Considerations for Fall-Arrest Anchor Points
This article is posted with permission from Windpower Engineering. Click here to read this article as it appeared in the May 2010 issue of Windpower Engineering.
By W. David Lough
All fall-arrest equipment must meet some sort of standard and pass sometests so equipment manufacturers can ensure end users and employers that the equipment will perform to specifications. These standards (ANSI Z359 Fall Protection Code) are the first defense in place to save lives while working at height. With all this thought, testing, and design put into equipment (harnesses, snaphooks, carabiners, lanyards, self-retracting lifelines, and ladder climbing sleeves), why would anyone connect to an anchor that is not designed, tested, or even certified for use as fall protection? Yet every day in the wind industry, workers are connecting compliant fall protection equipment to anchors that are not compatible, or were never designed or intended for use as a fall-arrest anchor. The only advantage to these anchors is that they are already in place. Just because an anchor is convenient, doesn’t mean you should depend on it to save your life.
Fall-arrest anchors are placed in three main areas in a wind turbine. There are anchorage points that support the ladder safety system for access to the nacelle. There are assorted fall arrest and rescue anchors used inside the nacelle, the hub and the blades and there are fall arrest anchors and rescue anchors on top of the nacelle. Each one of these areas creates different challenges for workers and it is unnerving to think that these anchors may not be sufficient, whether we are talking about strength, design, or a combination of both. In some instances, turbines are designed outside of the U.S. Hence, the safety systems are designed to the requirements of that country, which may not meet the requirements here. Some anchorages were originally designed for fall restraint but are being used for arrest, while others were not designed for fall protection at all.
Ladder safety systems: There are a number of ladder safety systems on the market. Rail or pipe units spread fall arrest loads over a number of rungs, while cable systems require a single anchor at the top of the ladder. Cable systems are the most prevalent in the wind industry. In many cases, these systems are connected to the rungs of the ladder or to a point at the top of the ladder. As a minimum, the anchor point at the top of the ladder must meet the requirements given by the manufacturer, a figure usually dependent on the number of workers climbing the ladder at any given time.
Therefore, for a one-person system with a top bracket connected to three ladder rungs (typical installation), each rung must support at least 1,125 lbs (5 kN). This can be of concern knowing that a 0.75-in. round steel bar with a 16-in. span (typical ladder rung) becomes fully stressed and enters into plastic deformation at about 1,000 lbs when applying that load to the center of the span. It is of even greater concern if the ladder is aluminum because these rungs will fail at much lower loads.
Anchors inside the nacelle: As a rule, fall arrest anchors must have a minimum strength of 5,000 lb, or be designed by a qualified person and have a factor of safety of at least two. In most turbines, finding this kind of strength is not an issue. The issue is that most anchors used for fall protection in a wind turbine, whether it be in the tube at the transition points, in the nacelle itself or in the hub, were not designed with fall protection in mind. Many were designed for equipment lifting or used during construction of the turbine. In many cases an incompatible connection is created when used with the large snap hooks that most wind turbine climbers use. An incompatible connection is one that allows the gate of the snap hook to contact the anchor point and apply a load on the gate in the event of a fall. New ANSI standards have increased the required design strength of snap hook and carabiner gates, however this increase in gate strength by no means eliminates the possibility of defeating the gate, especially when connected to a rigid anchor. The only way to limit the hazard exposure to the worker is with properly designed anchors intended for fall protection and designed for use with appropriate fall arrest equipment, along with education.
Anchors on the top of the nacelle: These anchors have the same design requirements as anchors located inside the nacelle. Anchorages located at foot level have additional considerations because the free fall distance may be greater, increasing the potential energy created during the fall. Nacelle anchorages are commonly a pipe design, strategically placed on the roof or surrounding the perimeter of the nacelle. Other designs involve a number of single anchorages placed around the roof. The same issues of compatible connections come into play and there is a great concern regarding the strength of the pipe rail and performance as a system. Depending on the pipe’s location, weight of the worker and lanyard used, forces on the anchorages could vary considerably because of the increased free-fall distance and potential to fully deploy an energy absorber. Many rails were never installed for fall arrest, yet they are being used with that intent on a regular basis.
More and more we see accidents in which the worker has an ANSI compliant harness and lanyard, however not enough thought was put into what the lanyard is connected to, and then a system failure occurs. Workers are making decisions about which anchor points are acceptable but they are not given the tools to make an educated decision or they are not given a properly engineered anchor they deserve. We are putting lives at risk as long as we believe that strength is the only requirement of a fall arrest anchor and we can connect to anything readily available as long as it looks good
ABOUT THE AUTHOR
W. David Lough is the General Manager for Gravitec Systems Inc., a Poulsbo, WA, consulting company that specializes in fall protection engineering, training and equipment. He has been designing, consulting and training on fall protection systems for more than 13 years and is on the ANSI Z359 Accredited Standards Committee. Lough has also aided in writing Canadian Standards Association documents and has written several articles on fall protection.