API RP‐2MIM (API 2019b) recommends monitoring of the mooring system to continouosly verify its condition and performance. This may include direct measurements of the mooring response (e.g. line tension, strains) or measures than can indirectly predict mooring response such as vessel offset. Data from a position‐monitoring system can be used to ascertain that the mooring system is meeting design requirements. Environmental monitoring may also be used to provide metocean data to support the position monitoring. Monitoring for individual line performance is also useful to detect a line failure or loss of tension in the mooring system.
API RP‐2MIM (API 2019b) recommends determining assessment initiators, based on a change of condition outside the limits of the original design basis or from a recent in‐service assessment. Possible assessment initiators could result from, for example, damage, changes in station‐keeping performance and stability. To demonstrate an acceptable risk level for the mooring system, the likelihood of failure can be estimated by the use of a suitable risk assessment method for several different cases. These range from as‐originally‐designed to the as‐is‐condition using current design criteria. For the risk acceptance criteria, pre‐requisites are listed including defining mooring system failures and acceptable risk levels and demonstrating acceptable levels for the as‐is or planned configurations using the results from the previous analyses. If a mooring system does not pass the fitness for service criterion, risk reduction measures that reduce risk to an acceptable level are required. These measures can include changed operational procedures and modifications to the system. In addition, modifications to inspection and monitoring programs can be implemented to identify any further deterioration and reducing the likelihood of any undesired consequences.
Unlike other integrity management standards, API RP‐2MIM (API 2019b) addresses emergency response planning based on unexpected failures of a mooring system. Single line failure can escalate to multiple line failure quickly, requiring a range of options to be considered to develop a response plan. API RP‐2MIM (API 2019b) recommends that at least a short‐term emergency response plan should be in place before operations commence. In addition, there is a requirement for an immediate operational response plan. This immediate response plan needs to provide operators with a set of operating procedures that address a range of reasonably foreseeable situations that could occur both under normal and severe weather conditions. Actions arising from the plan could include ceasing production or limiting operations with defined limits.
API RP‐2MIM (API 2019b) recommends that a conservative approach is adopted until the nature of the failure is understood and any escalation ruled out. Inspections can be undertaken to provide information of the degradation mechanism to be identified. This then allows an assessment to be made of the potential for multiple degradation mechanisms to exist, which would affect the emergency response planning.
The short‐term plan should include procedures to address situations of substantial or imminent loss in mooring capacity, requiring emergency repairs to be made to the degraded mooring system sufficient to create the necessary time for a more permanent repair.
The medium‐term emergency response could entail a complete repair of the mooring system, eliminating the recorded degradation mechanism. It is recognised that there may be an intervening period between the inspection finding and the short‐term repair response during which the mooring system could operate in a degraded state, where accepted procedures for stopping operations and production shutdowns depending on the observed degree of degradation are needed.
Overall, API RP‐2MIM (API 2019b) is a comprehensive document relating to the integrity management of mooring systems and the only (to the knowledge of the authors) integrity management standard that includes a proper guidance for incident response planning and execution.
2.4.3 IACS Guideline for Survey of Offshore Moorings
A set of guidance for surveys of the moorings of floating units has been produced by IACS (2010). It states the minimal requirements for an annual survey, the scope of which are mooring components adjacent to the winch or windlass. It should be conducted with the vessel at operational draft with the position mooring system in use. This is to be supplemented by a more extensive survey if typical damage is present such as:
chain: reduction in diameter exceeding 4%, missing studs, loose studs in grade 4 chain;
wire rope: obvious flattening or reduction in area, worn cable lifters causing damage to the rope, severe wear or corrosion, broken wires.
Special periodical surveys are also required at approximately 5‐year intervals. These special surveys are quite demanding and should include the following:
close visual examination of all chain links, with cleaning if required;
enhanced NDE sampling of:5% of the chain20% of chain which has been in the proximity of fairleads in the last 5 yearsall connecting links
dimensional checks, including lengths over five links;
checking looseness and pin‐securing arrangements of joining shackles;
measurement of thickness (diameter) of approximately 1% of all chain links distributed through the working length of the chain; and
dismantling and MPI of all Kenter‐type shackles which have been in service for more than 4 years.
2.5 Standards and Guidance Notes for Concrete Structures
2.5.1 Introduction
The international recognised standard for designing, fabricating and maintaining offshore concrete structures is the ISO 19903 (ISO 2019b), originally introduced in 2006 and updated in 2019. The ISO standard was based on the Norwegian standard NS 3473 with input from other relevant standards. Previously, the Department of Energy introduced a set of Guidance Notes for offshore concrete structures in 1974 which was significantly updated in the fourth edition, published in 1990 (Department of Energy 1990) and supported by a background document on concrete (HSE 1997). The emphasis of these documents, however, was on design and construction of offshore concrete structures with limited sections dealing with inspection and repair. In addition, the Norwegian standard NORSOK N‐005 (Standard Norge 2017b) has some international status. NORSOK N‐005 includes a special annexe covering inspection of concrete structures. The relevant sections of ISO 19903, the Department of Energy Guidance Notes and NORSOK N‐005 are discussed below.
2.5.2 ISO 19903—Concrete Structures
ISO 19903 (ISO 2019b) includes an inspection programme for concrete structures similar to that in, for example, ISO 19902 for steel structures, requiring an initial (baseline), periodic and special inspection to be performed. The initial inspection is required as soon as possible after installation to verify the original design and that all the major parts of the installed structure have no obvious defects. Following this initial inspection, inspection and condition monitoring of the structure shall be carried out regularly in accordance with the established programme. Special inspections should be conducted after direct exposure to an accidental or design environmental event (wave, earthquake, etc.). These inspections should encompass the critical areas of the structure. However, following an accidental event such as a boat collision and a dropped object, the inspection may, in certain circumstances, be limited to the local area of damage. In addition,