Two Recent Conversions Of Semisubmersible Drilling Units To Floating Production Facilities
Authors: McBee H.E. , Hill R.S. , Dove P.G.S. 
Published: October 2008

Two recent conversions of semisubmersible drilling units to floating production facilities were governed by a different set of design requirements and operator philosophies. This paper compares them and illustrates how fabrication philosophies were chosen to produce a cost-effective project for each.

INTRODUCTION

The current state of the offshore oil and gas production market includes a niche for a cost-effective means of developing and producing marginal deep-water fields. The range of water depths for these fields begins at the limit of commercial feasibility for flied platforms and extends to depths where station-keeping criteria makes relatively recent technological concepts, such as tension leg platforms, necessary. Additionally, many fields are being developed at the edge of mature shallow-water areas including the Gulf of Mexico. These areas have the existing export pipeline networks and infrastructure such that storage of production crude is not necessary. Conversion of existing emisubmersible drilling units to Floating Production Facilities (FPFs) has proven to be a viable means of filling this niche.

The semisubmersible hull as employed by the offshore exploration industry has evolved over 30 years and is a proven concept for providing a stable work platform with minimum motions. FPF technology has been in use since he Argyll Field FPF was installed in the North Sea in the mid-1970's.

Two recent examples of the conversion of semisubmersible Mobile Offshore Drilling Units (MODUS) to FPFs are the PETROBRAS XXIV, installed in the Albacora Field offshore Brazil in January 1993, and the ENSERCH GARDEN BANKS, currently undergoing conversion. Both semisubmersibles are second-generation designs. The PETROBRAS XXIV, formerly the PENROD 72, is owned and operated by Brasoil, part of the Brazilian national oil company, Petrobras. The ENSERCH GARDEN BANKS is owned by Enserch Exploration Partners, Ltd. (EPO or Enserch Exploration).

The primary goal in a successfulFPF conversion is timely delivery of the unit to achieve the earliest possible production from the field. The strategy employed to accomplish the conversion of the unit is key to meeting this goal. There are several factors that must be considered when developing the conversion strategy. After a review of the modifications required to accomplish the conversion, these factors will be discussed as they relate to the two conversion projects presented herein.

Although the story of the PENROD 72 conversion to an FPF is well known throughout the industry, in order to fully understand the capabilities of the PETROBRAS XXIV, the history of the PENROD 72 as an FPF must be reviewed. In 1985, operator Placid Oil Co. and rig owner Penrod Drilling teamed up to initiate the conversion of the PENROD 72 for the Green Canyon Block 29 field, located in 1,540 feet of water in the Gulf of Mexico.

This was the first semisubmersible-based floating production system in the Gulf of Mexico, as well as the world's first deep-water FPF.

The field development scenario included production from up to 24 high pressure wells from a subsea template. Well fluids were processed through a two-phase separator, and the gas dehydrated to pipeline quality. The dry gas and combined liquids were exported through, respectively, 12" and 8" diameter risers to a template on the sea floor.