Trailing Suction Hopper Dredgers

Trailing suction hopper dredger

For the Tuas terminal the main dredging ships that will be used are medium-sized trailing suction hopper dredgers and grab and backhoe dredgers. These dredgers will be used for the dredging of the clay. The clay will be used as filling material for the constructed basin. The clay can be dredged in the surrounding area where the basin will be constructed. The two different dredgers are now discussed on how they work and why they are used for this project.

The medium sized trailing suction hopper dredger

The suction hopper dredger is a self-propelled vessel for sea and waterway conditions. The vessel is equipped with a hopper and a dredge installation to load and unload the payload itself. For the rest of the website the trailing suction hopper dredger will be named a TSHD. A TSHD can unload the dredged material two ways, the first way is dumping. With dumping the bottom doors open and the clay and sand is dumped above the location where it will be used.

The second way is the pomp method. This method uses the pumps to get the material out of the ship and on the location where it is needed. Most commonly this method is revered to as the rainbow method. But for this project this is not the case. Because of the lack of materials in the surrounding area the method that will be used is the “persen over de pomp” method, with this method there will be less spill and the material will be denser.

The dredging process of a TSHD consist out of 4 steps:

Dredging
Sailing towards the discharge location
Discharging
Sailing back towards the dredging location

The dredging step consist out of the excavation of the material. For a TSHD the part the dredges the soil is the draghead. The power and capacity of the draghead depends on the pumps that are installed on the TSHD. For the Tuas mega port the TSHD from Boskalis that will be used is “the queen of the Netherlands”.

TSHD Pumps

For the loading of the dredged material the underwater pump and the pump on the TSHD work together to get the get material onboard. The pump pressure and the power of the pumps of a TSHD can be determined with the use of multiple formulas. The parameters of the used TSHD are shown in the table below. These parameters are for the Boskalis ship called “the queen of the Netherlands”.

Gross Tonnage	33,423	Ton
Length overall	230.71	m
Breadth	32	M
Moulded dept	15.90 (aft ship) 16.85 (Mid ship)	M
Max. draught Int. load line	10.387	m
Max. draught dredging load line	13.674	m
Carrying capacity	59,168	Ton
Hopper capacity	35,500
Suction pipe diameter	2 x 1,200	mm
Max. dredging depth	PS: 67, SB:67/83	m, with submergible dredge pump.
Discharge systems	24	Bottom door sections/Pump ashore/Rainbow installation/ Back dumping
Sailing speed loaded	16	kn.
Total installed power	27,634	kW
Dredge pump output	SB: 7,000 kW, PS: 6,000 kW	SB: inboard and outboard dredge pump in series, PS: inboard dredge pump only
Jet pump output	3 x 1,000	kW
Pump ashore output	2 x 6,000	kW
Propulsion power sailing	23,000	kW
Bow thruster	2,650	kW

From these parameters the pump pressure can be found. The surface area wof the pipe will be:

The flow rate and the Reynolds number of the pump can now be determined by:

Where D is the diameter of the pipe [m], is the kinematic viscosity 0,00001787 [m²/s] and Vls is assumed to be 10 [m/s]. This will give a Reynolds number of and a flow rate of 11.31 [m³/s].
From this the pump pressure can be determined. This is done with the formula:

Where is the angular velocity, because most pumps have a rpm of 200 the angular velocity will be:

The density of the mixture also needs to be determined. The density of the mixture can be found by:

In this formula the density of the seawater is 1025 [kg/m³]. H is the maximum dredging height, as can be seen in the table this will be 83 m. k is the depth of the submergence of the first pump and has a value of 16 m. is the total losses over the inlet and length of the pipe. This can be found by:

Here beta is the entree losses, it is assumed to be 0.01 [-]. Lambda is Darcy-Weisbach and will be 0.008558 [-]. This means that the total losses will be:

The allowable under pressure for the pumps determined how much water is dredged, is 20000 Pa. Furthermore, due to the angle of dredging the mixture that will be dredged has a different value, the angle of dredging is assumed to be 0.52 rad. can now be determined and will have a value of:

So the pump pressure will be:

The power of the pump will be: Power = P * Vls*1/4*pi*D^2 = 1983766.9 W. This means that the power that is needed for the dredging of this mixture is below the available power of the pumps according to the table above.

