Source-linked AI summary
Wind-induced drift of objects at sea: the leeway field method
Øyvind Breivik, Arthur A Allen, Christophe Maisondieu, Jens Christian Roth
TL;DR
The paper addresses the need for standardized, transferable leeway data to support stochastic forecasts of missing SAR and HAZMAT objects. It proposes a standardized field method and illustrates it with three objects, whose measured drift properties include distinct downwind and crosswind behavior.
Problem
Existing SAR and HAZMAT trajectory forecasts require object-specific drift properties, but older field studies used obsolete methods and lacked a common procedure for producing exchangeable data.
Method
The paper defines leeway operationally, standardizes 10 m wind measurements, recommends direct current-meter measurements where possible, classifies objects by instrumentation capacity, and condenses results into nine coefficients.
Results
The method was applied to a shipping-container model, an oil drum, and a World War II mine, with downwind drift of approximately 1.4%, 0.8%, and 2% of 10 m wind speed, respectively.
Takeaways & Limitations
The standardized procedure supports exchanging leeway data between trajectory models, while direct measurements can reduce experimental spread and produce tighter uncertainty estimates for search operations.
Takeaways & Limitations
Full unconstrained and unsymmetrical coefficient analysis may not be possible when field data are short or wind conditions show little variation.
Abstract
from arXiv · showhide
A method for conducting leeway field experiments to establish the drift properties of small objects (0.1-25 m) is described. The objective is to define a standardized and unambiguous procedure for condensing the drift properties down to a set of coefficients that may be incorporated into existing stochastic trajectory forecast models for drifting objects of concern to search and rescue operations and other activities involving vessels lost at sea such as containers with hazardous material. An operational definition of the slip or wind and wave-induced motion of a drifting object relative to the ambient current is proposed. This definition taken together with a strict adherence to 10 m wind speed allows us to refer unambiguously to the leeway of a drifting object. We recommend that all objects if possible be studied using what we term the direct method, where the object's leeway is studied directly using an attached current meter. We divide drifting objects into four categories, depending on their size. For the smaller objects (less than 0.5 m), an indirect method of measuring the object's motion relative to the ambient current must be used. For larger objects, direct measurement of the motion through the near-surface water masses is strongly recommended. Larger objects are categorized according to the ability to attach current meters and wind monitoring systems to them. The leeway field method proposed here is illustrated with results from field work where three objects were studied in their distress configuration; a 1:3.3 sized model of a 40-ft Shipping container, a World War II mine and a 220 l (55-gallon) oil drum.
IFREMER, France
The paper identifies Jens Christian Roth with the Royal Norwegian Navy, Norway, and cites a revised 2011 publication of The Leeway Field Method.
- Jens Christian Roth is affiliated with the Royal Norwegian Navy in Norway.
- The revised paper was published by Breivik, Allen, Maisondieu, and Roth in Applied Ocean Research in 2011.The citation gives volume 33, pages 100–109, and DOI 10.1016/j.apor.2011.01.005.
1. Introduction
The paper addresses the need for reliable, exchangeable drift data to support stochastic search and rescue and hazardous-material trajectory forecasts. It proposes a standardized leeway-field procedure spanning operational definitions, measurement methods, coefficient estimation, and object-size categories.
- Search and rescue and hazardous-material forecasts use stochastic trajectory ensembles incorporating currents, wind, and object-specific drift properties.
- Existing object databases are incomplete, and generic or regional categories are needed when an exact object match is unavailable.
- Field-work quality controls search-area expansion, so tighter confidence limits can reduce the scope and magnitude of search operations.
- The paper proposes a common procedure so field results can be exchanged between organizations and nations and used by trajectory forecast models.
- The recommended procedure is illustrated with shipping containers, oil drums, and drifting World War II mines, alongside instrumentation trade-offs.
2. Estimating the leeway of a drifting object
The paper defines leeway relative to near-surface ambient current, standardizes wind measurements, and recommends direct current-meter measurements where feasible. It then separates downwind and crosswind components, estimates regression coefficients, and matches methods to object size and available data.
- Operational definition: Leeway is wind- and wave-induced object motion relative to ambient current measured between 0.3 and 1.0 m depth.
- Operational definition: Standardized 10-minute vector-averaged wind measurements are scaled to the 10 m reference height.
