Safe Mooring Design (MSC.1/Circ.1175/Rev.1)
Safe mooring design is an important part of ship design, equipment selection and structural verification. The IMO Revised Guidance on Shipboard Towing and Mooring Equipment, MSC.1/Circ.1175/Rev.1, provides guidance for shipboard fittings and supporting hull structures associated with towing and mooring operations, including the design loads, safe working loads, safe towing loads and information to be included in the towing and mooring arrangements plan.
Purpose of MSC.1/Circ.1175/Rev.1
MSC.1/Circ.1175/Rev.1, titled Revised Guidance on Shipboard Towing and Mooring Equipment, was approved by the IMO Maritime Safety Committee on 9 December 2020.
The revised guidance was developed to support a uniform approach to the application of the revised SOLAS regulation II-1/3-8 concerning towing and mooring equipment.
The guidance applies to ships constructed on or after 1 January 2024. For ships constructed on or after 1 January 2007 but before 1 January 2024, the earlier MSC.1/Circ.1175 remains applicable.
The principal focus of MSC.1/Circ.1175/Rev.1 is the design and construction of shipboard fittings and supporting hull structures associated with normal towing and mooring operations in harbours or sheltered waters.
Scope of the Revised Guidance
MSC.1/Circ.1175/Rev.1 addresses shipboard fittings and the supporting hull structures that transfer towing and mooring loads into the ship's structure.
Shipboard Fittings
The guidance covers fittings such as bollards, bitts, fairleads, pedestal rollers and chocks used for mooring and similar components used for normal or other towing.
Supporting Hull Structures
The supporting hull structure beneath or around the fitting must be capable of safely transmitting the applicable towing or mooring forces into the ship's structure.
Winches & Capstans
The hull structures supporting winches and capstans used for normal or other towing and mooring operations are also addressed by the guidance.
Towing & Mooring Arrangements
The towing and mooring arrangements plan identifies the intended use, load limits and relevant characteristics of fittings and mooring arrangements.
Applicability to Ships Constructed on or After 1 January 2024
The revised guidance applies to ships constructed on or after 1 January 2024, in accordance with the revised SOLAS framework.
The guidance does not supersede the earlier MSC.1/Circ.1175 for ships constructed on or after 1 January 2007 but before 1 January 2024.
Application should therefore always be determined using the vessel's construction date together with the applicable flag Administration, recognized organization and statutory requirements.
Before 1 January 2007
Applicability must be assessed against the applicable statutory, flag, class and design requirements for the individual vessel.
1 January 2007 – 31 December 2023
The earlier IMO Guidance on Shipboard Towing and Mooring Equipment, MSC.1/Circ.1175, remains applicable.
1 January 2024 Onwards
MSC.1/Circ.1175/Rev.1 applies to the ships within its scope under the revised SOLAS framework.
Special Towing
Certain special towing services are outside the scope of this guidance and may be subject to separate requirements.
What MSC.1/Circ.1175/Rev.1 Covers
The revised guidance establishes design principles intended to ensure that shipboard fittings and their supporting structures have adequate strength for the loads associated with their intended towing and mooring functions.
- Shipboard fittings used for normal towing.
- Shipboard fittings used for other towing applications within the scope of the guidance.
- Shipboard fittings used for mooring operations.
- Supporting hull structures beneath fittings.
- Supporting structures for mooring winches and capstans.
- Design loads and load directions.
- Safe towing load (TOW).
- Safe working load (SWL).
- Towing and mooring arrangements plan information.
- Recommended characteristics of mooring and tow lines within the scope of the guidance.
What the Guidance Does Not Cover
It is important not to interpret MSC.1/Circ.1175/Rev.1 as a complete specification for every mooring operation or every towing service.
The circular specifically states that it does not require tow lines and does not mandate standards for mooring lines on board the ship.
The guidance is also not applicable to the design and construction of shipboard fittings and supporting hull structures used for certain special towing services.
- Escort towing is subject to applicable local or specific escort requirements.
- Canal transit towing is subject to the requirements applicable to the relevant canal.
- Emergency towing for tankers is addressed under the applicable SOLAS requirements, including SOLAS II-1/3-4.
- Other specialized towing arrangements should be assessed against their specific applicable requirements.
Shipboard Fittings for Towing
Shipboard fittings used for towing should be arranged and supported so that towing loads can be efficiently transferred into the ship's structure.
The guidance identifies fittings such as bollards, bitts, fairleads, pedestal rollers and chocks as shipboard fittings within its scope.
Towing fittings should be located on stiffeners and/or girders forming part of the deck construction, or on equivalent arrangements where adequate strength for the intended service can be demonstrated.
Towing Load Considerations
For normal towing operations, the minimum design load applied to supporting hull structures is based on 1.25 times the intended maximum towing load, such as the static bollard pull indicated on the towing and mooring arrangements plan.
