PIPELINE AND RISER DESIGN BASIS
DESIGN APPROACH
GENERAL
The pipeline will be designed considering the pipeline section is an offshore section with varying water depths along the pipeline route.
Routing
Pipeline routing will be finalized taking into account drilling rigs, supply boat operations/ dropped object have a look at, present and future centers, seabed situation, fishing regions, present coral reefs, environmentally sensitive regions and areas stricken by different restrictions.
pipeline routes shall be optimized to reduce production and set up cost. the following standards are to be followed;
Pipeline Wall Thickness Selection
Methodology for the selection of wall thickness will consider the following design conditions:
Wave Theory Selection
Wave conditions, which are to be considered for design purposes, may be described either by deterministic design wave methods (applicable wave theories) or by stochastic methods applying wave spectra. For the design of pipeline, applicable wave theories shall be used and be described by regular, periodic wave cycles, characterized by wavelength (period), wave height, wave direction and possible
shape and spreading parameters. The applicability of wave theories can be described as follows:
Pipeline On-Bottom Stability
Lateral Stability
The on-bottom stability analysis will be performed to ensure that the pipeline has both lateral and vertical stability under the given environmental conditions.
The following design condition shall be considered in the analysis.
The minimum water depth (LAT) along the pipeline route should be used for the on-bottom stability analysis. At each end of the line pipe a minimum concrete cutback shall be considered in the stability analysis. It shall also be ensured that the pipeline is not buoyant and installed air filled weight minimum specific gravity. For vertical stability, the immediate and the long term sinkage of the pipeline shall be evaluated.
Pipeline Vertical Stability
When pipeline is laid it may settle down into the soil depend upon the pipe weight (including contents) and soil condition. Therefore, the pipeline to be checked for possible sinking under flooded condition.
Pipeline Expansion and Expansion Offset
The pipeline expansion at the platform ends resulting from the effect of pressure and temperature shall be assessed. The calculation shall consider the design pressure, design inlet temperature, minimum seabed/ installation temperature and minimum contents density. The pipeline end expansion analyses under both operating and hydro test conditions shall be carried out.
Pipeline In-Situ Stress Analysis
The pipeline in-situ stress analysis under functional and environmental load conditions will be performed. The in-situ stress calculation involves computation of pipeline longitudinal, circumferential and equivalent stresses resulting from loading associated with hydrotesting and operating conditions. The analysis shall consider the pipeline under both fully and unrestrained conditions.
Pipeline Free Span Analysis
Pipeline free spanning analysis shall be performed. The analysis is to determine the maximum allowable free span lengths based on static span criteria, dynamic span criteria and Euler Buckling criteria.
Maximum wave heights (Hmax) and associated period (Tp) shall be used for environmental loading in static span calculations.
“Fixed-pinned” end restraint conditions may be assumed in assessing natural frequency of the pipe span. The vortex shedding calculations shall be performed for both in-line and cross flow oscillations. Both inline and cross flow spans will be determined for installation, hydrotest and operation design conditions. Inline and cross flow span analysis shall be performed by considering both design steady current plus wave-induced current (due to significant wave height with associated wave period).
RISER MECHANICAL DESIGN
The riser mechanical design scope shall include the following:
Riser Vortex Shedding Analysis
A vortex shedding analysis of the riser spans shall be performed using the design steady state current plus wave induced current due to significant wave height. Cross-flow oscillations shall not be allowed. Riser clamp locations shall be selected based on the vortex shedding analysis and riser flexibility (stress) analysis results.
The vortex-shedding analysis is to be carried out using a uniform metal loss equal to 50% of the corrosion allowance.
.
Riser Stress Analysis
The structural integrity of the riser system shall be proven for all design conditions. Where possible, riser support locations and riser wall thickness will be selected such that an expansion loop is not required between the riser and pipeline.
The riser model shall be designed to include any expansion loop/dog leg and a straight pipeline in order to minimize the end effects.
Riser Stress analysis shall be carried out for functional and operational loading up to the hang off sleeve clamp.
Riser Guards
it is advised to install riser guard at platform. Consideration shall be given for the future riser installation and riser maintenance while providing riser guards.
CORROSION CONTROL
External corrosion coatings shall provide the primary preventive measures against external corrosion. The pipelines shall be protected by a combination of coatings and cathodic protection system. Cathodic protection system will be provided as a secondary measure in the event of coating damage.
Cathodic Protection (CP)
Cathodic protection system will be provided as a secondary measure in the event of coating damage.
Aluminium (Indium activated) half shell bracelet anodes will be used as sacrificial anode system. The protection system will be designed to provide corrosion prevention throughout the lifetime of the pipeline. Both ends of the pipeline shall be electrically isolated from both platforms and jackets by installing insulation gaskets at downstream of inlet riser and upstream of outlet riser.
