Dynamic positioning systems (DP1, DP2, DP3)

01

Choose the DP equipment class and redundancy

IMO MSC/Circ.645 (adopted 6 June 1994) groups DP systems into three equipment classes by worst-case failure: for class 1, loss of position may occur in the event of a single fault; for class 2, loss of position is not to occur after a single fault in any active component or system (generators, thrusters, switchboards, remote-controlled valves), and IMO adds that "Normally static components will not be considered to fail where adequate protection from damage is demonstrated, and reliability is to the satisfaction of the Administration"; for class 3, a single failure includes the class 2 items, and "any normally static component is assumed to fail", plus all components in any one watertight compartment or fire sub-division, from fire or flooding. The class for an operation is agreed between vessel owner and customer from a risk analysis of the consequence of a loss of position. [1] For class 2 the power system should be divisible into two or more systems with bus-tie breakers that separate automatically on failures; for class 3 the divided systems should sit in spaces separated by A.60 class divisions, watertight below the operational waterline. Class 2 needs at least two independent DP computers; class 3 needs two plus a back-up DP control system in a room separated by an A.60 division; classes 2 and 3 need a consequence analysis function that alarms if the worst-case failure would leave insufficient thrust; and each DP computer needs a UPS giving at least 30 minutes after a mains failure. [1]

ABS's Guide for Dynamic Positioning Systems (2024) states that IMO MSC.1/Circ.1580, issued on 16 June 2017, updates MSC/Circ.645 and is to be applied to vessels and units constructed on or after 9 June 2017, while MSC/Circ.645 may continue to be applied to those built from 1 July 1994 to before 9 June 2017. Its optional notations DPS-1, DPS-2 and DPS-3 are in line with IMO equipment class 1, 2 and 3; DPS-0 is an ABS class for centralized manual position control with automatic heading. Its minimum-equipment table lists 0, 1, 2 and 2 + 1 (backup station) DP control computers and 1, 2, 3 and 2 + 1 (backup station) position reference systems for DPS-0, DPS-1, DPS-2 and DPS-3, with a power management system and an FMEA required for DPS-2 and DPS-3; a power management system is not required where all thrusters are direct diesel drive. [2]

Editorial note: class notations differ by class society, so state the society and notation the vessel must carry, and ask the DP supplier for the type-approval certificate covering that notation.

Give the supplier the vessel type and operation (floating crane, construction, drilling, supply), flag and class society, required IMO equipment class and class notation, thruster arrangement and drive type, power system split and bus-tie philosophy, the specified maximum environmental conditions for station keeping, required position reference systems and sensors, UPS arrangement, consequence analysis and FMEA scope, operator stations (main and backup), and delivery and sea-trial schedule.

02

Specify position reference systems, sensors and the MRU

MSC/Circ.645 asks that for classes 2 and 3 at least three position reference systems be installed and simultaneously available; that where two or more are required they not all be of the same type but based on different principles; and, for class 3, that at least one be connected directly to the back-up control system behind an A.60 division. Vessel sensors should at least measure heading, vessel motions and wind; where a class 2 or 3 system depends on sensor signals, they should come from three systems serving the same purpose, for example three gyro compasses. [1] ABS requires a motion reference unit (MRU) where position reference systems depend on roll and pitch correction, at least two references on different measurement techniques where more than two are required, UPS power for references and environmental sensors, and differential correction equipment for satellite-based systems. [2]

Praxis Automation's Mega-Guard DP page shows how one maker implements this: the DP controller uses an extended Kalman filter to estimate vessel motion and wind, wave and current forces; it accepts DGPS, GNSS, laser, radar, hydroacoustic, taut wire and winch references plus gyro, wind and MRU inputs, and supports up to 16 position reference systems weighted by reliability and accuracy. Its GNSS reference lists 2.5 m accuracy with GPS and Glonass, 0.6 m with SBAS and 0.2 m with a correction subscription (2DRMS, 95%); its hydroacoustic reference uses a USBL transceiver with 5000 m range and seabed transponders for 4000 m depths; its taut wire reference uses a 500 m, 6 mm steel cable with a clump weight, corrected for roll and pitch by the MRU; and it supports laser and radar references that measure range and bearing to reflectors or responders on another vessel or structure. The page quotes positioning to 1 dm with high-accuracy references, an interface to the power management system to prevent thrusters drawing more power than available, and a joystick control model plus DP0 to DP3 models, the DP3 model being triple redundant. [3]

Kongsberg's MRU 5 datasheet gives an example of motion sensor data: roll and pitch accuracy of 0.02° RMS for a ±5° amplitude, 0.001° resolution, real-time heave accuracy of 5 cm or 5% (whichever is highest) for 0 to 25 s periods, data output up to 200 Hz, 10 to 36 V DC supply at max 8 W, 2.2 kg, IP66, operating temperature −5 °C to +55 °C, and vibration and EMC to IEC 60945. [4]

03

Type approval, redundancy diagram and reference list to request from a DP supplier

When requesting quotations, ask each supplier for the class type-approval certificate, the redundancy block diagram for the required class, the list of supported position reference systems and sensors with their datasheets, consequence analysis and capability-plot functions, FMEA and proving-trial support, and service coverage for the vessel's operating area.

04

Controllers, operator stations, references, MRU, sensors, PMS interface, UPS and consequence analysis

The counts and the 30-minute UPS come from IMO MSC/Circ.645 and the ABS guide; the reference types and thrust limiting through the power management interface are as Praxis describes its own Mega-Guard system.

05

Where the DP figures come from: MRU 5 and Mega-Guard values, class rules and gaps

Where a row gives values for one unit they are Kongsberg MRU 5 or Praxis Mega-Guard figures, not whole-system data; heading accuracy, wave height, supply frequency and thruster response time are not stated in any source reviewed.

06

Send the vessel, operation and class together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Vessel: [type; thrusters; drive]
Operation: [crane lift; construction; drilling]
Class: [IMO class; society notation]
References: [types; count]
Sensors: [MRU; gyro; wind; count]
Power: [split; PMS; UPS]
Environment: [wind; current; sea state]
Quantity: [systems]

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