FLOATER RESPONSE FROM CRANE LIFTS

by J. Vazquez

August 20, 2019

This article is intended to provide a fun way of acquiring a better understanding on a recurring topic that somehow I've managed to live with, in the absence of absolute clarity. I am hoping I can get your participation, even if naval architecture is not necessarily your field of expertise. For the record, I am not a naval architect, though I have done a lot of work on offshore structures.

Consider the simplest of floating barges (a perfect rectangular shape), with known mass distribution, a crane and only two movable cargo boxes, as depicted in Figure 1, below. For simplicity, and to avoid any issues having to do with possible changes in the TCG, we'll assume that the crane is only capable of lifting both cargo boxes simultaneously. Assuming 4 possible conditions, as listed below, what would be the inclination angle, θ, due to a wind-induced Moment?

Just for "fun" question:

It should be obvious that if the cargo being lifted was originally resting on the deck of the barge, then the act of lifting this weight (assuming the crane tip is directly above it) will not cause any barge inclination. But, would the response of the barge to a given wind-induced moment, Mw, be different for the depicted conditions? In other words, does the vertical position of the weight being lifted affect the rotational stiffness (degree of stability) of the barge?

If you are in agreement that the answer is YES, THE ROTATIONAL STIFFNESS OF THE BARGE CHANGES FROM WHEN THE WEIGHT IS RESTING ON THE DECK VS WHEN THE WEIGHT IS LIFTED FROM THE DECK, then consider this follow-up question:

Once the cargo weight is lifted off the deck, does the system become more and more tender (i.e., does the rotational stiffness or GM value decrease more and more) as the cargo weight moves higher and higher from the deck? Or, once lifted, does the GM remain unchanged (provided the crane tip is not moved)?

Multiple Choice answers:

a) Yes, the system does become more and more tender as the lifting cable is reeled in.

b) No, the system does not become more tender as the lifting cable is reeled in. The rotational stiffness is unchanged once the weight is off the deck, even as the cable is reeled in.

c) No, the system does not become more tender as the lifting cable is reeled in. Instead, after the rotational stiffness changes (abruptly) the instant the cargo weight separates form the deck, the rotational stiffness increases as the calbe is reeled in (since the length of the hanging cable gets shorter and shorter).

Another way of looking at this is as follows:

Assume that each of the three cases depicted below have the exact same rotation angle due to an applied moment, and further assume that they all started from an even keel position. The left-most depiction has the weights resting on the deck. The middle depiction is for the case when the weights have been lifted a small distance above the deck and the right-most depiction is for the case when the weights have been lifted to a higher elevation, but not quite all the way to the crane tip. Assuming we are in agreement that it will take a larger applied moment to produce the same rotation on the left-most depiction (when the weights are resting on the deck) than for the other two cases (when the weights have been lifted off the deck), then the follow-up question is whether it takes a larger moment for the middle case than for the right-most case to produce the exact same rotation?

I'll provide my answer in the next issue, but I'd really like to get some answers (and preferably explanations) from you.