ISO 19905-3:2021 Petroleum and natural gas industries — Site-specific assessment of mobile offshore units — Part 3: Floating units.
The limit states enumerated above apply to the hull and stationkeeping system of the mobile floating unit. They can also apply to other activity critical components for which assessments are required by this document (see 10.2), e.g. marine drilling riser and wellhead for drilling units.
NOTE I The (JLS is normally based on the unit being in a survival condition with, for example, the marine drilling riser disconnected on drilling units or equivalent survival preparations being undertaken on other types of unit.
NOTE 2 The limits of the SLS arc often based on, for example, the capability of the connected marine drilling riser on drilling units, or other limiting equipment on other types of unit. The ASOG specifies the circumstances under which the unit transitions from the SLS to the ULS condition.
For moored units and thruster-assist moored units, the stationkeeping damaged case (e.g. a single mooring line damage or loss of thruster assistance component(s) as determined through FMEA) shall be assessed (see ISO 19901-7) as part of the ULS condition.
It can be acceptable to continue site-specific activities with the unit in an impaired state (e.g. with a single mooring line damaged); in such cases, it shall be shown that:
— the unit in the impaired/damaged condition meets all the requirements of this document, including the requirement for an additional stationkeeping redundancy case (e.g. another mooring line damaged), and
— the failure was not the result of a systemic defect or error that can lead to other stationkeeping components failing under non-extreme conditions.
The FLS is generally addressed at the design stage. This document contains no specific requirements that fatigue be considered.
The ALS are generally addressed at the design stage. This document contains no specific requirements that the ALS be evaluated in the site-specific assessment, unless there are unusual risks at the site under consideration that were not considered at the design stage (see lth.3).
NOTE 3 Changes to the grade or strength of the mooring system can affect the fatigue capacity of certain hull and equipment components, e.g. fairleads, mooring winches, etc.
5.5 Determination of assessment situations
5.5.1 General
Provisions in ISO 19900 related to metocean conditions and their application shall be conformed with in conjunction with the further requirements of ISO 19901-1 and those of this document.
According to ISO 19900, assessment situations include all the service and operational requirements resulting from the intended use of the floating structure and the metocean conditions that could, affect the stationkeeping system, and any activity specific requirement limitations.ISO 19905-3 pdf download.