7. Questions & Answers

Below is a multiple question test to check your knowledge on DP systems

7.1. General Questions on DP

What is the difference between ‘DP current’ and real current?

‘DP current’ is the current value that the DP system shows on the DP screen. It is equal to the total thruster force minus the Wind Force. DP current = Thruster force – Wind Load
In other words, the ‘DP current’ is the total of all external forces acting on the vessel (which are balance by the vessels’ trust force) minus the wind load. Those external forces are: Current load, Wave load, External forces (pipelaying loads, pull/push loads etc) and all non-model phenomena (hydraudynamic effects etc.)

What factors affect thruster efficiency during dynamic positioning?

1. Thruster – Hull interaction
2. Thruster – Thruster interaction
3. Wave effects + vessel motion
4. Current effects + forward speed
5. Free surface effects (air ventilation), Thrusters too close to the sea surface

What is dead reckoning?

If all position references are lost

A conventional propeller is typically about 30% more efficient when operating ahead or when operating astern?

It is more efficient when operating ahead.

Which company operates the DP (Class II) diving support vessel, DSV ‘Constructor’.

1. Subsea 7
2. Boskalis
3. Shell
4. Cal Dive

Which company operates the DP (Class II) diving support vessel, ‘Uncle John’.

1. Subsea 7
2. Boskalis
3. Shell
4. Cal Dive

7.2. Position and measurement Systems

All DP systems use wind measurements as one of the main inputs to the DP control system. Why?

Wind is one of the main environmental loads that change the position of a vessel. Since wind needs some time to move the vessel, wind measurements are fed into the DP computer and the required trust for correcting the anticipated vessel response is applied as necessary at the upcoming seconds. This a feed forward control; the variation at the process input (i.e wind) are detected and the required correction signal is applied before the process output is affected.

Name two factors that may accidentally affect the measurement of a wind sensor on DP vessel.

• The possiton of the sensors relative to the wind direction
• The obstruction of the sensors when the vessel is very close to a platfrom, or when cranes and other equioments shelder the sensor 

What is an inertial position monitoring systems? Can they be used for DP? What are the disadvantages?

Inertial reference systems are systems that make use of gyro-compases, accelorometers, velocity measurements etc in order to track the vessels position and heading. Once the initial position is inputed to the system it able to track the any movements and determine the relative position from a start point. The systems is accurate for a limited time period. It is however a reliable ‘dead-reckoning’ system since it relies only on the equipment onboard and it is also used in combination with other systems to increase the accuracy.

What is a DP capability curve?

A DP capability curve shows the maximum wind speeds at each direction for which the vessel can maintain position. The capability curves allows to find the limiting wind speed for safe DP operation of the vessel.