Page 18 - Offshore Well Construction

CHAPTER 14
Subsea Systems
Petroleum Extension-The University of Texas at Austin

INTRODUCTION
The exploration for oil and gas offshore began in the late 1800s, and in 1896 an offshore well was drilled off the coast
of California. In 1938, the discovery of the Creole field 2 km (1.24 ft) from the coast of Louisiana in the Gulf of Mexico
heralded the beginning of the move into open, unprotected waters. In this instance, a 20 m (65 ft) by 90 m (295 ft) drilling
platform was secured to a foundation of timber piles set in 4 m (13.12 ft) of water. Typically, these pioneering offshore
wells utilized piers to create a platform above the prospect which thus enabled them to drill vertical wells into the target.

As the search for oil and gas reserves has continued to intensify, so exploration has moved into increasingly deeper waters.
The first subsea well was completed by Shell in 1960 and came on stream in January 1961. This marked both the successful
conclusion of many years of R&D and the beginning of a new era in subsea production. Nowadays, subsea completions are
a commonplace option and it is the mode of production that is changing. Originally, such wells would have been tied back
directly to a platform but now alternatives exist and can be ranked for any particular field depending on cost and water depth.

CURRENT SUBSEA DEVELOPMENTS
Current subsea development options include:

• Tension leg platforms without storage (TLP)
• Floating production vessels without storage (FPV)
• Floating storage units (FSU)
• Floating production storage and offloading vessel (FPSO)
• SPAR buoys for storage or production and storage
• Deep draft semisubmersibles with storage and offloading (DDSS)

FPSO
The idea of FPSOs has been around for many years and the concept has been utilized since the 70s when conversions
from existing tankers was the norm. In the late 80s, the Petrojarl heralded the first of the turret systems and was marketed
as a testing and early production system. Since then, various turret designs have appeared and include those on the
Gryphon, Uisge Gorm, Captain, Anasuria, and Foinavon.

Definitions
Floating—the body is in equilibrium when floating. This excludes TLPs which use buoyancy to maintain equilibrium.
The unit must have a displacement and buoyancy compatible with its payload requirement, a form compatible with
its station-keeping requirement, and be able to provide a safe, stable platform as a working environment.
Production—the vessel could contain primary and secondary processing equipment to treat live well fluids; e.g., oil/
water separation. These are field specific and can range from a single stage separation to a full blown separation,
compression, and injection system with its associated power requirements.
Storage­—able to store significant quantities of oil until it can be removed by shuttle tanker. This could be due to the lack
of an effective export option in the vicinity other than a shuttle tanker or to the poor quality of the crude which would
incur a high pipeline tariff. Note that lack of sufficient storage could be detrimental in the long term if production has
to be halted because of a logjam in the export route (planned shutdown excepted).
Offloading—contains a means by which oil can be transferred from storage to either a shuttle tanker or alternative export
source. Direct unloading is permissible only if there are no weather implications; i.e., the FPSO can weathervane. If that
is the case, then a remote-loading buoy may be required. Such remote buoys would include:
• Surface loading buoys (CALM systems)
• Loading towers

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