Motion of ships in irregular wave

Discussion in 'Hydrodynamics and Aerodynamics' started by Rabah, Feb 7, 2022.

  1. Rabah
    Joined: Mar 2014
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    Rabah Senior Member

    Hello dear forum readers!

    I want to introduce you to 6 video clips about motions in irregular waves of two models of Sea Tech Ltd, Nizhny Novgorod, Russia - according to the SAM 1469A project and the HSC150B project:

    https://www.youtube.com/watch?v=uUQV-WrUq-о











    I want to express special gratitude to my grand daughter Ivayla for her help and for perfectly carried out work for sound effects of the clips.

    NA Razmik Baharyan

    Ruse-Bulgaria
     
    Last edited: Feb 13, 2022
    Will Gilmore likes this.
  2. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    I enjoyed watching those simulations very much.

    In the first four videos, the vessels were making around 30 knots of headway. The last video was of a vessel adrift?

    My question is around the apparent lack of wake. It seems like a wake would redistribute the points of buoyancy along the hull, which might have an effect on the motion of the vessel when surface waves are compounded over them.
     
  3. Rabah
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    Rabah Senior Member

    Dear colleague,

    The catamaran intended for operation in the rivers and lakes with maximum speed 50 km\h.

    The supply ship intended for marine oil (gas) production platform, maximum speed of 30 kn.

    The Motion in three versions is shown:

    - Along-side /ship's heading 90deg/- rolling;

    - Contrary to the wave /ship's heading 180 deg/ - combined heaving – pitching;

    - Under an oblique angle to the direction of wave travel /ship's heading 135deg/-combined longitudinal - rolling motion.

    Program Maxsurf Motion does not display overlapping surface tracks of the wavemaking at the movement of the ship with the profile of the irregular wave.

    Such surface tracks of the movement of the ship can be seen in other program - Maxsurf Resistance.

    Yours faithfully, NA Razmik Baharyan
     
  4. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    Mr Rabah, please could you clarify :
    Does not display , but take into account wavemaking forces on the motion
    or :
    Does not display and does not take into account wavemaking forces on the motion ?
    Anyway, impressive clips !
    Thanks !
     
  5. Rabah
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    Rabah Senior Member

    Thanks for the question. My answer is the following:

    First about the Maxsurf Resistance program!

    It displays the wave picture /see the file/ of the ship when moving on calm water - the movement of the ship generates the appearance of accompanying waves. Often such files are just for presentation, as it is written in the program manual.

    Now about the Maxsurf Motions program!

    Here the situation is completely different - the force of the wind generates the appearance of an irregular wave, and since this is considered a random process, it follows that, in turn, an irregular wave acts on the ship and causes its oscillatory movements as a rigid body with 6 degrees of freedom.

    The hypothesis of the Russian scientist Krylov is used by Salvesen in the Strip theory applied in the program.
     

    Attached Files:

    Last edited: Feb 9, 2022
  6. Rabah
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    Rabah Senior Member

    Let we go on with explanations:

    In the Manual of the program Maxsurf Motion it is written about forces of Frud-Krylov.

    In the enclosed file will see page 208, &12 „Hydrodynamic models of motion“ from the book under editions Voitkounski „ Ship theory Handbook “, vol.2, 1985. There it is written about model of Frud and model of Krylov.

    In the program two methods are applied:

    Strip theory - for speed of the ship with Fn (L) = 0 - 0,8;

    Panel Method - for speed of the ship with Fn (L) = 0 - 0,1;

    Method Strip theory has the name in Russian technical publications: „Hypothesis of flat sections", for the first time publication in the book from Salvesen, Tuck and Faltinsen “Ship motions and sea loads” (1970) and then in the books:

    Lloyd “Seakeeping: Ship behavior in rough weather” (1989 and 1998);

    Edward Lewis “Principles of Naval Architecture” Volume III (1989);

    Panel Method is usually applied to ships in the water, without speed. He is used at designing of ships to evaluate the structural strength, subject on impact of overload shock from slamming including constructions of Deck area at catamarans.

