Deepwater Geohazards: Geotechnical Concerns and Solutions
Authors: Kvalstad T.J. , Nadim F. , Arbitz C.B. 
Published: October 2008

Identification and quantification of natural and humaninduced hazards is an inevitable part of every field development project. Quantification of deepwater geohazard risks requires identification of potential failure modes and triggering sources, the related failure consequences and assessment of the uncertainties involved in the evaluation of site conditions, soil parameters and analysis methods. Field development activities on the continental slopes require special attention. Large slides (historic and ancient) have been observed on slopes with a very low inclination. The slides seem to develop retrogressively upslope and sideways and the run-out distance can be several hundred km. Slope instability may thus not only affect a local the field development project locally, but also have a potential impact on third parties in a much wider area. A rational explanation of observed phenomena in a combined effort from all relevant geosciences is required for calibration of risk assessment methods.

Introduction

Geohazards can be described as site and soil conditions having a potential (a certain probability) of developing into a failure event causing loss of life or investments. Assessment of geohazard risk will thus require identification and analysis of the relevant failure scenarios (i.e. failure modes, triggering sources and related failure consequences) that can give a significant contribution to the total risk. Furthermore, the annual probability that the associated triggering mechanisms would reach a level causing failure events will have to be evaluated.

The quantification of risk has to be based on site investigations with identification of bathymetry and local gradients, stratigraphy, assessment of soil and pore fluid properties and in situ stress, pore pressure and temperature conditions. An understanding of the regional and local geology, ongoing geological processes and type, locations and extent of anomalies is required to quantify the potential impact and rate or frequency of ongoing natural processes. Finally, evaluation of the potential impact of exploration, development and production activities on soil conditions is required for assessment of the human induced risk.

Geotechnical Concerns.

The concerns of geotechnical engineers involved in geohazard risk assessment are related to our ability to define the relevant failure modes, their consequences and their probability of occurrence.

The uncertainties involved in assessment of site and soil conditions are in many cases amplified by the areal extent and depth of sediments involved and the practical and economical limitation of the site investigations.

Assessment of the in situ effective stress conditions is essential for evaluation of soil strength and stiffness and requires assessment of the in situ pore pressure conditions. Deepwater conditions also imply that gas hydrates might be present, and this has brought a new complicating factor into material models involved in geotechnical engineering.

Finally the basic understanding and the modeling of triggering mechanisms of submarine slides on very gentle slopes, retrogressive slide development upslope and laterally as well as the evolution of slide masses into downslope mass wasting processes represent a challenge, requiring improved material models and mechanical analysis tools.