Vessel Sensors should at least measure vessel heading, vessel motions, and wind speed and direction.Other environmental sensors may be fitted, such as current meters, tension meters, but these are usually informational sensors and their output may not be fed into the DP.
There is a force acting on the vessel that no sensor can calculate. This force can be defined as the resultant of all other forces acting on the vessel apart from wind. The possible components of this force are numerous. It will also contain any errors in measurement, or unmeasured forces acting on the vessel.
Possible components:
• Surface current ,
• Subsea current,
• Waves,
• Swell,
• Effect of drag by attached equipment such as pipe or riser,
• Effect of current on riser,
• Workboats tied up to vessel,
• Wind (when wind sensors are deselected),
• Error in AWC calculation,
• Unidentified Forces, such as forces in a cable or pipeline being ploughed in
• Thruster errors.
When an equipment class 2 or 3 DP-control system is fully dependent on correct signals from vessel sensors, than these signals should be based on three systems serving the same purpose (i.e. this will result in at least three gyro compasses being installed).
Sensors for the same purpose, connected to redundant systems should be arranged independently so that failure of one will not affect the others. When class 2 and 3 is fully dependent on correct sensor readings, then 3 systems should be installed. For equipment class 3, one of each type of sensors should be connected directly to the back-up control system and separated by A.60 class division from the other sensors.
5.1. Means of obtaining Vertical Reference for input into a DP system. The importance of the provision of vertical reference
The latest version The Motion Reference Unit (MRU) measures pitch, roll and heave. Although a DP system does not control a vessel in the pitch, roll and heave axes, pitch and roll must be measured to provide accurate compensation for some position measurement equipment.
The VRU on the vessel determines the difference between the "local" vertical and reference plane of vessel. VRU signals are used for position holding rather than transit.
The compensation values of pitch and roll are used for:
• SBL and USBL acoustic,
• Inclinometer for slope of taut wire,
• Inclinometer for slope of riser,
• Compensation for aerials.
Heave is calculated by the double integration of the vertical acceleration of the unit. Heave is not needed for DP operation, but it is often useful for other purposes, e.g. advice to helicopters. A typical VRU provides heave readings in the range +10m with an accuracy of 5cm or 5%, and pitch and roll readings to ±30° down to accuracy 0.1°.
Figure 53. Vertical Reference Unit
5.2. The function of gyro compasses and their redundancy within a DP system
The most critical sensor for positioning will normally be the gyro, because the heading measurement is used to determine the position, for heading control and is needed to perform coordinate transforms. The gyrocompass is a pendulous suspend gyroscope which gravity controlled and damped. Gyrocompasses work over the range of 80N to 80S.
North speed, east speed, north acceleration, east accelerating gimbal all have automatic compensation with speed input. The normal startup cycle of a gyrocompass is 6 hrs. However, slew controls can override the automatic starting cycle after 5 mins. The vessel speed compensation is set to the vessel's average speed for the duration of the voyage.
Figure 54. Gyrocompass
If there is more than one gyro, either one will selected as a preferred gyro, or an average is used. If there are only two gyros, and one starts to drift, the system can only report a gyro difference. The operator has to decide which one is correct, this may be difficult. If three gyros are available select them all, this can allow the DP to vote, two out of three and a drifting or failing gyro can be voted off even if it is the preferred gyro.
5.3. Provision of wind sensors within the DP system
• Anemometer
An anemometer is a device for measuring both the speed and direction of the wind. Wind is a major disturbing element on the vessel. The wind speed and direction are used to improve position control by modifying thruster demands.
They have two main purposes: to assist in weathervaning for large stationary vessels such as shuttle tankers or production platforms, and to make allowance for gusty wind conditions. Separate sensors are provided for wind direction and wind speed.
Wind speed sensor can operate at wind speeds up to 60 metres/sec, and gusts up to 100 metres/sec. The lower threshold is around 1 metres/sec. The accuracy is +0.3 metres/sec. Wind direction sensor the accuracy is ±3°.
