Introduction The year 2018 edition of the Rules for Building and Classing Mobile Offshore Drilling Units consists of the nine (9) booklets as shown in Table 1. The purpose of the generic title ABS Rules for Conditions of Classification ?C Offshore Units and Structures (Part 1) is to reflect the expanded contents of PART 1@ as a result of including consolidated requirements for ??Classification?? applicable to all types of offshore units@ pipelines@ risers@ and other offshore structures@ as specified in the Foreword to Part 1. With regard to Part 2@ the purpose of the generic title ABS Rules for Materials and Welding is to emphasize the common applicability of the requirements to ABS-classed vessels@ other marine structures and their associated machinery@ and thereby make PART 2 more readily a common ??PART?? of various ABS Rules and Guides@ as appropriate. The numbering system applied in the Rules is shown in Table 2. 3. The primary changes from the 2017 edition of the Rules are identified and listed in Table The effective date of the indicated Rule Changes is 1 January 2018@ unless specifically indicated otherwise. The effective date of each technical change since 1993 is shown in parenthesis at the end of the subsection/paragraph titles within the text of each Part. Unless a particular date and month are shown@ the years in parentheses refer to the following effective dates: For certain changes that have a major impact on shipyards@ designers and vendors@ ABS has instituted a two-year Rule cycle to give the industry more time to prepare for these changes. The text of these changes will appear in a box along with the effective date just under the existing Rule requirement. Until the next edition of the Rules for Building and Classing Mobile Offshore Drilling Units is published@ Rule Change Notices and/or Corrigenda@ as necessary@ will be published on the ABS website ?C www. eagle.org ?C only@ and will be available free for downloading. It is not intended to publish hard copies of future Rule Change Notices and/or Corrigenda to existing Rules or Guides. The consolidated edition of the Rules for Building and Classing Mobile Offshore Drilling Units@ which includes Rule Change Notices and/or Corrigenda using different colors for easy recognition will be published on the ABS website only when RCN and/or Corrigenda are issued. The listing of CLASSIFICATION SYMBOLS AND NOTATIONS is available from the ABS website www.eagle.org for download.