Shankar Bhat - DeepFlex Inc.
Paul Jacob - MMI
The need for composite flexible fiber reinforced pipe (FFRP) becomes more critical as the industry moves out to 3,000 m (9,842 ft) water depths. Earlier generations of fiberglass reinforced plastic bonded pipe systems have been in use for over 40 years in shallow water applications. The challenge for the next generation was to design and produce a new all-composite type of pipe to withstand the greater external hydrostatic pressures, higher internal wellhead pressures, and temperature extremes that accompany deepwater.
Constructed from extruded polymeric layers reinforced with laminated glass-fiber tape stacks, DeepFlex Inc. has a next generation lightweight, nonmetallic, unbonded FFRP developed for subsea and deepwater floating system applications. To maximize the strength of its composite-only pipe, DeepFlex created overlapping layers of composite reinforcement, using multi-start stacks of specially made, pre-cured unidirectional glass fiber composite tapes. Pipe is continuous, and is made in lengths limited only by storage capacity.
All-composite flexible fiber reinforced pipe (FFRP) can be used for dynamic risers, subsea flowlines and pipelines, subsea jumpers and surface jumpers on hybrid risers or on platform decks.
Tests of the 2-in. (5-cm) pipe have demonstrated its ability to survive the pressure found in the Marianas Trench, the deepest spot in any ocean. A 4-in. (10-cm) pipe has been tested to a collapse pressure of 10,000 psi (689.5 bars), the equivalent of more than 6,000 m (22,482 ft) of seawater. The pipe is designed to take a tremendous compressive load with a generous safety factor.
While the plies within each FFRP stack are bonded together by epoxy resin, each stack remains unbonded from the others, ensuring true flexibility under extreme conditions and increasing fatigue resistance in dynamic applications. Unbonded construction also allows the pipe to be produced and installed in continuous lengths. In addition, the composite materials act as effective insulators, keeping product flowing through pipes at colder temperatures. The all-composite makeup results in pipe that is lighter than steel or other types of flexible pipe, allowing significant reduction of loads on host facilities in deep water.
Finite element analysis
The way FFRP is constructed permits tailoring to the variables of the particular environment. A cross section lay-up allows each layer to be designed to meet specific requirements for burst, collapse, axial extension, bending, and torsion. To meet exacting specifications, further insight was needed into the performance of each layer of composite to optimize pipe cross section configuration. DeepFlex worked with structural mechanics consultants at MMI Engineering Inc., who applied Abaqus unified finite element analysis (FEA) software, from Dassault Systèmes’ SIMULIA brand, to model FFRP.
Model creation, testing
As prototype testing began generating data during the design and development stages, DeepFlex supplied design information and pipe cross section data to MMI for the numerical model creation and testing. A complex model was needed to handle the internal interactions of the materials in a more complete way.
The company has a proprietary method of sizing its pipes. A cross section, and the properties of the pipe (for example, pipe makeup for 10,000 ft [3,048 m] of water and 5,000 psi [34.47 MPa] burst pressure), were supplied MMI for analysis. The company has an extensive prototype test program that provides overall results for product performance, but wanted to build on this and gain an understanding of how the various components in the pipe behave under loading. Abaqus FEA software provided insight into product performance.
MMI used the preprocessing capabilities of Abaqus/CAE to create meshed FEA models of the pipe to analyze for performance characteristics. The Abaqus analysis products were then used to conduct the simulations. A combination of Abaqus/Standard and Abaqus/Explicit were used. Abaqus/Explicit verified the interaction between components, as it is easier to “shake out” numerical problems with contact. Once confidence was gained in the contact interaction, Abaqus/Standard completed the main set of performance analysis runs.
SIMULIA/Abaqus finite element analysis (FEA) image of Deep/Flex pipe showing meshed representation of the layers of extrusions and reinforcement.
To model the composite components of the pipe, instead of creating the individual plies, orthotropic solids of each composite section were built. Abaqus could have modeled all the individual layers, but that level of detail at this point in the studies was not needed. Such detail could be included at a later stage of product development if required.
MMI began numerical analysis with sensitivity studies of 2D models to determine where to focus on the interactions between composite layers within a pipe structure under various loads. From these studies, 3D models were created with each composite component modeled explicitly with contact (such as friction between reinforcement stacks) where required. Boundary conditions and loads then were applied and benchmark tests were performed to confirm that the model behavior was realistic.
The FEA model included nearly one million degrees of freedom and the analysis was run overnight on a single processor 64-bit Intel Xeon processor machine with the Red Hat Linux 64 operating system. The FEA results were a starting point for an understanding of the failure limits of particular pipe specifications, simulating burst and collapse tests. The analysis indicated the mechanisms and responses of the structure under loading.
Efficient modeling design
Studying what the failure modes would be (how they would progress through the structure, for internal pressure, external hydrostatic collapse loads bending, torsion, and axial loads) helped the company design its pipe more efficiently. Individual components in the cross section can be tailored to meet the demands of the different layers in loading conditions such as burst or collapse. This is an advantage over steel pipe, where working with a single material and thickness can limit design flexibility.
MMI developed a method to assess failures between the individual layers, using the ‘Model Change’ command in Abaqus to alter the states between them and applying loads to the model structure gradually until components began to fail. This approach led to developing global characteristics for load extension and bending that took into account the effects of burst and collapse pressures.
DeepFlex used a length of composite pipe (in red) positioned in a dynamic test machine to carry out bending and torsional stiffness tests with and without internal pressure.
MMI provided its FEA analysis data to DeepFlex for use as part of its design process going forward. DeepFlex continues to work with MMI, and is also using Abaqus software in-house.
Pipes are offered at various internal pressure design ratings up to 10,000 psi (689.5 bars) working pressure. The FAT (Factory Acceptance Test) is carried out at 1.5 times the working pressure and burst ratings are a minimum of 2.5. No existing codes cover this new product directly, but Deep Flex’s goal is to meet or exceed API (American Petroleum Institute) 17 requirements when they are applicable.