Riser Cathodic Protection
Three additional anodes shall be installed on seabed pipeline (first three joints), adjacent to the riser base to provide Cathodic protection for the riser. In case of expansion offset, similar sacrificial anodes shall be provided. The riser and sub-sea pipeline shall be kept electrically isolated at both ends from the platforms/ jackets by installing insulating gasket set on topside interface.
Riser Clamp Cathodic Protection
All riser clamps installed shall be protected by jacket anode. The clamps shall be kept electrically connected with jacket bracings. Slender stand off type anode should be installed on the clamps for initial protection.
PIPELINE PROTECTION AGAINST FISHING GEAR AND VESSEL ANCHOR
Due to consequence (likelihood and severity) of anchor snagging or dragging cases by workboats or fishing net, protection against this impact is a concern. Movement of vessel in this area shall be monitored closely by SMR (Marine Operations).
PIPELINE PROTECTION AGAINST DROP OBJECT
protection against impact of drop object is a concern. Though concrete coating can provide some mechanical protection, the potential effect of any such impact on the safe operation of this pipeline shall be evaluated in Drop Object Study.
PIPELINE AND RISER INSTALLATION STUDY
The detailed installation engineering of pipeline (including risers) will be performed to analyse the pipeline and riser installation .
GENERAL
The pipeline will be designed considering the pipeline section is an offshore section with varying water depths along the pipeline route.
Routing
Pipeline routing will be finalized taking into account drilling rigs, supply boat operations/ dropped object have a look at, present and future centers, seabed situation, fishing regions, present coral reefs, environmentally sensitive regions and areas stricken by different restrictions.
pipeline routes shall be optimized to reduce production and set up cost. the following standards are to be followed;
- avoid unwanted seabed capabilities and obstructions
- avoid present structures, pipelines and structures, and so forth
- minimise pipeline crossings
- optimised (minimum) pipeline course length
- keep away from anchorage regions
- avoid rig foot prints
- provision of drilling barge approach
- recollect platform processes of destiny pipelines
- minimise topside piping necessities
- don't forget pipeline installation methods
- minimal radius
- preferred proper range for pipeline routing near existing platforms: 100m to
- distance among parallel pipelines should now not be less than 10m
- most fulfilling routing to hold the present anchorage regions and surroundings, minimum encouraged pipeline route curvature, laying tolerance and marine and production problems
- provide adequate area provision for pipeline enlargement offset
- keep away from underwater set up and particles
- avoid anchorage regions, environmentally touchy and coral reef area. in the event that they can not be avoided, minimise the effect and damage.
- at pipeline/riser interface the deviation will be sufficiently small to permit set up of the riser clamps with out introducing bending stresses inside the riser. in which the pipeline is mounted adjacent/parallel to an existing pipeline a minimal separation of 15m will be maintained, except at the platform approach.
Pipeline Wall Thickness Selection
Methodology for the selection of wall thickness will consider the following design conditions:
- Hoop stress due to internal pressure
- Collapse due to external pressure
- Propagation buckling
- Withstand installation stresses
- Withstand operational stresses
- Ensure on-bottom stability
- Ease the welding requirements
- The resulting pipeline internal diameter shall be suitable for passage of pig
Wave Theory Selection
Wave conditions, which are to be considered for design purposes, may be described either by deterministic design wave methods (applicable wave theories) or by stochastic methods applying wave spectra. For the design of pipeline, applicable wave theories shall be used and be described by regular, periodic wave cycles, characterized by wavelength (period), wave height, wave direction and possible
shape and spreading parameters. The applicability of wave theories can be described as follows:
- Linear wave theory, by which the wave profile is described as a sine function, can be applied to moderate and deep water;
- Stokes 5th wave theory, for high waves, can be applied to moderate and deeper water. For on-bottom application, Stokes 5th wave theory yields similar result as Linear wave theory;
- Solitary wave theory is applicable for very shallow water depth;
- Cnoidal is applicable to the intervening region between Linear/Stoke 5th and Solitary wave theories where their limits of applicability fail to overlap;
- Stream function waves, which are based on numerical methods, can be applied over a broad range of water depths. However, this will require a lot of computing time.
- Applicable wave theory shall be selected based on the nature of the intended application, on-bottom application.
Pipeline On-Bottom Stability
Lateral Stability
The on-bottom stability analysis will be performed to ensure that the pipeline has both lateral and vertical stability under the given environmental conditions.
The following design condition shall be considered in the analysis.
- Installation condition (pipeline empty) subjected to 1-year current + 1 year significant wave.
- Operational condition (pipeline full of product + Pipe weight subjected to 100 year significant wave + 100 year current.
The minimum water depth (LAT) along the pipeline route should be used for the on-bottom stability analysis. At each end of the line pipe a minimum concrete cutback shall be considered in the stability analysis. It shall also be ensured that the pipeline is not buoyant and installed air filled weight minimum specific gravity. For vertical stability, the immediate and the long term sinkage of the pipeline shall be evaluated.
