SAFE CRANE LIFTS NEED WIND SHADOW LIFTING TOOL™
Maximum safe peak wind velocity is based on the load density. It must be checked before every crane lift.
Are you checking for your maximum safe peak wind velocity before every lift? With Wind Shadow Lifting Tool™, we make this check easy, fast, and timely for you. Let’s look at how this works!
Understanding load density
Crane loading tables are engineered by most crane manufacturers for a load density of 1.2 square meters surface area per ton of load weight. Lifting buckets of concrete is a primary use for most cranes — a concrete bucket typically has a load density close to 1.2 m2/ton (12.92ft2/ton).
Crane rating tables also assume a peak wind usually around 21 mph or 9.8 meters per second.
When a load with a large surface area for the same weight is lifted, a new peak allowable wind speed must calculated for the crane rating tables to remain valid.
The shipping industry is working under new international regulations requiring the installation of stack exhaust scrubbers. Additional emergency generator capacity is also commonly being installed. As a result, the shipping industry is lifting large, low-density loads — scrubbers, equipment containers, decorative covers, weldments, and supplies — to the highest decks of very large ships. Lifts of this magnitude and to these heights have not been attempted as frequently as is now common.
A number of near misses in the shipping industry shows that a catastrophic event is a matter of time.
The inability to readily calculate the safe maximum peak wind velocity has led to an industry culture of flying blind.
Operators are usually unaware how frequently they are undertaking lifts based on invalid crane load rating tables. If an operator is not making load density calculations with every lift, he or she cannot know if the actual peak winds during the lift excedes the allowable peak winds for the lift based on that load density.
The Wind Shadow Lifting Tool™ smartphone app has two easy steps:
1. Quickly analyze whether a lifting operation is safe to proceed based on calculating the new maximum allowable peak wind velocity for the intended lifting load density.
2. Readily visualize the wind shadow provided by the structure on to which the load is being lifted for any given wind angle and elevation thereby safely increasing significantly the ability to complete the lift into adverse winds.
While the first step may reduce the window of opportunity to complete lifts, the second step significantly increases the ability to complete lifts, despite inclement winds, by being able to visualize the wind shadow cast by the structure onto which the load is being lifted.
A Safer and Simpler Way
To simplify the very important safety step of checking allowable peak wind limits and also to enable visualizing and leveraging structure wind shadow effects, the Shipwright team has developed the Wind Shadow Lifting Tool™ for Android and iOS.
Wind Shadow Lifting Tool™ is a two-part mobile phone app. First, the user can calculate the maximum allowable wind speed based on load density. Then the app provides visual graphics to interpret and use the wind shadow cast by the ship or structure for the current wind speed and angle.
With the wind shadow graphics and the new allowable peak wind speed, the lift team will have critically important information to make safe and efficient decisions during lifts.
Environmental Considerations for Crane Operations
The global shipping industry is routinely lifting equipment, supplies, weldments, and antennas to the highest decks of very large ships. Lifts of this magnitude, and to these heights, have never been attempted as frequently as today
Moreover, as more installations occur, the lifting programs gain more experience and confidence, larger lifts in a wider range of environmental conditions are planned and executed. Common to all of these lifts is that they are being conducted while the vessels are in service with very limited and rigid time window. Operators are under tremendous pressure to accomplish difficult lifts.
Industry awareness is growing of the importance and influence of environmental conditions on lift execution and project schedule.
Every opportunity to complete a lift is valuable both in the sunk costs of mobilizing people and equipment for the lift, and the delay in the schedule of planned lifts due to adverse environmental conditions.
Wind and Rain
Of the many environmental factors — light, temperature, fog, snow, icing, lighting, air, and particulates such as sandstorm or volcanic ash — the two dominant environmental conditions impacting lifts are rain and wind.
Rain increases the risk of slips by workers, especially on the higher decks. For crane operators, working visibility can be heavily impacted by rain.
Managing Adverse Wind
Wind is a significant environmental factor for crane operators: wind can lead to a catastrophic casualty with impact between large loads and the ship, uncontrolled release of the load, the collapse of the crane boom, or even the overturning of the crane itself.
“The rain will drive you crazy, but the wind will kill you.” — Lift industry saying
Through careful engineering analysis — including computational fluid dynamics (CFD) — Wind Shadow Lifting Tool™ quantifies the reduction in relative wind velocity due to the effect of the wind shadow created by the ship being worked on.
Wind Shadow Lifting Tool™ increases the safety of crane lifts. It also increases the window of lift capability.
Wind Shadow Phenomenon
The wind shadow phenomenon, or just wind shadow, is usually referred to in sailing as the region to leeward where the wind speed is reduced because of the interference of the ship. Not limited to nautical situations, this effect can be shown below in the example of wind flow over trees and a house.
