State-of-the-art drive technology guarantees efficient land reclamation

Like huge vacuum cleaners, trailer suction hopper dredgers suction-up the sand from the sea bed and empty it into their containers which can hold about 20,000m3

.

The ship, which with its freight weighs about 30,000t, steams to the area where the new land is to be reclaimed and ejects the sand in a huge arc away from the ship to deposit it precisely at the intended location

.

Large pumps, located to the port and starboard, both inboard and outboard, are used to suction up the sand and eject it

.

There are various ways in which these pumps can be driven

.

One possibility is to use mechanical pumps driven by a diesel engine through a gearbox

.

However the more modern and efficient alternative is to use electric

drives

.

In this case, the electric motors are directly supplied from the generators

.

In this type of system, the motor runs in approximate synchronism with the generator

.

This means the pump speed can be changed by varying the generator speed

.

However, a lower generator speed also means a lower voltage

.

With this concept, the generators normally supply the electric power for the whole ship ? which means that any voltage fluctuation effects all of the loads

.

This means that the speed control range is limited in practice

.

AC drive converters

The most effective solution is to use

AC drive converters

to drive the pumps

.

The reason for this is that the variable speed allows the process to be far better controlled and allows a significant amount of energy to be saved

.

While fixed-speed motors always provide their full power, variable-speed systems precisely adapt themselves to the operational requirements

.

This means that the motor draws only that amount of power which is precisely necessary at the particular operating point

.

The result, in partial-load operation, is a significantly lower power requirement than for fixed-speed

drives

with the same output

.

Further, the maintenance costs are lower which in turn reduces the operating costs

.

But

AC drive converters

have another significant advantage: The operator control and visualization functions can be integrated into the drive system ? for fast fault diagnostics and troubleshooting

.

Harmonized pump

drives

As a result of these clear benefits, more and more trailer suction hopper dredgers use variable-speed electric

drives

.

Some of the best examples include the Queen of Penta Ocean, owned by the Penta Ocean Company in Japan and the Rotterdam, owned by HAM Ballast Dredging in The Netherlands

.

Both of these vessels were built by the van der Giessen de Noord shipyard, at Krimpen aan de Ijssel in The Netherlands and are equipped with variable-speed drive solutions using Siemens high-voltage motors and Simovert MV medium-voltage

AC drive converters

.

With these medium-voltage drive systems, the motors and

AC drive converters

are always harmonized with one another

.

This is the reason that this combination, for example, does not require a special output filter

.

This reduces the costs and saves space, reduces the drive complexity, reduces losses and therefore permanently increases the

cost-effectiveness

of the complete system

.

Harmonised drive systems, either low or medium voltage, fixed or variable speed, are also used as propeller

drives

for fast boats, thruster

drives

for self-propelled drilling rigs,

drives

which boost the ship?s engines of some large freighters, and pod gondola

drives

for ferries or cruise ships

.

Speed control

The pumps of the trailer suction hopper dredger vessels also have such harmonized

drives

? however these have been configured so that four

AC drive converters

feed six motors

.

Each of the two inboard pumps are driven by two 3MW motors; each of the outboard pumps is driven by an underwater motor with a 2

.

2 MW output

.

In the dredging mode, one of the two inboard pump motors and the outboard pump motor run

.

In this case, the outboard pump operates as booster for the inboard pump

.

This allows sand to be drawn-up from more than 50 meters

.

In the rainbow or dumping mode, the underwater motor is switched-out

.

The Simovert MV drive converter which then becomes available, is switched to the second motor of the inboard pump

.

Now, the complete pump power for each side is 6 MW ? or 12MW for the total vessel

.

This is because the two inboard pumps are connected in series when used in the rainbow mode

.

Simovert MV is extremely flexible which means that it can feed various types of motors, ie it is easy to changeover to different motor data

.

Simovert MV is the first AC drive converter of its performance class with HV-IGBT power semiconductors

.

The HV-IGBT (High-Voltage Insulated Gate Bipolar Transistor) is the only component of its type which can be operated without any supplementary snubber circuitry

.

In conjunction with the three-level inverter, this allows an especially simple, compact and reliable modular design

.

Proven

cost-effectiveness

While high-voltage motors and Simovert MV are the most cost-effective solution for the large sand dredging pumps,

low-voltage

drive systems provide the better price-performance ratio for other applications

.

A typical example: Trunning winches raise and lower the suction pipe mounted to the ship?s frame, intermediate winches and draghead winches position the draghead at the end of the suction pipe, on the sea bed

.

Recently, variable-speed drive systems have also established themselves for these types of applications

.

They have proven themselves to be simple and therefore also more rugged and requiring less maintenance

.

They also have better operating characteristics

.

For the Rotterdam, six

low-voltage

drive systems, each of 300kW, power the winch

drives

.

They comprise N-compact

low-voltage

motors and Simovert Master

drives

low-voltage

AC drive converters

.

The Simovert Master

drives

units are equipped with an application-specific technological software and are coupled to a higher-level process control system

.

This allows the winch

drives

to quickly and automatically execute position changes

.

In this case, the software not only controls the speed and torque and the holding brakes, but it also monitors the braking torque as a function of the required speed

.

The winch drive system is directly connected to the local operator interface, the higher-level automation system and the emergency monitoring system via the communication interfaces

.

Commands from the process control – such as start, stop and setpoint signals – which can depend on the actual operating situation of the winch, can be received and the drive system can immediately respond to actual operating requirements

.

n

Wil Van Mol is senior sales manager, shipbuilding, Siemens Automation and Drives, Nuernberg, Germany