13.2 Footprint Plots & Weather-Optimal Heading

Footprint versus capability: two different polar stories

Candidates often mix capability plots with footprint plots. They may both appear as polar or plan-view diagrams, but they answer different questions:

A footprint (DP footprint, position plot, watch circle history, excursion plot) is the scatter of measured vessel position over a period on automatic DP (and often a heading scatter or thruster utilisation trend alongside). It is one of the DPO’s best performance monitors: small, centred scatter usually means healthy control; large, growing, or one-sided scatter means the plant is working hard or something is wrong.

What a footprint shows operationally

Typical footprint displays include:

• plan-view trail or cloud of position fixes relative to the setpoint or origin,
• optional rings at agreed watch-circle radii,
• time window (last 15 minutes, 1 hour, since “reset footprint”),
• sometimes thruster force vectors or percent utilisation overlaid,
• heading stability indicators.

[!NOTE] Footprint is not a guarantee of future safety. A tight footprint in moderate weather can explode after WCF or a squall. Use footprint with thruster percent, power reserve, consequence analysis, and ASOG — never alone.

Using footprint as a monitor (exam and bridge habits)

1. Reset or mark the footprint when you settle on a new setpoint, heading, or after major plant change so the cloud reflects the current phase.
2. Agree watch-circle / excursion limits with the activity (dive, gangway, crane) and ASOG — know when scatter becomes advisory or red.
3. Trend thruster utilisation with the footprint: a still-small footprint at 85% thruster average is a margin warning, not a success story.
4. Investigate bias early — do not wait until the vessel is at the edge of the permitted circle.
5. After failures, compare residual footprint growth to residual capability expectations; expanding footprint with saturated residual thrusters is a terminate/abort cue.

Weather-optimal heading — reduce thruster power by alignment

Weather-optimal heading (environment-optimal heading, minimum-power heading, weathervaning-style DP heading selection) means choosing a vessel heading that minimises the thruster force needed to hold position against wind, current, and waves. In practice the DPO (or an advisory function on some systems) seeks a heading where:

• windage moment and lateral force are reduced (often nearer head-to-wind or a known sweet spot for that hull),
• current load is less beam-on when current dominates,
• thruster geometry is efficient for the residual force direction,
• post-WCF residual envelope still has margin at that heading.

Benefits of a good weather-optimal choice:

Weather-optimal is not always pure head-to-wind. Current, waves, thruster forbidden zones, and hull-specific polar shape can shift the true minimum-power heading. Some DP systems provide a heading optimisation or power plot advisory; the operator still owns the decision when worksite geometry conflicts.

Trade-offs with the operation

Pure weather-optimal heading often conflicts with the industrial task. Exam scenarios love this tension.

The professional answer is a negotiated heading band: as weather-optimal as residual safety allows, as operational as the task requires, documented in the pre-job brief and ASOG. If the only heading the crane allows sits outside residual capability for forecast weather, the correct decision is not “force the crane heading and ignore residual plots.” Options include delay, reduce concurrent work, improve plant, change lift geometry, or abort the critical phase.

Exam-style heading choice scenarios

Scenario 1 — Free field, rising wind. Vessel on open-water standby, no hose or gangway. Wind increases and thruster utilisation climbs to 70% with a growing footprint. Best first heading action: rotate toward weather-optimal (typically more head-to-environment) while monitoring footprint and consequence analysis. No industrial geometry blocks the turn.

Scenario 2 — Crane over subsea asset. Forecast allows residual hold only within ±15° of head-to-wind. Crane needs beam-on for outreach. Beam-on residual plot is outside envelope. Correct decision path: do not accept beam-on “because the client wants the lift today.” Redesign lift geometry, wait for weather, or cancel — residual station-keeping after WCF outranks schedule pressure.

Scenario 3 — Gangway connected. Weather-optimal would reduce thruster load but swings the gangway outside the motion envelope. DPO holds gangway-compatible heading while weather is still inside residual capability; when wind trends such that time to disconnect will leave the vessel unable to hold residual, ASOG drives early disconnect then rotate to weather-optimal and move clear.

Scenario 4 — Footprint bias with thrusters at 40%. Small thruster demand but large position bias suggests reference or model problem, not “need weather-optimal heading.” Fix PRS/selection first; spinning the ship will not cure a bad laser target.

Linking footprint, weather-optimal heading, and capability

A clean mental model for the exam:

1. Capability polar tells you which headings can hold a given environment for intact/residual plant.
2. Weather-optimal choice picks, within allowed headings, the one that minimises thruster power and maximises margin.
3. Footprint shows whether the choice is working in reality.
4. If footprint grows or thrusters soar, either environment exceeded the plan, heading is no longer optimal, plant degraded, or sensors/references are lying — investigate and follow ASOG.

Operator checklist for heading and footprint

1. Before critical work: pick heading from residual capability + worksite geometry + escape route, not thruster economy alone.
2. Brief the allowed heading band and who can authorise a change.
3. On DP settle: reset footprint, record thruster baseline %.
4. On weather shift: re-evaluate weather-optimal within the band; if band is unsafe, terminate industrial interface first.
5. Never treat a tight footprint as permission to ignore post-WCF plots or consequence analysis alarms.

Bottom line: footprint plots monitor historical position scatter and real holding performance; weather-optimal heading reduces thruster demand by aligning with wind and current; operational needs create heading trade-offs; and exam answers choose headings that keep residual station-keeping credible while still serving crane, hose, gangway, and escape constraints.