Pipeline design challenges

JP Kenny is applying state-of-the-art technologies to the design of the seabed pipeline, its route mapping and the proposed carbon dioxide sequestration project.

In addition to the great length of the pipeline, other critical issues affecting the pipeline design are the depth of the sea bed (up to 1350m), the high temperature of the gas (up to 130C), and relatively high pressure (360bar).

The two largest upstream investments are the line pipe and pipe lay. According to JP Kenny pipeline business leader Dermot O’Brien, the mix of high pressure and high temperature make pipeline material selection and corrosion management a key design issue. Pipeline wall thickness, corrosion inhibitors and claddings all impact costs, and FEA analysis is helping in the selection of alternative materials.

“The deepwater location of part of the gas field presents clear challenges for the design and installation of large diameter pipeline,” O’Brien said.

“The pipeline requirements include about 260 kilometres of large gas delivery line and some 520 kilometres of small diameter pipe connecting the well heads, manifolds and other equipment.”

Other issues to be considered in the pipeline’s design are the effects of the local marine environment, which features steep escarpments at the continental shelf, the annual cyclone season, large tidal movements, and strong currents, which impact on the seabed and the pipeline itself.

Several design criteria and equations fall outside the current design codes, so specialised engineering assessments are being applied.

Optimising designs through virtual testing

JP Kenny used the Abaqus FEA technology to help minimise the design time for components, minimise the number of prototypes to be fabricated, and provide a virtual test facility that can measure several criteria at once.

Abaqus, a complete suite of finite-element analysis software from Simulia, the Dassault Systèmes brand for realistic simulation, was used to study the performance of alternative designs. The software provides solutions for modelling and visualising a design’s behaviour for structural integrity when the structure is subjected to loads and contact.

Abaqus is used to evaluate conceptual designs, front-end engineering design and detailed design of the pipeline.

The characteristics of the high-pressure, high-temperature gas flow and the natural sea bed behaviour could combine to make predicted pipeline end expansions of 7m for the main line. The forces associated with this expansion include lateral displacement cycles of the pipe on the sea bed of up to 10m.

The Abaqus suite is helping the engineers develop and test designs to withstand the pipeline dynamics and the forces operating at the continental shelf crossing.

For example, detailed lateral buckling analyses are being done to assess forces, movements and strains across the full range of behaviours, including ratcheting due to start-up and shutdown cycles, the cumulative effect that pressure and temperature fluctuations have on the highly stressed apex of the buckle, and the potential for pipeline creep or walking.

“Modelling of pipeline and pipeline-to-seabed interaction is helping to define appropriate locations for built-in buckle points,” O’Brien said. “It is also helping to identify expansion spool sizes, and Abaqus elbow elements are used to predict bending and ovalisation of the pipeline.”

Abaqus is also being used to confirm the location where the pipeline should span the sea floor escarpment at the continental shelf, which drops from 200m to 800m water depth.

The pipe lay team integrated the Fledermaus interactive 3D visualisation system with the Abaqus FEA tool to accurately map the escarpment. At the scarp crossing there is a potential pipeline span of 200-300m and there are local hazards, including mudflows on the scarp face. Crossing the escarpment at the optimum point will reduce the pipe lay for the project by up to 40km.

FEA analysis is being done for a complete range of conditions, including with the pipeline empty, with operating contents, and with flushing media.

The sensitivity cases being considered are different pipe outer diameters, wall thicknesses, addition of concrete coating and residual lay tension. Results from the span analysis reveal bending moment distribution, longitudinal strain profile and spanning pipeline profile along the route, and modal shapes and frequencies.

“To make the undersea infrastructure more secure, we have used Abaqus to plan for major event scenarios, including the impact of cyclones on pipeline dynamics,” O’Brien said.

“Abaqus has reduced simulation times, and improved the efficiency and accuracy of pipeline design and route mapping since the team switched from our former FEA tool.”