The phenomenon can also be seen in standard blunt body wind tunnel experiments which are commonly used to study the characteristics of air flow over and around objects.
As the flow develops around the object and separates, various regions of low velocity, or shadows, can usually be observed. The picture below shows the fluid flow in a typical tunnel experiment using a blunt block, where the flow is coming from left to right.
While the wind shadow phenomenon and the characteristics associated with it are usually studied for improvements in drag reduction, they can also be studied and implemented when planning lifts onto ships.
The large profile of a typical ship usually provides an area of decreased wind speeds behind the ship. Knowing this, a computer-based flow analysis can be created to very accurately predict this flow regime and quantify the new wind velocities throughout the wind field. Wind Shadow Lifting Tool™ was built to do exactly these calculations on the fly, from a hand-held smartphone in the field.
Specific knowledge of the extent and limits of this reduced velocity air flow can be utilized to plan and execute lifts from the lift launching point to the flight route and finally to the landing location of the load.
The utilization of this effect can allow for crane lifts in relative wind fields often 10-15 miles per hour less than the prevailing wind velocity outside the wind shadow. A typical CFD render for a cruise ship is shown below, where the velocity is represented in the color gradient and the flow is perpendicular to the ship (grey).
Wind Shadow Lifting Tool™ app is an easy-to-use tool can be used in the field to quickly show the shape, size, and velocity of the wind shadow for a given angle of wind window.
The on-site team can plan specific lift movements that would significantly reduce the relative wind velocity for many lifts. This increases the safety of the lift while expanding the of opportunity for completing the lift in greater wind states.
Crane Manufacturer’s Rated Window Speed
While many aspects of crane design, maintenance, and operation are closely regulated, the maximum wind speed for crane operation is left to the crane manufacturer to provide a guideline and ultimately the crane operator makes the final determination in the field.
Crane manufacturer wind limits are typically 14 to 21 miles per hour for a mobile crane. Additionally, these limits are not usually added to the load tables, but are instead provided as a note like in these examples:
However, a key factor determining maximum wind limit for lifting is the actual load being lifted. A primary use of cranes hired by the shipping industry is lifting concrete buckets on construction sites. A typical crane operator has to switch modes from those typical construction site loads to the wide variety of loads found on shipping industry projects.
For example, scrubber and antenna lifts can be infrequent for these cranes and their operators. These objects almost always have a much larger surface area for their weight than a comparable concrete bucket lift.
The result is that an often obscure and uncommon operating limitation, load density, becomes the dominant lifting limit criteria.
A dense load with a small profile relative to its weight can be lifted in higher winds than a load with a large profile relative to its weight.
Most crane manufacturers use 1.2m2/ton as the standard load density. If a load has a exposed-surface-area-to-weight ratio greater than this standard, then the wind speed criteria provided by the crane manufacturer for the standard load tables is not valid. A lower wind limit must be used. This lower wind limit can be calculated by Wind Shadow Lifting Tool.™
Lifting operations for the shipping industry almost always trigger a lower wind limit than provided by a manufacturer’s standard. Large structures with relatively low weights for their size result in large wind effects. The load is more likely to shift, sway, and move instead of remaining directly below the crane tip.
An example of the reduction in maximum wind velocity due to the surface area to weight ratios is a typical scrubber tower: with a weight of 7 tons and exposed surface area of 85 m2, the load density of 1.2 m2/ton is exceeded and a lower peak wind speed limit is required. Wind Shadow Lifting Tool™ allows the calculation of this new lower wind speed limit.
With the given surface area and weight of the scrubber tower, the new calculated wind velocity limit is approximately 2.81 m/s or 6.29 mph. The maximum wind velocity equation and example calculation for other typical lifts is shown in the table below.
Load density calculation:
Where:
Vmax = New max wind speed
Vmax_TAB = max wind speed from the table
mh = hoist load
Aw = the exposed surface area
The calculation shows that the maximum wind velocities for typical scrubber tower is much lower than the manufacturers rated wind table suggests.
Crane load cells unable to detect wind-imposed stress
While cranes providing support for shipping industry lifts are required to have load cell displays in the cab for the operator, the additional loading imposed by wind does not show in the display. The gravitational load on the hook, which is what the load cell is measuring, may not change even with additional loading due to the wind.
Rather, the bending moments on both the crane boom or derrick and the overturning moments on the crane base are increased by wind in a way that is not measured by the load cell display in the cab. This makes the danger due to wind loading insidious in that the crane operator won’t be aware of the excess loading until a casualty results.
The Wind Shadow Lifting Tool™ assists by letting the client and operators know the adjusted wind speed and the projected wind speed in the proposed lifting locations.
