In the previous report, we derived extensible catenary equations and discussed the fundamental characteristics of catenary. We also descrived the ways to solve the equations under various boundary conditions. In this paper, we introduce a program package based on the extensible catenary theory. Next, a simplified dynamic analysis method is shown. The accuracy of calculation by the program package is verified by comparing with the results of tank tests or calculations with other programs. Finally, initial design method of underwater line structures using the program package is shown and typical line structures such as mooring lines, towing lines and flexible marine risers are designed easily applying this method.