    In the enclosed file it is possible to see in the Manual of the program, page 16, what kinds of motion it is possible calculation on Strip theory and what on Panel Method.

    I display you and the file from the book written Volker Bertran “Practical ship hydrodynamics” (2000), p.123, fig.4.16 in which it is visible that presents „ Strip theory “.

    Yours faithfully, NA Razmik Baharyan
     
  7. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    Thanks for your thorough answer Mr Rabah !
     
  8. Rabah
    Joined: Mar 2014
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    Rabah Senior Member

    If I yet have not bothered you, I want to find out the problem about a ship drift at heaving.

    First it is necessary to reply: How originates drift?

    See the enclosed files from the book written by Chijiumov "Fundamentals of dynamics of ships on heaving “(2010), p.66.

    On Fig.3.11 the reference effect on a small ship abrupt surf waves is shown. In the first phase happen hydrodynamic impact on the windward side, then the ship drifts.

    On Fig.3.12 the operation squall wind on major ship is shown.

    Force of drift itself is not hazardous. Hazardous is receiving heeling moment and if he exceeds on value of the restoring moment, then there is an actual hazard from capsizing ship.

    Suddenly an applied force of the wind immediately reduce to origin of heaving and to roll.

    If for ship on what to reasons all have broken down propulsion system and steering gear, including the bow thruster, unmonitored free floating without own move (adrift) is shown as the captain does not have any capability starts to alter the heading of the ship towards waves/counter or under an oblique angle/.

    It presents an emergency event, one of most to the poor.

    My published video clips not concern such emergency event.

    Outcomes from my calculations on Strip theory can be seen on enclosed files with polar charts, whence it is visible that the captain should to shun the ship's heading under an angle 90 deg. after relation of the direction of wave travel /orange color of lines on the diagram / .

    Following wave catch up on the ship /heading 0 deg.-the heaving and the ship are moving in one-way/ too it is necessary to shun.

    NA Razmik Baharyan
     

    Attached Files:

    Last edited: Feb 11, 2022
  9. patzefran
    Joined: Feb 2011
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    patzefran patzefran

    If I have well understood, your calculations shows the worst case dynamic motion when the ship is drifting under waves action is waves abeam (90°), which looks logical to me.
    Unfortunately when there is no power to maneuver , most likely, it is also the equilibrium direction of drift ! On a sailboat, provided the wind and waves are about the same direction, there is a possibility to control the direction of drift, using a sail or a floating anchor (also possible on a powerboat !).
    Patrick Gallais (Ing retired !)
     

  10. Rabah
    Joined: Mar 2014
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    Rabah Senior Member

    I think that I was with very much exhausting image answered on all your questions.
    In my calculations force of drift is not determined but if there are doubts about transversally - horizontal fluctuations of the ship /Swaying / , they can be defined only with Panel Method instead of on Strip theory as already I was shown in the manual.
    The program is not interested from permanently applied force of drift one-way, but only oscillatory horizontal movement, alternating on one and then on other sides.
    If there was that not clear, I am ready to answer, if you precisely will formulate your problem.
    I repeat: In my video clips it is not shown anywhere drifting ship! You obviously are mistaken!
    Separately that in my calculations there is a case of calculation at condition of ship at speed 0 kn (or km/h) and ship's route concerning heaving 90 degrees, as shown in polar charts, but in the six publication of clips no such case.
    I shall prompt you how easily to perceive moving the ship and heaving on clips:
    Movement of the heaving is well visible!
    All oscillating motions of the ship too are well visible!
    But for the ship on direction of the course, there is a false sensation that the ship is not heaved ahead!
    It depends only from the watching!

    Imagine that your eyes in effect present a video camera installed on drone, flying above ship and moving with too speed as well as ship!

    NA Razmik Baharyan
     
    Last edited: Feb 11, 2022
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