Dredging Coastal Ports An Assessment of the Issues (1985) / Chapter Skim
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5. Options for Handling Large Vessels
5. Options for Handling Large Vessels
Pages 40-58
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From page 40... ...
The five principal options considered by the committee are: Construction dredging, that is, underwater excavation of materials to create a wider/deeper channel for larger vessels, can be undertaken at existing ports. It should be understood for the comparative purposes of this chapter that the other options described are not necessarily exclusive of dredging.
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From page 41... ...
In the proposals of some ports for construction dredging, enlarging the navigational facilities and creating landfill for new or expanded terminals are equally important in accommodating large vessels. OFFSHORE TERMINALS Offshore terminals have long been recognized as a potentially optimum solution for bulk cargoes.
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From page 42... ...
emphasizes the protection of coastal resources as the primary advantage, and among the problems, lists the following for marine operations: The tug-ship operation in the open sea The docking approach, especially with adverse winds, waves, and currents Attachment of mooring lines to buoys, dolphins, and pier structures, and detachment, especially from mooring buoys Decision about when to leave berth owing to worsening weather The principal constraint acting against construction of offshore terminals is their high capital costs. The Japanese contribution to an international study (Permanent International Association of Navigation Congresses, 1977)
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From page 43... ...
The offshore iron ore export terminal at Port Latta, Tasmania, Australia, where pelletized iron ore is conveyed two miles out to a terminal in the open sea for loading into large ore carriers. A fixed terminal was proposed for liquefied natural gas (LNG)
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From page 44... ...
Similar floating terminals have been proposed for LNG service. Offshore Storage Caisson Terminals This category includes the Dubai steel tanks placed underwater for oil storage, and the many large concrete caisson structures in the North Sea, which store oil for transfer through an associated articulated mooring buoy.
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From page 45... ...
describe the development and analysis of designs for dry bulk carriers of 60,000 DWT to 200,000 DWT, having restricted drafts of 35 to 55 ft. Relatively wide-beam vessels have been built and operated, but most of those 100,000 DWT or more are not restricted-draft designs, and exceed the water depths of almost all U.S.
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From page 46... ...
Midstream transfer facilities to handle barge-to-ship transfer at high cargo rates are now in operation in New Orleans for most bulk commodities. A topping-off service to load coal into large bulk carriers unable to load fully in the ports of Hampton Roads, Virginia, has been developed for the lower Delaware Bay, in an anchorage area used to lighter oil from larger to smaller tankers (Dowd, 1983~.
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From page 47... ...
Roseman (1979) states that slurry carriers for pelletized ores and other dry bulk commodities in converted or new special bulk carriers can be considered state-of-the-art technology, owing to the following advantages: Reduced cargo handling time between ship and terminal Elimination of airborne pollution from mechanical handling of coal Ability to operate from offshore terminals employing single-point moorings, permitting the use of large deep-draft vessels, with corresponding economies of scale System compatibility with proposed coal slurry pipelines Among the several proposals, coal slurry would most likely be pumped in 50 percent concentration (by weight)
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From page 48... ...
Economics Of the alternatives considered in this chapter, the construction costs of a new deepwater port would appear to be the greatest and those of topping-off services the least. Estimates have not been developed for the only recent new-port proposal in the United States (in the lower Delaware Bay)
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From page 49... ...
For containerships, there appear to be no effective alternatives (other than new deepwater ports) to dredging existing ports.
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From page 50... ...
cannot exceed the transportation cost-savings per ton of using larger vessels, and for the past three years, this has represented a relatively small difference. The economically most attractive alternatives for accommodating large vessels appear to be construction dredging of existing ports and lightering/topping-off.
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From page 51... ...
Thus, the contribution to navigational safety of new construction dredging in existing ports depends on adequate design, maintenance, and operational practices. Offshore Terminals Offshore terminals are located in deep water, as opposed to the protected waters of coastal ports and harbors, but their relatively greater exposure has not resulted in higher rates of casualties than similar port and harbor operations.
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From page 52... ...
Generally, the navigational safety of lightering, topping-off, and midstream transfer are about the same as for offshore terminals, with the obvious difference of involving two vessels, and for oil transshipments, have been carried out for many years without major casualties. Environmental Issues While the potential environmental effects of some alternatives for handling large vessels are associated with construction or the disposal of dredged material, others are principally associated with the vessels and their cargoes.
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From page 53... ...
, questions of environmental implications might be raised about the fluid medium in the slurry, and its ultimate fate and effects. New Deepwater Ports It is likely that the greatest change to the local environment from the creation of a new port would occur with shoreside developments, particularly as the sites proposed for new-port development have little existing landside infrastructure.
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From page 54... ...
Future Flexibility As implied in preceding sections, new construction dredging of existing multicommodity ports gives the nation the greatest future flexibility among the options. Assuming that offshore terminals are for single commodities, they offer little additional flexibility for
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From page 55... ...
CONCLUSIONS Of the five options for increasing the nation's capacity to handle large vessels, measured against the criteria of economics, navigational safety, environmental implications, national security/defense needs, and future flexibility, two of the options stand out as being the most attractive. That is, assuming the conclusion drawn in Chapter 4 is correct that a prudent society can ill afford to move into the future without the capacity to handle large ships, then lightering/topping off and dredging existing ports are clearly the most attractive two options.
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From page 56... ...
Criteria Applicable to Selection of Ports for Deep Construction Dredging If, in the face of uncertainty, prudence suggests that additional dredging of existing ports to achieve deep facilities is necessary, what criteria might be used in establishing priorities for construction dredging? Four criteria seem compelling.
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From page 57... ...
The third criterion that should be considered is the comparative cost of construction dredging and the additional maintenance dredging costs that will have to be met annually (or at whatever the maintenance dredging interval)
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From page 58... ...
(1985) , "The Design of Tankers for Restricted Draft Service," Paper presented to STAR Symposium, Society of Naval Architects and Marine Engineers, Norfolk, Va., May 21-24, 1985.
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