Reference is made to the Preamble of 1989 MODU Code.
These guidelines supplement Section 4.11 of the 1989 MODU Code.
When an assessment of a MODU's anchoring performance according to the 1989
IMO MODU Code is required, these guidelines may be applied for units whose primary
method of position keeping is provided by a spread catenary anchoring system.
These guidelines may also be used for mobile offshore units (MOUs) other than
MODUs, e.g., accommodation units, tender vessels, crane barges, etc.
The design of a spread catenary anchoring system should be based upon the unit's
operating requirements in relation to the design specification of the anchoring system, the
system capability/operability, integrity and reliability. There should also be means of
relating the design limitations to the proposed locations and site specific environmental
Anchoring analysis should be developed in parallel with such operational practices
(e.g. line adjustment, and use of propulsion) as are achievable in a timely manner and
which are intended to be used in practice.
Anchoring analysis should be conducted to demonstrate that the unit can, either at
proposed locations and site specific conditions or generally, withstand:
- acceptable environmental conditions during operation; and
- maximum design environmental conditions.
The anchoring analysis should also cover the effect of single line failures.
The maximum design environmental condition used in the analysis should be
determined on the basis of a risk analysis considering both the local environmental
conditions (either site specific or generally for the area) and the consequences of anchoring
failure. In areas of tropical cyclone the maximum environment used in the analysis may
be lower than the actual extreme environment, provided that evacuation of personnel or
move of the unit is planned and will be executed before arrival of the tropical cyclone.
Factors of safety and other such criteria should be determined using appropriate
codes and practices relevant to the design specification, the location of the unit and the
hazard consequences of an anchoring failure. Applicable criteria may be found in coastal
State, flag State, classification society publications and/or ISO Floating System Standard.
The anchoring analysis employed should be fully compatible with the operational
requirements, e.g. riser analysis, riser angle limits for hanging off, disconnecting, loading
and unloading of hydrocarbons, or gangway connection.
Anchoring failure/risk evaluation
Anchoring failure can be considered as any situation arising outside of the design
specification, e.g. excessive excursion as a result of insufficient tension, single line failure
and progressive line failure leading to total system failure.
Hazards associated with anchoring failure should be identified, risk evaluated and
measures taken to reduce the risk to persons affected by those hazards to the lowest level
that is reasonably practicable.
Typical consequences which may result from the anchoring hazards could include:
- riser, BOP or wellhead problems;
- structural damage;
- grounding; and
- reduced stability, etc.
In determining anchoring failure consequences, consideration should be given to all
foreseeable hazards, including, but not limited to:
- failure of the emergency release mechanism, premature or delayed release;
- failure of structures supporting anchoring equipment, fairleads, winches, etc.;
- manufacturing defects including material and manufacturing processes;
- mechanical, electrical and hydraulic equipment failures, etc.;
- overloading the anchor holding capacities;
- in-service degradation due to corrosion, abrasion deployment retrieval, etc.;
- inadequate operating procedures, maintaining procedures handling procedures,
- exceeding the capacity of the system to perform its function; and
- operator error.
Note: Whilst such failures need to be recognised as possibilities in risk assessment,
measures should be adopted to prevent them wherever possible.
Specifications of the anchoring components such as anchor, chain, wire rope,
connection link buoys, winches and monitoring equipment should be in accordance with
those assumed or required by the anchoring analysis and should comply with the IMO
1989 MODU Code, Section 4.11. Attention should be given to the ability of the chain to
tolerate the heavy bending it suffers in winch and fairlead.
Surveys and inspections
An inspection/maintenance system should be established. All components in the
system are to be periodically inspected and maintained taking into consideration the manner
in which components may deteriorate (e.g. through wear and fatigue), the capability of the
inspection method, and the implications for component and system performance.
Operating procedures should be developed, on a site specific basis if necessary,
which include criteria for the safe operation of the system having regard to the activities
in which the unit is engaged (e.g. criteria for gangway or marine riser disconnection).
The assumptions made in the anchoring analysis and the conclusions of the analysis
should be reflected by the manner in which the actual anchoring system is set up and
maintained. In order to achieve this aim, any conclusions or limitations derived from the
anchoring analysis should be described in terms of procedures which can be successfully
Operational procedures for reducing anchoring line tensions such as active winching,
or using thrusters to redistribute tension loads should be clearly defined with written
procedures onboard. Personnel likely to be involved with these practices should be
competent, adequately trained, and aware of their roles.
Any extension of working limits on implementing operational procedures should be
clearly defined, with adequate justification.