A small presentation on OFFSHORE PIPELINE DESIGN

This presentation will explain the following points in brief

  • Design  criteria and considerations for pipe wall thickness
  • Pipeline stability calculations
  • Cathodic protection and anode sizing calculations.
  • Pipeline laying analysis
  • Construction methods and procedure
  • Repair procedure

Categorization of Fluids

Fluids to be transported by the pipeline system shall be categorized according to their hazard potential.

Category Description
A Non-Flammable water-based fluids
B Flammable and/or toxic substances which are liquids at ambient temperature and atmospheric pressure conditions.
C Non-Flammable substances which are non-toxic gases at ambient temperature and atmospheric pressure conditions.
D Non-toxic single-phase natural gas
E Flammable and/or toxic fluids are gases at ambient temperature and atmospheric pressure conditions.

Location Classes

The pipeline system shall be classified into location classes as defined in the table.

Location Definition
1The area where no frequent human activity is anticipated along the pipeline route.
2The part of the pipeline in the near platform (manned)  area or in areas with frequent human activity. The extend of location class 2 should be based on appropriate risk analyses. If no such analyses are performed a minimum distance of 500 m shall be adopted.
SUBSEA PIPELINE ZONES LOCATION REASON
LANDFALL AREA 2 FOR FREQUENT HUMAN ACTIVITY ALONG THE PIPELINE ROUTE. NOTE: SHIPS ARE TREATED UNDER HUMAN ACTIVITY
500 m NEAR SHORE ADJACENT TO LANDFALLS 2
SHIPPING CHANNEL 2
ANCHORING AREA 2
REST ZONE 1 NO FREQUENT HUMAN ACTIVITY ANTICIPATED.

Safety Classes

Pipeline design shall be based on potential failure consequence.

Safety Class Definition
Low Low risk of human injury and minor environmental and economic consequences. (Installation phase)
Normal Considerable risk of human injury, significant environmental pollution or very high economic and political consequences. (Operation outside platform area)
High High risk of human injury significant environmental pollution or very high economic and political consequences. (Operation in location class 2).
SUBSEA PIPELINE ZONES LOCATION SAFETY CLASS (FLUID CATEGORY E)
CONSTRUCTION PHASE OPERATION PHASE
LANDFALL AREA 2 LOW HIGH
500 m NEARSHORE ADJACENT TO LANDFALL 2 LOW HIGH
SHIPPING CHANNEL 2 LOW HIGH
ANCHORING AREA 2 LOW HIGH
REST ZONE 1 LOW NORMAL

Note: construction phase means up-to pre-commissioning

Minimum and Maximum Water Depths

The maximum and minimum water depths for various subsea pipeline zones shall  be listed as below:

SUBSEA PIPELINE ZONES LOCATION WATER DEPTHS RANGE (m)
MAXIMUM MINIMUM (*)
LANDFALL AREA 2
500 m NEARSHORE ADJACENT TO LANDFALLS 2
SHIPPING CHANNEL 2
ANCHORING AREA 2
REST ZONE 1

Installation

  • Pipeline route survey
  • Marine operations
  • Pipeline installation
  • Tie-in operations
  • As-laid survey
  • Span rectification and pipeline protection
  • Installation of protective and anchoring structures
  • Installation of risers/ platforms/ buoys
  • As-built survey
  • Final testing and preparation for operation

Operation

  • Organization and management
  • Start-up and shutdown
  • Operational limitations
  • Maintenance
  • Corrosion control, inspection and monitoring
  • General inspection
  • Special activities

Abandonment

  • Environment and especially pollution
  • Obstruction for ship traffic
  • Obstruction for fishing activities and
  • Corrosion impact on other structures

Design Principles-System Integrity

  • Fulfill the specified transport capacity
  • Fulfill the defined safety objective and have required resistance against loads during planned operational conditions and
  • Have sufficient safety margin against accidental loads or unplanned operational conditions

Monitoring/Inspection during operation

  • Parameters which could violate the integrity of a pipeline system shall be monitored and evaluated with a frequency which enables remedial actions to be carried out before the system is damaged.
  • Instrumentation of the pipeline system.
  • The pressure in a pipeline system shall not exceed the design pressure during normal steady-state operation.

Pressure Control System

  • A pressure control system shall be used to prevent the internal pressure at any point in the pipeline system rising to an excessive level.
  • The pressure control system comprises the pressure regulating system, pressure safety system and associated instrumentation and alarm system.

Pipeline Route-Route Survey

  • A survey shall be carried out along the planned pipeline route to provide sufficient data for design and installation related activities.
  • The survey corridor with sufficient width.
  • To identify possible conflicts with existing and planned installations and possible wrecks and obstructions.
  • All topographical features which may influence the stability and installation of the pipeline – unstable slopes, sand waves, deep valleys,large boulders etc.

