8.2 Thrust Allocation, Forbidden Zones & Bias

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:

1. Which thrusters are available (enabled, ready, not faulted, deployed if retractable).
2. Each thruster’s force envelope (max RPM/pitch, direction limits for tunnels, azimuth range).
3. Forbidden/prohibited zones on azimuth angles.
4. Rate limits (how fast RPM, pitch or azimuth may change).
5. Power available (per thruster, per bus, or plant-wide caps from PMS).
6. Bias / force-opposition settings when configured.
7. Interaction losses modelled or mitigated by zones and allocation preferences.

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.

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 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.

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

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.