Mooring loads

The general design philosophy of a marine structure equipped with bollards / quick-release hooks considers a specific sequence of structural failure for elements within the mooring system. This sequence typically includes the mooring lines, winches, bollards, and connections between the bollards and flexible dolphins. The philosophy assumes an inherent ‘fail-safe’ design where the mooring line starts to render out before; the mooring line breaks, the mooring hook/bollard fails, and the stability of the whole mooring structure is compromised.

BS6349-1-2 [6-8], OCIMF MEG4 [6-9], PIANC 153B [6-10] and EAU 2020 [6-11] provide comprehensive guidance for designing marine facilities equipped with bollards and quick-release hooks. Additional guidance can be found in BS 6349-1-2 and BS 6349-2, which outlines five methods for determining mooring loads, with two methods being particularly relevant. Those two relevant methods are (1) mooring actions based on a (dynamic) mooring analysis and/or (2) mooring actions based on the ship’s equipment.

Mooring loads can be derived through either static or dynamic mooring analysis methods. These approaches involve a thorough assessment of wind, wave and current-induced forces acting on the moored vessel. The distribution of these forces is estimated on the basis of an elastic calculation taking into account the actual mooring configuration, such as pattern and number of mooring lines. This can be (as per BS6349-1-2):

Another approach is the assessment of mooring load actions based on the ship’s operational mooring equipment capacity. This is considered to be an extreme operating environmental condition.

The accidental situations may be assessed based on the maximum total load on a mooring point by:

For vessels up to 20,000t loaded displacement the following nominal bollard load can be maintained in accordance with BS 6349-1-2, PIANC 153B and EAU 2020. Standard partial safety factors for variable loads can be applied. For vessels over 20,000t these values should be considered for preliminary designs only.

The loads acting on bollards/mooring hooks can be determined by (dynamic) mooring analysis (Table 6-16) or in accordance with the ship’s equipment philosophy of BS 6349-1-2 (Table 6-15). The following partial safety factor approach can be used for determining loads on bollards and mooring hooks:

Some definitions:

Note that a different consideration of extreme operating mooring loads based on equipment may apply for bollards on continuous quay structures, because different use and different angles of mooring lines result in slightly different failure modes. The EAU 2020 provides a more detailed representation of mooring loads for continuous quays based on the ship’s equipment, while this handbook focuses on flexible dolphins, often being part of jetty structures.

¹⁾ The actual loads depend on the location where the ship is moored (sheltered, exposed) and the number of lines deployed. Loads may also be significantly lower if the location is very benign

²⁾ Based on an operational brake render load of 60% x MBL (this also applies for 1 line, but since the hook/bollard capacity should always be larger than 1 x MBL, this is selected as minimum)

³⁾ The factor 1.18 is based on specified minimum yield stress of winch brake and winch foundation, with mooring line loaded to 60% MBL (operational brake render load), 80% MBL (design brake maximum holding load) and 100% MBL (minimum braking load)

⁴⁾ Bollard SWL is based on extreme operating condition, while the ultimate load on the bollard could be higher for accidental situations; this ultimate load should also be taken into account for bollard design

⁵⁾ The SWL of an individual hook is always larger than the MBL of the line

⁶⁾ The working load/operating limit varies per mooring line type (steel wire line 55% and all other lines such as synthetic lines 50%), a safe mooring limit exists based on line and fender forces, and a safe loading limit based on vessel excursions during loading operations