Roncador Field: Transport of P-36 and Installation of the Mooring System
Authors: Henriques C.C.D. , Fachetti M.B. 
Published: October 2008

This paper describes the transport and installation phases of the semi-submersible PETROBRAS 36, since the unit's depart from Davie shipyard in Canada, where the conversion had been undertaken, up to the hook-up of the mooring system in Roncador Field.

The selection of dry transportation of P-36 from Canada to Brazil, using the heavy lift carrier Mighty Servant 1 from Dockwise, was driven by a combination of technical and strategic issues. A set of technical studies was performed to make this 32,700-ton heavy transport feasible, taking into account the risk factors involved in the operation.

P-36 mooring system comprises 16 lines in taut leg configuration, using polyester ropes and Vertically Loaded Anchors (VLAs). The water depth at the anchor locations ranges from 1200 to 1520m. The installation of a mooring system of this size in such depths represented an enormous challenge. The irregular soil characteristics in Roncador where two subsea canyons converge near to the platform location presented an additional complication factor.

Background

Petrobras-36 Production Unit.

The Spirit of Columbus semisubmersible platform was originally built as a drilling and production unit, able to process up to 100,000 bpd. Twelve mooring legs composed its original mooring system that was designed for a maximum water depth of 500m. The construction of the unit was concluded in 1994 but, up to 1996, when it was contracted by Petrobras and renamed to Petrobras-36, it had never operated.

In order to operate in Roncador Field, located in Campos Basin, the Spirit of Columbus undertook an extensive upgrade, performed at Davie shipyard in Canada. The upgrade included the installation of new process facilities for 180,000 bpd of oil and 7,200,000 m3/d of gas and 24,000 m3/d water injection capacity and facilities for risers' support. Structural modifications to support new equipment and to comply with stability and buoyancy requirements were also executed. Paper OTC 12136 (Ref.3) gives an overview of Roncador development program and a detailed description of the platform upgrade is given in paper OTC 12140 (Ref. 4).

Mooring System Upgrade.

The water depth increase (from 500 to 1360m), the huge number of flexible risers supported by the Unit and the strong currents in Roncador resulted in the need to upgrade the Unit's mooring system to 16 legs. Following Petrobras specifications, the system was designed in a taut leg configuration, using polyester ropes 1800m long with a Minimum Break Load (MBL) of 1,000 tons.

Project Design

Mooring Ropes.

The 1,800m polyester legs were divided in 3 segments in order to turn easier the operations with the ropes inside the Anchor Handling Vessels. The segments were supplied in different lengths, to enable the abandonment of the lines in buoys, after the pre-setting (see below). The ropes were designed including a surplus of 25% on top of the necessary Safety Factors to comply with eventual resistance losses during the Unit's lifetime. The amount of polyester to be purchased was enormous. So, Petrobras decided to split the purchase among three different suppliers.