Documentação Técnica

Documentação Técnica
* Engenharia de Dragagem, Sinalização Náutica, Batimetria, Projetos de Canais Navegáveis, Meio Ambiente, Cartas Náuticas, Software de Navegação, Topografia Básica e outros assuntos técnicos.

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sábado, 7 de maio de 2011

Projetos de Canais Navegáveis: Qual os fatores que determinam a profundidade de um canal ?

CHANNEL DESIGN PARAMETERS
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3 - DEPTH

Minimum Waterway Depth for safe navigation is calculated from the sum of the draught of the design vessel as well as a number of allowances and requirements as seen in the following formula:

Actual Waterway Depth * = Target Vessel Static Draught + Trim + Squat + Exposure Allowance + Fresh Water Adjustment + Bottom Material Allowance + Overdepth Allowance + Depth Transition - Tidal Allowance, (see Figure 5: Components of Waterway Depth)

* In the application of the formula, a decision should be made as to whether the trim and squat values should be added. In the standard case only, the squat value is used to determine the “actual channel depth.”

Project (Advertised) Waterway Depth = Waterway Depth - Overdepth Allowance

In addition to the factors affecting Waterway Depth included in this section, others that should also be taken into account include:

· the effect of currents in the waterway;
· the effect of water levels in the waterway and adjoining water bodies, by such changes as river flow and wind set up;
· environmental effects; and
· limiting depths elsewhere in the waterway.

In the determination of the design draught, it should be realised that the depth does not necessarily have to be available 100 percent of the time. This may require the deepest-draught vessel to schedule passage during high water levels. Selection of the design depth should be based on an economic analysis of the cost of vessel delays, operation and light load, compared with construction and maintenance costs.

3.1 TARGET VESSEL STATIC DRAUGHT

The draught of the target vessel that will be using the waterway is based on the anticipated ship traffic for the proposed waterway. These dimensions are selected by an economic evaluation of the ship traffic for the waterway.

3.2 TRIM

Trim is generally defined as the longitudinal inclination of a ship, or the difference in draught from the bow to the stern. It is controlled by loading. In general, at low speed, a ship underway will squat by the bow. The practice is to counteract this squat by trimming the ship by the stern when loading. The rule of thumb is to provide an allowance of 0.31 m to account for trim in waterway design (Ref.: 5,9).

The normal approach for a vessel is to assume a trim rate of 3"/100 ft of length or 0.25 m/100 m (Ref.: 3,5,9).

Figure 5: Components of Waterway Depth

3.3 TIDAL ALLOWANCE

The selection of an allowance for tidal effect should be derived from examination of a statistically significant sample of tidal records during the navigation season to determine to what extent tidal height above the chart datum should be included as part of the normally available water depth. The allowance selected should give the required level of waterway availability based on tidal scheduling determined through optimization analysis.

3.3 SQUAT

Squat refers to the increase of a ship’s draught as a result of its motion through water. It is a hydraulic phenomenon whereby the water displaced creates an increase in current velocity past the moving hull causing a reduction in pressure resulting in a localised reduction of the water level and, consequently, in a settling of the vessel deeper in the water. For various reasons—having to do with hull design, trim and other physical and operational factors—squat may be different at the fore and aft.

Recently, a new equation was developed on the basis of extensive research by Waterways Development to specifically target commercial waterways with vessel traffic and conditions representative of most major Canadian waterways. This equation takes into account the vessel beam in relation to the channel width, contrary to earlier equations that supposed infinite width. This new parameter is of importance since most Canadian waterways have limited width. The equation, known as Eryuzlu Equation # 3 (Ref.: 4, this reference is attached to this manual as Appendix 4), is therefore recommended as the one providing the most reliable results in waterways of limited dimensions. The equation is written as follows:

Z(d/D²)=a[vs/√gd]b/[D/d]cFw

where:
Z = squat;
d = vessel draught;
D = channel depth;
vs = vessel speed;
g = gravity acceleration;
W = channel width;
B = vessel beam; and
Fw = channel width factor.

With Fw = 1, where W > 9.61 B;
a, b, c are common coefficients: a = 0.298, b = 2.289,
c = -2.972
Fw = 3.1/√W/B , where W < 9.61 B; and

The equation is non-dimensional and therefore, can be used universally with any system of measurement units.

Applications *

The formula applies for:

1. vessels ranging from 19,000 DWT to 227,000 DWT, representing general cargo or crude carriers (block coefficient over 0.80);
2. a channel that is shallow and relatively straight;
3. the channel width may range from unrestricted to four times the vessel beam;
4. speeds ranging from about 2 knots to about 14 knots;
5. maximum trim of about 10 % of draft;
6. the predominant squat is fore squat; and
7. vessel loaded draft equal to or greater than 80% of the registered draft.

