A Reader's Question: Free Surface
William George
Marine Educator, Consultant and Expert Witness at William E. George and Associates, LLC
I never know what my readers are going to ask:
Student: Good day, I have a question on MODU stability. I am working on 6th Gen Semi-sub drilling rig. There are ballast tanks in pontoons, which are submerged during operations. We have upper and lower ballast tanks in both pontoons and most of times they are slack. I am in doubt about free surface effect in our case, how to calculate and how it will affect the rig if ballast tanks will not be completely full or empty?
Is a free surface effect critical in such case? I know, that you were teaching MODU stability, and I hope you have practical answer for this question. Thank you.
William George: You need to consult the Rig's Operating or Loading Manual!
Do you have Free Surface Moments for these tanks? If so, consult the Loading Manual for the Rig. There should be directions.?Most likely you have FS Moments for both transverse and longitudinal direction for each tank. (These FS Moments are illustrated above. FSML = Longitudinal Free Surface Moment. FSMT = Transverse Free Surface Moment.)
The Rig has one solid Center of Gravity, G. You will need to correct it for free surface transversely and longitudinally. Normally this is done by summing the FS Moments and dividing by the total displacement.
Use this KG corrected for FS to calculate the Transverse GM and Longitudinal GM using the Transverse KM?and Longitudinal KM.
Student: Thank you, Sir. I will check it. This is Chinese built rig, and most of documentation is in very bad order. Currently I am using stability program only, and it is also not reliable, since most of tank sensors showing wrong readings. I will try to find out manuals. Many thanks.
William George: Once you know the Transverse GM and Longitudinal GM you can calculate list transversely and longitudinally with:?
Tan (angle) = W x D /?Displ x GM
About 30 minutes later:
Student: Thanks. Finally found loading tables, with FS moments. Thank you, Sir. I appreciate your assistance.
William George: One more thing. The FS Moments usually assume an angle of Heel of 45 degrees relative to the horizon. This is excessive and the error is "supposedly on the side of safety". For full FO and DO tanks the assumed condition can be 98% full and 5 degree angle of list.
The FS formula?= lb^3/12*Displaced Volume
What you do not see above is, it being multiplied by the tangent of 45° because it equals 1. For small vessels the liquid in the tanks is relative to the horizon. When on the side of a steep wave the boat does not know its inclined and only slides down the side of the wave. On large ships excessive Free Surface Corrections can trick a Chief Mate or Master into taking more Ballast than actually needed. A still water rolling test can prove this out.
At the assumed full FO and DO conditions it is reduced by multiplying it by tangent of 5°. Full ballast tanks have no assumed Free Surface because they can be overflowed.
So for small angles of list the FS correction can appear to be excessive!
For the International Grain rules the Volumetric Heeling Moments includes the tangent function for the assumed angle of repose of the grain.
Student: Thank you Sir, I was trying to find out at what conditions we could keep ballast tanks during drilling, since we had excessive deck load and draft limitations. For me it is my first semi-sub, so calculation of KG and GM was a little bit unclear due two her construction: all ballast tanks are in pontoons, pontoons are fully submerged. Now after I got your advices and finally found documentations. I can assess the situation. Normally rig is operating at minimum angles of list and trim, less then 2 degrees. But due to excessive deck loads, stability is low. Another restriction was about % of filling ballast tanks, CE and Master did not want to keep them completely full, to avoid problems with sensors, which actually does not work well.
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William George: Interesting. Study the general arrangement plan. It looks different than a ship but it all works the same way. Usually the assumed condition is zero list and trim, no hog or sag, true draft is at the LCF, sea water density is 1.025 and the vessel is floating. By the way the Double Bottom Tanks on a Ship are also fully submerged! The only real thing different is the drawing of the vessel on the Capacity Plan and the General Arrangement Plan.
To learn more about MODU, Mobile Offshore Drilling Units, check out the following:
https://www.dco.uscg.mil/Portals/9/NMC/pdfs/examinations/12_operating_manual_for_deep_driller_v2.pdf
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My?Bunker and Draft Survey Course?is a narrated correspondence course done via email worldwide. Sadly, most Maritime Schools and Academies do not include Draft and Bunker Surveying in their curriculums because it is not included on the License Exams. As a result they do not prepare you to work on this type of ship.
Did you know that a Draft Survey properly done can yield the Total Displacement within 0.5% of the true cargo figure or Displacement if done properly? You can determine a vessel's total cargo weight by reading the drafts and sounding the tanks instead of the cargo manifest which may or may not be correct or verify a Shore Scale Weight. This method works on all vessels as long as they are afloat. You can also use Draft Surveying to independently confirm a vessel's Stability Calculations.
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Consultant
2 年Capt George. I for one compliment you Sir on all your hard work. I rarely comment or post anything. But I will say this, your work is the "Backbone" of what I believe ultimately affects the Safety of the Crew, the Ship, The Owners, the Insrance Agencies and more. Thank you for caring for their safety. Chris
Contractor at Self-Employed
2 年I was a Ballast Control Operator & Mate in the Gulfs of Mexico & Alaska doing this ??
Senior Vice President, EFU General Insurance Ltd.
2 年Thanks for sharing valuable article.