Standards Development

Scope

This part of ISO 19901 specifies methodologies for:

a) the design, analysis and evaluation of station-keeping systems for floating structures used by the oil and gas industries to support any combination of:

1) production,

2) storage,

3) offloading,

4) drilling and well intervention.

b) the assessment of station-keeping systems for site-specific applications of mobile offshore units and construction units.

Most station-keeping systems used with the class of floating structures covered by a) are termed “permanent mooring systems”, for which this part of ISO 19901 is applicable to all aspects of the life cycle and includes requirements relating to the manufacture of mooring components, as well as considerations for in-service inspections. Most station-keeping systems used with mobile offshore units, the class covered by b), are termed “mobile mooring systems”. Throughout this part of ISO 19901, the term “floating structure”, sometimes shortened to “structure”, is used as a generic term to indicate any member of the two classes, a) and b).

This part of ISO 19901 is applicable to the following types of station-keeping systems, which are either covered directly in this part of ISO 19901 or through reference to other guidelines:

i) spread moorings,

ii) single point moorings

iii) dynamic positioning systems,

iv) thruster-assisted moorings.

This part of ISO 19901 is not applicable to:

— station keeping systems which do not have redundancy against failure of any single component (e.g., single anchor leg moorings (SALMs)),

— station keeping systems which use any means other than mooring lines or thrusters such as tower soft yoke systems, or tension leg platforms (TLPs) that are using tendons.

The requirements for this part of ISO 19901 address spread mooring systems and single point mooring systems with mooring lines composed of steel chain, steel wire or synthetic fibre rope.

Descriptions of characteristics and typical components found in these systems are given in Annex A.

This document includes requirements relating to the selection of mooring components, mooring system configuration and performance, components design, installation, post-installation survey, and as-installed assessments as needed for mooring integrity management.

The procedures for the design of permanent or site assessment of mobile mooring systems specified in this document are based on a deterministic approach where mooring system responses (such as line tensions, vessel offsets, and anchor loads) are evaluated for a design environment defined by an annual probability of exceedance or return period. Mooring system responses are then checked against acceptance criteria for mooring strength, offsets and orientation, clearances, anchor capacity, fatigue resistance, etc. The minimum acceptance criteria are either defined in this document or are to be specified by the Operator.

NOTE 1 Station-keeping systems designed based on this deterministic approach might have differing levels of reliability.

For moored structures (vessels), system responses are calculated and compared to minimum acceptance criteria for:

— Ultimate limit states (ULS): Mooring component strength. Vessel offset, orientation, and clearance constraints. Herein the ULS includes both intact and single failure condition for station-keeping systems.

— Serviceability limit states (SLS): Vessel offset, orientation, and clearance constraints. For mooring components this includes clearances with the vessel, risers, umbilicals, seabed, water surface, field infrastructure, exclusion zones, etc.

— Fatigue limit states (FLS) : Cumulative mooring component fatigue damage.

— Accidental limit state (ALS): no criteria are given for accidental or abnormal limit state which are left to owner decision or local Authorities requirements.

The methodology described in this part of ISO 19901 identifies a set of coherent analysis techniques that, combined with an understanding of the site-specific metocean conditions, the characteristics of the floating structure under consideration, and other factors, can be used to determine the adequacy of the station-keeping system to meet the functional requirements of this document.

NOTE 2 For moorings deployed in ice-prone environments, additional requirements are given in ISO 19906 subclause 13.7.