Formulae, by definition, tend to generalize the real situation. Therefore, good judgement, experience and common sense are required in the use of this and any formula.

* The planner should consider these when undertaking the determination of the squat.

3.5 DEPTH ALLOWANCE FOR EXPOSURE

The selection of the exposure allowance should take into account the movements of heaving, pitching and rolling caused by local conditions, and should be based on available information on the local wave climate and vessel traffic considerations.

The allowance should be selected so as to minimize arrival and departure delays accounting for economic considerations. If a substantial allowance is required for a minimal reduction in delays or the delay problems are minimal with low traffic, the allowance can be omitted. However, for other cases, the supplementary depth can be based on the information provided in Table 14. (Larger values may be required in waterways on the East and West Coasts).


* (8) These values represent typical allowances for the Great Lakes waterways.

3.6 FRESH WATER ADJUSTMENT

Salinity increases the density of water, in turn reducing the draught of the vessel in the waterway. Design of the waterway depth should account for fluctuations in the salinity that may occur in an estuary exposed to tidal influences and river discharges. An adjustment for fresh water should account
for the decreased buoyancy of the vessel.

A rule of thumb to determine the additional loading allowance for vessels in fresh water is to set it at 2-3% of the salt water draught (Ref.: 1,5,9).

3.7 BOTTOM MATERIAL ALLOWANCE

This allowance, also known as the Net Underkeel Clearance, is by definition the minimum safety margin between the keel of the vessel and the project (advertised) waterway depth. This allowance is provided in addition to the allowances for squat, trim, freshwater and the influence of the design wind and wave conditions in order to ensure a safety margin against striking the bottom. The value is a function of the nature of the bottom, the handling characteristics of the vessel and the operational character of the waterway. Table 15 summarises the values that may be used as a function of the Bottom Material.

                                                                 (Ref: 2,7,8,9)

3.8 MANOEUVRABILITY MARGIN

The Manoeuvrability Margin is made up of the allowance for bottom material (or the Net Underkeel Clearance) and the exposure allowance. This margin is a measure of the minimum required to allow the vessel to manoeuvre adequately in the waterway. A minimum margin of 1.0 m is generally used for the operation of large vessels. Therefore, the sum of the Bottom Material Allowance and Exposure Allowance should be at least 1.0 m to accommodate the Manoeuvrability Margin for vessels of 250,000 DWT and greater (Ref.: 10).

3.9 OVERDEPTH ALLOWANCE

Overdepth Allowance refers to an allowance to account for waterway siltation between dredging and tolerance of sounding and dredging. 

The dredging tolerance varies with the type of dredging plant employed and the bottom conditions. The average acceptable tolerance is 0.3 m. If the bottom material is soft and can be displaced by a ship, no tolerance allowance is necessary (Ref.: 1).

An allowance for siltation is usually based on the anticipated accumulation patterns of the silt. The allowance is designed to accommodate the siltation between dredging operations.

3.10 DEPTH TRANSITION

All reaches of the waterway must be examined and depths set according to the varying conditions encountered. This, and the natural bathymetry of the waterway, will lead to the provision of different depths in adjacent sections of the waterway.

If the transition between adjacent reaches is large, the sudden change in Underkeel Clearance will have an effect on the current velocities and hydrostatic pressure on the hull. The result will be a change in the ship’s performance, manoeuvrability and draught.

Vessel squat in a transition area is presently being evaluated by Waterways Development. The preliminary analysis shows that the squat would increase by 15% to 20% when the transition is from deep water to shallow water.

FonteCANADIAN WATERWAYS NATIONAL MANOEUVRING GUIDELINES, Canadian Coast Guard, 1999.

NOTA DO EDITOR

A tolerância denominada "overdepth allowance" (overdredging) é devida ao assoreamento entre dragagens e aos erros nos levantamentos batimétricos e na execução da dragagem. A folga devido aos erros de dragagem depende do tipo de operação de escavação e da natureza do fundo do canal, sendo que nos fundos lodosos a tolerância pode ser desprezada. Considerando-se esses dois aspectos, é comum usar em média uma tolerância de 0,30 m (1 pé).

A folga a ser observada devido ao assoreamento entre dragagens vai depender da taxa de assoreamento por unidade de tempo, por unidade de extensão/área do canal (metros cúbicos/metro/ano, por exemplo). Por seu turno, a taxa de assoreamento pode ser obtida de forma direta, por medição, ou indiretamente através dos volumes escavados entre dragagens. A taxa de assoreamento varia de canal para canal, mas aqui no RS costumamos adotar uma "overdredging" de 0,50 metros para compensar o volume sedimentado entre dragagens e os erros de batimetria e da operação de dragagem.

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