For other towing services within the scope of the guidance, the design basis is related to the ship design minimum breaking load of the tow line.
Safe Towing Load (TOW)
Safe Towing Load (TOW) is the safe load limit of shipboard fittings used for normal and other towing operations.
Under MSC.1/Circ.1175/Rev.1, TOW for normal towing operations should not exceed 80% of the applicable design load. The same 80% principle applies to other towing operations within the scope of the guidance.
The TOW of each fitting intended for towing should be marked in tonnes on the fitting. Where a fitting is intended for both towing and mooring, the applicable SWL should also be marked.
TOW
The safe towing load limit applicable to a shipboard fitting when used for towing.
Design Load
The load used as the structural design basis for the fitting and supporting hull structure.
Mooring Fittings & Supporting Structures
For mooring operations, the strength of shipboard fittings and their supporting hull structures must be considered together. The design should ensure effective distribution of mooring loads into the ship's structure.
Mooring fittings, winches and capstans should be located on suitable stiffeners and/or girders forming part of the deck construction, or on equivalent arrangements where the required strength can be demonstrated.
- Bollards and bitts.
- Fairleads.
- Chocks.
- Pedestal rollers.
- Mooring winches.
- Capstans.
- Foundations and supporting hull structures.
Mooring Design Loads
For shipboard fittings used for mooring, the minimum design load applied to the supporting hull structures should be based on 1.15 times the ship design minimum breaking load (MBLSD) of the mooring line.
For winches, the minimum design load applied to the supporting hull structure should be 1.25 times the intended maximum brake holding load.
The maximum brake holding load should be assumed to be not less than 80% of the ship design minimum breaking load (MBLSD) of the mooring line.
For capstans, the minimum design load applied to the supporting hull structure should be 1.25 times the maximum hauling-in force.
| Equipment / Structure | Design Basis | Principal Requirement |
|---|---|---|
| Mooring fittings | Ship Design Minimum Breaking Load (MBLSD) | 1.15 × MBLSD |
| Mooring winch supporting structure | Maximum brake holding load | 1.25 × maximum brake holding load |
| Capstan supporting structure | Maximum hauling-in force | 1.25 × maximum hauling-in force |
Safe Working Load (SWL)
For the purpose of marking, the Safe Working Load (SWL) of each shipboard fitting intended for mooring should be equal to the ship design minimum breaking load of the mooring line in accordance with the applicable guidance.
The SWL should be marked in tonnes on each fitting intended for mooring. Where a fitting is intended for both mooring and towing, the applicable TOW should also be marked.
The SWL provisions apply to the use of no more than one mooring line. The towing and mooring arrangements plan should define the intended method of using the mooring lines.
Load Direction, Alignment & Structural Reinforcement
Mooring and towing forces do not necessarily act in a single horizontal direction. The arrangement of fittings and their supporting structures should therefore consider the range of horizontal and vertical force directions that may occur.
The reinforcing members beneath fittings, winches and capstans should be effectively arranged for variations in the direction of the applied forces.
Proper alignment between the fitting and supporting hull structure is an important part of the design. Poor alignment or inadequate structural load paths can increase local stresses and compromise the intended performance of the arrangement.
Structural Verification
Under the design load conditions specified in the guidance, the allowable normal stress is taken as 100% and the allowable shearing stress as 60% of the specified yield point for the material used, subject to the provisions of the guidance.
Towing & Mooring Arrangements Plan
The towing and mooring arrangements plan is a key document for communicating the intended use and load limits of shipboard fittings.
The plan should be available on board for the guidance of the master and should identify the SWL and TOW applicable to each shipboard fitting.
Information for Each Fitting
- Location on the ship.
- Type of fitting.
- Safe Working Load (SWL).
- Safe Towing Load (TOW).
- Intended purpose: mooring, normal towing or other towing.
- Method of applying the towing or mooring line load.
- Applicable limiting fleet angle or change in direction of the line at the fitting.
Mooring Arrangement Information
- Arrangement of mooring lines.
- Number of mooring lines.
- Ship Design Minimum Breaking Load (MBLSD) of each mooring line.
- Length of each mooring line.
- Restrictions on material, construction, stiffness and diameter of compatible mooring lines.
- Applicable environmental conditions where required by the guidance.
Mooring Lines and MBLSD
Ship Design Minimum Breaking Load (MBLSD) means the minimum breaking load of new, dry mooring lines for which the shipboard fittings and supporting hull structures are designed to meet the mooring restraint requirements.
The selection of mooring lines is therefore closely connected with the design basis of the ship's mooring equipment and fittings.
The guidance provides recommended numbers and minimum strengths for mooring lines based on the ship's Equipment Number and, for ships with an Equipment Number above 2,000, the applicable side-projected area and other specified parameters.
