Drilling offshore oil and gas wells has become increasingly challenging as water and well depths have increased. In these areas drilling hazards resulting from narrow margins between pore and fracture pressures, pore pressure uncertainty, pore pressure regression, high pressure and high temperature have resulted in significant additional cost due to the time required to rectify problems associated with these hazards. As a consequence, industry has focused its efforts on developing mitigations to these challenges. Typically these developments have tended to focus on down hole solutions such as Managed Pressure Drilling and Dual Gradient Drilling; developments in drilling fluid management technologies and wellbore strengthening etc.

One area which has not received as much focus recently is that of managing the drilling process and in particular the unique aspects relatedto working in deep water from Mobile Offshore Drilling Units. In particular theprobability of relatively small undetected hydrocarbon influxes abovethe subsea BOP.

Depending on the drilling fluid and influx composition, unconstrained expansion of the influx (gas) may exceed the surface equipment handling capability (i.e. diverter system, capability and performance). Additionally, depending on the bubble pointof the drilling fluid / influx composition unconstrained evolution of fluid fromthe riser can result in a significant reduction in riser fluid level and thereforereduction in hydrostatic pressure which may impact the riser BOP system as wellas increase the probability of additional influxes entering the wellbore.

This paper describes the design, development and construction of a riser gas managementsystem which addresses these concerns.

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