3103F.5 Mooring Loads on Vessels
Forces acting on a moored vessel may be generated by wind, waves, current, tidal variations, tsunamis, seiches and hydrodynamic effects of passing vessels. Forces from wind and current acting directly on the MOT structure (not through the vessel in the form of mooring and/or breasting loads) shall be determined in Section 3103F.7.
The vessel's moorings shall be strong enough to hold during all expected conditions of surge, current and weather and long enough to allow adjustment for changes in draft, drift and tide (2 CCR 2340) [3.4].
The design wind speed is the maximum wind speed of 30-second duration used in the mooring analysis (see Section 3105F).
The operating condition is the wind envelope in which a vessel may conduct transfer operations. It is determined from the mooring analysis (Section 3105F). Transfer operations shall cease, at an existing MOT, when the wind exceeds the maximum velocity of the envelope.
The survival condition is defined as the state wherein a vessel can remain safely moored at the berth during severe winds. For new MOTs, the survival condition threshold is the maximum wind velocity, for a 30-second gust and a 25-year return period, obtained from historical data.
For an existing MOT, a reduced survival condition threshold is acceptable (see Figure 31F-2-1). If the wind rises above these levels, the vessel must depart the berth; it shall be able to depart within 30 minutes (see 2 CCR 2340) [3.4].
The 30-second duration wind speed shall be determined from the annual maximum wind data. Average annual summaries cannot be used. Maximum wind speed data for eight directions (45-degree increments) shall be obtained. If other duration wind data is available, it shall be adjusted to a 30-second duration, in accordance with Equation (3-12). The 25-year return period shall be used to establish the design wind speed for each direction. In order to simplify the analysis for barges (or other small vessels), they may be considered to be solid free-standing walls (Chapter 29 of ASCE/SEI 7 [3.5]). This will eliminate the need to perform a computer assisted mooring analysis.
Wind speed measured at an elevation of 33 feet (10 meters) above the water surface, with duration of 30 seconds shall be used to determine the design wind speed. If these conditions are not met, the following corrections shall be applied.
The correction for elevation is obtained from the equation:
|Vw||=||wind speed at elevation 33 ft. (10 m.)|
|Vh||=||wind speed at elevation h|
|h||=||elevation above water surface of wind data [feet]|
The available wind duration shall be adjusted to a 30-second value, using the following formula:
|Vt = 30 sec||=||wind speed for a 30-second duration|
|Vt||=||wind speed over a given duration|
|ct||=||conversion factor from Figure 31F-3-1|
If wind data is available over land only, the following equation shall be used to convert the wind speed from over-land to over-water conditions [3.6]:
|Vw||=||over water wind speed|
|VL||=||over land wind speed|
Environmental loads induced by currents at MOTs shall be calculated as specified in this subsection.
Maximum ebb and flood currents, annual river runoffs and controlled releases shall be considered when establishing the design current velocities for both existing and new MOTs.
Local current velocities may be obtained from NOAA [3.9] or other sources, but must be supplemented by site-specific data, if the current velocity is higher than 1.5 knots.
Site-specific data shall be obtained by real time measurements over a one-year period. If this information is not available, a safety factor of 1.25 shall be applied to the best available data until real time measurements are obtained.
Operational dates need to be clearly stated in the definition of the terminal operating limits (see Section 3102F.3.6).
An average current velocity (Vc) shall be used to compute forces and moments. If the current velocity profile is known, the average current velocity can be obtained from the following equation:
|Vc||=||average current velocity (knots)|
|T||=||draft of vessel|
|vc||=||current velocity as a function of depth (knots)|
|s||=||water depth measured from the surface|
If the velocity profile is not known, the velocity at a known water depth shall be adjusted by the factors provided in Figure 31F-3-2 to obtain the equivalent average velocity over the draft of the vessel.
The OCIMF MEG3 [3.7] or the UFC 4-159-03 [3.10] procedures shall be used to determine current loads for moored tank vessels.
All MOTs shall consider the predicted SLR over the remaining life of the terminal, due to subsidence or climate change combined with maximum high tide and storm surge. Consideration shall include but not be limited to variation in fender locations, additional berthing loads (deeper draft vessels) and any components near the splash zone.
