Section 3105F Mooring and Berthing Analysis and Design
This section establishes minimum standards for safe mooring and berthing of vessels at MOTs.
This section applies to onshore MOTs; Figure 31F-5-1 shows typical pier and wharf configurations.
Multiple berth MOTs shall use the same conditions for each berth unless it can be demonstrated that there are significant differences.
MOTs shall have the following equipment in operation:
- An anemometer (N/E).
- A current meter in high velocity current (>1.5 knots) areas (N/E).
- Remote reading tension load devices in high velocity current (>1.5 knots) areas and/or with passing vessel effects for new MOTs.
- Mooring hardware in accordance with Section 3103F.10 (N/E).
Berthing systems shall be in accordance with Section 3105F.4 (N/E).
Quick release hooks are required at all new MOTs, except for spring line fittings. Quick release hooks shall be sized in accordance with Section 3103F.10. To avoid accidental release, the freeing mechanism shall be activated by a two-step process. Quick release hooks shall be insulated electrically from the mooring structure, and shall be supported so as not to contact the deck.
Section 3105F.5 and the OCIMF guidelines [5.4] shall be used in designing the mooring layout.
The existing condition of the MOT shall be used in the mooring analysis (see Section 3102F). Structural characteristics of the MOT, including type and configuration of mooring fittings such as bollards, bitts, hooks and capstans and material properties and condition, shall be determined in accordance with Sections 3107F.4 and 3103F.10.
The analysis and design of mooring components shall be based on the loading combinations and safety factors defined in Sections 3103F.8 through 3103F.10, and in accordance with ACI 318 [5.1], AISC 325 [5.2] and ANSI/AWC NDS [5.3], as applicable.
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).
MOTs are generally located in sheltered waters such that typical wind waves can be assumed not to affect the moored vessel if the significant wave period, Ts, is less than 4 seconds. However, if the period is equal to or greater than 4 seconds, then a simplified dynamic analysis (See Section 3103F.5.4) is required. The wave period shall be established based on a 1-year significant wave height, Hs. For MOTs within a harbor basin, the wave period shall be based on the locally generated waves with relatively short fetch.
These forces generated by passing vessels are due to pressure gradients associated with the flow pattern. These pressure gradients cause the moored vessel to sway, surge, and yaw, thus imposing forces on the mooring lines.
Passing vessel analysis shall be conducted when all of the following conditions exist (See Figure 31F-5-4):
- Passing vessel size is greater than 25,000 DWT.
- Distance L is 500 feet or less
- Vessel speed V is greater than Vcrit
L and B are shown in Figure 31F-5-4, in units of feet. V is defined as the speed of vessel over land minus the current velocity, when traveling with the current, or the speed of vessel over land plus the current velocity, when traveling against the current.
When such conditions (1, 2 and 3 above) exist, the surge and sway forces and the yaw moment acting on the moored vessel shall, as a minimum, be established in accordance with Section 3103F.5.5 or by dynamic analysis.
For MOTs located in ports, the passing distance, L, may be established based on channel width and vessel traffic patterns. The guidelines established in the Department of Defense, UFC 4-150-06, Figure 5-17 [5.6] for interior channels may be used. The "vertical bank" in Figure 5-17 of [5.6] shall be replaced by the side of the moored vessel when establishing the distance, "L."
For MOTs, not located within a port, the distance, "L," must be determined from observed traffic patterns.
The following passing vessel positions shall be investigated:
- Passing vessel is centered on the moored ship. This position produces maximum sway force.
- The midship of the passing vessel is fore or aft of the centerline of the moored ship by a distance of 0.40 times the length of the moored ship. This position is assumed to produce maximum surge force and yaw moment at the same time.
A seiche analysis is required for existing MOTs located within a harbor basin and which have historically experienced seiche. A seiche analysis is required for new MOTs inside a harbor basin prone to penetration of ocean waves.
The standing wave system or seiche is characterized by a series of "nodes" and "antinodes." Seiche typically has wave periods ranging from 20 seconds up to several hours, with wave heights in the range of 0.1 to 0.4 ft [5.6].
The following procedure may be used, as a minimum, in evaluating the effects of seiche within a harbor basin. In more complex cases where the assumptions below are not applicable, dynamic methods are required.
- Calculate the natural period of oscillation of the basin. The basin may be idealized as rectangular, closed or open at the seaward end. Use Chapter 2 of UFC 4-150-06 [5.6] to calculate the wave period and length for different modes. The first three modes shall be considered in the analysis.
- Determine the location of the moored ship with respect to the antinode and node of the first three modes to determine the possibility of resonance.
- Determine the natural period of the vessel and mooring system. The calculation shall be based on the total mass of the system and the stiffness of the mooring lines in surge. The surge motion of the moored vessel is estimated by analyzing the vessel motion as a harmonically forced linear single degree of freedom spring mass system. Methods outlined in a paper by F.A. Kilner [5.7] can be used to calculate the vessel motion.
- Vessels are generally berthed parallel to the channel; therefore, only longitudinal (surge) motions shall be considered, with the associated mooring loads in the spring lines. The loads on the mooring lines (spring lines) are then determined from the computed vessel motion and the stiffness of those mooring lines.
The kinetic berthing energy demand shall be determined in accordance with Section 3103F.6.
For existing MOTs, the berthing energy capacity shall be calculated as the area under the force-deflection curve for the combined structure and fender system as indicated in Figure 31F-5-5. Fender piles may be included in the lateral analysis to establish the total force-deflection curve for the berthing system. Load-deflection curves for other fender types shall be obtained from manufacturer's data. The condition of fenders shall be taken into account when performing the analysis.
When batter piles are present, the fender system typically absorbs most of the berthing energy. This can be established by comparing the force-deflection curves for the fender system and batter piles. In this case only the fender system energy absorption shall be considered.
A continuous fender system consists of fender piles, chocks, wales, and rubber or spring fender units.
The contact length of a ship during berthing depends on the spacing of the fender piles and fender units, and the connection details of the chocks and wales to the fender piles.
The contact length, Lc, can be calculated using rational analysis considering curvature of the bow and berthing angle.
In lieu of detailed analysis to determine the contact length, Table 31F-5-1 may be used. The contact length for a vessel within the range listed in the table can be obtained by interpolation.
|VESSEL SIZE (DWT)||CONTACT LENGTH|
|1,000 to 2,500||35 ft|
|5,000 to 26,000||40 ft|
|35,000 to 50,000||50 ft|
|100,000 to 125,000||70 ft|
For discrete fender systems (i.e., not continuous), one fender unit or breasting dolphin shall be able to absorb the entire berthing energy.
The longitudinal and vertical components of the horizontal berthing force shall be calculated using appropriate coefficients of friction between the vessel and the fender. In lieu of as-built data, the values in Table 31F-5-2 may be used for typical fender/vessel materials:
|CONTACT MATERIALS||FRICTION COEFFICIENT|
|Timber to Steel||0.4 to 0.6|
|Urethane to Steel||0.4 to 0.6|
|Steel to Steel||0.25|
|Rubber to Steel||0.6 to 0.7|
|UHMW* to Steel||0.1 to 0.2|
*Ultra-high molecular weight plastic rubbing strips.
F = longitudinal or vertical component of horizontal berthing force
µ = coefficient of friction of contact materials
N = maximum horizontal berthing force (normal to fender)
For guidelines on new fender designs, refer to UFC 4-152-01 [5.8] and PIANC [5.9]. Velocity and temperature factors, contact angle effects and manufacturing tolerances shall be considered (see Appendices A and B of PIANC [5.9]). Also, see Section 3103F.6.