Secondary Stabilisation Design

One of the means to provide stability to the unstable flowline, pipeline and umbilical is by laying the concrete mattress above it.

The concrete mattress sizing and required spacing will be determined based on the following assessments:
  • Mattress vertical edge stability check - to ensure mattress edge is not lifting due to vertical hydrodynamic loads;
  • Mattress lateral self-stability check - to ensure mattress is not wash-offed due to lateral hydrodynamic loads;
  • Pipeline pull-out check - to ensure there is no mattress dislodging from pipeline due to lateral active pipeline forces active on the mattress;
  • Mattress and pipeline sliding together (system stability check) - to ensure mattress submerged weight is sufficient to prevent lateral movement of mattress and pipeline;
  • Pipeline local buckling check - to ensure integrity of the pipeline located between the mattresses throughout design life (FE dynamic analysis will be required).
The mattress stability assessments as described above are based on static force balance method. One of the references for the concrete mattress stability design is:

Introduction - Pipeline Expansion Design

Pipeline tends to move when operating temperature and pressure is higher than the temperature and pressure when the pipe is installed and tied in.

For long pipeline far away from the platform, the pipe is constrained so there is no expansion. (This is because the expansion of the pipeline is restrained by soil friction between seabed and pipeline and concrete weight coating.) However, for the pipeline near the platform, the pipe is partially constrained and it can expand freely towards platform. (The vertical riser leg is relatively flexible and does not provide much friction between seabed and pipeline)

Longitudinal expansion in a pipeline depends on:
  • Thermal expansion force (temperature)
  • Poisson’s force (pressure)
  • Lay tension (leftover by installation)
  • Soil friction resistance force (between pipeline and seabed)

References:
  1. A.C. Palmer (1981) Movements of Submarine Pipelines Close to Platforms, Offshore Technology Conference, Houston, Texas, OTC 4067, May 1981.
  2. DNV OS-F101 (2007) Submarine Pipeline Systems.

Field Joint Coating (FJC)

Nominal length of one pipeline joint is 12.2m. Field Joint is the end of each pipeline joint where it is left bare without external coating for the purpose to facilitate girth welding to join 2 pipe section together. Field joint needs to be coated to provide protection to pipeline and reduce CP system drainage.

The objective of FJC, Ref. 2:
  • Corrosion control
  • Mechanical protection
  • Thermal insulation
  • Smooth transition to a concrete weight coating of linepipe (Infill)
The welded pipe/field joint is cleaned before application of field joint coatings.
Some of the popular field joint coatings include:
  • Tape wrap
  • Heat Shrink Sleeve
  • FBE
  • 3 Layer Polyolefin

Tape Wrap
Tape wrap is an adhesive backed heavy-duty plastic tape consists of a spirally wound tape made of PVC, polyolefin or fiberglass, with or without mastic primer is applied to cold or preheated field joints (cold applied). Tape wrap is applied as a double wrap around the pipe joint. The tape shall be sufficient to cover the full joint and overlap the external coating on the pipeline beyond the cutback on both side of the joint. (The length of overlapping needs to refer to codes and standards being used)


Heat Shrink Sleeve, HSS
HSS is formed by preheating and stretching the main coating material which is PP or PE in the circumferential direction. HSS will return to original size on reheating.

Liquid Epoxy is used as a primer to apply on the field joint surface before application of HSS to produce a better corrosion protection.


FBE
FBE FJC application onto the field joint is very similar with the parent coating (FBE external coating) where the field joint is cleaned, heated up, then the epoxy particles/powder are flocked onto the field joint.

Field Joint Infill Material
-


References:
  1. Palmer, A. C., King R. A. King (2004) Subsea Pipeline Engineering. Tulsa, Oklahoma: PennWell.
  2. DNV-RP-F102, Pipeline Field Joint Coating and Field Repair of Linepipe Coating, 2003

External Coatings

The main objective of external coatings on pipelines is corrosion control. It can also provide mechanical protection to linepipe. Anti-corrosion coatings available for offshore pipelines are shown below:
  • Asphalt
  • Coal tar enamel (CTE)
  • Fusion Bonded Epoxy (FBE)
  • 3LPP
  • 3LPE
For pipelines that require internal and external coating, normally internal coating is done before application of external coating unless the external coating requires pipeline heating that would damage internal coating.


