Petroleum and natural gas industries -
Specific requirements for offshore
structures - Part 2: Seismic design
procedures and criteria
(ISO 19901-2:2017, MOD)
This document contains requirements for defining the seismic design procedures and criteria for
offshore structures; guidance on the requirements is included in Annex A. The requirements focus on
fixed steel offshore structures and fixed concrete offshore structures. The effects of seismic events on
floating structures and partially buoyant structures are briefly discussed. The site-specific assessment
of jack-ups in elevated condition is only covered in this document to the extent that the requirements
are applicable.
Only earthquake-induced ground motions are addressed in detail. Other geologically induced hazards
such as liquefaction, slope instability, faults, tsunamis, mud volcanoes and shock waves are mentioned
and briefly discussed.
The requirements are intended to reduce risks to persons, the environment, and assets to the lowest
levels that are reasonably practicable. This intent is achieved by using:
a) seismic design procedures which are dependent on the exposure level of the offshore structure and
the expected intensity of seismic events;
b) a two-level seismic design check in which the structure is designed to the ultimate limit state (ULS)
for strength and stiffness and then checked to abnormal environmental events or the abnormal
limit state (ALS) to ensure that it meets reserve strength and energy dissipation requirements.
Procedures and requirements for a site-specific probabilistic seismic hazard analysis (PSHA) are
addressed for offshore structures in high seismic areas and/or with high exposure levels. However, a
thorough explanation of PSHA procedures is not included.
Where a simplified design approach is allowed, worldwide offshore maps, which are included in
Annex B, show the intensity of ground shaking corresponding to a return period of 1 000 years. In
such cases, these maps may be used with corresponding scale factors to determine appropriate seismic
actions for the design of a structure.
For design of fixed steel offshore structures, further specific requirements and recommended values
of design parameters (e.g. partial action and resistance factors) are included in ISO 19902, while those
for fixed concrete offshore structures are contained in ISO 19903. Seismic requirements for floating
structures are contained in ISO 19904, for site-specific assessment of jack-ups and other MOUs in
ISO 19905 (all parts), for arctic structures in ISO 19906 and for topsides structures in ISO 19901-3.
