FPSO Field Development System for Large Riser Count and High Pressures for Harsh Environments

Abstract

In an offshore production system, a semisubmersible supports the risers and provides the service requirements for the wells, while an associated FPSO provides the processing functions and storage of the produced hydrocarbons. As a result, a greater number of risers can be supported in a harsh environment while the turret and swivel requirements of the system are significantly reduced.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the offshore production of oil and gas. More particularly, it relates to surface vessels used to receive, process and store hydrocarbons from subsea wells.

2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98

A Floating Production Storage and Offloading system (“FPSO”) is a floating facility installed above or close to an offshore oil and/or gas field to receive, process, store and export hydrocarbons.

It consists of a floater, which is either a newly built or converted tanker, [permanently] moored on site. The cargo capacity of the vessel is used as buffer storage for the oil produced. The process facilities (topsides) and accommodation are installed on the floater. The mooring configuration may be of the spread mooring type or a single point mooring system, generally a turret.

The high pressure mixture of produced fluids is delivered to the process facilities mounted on the deck of the tanker, where the oil, gas and water are separated. The water is discharged overboard after treatment to eliminate hydrocarbons. The stabilized crude oil is stored in the cargo tanks and subsequently transferred into shuttle tankers either via a buoy or by laying side by side or in tandem to the FPSO. The gas is used for enhancing the liquid production through gas lift, and for energy production onboard the vessel. The remainder is compressed and transported by pipeline to shore or re-injected into the reservoir.

A Steel Catenary Riser (“SCR”) is a steel pipe hung in a catenary configuration from a floating vessel in deep water to transmit fluids to or from the seafloor. A Lazy Wave Steel Catenary Riser (“LWSCR”) is an SCR having an elevated intermediate portion that acts to minimize motion of the SCR on the seafloor when the floating vessel is subjected to heave.

BRIEF SUMMARY OF THE INVENTION

In an offshore production system according to the invention, a semisubmersible supports the risers and provides the service requirements for the wells, while an FPSO provides the processing functions and storage of the produced hydrocarbons. As a result, a greater number of risers can be supported in a harsh environment while the turret and swivel requirements of the system are significantly reduced.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a side schematic view of one embodiment of the present invention shown in operating conditions.

FIG. 2 is a side schematic view of the embodiment illustrated in FIG. 1 during extreme metocean conditions.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a field development methodology for offshore oil and gas fields in harsh environments (e.g. hurricane/cyclonic) using FPSO technology.

A system according to the invention comprises a semisubmersible, a disconnectable buoy, an FPSO and a bundle of fluid transfer lines. The system is designed to be deployed in deep water harsh environments where riser performance (either SCR or LWSCR) is a key performance parameter. The power and pressure requirements for gas and/or water injection are key drivers of having those services on the semisubmersible rather than being, supplied by an FPSO through a swivel, yoke or equivalent system.

The semisubmersible is used to support the risers because of its favorable motion characteristics under harsh conditions. The semisubmersible also supports the water injection equipment, gas injection, the High Integrity Protection System (“HIPS”) and any chemical injection equipment. The power for this equipment may be generated on the semisubmersible. The produced hydrocarbons may be manifolded into a fluid transfer line. The fluid transfer line may have a predominantly-oil line, a predominantly-gas line and a communications line. The pressure in the fluid transfer line is much lower than that of the producing wells. The fluid transfer line is connected to a moored submersible buoy. The submersible buoy is connected to an FPSO via a bow-mounted turret. All processing and storage is performed on the FPSO. The FPSO may disconnect from the buoy if the environmental conditions become too extreme. The buoy drops below the water surface to a depth with reduced hydrodynamic loading. The rotational functionality is provided by the bow-mounted turret. The swivel is mounted onto the FPSO; no rotating parts are on/in the buoy.

In a production system according to the invention, the semisubmersible supports the risers and provides the service requirements for the wells, while the FPSO provides the processing and storage of the produced hydrocarbons. As a result, a greater number of risers can be supported in a harsh environment while the turret and swivel requirements are significantly reduced.

A sample schematic of a system according to the invention under operational conditions is shown in FIG. 1. FIG. 2 is a schematic of the methodology under extreme conditions.

The invention is intended for use in harsh environments. The methodology is highly applicable to locations where the harsh environments are rare but intense (a benign methodology might work except for those rare occasions). Such locations include many of the hurricane/cyclonic areas of the world. Competing systems include the FPSO or semisubmersible by itself, a disconnectable buoy similar to White Rose or Stones or the MoorSpar™ mooring buoy.

The advantages of a system according to the invention include:

• • Support for a large number of risers under harsh conditions, including high pressure, sour service, and high H2S content
  • A HIPS that is located above the water line in a serviceable environment
  • A water injection system that is located above the water line in a serviceable environment
  • A fluid transfer system between the semisubmersible and FPSO that is “low pressure”, and is comprised of only a few lines, thereby simplifying the design and allowing for more motion.
  • A disconnectable buoy which supports only a few lines and doesn't require any rotational parts.
  • A swivel requiring only a few flow paths and an electronic data path
  • A bow-mounted turret, reducing the construction and/or modification costs of the FPSO and enhancing storage capacity.
  • The entire system is comprised of field-proven hardware
  • Ability to start producing a field before pipeline infrastructure

Potential disadvantages of such a system include:

• • Requirement for two vessels—an FPSO and a semisubmersible, with associated mooring line infrastructure; and,
  • Operation of two vessels in proximity to each other.

Systems or methodologies that perform similar functions include the FPSO, the semisubmersible, the disconnectable FPSO, the MoorSpar™ mooring buoy and riser tower. Each system has its own advantages and disadvantages. Most of these systems cannot meet at least one of the following requirements: many risers, high pressure, sour service, high H₂5 content, large quantities and/or high pressure water injection, harsh environment, storage & offloading.

The independent innovative aspects of the invention include the following:

• • A system comprising at least a semi-submersible and FPSO wherein the processing of the hydrocarbons is not done on the semisubmersible and the water injection and/or gas injection is not done on the FPSO;
  • A system that supports deep-water risers in a harsh environment without an initial pipeline infrastructure;
  • A system that supports deep-water risers that will have to withstand high H₂5 level and/or high temperatures and/or sour service, without an initial pipeline infrastructure; and,
  • Use of one or more disconnectable FPSOs in conjunction with a semisubmersible.

Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.

Claims

1. An offshore oilfield development system comprising: a semisubmersible having equipment for water and/or gas injection, chemical injection, and manifolding;a plurality of subsea risers in fluid connection with the equipment on the semisubmersible and wellheads on the seafloor; anda submersible mooring buoy having means for mooring an FPSO said means including means for fluid transfer from the mooring buoy to an FPSO moored thereto; andat least one fluid conduit connecting the mooring buoy to the subsea risers via the semisubmersible.