Dimensions – The Port, the Ship, the Canals and More
Dave Cooley, GHPB
Ocean-going ships are constructed to satisfy a certain product demand, to serve a specific need or, to serve a particular market. With this thought in mind, ship owners continually seek to optimize their fleet by striving to match the dimensions of the ship utilized on a particular route, or serving a specific trade with the dimensional limitations of the various ports-of-call, as well as the limits of any canals, whose use would offer tangible economic incentives when compared to other trade route alternatives. Likewise, port and canal operators also strive to construct navigable channels, given the limits of seabed geology and economics, within the port or the canal that would accommodate the dimensions of the largest ships feasible.
The two key considerations, the parameters of the ports and the limits of any canals, are vital factors when deciding the final dimensional parameters of a ship. Once these dimensional variables are decided and the ship is built, the length overall (LOA) and the breadth (beam) are fixed. The only variable is the vessel’s draft (depth whether laden or not). Therefore, the goal of the ship owner is to schedule cargo such that the ship arrives at or near the maximum draft of the port, discharges cargo, loads cargo and departs also at or near the maximum draft of the port. While theoretically optimum, achieving this goal is also a function of the nature of trade and the type of cargo carried.
For example, containerships operating in a liner service where the nature of the trade at each port is generally bilateral (containers are discharged and containers are loaded). While the types of cargo shipped by container are indeed quite variable, it does, however, all fit nicely into a box, perhaps along with other goods, and is destined for another foreign port along the liner route. As a result, containerships will have a high probability of achieving optimum operations.
On the other hand, transporting bulk commodities such as oil and grain, for example, is a port-to-port operation originating in areas where the commodity is produced and terminating in areas where the commodity is consumed. Furthermore, the nature of the bulk commodity trade is generally unilateral as the ship arrives in port laden, discharges cargo, and leaves port in ballast or the ship arrives in ballast, loads cargo, and departs laden. Optimum operation is again noted as employing the largest ship possible compatible with the smallest limits imposed by the load port, the discharge port, or any canal available for transit along the route. After discharging cargo, further optimization may be available based on where the ship is positioned for the next voyage.
The dimensions of the Houston port have changed over the years to accommodate the ever increasing size of ships necessary to handle the ever-rising growth in waterborne trade. After the 1900 Hurricane that devastated Galveston and following intensive lobbying by Houstonians, Congress, in 1914, authorized the dredging of Buffalo Bayou and Galveston Bay to create the Houston Ship Channel with an initial depth of 25 feet and a width of 100 feet. Over the years, six dredging projects expanded the dimensions of the Houston Ship Channel gradually increasing the channel’s depth and width in order to accommodate larger ships necessary to handle the increased trade. The latest dredging project occurred in 2005 that resulted in a depth of 45 feet and a channel width of 530 feet.
Changes in the channel depth and width along with the current profile of the Houston Ship Channel in Galveston Bay are shown by Figure 1 and Figure 2, respectively.
The dimensions of the Houston Ship Channel that provides access to the docks, piers, and wharves along that waterway vary according to the available width (generally narrowing upstream of Morgan’s Point) and depth (progressively shallower upstream of Boggy Bayou). Table 1 shows the various dimensional limits of the Houston Ship Channel traveling upstream.
The dimensions of the Houston Ship Channel offer ship-owners five distinct opportunities to optimize the size of ship calling at Houston as different types of ships call at the various wharves and docks located within each segment. The trend is that larger ships, generally containerships, tankers (oil, refined products, and chemicals), gas carriers, and bulk carriers are more prevalent from Bayport to Boggy Bayou. Upstream of Boggy Bayou are lesser-sized ships carrying general cargo, bulk carriers handling break-bulk cargos and pure car carriers delivering automobiles. Table 2 displays this dispersion.
In addition, the dimensions of ships calling at Houston are also influenced by the dimensions of the Panama Canal. The beam and draft at the “old lock” is the primary limiting factor for ships transiting the Panama Canal on the way to Houston. Table 3 presents the dimensions of the Panama Canal for both the old and new locks. (The new locks were inaugurated on 26 June 2016).
Analyzing the ships calling at Houston from January through September 2016 indicate that the ship dimensions generally aggregate around various key dimensional limits established for not only the port itself, but also around the beam and draft limits for the Panama Canal (old locks).
The draft of vessels both arriving and departing ranges from about 17 feet to 45 feet, where the latter is the draft limit of the Houston Ship Channel. The average arrival draft is 27.4 feet while the average departure draft is 29.3 feet. The difference of about 2 feet suggests that ships are departing with heavier cargo than ships arriving. Comparing the draft of ships arriving with the draft of ships departing shows there are more ships departing on drafts between 40 feet and 45 feet than the drafts of ships arriving. Also, there is a heavier concentration of ships departing on drafts at or very near 40 feet than those arriving. Exports to Asia and Australia-Oceania rising in both absolute and percentage terms indicates that ships leaving Houston bound for the Pacific are generally loaded to the maximum depth of the old locks on the Panama Canal (40 feet). See Figures 3, 4, and 5.
The length overall (LOA) of ships transiting the Houston Ship Channel predominantly aggregate around three basic levels; 600 feet, 750 feet, and 1,000 feet. The first value (600 ft) is simply the average of the LOA of the ships arriving and departing Houston. The remaining two values (750 ft and 1,000 ft) are limits established by the Houston Pilots for ships transiting between Boggy Bayou and the Turning Basin (750 ft) and for ships transiting to either of the two container terminals, Bayport or Barbour’s Cut (1,000 ft). In addition, there are also a reasonable number of ships with LOA of around 965 feet, the limit of the old locks at the Panama Canal. See Figure 6 and Figure 7
The beam of ships arriving and departing Houston are relatively clearly defined. The first tranche is a beam of up to 106 feet, which is the beam limit for the old locks at the Panama Canal as well as for those ships that can be accommodated between Sim’s Bayou and the Turning Basin at the upper reach of the Houston Ship Channel. The second tranche is at 120 feet; the third at 135 feet, and the fourth at 138 feet. Each of these values is a limit established by the Houston Pilots for various reaches as ships move upstream on the Houston Ship Channel (see Table 1 above). The fifth tranche are ships with a beam of around 160 feet; all of which require special approval and handling by the Houston pilots.
Statistics by Ship Type
Statistics by ship type for vessels arriving and departing Houston are displayed by Table 4.
The difference between the arrival draft and the departure draft for the deepdraft ships calling at Houston are 3 feet or less, for all ship types except LPG carriers, with a draft difference of 7.4 feet. This suggests that LPG carriers generally arrive in ballast, load cargo, and depart.
Similarly, bulk carriers loading grain generally also arrive in ballast, load cargo, and depart. Conversely, large oil tankers discharging foreign crude oil arrive in port fully laden, discharge cargo, and depart. Smaller oil tankers loading or discharging black oil may or may not have the opportunity to optimize cargo operations by arriving in port laden, discharging cargo, loading cargo, and departing.
Containerships, general cargo ships, break-bulk ships, and chemicals-refined product carriers generally optimize cargo operations by arriving in port laden, discharging cargo, loading cargo, and departing. This scenario is supported by the narrow difference between the arrival draft and the departure draft of these types of ships.
The Houston port is quite dynamic! Various parameters influence the dimensions of ships, and the dimensions of ships push to the limits of economics and hydrography, the depth and width of port channels and canals. As the dimensions of ships become larger, ports and canals adjust by offering deeper and wider channels, which results in ship owners developing ever larger sized ships to capture the economic benefits of these navigational opportunities.
- Date December 15, 2016
- Tags December 2016