RISER STRESS ANALYSIS
Riser Arrangement in FPSO field
Platforms are producing and flowing oil and gas to the FPSO for processing and storage, which is located at a distance from field. Full Well Stream (FWS) pipeline is running from well to FPSO and Gas Lift (GL) pipeline from the FPSO back to field. Production will flow to the FPSO and commingle with the production for final processing, storage and offloading. The compressors on the FPSO will supply all fuel gas, instrument and Gas Lift gas required . Source of this gas will be from the existing wells and any additional associated gas from the other wells. The pipelines will tie-in to the FPSO FWS pipeline and GL pipeline. The subsea tie-in will consist of a pipeline end manifold (PLEM), which will be designed to enable minimum interference of the production from the incoming production.
PURPOSE OF RISER STRESS ANALYSIS
This study presents the design of the riser and subsea tie-in spools for the following pipelines:
- FWS pipeline connecting field to PLEM.
- Gas Lift pipeline connecting PLEM to field well
Riser stress analysis covers:
- Vortex shedding analysis of the riser span to determine suitable riser clamp locations.
- Stress analysis at the pipeline expansion spool and riser at the platform.
- Stress analysis at the pipeline expansion spool at the PLEM for spool
- Verification of the sliding/guide clamps and load-bearing clamps.
- Verification of the standard subsea flange and their suitable application at the expansion spool.
RISER SPAN
The vortex induced vibration analysis can be performed by Excel spreadsheets for the 1-year installation (empty), hydrotest and 100-year conditions (operating).
The vortex induced vibration analysis can be performed by Excel spreadsheets for the 1-year installation (empty), hydrotest and 100-year conditions (operating).
RISER/SPOOL STRESS
The stress analyses for FWS and Gas lift pipeline risers and PLEM tie-in spools shall be performed to determine the maximum induced stress within the system and hence determine the requirement of expansion offset and the maximum loads for riser clamp design. The analysis takes into account of both functional (self weight, temperature and pressure) and environmental loading.
The riser/spools stress analysis can be carried out using the commercial finite element software Autopipe for the following modes of operations:
The riser/spools stress analysis can be carried out using the commercial finite element software Autopipe for the following modes of operations:
- Operating with Functional Loads (self-weight + pressure + temperature + expansion) plus 100-year Environmental Loads.
- Hydrostatic Test with Functional Loads (self-weight + pressure) plus 1-year Environmental Loads
RISER CLAMP LOADS
The maximum loads for the hanger flanges and riser clamp supports can be established based on results from the riser stress analysis.
RISER STRESS ANALYSIS CONCLUSION AND RECOMMENDATION
Based on the results of the analyses, the following conclusions and recommendations shall be made.
RISER SPAN
Based on the vortex induced vibration analyses, the span between the proposed riser clamps shall not suffer from vortex shedding induced vibrations at platforms.
Based on the vortex induced vibration analyses, the span between the proposed riser clamps shall not suffer from vortex shedding induced vibrations at platforms.
RISER STRESS
The analysis shall be carried out for hydrotest and operational phase for the functional; and functional and environmental conditions.
RISER CLAMP LOADS
The loads for the riser sliding clamps and hanger clamps shall be taken from the software output.
TIE IN FLANGE LOADING AND STRESS CHECK
The flange supplier shall perform calculations in order to check the strength of the proposed flange rating when subject to these forces and moments.
JACKET DISPLACEMENT
The jacket displacement will be applied to the guides and clamp locations during the stress analysis of the risers.
TOPSIDE LOADING
Topside loading is imposed to simulate the loading that cause by the topside piping. Only the maximum loading will be used in the riser stress analysis.
METHOD OF ANALYSIS AND DESIGN
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The risers are designed to meet the following design requirements:
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Riser load Boundary Condition
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VORTEX INDUCED VIBRATIN
The allowable span lengths for the vortex induced vibration criteria are calculated based on DnV guidelines, whereby the reduced velocity (Vr) is used in determining when vortex-induced vibration will occur. The reduced velocity is defined as:
the in-line oscillations of a free span are initiated at lower velocities than those required for the onset of cross flow motion. Therefore, the maximum allowable span length for the in-line motion criterion will automatically satisfy the cross-flow criterion.
The program calculates the allowable riser span length to avoid the onset of pipeline in-line and cross flow oscillations induced by vortex induced vibration, which complies with the DnV method. Based on the calculated span length, the riser clamp elevation is then identified
such that the clamp elevation spacing is always shorter than the riser maximum span length.
IN PLACE STRESS ANALYSIS
AUTOPIPE works on the basis of a global (right-hand) co-ordinate system that can be located at any point along the model. Nodes are established at important locations such as valves, mean sea level, and riser clamps. In addition, nodes are used to define the length of riser elements. Where large changes in stress can occur over a very short distance, nodes are necessarily placed closer together to improve the resolution of the calculated stress profile.
Defining Riser Components:
AUTOPIPE has a series of elements that can be used to model the various components of the riser. Most of these elements are ‘lower order’ beam theory and incorporate stress intensification factors, as appropriate at pipe and bends. The basic pipe element itself does not include deformations of the pipe cross section. It will, however, account for linear changes of shear and moment along the element length.
