An Innovative Deepwater Geotechnical Tool
Authors: Bienvenu V. , Bessonart J. 
Published: October 2008

The today's challenge for the Offshore Industry is the fields' development in deep and ultra-deep seas.

A new generation of geotechnical tools for soils identification is being developed for these water depths.

If tools as cone penetrometers have already been largely studied for deep seas, no static sampler has yet been planned. MARINE GEOSYSTEM, a new established French Geotechnical Consulting and Services Company, has patented a new concept of geotechnical static sampler, capable to work by water depths up to 6000 meters with a modular depth of investigation up to 30 meters below seabed.

Introduction

With the increasing need of reliable soil information in deeper seas, a high level of new technologies developments and methods of work is expected by the Offshore Industry.

In the field of deep geotechnical surveys, the existing tools are well identified and classified under two categories, (1) the in-situ measurement performed into the soil and (2) the sampling.

The measurements of the in-situ soil characteristics are mainly obtained thanks to a cone penetrometer. Other tests as the vane test or the seismic cone may also be performed. The penetrometer technology is widely used on the continental shelf but has also been developed for deep seas (mini-cone, coil tubing systems") [Meunier and al, 2000].

Existing sampling equipments have been conceived including gravity corer systems.

The tool planned by MARINE GEOSYSTEM, called STARFISH, is mainly developed to perform a static penetration into the soil at constant speed, to come back to sampling specifications applied on the continental shelf.

Existing tools

All of the existing samplers dedicated to water depths greater than 1000 meters, excepted traditional wireline methods used on the continental shelf, do utilize a dynamic impact to penetrate into the soil.

The Giant Corer CALYPSO has been developed by the IFRTP (French Institute of Research and Technology for Austral Territories) and is operated onboard the scientific vessel Marion-Dufresne. It is one of the most referenced corers and can be fitted up to 10 tons and 60 meters long. The results achieved by the international campaigns of IMAGES Program confirm the exceptional performances of the CALYPSO corer [IFRTP, Calypso documentation].

The STACOR is a large diameter gravity corer developed by the French Institute of Petroleum between 1987/1990 [IFP, Stacor documentation]. The STACOR presents a true immobility of the piston during the penetration. Cores were taken in abyssal plains through more than 5800 m of water.

Others corers as the JUMBO piston corer (JPC) [Silva and al, 1999] or SELCORE (developed by SELANTIC) could be mentioned.

It was then important to develop a sampler offering the same type of specifications of what was performed and recovered on the continental shelf including recovery of a 4" (101.6 mm) static sample. With the STARFISH system the operations are entirely controlled (penetration speed, position and verticality of sampler") with a recovery from the mudline to a maximum depth of 30 meters below seabed.