3105F.2 Mooring Analyses
A mooring analysis shall be performed for each berthing system, to justify the safe berthing of the various deadweight capacities of vessels expected at the MOT. The forces acting on a moored vessel shall be determined in accordance with Section 3103F.5. Mooring line and breasting load combinations shall be in accordance with Section 3103F.8.
Two procedures, manual and numerical are available for performing mooring analyses. These procedures shall conform to either the OCIMF (MEG 3) [5.4] or UFC 4-159-03 [5.5]. The manual procedure (Section 3105F.2.1) may be used for barges.
A new mooring assessment shall be performed when conditions change, such as any modification in the mooring configuration, vessel size or new information indicating greater wind, current or other environmental loads.
In general, vessels shall remain in contact with the breasting or fendering system. Vessel motion (sway) of up to 2 feet off the breasting structure may be allowed under the most severe environmental loads, unless greater movement can be justified by an appropriate mooring analysis that accounts for potential dynamic effects. The allowable movement shall be consistent with mooring analysis results, indicating that forces in the mooring lines and their supports are within the allowable safety factors. Also, a check shall be made as to whether the movement is within the limitations of the cargo transfer equipment.
The most severe combination of the environmental loads has to be identified for each mooring component. At a minimum, the following conditions shall be considered:
- Two current directions (maximum ebb and flood; See Section 3103F.5.3)
- Two tide levels (highest high and lowest low)
- Two vessel loading conditions (ballast and maximum draft at the terminal)
- Eight wind directions (45 degree increments)
- The maximum allowable extension limits of the loading arms and/or hoses.
- The maximum allowable compression/deflection of the fender system.
Simplified calculations may be used to determine the mooring forces for barges with Favorable Site Conditions (see Table 31F-3-8) and no passing vessel effects (see Section 3105F.3.2), except if any of the following conditions exist (Figures 31F-5-2 and 31F-5-3).
- Mooring layout is significantly asymmetrical
- Horizontal mooring line angles (α) on bow and stern exceed 45 degrees
- Horizontal breast mooring line angles exceed 15 normal to the hull
- Horizontal spring mooring line angles exceed 10 degrees from a line parallel to the hull
- Vertical mooring line angles (θ) exceed 25 degrees
- Mooring lines for lateral loads not grouped at bow and stern
When the forces have been determined and the distance between the bow and stern mooring points is known, the yaw moment can be resolved into lateral loads at the bow and stern. The total environmental loads on a moored vessel are comprised of the lateral load at the vessel bow, the lateral load at the vessel stern and the longitudinal load. Line pretension loads must be added.
Four load cases shall be considered:
- Entire load is taken by mooring lines
- Entire load is taken by breasting structures
- Load is taken by combination of mooring lines and breasting structures
- Longitudinal load is taken only by spring lines
A numerical procedure is required to obtain mooring forces for MOTs that cannot use manual procedure. Computer program(s) shall be based on mooring analysis procedures that consider the characteristics of the mooring system, calculate the environmental loads and provide resulting mooring line forces and vessel motions (surge and sway).