D. B. JOHNSON, D.D. BALDWIN, J.R. LONG
The current project began in 1995 as a joint-industry project under the U.S. Department of Commerce's National Institute of Standards and Technology (NIST) Advanced Technology Program (ATP). The goals of the project are to design, develop, manufacture, test and qualify a TLP production riser made with fiber-reinforced polymeric composites. A concentrated effort was conducted in the first phase of the project to design a low-cost, light-weight CPR suitable for deep water (3000 to 5000 feet). Both single casing and dual casing risers were considered (Refs. 4 and 5). Subsequent phases included fabrication and testing of pre- production prototype specimens to characterize the design (Refs. 6 and 7). In the last phase of the program, full-length CPR joints were fabricated and tested to demonstrate compliance with the key requirements of the design.
CPR Requirements
The project team began by developing a Functional Specification and Performance Criteria document which provided guidance during the production and demonstration of a CPR joint. The objective was that the CPR be fully ready for use by the offshore industry at the completion of the project.
Functionally, the CPR must perform the same duties as a steel riser casing. Generally, these functions are: pressure and fluid containment for well-control purposes; a structural member of the well riser system; the shroud containing the conduit for conveying fluids to and from the reservoir; and, equipment for guiding drilling and workover tools and tubulars into the well. In these functions, the CPR is not to require any special handling techniques or equipment,
Preliminary composite tube body designs were produced for three different riser configurations: a 10 3/4-inch single/dual casing riser; a 9 5/8-inch single casing riser; and a 10 3/4-inch dual casing riser. The 10 3/4-inch single/dual casing riser was selected for detailed design and prototype testing. This configuration allows for the demonstration of a CPR which meets the most severe requirements of CPR service. Table 1 lists the casing parameters of a 10 3/4-inch CPR meeting the requirements of both the single casing and dual casing applications. While this configuration was an excellent vehicle for demonstrating that a CPR can meet the structural requirements of offshore service, the project participants recognize that a CPR designed specifically for either the single casing or dual casing application would be more cost effective. At this point, the CPR design has been shown to meet the cost, weight and performance goals of the project.
CPR Design Development
The use of well-established advanced composite structural design methodologies has resulted in a CPR design with predicted capabilities that exceed the expected loadings. The design of the CPR joint requires three separate efforts: composite tubular design metal-to-composite interface design; and metal connector design. The composite tubular wall is a hybrid composite structure, with carbon fiber and glass fiber reinforcements in an epoxy matrix. The metal-to-composite interface is a multiple traplock configuration, well-suited for supporting axial and pressure loadings. A premium threaded connection is used for the metal connector design.
Composite Tubular Wall. The CPR tube body, as shown in Fig. 1, is a hybrid composite structure, consisting of carbon and E-glass fibers in an epoxy matrix. The tube is manufactured using the filament winding process, in which the reinforcing fibers are impregnated with the uncured epoxy resin and applied to a rotating mandrel in precise orientations and thicknesses. The resulting composite structure is then cured by the application of heat energy, resulting in a solid structural tube. After extraction of the mandrel, both internal and external thermoplastic or elastomeric liners are installed.
In the CPR design, the amount of the circumferential carbon fiber is determined by the Shut-in Well Head Pressure (SIWHP). This material is equally distributed between the inside and outside surfaces of the tube wall to maximize external pressure buckling capability. These layers are hybridized with E-glass fiber to increase the damage tolerance and impact resistance of the tube wall. The circumferential layers of E-glass at mid-wall are also used as a “core” material to increase the stability of the cross-section by increasing the separation distance between the inside and outside carbon circumferential reinforcements.
The low-angle carbon helical layers in the CPR provide axial strength and stiffness. The minimum axial stiffness requirement of 100 (msi)(in2) dictates that at a minimum cross- sectional area of low-angle carbon reinforcement be included in the CPR cross-section. This amount of axial reinforcement results in a tube wall with more than adequate strength to resist the axial load generated by the SIWHP and the riser top tension.
Metal-to-Composite Interface. In selecting a cost-effective metal-to-composite connection for the CPR. it was recognized that the riser is expected to support primarily axial tension and pressure loading (both internal and external). The selected interface design is a multiple traplock configuration, as illustrated in Fig. 2, in which one-piece end fittings are incorporated into each end of the composite tube during the filament winding process. The absence of significant torsional loads made the use of the traplock attachment attractive in the CPR application.
The mechanism of load transfer in a multiple traplock can vary greatly, depending on part geometry. Axial load is transferred between the composite tube and steel fitting across the trap faces. In order to achieve a good distribution of load between multiple traps, the designer must balance the relative stiffnesses of the steel and composite materials.
