General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)

SEA /REL (part 2) for API 2C
(Requires GHS version 16.50 or later with SK module)

Despite the name of this week's COW, we won't actually be using the /REL parameter directly. Instead, we will use a run file to compute the relative velocities required by the API 2C Vessel Specific Method for on- or off-board crane design loads. The concept of relative motion (as introduced in COW127) is still important, however, as we need to understand the physics of the underlying scenario before we start plugging things into SET commands.

In this example, we will only focus on the vertical design loads for an off-board lift, assuming a known Safe Working Load (SWL), but the concepts herein can be easily extended to compute horizontal design loads as well.

The primary goal is to compute the Dynamic Coefficient, Cv, according to Equation 4.1 in API 2C. This equation (given below) requires the input of a relative vertical velocity, Vr, as shown:

[We must admit a great deal of satisfaction in sneaking not one, but two, equations into this week's COW.]

Drawing on our knowledge of relative motions from COW127, it is clear that Vr is simply a vector summation of the deck and crane tip velocities (which are assumed to have a random phase relationship) and then the addition of the vertical hoist velocity (which is constant and in the vertical direction) to yield the expected maximum relative vertical velocity of the lift, Vr.

The first two velocity terms, Vh and Vd, can be computed using Equation 4.2 and Table 2 from API 2C. Vc, on the other hand, must be computed using the SEAKEEPING command. To do this, we place a Critical Point at the boom tip in the expected operating condition. We also specify a seaway using a JONSWAP "J2" spectrum with a significant wave height of 5 feet, a peak period of 6 seconds, and a heading of 165 degrees, as shown below.


.operating_cnd `set operating load condition
.move_crane `move crane sail parts for visual effect

crt (1) "Boom Tip" 253a 117s 132

set Hsig=5 `ft

st crt
wave (spe) j2 6,{Hsig} `peak period, sig. wave height
sea /heading:165 /samp:15 /crt:1 /noplot

report /preview
input "Enter Vc: " Vc

For this example, we will select a design amplitude equal to the average maximum in 1000 cycles. After running SEAKEEPING, Vc is found in the Velocity Statistics Table in the report as shown:

The equations for Vr and Cv are then implemented in macros called get_Vr and get_Cv. After inputing Vc from the table above, and any additional parameters, we obtain the Dynamic Coefficient (Cv) and the Vertical Design Load, as shown below:

The files needed to recreate this example may be found here.

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