Boundary conditions can be specified in AUTOPIPE at any of the nodes to restrain either translation or rotation of a particular node in any of the three global (or local) co-ordinate directions. This feature is used to simulate riser clamps and supports.
The boundary conditions for the hanger clamp restrain the riser against lateral and downward translation and rotation in all directions. Consequently, the weight of the riser is transferred to the jacket at this clamp. Underwater riser guide clamps are assumed to allow movement along the axis of the pipe but restrain the pipe against translation in the other directions. Topsides piping supports allow the pipeline to move upward.
Boundary conditions that specify a fixed displacement or rotation can also be applied. This feature is used to apply displacements arising from thermal expansion of the pipeline as well as jacket displacements at the riser clamps and topside supports during the design storm event.
In the vertical plane, the seabed soils are assumed to support the pipe with an elastic foundation support and the elastic sub-grade modulus is specified as an input parameter. In the plane of the seabed, the soil is assumed to provide a bilinear approximation to Coulomb friction. This approximation is necessary to get numerical convergence of the solution. The calculation of the soil friction forces is based on the soil friction factors multiplied by the pipeline submerged weight per unit length.
Definition of Special Features in AUTOPIPE
Coatings:
External coatings are considered as part of the overall structural model of the riser. The pipe weight is computed directly by the program.
Fluid Properties:
Both internal and external fluid properties influence the riser. By specifying the water depth relative to one of the global co-ordinate directions, the external loading is automatically accounted for by AUTOPIPE using the hydrodynamic diameter.
Internal pressure is applied and results are in both circumferential and longitudinal stresses. The specific gravity of the pipe contents can be specified in order to calculate its weight.
Riser Loading
Gravity:
For this analysis, gravity loads due to each different section of pipe, pipe contents and coatings are captured in AUTOPIPE by specifying the material densities.
Temperature and Thermal Expansion:
Constant temperature or variable temperature profiles can be specified throughout the model for automatic thermal expansion calculations. For this analysis, a constant temperature is applied to each riser model, and the overall pipeline expansion is applied as a displacement at the end of the pipeline. The riser temperature is assumed conservatively to be the design maximum temperature.
Pressure:
Constant internal pressure is applied to the whole model for hoop stress calculations.
Hydrodynamic Loads:
Hydrodynamic loads include the forces due to wave action and steady currents. Three wave theories are available in AUTOPIPE: Airy (linear), Stokes’ (up to 5th order) and Stream function. For this analysis, Stokes’ function wave theory is used. In AUTOPIPE, the water particle velocities computed with the selected wave theory are combined with the steady current to determine the distribution of instantaneous water particle velocity at the centerline of the submerged pipe body. Inertia, drag and lift forces due to waves and currents are automatically calculated using the Morison equation and the selected hydrodynamic force coefficients.
Output
AUTOPIPE has both hardcopy and screen output which can list the displacements, forces, moments, and stresses of each node point; the natural frequencies and corresponding translation and rotation point displacements for each active mode shape; the model listing; results and analysis summaries. Stresses can also be sorted from the highest to lowest stressed points (instead of by segment name and numerical sequence). Code checks are performed by AUTOPIPE by specifying applicable design codes.
Riser Clamp Loads
Standard Riser Clamp Design Loads
A series of standard riser clamps for pipe sizes diameter have been can be developed.
Flange Analyses
Subsea Flange
The maximum loadings at the flange nodes are taken to check the flange and bolt integrities under the extreme loads condition.
ASSUMPTIONS
Riser Span
- The ambient temperature is taken as the minimum seabed surface temperature
- For VIV analysis, the end fixity is assumed to be pinned-fixed condition for underwater clamps
- No marine growth is considered during installation and hydrotest.
- At least 50% wall loss is taken into consideration during operations
- The hydrotest temperature is assumed as 30°C.
Riser Stress Analysis
- The ambient temperature considered in the analysis is the minimum seabed
- The pipeline is assumed to be straight with no lateral or vertical curvature. As such the full pipeline expansion is applied.
- The pipeline is not buried and resting essentially on the seabed with minimal penetration into the seabed.
- Pipeline and riser sections are assumed to be internally corroded
corrosion allowance for operating condition.
- The maximum wave height is applied to the riser.
- No marine growth is considered during installation and hydrotest.
- The riser is assumed to be operating at the design pressure and design
- Jacket displacement for platform shall be considered in this stress
RESULTS AND DISCUSSION
Vortex induced vibration, riser in-place stress analyses and flange design calculations shall be carried out on both risers.
Subsea Tie-In Flanges
The AUTOPIPE stress analysis output also provides the forces and moments at any defined location.The flange supplier shall perform calculations in order to check the strength of the proposed flange rating when subjected to forces and moments.
Flange dimensions shall be such that the stresses in the flange calculated in accordance to regulations, do not exceed the allowable flange stresses.The stress check is done on the weld neck flange. The supplier should finalize the stress check on the weld neck, swivel and misalignment flanges.
