Drilling From an FDPSO: A Two-Stack Approach
- Karen Bybee (JPT Assistant Technology Editor)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- April 2011
- Document Type
- Journal Paper
- 61 - 62
- 2011. Offshore Technology Conference
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This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20491, "Drilling From an FDPSO: A Two-Stack Approach," by Kenneth C. Hampshire, SPE, and J. Greg Noles, Murphy West Ltd., and Albert Kachich, Doris Inc., originally prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed.
One reason for the success of the Azurite floating drilling, production, storage, and offloading (FDPSO) vessel was the innovative approach in specifying the equipment to fulfill the drilling requirements. This resulted in the specification and deployment of many unique drilling components that combine to make up the drilling structure that extended from the seafloor to the surface.
The Azurite field is in 4,600-ft water depth in the Mer Profonde Sud Block offshore Republic of Congo, a part of the world that is without significant infrastructure to transport or store hydrocarbons. As a result, a requirement for developing the Azurite field was to find a way to combine drilling activity and oil storage economically. To do this, a drilling rig was installed on a floating production, storage, and offloading (FPSO) vessel—making it an FDPSO. This unique and innovative combination presented a variety of challenges. To address these challenges, one solution was to use two blowout-preventer (BOP) stacks while drilling, to ensure well control and reduce drilling risk.
Controlling wellbore pressure is paramount to safety in drilling operations. Conventional deepwater-drilling operations use a subsea stack of BOPs to achieve this. Many combinations of closure scenarios are available with a properly specified stack to reduce or remove internal pressure in the riser column above the stack. Well control requires frequent actuation of BOP functions.
Functionality and ease of maintenance would be enhanced greatly if the stack could be installed above the water line. Additionally, the size and weight of the surface stack would be significantly smaller than a subsea stack. The riser also would be simplified in that it would not need to carry hydraulic control lines down to the stack, and the riser would not need to have a choke or kill line.
The idea of having a wellhead connected to a stack mounted above the water line with a high-pressure riser system was appealing, but having no protection at the seabed was not. The Azurite project team recognized that if the integrity of the riser was compromised for any reason and drilling or reservoir fluids escaped, the environmental consequences would be unacceptable. To address this concern, another stack, customized to safeguard against such an occurrence, was specified that would be a standalone module that would not require the rigorous functionality of the surface stack or require control lines running down to it from the surface. This package was called the subsea isolation device (SID). Fig. 1 shows the surface stack (left) and the SID.
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