The designer should also consider the adequacy of the mooring lines based on the individual mooring arrangement, expected shore-side mooring facilities and prevalent environmental conditions.
Equipment Number and Mooring Arrangement Design
For ships within the applicable scope, the Equipment Number (EN) is used in determining recommended mooring line arrangements and strengths.
For ships with an Equipment Number of 2,000 or less, the guidance provides tables for recommended numbers and minimum ship design minimum breaking loads of mooring lines.
For ships with an Equipment Number greater than 2,000, the guidance uses the side-projected area and identifies different mooring line functions, including head lines, stern lines, breast lines and spring lines.
Breast Lines
Breast lines primarily provide transverse restraint against movement away from the berth.
Spring Lines
Spring lines primarily provide longitudinal restraint against movement in the fore-and-aft direction.
Head Lines
Head lines contribute to restraint in both longitudinal and transverse directions, depending on their angle.
Stern Lines
Stern lines contribute to restraint in both longitudinal and transverse directions, depending on their angle.
Environmental Conditions & Mooring Design
Mooring arrangement design should take into account the environmental conditions relevant to the intended operation and the applicable design basis.
For ships with an Equipment Number greater than 2,000, the guidance identifies environmental parameters including wind speed and current conditions for the determination of recommended mooring line characteristics.
Where the applied design environmental criteria exceed the recommended criteria in the guidance, the towing and mooring arrangements plan should identify the applicable design environmental criteria.
- Wind speed and direction.
- Current speed and direction.
- Vessel side-projected area where applicable.
- Mooring location and shore-side facilities.
- Expected operational environmental conditions.
Relationship with Other IMO Safe Mooring Guidelines
MSC.1/Circ.1175/Rev.1 should not be considered in isolation from the other IMO instruments developed as part of the revised safe mooring framework.
MSC.1/Circ.1175/Rev.1
Revised Guidance on Shipboard Towing and Mooring Equipment. It focuses on shipboard fittings, supporting hull structures, towing and mooring loads, SWL, TOW and the towing and mooring arrangements plan.
MSC.1/Circ.1619
Guidelines on the Design of Mooring Arrangements and the Selection of Appropriate Mooring Equipment and Fittings for Safe Mooring.
MSC.1/Circ.1620
Guidelines for Inspection and Maintenance of Mooring Equipment Including Lines.
SOLAS II-1/3-8
The SOLAS regulatory framework establishes the mandatory requirements concerning towing and mooring equipment and safe mooring arrangements.
Safe Mooring Design and Occupational Safety
Safe mooring is not limited to the strength of individual fittings. The overall arrangement should also support safe operation and reduce exposure of personnel to foreseeable hazards during mooring activities.
The broader IMO safe-mooring framework places increased emphasis on the design of mooring arrangements, the selection of suitable equipment and fittings, and the inspection and maintenance of equipment including mooring lines.
Designers and ship operators should therefore consider the interaction between equipment arrangement, line leads, access, operational procedures and the areas where personnel may be exposed to mooring-line failure or other hazards.
Survey & Technical Verification Considerations
During a technical survey or pre-survey assessment, the physical condition of the mooring and towing arrangement should be considered together with the applicable design documentation.
- Towing and mooring arrangements plan.
- Identification and marking of SWL and TOW.
- Fitting type and intended operational purpose.
- Condition of bollards, bitts, fairleads, chocks and rollers.
- Condition of winches, capstans and associated machinery.
- Foundations and supporting hull structures.
- Mooring line specifications and MBLSD information.
- Applicable design limitations and fleet angles.
- Maintenance and inspection records where applicable.
- Consistency between the physical arrangement and approved documentation.
Common Technical Issues Identified During Mooring Verification
A technical review may identify discrepancies between the approved design information, the physical arrangement and the condition of the equipment.
- Missing or unclear SWL or TOW markings.
- Inconsistencies between fittings and the towing and mooring arrangements plan.
- Corrosion, deformation or structural deterioration of fittings.
- Cracking or deterioration in supporting structures.
- Damaged fairleads, chocks, rollers, bollards or bitts.
- Winch or capstan foundation deficiencies.
- Equipment whose intended operational use is unclear.
- Mooring line characteristics that are not compatible with the documented design basis.
- Missing or outdated technical documentation.
- Inadequate identification of operational limitations.
Preparing a Vessel for Mooring Design Verification
Owners and technical managers can improve survey readiness by reviewing the vessel's mooring and towing documentation before the attendance of the classification society, flag Administration or other authorized surveyor.
Documentation Review
Confirm that the towing and mooring arrangements plan, equipment information, load limits and applicable design documentation are available and consistent.
Physical Inspection
Examine fittings, foundations, winches, capstans, line leads and associated equipment for visible deterioration or damage.
Line Verification
Verify that installed or replacement mooring lines are compatible with the documented design requirements and equipment arrangement.