Pipeline Route- Seabed properties

  • Geotechnical properties necessary for evaluating the effect of relevant loading conditions shall be determined for the seabed deposits
  • Soil parameters
  • Problems with respect to excavation and burial operations
  • Problems with respect to pipeline crossing
  • Problems with the settlement of pipeline system and /or the protection
  • Possibilities of mudslides or liquefaction as the result of repeated loading and
  • Implications for external corrosion

Environmental Conditions

WIND TIDE WAVES INTERNAL WAVES AND OTHER EFFECTS DUE TO DIFFERENCES IN WATER DENSITY CURRENT ICE EARTHQUAKE SOIL CONDITIONS TEMPERATURE MARINE GROWTH

Determination of pipe wall thickness for subsea pipeline section following failure modes to be considered:

Failure Mode

  • PRESSURE  CONTAINMENTS
  • SYSTEM COLLAPSE
  • PROPAGATION BUCKLING

Governing Factors

  • Pressure containments (bursting) limit state  is not the governing case in determining the pipe wall  thickness
  • Both  pipeline installation and propagation buckling  criteria are important in governing the pipe wall thickness
  • Buckle arrestor if necessary.

Governing Codes

The pipeline wall thickness for the subsea pipeline section is designed in accordance with the guidance provided in DNV OS – F101 submarine pipeline systems

Pipeline Diameter

The inside diameter is generally taken as constant along the entire pipeline length in order to facilitate the piging operation.

Steel Grade

The subsea pipeline steel  (of C-Mn)grade is selected as API-5L X65 considering the probability of accidental events (e.g. from earthquakes, ships, unauthorised trawlling etc.).

DESIGN PRESSURE

DESIGN TEMPERATURE RANGE

PRODUCT DENSITY

CORROSION ALLOWANCE:

  • For gas composition (no water present), it is expected that there will be no internal corrosion. Thus no internal corrosion allowance may be taken.
  • For fluid composition (water present), it is expected that there will be internal corrosion. Thus internal corrosion allowance may be taken.

Design Philosophy

The required pipewall thickness is established considering the following:

 Pressure containment (bursting) limit state

  • For operation phase
  • For system pressure test

Local buckling limit state

  • External pressure ) collapse
  • Propagation buckling

Pressure Containment (BURSTING)

Calculation steps are listed as below…

  • Step 1:  determine thickness requirement at landfall area
  • step 2:  verify the above thickness from                system test       
  • step 3:  verify the above thickness from operation condition at maximum Water depth
  • step 4:  verify the above thickness for system test pressure at maximum water depth

Change the thickness value, if necessary to satisfy the checks and conclude the minimum thickness requirement from pressure containment (bursting).

System Collapse and Propagation Buckling

Calculation steps are listed as below…

  • Step 1:  verify the above determined thickness for system collapse in maximum water depth condition
  • step 2:  change thickness, if necessary to satisfy the system collapse check
  • step 3:  verify the above determined thickness for propagation buckling in maximum water depth condition
  • step 4:  change thickness, if necessary to satisfy the propagation buckling check.

Conclude the pipewall thickness requirement from system collapse and               propagation buckling criteria.

PRESSURE CONTAINMENT (BURSTING)-AT ABOVE WATER LOCATIONS:

  • For landfall locations which are of safety class “high”, the pipeline thickness requirement is calculated for operating condition
  • If this thickness is more than the minimum requirement of 12 mm, this thickness is retained for further checks.
  • To complete the installation phase of the subsea pipeline system, a pressure test would be  performed. The pipeline is filled with water during pressure test. Find out whether it is accpetable or not. 

STABILITY CALCULATIONS FOR THE SUBSEA PIPELINE – CONCRETE THICKNESS DESIGN:

BASIC CONSIDERATIONS:

  • The water depth variation along the subsea pipeline route is considered.
  • In the shipping channel and in the ship anchoring area, pipeline must be in a trench with a cover of 1.3 D, D being the overall coated pipe diamter.
  • Postlay trenching may be considered if soil condition permits.
  • Environmental data (wave and current).
  • Current is considered to be always acting perpendicular to the pipeline.
  • Most conservative wave with its direction/ approach with respect to the pipeline axis is considered.
  • Wave refraction is neglected.
  • Concrete thickness shall be designed in multiple of 5 mm (e.g. 50  mm, 55 mm, 60 mm etc. and shall be optimised to reduce strain concentration at field joints during pipelaying.

Environmental conditions for different pipe burial conditions:-

When pipeline section is designed as Wave and current return period conditions to be considered
1 year 10 year 100 year
Exposed section Yes
Postlay trenched section Yes (Exposed) Yes (in trench) Yes (in trench)
Prelay trenched section and mechanically backfilled following pipeline installation Yes (in trench) Yes (in trench)

Design Methodology:

Step – 1:

  • First check whether the steel pipe is having a negative buoyancy force or not (in absence of wave and current). If not, find out the minimum          concrete thickness to make the pipeline negatively buoyant.

