Dynamic Design Tools Increase Safety

Risers and flowlines are integral components of subsea developments. The design of these components requires extensive analysis that involves taking into account a complex set of interacting gravity and hydrostatic and dynamic effects in addition to considerations regarding the response of the materials of these components and their interaction with the seafloor. The use of numerical, in particular finite element (FE)-based analysis tools, in designing these components is essential.

PIP Flowlines, SCRs

Designing pipe-in-pipe (PIP) flowlines and steel catenary risers (SCRs) requires engineers to consider effects such as the interaction between unbonded nested pipes and assessing necessary pretension of the inner pipe to alleviate adverse effects of locked-in compression after pipelay is complete.

Analysis tools based on state-of-the-art 3-D continuum FE, which ensures highest accuracy in the simulation, have proven critical to accurate and realistic pipeline assessment subject to such events. (Images courtesy of Wood Group, MCS Kenny)

Interaction between inner (carrier) and outer (jacket) pipes of an unbonded PIP flowline or SCR is complex and requires specifying axial (e.g., friction characteristics) and radial (e.g., relative movement) contact characterization. This is modeled in Abaqus using the ITT elements. Modeling the contact between the inner and outer sets of pipes introduces a great level of nonlinearity to the model and adds computational cost. However, FE analysis that uses such advanced techniques is essential to assessing required pretension of the inner pipe so that the final locked-in compressive stresses are acceptable.

Pipelines For The Arctic

Designing pipelines and risers for arctic conditions introduces challenges, such as ice-loading modeling, ice-gouging and/or permafrost thawing, and frost heave. Several codes have been developed during the last two decades that have addressed ice modeling and loading, including API RP 2N, IEC and, most recently, ISO/DIS 19906. These codes provide ice static and dynamic loading characterization on common structural shapes and sizes.

Ice-loading modeling in numerical schemes (e.g., FE) has developed over the years, yet it still encounters challenges. Different material models have been shown to suit variousapplications, depending on the type of ice under consideration, size of the loaded area, and expected level of stress and rate of loading. For example, elasticity has been adequate for solid/consolidated ice under low levels of stresses and relatively rapid loading, unconsolidated ice under higher levels of stress can be better modeled as a plastic material, low loading causes ice to behave in a viscous manner, and damaged elasticity has been successful in describing internal damage of the ice due to brittle cracking.

VIV of complex-shaped (e.g., multiplanar) jumpers and FIV normally require numerical analysis such as CFD.

Ice gouging is a natural phenomenon during which a large floating ice mass gouges the seabed, threatening the integrity of subsea structures and pipelines. Modeling the impact of the phenomenon on these structures is challenging as it involves a number of complex interactions, namely between the aero- and hydrodynamic forces and ice mass, between the ice mass and seabed, and between the seabed and subsea structures. Analysis tools based on state-of-the-art 3-D continuum FE, which ensures highest accuracy in the simulation, have proven critical to accurate and realistic pipeline assessment subject to such events.

A pipeline buried in the permafrost can be subject to overstress if the pipeline degrades the integrity of the permafrost by warming and thawing it. Depending on properties of the frozen soil, thawed permafrost could collapse and subject the pipeline to excessive deflection and strains. Simulating this process and the impact on the pipeline involves multiple physical processes, including heat transfer, soil consolidation, and soil-pipeline interaction. Pipelines for HP/HT conditions

Designing a pipeline for HP/HT conditions should account for thermal expansion and degradation of material properties because of elevated temperature as well as possible upheaval and/or lateral buckling due to both high temperature and high pressure. The concept of effective forces and true stresses should be clear and addressed correctly. Also, accounting for the behavior of the inner and outer pipes of a PIP configuration usually necessitates modeling the two pipes and complex interaction between them. Interaction between the pipeline and seabed also should be accounted for, particularly the soil resistance to pipeline movement and resulting buildup of compressive forces.

Pipeline-soil interaction is one of the more complex and less established aspects of the process; a number of joint industry projects (JIPs) have addressed the issue. User-defined subroutines have been used to implement recommendations of these JIPs to model the pipeline-soil interaction in both upheaval and lateral buckling as well as potential axial movement (e.g., walking).

Components Of Pipelines, Risers

Besides pipes, pipelines and risers incorporate a number of components that work as connecting points between different parts of the pipeline/riser system and at the boundaries of the system. Components include flanges, bulkheads, load-shares, tapered stress joints, flex joints, and bend stiffeners. These usually are complex and can involve parts connected loosely, by way of bolts, and can move relative to one another. This makes the analysis particularly involved and requires numerical simulation of nonlinear behaviors, such as contact between surfaces, friction and sliding, and bolted connections. Analyzingsuch components using FE analysis requires advanced techniques of solid modeling, nonlinear material modeling, and contact, which usually entails significant degrees of nonlinearity and demand for computational power and proper engineering judgment.

CEL models simulate pipeline interaction with the seabed, including the impact of mud flow on pipeline and the response of pipeline to lateral and upheaval buckling.

Jumpers

Designing jumpers requires considering numerous static and dynamic effects. Static and quasi-static loads include gravity, current, and waves; dynamic loads include flow-induced vibration (FIV) and vortex-induced vibration (VIV). VIV for simple-shaped jumpers usually is analyzed using code-based equations, whereas VIV of complex-shaped (e.g., multiplanar) jumpers and FIV normally require numerical analysis such as computational fluid dynamic (CFD), which simulates induced turbulent flow and vortices and resulting vibration of the jumper.

Pipeline, SCR Seabed Interaction

Proper modeling of the SCR touchdown zone (TDZ) is essential to realistic assessment of the fatigue life of that structure. However, the SCR TDZ modeling also is challenging, as it requires modeling the highly complex response of the soft seabed and potential forming of a trench in the seabed over time.

A number of simplified models have been developed for implementation in global SCR analysis; however, these models usually are limited to specific ranges of soil types and SCR pipe parameters. Currently, new FE techniques (such as coupled Eulerian Lagrangian, or CEL, FE analysis) are used to model the response of the SCR when interacting with the seabed for better understanding of that interaction and then simplifying it for global analysis of the SCR.

MCS Kenny has developed CEL models to simulate pipeline interaction with the seabed, including the impact of mud flow on pipeline and the response of pipeline to lateral and upheaval buckling.