8.2 Thrust Allocation, Forbidden Zones & Bias

Nautical Institute DP Operator DPO

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1 Introduction & NI Scheme

1.1 Current Exam Facts 1.2 Phases A–E Pathway 1.3 Certificates, Eligibility & Revalidation

2 DP Principles Fundamentals

2.1 What is Dynamic Positioning & Degrees of Freedom 2.2 Seven Components of a DP System 2.3 The DP Operator & Human-Machine Interface

3 Controller, Model & Environmental Sensors

3.1 DP Controller & Mathematical Model 3.2 Kalman Filtering & Current Estimation 3.3 Gyrocompass & Heading Feedback 3.4 Wind Sensor, MRU & UPS

4 GNSS & Acoustic Position References

4.1 DGNSS & Differential Correction 4.2 Relative GNSS / RTK Concepts 4.3 Hydroacoustics USBL/SSBL 4.4 Long Baseline (LBL) Systems

5 Relative PRS, Selection & Voting

5.1 Light Taut Wire 5.2 Laser Relative References (Fanbeam/CyScan) 5.3 Radar/Microwave Relative References (RadaScan) 5.4 Reference Selection, Voting & Common-Mode Failure

6 Power Generation & PMS

6.1 Diesel-Electric Power Architecture 6.2 Power Management System 6.3 Load Sharing, Load Shedding & Spinning Reserve

7 Bus Configuration & Power Failures

7.1 Open vs Closed Bus-Tie 7.2 Blackout & Blackout Recovery 7.3 Drive-Off vs Drift-Off

8 Thrusters & Allocation

8.1 Tunnel, Azimuth & Retractable Thrusters 8.2 Thrust Allocation, Forbidden Zones & Bias 8.3 Thruster Interaction & Faulty Thruster Response

9 Control Modes

9.1 Auto DP Mode 9.2 Joystick & Independent Joystick System 9.3 Mode Selection & Practical Setup

10 DP Equipment Classes

10.1 IMO Equipment Classes 1, 2 and 3 10.2 MSC.1/Circ.1580 vs MSC/Circ.645 10.3 Sensor Redundancy & 2-out-of-3 Voting

11 FMEA, Worst Case Failure & Consequence Analysis

11.1 FMEA & Annual DP Trials 11.2 Worst Case Failure 11.3 Consequence Analysis

12 ASOG, CAM & TAM

12.1 Activity Specific Operating Guidelines (ASOG) 12.2 Critical Activity Mode (CAM) 12.3 Task Appropriate Mode (TAM)

13 Capability Plots & Approach Planning

13.1 DP Capability Plots 13.2 Footprint Plots & Weather-Optimal Heading 13.3 500 m Zone Approach & Escape Routes

14 Watchkeeping & Industry Guidance

14.1 Watchkeeping & Handover 14.2 Checklists, Alarms & Time to Terminate 14.3 IMCA M103/M117 Guidance Context

15 Integrated Operations & Assessment Strategy

15.1 Multi-System Failure Scenarios 15.2 Integrated Operations Case Studies 15.3 Induction Assessment Strategy

8.2 Thrust Allocation, Forbidden Zones & Bias

Key Takeaways Explain This Topic AI

Thrust allocation logic (TAL) converts controller force/moment demand (Fx, Fy, Mz) into individual thruster RPM/pitch/azimuth commands

Forbidden (prohibited) zones stop azimuth thrusters aiming into angles that cause thruster–thruster wash, hull impingement, or wash into PRS/divers/ROVs

Thruster bias (force opposition) runs opposing thrusters against each other so net force is small but response in either direction is fast — at a cost in fuel and wear

Power limits from PMS/thruster ratings cap what TAL may demand; available thrust can be less than the theoretical maximum of enabled units

TAL re-optimises continuously when thrusters are enabled, disabled, faulted, or constrained by zones and rate limits

Last updated: July 2026

From force demand to thruster orders

The DP controller decides how much total force and yaw moment the vessel needs: typically Fx (surge), Fy (sway) and Mz (yaw). It does not permanently hard-wire “bow thruster always does all sway.” Instead, thrust allocation logic (TAL) — also called force allocation — solves, every control cycle, how to share that demand across the enabled, healthy thrusters within their physical and operational limits.

If you only remember one sentence: TAL turns a three-axis force demand into many thruster commands. Everything else in this section is a constraint on that optimisation.

What TAL optimises

TAL aims to meet Fx, Fy and Mz as closely as possible while respecting:

Which thrusters are available (enabled, ready, not faulted, deployed if retractable).

Each thruster’s force envelope (max RPM/pitch, direction limits for tunnels, azimuth range).

Forbidden/prohibited zones on azimuth angles.

Rate limits (how fast RPM, pitch or azimuth may change).

Power available (per thruster, per bus, or plant-wide caps from PMS).

Bias / force-opposition settings when configured.

Interaction losses modelled or mitigated by zones and allocation preferences.

• TAL input Source
• Force/moment demand Fx, Fy, Mz DP controller
• Thruster enable/status Operator + thruster systems
• Feedback (RPM, pitch, azimuth) Thruster sensors
• Power limits PMS / drive ratings
• Zones, bias, priorities Configuration / operator settings

When the demand cannot be met fully — weather exceeds capability, too many thrusters offline, power capped — TAL saturates : thrusters go to limits, residual force remains, and the vessel’s footprint grows . That is expected physics, not a mysterious software bug.

