Buoys – rafts – and aquatic devices – Buoy
Reexamination Certificate
1999-08-20
2001-03-27
Basinger, Sherman (Department: 3617)
Buoys, rafts, and aquatic devices
Buoy
C441S023000, C441S133000, C405S195100
Reexamination Certificate
active
06206742
ABSTRACT:
The present invention relates to a method for locally imparting additional buoyancy to a longitudinal body emerged in water, and also relates to a buoyancy device adapted to perform said method.
The invention in particular relates to use in connection with plants comprising risers and/or umbilicals arranged between a submarine connection and a floating equipment positioned at the surface. The invention in particular relates to a plant comprising dynamic risers of a flexible type or so-called “umbilicals”, passing from the seabed to a vessel or to a platform not standing on the seabed, but moving in a flexible mooring. A buoyancy device according to the present invention will reduce the strain in the risers, a strain caused by the weight of the risers themselves and possible loads. The riser cables and/or the pipes will in a conventional manner rest on the buoyancy device having the shape of a buoy, and enclose the same along an angle extending to a maximum of 180°. Conventionally such buoyancy devices are anchored to the seabed by wires, steel ropes or chains, so that the buoyancy devices are positioned and maintained in the water between the seabed and the surface.
In connection with previously known buoyancy devices used in connection with dynamic risers, e.g. a buoy developed for use on the Guillemot oil field, separate pressure tanks made of steel have regularly been used, and these tanks are in turn connected to a steel structure including a frame and recesses with a shape adapted to risers. Such previously known pressure tank systems lead to many disadvantages, of which the most important ones are mentioned below. It is also referred to U.S. Pat. Nos. 4,793,737 and 5,505,560, giving examples of similar techniques.
Conventional pressure tanks are often made from steel. Steel is heavily corroded when exposed to sea water, and accordingly the tanks have to be dimensioned to resist the pressure of water at the working depth. As a result the buoy will be very heavy and must be installed by means of specific vessel having a derrick with a sufficient lifting capacity for the heavy steel buoys. In addition the buoys have to be filled by air already on the surface to avoid internal corrosion problems that have to arise if water would be pumped out of the tank after installation. Accordingly the buoy has to be pulled down to its desired position due to the large buoyancy, before installation. The buoyancy and therefore also the volume must be exceptionally large, as the buoyancy must compensate the high intrinsic weight of the buoy, again due to the use of steel. Even if internal corrosion is avoided as seawater never comes in contact with the inner side of the buoyancy device, a thorough external corrosion protection must be obtained by means of surface protection and sacrificial anodes. All these precautions result in very high costs during the mounting process and during maintenance. Regular inspections are also required to avoid damages due to corrosion.
The object of the present invention is to provide a new buoyancy device adapted to be used in connection with dynamic riser systems where the above-mentioned disadvantages are avoided. This is partly obtained by using a new method during deployment, as the buoyancy device is laid out while the substantial part of the internal volume of the buoyancy device communicates freely with the surroundings. This feature ensures that the structure of the buoyancy device is not exposed to large and detrimental external pressures.
Accordingly also the internal volume of a buoyancy device according to the present invention will be exposed to seawater during the laying out operation. Such exposure is accepted as the new construction preferably is manufactured from a material being corrosion resistant against sea water. A preferred material may be glass reinforced plastics (GRP), however, other composites reinforced by fibres may also be used.
The features mentioned above also give other advantages for buoyancy devices according to the present invention. As composite materials having fibre reinforcement, e.g. built up from KEVLAR or GRP are materials with a low density, the requirements to hoisting capacity are reduced drastically. The low weight also makes it possible to collect several buoyancy devices on the site by means of one minor vessel, which again reduces the on-site mounting costs further. In addition the buoyancy device may be installed in a completed version, i.e. including the anchoring lines connected to the buoy while the weight of this line may be compensated in advance by means of internal or external buoyancy elements. However, this does not exclude that the anchoring line instead may be connected first when the buoyancy element has been lowered down to the site. Accordingly the mounting method will be very flexible and may be adapted to local conditions. The buoy may be designed so that it is neutral (neither sinking nor floating) or has limited buoyancy when submerged in water.
The selected material ensures that corrosion problems will not arise, and this again makes it possible to use later filling with air and controlling of the overpressure in the buoyancy chambers. Even ballasting by use of seawater may take place without problems.
The shape of the design also gives the solution according to this invention a very high flexibility and freedom to select shapes and designs appropriate for the using conditions. As an example the saddles by which the risers are supported may be implemented directly on the external surface of the buoyancy device. The design of the body of the buoyancy element itself, accordingly may be adapted to the minimum accepted bending radius of the dynamic riser or umbilical used. Integrating the buoyancy tank or the buoyancy tanks in the buoyancy element will also be simple, and the buoyancy device may be moulded as one single unit of GRP material or a similar suitable artificial material, such as a composite material comprising reinforcing fibres.
Finally the buoyancy device may comprise a plurality of internal chambers of suitable shape and arrangement, and each such internal chamber may be provided with valves which again allow filling of selected chambers with seawater when used as ballast chambers, while other chambers may be filled by a gas, preferably air, to adjust the buoyancy. When the buoyancy tank or tanks consisting of GRP material are filled with air, they may be filled until the air pressure corresponds to the prevailing water pressure at this depth, and therefore the walls of the buoyancy device will not be exposed to a large, external pressure, which, in connection with conventional solutions, could bring the buoyancy chambers to implode.
Finally the tank or the tanks may be equipped with excess pressure valves to prevent over-pressure within the tank during filling with air. If an internal excess pressure value is used, a possible leak will result in some air bleeding out before the device reduces its buoyancy. Therefore, a possible leak may be detected before a detectable reduction of the buoyancy itself has ocurred.
REFERENCES:
patent: 4400110 (1983-08-01), Beynet et al.
patent: 4793737 (1988-12-01), Shotbolt
patent: 5957074 (1999-09-01), De Baan et al.
Bull Henrik
Ingebretsen Helge
ABB Offshore Technology AS
Basinger Sherman
Cohen & Pontani, Lieberman & Pavane
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