Patent Application
20050067037
The present invention relates to composite risers and in particular composite risers particularly suited for deep sea mineral exploration.
In recent years the interest in composite risers has increased dramatically. This increased interest is a result of technical advances as well as the desire to exploit gas and oil reserves located in deeper water offshore. As the exploration and production of oil and gas move into deeper water, the weight, cost and reliability of risers have become increasingly important. These deep water applications significantly increase the pressures exerted upon the riser systems. The term “composite risers” refers to a variety of different types of high pressure pipes with metal connectors which extend from the sea bed to the surface. These risers can include but are not limited to drilling risers, production risers, workover risers, catenary risers, production tubing, choke and kill lines, as well as mud return lines.
Composite risers offer a number of advantages for deep water implementation such as high specific strength and stiffness, lightweight, corrosion resistance, high thermal insulation characteristics, high vibrational dampening and excellent fatigue performance. Additionally, composite risers can be specifically designed for each application to provide a better system oriented solution. The potential weight reduction can provide significant benefits for floating systems.
Currently, the risers predominantly in commercial use are made of steel or titanium. To achieve widespread acceptance, composite risers must show reliability equivalent or higher to the current risers. Currently, composite laminates used to make a composite riser are not fluid tight. Further, the composites have poor resistance to abrasion from drilling tools in the apparatuses used in drilling and production. Thus, the design of composite risers has included an inner liner to provide fluid integrity and/or wear resistance. Many different materials have been suggested for use as a liner, but metal liners appear to have certain advantages. Steel and titanium have been used as liners for their strength. The metal liners are relatively thin in comparison to a riser made entirely from metal.
It is important for the riser to remain fluid tight for successful operation. The integrity of composite risers is dominated by the fatigue resistance of the liner, the weld connecting the liner to the metal composite interface, the weld connecting the liner and transition ring, and the weld connecting the transition ring and metal composite interface. The failure of a weld, or other portion of the metal liner, results in a loss of pressure integrity. Thus, back-up sealing systems to maintain fluid tight passageway in composite risers have been suggested. This back-up seal is incorporated in the elastomeric liner that is placed between the metal liner and the composite structure to accommodate differential movements between the metal and the composites.
In the case where there is no back-up seal, the internal pressure will be relieved by venting to the outside of the riser if the metal liner is perforated by fatigue, wear or other damage forms. This loss of pressure containment is not operationally acceptable. But, in the case where there is a back-up seal, the internal fluids will be retained inside the riser not only inside the metal liner but also between the metal liner and the elastomeric liner. Under this condition, the pressure inside the metal liner and the pressure outside the metal liner (i.e. between the metal and the elastomeric liners) will be basically the same. However, when the internal fluids are removed during the depressurization of the riser as would be required during operations, the trapped fluid between the metal and elastomeric liners may not flow fast enough to keep a balanced pressure around the metal liner resulting in a condition where the pressure outside the metal liner is higher than the pressure inside the metal liner. In this situation, the metal liner can collapse. If the metal liner collapses, the inner bore will be reduced or blocked making it difficult to retrieve drilling tools and other equipment contained within the riser or equipment being used below the riser. In addition, it is desirable to prevent sea water from breaching the composite riser which can cause the liner to collapse.
Thus, there has been a continuing need for a lightweight collapse resistant liner. The present invention offers the advantages of improving the collapse resistance of a metal liner, with minimum additional weight.
The invention will be better understood with reference to the drawings taken in conjunction with the detailed description which follows. The figures are representative of preferred embodiments of the invention as follows:
The invention is a collapse resistant riser especially suited for composite risers having a metal liner. The invention consists of a metal liner, a thin reinforcing layer of dry fibers or composite on the outside of the metal liner, a shear ply layer (elastomeric layer) over the reinforcing layer, and an outer main structural layer of composite material over the shear ply. In a preferred embodiment, the metal liner can have an inner performance enhancing layer which serves as a corrosion resistant layer and/or wear resistant layer positioned adjacent to the inside wall of the metal liner. Also, in a preferred embodiment, there is a fluid impermeable layer, such as rubber, over the outer main structural layer of composite. And in yet another preferred embodiment, a scuff absorbing layer, such as fiber glass composite or aramid fiber composites, is applied over the fluid impermeable layer. The preferred materials for the metal liner are steel, aluminum and titanium.
In another embodiment of the invention, the composite riser has a metal assembly having a metal liner having two ends and a metal composite interface section attached to each end. The metal composite interfaces are attached to the mechanical connector that is used to connect different riser joints to form the riser string. The mechanical connectors are a threaded oil-field connectors or flanges. The metal composite interface and the mechanical connector can be machined as one pipe from a single forging or portions can be machined separately and then welded together. A reinforcing layer of dry fiber or of composite material is applied on the outside of the liner, and a shear ply of elastomeric material (e.g. rubber) is positioned over the reinforcing layer and at least a portion of each metal composite interface. In a preferred embodiment, the composite metal interface contains one or more grooves (trap locks) for attaching the composite outer layer to the metal assembly and thus transferring the loads from the mechanical connector to the composite layer. The shear ply is covered by the outer main structural layer which is connected to the metal composite interface. In preferred embodiments, a fluid impermeable layer is provided over the outer main structural layer of composite, and a scuff resistant layer is applied over the fluid impermeable layer.