Forbidden zones (prohibited zones)

A forbidden zone (also called a prohibited zone) is a configured sector of azimuth angles that a thruster must not aim into while under DP allocation. Reasons include:

Thruster–thruster interaction — wash from one thruster into another reduces net force and can cause vibration or load instability.

Thruster–hull interaction — wash hugging the hull (related to Coanda-type effects) wastes thrust.

Wash into PRS — e.g. thruster race disturbing a taut wire, acoustic transducer, or laser/radar target path.

Dive/ROV/subsea asset protection — wash hazardous to divers, ROVs, or umbilicals in defined arcs.

Mechanical or structural limits — hard stops, nozzle clearances, ice knives, etc.

• Concept Operator meaning
• Forbidden zone Do not command thrust in these azimuth angles
• Effect on TAL Allocator finds other thrusters or other angles to meet demand
• Capability impact Effective envelope may shrink even though thruster is “online”
• Not the same as Max RPM limit, generator load limit, or thruster deselection

Exam trap: forbidden zones are about direction , not a blanket RPM cap on every thruster. RPM limiting is separate. Forbidden zones also are not a transit park mode — they apply during DP allocation while the thruster remains selected.

If zones are too wide, or many thrusters share overlapping constraints, TAL may struggle to produce pure sway or pure yaw without residual coupling. If zones are too narrow (or missing where interaction is severe), you get efficiency loss and possible PRS pollution . Vessel FMEA notes and DP trials establish practical zone settings; the DPO’s job is to understand that zones trade freedom of azimuth for safer, more predictable force .

Bias and force opposition

Thruster bias (force opposition) deliberately runs thrusters against each other so that large opposing forces cancel to a small net force, while each thruster sits part-loaded and can ramp up or down quickly in either direction.

Why bother?

Thrusters (especially fixed-pitch with slow RPM build, or units with slew delays) have response lags .

Near zero net demand, unbiased thrusters may sit at idle and then be slow to produce force when weather shifts.

Bias keeps actuators “pre-tensioned” so small demand changes produce fast net force changes — improved control authority and often tighter position control in demanding conditions.

• Bias characteristic Detail
• Net force Near zero if perfectly opposed
• Individual thruster load Elevated — thrusters work hard against each other
• Benefit Faster response; better fine control
• Cost Higher fuel, wear, power draw, heat

When used Often in tight DP work or high-precision modes; reduced or off in calm/low-demand states per procedures

Bias is not “set all thrusters to maximum” and not “disable redundant thrusters to save fuel.” Those are opposite ideas. Bias spends fuel to buy responsiveness.

Tip

If thrusters look busy but the vessel is holding well with small footprint, bias may be active. If power margin is tight, reducing bias can free capacity for net weather force — a deliberate operational trade-off with the engineer and company procedures.

Power limits and allocation

TAL cannot invent kilowatts. Interfaces from the PMS and thruster drives impose power limits :

• per-thruster maximum electrical or hydraulic power,
• bus-section available power after generator trips,
• dynamic load limitation when frequency is threatened,

operator or automatic thruster power reduction.

When power-limited, commanded thrust is capped even if the hydrodynamic “ideal” allocation wanted more. The controller may still demand large Fx/Fy/Mz, but delivered force falls short — footprint increases, consequence analysis may go red, and the correct response is operational (reduce risk, improve power plant state, change heading, stop critical work), not “force more RPM” against PMS protection.

• Limit type Typical effect
• Thruster rating Single unit cannot exceed nameplate force
• Plant / PMS cap Whole set or bus limited after power loss
• Rate limit Force builds slowly; short gusts not fully countered
• Zone constraint Force direction restricted; other thrusters overloaded
• How TAL behaves when the thruster set changes

TAL re-solves allocation when:

• a thruster is deselected or trips,
• a thruster is enabled after deployment,
• feedback shows a thruster is not following command,
• power available changes,

bias or mode settings change.

Remaining thrusters pick up the load. If redundancy was designed correctly, position may still hold within a smaller capability envelope. If the lost thruster was critical for the current weather direction, residual force appears immediately — this is the link between allocation and capability plots (intact vs post-failure).

Worked allocation scenario

Demand is strong beam wind (large Fy) plus small heading error (Mz). TAL loads bow tunnels hard to starboard, aims both stern azimuths with lateral components, and avoids a forbidden sector on the port stern azi that would wash the starboard azi. Bias is moderate so tunnels are not starting from zero. A generator trips; PMS imposes thruster power limit. TAL reduces magnitudes; footprint grows; DPO confirms heading still optimal on the capability plot and delays the next critical lift until reserve generation is restored. Allocation did its job within the new power constraint — it did not violate PMS to “keep the old footprint.”

Exam traps for allocation items

• Trap Correct framing
• TAL selects which DGNSS to use That is reference management / voting
• TAL starts standby generators That is PMS
• Forbidden zone = max RPM for all thrusters Zones constrain azimuth direction
• Bias saves fuel by idling thrusters Bias uses fuel for opposing load
• After thruster loss, TAL freezes old commands TAL re-allocates remaining units

Bottom line: TAL is the optimiser that turns Fx/Fy/Mz into thruster orders under zones, bias, power and rate limits . Know what each constraint buys you and what capability it costs.