Keywords : penfeld, penetrometer

PENFELD penetrometer

The Penfeld penetrometer is a geotechnical measurement tool. Resting on the sea floor, it takes measurements in the sediment using an instrumeted tip.

 

The penetrometer is designed to work at depths of up to 6,000 metres. It is equipped with a rod which penetrates the sediment to a depth of 30 metres. The 36mm diameter rod is coiled around a 2.20m diameter drum and is straightened with plastic during penetration using the 'coiled tubing' technique.

 

The device is secured to the ship with a cable. It has its own energy source (equal pressure lead batteries).

 The weight of the device in water is 45kN. Its weight in air is 65kN. The penetration force can reach 40kN. The machine's footprint is 2.2 x 2.7m2 (or 6m2) and it is 4.7m high. The upper section of the structure tilts, allowing access to the battery pack or to set the rectifier parameters. Its overall length on the ground therefore totals 6.3m.

Penetrometer tip

A multisensor tip, specially designed for working at great depths, was developed by Ifremer and produced by Créatech Industrie.

It includes the following sensors:

  • tip resistance (measuring range 0-10kN),
  • lateral friction (measuring range 0-10kN),
  • differential interstitial pressure (-3,000kPa to +5,000kPa),
  • gamma density (1000 to 2700kg/m3),
  • temperature (-2° to +32°),
  • gradients in two perpendicular directions to calculate the trajectory of the tip.

Other tips are being developed, in particular for measuring speed in sediments.

Deployment on Ifremer ships

The RV Atalante has been adapted to operate the Penfeld penetrometer, as will the Pourquoi Pas?.

 

The rig comprises a 19mm steel deep sea cable (376kN breaking force), a swivel, 500kg ballast to keep the cable under permanent tension, and a 60m long synthetic cable acting as a stabiliser.

 

The Atalante's steel cable is capable of working at depths of up to 2,500m.

 

The penetrometer is deployed via the stern A-frame.

 

It is launched and recovered using a synthetic handling cable called a 'lift-line'.

 

A pinger on the ballast helps monitor the seabed approach phase.

 

The penetrometer is held by a central cable and lateral ropes during launching and recovery operations, when it is suspended beneath the stern A-frame.

 The positioning of the penetrometer is monitored by the UBSL system.

Measuring sequences

During a dive, the penetrometer can make five to seven insertions in different locations.

 

The sequences (number of insertions, number of stages during each insertion, duration of each insertion, duration of the stages) are programmed before it is launched. The stages (stop during penetration) allow the development of interstitial pressure to be measured.

 Measurements are recorded on the seabed and are available as soon as the device returns to the surface. A bi-directional acoustic link is used to monitor operations and the condition of the device. It also enables real time programming (intervals between two stages).

Examples

The penetrometer was tested during inshore trials at the Pont de Cran site on the Vilaine river (Morbihan) and during the PENETRESS1 (November 2002) and PENETRESS2003 (August 2003) technological missions.

 A sample result is given: penetration of 22.5m, stopped under a total force of 30kN.

Publication

MEUNIER J., SANGOUARD D., LHUILLIER B., " A new sea-bed penetrometer" OTC, Houston - may 2000