Drilling From a FDPSO: A Two-Stack Approach
Authors: Hampshire Kenneth Christopher , Noles Greg , Kachich Albert John 
Published: July 2010

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 sea floor to the surface.

This paper focuses on the first stage rationale, the planning, the preparation of this equipment for service, and the method by which it was installed. The equipment discussed constitutes everything in-line, from the wellhead up to the rig floor, for the drilling campaign.

Introduction

The Azurite field is located in the Mer Profonde Sud block offshore Republic of Congo, in 1400m water depth, a part of the world that is without significant infrastructure to transport or store hydrocarbons. As a result, a requirement of developing the Azurite field was to find a way to economically combine drilling activity and oil storage. To realize this, a drilling rig was installed on a Floating, Production, Storage and Offloading (FPSO) -making it the world's first FDPSO. This unique and innovative combination presented a variety of challenges. To address these challenges, one solution was the idea to use two blowout preventers (BOP) stacks while drilling, in order to reduce well control and drilling risk.

Two Stack Rationale

Controlling well bore pressure is paramount to safety in drilling operations. Conventional, deepwater, offshore, drilling operations employ the use of a subsea stack of blowout preventers (BOPs) to do 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. This well control requires frequent activation of BOP functions. The cost and dependability of activating such equipment subsea continues to warrant finding alternate methods of well control.

Functionality and ease of maintenance are greatly enhanced 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 would also be greatly 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, which are conventionally needed for mud circulation in subsea stacks. Running such a riser string also pays dividends by drastically simplifying, and reducing, the running time.

However, a surface BOP stack would require a riser system that could withstand the reservoir pressure. This new requirement would have to be addressed.

The idea of having a wellhead connected to a stack, mounted above the water line, with a high pressure riser system in between, was appealing. But having no protection at the sea bed 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. The need to safeguard the environment could not be overlooked.

To address this concern, another stack - customized to safeguard against such an occurrence was specified. It would be a stand-alone module that would not require the rigorous functioning of the surface stack, or control lines running down to it, from the surface. This package was dubbed the subsea isolation device or SID. The two stacks are pictured in Figure 1.