Feature ArticleVGP 30/3 and SHSBD-A: New Innovations in Sediment Coring
By Francesco Paolo Galloni
It is well known that researching marine sediments provides considerable amounts of data on the environmental effects of human activity on the underwater ecosystem, particularly concerning pollution. The need for rapid removal of polluting substances from the water makes a preliminary analysis of the type and/or quantity of the materials necessary, from the geological, chemical, physical and biological point of view.
Normally, obtaining a six-meter-long seabed sample with standard coring equipment requires the use of a survey vessel or a large pontoon equipped with a lifting system (crane or A-frame) that has a minimum of five tons of force. Additionally, to permit a vertical extraction of the core from the sediment, the vessel needs a mooring or dynamic positioning, wasting time and increasing cost.
Due to these limitations, there is an ever-increasing need for lightweight equipment in the field of seabed core sampling, ideally an apparatus with minimum bulk that is as independent as possible from the nautical means it rests against. If this equipment is able to be housed on vessels less than 15 meters long or on small pontoons, it can be used for core sampling operations in any work area, from the coastline to the edge of the continental shelf, or in rivers, lakes, lagoon areas and artificial basins.
Diagram of the VGP 30/3: (A) barrel (B) frame base (C) fixed portion (D) first mobile portion (E1 and E2) hydraulic actuators (F) second mobile portion, or cart (G) penetration tools (H) control unit (I) stabilizer feet.
Evolution of GeoPolaris Equipment
GeoPolaris has recently engineered improvements to its self-sheathing hydraulic sampling bottom device (SHSBD), which was discussed in the January 2008 issue of Sea Technology.
The new Vibrocorer Geo Polaris (VGP) 30/3 and SHSBD-A corers can be transported in a standard truck or container, do not require long assembly operations and can be quickly set up and tested on site. The VGP 30/3 extracts three-meter-long core samples in up to 100 meters of water, and the SHSBD-A extracts nine-meter-long samples in water depths of up to 250 meters or more.
Their innovative hydraulic self-extraction system and relatively light weight allows the VGP 30/3 and SHSBD-A corers to be used on small, shallow-draft vessels equipped with pulling systems with a lifting power 70 percent less than that required for standard coring equipment, and the dimensions of the pulling cable and winch of the pulling system can be reduced. Additionally, the self-extraction of the barrel from the seabed means that the pulling system is not required to be in a vertical position, allowing the vessel to be independent from the device. Finally, the extraction procedure does not require a violent pull of the barrel out of the seabed, which ensures that nearly all of the sample is recovered. As an added environmental measure, the corers' hydraulic system uses biodegradable vegetable oil as lubricant.
When designing the corers, particular care was given to the quality of samples—the devices are designed to reduce sample contamination and disturbance to a minimum. They do this by featuring four different penetration modes (gravity, vibration, percussion and hydraulic thrust), customizing the coring procedures for the type of sediment to be sampled. Moreover, the dedicated variable-frequency vibration system allows operators to take samples from harder layers of sediment, such as beachrock. All of the sampling options can be chosen and changed on site by quick modifications.
Building on the 2008 version of the SHSBD, technical developers worked to build fully automated devices. The new VGP 30/3 and SHSBD-A corers are surface-controlled systems; all the coring activities are controlled by an operator on the vessel with a touch screen. During sampling phases, the corers transmit and display the following data: continuous measurement of the penetration thrust applied; continuous measurement of penetration; recording of penetration/extraction time; and data required to fill in the core sample log (coordinates, depth, name of the point, etc.).
Data may be correlated to the time function and managed through dedicated software, which enables users to obtain real-time stratigraphic indications and immediately fill in the core sample log. All data can be downloaded via USB port at the end of sampling activities.
Finally, the VGP 30/3 and SHSBD-A corers can remotely disconnect the barrel from the frame if it is impossible to carry out the hydraulic self-extraction due to circumstances beyond the operator's control. This makes it possible to protect the core-sampling apparatus by sacrificing a relatively inexpensive part—the barrel.
Diagram of the SHSBD-A: (A) barrel (B) sliding cart (C) vibro-percussive system (D1 and D2) sliding chains (E1 and E2) motors (F) power pack (G) control unit (H) extendible legs.
The VGP 30/3 Corer
The VGP 30/3 design allows a three-meter-long, low-disturbance sample to be extracted from the seabed with the help of a small survey vessel or plastic modular pontoon equipped with a crane or A-frame capable of providing 600 kilograms of pull force. It can be operated at depths up to 100 meters.
The VGP 30/3 corer is comprised of a core sampler, or barrel, designed to be driven into the seabed to collect soil samples. It is linked to a frame with multiple sliding portions, which features a base designed to rest on the seabed, a fixed portion connected to the base, a mobile portion connected to the fixed portion by two hydraulic actuators, and a second mobile portion, or cart, linked to the first mobile portion by a sliding system. The second mobile portion of the frame holds the barrel and the penetration tools and is connected to the first mobile portion through a system of pulleys and rails between the mobile portions. The control unit is stored in a box connected to the frame.
The main advantage offered by the VGP 30/3 is that it is shorter, which facilitates transportation and movement operations considerably. This is achieved thanks to the pulley and rail system, which multiplies the displacement of the sampler with respect to the displacement of the first mobile portion. Consequently, for one vertical stroke of the first mobile portion of the frame, it is possible to (at least) double the displacement (and the length) of the sampler in the longitudinal direction.
