This article is written from a point of view onboard TSHD "Alexander Von Humboldt", dredging off the Belgian coast, and subjected to frequent gales.
TSHD "Alexander Von Humboldt" during maintenance dredging off Zeebruges, view from the wqheelhouse.
Trailer dredgers (TSHD) are quite tolerant to bad weather, compared to other types of dredgers.
Yet, dredging in adverse weather conditions has it chores.
Let's look into weather delays by wind and sea state:
The determining factors are:
1. Swell height, direction, and period
2. Wave height, direction, and period
3. Wind speed and direction, wind gusts
4. Ship's dimensions, windage, and available power
5. Forecasted weather
Too often hopperdredgers stop operations during worsening weather conditions, either too late or too soon.
Stopping operations may be the safest option, but is not the most productive. (Unless the dredge contract stipulates payed weather delays.)
And stopping too late, suffering serious damage, is a no-brainer.
This dilemma is a fine balancing act between the everlasting quest for more production and the want to keep the ship damage-free. (Imaginary) pressure from shore staff, should not influence that decision making process.
Take also in account that dredging in high seas will reduce the cycle production anyway:
* Sailing times are longer due to wave resistance.
* Operators will have to reduce the ships' speed, to protect the engines from over load and avoid heavy slamming on the bow.
* The settling process is more hindered in the hopper (the theory of hindered settlement).
* Sand gets more compacted in the hopper, and it takes more time to mix it with water, for discharging.
* Overflow-losses are increased due to rolling and pitching.
* Pipe production will not be as consistent due to heaving of ship and pipe.
* Manoeuvres for positioning, dumping, connecting or lowering the tubes will take longer.
It is important to establish a clear set of quantified limits ( a “weather envelope”) for the dredging activities.
Here follows an example: "Alexander Von Humboldt" ( a 9000 m3 trailer) has limited power for station-keeping, during discharge, in an exposed part of the Zeebruges outer harbor.
At about 5 Beaufort, discharge operations have to be stopped. This dredger has -however- a fallback operation with a larger weather window: maintenance dredging in the Zeebrugge access channel.
The operational limits are clearly illustrated in the above graph.
Such a graph can be compounded from previous experience, from near-misses, etc... Nowadays most dredgers have DP-systems, with a "capability-plot function". This will simply calculate how close are the edges of the weather envelope.
A capability plot for a "Bourbon" class anchorhandling vessel.
One step further is to have good real-time wave- and wind-data available to the operator. On top of that a good local prediction model, is even better, because it allows proper work planning, in advance of weather developments.
On the Belgium coast, government has set up a highly precise measuring network over the whole near-shore sea area. All the assembled data -even the salinity- can be found real-time on their website: http://www.meetnetvlaamsebanken.be/
Such information takes away margins of error in the decision to continue dredging in worsening weather.
It allows for a simple "go" or "no go" decision, based on facts and figures.
TSHD "Gerardus Mercator" during deepdredge operations offshore Taiwan.
Bart Van de Velde
2nd Mate THSD Alexander Von Humboldt