Improved subsea riser system

Abstract

An improved riser system which comprises a connector for connecting conduits and a mooring system for mooring the connector to the floor of a body of water. The connector may include a pivoting device. The improved riser system also comprises a buoy system for supporting the connector. The buoy system is configured to provide a fixed buoyancy for the connector, the mooring system and at least a portion of the conduits and for providing variable buoyancy for placement of the connector at a predetermined water depth. The improved riser system may also include a flexible conduit made of titanium.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application claims priority to and is a continuation-in-part of U.S. application Ser. No. 12 / 785,221, filed May 21, 2010, entitled “IMPROVED SUBSEA RISER SYSTEM,” the disclosure of which is incorporated herein by reference.TECHNICAL FIELD[0002]The present invention relates generally to systems for fluid transportation in deepwater environments. Specifically, the present invention relates to a subsea riser system for the transportation of fluids from, for example, a sea floor to a floating vessel or from the floating vessel to the seafloor.BACKGROUND OF THE INVENTION[0003]Within various industries, pipes are used to transport fluids from one location to another. In the petroleum industry, for example, pipes are used to transport crude oil and gas from wells on the seafloor to the sea surface, and to a distribution network at least for some distance between the fluid's source and its destination. Proper design of piping systems is impo...

AI Technical Summary

Benefits of technology

[0014]The present invention is directed to an improved riser system and method of installation. Embodiments of the invention reduce the transmiss

Problems solved by technology

Consequently, designing risers to withstand the internal pressures, hydrostatic pressures and hydrodynamic forces of deep water can be challenging.

This challenge is exacerbated when the surface facility to which the riser is connected is a floating platform because movement of the floating platform due to the wave, wind and sea currents can transmit significant stress to the riser.

Continuous application of stress to the riser causes fatigue and eventually could rupture the riser.

Below the surface currents, there may be submerged currents that cause vortex induced vibrations.

Because the flexible conduit is limited in its ability to withstand hydrostatic pressures and axial tension capacity, the flexible conduit is connected to a catenary riser located in the deeper zone of the water (the catenary riser normally curves gently upward from the sea floor).

However, because the flexible conduit is in the upper zone of water, i.e. the first 200 feet of water depth in the Gulf of Mexico, it moves with the currents and this movement causes stress on the catenary riser because the moving flexible conduit is attached to the catenary riser.

What is more, the demands on riser systems are changing, in part, because drilling is increasingly occurring in deeper and more hostile water depth locations.

This development has made it more challenging to provide cost effective riser systems because of the corresponding increase in hydrostatic pressure and hydrodynamic forces as riser systems are deployed in deeper and more hostile water depth locations.

An additional challenge in designing current riser systems is a need to accommodate subsea systems that permit the size of gas and oil risers to be on the order of 16 inches in diameter and larger.

In view of the bore size limitation, it should be appreciated that any change in internal diameter between the catenary riser and one or more flexible conduits connected to the catenary riser makes pigging a complex operation.

As the complexity of the pigging operation increases, so does a riser's operational costs.

Though it is possible to pig composite flexible conduits having an internal diameter less than the catenary riser, such an operation adds complexity.

What is more, pigs that jump in diameter usually do not work as efficiently as pigs that maintain a constant diameter.

Composite flexible conduits are susceptible to high temperature production fluids.

As such, a composite flexible conduit is usually the component that limits a riser system's ability to handle such fluids.

Therefore, as the contents of the different conduits change, the relative loads exerted by the conduits against each other change and cause fatigue of components of the riser system.

The current technology of suspending an SCR directly from a host facility is limited due to motions caused by ultra deepwater host facilities.

The use of flexible pipe directly suspended from the host to the seafloor has different limitations due to its own weight, collapse pressure and temperature restrictions.

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