What are Vibracores and How are They Used?
Vibracoring is a technology and a technique for collecting core samples of underwater sediments and wetland soils. The attached core tube is driven into sediment by the force of gravity, enhanced by vibration energy. The vibrations cause a thin layer of material to mobilize along the inner and outer tube wall, reducing friction and easing penetration into the substrate. A simple way to understand this is to imagine putting all of your laundry on one side of the washer and spinning it very fast. The offset of weight causes the washer to shake violently – similar to a vibracorer. With coring, the liquid spaces in the matrix allow sediment grains to be displaced by the vibrating tube. Vibracoring works best on unconsolidated, heterogeneous sediments and soils.
Although vibracoring is not a system for hard material, drilling it has become the industry standard for the collection of environmental samples in marine depositional sediments. This is partly due to the efficiency of vibracoring – with cores up to 20’ able to be collected in under 10 minutes in many cases but also due to the high recovery of vibrated samples. In traditional drilling or direct push boring, samples of recovered soils are often less than 50% – for example if you drive 10 feet you may only recover 5 feet of sample. With vibracores, this sample recovery tends to be much higher due to the reduced friction on the core liner from vibrations. For marine environmental studies – particularly where chemistry is needed, the higher recovery of sediments is critical in understanding the history of the deposited sediments. The down-side of vibrating material is that the sample collected maybe disturbed and many agencies will not accept vibracorer samples for geotechnical testing.
There are numerous types of vibracorers that have been developed over the years with many custom systems to suit site-specific requirements. Below are some key factors when considering the selection of a vibracore system:
Vibracoring Key Factors
- Depth – all designs must start with how deep the coring system must go in the water. Beyond the challenges of proper pressure, housings are the logistical challenges of power transmission to deep water. Although most stock systems are used in depths averaging less than 100 meters, coring systems have been used on ROVs going to depths of 3000+ meters.
- Frequency – this is the speed at which the vibrator motor runs (RPM) that yields vibrations per minute (VPM) value. Higher frequency systems (3000 VPM) are generally used with soft silts and clays while lower frequency systems (1500 VPM) tend to work better for sands or in hard substrates that do not respond to vibration well. Another component of frequency is the bias caused by high vibration on sample representativeness. For projects where less disturbed samples are required or geotechnical parameters are critical, lower frequency vibracorer units may be preferred.
- Impact Power – this is a less understood and documented factor in vibracoring technology which considers the downward force of the coring system. Higher force units either use larger swing weights in the coring motor (amplitude) or a heavier head weight or a combination of both. Some companies also use a dynamic system for increasing force such as a winch that pulls the corer down into the substrate. Note that there is a distinctive difference between force lbs in a vibrator motor which is the amount of material a unit can move and amplitude force which is the amount of impact each vibration creates.
Some of the earliest designed vibracorers came from the construction industry using impact hammers and vibratory motors to run pile drivers. Most of these tools were run by large air compressor systems and used pneumatic motors. The benefit of the pneumatic vibracorer systems is that the motor does not need to be pressure sealed and the systems are very simple to build. In addition, very high impact vibration can be achieved with these systems although most do not achieve as high of VPM. The downside is that they require large specialized vessels to operate on and they are very loud both on the boat and in the water – which can be a major problem for marine mammals. These systems are also limited to shallower water with hose lines being very cumbersome for any deep-water applications.
Similar to pneumatic coring, most hydraulic vibracorer systems came from the marine construction industry, particularly with the design of the hydraulic pile drivers and compactors now on the market. Hydraulic vibracorers are some of the most powerful hydraulic rigs because large offset weights can be run along with high frequency using the benefits of the hydraulic torque curve. Hydraulic vibrators are often used only in shallow water as the hydraulic hoses are cumbersome for deeper depths. That said, when vibracorers are used at very deep depths most systems revert to hydraulic as the fluid is not compressible and deep-water housings are unnecessary. Many deepwater ROV systems utilize hydraulic pumps which has popularized the design of using ROVs for powering deep coring rigs. Hydraulic systems have come under some criticism due to the risk of fluid leaks in sensitive environments.
Most of the modern vibracorer systems have gone to electric operation with the exception of deeper water and heavy offshore systems. Electric can be broken up into DC and AC systems with both having very different profiles.
The early AC systems were designed by a very famous Frenchman named Andrew Rossfelder who had a colorful past that included being a resistance fighter in WW2 and being arrested for trying to kill the French President Charles De Gaulle after his home country of Algeria was abandoned by France. Rossfelder went on to develop the early electric vibracorer systems which are still in use today. The larger benefit of the electric motor is that you can achieve portability with both high VPM and high force output. In addition, you have infinite control of both frequency and in some systems impact force which is key to working in mixed sediments with sands and clays. Another unique factor with electric systems is that the motors can be designed with a dual-axis allowing for two offset weights to be run on a single motor. This both helps direct the vibration force down while acting like a gyroscope which helps keep the power head vertical while driving underwater. The downside is that most systems are rated to less than 1000 feet due to pressure housings and electric cabling.
The DC vibracoring systems are usually modified truck bed vibrators with high frequency and low impact force. These are great for shallow water silts but due to the low impact and minimal duty cycles they are not very viable for most commercial applications.
Peripheral Tools For Vibracoring
Penetrometer – this tool logs depth versus time to provide a qualitative measurement of the sediment structure. The Penetrometer is typically a digital depth sensor attached to the vibracorer head. This sensor transmits data and time to a logger either on the surface or at the tool. Although a simple concept, this data can be useful in understanding sedimentation layers, particularly if samples are not easily collected – some companies use this similar to a Cone Penetrometer for real-time sediment measurements.
Tilt Sensors – a number of motion reference units are available on the market today (including those in your phone!) that can be used with vibracores. These motion sensors can log the vertical control of the head while driving. This data can ensure that the correct profile of the seabed is collected and aid in real-time operation.
Camera Systems –Custom camera systems on many vibracorers have been employed to better understand what is happening while samples are collected. A number of systems allow visual logging of recovery length versus penetration as the core sample is driven. This can greatly aid the understanding of geotechnical properties of the core as it is pushed into the sediments..
Ocean Instruments builds both custom and stock coring systems with our specialty in AC electric vibracorer units for water depths to 2000 feet and deep-water systems using hydraulic power. OI has 3 stock systems that can core to depths exceeding 40 feet BGS.