DP System on Drillship CHIKYU: Dynamic Positioning System- Part Two

FLASHBACK

This article is the second part on the series of "Dynamic Positioning System". The first part of this series had focused on an introduction to DP Systems and their application in pipe-laying and cruise ships. The second part is a case study on the DP System on Drillship CHIKYU.

PURPOSE OF DP SYSTEM ON DRILLSHIPS

A drillship requires to maintain her station or heading during underwater drilling operations. The dynamic effect of waves, wind and underwater currents take huge toll on the safety of the operation of the vessel. The drill and riser pipes are nothing but like strands of thread when they go down to depths of the order of 2 to 3 kilometers below the sea level. Any significant disturbance in the orientation of these structures due to the ship's uncontrolled motions during drilling operations have every possibility to trigger a major maritime disaster. 

The Dynamic Positioning System ensures that the ship maintains her station or a pre-defined heading. Drillship CHIKYU is Japan's scientific drilling vessel designed for drilling operations up to depths of 7 kilometers. This article is a case study on the design of DP system used on-board the ship.

DESIGN OF CHIKYU'S DP SYSTEM

MAIN PARTICULARS:

Main Particulars of Drillship CHIKYU (Info-graphic)

DP SYSTEM DESIGN PROCEDURE

The design process of the DP system of the ship included results from tank tests and a combination of static and dynamic simulation of the model using its thrusters, generators, and position keeping ability. So the basic control flow of the system was generated (refer the flowchart below).

The forces which control the motion of the vessel can be categorized as:

• Feed forward forces: The forces of waves, currents and wind, which are basically the external forces acting on the system, initiating deviation. They were estimated using the Kalman Filtering Technique (refer to the link for the calculations based on Kalman Filtering Technique)
• Feed back forces: These include the forces required to be generated by the thrusters in order to compensate the feed forward ones, to minimize deviation or maintain required heading. 

Consequently, there is a constant monitor on the electric load on the power system, according to the number of thrusters, the azimuth angle and RPM of the thrusters required to maintaining station.

POSITION REFERENCE SYSTEM:

In order to maintain station or predefined heading, the DP system takes help from a series of position reference systems used on board the ship. These systems constantly feed the ship's position to the DP sensors on-board the ship. Each position reference system in use, has its own set of advantages and disadvantages. Therefore, a ship using DP system, makes use of multiple position reference systems to maintain redundancy and accuracy.

The ones used on-board Drillship CHIKYU are:

• Network DGPS:  In a Network Differential Global Positioning System, the vessel obtains its co-ordinates in terms of latitudes and longitudes with reference to the World Geodetic System. It uses a network of multiple satellites (CHIKYU uses INMARSAT satellites) and land reference stations called 'links'. The choice of links depends on the operational area of the ship. 

Layout of DGPS Configuration

• GPS-GLONASS System: The Global Navigation Satellite system is the Russian powered counterpart of USA's GPS system. The advantage in using a GLONASS system is that, the GLONASS satellites have higher inclination (upto 65 degree latitudes) as compared to GPS satellites which have maximum inclination of 55 degree latitudes. This enables better connection availability even in higher latitudes. A GPS-GLONASS System therefore uses the feeds from both, GPS and GLONASS reference systems for position reference. 
• HAPRS (Hydro Acoustic Position Reference System): HARPS applies the theory of acoustic underwater ranging by communication at hydroacoustic frequencies between hull mountd transducers and sea-bed located transponders. 

CONFIGURATION OF THRUSTERS:

CHIKYU uses a combination of thrusters for its dynamic positioning. The layout of the azimuth thrusters and a side thruster under CHIKYU's hull can be clearly understood in the following graphic representation. 

Layout of thrusters in Drillship CHIKYU

DP CONTROLLING FUNCTIONS:

Based on different sea conditions and operational requirements, the dynamic positioning system on Drillship CHIKYU is set to various modes of operation, each having a specific characteristic and purpose:

A) DP Mode: 

• All thrusters controlled automatically to maintain station keeping or to keep the vessel's motion steady depending on the external forces on the ship.
• If the target position is altered, the ship will move towards the target with steady speed maintaining controlled deviations from its defined path. The steady speed can vary from 0.1 knots to 0.5 knots.
• Even for adverse sea conditions like 2.5 knots currents (upto 30 degrees against heading angle), 23 knots wind (upto 30 degrees against heading angle) and 5.5 m waves (upto 30 degrees from heading angle), the vessel can be safely operated in DP mode by increasing the control gain of the DP system. 
• In calm sea conditions, the vessel's position can be fixed by automatically adjusting the azimuth angles. This is also called "Azimuth Fixed Mode".

B) AUTO HEAD Mode:

• Vessel's target point can be set, and vessel moves steadily towards the target even in rough sea conditions.
• The instantaneous direction of the vessel's heading can be controlled using joysticks on the DP control panel.

C) JOY Mode:

• Vessel's heading and direction can be controlled using joystick on the DP control panel.
• The thrust can be increased upto four times normal, using thrust power up button on the joystick itself.

D) NAVIGATION Mode:

• While positioning the vessel on its drilling site, this mode enables auto piloting and navigating operations for precise positioning.

E) RACS Mode:

• After the riser pipes are lowered, it is necessary to control the riser angle with respect to the vessel.
• This mode enables the positioning of the ship with respect to the top and lower riser angle feeds from the DP sensors.
• Thus, the riser angle always remains minimised during its drilling operations, irrespective of the sea conditions.

Concept Layout of RACS Mode operation

F) CONSEQUENCE ANALYSIS:

• During operation of the DP system, it also analyses its position keeping capability.
• In case the present hading or position keping falls below a preset value, the system automatically triggers an "Insufficient Thrust" warning to the operators.
• This adds an additional layer of redundancy to the entire system.

RELIABILITY:

After the DP system was installed on the ship in 2004, numerous sea trials have been carried out ever since to maintain a check on the reliability of the system. Its position keeping ability has remained outstanding, and it has not shown a standard deviation of more than 5 meters even in the roughest sea conditions till date. LSD

Article By: Soumya Chakraborty

Author's Note: The case study on the DPS on drillship CHIKYU has ben carried out based on the tank test and sea trial results published by the concerned organizations. Flowcharts and pictures in this article do not belong to LSD. Full credit goes to their respective owners. Thank You for reading. If you have any doubts and queries, please comment or write to me at learnshipdesign@gmail.com