017335" `kN roll imo /s /ak:0 /L:34.5 /B:7.6 /CB:0.6 /T:25.1 dynstab sb /speed:14.29 /res:{res} |
|
One will notice we also defined the service speed of the vessel via /SPEED and asked for an SB failure mode analysis. The call to ROLL IMO is an important step, as this defines a number of IMO-specific parameters which DYNSTAB respects when set by the user. And that's really all that is needed to run an SB evaluation. This run file will first check the applicability of the SB failure mode to the vessel (level 1): |
|
| ...and then run a long-term vulnerability analysis (level 2) in the IACS wave scatter data prescibed by the IMO SGIS criteria: |
|
|
The result in this case is that the vessel at the stated service speed is vulnerable to Surf-riding and Broaching. Reducing the forward speed would probably reduce vulnerability. Results from running several speeds could be used to build a chart of vulnerability index vs. speed which could be used to guide safe operations.
A complete run file of this example can be found here. |
|
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 |