Ships – Floating platform – Multiple leg
Reexamination Certificate
2000-02-28
2001-06-19
Sotelo, Jessica D. (Department: 3617)
Ships
Floating platform
Multiple leg
C114S114000, C114S06500R
Reexamination Certificate
active
06247421
ABSTRACT:
TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY
The present invention relates to a method for converting an existing moored semi-submersible drilling- or accommodation-rig including at least two submerged pontoon bodies, a work deck raised to a secure level above the surface of the water that has power generation facilities, and upright support columns between the pontoon bodies and the work deck. This structural combination is established as a dynamically positioned drilling vessel having thrusters and additional power generation and distribution facilities for station keeping; that is, stationary location of the platform in the water using the thrusters. The invention can alternatively be used for conversion of a semi-submersible rig to a dynamically positioned floating production vessel.
BACKGROUND ART
Usually, when converting an older existing moored drilling rig to a dynamically positioned (DP) drilling vessel, a number of thrusters are added to the pontoon bodies and a large amount of power generation and distribution equipment is necessarily added to the work deck. This equipment includes, among other things, diesel generators, switchboards, transformers, variable speed drives for thrusters, support equipment for the diesel generators and control equipment for keeping the rig on station.
Additionally, the drilling deck's capability for carrying load has to be increased to accommodate the DP support equipment and the possibility of additional drilling equipment. Usually the increased weight and payload are compensated for by adding sponsons to the submerged pontoon bodies and by adding fenders to the support columns. Both measures enhance the carrying and stabilizing capacity of the vessel. Also, the deck structure and the bracing system usually will have to be upgraded. All these structural alterations lead to extensive and expensive welding labor.
The integration of new thrusters into the pontoon bodies and the installation of new power generation and distribution equipment into the existing pontoon and deck layout (while at the same time meeting separation and redundancy requirements for a dynamically positioned rig) involves substantial time, complex routing and installation of piping and cables. The time schedule for performing such a conversion normally means that the existing rig must be taken out of operation for a long time period for the necessary construction to be completed.
In order for a conversion to be cost-effective, one usually desires to start with an existing, but comparatively modern drilling rig having a large deck-load capacity. Such rigs, however, are usually profitable in their normal role and original configuration and therefor are seldom cheaply available for conversion.
Other types of semi-submersible conversions are known; for example, the owner of the present invention is also the owner of the invention disclosed in U.S. Pat. No. 5,036,782 entitled Method For Converting a Semi-Submersible Vessel. The '782 patent, however, discloses an enhancement to an existing semi-submersible drilling- or accommodation-rig for accomplishing a conversion into a floating production plant having apparatus for extracting and processing oil and/or gas, or other heavy and bulky deck loads. The disclosed apparatus and method for the conversion taught by the '782 patent, however, is distinctly different from the apparatus and method disclosed herein. Importantly, the conversion taught by the '782 patent does not enable dynamic station keeping and/or positioning.
DISCLOSURE OF THE INVENTION
The main objective of the present invention is to provide apparatus and method that can be used for a cost effective conversion to a deep water dynamic positioned drilling vessel of both modern semi-submersible rigs and older, previously less attractive rigs that had limited water-depth operating capability, but which are still suitable for conversion according to the teachings of the present invention.
According to the invention, this objective may be accomplished by the pre-fabrication of rig extensions for the corners of an existing semi-submersible rig. Such a method for converting semi-submersible rigs to dynamically positioned vessels can advantageously include: (1) adding “extensions” to both ends of existing pontoon bodies; (2) adding work deck extensions to the original work deck; (3) adding supporting structures between the new pontoon body extensions and the work deck extensions; and (4) adding thrusters at the pontoon body extensions for providing dynamic station keeping capabilities to the resulting converted vessel. Preferably, the support structures between the pontoon body extensions and the respective work deck extensions positioned thereabove are configured as columns, rendering the necessary water-line area increase required because of the additional mass of the newly added structures.
Thrusters, and a majority of the power generation and distribution equipment required for dynamic position station keeping are arranged on and in the rig extensions themselves. The rig extensions increase the displacement of the vessel in a way substantially corresponding to the dead weight of those extensions, including thrusters and power generation facilities. It also enables an increase in the water line area of the vessel so that necessary deck-load capacity and stability is achieved.
A preferred embodiment of the invention adds the new thrusters and power generation and distribution equipment that supply the required power for dynamic station keeping. The existing power generation and distribution equipment normally continues to be used primarily for powering the drilling equipment and existing rig systems, with only limited requirements for interconnection between the new and old power generation and distribution equipment.
Preferably, the supporting structures between the pontoon body extensions and the respective work deck extensions are arranged as columns which also contribute to giving the required water-line area increase. According to an optional variation to the invention, connective bracing elements may be arranged between the pontoon body extensions and/or the supporting columns.
Still another possible variation to the invention includes the utilization of existing thrusters and existing power generation and distribution equipment as one or more DP-failure backup group(s) for a converted dynamically positioned vessel.
According to a preferred variation of the invention, each rig corner extension, including a pontoon body extension, a work deck extension and the supporting structure therebetween is pre-fabricated as an integral unit with even the power generation equipment pre-installed. These integral units are attached to an existing rig in a dry dock and the systems are interconnected as necessary. These pre-fab characteristics account, at least partially, for the expedient nature of the present conversion technique.
Alternatively, the submerged pontoon bodies are extended in sections by means of habitats upon the floating existing rig. Support columns are mounted in sections onto the extensions of the submerged pontoon bodies and the work deck is extended out above the support columns. The power generation and distribution equipment are then installed and the various systems interconnected as necessary.
The general beneficial effects described above apply generally to each of the exemplary descriptions and characterizations of the structures and devices disclosed herein. The specific structures through which these benefits are delivered will be described in detail hereinbelow.
REFERENCES:
patent: 3978807 (1976-09-01), Nitzki
patent: 4436050 (1984-03-01), Liden
patent: 5036782 (1991-08-01), KInde et al.
GVA Consultants AB
Kilpatrick & Stockton LLP
Sotelo Jessica D.
LandOfFree
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