The International Ship and Offshore Structures Congress (ISSC) is a forum for the exchange of information by experts undertaking and applying marine structural research. The aim of the ISSC is to facilitate the evaluation and dissemination of results from recent investigations, to make recommendations for standard design procedures and criteria, to discuss research in progress and planned, to identify areas requiring future research and to encourage international collaboration in furthering these aims. Ships and other marine structures used for transportation, exploration and exploitation of resources in and under the oceans are in the scope of the ISSC.The 20th International Ship and Offshore Structures Congress (ISSC 2018) was held in (Liège) Belgium and Amsterdam (The Netherlands), 9-14 September 2018. The first volume of the proceedings contains the eight Technical Committee reports presented and discussed at the conference and the second volume contains the reports of the eight Specialist Committees. This third volume contains the Official discusser's reports, written discussions and floor discussions, and the replies by the committees.