Floating Offshore Structures – Offshore Structure Series

There are many oil and gas discoveries which are out of reach of fixed structures for one reason or another. They may be in extremely deep water, or the oil or gas deposit might be too small or too widely spread to warrant the high cost of building a fixed structure. In these cases, seabed-completed wells may be connected to a floating platform moored above the field, using a production marine riser. The limiting conditions for fixed installations are not clearly defined, and they have been used in some cases for depths of over 250 meters, although this is in a benign environment. Floating platforms can also be used as the basis for an Early Production System (EPS), in which the appraisal wells drilled from a floating drilling vessel are completed at the seabed and produced to a floating platform carrying the required process plant and other facilities. This allows production to begin and create income whilst a fixed platform is being designed and installed for full field development. In this article, there are some discussions about three main types of floating offshore structures which are Tension Leg Platforms, SPAR and FPSO.

Tension Leg Platforms (Tethered Buoyant Structures)

One more form of offshore platform is what is known as the Tension Leg Platform, or Tethered Buoyant Structure. This method is intended for oil and gas production from water depths of over 500 meters. The platform works in much the same way as a taut moored buoy, which is anchored to the seabed using a vertical wire. The Tethered Buoyant Structure is basically a large, semi-submersible floating vessel, which uses a heavy gravity anchor to moor it to the seabed. Tension force is maintained in these vertical cables by adjusting the buoyancy of the floating platform, to ensure positive tension at all times. This method reduced marine response in the platform to effectively zero in vertical terms, and very little in horizontal terms. Horizontal drift can be further reduced as necessary. By using buoyance against a tension mooring system, this allows the use of a semi-submersible floating platform which can carry an additional load, balancing this out by increasing the buoyancy.

This type of structure is still under development, and there are still many problem points to iron out. How widespread it will become in the future is largely dependent on solving these issues, along with a thorough economic assessment comparing the system to other available ones.

Figure 1 – Tension Leg Platform

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