13.1 DP Capability Plots

Capability plots: the planning picture of “can we hold?”

A DP capability plot (also called a DP capability diagram, polar capability plot, or environmental-force envelope) answers a planning question in geometric form:

For a defined thruster and power configuration, and for a stated environmental model, what is the maximum environmental force the vessel can still resist while holding position and heading — as a function of vessel heading?

On the Nautical Institute induction and simulator path you are not expected to generate plots from first principles. You are expected to know what the polar diagram shows, why intact and post-WCF sets both exist, where the numbers come from, and how a DPO uses the booklet (or electronic plot library) before committing to weather, heading, and critical work.

What the polar diagram shows

Capability plots are almost always drawn as polar diagrams:

If the present (or forecast) environment, converted into the plot’s units and assumptions, sits inside the envelope for the planned heading, the configuration is theoretically capable of holding under those assumptions. If it sits outside, the plant as modelled cannot produce the required surge/sway/yaw forces at that heading — station-keeping will fail or thrusters will saturate even before any equipment failure.

[!IMPORTANT] Capability is heading-dependent. The same wind speed that is acceptable bow-to-wind may be unacceptable beam-to-wind (or the reverse) depending on thruster layout, superstructure, and residual set after WCF. “We held 30 knots last week” without matching heading and configuration is not a capability argument.

Intact versus post-WCF capability plots

Two families of plots matter on every Class 2/3 vessel:

Critical activities (diving, close proximity to a platform, heavy lift over a live asset, gangway connected, hose connected under tension, etc.) must be planned against the more restrictive residual envelope, not only the optimistic intact plot. Section 11 already defined WCF and online consequence analysis; this section is the offline polar twin of that residual thinking.

Practical reading order for a DPO before a critical task:

1. Identify which residual case applies (open-bus section loss, proven closed-bus residual, thruster group loss, etc.).
2. Open the matching post-WCF plot (and any intermediate degraded-case plots company keeps).
3. Convert forecast wind / current / sea into the plot’s units and assumptions (or use company conversion tables).
4. Check the planned heading against the residual envelope with margin.
5. If the residual envelope is tight, change heading, reduce concurrent thruster load, improve plant (more thrusters enabled, correct bus), or delay/abort the critical phase.
6. Once on location, treat online consequence analysis + ASOG + footprint as the live confirmation — plots do not auto-update every sea-state sample.

Generated from design data and FMEA — not live sea state alone

Capability plots are engineering products. Typical inputs include:

• thruster type, location, max thrust, forbidden zones, and interaction limits,
• power available to thrusters for each configuration case,
• vessel windage and current drag models (area, coefficients, draft),
• sometimes wave drift force assumptions for a design sea state,
• residual thruster/power sets taken from the FMEA / WCF analysis,
• allocation logic assumptions (how the DP system will use remaining thrusters).

Exam-critical distinction: capability plots are pre-calculated planning envelopes. They are not generated solely from the live wind sensor and wave buoy at the moment you look at the screen. Online consequence analysis uses present plant and present environment estimates to re-test residual hold continuously; the paper/electronic capability plot booklet is the offline design envelope for planning and for understanding shape of the polar.

If thrusters have been deselected “to save fuel,” a drive is degraded, a retractable thruster is stowed, or the bus configuration differs from the plotted case, the published plot may overstate real capability. The DPO must match plot case ↔ plant state. A beautiful intact polar is worthless when you are already operating on half the residual thrusters assumed in the FMEA case.

How the DPO reads “can we hold?”

Reading a capability plot is a disciplined checklist, not a glance at the outer ring.

Step 1 — Identify the case. Intact? Post-WCF bus A lost? One thruster group out? Power-limited residual? Wrong case → wrong answer.

Step 2 — Align environment direction. The polar is usually relative to wind or a defined environment vector. Convert “wind from 040° true, vessel heading 220°” into the relative angle the plot uses (bow-on, quartering, beam, etc.).

Step 3 — Read radial capability at that angle. Note wind-speed (or force) limit on the curve. Apply company safety margin if required (never plan to sit exactly on the line).

Step 4 — Compare to forecast and actual. If forecast peak exceeds residual capability at the only headings the operation allows, the operation is not weather-viable on residual criteria.

Step 5 — Cross-check operational constraints. Crane boom, hose, gangway, pipeline, or client heading restrictions may force a heading that is weak on the residual plot. That is a planning conflict to resolve before entry — not a surprise at green status on the first watch.

Step 6 — Confirm with live tools. On watch: thruster utilisation, power margin, footprint growth, online consequence analysis colour/alarm, ASOG environmental rows. Plots answer planning “can we hold?”; live tools answer now “are we still inside residual safety?”

Environmental force components the plot may include

Plots vary by vendor and company standard. Common framings:

Never assume a wind-only polar includes your actual 2-knot beam current and long swell. Read the legend and assumptions on the plot sheet. Exam stems that hide a strong current while the candidate only quotes a wind curve are testing whether you treat the plot as a complete real-world weather oracle — it is not.

Worked bridge scenario

A dive support vessel plans 18 hours of diving 30 m from a jacket in forecast 28-knot winds. Intact capability at the preferred beam-on working heading shows hold to ~40 knots wind-equivalent. Post-WCF (loss of one open-bus thruster group) shows only ~26 knots at that same heading, but ~34 knots if the vessel can lie ~20° closer to head-to-environment. The DPO refuses to accept the intact 40-knot story for dive CAM. Options discussed with Master and dive control: change heading toward the residual strong sector (if hose/umbilical geometry allows), reduce weather acceptance window to residual-safe levels, or delay the dive. They select a compromise heading still acceptable to the worksite and still inside the post-WCF envelope with margin, document it in the ASOG brief, and set abort criteria if wind trends above residual limit allowing for time to terminate diver recovery. Online consequence analysis is monitored continuously once on station.

Exam traps for capability plots

Operator habits

1. Know where the vessel’s intact and post-WCF plot sets live (DP ops manual, electronic library) and which residual case matches CAM configuration.
2. Match enabled thrusters and bus state to the plotted case before quoting numbers.
3. Plan critical weather against residual polar, not only intact.
4. Treat weak residual headings as operational constraints equal to crane or hose limits.
5. Use plots for planning; use consequence analysis, footprint, and ASOG for execution.

Bottom line: DP capability plots are polar envelopes of maximum environmental force versus heading for defined thruster/power configurations. Intact and post-WCF plots support heading and weather planning; they are generated from design data and FMEA residual sets, not from live sea state alone; and the DPO uses them to answer planning “can we hold?” while live consequence analysis and ASOG answer whether residual safety still holds on watch.