- Leeway components: Downwind and crosswind decomposition supports separate regression analysis, including distinct left- and right-drifting crosswind coefficients.
- Coefficient estimation: Nine coefficients condense linear relationships between 10-m wind speed and downwind and crosswind leeway components, with associated errors.
- State changes: Jibing, capsizing, swamping, and tumbling require sufficient repeated or long-duration observations to establish their frequencies.
- Measurement methods: The direct method attaches or tows a current meter with the object, while the indirect method subtracts separately estimated current from total drift.
- Measurement methods: Direct measurements showed dramatically lower experimental spread than older indirect measurements, although this comparison estimates only regression deviations.
- Data requirements: Limited data duration or wind-speed variation may prevent unconstrained, unsymmetrical coefficient estimation and require constrained alternatives.
3. A field study of the drift properties of three objects
Field trials measured the leeway of a scaled container, an oil drum, and a WWII mine using object-specific instrumentation and standardized wind-based analyses. The results generally showed linear downwind drift, little crosswind motion for the container and mine, and agreement between the container measurements and earlier theory.
- Field setup: Four drift runs tested three objects near Fedje, Norway, under moderate seas with significant wave height about 2.3 m and peak periods of 6.5–9.5 s.The objects were prepared to approximate distress configurations, and wave-drift forces were considered negligible for these tests.
- Analysis: Leeway and wind were matched as 10-minute vector averages, decomposed into downwind and crosswind components, and fitted with constrained and unconstrained linear regressions against 10-m wind speed.The analysis produced regression slopes, offsets, error measures, and related statistics for the leeway components.
- Scaled-down model container: The 70%-immersed model container drifted at approximately 1.4% of wind speed in unconstrained regression and 2% in constrained regression.The unconstrained result agreed reasonably with an earlier theoretical estimate of about 2.2%; differences were within the reported experimental error range.
- Scaled-down model container: The model container exhibited very little crosswind drift, consistent with earlier assumptions, although full-scale containers and additional immersion levels require further measurements.The authors specifically identify 10–20% and 80–95% immersion as useful conditions for future study.
- World War II mine: The WWII mine had very little crosswind leeway and a downwind component of approximately 2% of wind speed.Its downwind relationship was evaluated against 10-m wind speed using constrained and unconstrained regressions with 95% prediction limits.
- Uncertainty of the leeway estimates: The three objects clustered well in two-dimensional leeway space and had comparatively low error ellipses because their leeway was measured directly.All three objects were heavy and well immersed in their distress configurations.
4. Conclusion
The paper presents a standardized leeway field method that converts measurements into coefficients usable in trajectory models while retaining raw data for future model developments. Results from three objects support the method, but uncertainty remains sensitive to data quantity, object loading, and wave effects.
- Conclusion: The method standardizes leeway definitions and 10-m wind measurements so field data can be exchanged between trajectory models.It condenses results into nine coefficients while recommending that raw experimental data also remain available.
- Conclusion: The three objects showed distinct downwind leeway rates: 1.4% for the container, 0.8% for the oil drum, and 2% for the mine.The mine and container had negligible crosswind leeway, whereas the drum had 0.4–0.6% crosswind leeway; constrained regressions produced higher ratios.
- Instrumentation: Towed current meters appeared superior to gimbled meters, while using one wind anemometer worked here because winds were uniform and objects stayed relatively close.Flow distortion around object undersides may nevertheless cause deviations from actual motion through the water.
- Limitations: Experimental spread was extensive, and the influence of off-wind swell on residual variance and object jibing remained unclear.Resolving wave effects would require significant-wave-height time series and possibly a full two-dimensional wave spectrum.
- Limitations: Different loading conditions should also be tested because distress-object loading can vary greatly, especially for containers and oil drums.The paper offers no final operational solution, but suggests including multiple loading ranges.
- Conclusion: Directly measured leeway estimates had low error ellipses compared with older categories studied using the indirect method.The paper links search-operation success to the quality of the preceding field work.
Tables
The tables report regression parameters for the three studied objects using both unconstrained and zero-constrained fits, with a separate breakdown of oil-drum crosswind events.
- Table 1: Table 1 lists unconstrained linear-regression parameters for the three objects studied.The wind range is specified in Table 2.
- Table 3: Table 3 separately regresses positive and negative 10-minute crosswind events for the oil drum under constrained and unconstrained fits.The total average of left- and right-drifting events appears in Table 1.