Science Centres: Vessels

More information about Tangaroa's dynamic positioning system and what it can do.

1. What is a dynamic positioning (DP) system?

2. What does the installation include?

3. What kind of conditions can the DP2 withstand?

4. How accurately can the DP2 hold the vessel in a fixed position?

5. What other advantages does the DP2 system provide?

1. What is a dynamic positioning (DP) system?

A dynamic positioning (DP) system is like a computerised anchor. It’s a computer controlled system to automatically maintain a vessel in a fixed position at sea (within a few metres) by using its own propellers and thrusters.

A DP system typically includes underwater propulsion hardware, diesel-electric generators to supply power to drive the propulsion hardware, control computers and software, and high-resolution positioning systems (usually using GPS and/or transponders moored to the seabed).

There are three different classes of DP system available:

1. DP1 - has a computerised system to maintain the vessel on location.

2. DP2 - has identical underwater components to a DP1 but also has two completely independent diesel-electric power generators, switchboards, computer and positioning systems, so that if any of these fail the vessel will remain in position.

3. DP3 - houses all its dynamic positioning equipment in separate, sealed engine rooms as an extra safety precaution. Generally DP3 vessels must be purpose-built and are rarely used.

In New Zealand there are no other vessels with DP2 capabilities, although a few have DP1 capabilities.

2. What does the installation include?

The new DP2 system on Tangaroa includes:

• A new stern thruster (800 kW).
• An increased capacity bow thruster (368 to 600 kW).
• A retractable azimuth in the bow. (A retractable azimuth is an electronically driven screw system that can be lowered beneath the vessel and rotate 360⁰. The screw then pushes the vessel in the desired direction – forwards, backwards, or even sideways).
• A new bow module to house the azimuth and bow thrusters.
• Two 1440 kW generators to power the new thrusters.
• A High Precision Acoustic Positioning (HiPAP) system.
• A computerised control system.
• A pipe through the hull to allow sensitive electronic equipment to be deployed below the vessel. 

See this diagram for where these are located on the vessel 

3. What kind of conditions can the DP2 withstand?

No vessel can maintain a fixed position in extreme weather conditions, even with a DP system. However, the DP2 system installed on Tangaroa means the vessel can stay in a fixed position in currents up to 2 knots, a swell of up to 3 metres, or winds up to 45 knots. These are typical marine conditions Tangaroa might encounter on voyages around the world.

4. How accurately can the DP2 hold the vessel in a fixed position?

The DP2 can hold Tangaroa within a few metres of a fixed position (which is very accurate considering Tangaroa is 70 metres long and weighs more than 2000 tonnes). The system also allows the vessel to accurately move along a precise path. These added functions will greatly enhance the vessel’s ability to deploy or operate scientific and other equipment including remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), manned submarines, seafloor observatories, and other equipment temporarily fixed to the seabed, such as seabed samplers and rock-coring equipment. Being able to hold a steady position is also very important for safety when deploying divers or working around marine construction works, such as oil and gas platforms or pipelines.

5. What other advantages does the DP2 system provide?

Before the upgrade, Tangaroa had only one engine, gearbox, shaft, propeller and rudder, and the failure of any of these parts would have incapacitated the vessel. Though Tangaroa has not had any serious incidents of this type, it is a significant risk (both to safety and financially), especially when the vessel is operating in high risk areas such as Antarctica. Installation of the DP2 provides an alternative propulsion system if any of these components fail.

Other advantages of the new system include:

• Increased manoeuvrability – making it easier to change position quickly and accurately. The new system also allows the ship to move backwards, forwards and even sideways if needed.
• Unlike a physical anchor it is not dependent on water depths and cannot be obstructed by debris on the seabed. The system allows the vessel to be ‘anchored’ at depths of 4000 metres or more.