When the significant wave period, Ts, is greater than 4 seconds (see Section 3105F.3.1), the transverse wave induced vessel reactions shall be calculated using a simplified dynamic mooring analysis described below.
The horizontal water particle accelerations shall be calculated for the various wave conditions, taken at the mid-depth of the loaded vessel draft. The water particle accelerations shall then be used to calculate the wave excitation forces to determine the static displacement of the vessel. The Froude-Krylov method discussed in Chakrabarti's Chapter 7 [3.11] may be used to calculate the wave excitation forces, by conservatively approximating the vessel as a rectangular box with dimensions similar to the actual dimensions of the vessel. The horizontal water particle accelerations shall be calculated for the various wave conditions, taken at the mid-depth of the loaded vessel draft. The computed excitation force assumes a 90-degree incidence angle with the longitudinal axis of the vessel, which will result in forces that are significantly greater than the forces that will actually act upon the vessel from quartering seas. A load reduction factor may be used to account for the design wave incidence angle from the longitudinal axis of the ship. The overall excursion of the vessel shall be determined for each of the wave conditions by calculating the dynamic response of the linear spring mass system.
When required in Section 3105F.3, the sway and surge forces, as well as yaw moment, on a moored vessel, due to passing vessels, shall be established considering the following:
- Ratio of length of moored vessel to length of passing vessel.
- Distance from moored vessel to passing vessel.
- Ratio of midship section areas of the moored and passing vessels.
- Underkeel clearances of the moored and passing vessels.
- Draft and trim of the moored vessel and draft of the passing vessel.
- Mooring line tensions.
The passing vessel's speed should take into consideration the ebb or flood current. Normal operating wind and current conditions can be assumed when calculating forces due to a passing vessel. Either method of Kriebel [3.12] or Wang [3.13] may be used to determine forces on a moored vessel. Kriebel's recent wave tank study improves on an earlier work of Seelig [3.14].
The penetration of long period low amplitude waves into a harbor can result in resonant standing wave systems, when the wave forcing frequency coincides with a natural frequency of the harbor. The resonant standing waves can result in large surge motions if this frequency is close to the natural frequency of the mooring system. Section 3105F.3.3 prescribes the procedure for the evaluation of these effects.
A tsunami may be generated by an earthquake or a subsea or coastal landslide, which may induce large wave heights and excessive currents. The large wave or surge and the excessive currents are potentially damaging, especially if there is a tank vessel moored alongside the MOT wharf.
Tsunamis can be generated either by a distant or near source. A tsunami generated by a distant source (far field event) may allow operators to have an adequate warning for mitigating the risk by allowing the vessels to depart the MOT and go into deep water. For near-field events, with sources less than 500 miles away, the vessel may not have adequate time to depart. Each MOT shall have a "tsunami plan" describing what actions will be performed, in the event of a distant tsunami.
Recent tsunami studies have been completed for both Southern and Northern California. For the Ports of Los Angeles and Long Beach, one of these recent studies focused on near field tsunamis with predicted return periods of 5,000 to 10,000 years [3.15]. These maximum water levels (run-up) would not normally be used for MOT design. However, because the study also provides actual tidal records from recent distant tsunamis, it should be used for design.
The run-up value for Port Hueneme was obtained from an earlier study by Synolakis et al. [3.16].
Run up-values: Port of Los Angeles and Long Beach = 8 ft.
Port Hueneme = 11 ft.
For the San Francisco Bay, a recent study provides the maximum credible tsunami water levels and current speeds. These results are deterministic and are based on the most severe seismic sources that could reasonably impact MOTs in the San Francisco Bay [3.17]. Table 31F-3-6 provides values for the marine oil terminal locations within San Francisco Bay. Water levels could be positive or negative and current velocities may vary in direction. In order to determine the maximum run-up at a MOT, the largest values should be added to the mean high tide. Further details are available in [3.17].
Loads from tsunami-induced waves can be calculated for various structural configurations [3.18]. Tsunami wave heights in shallow water and particle kinematics can also be obtained. Other structural considerations include uplift and debris impact.
|S.F. BAY LOCALE||MAXIMUM WATER LEVELS (ft.)||CURRENT VELOCITY (ft/sec)|