Chapter 10
Offshore testing
Seabed soil samplers
SEABED SOIL SAMPLERS 10.3 | |
| Grab sampler The grab sampler (Figure 96) is one of the simplest forms of seabed sampling. It is a grab bucket very similar to that used on land. The grab units tend to be either hydraulically or manually operated. The advantage of the manually operated version is that they are very simple and not really restricted to any water depth other than length of wire and winch capacity. These units can work in up to 4000 m of water. They give a good idea of the index properties of the seabed material. The unit is typically deployed from a vessel’s crane or A-frame to recover the samples back to deck. These units are also useful in obtaining bulk samples that can be used back in laboratories for model testing such as soil pipeline interaction. |
Figure 97 | Box corer The box corer has become a standard sampling tool for surveys in soft or deep sediments. The enlarged surface area of the box (0.25 m2) allows for relatively large sample sizes to be recovered in deep water where the time required to deploy and recover the instrument is significant. The box corer is built within a gimballed hexagonal frame. The instrument is triggered by a trip as the main coring stem passes through its frame. The depth of penetration (maximum 50 cm) can be controlled to prevent over-penetration in softer sediments (Figure 97). The recovered sample is completely enclosed after retraction, reducing the loss of finer materials during recovery. Stainless steel doors, kept open during the deployment to reduce any ‘bow-wave effect’ during sampling, are triggered on sampling and remain tightly closed, sealing the sampled water from the water column. On recovery, the sample can be processed directly through the large access doors or via the removal of the box completely, together with its cutting blade. A spare stainless steel box and galvanised cutting blade can then be added, ready for an immediate re-deployment. Box corers are available in different sizes. The largest is the 0.25 m2 type, while smaller 0.06 m2 mini box corers can also be provided. Box corers provide a very high quality bulk sample which again, like the grab sampler, has no electronics fitted, so is only limited in depth of water for operation by the length of wire and winch. |
Gravity and piston corers
| Figure 98 |
Corers have now been developed into jumbo piston corers (JPCs) which
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Figure 99 | Vibro-corer A variation of the gravity core is the vibro-corer (Figure 99). This corer uses motors to generate a centrifugal force to vibrate the sample barrel into the ground. It enables samples to be taken in granular material and in stiff clays where free-fall devices, such as a gravity corer, would meet refusal. The high-powered vibro-corer is powered by an electric twinlinear vibrator motor delivering over 9000 kg of centrifugal force. Standard size vibro-coring equipment will produce 86 mm diameter core samples to a maximum depth of 6 m. In coarse aggregates larger diameters up to 150 mm can be obtained. To minimise sample disturbance and coring time, the units can be fitted with an integral penetrometer and data recorder supplying online information on penetration against time and penetration rate (m/sec). Data is subsequently used to assist in the evaluation of actual layer thickness compared with recovered length. Typical sea bottom vibrating time is up to ten times less than with a standard vibro-corer, improving performance rates and minimising core disturbance. |
The corer is capable of coring up to 6 m into the seabed in water depths
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Deep water sampler With the need for higher quality samples in the deepwater environment, a new deep water sampler has been developed. This is a variation on the jumbo piston corer but intended to provide higher quality samples. The deep water sampler was designed in cooperation with the Norwegian Geotechnical Institute to sample soft soils in deep water. The aim is that the sample length will be at least 10 m and with a sample diameter of 110 mm. One of the most important goals is the recovery ratio of 95% or higher of the soil. This means that the sampler has to penetrate the soil with minimal disturbance and handle the sample with care during retraction. |
| Figure 100 |
Cutting shoe The cutting shoe (Figure 101) protrudes into the soil and guides a soil sample into the sample tubes behind the cutting shoe. The shape of the cutting shoe is designed to obtain the least possible deformation of the soil. Another function of the cutting shoe is to lock the spring loaded core retainer during penetration. As soon as the whole sampler is pulled back, the friction force on the cutting shoe triggers the core retainer. Figure 101 |
Figure 102 | Core retainer The core retainer (Figure 102) consists of several fingers made out of spring steel. The function of the core retainer is to cut the soil sample and support it, while lifting the whole sampler. The core retainer is spring loaded, which gives it its strength. The retainer is triggered by the retraction of the sampler. |
Sample tube The sample tube (Figure 103) consists of an outer tube, a liner and a set of rings and seals to connect the liners. The outer tube protects the liner and guides the pushing and retracting forces which are being imposed upon it. The liner guides the sample and acts as a container for the retrieved sample. Figure 103 Piston The piston (Figure 104) fits closely into the liners. A seal on the piston can withstand possible pressure differences between the soil and the inside of the empty liners. This seal is also used as a one-way brake so that the piston can only move upwards in the liners. Figure 104 The piston stays stationary to the soil during penetration. The reaction force is lead through a load cell and chain to a fixed point above the sampler. The piston is used to seal the sample and to keep it steady inside the liner. Pressures are monitored by sensors inside the piston. If the pressure is too low or too high, a safety valve will open to avoid damage to the sample and sampler. |