Step – 2:

  • Consider exposed pipeline on sea bed and carry out stability assessment. For the purpose consider the deeper part of the pipeline route.
  • Find out the minimum required concrete thickness for various water depth  upto  maximum water depth. Decide on the exposed pipeline section length (with its minimum and maximum water depth) and an uniform concrete thickness (if possible) for this section.

Step – 3:

  • Find the concrete thickness requirement  (for the pipe exposed installation period – 1 year return period conditions for wave and current) for various water depths shallower than the above determined minimum water depth of the exposed pipe section.
  • Consider postlay trenching for this zone and check the pipe stability (with the above designed concrete thickness) under 10 years and 100 years return period environmental conditions (to check pipeline stability for backfill with excavated soil and naturally backfilled condition, respectively) to conclude the concrete thickness requirement for this postlay trench zone. Minimum and maximum water depths are to be noted for this zone.
  • The pipeline in the rest water depths shallower  than the above minimum water depth limit shall be in a prelay trench which will be mechanically backfilled following pipeline installation.

Step – 4:

  • Check sample pipeline stability in the prelay trench zone for the installation period (1 year and 10 year return period conditions for wave and current) and decide on the concrete thickness requirement for this zone.

Step – 5:

  • Check vertically stability for the pipeline (sinking in case of exposed pipe section and sinking as well as floatation in case of trenched and backfilled pipe section).

Step – 6:

  • Summarise the above results to highlight the concrete thickness design along the pipeline route, with the trenching (prelay and postlay) requirement as envisaged (from pipeline protection from ship hazards).

DESIGN DATA AND BASIS: LINE PIPE CHARACTERISTICS:

  • Pipe characteristics
  • Anti corrosion coating
  • Field joint coating
  • Concrete coating
  • Pipe contents
    • During installation phase pipeline is air filled.
    • During operation phase pipeline is filled with product

ENVIORNMENTAL DATA AND HYDRO DYNAMIC LOADS:

  • The pipeline on bottom stability assessment is based on a given return period of  near-bottom environmental loads acting on the pipe. These hydrodynamic loads are defined as flow-induced  loads caused by the relative motion between the pipe and the surrounding water. Most critical combination of near bottom wave induced water particle velocity (due to wave) and near-bottom current is thus considered.
  • For waves, the angle of attack  with respect to the pipe  axis is considered  to evaluate the water particle velocity and acceleration  normal to the pipe axis. Current are considered to be always to be acting perpendicular  to the pipeline.

DESIGN DATA AND BASIS: WATER DEPTHS:

  • Water depth variation along subsea pipeline route  is considered.

DESIGN DATA AND BASIS: ARTIFITIAL EMBEDMENT:

  • It is well known that the pipeline in a trench (prelay or postlay with backfilling) facilitate the lateral stability of a pipeline during its life time.
  • In order to ensure lateral stability of the pipe with a reasonable concrete  thickness, artificial embedment  may be emphasized. In the  near shore and in areas having the probability of ship hazards.

DESIGN DATA AND BASIS: SOIL FRICTION:

  • The lateral soil friction factor for exposed pipe section is taken as 0.7 corresponding to a sandy soil sea bed.
  • For the postlay trench case, the lateral soil friction factor is increased to 2.144 using the slope of the trench as 30 degree. For prelay trench case (I.e. flat bottom surface) no change in soil friction is considered.

DESIGN DATA AND BASIS: CURRENT DATA:

  • Current velocity at the near bottom (1 meter above sea bed) shall be considered.
  • 1 year, 10 year and 100 year return period extreme wave characteristics are selected on the basis of most critical wave case on the pipeline. 

DESIGN DATA AND BASIS: Hydrodynamic  force coefficients-

In order to calculate the hydrodynamic loads acting on the pipeline, the following dynamic force coefficients to be considered in accordance with following-

Pipeline case Exposed pipe section  on the sea bed Prelay and postlay trench section with cover= 1.3XD, D being coated pipe diameter.
Drag coefficient 0.7 0.28
Lift coefficient 0.9 0.36
Inertia coefficient 3.29 1.81

DESIGN DATA AND BASIS: VERTICAL STABILITY-

It is necessary to check the following:

  • Sinking check for exposed pipeline section (water filled condition)
  • Sinking and floatation checks for trenched and backfilled pipeline sections (for sinking pipe water filled condition and for floatation pipe air filled condition)
Print Friendly, PDF & Email

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

Leave a Reply

Your email address will not be published. Required fields are marked *

Recent Content

Enable Notifications    Ok No thanks