Slip-and-Fall Hazards in the Marine Environment
While 0.5 and above for the static anti-slip coefficient of friction may be safe on a dry walkway on land, for several reasons it may not be safe on vessels.
By Hector V. Pazos
GIVEN the exposure to the marine environment and the motions of a vessel, slips and falls are a significant source of personal injuries among employees on floating equipment, probably substantially greater than for most industry groups.
Although there are a good number of publications and articles written regarding the subject of slips and falls, there are almost none addressing the conditions influencing slips and falls in a marine environment.
The following glossary is included to avoid misinterpretations of the terminology used:
Friction--The force that resists the relative movement of two surfaces in contact with one another.
Static Friction--The resisting force at the instant relative motion begins.
Dynamic Friction--The resisting force when movement is occurring without interruption. Dynamic friction is usually less than static friction.
Anti-Slip Coefficient of Friction--The ratio of the force required to move one surface over another to the total force vertically applied to the two surfaces.
The anti-slip coefficient of friction is commonly identified as "u":
U = Horizontal Force/Vertical Force
COF Consensus
The generally accepted industry standard is that a static anti-slip coefficient of friction of 0.50 or above is safe on a dry walkway surface. A value below 0.50 indicates an unsafe walkway surface. There are some exceptions: The American with Disabilities Act Accessibility Guidelines indicate a minimum coefficient of friction of 0.6 for level surfaces and 0.8 for ramps.
But this guideline should not be used alone when evaluating slip resistance. The influence of footwear must be included. Also, the above consensus applies to same-level falls on floors that are, for all practical purposes, horizontal.
The floors or decks on a vessel are rarely horizontal, however. There is almost always some trim and list/sheer and camber, and there are motions. Furthermore, structural deformations are created during and after construction that may affect the anti-slip coefficient of friction. Finally, the presence of water on exterior walking/working areas is more common than in land-based facilities.
Slip-and-Fall Standards
The American National Standards Institute in 2001 approved a standard for workplace slips and falls. It was drafted by the American Society of Safety Engineers and is designated as A1264.2, "Standard for the Provision of Slip Resistance on Walking/Working Surfaces."
The American Society for Testing and Materials (ASTM) has a standard method for static coefficient of friction determination, ASTM C 1028-84, and several related standards developed through many years.
Other organizations, including Underwriters Laboratories (UL), have developed similar standards.
Over the years, there have been several approaches to measure the anti-slip coefficient of friction. Various organizations in the United States and overseas have developed apparatus to measure slip resistance. Some measure the dynamic rather than the static coefficient of friction.
The hard fact is that different machines may yield different results on the same test surface. Furthermore, the dynamics of a pedestrian's foot are quite different from the dynamics of a weighted object or a shoe being dragged across a surface.
ANSI standard A1264.2 discusses the slip resistance testing of floors using four approved slip measurement instruments:
Horizontal pull slipmeter, which can be used only on dry floors.
Brungraber Mark II Portable Inclinable Articulated Slip Tester.
Brungraber Mark I Portable Articulated Sliptester.
Variable Incidence Tribometer.
It should be mentioned that no two walkway material industries use the same method of measuring slip resistance. The resilient tile industry's method is different from that of the commercial floor polish industry. To attempt to correct this situation, the National Floor Safety Institute of Bedford, Texas has developed a tester for wet Static Coefficient of Friction (SCOF) and provides a certification status when the value of the SCOF meets 0.6 or better.
Other factors have to be kept in mind when dealing with slip resistance. If an object such as a meter (tester) is allowed to remain unmoving on a wet surface for a period of time, adhesion can develop between the object and the surface, resulting in high readings of the SCOF--seemingly indicating the wet surface is more slip resistant than the same surface is when dry.
COF in Dry Areas Aboard Marine Vessels
At least three factors affect any coefficient of friction that may be attempted to be measured in any dry area of a vessel. (There are other factors, besides, in wet areas.)
1. Fabrication tolerances of steel or aluminum vessels: The American Society for Testing and Materials has developed a standard guide for steel hull construction tolerances, designated as ASTM F1053/F 1053M-94, which presents permissible deviations, distortions, unfairness and construction inaccuracies in principal strength members in new construction of steel hulls. This guide is restricted to principal strength members and there is almost no provision for local waviness of deck plates.
The waviness of deck plates can result in an increase or decrease of the measured coefficient of friction of 5-10 percent, when superimposed to alignment and distortion of the hull.
2. List and trim/sheer and camber: The walk areas of a vessel are intended to be as horizontal as possible when the design condition is being developed. But there are infinite loading conditions that result in any deck of a vessel not being horizontal at almost any time. Additionally, most vessels' decks have some sheer and camber.
The combination of these factors may affect the measurement of the coefficient of friction by a substantial amount, depending on the location and the type of vessel.
3. Vessel motions: A floating vessel is never at rest. Even alongside a dock, there are constant motions created by wind, currents, waves, and the wake of other vessels. The roll and pitch motions will affect the coefficient of friction being measured.
Effects of Hydroplaning
In addition to the factors mentioned for dry areas, there is one more factor of importance regarding slip and fall in wet areas where the amount of water is substantial: hydroplaning.
The problem of hydroplaning has been carefully investigated both experimentally and analytically regarding rubber tires on pavement surfaces, but there has been no mention regarding slips and falls on wet decks of vessels. Yet the sole of a shoe in motion has a similar pressure buildup on the sole's surface because of its collision with the standing fluid to the pressure buildup on a tire's surface. Hydroplaning can occur when a person moves fast on a deck where the amount of standing water is significant.
The marine environment introduces several additional factors in the analysis of slip-and-fall accidents that are normally not considered in studies of slip-and-fall situations in other industries.
The generally accepted industry standard that a value of 0.5 and above for the static anti-slip coefficient of friction may be safe on a dry walkway surface, in a non-marine environment, but it may not be sufficiently safe in most marine environments.
This article originally appeared in the March 2003 issue of Occupational Health & Safety.