NORWAY:

A semisubmersible for all seasons

The Octabuoy is a semisubmersible production platform that also performs drilling operations, well completion, workover and maintenance. Working with the Krylov Shipbuilding Research Institute, Norwegian-based Moss Maritime designed the Octabuoy principally for environmental conditions typical in areas such as offshore West Africa, Brazil and the Gulf of Mexico. Its main objectives are to:

• Eliminate expensive offshore installation and commissioning.
• Provide ample topsides weight capacity and deck area for maximum flexibility.
• Give a choice of rigid or flexible risers for dry or subsea completions.
• Provide good motion characteristics in the design, together with a taut leg mooring system, keeping offsets within margins comparable with the tension leg platforms (TLPs).
• Obtain favorable pitch and roll motions to avoid typical riser fatigue problems.

Semisubmersible production platform performs drilling, completion / workover and maintenance operations.

Motions. The key to the platform’s good motion characteristics is an accurate fine-tuning of its hull parameters and stability. The vessel’s shape reduces pitch and roll motions to a level that allows the use of rigid risers and, thus, dry wellheads located at the main deck level.

The vessel’s design seeks to eliminate 2nd order roll/pitch motions, known as the Mathieu effect, which affect semis and spars but cannot be revealed by linear hydrodynamic calculation. The unit is not sensitive to heave resonance effects because of a relatively high natural period in heave. This is very important in areas such as offshore West Africa, which has wave data dominated by swell and long wave periods.

Structure. The vessel consists of an octagonal ring pontoon, four columns and two deck sponsons on top of columns to provide for buoyancy and waterline area during mating operation. The lower hull is 83 m high, and the pontoons are 12 m high and 90 m long/wide. The deck structure is 8 m high and has a main deck area of 6,200 m². Topside weight capacity including deck structure is 28,000 t.

The Octabuoy is moored by a passive 16- to 20-point, taut leg mooring systems consisting of chain/fiber/chain.

Able to accommodate a wide range of production rates and specifications, the unit will also have facilities to allow the operator to carry out drilling, completion, workover and maintenance of wells.

Construction and installation, flexibility and re-use. The Octabuoy’s hull and deck are to be built and outfitted independently – even at two locations.

Hull and deck have been designed to float at less than 30 ft draft. Once completed, the barge-type deck structure can be built and moved to the place where process equipment and other topsides facilities are to be installed. The lower hull will be moored and ballasted to required mating draft and the outfitted deck would then float over.

By allowing full installation and outfitting of the deck barge equipment at quayside inshore, the concept minimizes or avoids expensive offshore installation and hook-up operations.

This installation procedure can be reversed. At the end of a field life the deck barge can be released and re-equipped to accommodate new specifications for moving into a new field.