[HELP] Confusions regarding ship stress calculating equipment

Can someone please answer these questions of mine:-

Why there are TWO sets of values of maximum permissible shear forces and bending moments on a hull stress indicator and what do these stress values represent?

Why the maximum permissible Shear Force and Bending Moment values may be adjusted on the advice of the classification society during the service of the ship?

What are the typical assumptions made by a loading instrument with respect to the wave profile for assessing ship stresses when the ship is at sea ?

Why are there consistent inaccuracies between the draughts as calculated by the ship stability software and the draughts as read, despite dock water density being correctly input into the programme on each occasion? And how can they be compensated to ensure that the draughts as calculated by the software are the same as the draughts as read.?



View Reddit by Tri_WorlderView Source

4 thoughts on “[HELP] Confusions regarding ship stress calculating equipment

  • May 23, 2020 at 10:18 pm
    Permalink

    There’s normally an “at sea” and an “in port” condition I.e. the hull is expected to tolerate more stressors in port due to less force from the waters. That wider margin helps when transitioning through partly loaded conditions in port.

    Reply
  • May 23, 2020 at 10:18 pm
    Permalink

    For the torsion forces, shear forces, bending forces and GM, there are 2 permissible limits. As Stockless explained. When the ship is in port it is assumed that the forces on a ship are a whole lot lower than when the ship is at sea battling waves.

    The changes in maximal permissible forces on the ship can indeed be done by the class society. Usually this happens when the ship changes class society or flag. The reason for this is because different societies and flags use different calculations or allow different calculated forces.

    I do not understand your third question.

    The consistent inaccuracies have a plethora of reasons.

    * For example something as easy as the hydrometer being damaged, using that you will get a different water density reading.
    * Taking the water from a wrong location, near an overboard for example. Local density in the water can be different than for the water in the entire port.
    * Wrong reading of the draft marks or the water marks being painted in wrong places. Yes, this does happen.
    * Unaccounted ballast. Usually ships have the drain for ballast tanks in the back of the tank. But most ships will trim by the bow for fuel efficiency. This means when a tank is registered as empty, there can still be tons of water in the tank that aren’t registered. Or a tank sensor can be damaged or wrongly calibrated.
    * The volume calculation tables of a tank can be wrong, leading to a wrong amount of water measured in the tank compared to the actual amount.
    * But the most important one is wrongly declared cargo weights. This is a real plague in the container industry. A lot of customers have the annoying habit of under-declaring the weight of a container. This does not just mean that you get different measurements when arriving, it can bring the entire stability of ships in danger.
    * Ship construction weights in reality not being the same as the calculated weights.

    To compensate for this we do a manual calculation of the calculated draft and actual draft. We then use this ‘offset’ to the new calculated drafts upon departure. Generally the new departure drafts is quite close to this new compensated calculation. It isn’t an exact science, it can’t be.

    ​

    Hope this answers your questions.

    Reply
  • May 23, 2020 at 10:18 pm
    Permalink

    Regards the second question, conditions of class can be applied to a vessel for reasons such as age, condition of steelwork, engine problems, grounding or collision. If the vessel can’t perform as designed it doesn’t automatically mean she needs to immediately go to drydock, the classification society can impose restrictions on speed, draft, or anything else that could impact stability.

    Reply
  • May 23, 2020 at 10:18 pm
    Permalink

    And for the last question, readings and physical values can be compared throughout loading and discharge to calculate the offset. Another reason for inconsistent readings could be faulty, fouled or uncalibrated draught pressostats.

    Reply

Leave a Reply

Your email address will not be published. Required fields are marked *