Deepwater turret pillar supports production riser

Dag Thulin
Maiacs Ltd

• All connections on the DUS turret pillar between the surface and seabed are molded into a single pillar. Water depth is not a limiting factor. The pillar can be towed out for installation at the production site. [66,112 bytes]

Conventional turret solutions for floating production systems in deepwater often have flexible connections through the water column from surface to seabed. These connections will create problems as a result of high environmental pressures (200 - 400 bar).

A number of riser pipes, cables, and umbilicals run down to the seabed from the surface production vessel, which raises the risk of production disruption and accidents. One solution to solve that problem has been to gather all the pipes into a wide tube at the surface and split them out at the seabed. However, the one remaining problem is the high pressures.

One design - the DUS pillar - developed by Maiacs Ltd of Sweden attempts to deal with the complexitt and high pressures. With the DUS, all of the piping is molded into a one compact pillar. The mold is a FRP HDPE (high density polyethylene) with a zero balanced density.

A 3,000-meter-long riser system has been derived. The system is described as follows:

• The DUS pillar is anchored on the seabed and is freestanding.
• At a depth of 100 meters below the surface, the flexible risers are connected to the turret.
• The DUS pillar is moulded into one piece; there are no joints.
• Lengths of 3000-4000 meters are available.
• The DUS is towed to the site and positioned on the seabed with an ROV.
• If needed, a separator could be installed at the lower end of the pillar on the seabed and a diesel generator on top of the pillar.

By having all the pipes and cables secured inside the thermoplastic, which also is watertight, the risk for hazardous surprises are very limited. Extra pipes and cables are installed for all eventualities. The DUS pillar could never be broken off thanks to the fiber reinforcement and a number of carbon cables inside the HDPE from top to seabed. These carbon cables are also very effective for towing purposes.

Wax deposits and other flow reducing influences are one of the most costly problems that could affect a riser system. The advantage with the DUS pillar is the possibility of designing relevant heating systems in combination with the low thermal factor of 0.3 for the HDPE.

Installing the entire production riser package in one solid piece, from top to bottom, is an advantage for ultra-deepwater. The DUS pillar offers one-piece installation for almost any depth. The investment in a DUS pillar system appears to be less than most conventional riser installations, for the following reasons:

• Building of the pillar is done on shore.
• All systems could be tested in advance.
• Towing out to the site is less expensive.
• The seabed is well prepared to accept the DUS pillar.
• The DUS pillar is freestanding, when installed at the seabed.

The DUS pillar could be built at almost any site in the world. The construction operation is simple and does not include a lot of complicated machinery. The production unit could also be transfered into a container package for easy transportation to the production site.

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