HORUS Towards Permanent Monitoring of Underwater Geohazards
Authors: Grimaud Sylvie , Adamy Jerome , Unterseh Stephan , Rolin Jean-Francois , Geli Louis 
Published: April 2012

Geohazards can significantly affect the underwater developments and regularly, there is no possibility to avoid to be exposed to those geohazards. In such acase, mitigations are usually designed and implemented to limit theconsequences of the geohazards. They rely on our understanding of geohazards, our ability to model their behavior as a function of time, and our capabilityto predict any possible evolution during the field life. It is the aim of TOTALeffort over the past 20 years.

One way of improving our knowledge is to develop long-term monitoring systemsfor critical areas. Inspired by scientific sub-sea observatories for naturalhazards, TOTAL is developing a monitoring station called HORUSTM, for HazardsObservatory for Risk analysis by Underwater System, specifically dedicated togeo-hazards for industrial applications.

This station can be equipped with a variable but multi-sensors payloaddepending on the type of geohazards that affect the area. Among the differentpossibility of permanent in-situ measurements, pore pressure, temperature, gascontent, microseismicity and seabed deformations are considered. Theseparameters will provide early warnings for potential slope instabilities, gasescapes, modification of the soil properties, or any evolution of thegeological features.

This cabled station is connected to the production facilities where data arerecorded and real time preprocessed in order to detect anomalies, and possibly, to raise alarms. If the parameter monitoring seems relatively easy, datamanagement and data processing are more of concern: alarm levels need to beassociated to reliable indicators. Multi parameter cross-correlation andcomparison to pre-defined " normal" thresholds is part of the preprocessing thatis automatically performed offshore. The station is designed to facilitatemaintenance and sensor replacement by an ROV. A connecting/disconnecting rackis available for the installation of new equipment. Additional battery pack canalso be set on the structure for a short-term autonomous use in explorationareas.

This monitoring concept is currently under development in Nigeria.

Introduction.

Seabed geohazards are more and more frequent in the deep offshore where subseadevelopments are planned. Facing the issue of particularly challenging seabedconditions offshore Nigeria (Figure 1), the decision has been taken to developa continuous real time monitoring system devoted to geohazard monitoring duringproduction time.

This paper presents this new approach for real time and continuous monitoringof geohazards during the field production. The ultimate goal of this real timemonitoring, is to help operating companies to be more predictive and to be ableto anticipate any risk for production installations due to changes inequilibrium of geohazards.

Geohazards are geological features affecting the seabed such as pockmarks, mudvolcanoes, gas hydrates, slope unstability, faults and few more. If they arelocated over an area where we plan to develop an oil or gas field (Figure 2), one must assess all the potential risks they represent for the human activity, the environment, and the integrity of the production facilities.