Subsea well

Details Subsea well Subsea well

1999-02-26

2001-04-03

Bagnell, David (Department: 3673)

Wells

Submerged well

Wellhead

C166S345000, C166S355000

Reexamination Certificate

active

06209650

ABSTRACT:

FIELD OF INVENTION
This invention relates to a subsea well arrangement for offshore production of oil or gas, comprising a wellhead, a christmas tree mounted on the wellhead and at least one riser for connection with a production vessel at the sea surface.
BACKGROUND OF THE INVENTION
When developing marginal offshore production fields it is important to keep the costs low. A substantial cost factor with known forms of development, is due to subsea pipelines and cables between christmas trees at the wells and product receiver sites, for example a platform or a floating production vessel. Typically there may here be the question of distances like 2 km. In this connection it is to be noted that modern drilling technology makes it possible to produce by means of a reduced number of christmas trees, because the boreholes or wells can have several branches. This will make it possible to develop subsea fields in some instances by employing only one subsea christmas tree.
International patent application PC/NO96/00201 dated Jul. 8, 1996 relates to a system for offshore production of hydrocarbons by employing a moored production vessel or ship. The present invention can be regarded as a further development thereof and is based on the idea that the production vessel can by lying directly above or in the immediate vicinity of the christmas tree at the seabed and that the well fluid flows are transferred directly between the christmas tree and the vessel without relying on pipelines or cables on the seabed. In this connection the invention is not exclusively based upon location of the production vessel by means of mooring, as according to the international patent application mentioned above, but can also employ means as known per se for dynamic positioning of the vessel.
BRIEF SUMMARY OF THE INVENTION
The solution to be described in the following specification can either be utilized in production from an existing or already installed and also conventional christmas tree at the seabed, or by employing a particular and preferably simplified form of christmas tree, being more or less integrated into an arrangement according to the invention, as will appear from the following description.
In an arrangement as stated in the introduction above, the novel and specific features according to the invention primarily comprise that on top of the christmas tree there is provided a swivel device communicating with the christmas tree and that the swivel device is provided with preferably laterally directed connection members for risers or hoses and for an umbilical or control cable.
On the basis of this fundamental solution, which in actual practice can be embodied in various forms, it is possible to obtain a number of advantages, of which in particular the following are mentioned:
The production vessel employed does not need any modifications worth mentioning, in relation to common ship designs, and therefore will be relatively inexpensive,
the same production vessel can be used for installing and possibly retrieve the swivel device, or also for well maintenance, which contributes to reduced operational costs,
very small marginal subsea fields can be profitable by using this novel arrangement, so that the degree of extraction can be increased for the fields,
the equipment employed can be re-used by moving it from field to field.
A preferred embodiment of the present invention comprises a subsea well arrangement for offshore production of oil or gas, comprising a base structure having its foundation on a seabed, a housing rotatably mounted about a first central, vertical axis on the base structure, a wellhead, a christmas tree mounted on the wellhead, at least one riser for connection to a production vessel at sea surface, a swivel device provided over the christmas tree, the swivel device communicating with the christmas tree and the swivel device being rotatable about the first, central vertical axis, at least one connecting member provided on the swivel device for connection with the at least one riser or hoses and with an umbilical or control cable, at least one attachment member provided on the housing, a yoke rotatable within a first angle about a horizontal axis and connected to the at least one attachment member, the yoke adapted to be connected at its outer end to at least one mooring line of the production vessel and wherein the housing rotatably mounted on the base structure having its foundation on the seabed is adapted to transfer mooring forces directly from the attachment members to the foundation without any noticeable stress applied to the christmas tree or the swivel device.
In one aspect of the preferred embodiment, the yoke contains yoke legs connected to attachment members and a lower end of the at least one riser is connected centrally to the swivel device and extends outwards from the swivel device centrally between the yoke legs.
In another aspect of the preferred embodiment, the connecting member for the umbilical cable is located at an upper part of said swivel device, and the swivel device contains a swivel member for cable connections at the same level as the connecting member for the umbilical cable.


REFERENCES:
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patent: 4668126 (1987-05-01), Burton
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patent: 177780 (1995-08-01), None
patent: WO95/02146 (1995-01-01), None
patent: WO 95/01904 (1995-01-01), None

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