1.1. Dynamic Positioning Systems
For many offshore operations it is necessary to keep a vessel at a fix position and heading. Traditionally this has been done using an anchor spread. Nowadays, Dynamic Positioning (DP) systems are replacing anchors.
A Dynamic Positioning system is able to control the position and heading of a vessel by using thrusters that are constantly active and automatically balance the environmental forces (wind, waves, current etc.). Environmental forces tend to move the vessel off the desired position while the automatically controlled thrust balances those forces and keeps the vessel in position.
The main components of any DP system are the positioning system, the DP computer and the thrusters. The positioning system, usually a GPS, monitors the position of the vessel. When the vessel moves off the intended position the DP computer will calculate the required thrust which will then be applying by the thrusters in order to maintain the position of the vessel.
1.2. The use of DP systems
Dynamic positioning systems are typically used by offshore vessels for accurate maneuvering, for maintaining a fixed position or for track keeping (pipe/cable laying). We usually find DP systems on:
- Offshore drilling vessels (Drilling ships and Semi-submersibles). A Drilling vessel will use DP to remain in a fix location while drilling in deep water.
- Offshore support vessels: Platform supply vessels (PSVs), Well intervention vessels, Diving Support Vessels. Support vessel use DP to stay in a safe distance from offshore platforms and drilling rigs.
- Pipe-laying and offshore construction vessels. Pipe-laying vessels use DP for position keeping and track keeping.
- Dredging vessels. Suction Hopper dredgers, Rock-dumping vessels, Trenching vessels
- Shuttle Tankers. Shuttle tankers during offloading of FPSOs.
The first DP system was installed in 1961 on the vessel ‘Eureka’. Nowadays (2022), there are over 1000 DP-capable vessels and DP is consider indispensable for deep-water operations.
1.3. Pros and Cons of using Dynamic Positioning
Dynamic Positioning is not always the best of the most economical option. Mooring lines are usually a better option for shallow water or for operation that do not require frequent relocation of the vessel (e.g. drilling at shallow water, diving operations in shallow waters). On the other hand DP is the best option for deep water operations, for congested seabeds and in situations where to vessel needs to relocated frequently
- Quick and easy positioning and maneuverability of the vessel. No need for mooring lines, tugs boats and time consuming anchor handling operations.
- Offshore operations can take place in ultra-deep waters were mooring lines are difficult to installed.
- Easy to change location or weather vane in order to avoid the effects of bad weather. Quick disconnect and sail away in case of emergency.
- Very safe when working in congested seabeds with many pipelines, mooring lines from other vessels or subsea structures such as manifolds, wellheads, risers etc.
- High Capital expenditure for designing and installing a DP systems. High CAPEX.
- High fuel consumption and increased maintenance cost. High OPEX.
- It poses limitations in very swallow waters and situations were diving operations must take place close to the thrusters
- Potentially severe consequences in case of equipment failure during pipe-laying or during operations near fixed platforms.
1.4. The components of a DP systems
There are 5 main component in a DP systems:
- Control Systems. The DP control system calculates the offets between the measured values of position and heading and the required values (setpoint values). Based on the calculated offsets the control system calculated the forces that the thrusters must generate in order to reduce the errors to zero.
- Power generation
- Thrusters and propulsion
- Environmental reference
- Position and Heading reference
1.5. How DP systems work
The video below explains where and how DP systems are used
1.6. Thrusters and propulsion
The initial design of a DP system sets the environmental conditions at which the vessel is intended to work on DP. Based on this operational requirement the number, size and location of thrusters is determined.
1.7. Weathervaning DP
Weathervaning DP is the use of dynamic positioning to keep the vessel at fixed position while allowing the heading to change to get heading with the least environmental load. This function is similar to single point mooring systems and results into minimum power requirements.
Weathervaning DP was introduced and tested already in the late 1980s by Pinkster and Davison. The basis of this control
is that the vessel is freely weathervaning, similar to single point moored vessels. This type of control is also called weathervaning DP. It must not be confused with the weathervaning mode of normal DP vessels, since bi-axial DP is a passive heading control