The present invention will be described in relation to the expected predominant use of the composite riser which is in offshore oil and gas exploration. However, it should be understood that the invention would have application in other offshore mining techniques or in other applications where the composite riser is exposed to pressures exceeding about 1,500 psi where internal liner collapse is of concern. The use of the term “riser” is not meant to be limiting but is used in the context to describe a conduit including as described in the preferred embodiment the conduit used in offshore mineral exploration and mining.
The metal composite interface sections 22 provide outer connecting surfaces 26 for engaging with the composite laminates 28, and attaching the composite laminates 28 to the riser core 12. The details of the construction of the composite laminates have been omitted from
A portion of the composite laminates 28 will be made of fibers impregnated with resin which are then cured. The fibers can be any of those known for use such as carbon, aramid, S-glass, E-glass, boron, or combinations thereof. These will be bound together by a polymeric resin such as an epoxy resin, or vinyl ester resin which is cured.
Once a composite material is cured, the cured resin will normally crack at a lower pressure than the fibers. Thus, in riser applications, the pressure exerted will likely be sufficient to crack the resin meaning that the composite material in the riser will not be fluid tight. The liner 16 of the riser core 12 is utilized to maintain the fluid tight integrity of passageway 32.
On the outer surface of the metal liner 34 is positioned reinforcing layer 38 of composite material or dry fiber laminate. The reinforcing layer can be applied as dry fiber laminates, that is fibers are spun around the line but are not adhered together with a binder resin or it can be applied as a typical composite layer, fiber laminate impregnated with resin or combinations thereof. The reinforcing layer will typically have multiple plies of fiber. When the fibers are impregnated with resin the reinforcing layer can be cured. The reinforcing layer can be adhesively bonded to the metal liner 34 if desired. The structure and dimensions of this reinforcing layer are determined taking into account expected pressures to be encountered in the use. The reinforcing layer is applied adjacent to the metal liner to provide sufficient strength to resist collapse if water or other fluid penetrates the outer composite layer, or if internal fluids leak through the liner. The reinforcing layer should be designed to provide sufficient hoop strength to avoid collapse from the expected ambient pressure extreme of use. Also, in a preferred embodiment, the fibers used are preferably those that do not cause a galvanic reaction with the metal liner.
Positioned over reinforcing layer 38 is shear ply layer 40. Shear ply layer 40 is of natural rubber layer, or modified natural rubber layer or other polymeric material. Hydrogenated acrylonitrile butadiene rubber (HNBR) is useful for the shear ply. The purpose of shear ply is to allow the outer layers to move separate from the layers inside the shear ply. Further, shear ply layer 40 also serves as a fluid tight barrier to prevent water from penetrating reinforcing layer 38 from the outside and serves as a seal to contain any internal leakage within the riser.
Positioned on the outer side of shear ply layer 40 is outer main structural layer 42 of composite material. Outer main layer 42 is made from a composite material with plies of fibers impregnated with a resin which has been cured. The design, orientation, and thickness of the various plies of composite fibers making up the outer main layer 42 is determined in accordance with the strength requirements to support the composite riser when it is installed in a string of risers connecting the sea floor to the surface. The outer main layer can be of any desired configuration and mixture of plies of various fibers. For example, a portion of the outer main layer can be a carbon fiber helical and hoop composite. The outer main layer 42 is preferably covered by fluid impregnable layer 44. Fluid impregnable layer 44 is made from any fluid impermeable material such as rubber or polymeric material. The same material as used for the shear ply can be used as the fluid impregnable layer. The purpose of layer 44 is to prevent water from impregnating the main outer layer 42. If fluid impregnable layer 44 was not present, water would flow through any cracks that develop in the resin bonding the fibers in the main outer structural layer and exert pressure against the shear ply and the layers inside the shear ply. If the reinforcing layer 38 were not provided that pressure could exceed the collapse strength of metal layer 34.
In a preferred embodiment, a scuff absorbing layer 46 is positioned over fluid impregnable layer 44. Thus, scuff absorbing layer 46 is designed to protect the fluid impermeable layer 44 to prevent cuts and gouges which would compromise the integrity of fluid impermeable layer 44. Scuff absorbing layer 46 is preferably made of a composite material. In a preferred embodiment, the scuff absorbing layer is a glass fiber composite.
In a preferred embodiment, the metal liner has one or more engaging surfaces on the outside of the metal liner. These engaging surfaces improve the adhesion or engagement of the reinforcing layer 38 to the liner.
The invention increases the reliability of the composite riser while minimizing weight. A composite riser constructed without the reinforcing layer would either provide a metal liner of sufficient strength to withstand the expected ambient pressure or would assume that the outer fluid impermeable layer would not be breached or compromised. The assumption that the outer fluid impermeable layer will not be breached is not a sound long term assumption in the commercial environment. Dimensioning the metal liner to resist the pressure has the disadvantage of sacrificing weight which is a primary advantage of the composite riser.
While the present invention has been described in relation to certain preferred embodiment, the description is not limiting of the invention and those skilled in the art will appreciate variations from the disclosed embodiments without departing from the nature of the invention.