Asphalt and Coal Tar Enamel
Asphalt, bitumen, and coal tar enamel (CTE) coating is 5-6 mm thick flood coating applied as molten material onto a rotating linepipe. This coating is reinforced with 1 or 2 layer of fiberglass to secure a better cohesion. Asphalt Enamel (AE) is a hot applied coating that has long been used in the offshore pipeline coating industry.

Pros:
  • Inherently rough surface (good in preventing slippage between concrete and coating for concrete coated pipe)
Cons:
  • Coherent is not strong and provide relatively poor adhesion to steel.
  • Poor cathodic disbandment resistance compare to FBE and 3 layer polyolefin systems (3LPP and 3LPE).
  • Prone to mechanical damage during construction and installation if applied on pipeline or riser without concrete coating.

Fusion Bonded Epoxy
FBE coating is a 0.4-0.6 mm thick, thin film coating with strong chemical bond with steel. It is a thermosetting resin applied in the form of dry fine powder onto the rotating linepipe being heated up to approximately 260 Celsius. At this temperature, the epoxy powder melts and flow all over the pipe surface. The pipe is then cooled down by water quenching. FBE coating can withstand temperature up to 95 Celsius. FBE top coated with plasticized formulation can be applied for HDD installation pipeline.

According to Ref. 3, FBE is not much used for marine pipelines as it is prone to impact damage and does not resist concrete coating by impingement.

Pros:
  • Good adhesion to steel (suitable for usage on offshore pipelines and components, i.e valves and pumps.
  • Flexible. (suitable for reeled pipelines application)
  • Can be used with or without concrete coating.
Cons:
  • FBE coating is slippery (For application on concrete coated pipe, it is recommended to provide anti-slip bands to reduce the risk of slippage between concrete and FBE coating)
  • Susceptible to mechanical damage during handling and transportation

Multilayer Polyolefin System
3-layer Polyolefin Coating (3LPP, 3LPE) consists of:
  • Base layer: FBE
  • 2nd layer: polyethylene or polypropylene
  • 3rd layer: polyethylene or polypropylene
It is called 3LPP if polypropylene is being used; 3LPE if polyethylene is used. Both Polypropylene and polyethylene are suitable for HDD installation pipeline coating as the natural self-lubricating characteristic of these materials minimizes the damage to the coating during installation process. 3LPP and 3LPE can withstand design temperature up to 130 Celsius.

Pros:
  • Low moisture and oxygen permeation
  • Strong adhesion properties of FBE
  • Toughness characteristics of Polyolefin (high electrical resistance and long life expectancy)
  • Resistant to handling and transportation damage

Cons:
  • Polyolefin coating is slippery (For application on concrete coated pipe, t is recommended to provide anti-slip bands to reduce the risk of slippage between concrete and polyolefin coating)

References:
  1. Palmer A. C., King R. A. (2004) Subsea Pipeline Engineering. Tulsa, Oklahoma: PennWell.
  2. DNV-RP-F102, Pipeline Field Joint Coating and Field Repair of Linepipe Coating, 2003.
  3. Braestrup M. W., Andersen J. B., Anderson L. W., Bryndum M. B., Christensen C. J., Niels Rishoj (2005) Design and Installation of Marine Pipelines. Oxford, UK: Blackwell Science.

Design Stages

Offshore Pipeline Designs are usually carried out in 3 stages:
  1. Conceptual Design
  2. Preliminary Engineering (FEED)
  3. Detail Design


Conceptual Design
  • To perform technical feasibility studies and constraints on system design and construction.
  • To identify potential difficulties.
  • Basic cost estimation.

Preliminary Engineering
  • To determine linepipe sizing, material and grade.
  • Prepare authority approval
  • Perform MTO to order pipeline

Detail Engineering
  • A complete linepipe design. (material, grade, size, hydrodynamic stability, free span, CP, thermal expansion, Riser VIV and Riser Stress)
  • Route Selection or Optimization.
  • Preparation of Specifications.
  • Compilation and finalize of MTO report for procurement of materials.
References:
  1. Buyon Guo, Shanyong Song, Jacob Chacko, Ali Ghalambor (2005) Offshore Pipelines, Standards and Technical Publications, South Australia.
  2. Yong Bai (2001) Pipelines and Risers, Elsevier Ocean Engineering Book Series, Volume 3, 498 pp.

Previous Posts