Excess Pore Pressure Measurement and Monitoring for Offshore Instability Problems
Authors: Strout J.M. , Tjelta T.I. 
Published: October 2008

In situ pore pressure is one of the fundamental parameters required for assessing the strength and stability of geomaterials. In particular, identification of pore pressure regimes is necessary in geohazards studies. Offshore geological processes such as rapid sedimentation, minor slides, erosion, and fluid or gas seeps can result in non-hydrostatic pore pressures in critical layers, leading to strength reductions and increased risk for instability. This paper presents new technologies for the in situ measurement and long term monitoring of pore pressure and gas seepages in seabed sediments. The piezometers presented are single point borehole piezometer, the multilevel borehole piezometer, and a new CPT lance piezometer. The piezometers are designed for autonomous subsea deployment in long-term monitoring campaigns. Pore pressures can be measured to depths up to 200m below mudline (bml) in drilled geotechnical boreholes, or up to 40m bml using hydraulic standpipes based on CPT rods installed using a seabed CPT frame. A novel solution using production well technology to construct a geotechnical observation well is also presented. Two applications for gas leakage detection are presented; the first using commercially available sensors for monitoring methane in the drill shafts of a gravity based platform, and the deployment of passive gas traps to attempt to identify periodic leakage from seabed pockmark features. The equipment and techniques presented have been developed in conjunction with various geohazard assessment studies for developed and undeveloped offshore locations. Details of the technology are described, including basic technical specifications, installation approaches, and experience from actual installations. The application of some of this instrumentation in a scientific study of a North Sea pockmark seepage study are also presented and discussed.

Introduction to geohazards monitoring

One of the fundamental concepts in geotechnical engineering is the effective stress principle: ?' = ?-u The total stress component (s) can be calculated based on the mass of the soil and estimated (or measured) values of the lateral stress coefficients. Pore pressure (u) is the interstitial pressure between the individual grains in a soil matrix. Pore pressure may be due to fluid or gas and in an unsaturated soil a combination of both. In this article we make no specific distinctions between pore gas pressure or pore fluid pressure, both are collectively referred to as pore pressure.

Estimates of pore pressure may be made in some cases, for example using basin sedimentation models or hydrogeological models, but generally it is difficult to make a reliable estimate of pore pressure. Often pore pressure will be assumed to be hydrostatic, or with a moderate overpressure (say 5-10% over hydrostatic). Direct measurement of pore pressure is the best approach to obtain reliable pore pressures, and thus establish the effective stress state in the soil in addition to the total stress state.

Virtually all geotechnical calculations regarding strength and deformation require consideration of the stress state of the oil. In classical slope stability calculations it is possible to adopt either a total stress or effective stress approach, however it is usually necessary to consider both.