The particular configuration of the mobile portions' movement halves the traction loads on the pulley system's ropes, making it possible to use smaller-diameter ropes and making the apparatus lighter and more cost-effective. Another fundamental requirement of core sampling devices is stability. Indeed, such apparatuses are intended to operate on sloping or inconsistent seabed that can cause sudden displacement of the equipment, jeopardizing its correct operation or safety. VGP 30/3 is equipped with three touch screen-adjustable stabilizer feet. Stabilizers modify and adjust the inclination of the device in relation to the seafloor, thus ensuring vertical penetration, even on a sloping seafloor. This provides the advantages first of allowing coring on seabeds with gradients up to 10°, which are often found close to jettys and generally in harbor areas, and, second, ensures that the sediment core will be a faithful representation of the seabed.
The penetration tools (hammer or a vibrator) can be changed on site to suit the seabed type. In the case of very soft sediment, it is possible to use only the push of the hydraulic actuators as a controlled gravity core.
The SHSBD-A Corer
The SHSBD-A modular design allows a sample of up to nine meters in length to be extracted from the seabed with the help of a small survey vessel—without dynamic positioning or mooring—equipped with a crane or A-frame with 3,000 kilograms of pull force. The device can be operated in water depths up to 250 meters, with options for deeper water.
The SHSBD-A corer is a direct evolution of the SHSBD. The new device maintains the simplicity, modularity and the general design of the original, but features remote control capabilities to eliminate the need for a diver. Combined with increased strength, this gives the system the ability to perform a range of applications.
The SHSBD-A corer is comprised of a barrel linked to a sliding cart containing the vibro-percussive system. The cart slides along the modular frame on a rail system and it is linked to a double sliding chain driven by two motors mounted on the top of the fixed frame. The hydraulic power pack and control unit are connected to the base of the frame. Four extendible legs stabilize the device on the seabed.
The vibration, combined with the controlled thrust applied on the cart from the motor-driven sliding chain, allows the barrel to penetrate the seabed. By inverting the direction of thrust, the device then extracts the sample. The structural design and the size of the tools generates a remarkable penetration/extraction force.
The SHSBD-A corer is a modular system, composed of elements with a length of 1.07 meters, 1.67 meters and 2.05 meters. Therefore, it may be fitted according to the required penetration depth, adding or removing elements and resizing the sliding chain, which reduces size and weight.
Field Test Performance
Since 2009, the VGP 30/3 and SHSBD-A corers have been used in several campaigns to investigate about 700 sampling stations in offshore areas, coastal areas, harbors, rivers, lakes and artificial basins, providing technical assistance in environmental characterization activities for research and institutional bodies, such as Italian port authorities, Centro Interuniversitario di Biologia Marina di Livorno, the Italian Regional Agencies for Environmental Protection, the Italian Ministry of Environment, The Institute for Environmental Protection and Research, Voies Navigables de France, and other private companies and international groups.
The VGP 30/3 and GEO1 modular pontoon for shallow-water coring activities.
For example, at the end of 2010, a test activity was carried out with the VGP 30/3 in the Baltic Sea in cooperation with the Geological Survey of Finland (GTK). The purpose was to study the corer's compatibility with the GTK vessel and its performance in glacial sediment sampling, and it achieved a very good penetration/recovery ratio.
The VGP 30/3 corer has also been used with excellent results on different shallow-draft vessels that had never before been employed for coring activities, such as small fishing boats, multicats or pontoons. An additional benefit of the use of small vessels with high maneuverability is that an anchoring permit to operate in intense ship traffic areas is not needed, reducing interference and standby time.
The integration between the VGP 30/3 and plastic modular pontoons has proven particularly beneficial. This configuration can be easily transported and assembled in difficult-to-access areas (lakes, river and basins) and perform coring activity at almost any depth with minimum cost. The GEO1 plastic modular barge is one example. With a draft of only 50 centimeters while carrying a 1,500 kilogram payload, the vessel is well-equipped to operate at very shallow depths, making sediment coring possible in previously inaccessible regions.
The SHSBD-A corer has also been used on different types of vessels, such as small tugs or barges equipped with three-ton-pulling-power cranes, whereas traditional corers need a minimum lifting power of about eight tons. Moreover, no dynamic positioning systems or moorings are required.
During a recent environmental survey for the regional government of Tuscany, for example, GeoPolaris was able to procure core samples of sand deposits at a depth of 100 meters in the north Tyrrhenian Sea without anchoring the tug vessel that was hosting the corer.
Besides the benefits of their compact size, the VGP 30/3 and SHSBD-A feature a low rate of disturbance to the sedimentary structure, allowing operators to obtain a continuous core of material. Field testing has shown that the corers maintain a 90 percent recovery-to-penetration ratio in more than 95 percent of the samples performed on any type of sediment and up to 10-inch-thick concretions, aiding in the use of these samples for environmental and geotechnical analyses.
Future Technological Development
The versatility of the VGP 30/3 and SHSBD-A designs permit adaptation of different tools to the base frame, which expands the possibilities for future use.
For the VGP 30/3 corer, the presence of remote controlled hydraulic actuators allow operators to add a piezocone to perform cone penetration tests with piezometer data or to add a probe and perform chemical/physical measurements. With the next version of the VGP, it could be possible to use a unique device to carry out a complete geognostic investigation of the surface seabed on site, adapting different tools and ballast on the same frame. Updated software installed on the touch screen will allow users to manage various sets of data. Finally, the hydraulic power pack will be installed on the device.
For the SHSBD-A corer, the next step will be to combine rotary and vibrocoring penetration modality.
Francesco Paolo Galloni is a marine geologist and technical manager at GeoPolaris Srl. He began working in the offshore survey field in 2001 and joined GeoPolaris in 2008.
Giovanni Catone is operations manager at GeoPolaris Srl. He received habilitation as an expert mechanical technician and joined GeoPolaris in 2005.