Pipeline Vertical Stability
When pipeline is laid it may settle down into the soil depend upon the pipe weight (including contents) and soil condition. Therefore, the pipeline to be checked for possible sinking under flooded condition.
Pipeline Expansion and Expansion Offset
The pipeline expansion at the platform ends resulting from the effect of pressure and temperature shall be assessed. The calculation shall consider the design pressure, design inlet temperature, minimum seabed/ installation temperature and minimum contents density. The pipeline end expansion analyses under both operating and hydro test conditions shall be carried out.
Pipeline In-Situ Stress Analysis
The pipeline in-situ stress analysis under functional and environmental load conditions will be performed. The in-situ stress calculation involves computation of pipeline longitudinal, circumferential and equivalent stresses resulting from loading associated with hydrotesting and operating conditions. The analysis shall consider the pipeline under both fully and unrestrained conditions.
Pipeline Free Span Analysis
Pipeline free spanning analysis shall be performed. The analysis is to determine the maximum allowable free span lengths based on static span criteria, dynamic span criteria and Euler Buckling criteria.
Maximum wave heights (Hmax) and associated period (Tp) shall be used for environmental loading in static span calculations.
“Fixed-pinned” end restraint conditions may be assumed in assessing natural frequency of the pipe span. The vortex shedding calculations shall be performed for both in-line and cross flow oscillations. Both inline and cross flow spans will be determined for installation, hydrotest and operation design conditions. Inline and cross flow span analysis shall be performed by considering both design steady current plus wave-induced current (due to significant wave height with associated wave period).
RISER MECHANICAL DESIGN
The riser mechanical design scope shall include the following:
- Riser vortex shedding analysis
- Riser stress analysis
- Determination of riser clamp and hanger clamp/ flange locations
- Determination of riser/piping tie-in, isolating Flange/ gasket locations.
Riser Vortex Shedding Analysis
A vortex shedding analysis of the riser spans shall be performed using the design steady state current plus wave induced current due to significant wave height. Cross-flow oscillations shall not be allowed. Riser clamp locations shall be selected based on the vortex shedding analysis and riser flexibility (stress) analysis results.
The vortex-shedding analysis is to be carried out using a uniform metal loss equal to 50% of the corrosion allowance.
.
Riser Stress Analysis
The structural integrity of the riser system shall be proven for all design conditions. Where possible, riser support locations and riser wall thickness will be selected such that an expansion loop is not required between the riser and pipeline.
The riser model shall be designed to include any expansion loop/dog leg and a straight pipeline in order to minimize the end effects.
Riser Stress analysis shall be carried out for functional and operational loading up to the hang off sleeve clamp.
Riser Guards
it is advised to install riser guard at platform. Consideration shall be given for the future riser installation and riser maintenance while providing riser guards.
CORROSION CONTROL
External corrosion coatings shall provide the primary preventive measures against external corrosion. The pipelines shall be protected by a combination of coatings and cathodic protection system. Cathodic protection system will be provided as a secondary measure in the event of coating damage.
Cathodic Protection (CP)
Cathodic protection system will be provided as a secondary measure in the event of coating damage.
Aluminium (Indium activated) half shell bracelet anodes will be used as sacrificial anode system. The protection system will be designed to provide corrosion prevention throughout the lifetime of the pipeline. Both ends of the pipeline shall be electrically isolated from both platforms and jackets by installing insulation gaskets at downstream of inlet riser and upstream of outlet riser.
Riser Cathodic Protection
Three additional anodes shall be installed on seabed pipeline (first three joints), adjacent to the riser base to provide Cathodic protection for the riser. In case of expansion offset, similar sacrificial anodes shall be provided. The riser and sub-sea pipeline shall be kept electrically isolated at both ends from the platforms/ jackets by installing insulating gasket set on topside interface.
Riser Clamp Cathodic Protection
All riser clamps installed shall be protected by jacket anode. The clamps shall be kept electrically connected with jacket bracings. Slender stand off type anode should be installed on the clamps for initial protection.
PIPELINE PROTECTION AGAINST FISHING GEAR AND VESSEL ANCHOR
Due to consequence (likelihood and severity) of anchor snagging or dragging cases by workboats or fishing net, protection against this impact is a concern. Movement of vessel in this area shall be monitored closely by SMR (Marine Operations).
PIPELINE PROTECTION AGAINST DROP OBJECT
protection against impact of drop object is a concern. Though concrete coating can provide some mechanical protection, the potential effect of any such impact on the safe operation of this pipeline shall be evaluated in Drop Object Study.
PIPELINE AND RISER INSTALLATION STUDY
The detailed installation engineering of pipeline (including risers) will be performed to analyse the pipeline and riser installation .
