API RP 2I: THIRD EDITION
DOD Adopted ANSI Approved Approved
This recommended practice provides guidelines for inspecting mooring components of mobile offshore drilling units (MODUs) and permanent floating installations. Although this document was primarily developed for the moorings of MODUs and permanent floating installations, some of the guidelines may be applicable to moorings of other floating vessels such as pipe-laying barges and construction vessels. Furthermore, some of the guidelines may be applicable to secondary or emergency moorings such as moorings for jack-up units, shuttle tanker moorings, and dynamic positioning (DP) vessel harbor mooring.
The applicability of this document to the moorings of other floating vessels is left to the discretion of the user.
The need for rigorous, effective inspection of mooring hardware is apparent because most of the mooring failures involved faulty mooring components including corroded or physically damaged wire-rope or chain, defective connecting links, or mooring hardware of inferior quality. This document should be useful to engineers and operating personnel concerned with the following:
a) planning a mooring inspection;
b) conducting or supervising a mooring inspection;
c) deciding whether to reject, repair, or replace mooring hardware;
d) communicating with others concerning acceptable mooring hardware.
Inspection Philosophy and Exception to This Document
The inspection philosophy of this document is to remove a mooring component with excessive deterioration from service. Based on this philosophy, a criterion of limiting the strength reduction to 10 % minimum breaking strength (MBS) was established in the first edition of this document. This criterion has been used by the industry for more than 20 years with generally satisfactory results, and it has become a long standing and widely accepted criterion.
Inspection and Design Check
It should be emphasized that this document does not address the critical design issues such as tension factor of safety and fatigue, although some discussion is given to the design issue of corrosion allowance. Any attempt to link inspection with these critical design issues will make discard criteria a moving target, depending on design assumptions, analysis software used, margin of safety, and location of the operation, etc. Setting an industry inspection standard in this case is impossible. The design check should be conducted separately. If the design check indicates that the reliability of the mooring system can be overly compromised, the acceptance of a mooring component that passes the discard criteria should be carefully re-evaluated. On the other hand, if the design check indicates that the mooring component is significantly over-designed, and it can tolerate much more damage than allowed by this document, design calculations should be submitted to the appropriate authority asking for permission to take exception to API 2I. This process has been used by the industry under various conditions, and some examples are provided below.
— A MODU chain was found to have a large number of loose studs that exceeded the discard criteria and therefore should be replaced. However, the chain was accepted for continued service based on: 1) Break test of samples taken from the problem area indicated the chain retained more than 90 % MBS. 2) A fatigue analysis, taking into consideration the additional stress concentration at the stud footprint due to loose stud, indicated sufficient fatigue life for continuous operation.
— A mooring component was found to have lost 15 % of its strength, well exceeding the discard criteria of 10% MBS. A design check indicated the factors of safety were twice the required factors of safety for the operation. The component was accepted for continued service.
Safety of Inspection Personnel
Safety should be given high priority during mooring inspection. If a certain recommended inspection procedure poses a significant risk of jeopardizing the health and safety of the inspection personnel, the procedure should be modified to minimize the risk. However an effort should be made to ensure the inspection objectives are not compromised.
Mooring Component Traceability and Inspection Documentation
Since the inspection philosophy of this document is to remove a mooring component with excessive deterioration from service, it is important to keep a complete and auditable record of the component history. This component history shall be maintained in accordance with Annex A and shall include manufacturing, inspection, usage, and retirement records.
In cases where a complete component history for in-service mooring components is not available, decisions to keep a component in service should be based on its present condition and experience with components in similar services. Furthermore, a lack of historical documentation does not eliminate the need to maintain on-going documentation for future use.
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