Upgrading existing Drilling Units has been considered an attractive option for offshore oil exploitation, since costs and time to be spent at shipyards can be lower than for new building. Moreover, use of a Dynamic Positioning System and automated drilling facilities are essential for the performance of the Unit when operating in ultra deep waters.
The demand of drilling offshore Brazil was the motivation to evaluate the feasibility of upgrading a Mobile Drilling Unit to operate in water depths up to 2500 m, with slender drilling risers.
This paper focuses on the main aspects considered during the basic design phase of the upgrading of a Semi-submersible Drilling Unit as follows:
Additional utility systems;
New Bilge and Ballast system;
Increase in Payload;
DP system with addition of thrusters;
Naval architecture, including blisters, pontoonsenlargements, thrusters locations and new compartmentation;
Upper hull arrangement;
Power demand and Generation;
Integration between systems;
With all those aspects addressed, the objective of providing a design of a Unit feasible to operate in ultra deep waters has been reached, from safety, structural, stability, station keeping, arrangement and drilling performance points of view.
For drilling operations in offshore areas with deep waters, it will be essential using automated drilling facilities and Dynamic Positioning system in the vessels that will prospect in such areas, since conventional mooring systems are impractical for these water depths.
Besides that, the time to retrieve and deploy anchors can take many hours or even days, not mentioning the operations related to retrieval and connections of riser joints and lowering and hoisting of the BOP from moon-pool area. Additionally, for operating in ultra deep waters, the deployment of anchors to maintain the Unit on station can cause damages to subsea installations above well heads or to the pipelines connected to other platforms.
To prospect in such areas, upgrading an existing drilling Unit can be an attractive solution, mainly if it has limited operational performance and there is a lack of opportunities for using its original characteristics.
Upgrading an existing platform will require an extensive knowledge on all its systems, mainly if the Unit operates as a conventional moored platform and does not have sophisticated drilling equipment and monitoring systems, which is the case covered by this paper. For its new expected performance, the platform to be upgraded will experience dramatic modifications and additions in all systems. The integration between new and existing systems is critical and shall be carefully planned and detailed, even during basic design phase.
Such type of upgrade leads also to dramatic increase in the payload, being necessary to redesign all architecture of the vessel, including upper and lower hulls, once the structure will be subjected to considerable increased loads, both from larger payload and hydrodynamic actions.