Large Size Sampling Tools for Offshore Exploration
Authors: Schwank Stefan K. 
Published: October 2008

Subsea bulk sampling operations face ever increasing demands on quality and economy which can only be satisfied by the deployment of the latest sampling technology. The drilling technique used emanates primarily from the large diameter borehole technology onshore which is modified for offshore applications. The drilling principles are, however, valid for both types of application. Different drilling techniques together with the necessary equipment or equipment systems are outlined both for onshore and offshore drilling operations. Specialist offshorebulk sampling equipment, such as the cutter or the spherical cutter. were specifically developed for operating in great depths of water or for drilling to great depths under water.

Introduction

(ieologists, too. set ever higher targets for hulk sampling operations in order to explore suspected mineral reserves as accurately as possible. AS-bulk sampling represents the secondstage towards commercial extraction of mineral deposits after stage one. which for example may be soil exploration by samll diameter boreholes. the large diameter boreholes deployed in this process are being positioned precisely in those locations where concentrations of mineral reserves are being suspected or which arc representative for the area under consideration for mineral extraction. When calving out subsea bulk sampling, consideration should be given under the following criteria:The area covered by each individual sample must be as large as possible The larger the area, the more representatives the result. Inconsistencies such as overbreak for example influence the result obtained from a large sample less than that of a smallone. At the same time. a large cross-sectional area results in a large material sample being extracted from the horizon under investigation.The extraction depth of each sample and the thickness of each stratum must both be determinable accurately. The method of extraction and. therefore, the sampling equipment must be capable of indicating the precise depth of each individual bulk sample automatically. Only then can the horizons of individual mineral deposits be accurately profiled.The extracted material should correspond as accurately as possible to the theoretical volume of the borehole. Any overbreak distorts the statistical result and should, therefore, be minimised.The capacity of the drilling equipment as regards drilling depth must be large enough to accommodate unforeseen geological features without difficult. A safety margin of between 10 and 20 ?/o of the anticipated maximum depth is recommended. However, as equipment costs increase disproportionately with drilling depth, it is essential that preliminary survey data made available are as accurate as possible.Drilling equipment and its (floating) equipment carrier must be easy to maneuver and able to be positioned accurately.Depending on the specified accuracy and the prevailing offshore conditions the most appropriate positioning systems must be selected.Drilling progress rates must be high. Ancillary equipment such as the processing plant must be tailored towards the production capacity of the drilling rig in order to avoid unnecessary downtimes for the drilling equipment.