Corrective Action
Record deficiencies and establish appropriate corrective actions before formal survey or class attendance where practicable.
Technical Importance for Shipowners & Managers
Correctly designed and properly maintained mooring arrangements are fundamental to safe vessel operations in port and sheltered waters.
Structural Integrity
Appropriate load paths and supporting structures help transfer towing and mooring forces safely into the hull.
Operational Safety
Clear equipment limitations and appropriate arrangements support safer mooring and towing operations.
Survey Readiness
Consistent documentation and physical condition can help demonstrate compliance during applicable surveys.
Risk Reduction
Early identification of structural, equipment or documentation deficiencies can support timely corrective action.
Frequently Asked Questions
What is MSC.1/Circ.1175/Rev.1?
MSC.1/Circ.1175/Rev.1 is the IMO Revised Guidance on Shipboard Towing and Mooring Equipment. It provides guidance on shipboard fittings and supporting hull structures associated with towing and mooring operations and includes provisions concerning design loads, TOW, SWL and the towing and mooring arrangements plan.
Which ships does MSC.1/Circ.1175/Rev.1 apply to?
The revised guidance applies to ships constructed on or after 1 January 2024 within its scope. The earlier MSC.1/Circ.1175 remains applicable to ships constructed on or after 1 January 2007 but before 1 January 2024.
Does MSC.1/Circ.1175/Rev.1 apply to mooring lines?
The guidance includes design information and recommendations concerning mooring and tow lines, including the ship design minimum breaking load used as a design basis. However, the circular specifically states that it does not mandate standards for mooring lines on board the ship.
What is Safe Working Load (SWL)?
SWL is the safe load limit of shipboard fittings used for mooring operations in harbours or similar sheltered waters. Under the guidance, the SWL for marking purposes is equal to the ship design minimum breaking load of the applicable mooring line.
What is Safe Towing Load (TOW)?
TOW is the safe load limit of shipboard fittings used for normal and other towing operations within the scope of the guidance. The applicable TOW should be marked on fittings intended for towing.
What fittings are covered by the guidance?
Shipboard fittings include bollards, bitts, fairleads, pedestal rollers and chocks used for mooring and similar components used for normal or other towing. The supporting hull structures and the structures supporting relevant winches and capstans are also addressed.
What is the design load for mooring fittings?
For mooring fittings, the minimum design load applied to the supporting hull structure should be 1.15 times the ship design minimum breaking load of the mooring line, subject to the provisions of MSC.1/Circ.1175/Rev.1.
What is the design load for mooring winches?
The minimum design load applied to the supporting hull structure of mooring winches should be 1.25 times the intended maximum brake holding load. The maximum brake holding load should be assumed not less than 80% of the applicable ship design minimum breaking load of the mooring line.
Does the guidance cover special towing operations?
Not all special towing operations are covered. Escort towing, canal transit towing and emergency towing for tankers are specifically outside the applicable scope of this guidance and are subject to their respective requirements.
What is the towing and mooring arrangements plan?
It is the onboard plan identifying the location, type, SWL, TOW, intended purpose and method of applying loads for the shipboard fittings. It also provides relevant information on mooring line arrangements, MBLSD, line lengths, compatibility restrictions and applicable environmental criteria.
How does MSC.1/Circ.1619 differ from MSC.1/Circ.1175/Rev.1?
MSC.1/Circ.1175/Rev.1 focuses on shipboard towing and mooring equipment, fittings and supporting hull structures. MSC.1/Circ.1619 provides broader guidelines on the design of mooring arrangements and the selection of appropriate mooring equipment and fittings for safe mooring.
Can PAMS assist with mooring design and technical verification?
PAMS Pacific Admiralty Maritime Services provides independent marine surveying and technical consultancy support for vessel condition assessment, mooring and towing equipment verification, technical inspection and survey preparation. PAMS does not replace the flag Administration, classification society or authorized statutory surveyor.
PAMS Pacific Admiralty Maritime Services
PAMS Pacific Admiralty Maritime Services is an independent marine technical consultancy based in Manila, Philippines, providing marine surveying, marine technical consultancy, offshore engineering consultancy, FPSO engineering support, marine audits, technical inspections and maritime compliance services.
PAMS can provide independent technical support for shipowners, ship managers, operators, insurers and other maritime stakeholders, including vessel condition assessment, technical inspections, survey preparation and review of mooring and towing arrangements.
PAMS operates independently and does not provide ship agency, crewing or manning, recruitment or staffing, bunker supply, freight or logistics services, STCW training or commercial ship management.
Need Mooring or Towing Technical Support?
Contact PAMS Pacific Admiralty Maritime Services for independent marine surveying and technical consultancy support relating to mooring and towing equipment, vessel condition assessment, technical inspection and survey preparation.