Care must be taken in installing the anemometers to avoid wind shadow from the vessel superstructure and spars. An anemometer should be at least 10 diameters from any spar or mast.
5.4. Wind Feed-Forward facility, and its importance within the DP system When a DP system is being designed for a particular vessel, the programmer will calculate the effect of wind direction and speed on that vessel taking into account variables such as draft and affected hull area. This calculation, called the Aerodynamic Model,- allows for the wind effect through 360 degrees as it will exert a greater offsetting force acting on the beam of a ship for example than it would acting from the bow.
The amount of thrust required to maintain position with the wind from any direction and any speed (up to design limit) is then programmed into the system and the DP system will automatically and instantaneously use that amount of thrust to compensate for wind force. This is known as Automatic Wind Compensation (AWC) or Wind Feed Forward. Sudden changes of wind direction or speed are instantly compensated for by AWC commanding appropriate thrust, ensuring that position loss is minimised. It can be seen from this why it is important that the system receive the most accurate wind information possible.
5.5. The limitations of wind sensor inputs, and the consequences of de- selecting the wind sensor input
Wind Sensors are very important in that the DP treats wind as being one of the major forces affecting the vessels position keeping (position keeping meaning both heading and position). Care must be exercised in both positioning them on the vessel and also in selecting which one to use in the system. Ideally the anemometers should be sited where they are totally unobstructed e.g. on top of the derrick on a drilling rig.
On many types of vessel it is impossible to find an unobstructed position and the anemometers may have to be sited at all four corners of a barge for example. If this is the case the operator must be aware at all times of wind direction and select the anemometer which is reading the true wind speed and direction. There are also times when the wind sensors are being masked e.g. alongside the lee side of a platform or they give a false reading e.g. down draft from a helicopter. The operator should again be aware of anything which may give a false reading and take appropriate action when required.
A typical example is downdraft from a helicopter. When there is doubt regarding the accuracy of wind information from the sensors, they should be deselected from the system. Then reselected once the situation has been sorted out.
5.6. Interpreting messages provided on the DP system displays and on the printer
Environmental forces are never constant. Wind, current and swell should be monitored continuously as should their effects on position keeping. Electronic monitoring methods, such as wind sensors and resultant force vectors provide the DP control system with inputs, but these methods should be supported by visual monitoring and forecasting. Preventative measures may require the vessel to cease operations during these periods and move off to a safe location.
5.7. The alarms and warnings associated with catastrophic failure, i.e. position and/or heading Dropout
Status lights should be provided in the operation control rooms, if necessary repeated onto working areas, ECR, and supervisor and Masters cabin. They should be manually activated from, and repeated in, the DP control room. Status lights should be checked for operation prior to commencing operations. Operations should not commence to switching on the green light.
Steady green light to indicate vessel under full DP control, normal operational status, operations may commence. Yellow light to indicate the DP operating system is in a degraded status, operations may be stopped operations should be prepared to stop, divers move to a place of safety.
Flashing red light to indicate DP emergency, operations to be stopped and equipment personnel recovered.
A distinctive alarm should sound in the saturation control room, air diving control area, the Master's cabin, Operations Superintendent's cabin (if applicable) and the senior Diving Supervisor's cabin in conjunction with the flashing red light. Provision of a means of cancelling the audio and flashing functions of the signals from the receiving positions when they have been noted should be made.
5.8. Corrective actions to accept and remedy any alarm or warning condition
DP control location requires the DP watchkeeper to be in attendance at the DP control console at all times the vessel is operating in DP mode.
DP Alert Level Responses;
• Green – Normal. No action. Operations progress.
• Yellow – Degraded. This may mean cessation of all supply operations,
movements of the vessel away from the installation to a safe position, or to take manual control, for example in case of connected hose operatins.
• Red – Emergency. Take whatever action necessary to prevent human injury,