Clamshell buckets and rock grapples generally fall into two categories:
- Mechanical
- Hydraulic
In this blog, we’re going to take a deep dive on safely rigging a mechanical system.
Conventional cranes with lattice booms are considered the standard for 2-line cable operation. An additional requirement for these cranes is a minimum of two drums for dredging and other heavy material handling projects.
A properly rigged system will ensure workplace safety, effective workflow, and the success of any job.
Maritime Rigging for Clamshell Buckets and Rock Grapples on Conventional Cranes
There are required guidelines to follow and dedicated machinery to use during any dredging project.
During a marine construction project, conventional cranes can be equipped with mechanical grabs, providing better operator feedback when there is no view of the clamshell bucket while underwater.
One line from the crane is used as the holding line attached to the head of the bucket through a shackle or wedge and socket. The closing line enters through the rollers of the bucket to the bottom sheave nest then back up until the necessary parts of line are achieved. The best practice is to use Crosby clips to secure the cable to itself before and after it is routed through the terminating wedge and socket.
Maritime dredging projects present unique circumstances, and by having a properly rigged crane, operators can work safely, quickly, and effectively.
Because of their immense power and lack of feedback, hydraulic cranes can be difficult to operate with mechanical clamshell buckets. Hence, an adequately rigged bucket or grapple is the top priority.
Consult with OSHA guidelines to ensure that everything is rigged correctly before starting any dredging or material handling project.
Lifting Speed vs. Closing Power
Reeving affects your headroom and lifting capacity, as well as your speed, by expanding the hoists’ mechanical advantage.
The mechanical advantage measures the output force to input force ratio in a pulley and conserves energy. Reeving also controls lateral movement and provides true vertical lift.
The three most common terms when discussing reeving are:
- Single
- Double
- Part
Single or double refers to the number of ropes that come from the drum, while part describes the mechanical advantage one gains with multiple reeving rigs.
In a system with a two-part single reeving, the load is distributed over both parts, which doubles the capacity of a one-part system. In addition, this simultaneously reduces the lowering or lifting speed of the rig by one-half.
Certain distinct circumstances require true vertical lifting, which can only be accomplished with a two-part rig.
Another benefit of double reeving is it reduces the overall headroom than a single-reeved clamshell bucket or rock grapple.
Inspection your Cables
Wire ropes can deteriorate over time, and like most pieces of equipment, preventative maintenance and routine inspections are the best way to prevent a critical failure.
Common causes of deterioration include:
- Corrosion
- Kinking
- Peening (Hammering)
- Overloading
- Other exposure to the elements (i.e., excessive heat, freezing temperatures, and salt water)
Replacing wire ropes is costly but, like most maintenance, it is far cheaper when you consider potentially dangerous malfunctions, extensive damage to equipment, and lengthy project delays.
Consult with a local service technician or develop and implement a comprehensive wire rope inspection checklist to ensure safe, efficient operation.
It’s important to note that most clamshell buckets require less wire rope than hook block operations.
Dredging Water Saturated Materials
Depending on the unique conditions of your project, it’s essential to choose the right attachment to deliver the best results.
If you are dredging in mud, you should opt for a bucket that has a square nose with teeth. For sand, clay, or gravel, choose a heavy duty round nose clamshell bucket that has aggressive teeth so that your rig can dredge through the earth. Also, saturated materials weigh significantly more than dry materials, so be sure to consult with your engineering team to adjust the load capacity
A rock grapple is best utilized if your site conditions have larger rocks that could cause a clamshell to get stuck.
Another benefit of rock grapples is their ability to move materials that would not fit in a clamshell bucket.
Clamshell Buckets and Rock Grapples: To Sum It All Up
When you’re working in a marine setting with a dredging clamshell bucket or rock grapple, properly rigging and reeving your wire rope is one of the most critical aspects of your operation.
Conduct regular inspections of your wire rope to reduce the chance of malfunction, injury, or unnecessary delays.
Explore your options given the specific conditions you’re going to be dredging and identify the best attachment for your project.
Always consult with your local, state, and federal guidelines to ensure that your job site is OSHA compliant.
When possible, consult with local equipment service technicians if any concerns, questions, or breakdowns in equipment occur.