In the calculator that can be found with the following link three different TSHD from Boskalis are compared with each other. In this calculator the pump parameters can be changed to find different power outputs.

Settling velocity and mass of the dredged material

The settling velocity of the dredged material determines how fast the loading cycle of the TSHD need to be. The settling velocity can be found by the size of the tank and by the flow rate of the dredged material. This means that the settling velocity will be:

With the dredging of the soil the material is stored in the hopper. The added mass of the dredger can be determined and check if it is less than the carrying capacity.

To determine the mass of the load, the filling of the TSHD must be determined. The important parts for the filling are.

Loading time of the hopper
The overflow losses
Intake capacity of the hopper

To determine the filling time of the hopper the mass and volume of the dredged load needs to be determined. For the queen of the Netherlands the carrying capacity is 59,168 Ton. This means that with the density of the mixture and size of the hopper the Total weight of the mixture material can be determined and checked if it is less than the carrying capacity.
The loading time of the dredger is assumed for an average TSHD to be around the 2 hours. But the filling time can also be determined. This is done by the flow rate of the pumps.

During the next step the overflow losses are determined. The overflow losses or in other word the losses of material the leaves the basin through the overflows is dependent on the porosity of the mixture. Because the porosity of the mixture is assumed to be 0.05. The overflow losses are expected to be round the 40%. The intake of the hopper is assumed to be 0.9 m³/s. With this all known the Mass of the load can be determined.

With the mass of the load determined it shows that the carrying capacity of the TSHD is larger than the mass that is dredged.

In this calculator, three different TSHD from Boskalis are compared with each other. In this calculator the differed settling velocity and mass of the loads can be determined.

Draghead

There are many different types of draghead, the use of the draghead depends on the soil that needs to be dragged. If the soil is non cohesive the draghead uses jets to loosen the soil. If the soil is cohesive the draghead with teeth’s or blades are used to cut the soil.

The material that is cut is then mixed with the water form the jets and pumped through the suction pipe. The TSHD moves continuously along the seabed until the hopper is filled up. The dredged material will have a density of:

And this mixture has a density of 1256.23 kg/m³.

With the mixture density found it is possible to find the suction time. This is because the suction time depends on the volumetric concentration of the flow rate of the soil.
When dredging more cohesive soils the suction time is mostly depended on the cutting speed. For the cutting of the soil a force is required. At the location of the Tuas mega the soil consists out of non-cohesive materials. For the dredging of non-cohesive soil types water jets are used. The mass that is loosened by the sand can be found:

In this formula alpha is the coefficient that depends on the particle size, jet pressure and trail speed. It is assumed to be 0.1[-]. The jet capacity is assumed to be equal to the flow rate. The jet pressure is assumed to be 5000 Pa. With these values the mass of the sand can be determined.

But because the “the queen of the Netherlands” has two dragheads the total sand intake per second will be 7241.76 kg/s.

The eroded layer thickness of the draghead can also be determined for the trailspeed of the TSHD. The formula to find the thickness is:

B is the width of a draghead, d is the thickness of the dredged layer, V is the trailing speed and the is the densities of Water [1.025 kg/m³], the density of the particle [2600 kg/m³] and the density in situ [1256.23 kg/m³]. For this example, the trailspeed is assumed to be 2 m/s. This will give a eroded layer with the thickness of:

In the calculator, three different TSHD´s from Boskalis are compared with each other. In this calculator the thickness of the dredged layer can be found for different parameters.

The video below shows how Trailing Suction Hopper Dredgers go about their work:

Other Construction Aspects

Backhoe Dredgers

Magnor is one of the backhoe dredgers that is being used at the port construction. For more details click here.

Caissons

The caissons are one of many aspects which make this construction process unique. The caissons are made in a huge factory that has been set up on the project site. For more details click here.

Soil Improvement

The soil at the project site consists mainly of clay. Therefore soil improvements are necessary to create a stable foundation for the port. For more details click here.