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• The qualitative response is the primary focus and therefore no specific dimensions and properties are shown in Figure 1. If needed, these properties can be obtained directly from the attached model file.
• The cable is not being modeled as moving over the pulleys. Instead, it is directly connected to the pulleys (joints 2 and 3) represented as springs which model the flexibility of the supporting structure. These springs resist load normal to the cable at the middle point of contact, but not tangentially. The local axes of the joints are chosen to bisect the angle between the cable segments entering and leaving the pulley. There is no consideration of the radius of the pulley except to locate the middle point of contact.
• Winching (lowering of the mass) is modeled by applying deformation load to the vertical cable. This moves the mass downward. Dynamical behavior of the mass is captured. Dynamics of the cable itself is not, presumably since it is resumed to be insignificant.
• Time history load Load case "Payout NL" is used to increase the deformation load of the vertical cable linearly in time, which would correspond to a constant speed of winching. The "Payout NL" load case uses time function "Payout start" at the beginning and the end of the winching to smooth the response to eliminate high frequencies in the results. This corresponds to the behavior of the actual structure for which the winching would start and stop with a gradual increase or decrease of speed.
• Since the model utilizes only cables cable elements which inherently include P-Delta and large-displacement effects, it is not necessary to specify these effects in the definition of the "Payout NL" load case. However, nonlinear direct-integration time-history is required to capture cable nonlinearity. A nonlinear static load case is used as initial conditions to apply dead load before beginning the winching operation.

Discussion of Results

The analysis results are presented in Figures 2 to 5.

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Figure 5 shows the forces in the vertical cable and the feeder cable that is connected to the winch. These cable forces are consistent with the mass displacement, velocity and acceleration plots. Note that the tension in the cable decreases somewhat during winching due to damping forces that are resisting part of the load. These forces will act on the supporting structure, which is a primary goal of the analysis.

Results will be sensitive to the damping assumed and to the shape of the transitions at the start and end of winching, so sensitivity studies may be warranted.

Figure 2: Mass displacement

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