A jack-up with displacement of 10,000 kips has its LCG 100 feet aft of frame zero (AFO). If 200 kips are loaded at 60 feet AFO and 100 kips are discharged from 20 feet AFO, what is the new LCG?
• Longitudinal Center of Gravity (LCG) calculation using moments about a reference frame • Effect of adding a weight versus removing a weight on the vessel’s total moment and displacement • Using the relationship: LCG = (Sum of longitudinal moments) / (Total displacement)
• First, compute the original total longitudinal moment of the jack-up using the given displacement and original LCG. What is that value? • Next, determine the moment effect of adding 200 kips at 60 feet AFO and the moment effect of discharging 100 kips at 20 feet AFO. How do you treat the sign of these changes? • After adjusting both the total displacement and the total moment, how do you calculate the new LCG from these updated values?
• Be sure you use consistent units (kips and feet) when calculating moments (kip-feet). • When discharging a weight, verify that you are subtracting both the weight and its moment from the totals, not just the weight. • Before picking an answer, double-check that your final LCG is reasonable compared to the original 100 feet AFO — is it expected to shift forward or aft based on where the weights were added/removed?
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