Offshore structure heavy-lifts, i.e., drilling platforms, oil and gas production topsides, offshore wind turbine equipment, etc. are used to be transported by semi-submersible ships or self-propelled barges nowadays owing to their broad and flat cargo decks. Available securing is needed for cargo on deck due to sea environment loads. The inertial force to which heavy-lifts are subjected and caused by ship motion during transportation affects securing effectively. There have been three kinds of common methods for inertial force calculation, which are: 1) A 10-year-recurrent wave prediction based on sea area wave scatter diagram is taken as the designed wave for ship motion and loads response numerical simulation, calculating the inertial force via utilizing the simulation results. 2) A designed wave decided by wave scatter diagram, navigation time in targeted area and design probability of exceedance is used for ship motion and loads response numerical simulation so as for the calculation of inertial force. 3) The inertial force is calculated by the theoretical formulas in CCS guideline named GUIDELINES FOR TOWAGE AT SEA, according to navigation features and ship parameters and assuming that the ship is stuck in the most dangerous condition. It is different of calculation principle with each other for the three methods mentioned above. This paper simulated the ship motion and loads response utilizing the wave scatter diagrams in three typical sea areas, then it compared the simulation results of inertial accelerations with formular calculation results, finding that the theoretical calculation results are obviously bigger than the simulation results in a typical sea area of low latitude while are smaller than the simulation results in a typical sea area of middle or high latitude where is rough sea. Application of method 1 or method 2 is more economic for securing in a typical sea area of low latitude, considering safety of cargo transport and is safer in typical sea areas where are rough sea from middle latitude to high latitude. In addition, this study made it different in direction of wave for ship navigation with simulation of ship motion and loads response. To keep rolling amplitude relatively small in ship navigation, a proposal is come up with on the basis of simulation results in three typical sea areas from low latitude to high latitude, which is the semi-submersible ship should navigate in head sea or bow sea from 180 degrees to 140 degrees of wave directions at portside or starboard side in areas from middle latitude to high latitude where are rough sea with a different case that the range can be increased from 180 degrees to 120 degrees of wave directions in a typical sea area of low latitude, while the self-propelled barge can keep in a relative small rolling amplitude in wave directions from 180 degrees to 160 degrees.

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