Propulsion & Dynamic Positioning (DP)

How the three parts share the work

The DP controller compares the measured position and heading with the set point. It adds a wind feed forward and a vessel model, then allocates thrust across the thrusters. Each thruster answers with propeller speed or pitch and an azimuth angle. If a reference drops out, the vessel model lets the controller hold position for a short time in dead reckoning.

A fault anywhere in that loop can push the rig off location and force an emergency disconnect of the lower marine riser package. That is why you judge a used thruster, controller or reference against the whole system, not on its own.

Equipment classes and the redundancy concept

IMO defines three DP equipment classes. In Class 1, a single fault may cause loss of position. In Class 2, a single fault in any active component, such as a thruster, generator, switchboard or remote-controlled valve, must not cause loss of position. Class 3 adds fire or flooding in any one compartment, so redundant groups sit in spaces separated by A-60 boundaries and watertight bulkheads. Deepwater drilling units are built to Class 2 or Class 3.

The redundancy concept splits power, thrusters and control into groups. The failure modes and effects analysis (FMEA) proves that the worst-case single failure leaves enough thrust to hold position. Any used component must fit that concept. A thruster with a different power draw, or a controller with a different network layout, changes the FMEA.

Standards and class notations

• IMO MSC/Circ.645 covers vessels constructed from 1 July 1994. MSC.1/Circ.1580, issued in June 2017, applies to vessels and units constructed on or after 9 June 2017.
• MSC.1/Circ.1580 calls for an initial survey, periodical surveys at intervals of no more than five years, and an annual survey. It also asks for an additional survey, general or partial as the case needs, after a defect is found or the DP system is repaired or altered.
• Class societies turn the IMO classes into notations, such as ABS DPS-2 and DPS-3, or DNV DPS(2) and DPS(3) alongside the long-standing DYNPOS(AUTR) and DYNPOS(AUTRO).
• IMCA M 103 gives design and operating guidance for DP vessels. IMCA M 190 covers annual DP trials, and IMCA M 166 covers FMEA.
• The MTS DP guidance documents set out design philosophy and operating guidance, including activity-specific operating guidelines (ASOG).

A used DP component installed on a classed rig means a class survey, an FMEA update and, in most cases, proving trials before the notation is confirmed.

Integration questions that cut across the group

• Thrusters are the largest consumers on a DP rig. Their drives sit on the main switchboards, and the power management system must keep enough online generation for the worst-case failure.
• Thruster interface to the controller, by hardwired signals or a fieldbus, and the feedback the controller expects.
• Sensor inputs from gyrocompasses, motion reference units and wind sensors, which need the same redundancy as the references.
• Software generation. A spare controller card with older firmware than the installed system may not run with it.

Sourcing used DP equipment

Like-for-like is the low-risk path, meaning the same OEM, model and software release as the installed equipment. Unused surplus from cancelled drillship newbuilds often matches units on sister hulls and keeps its OEM records. Equipment from cold-stacked rigs needs preservation records, because seals, bearings and electronics age in storage.

Ask for the class certificates, the FMEA and trials reports from the source rig, running hours and the OEM service history. Naventr can arrange an independent inspection on request and pass your questions to the Seller.