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1.1.             
Pipeline Decommissioning

 

Pipeline decommissioning is a major part of
decommissioning facilities of offshore platforms. This paper will look at
trunklines, rigid flowlines, flexible flowlines, umbilical and power cables.
(Table 3)

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Table 3 Offshore Pipelines and Relevant Descriptions

(Shen, Birkinshaw , &
Palmer, 2017)38

Pipelines

Typical Dimensions

Applications

Trunklines

Up
to 44inches, up to 840km

Major
export infrastructure for oil and gas

Rigid
Flowlines

Up
to 16 inches, less than 50 km long

Infield
flowlines and tie-in spools

Flexible
Flowlines

Up
to 16 inches, up to 10 km long

Infield
flowlines and tie-in spools

Umbilical

2
to 8 inches, up to 50 km long

Chemical,
hydraulic and communication distribution

Power
Cables

2
to 4 inches, up to 300 km long

Power
distribution between and within fields

 

There are many techniques are available for
subsea pipeline decommissioning .However, there is no one size fits all
approach and techniques depends on many factors including the pipeline size,
water depth, type of pipeline, type of conveying fluid, technical limitations,
regulatory framework, risk factors, environmental effects, stakeholder
interests and cost.

There are no internationally accepted patterns
for pipelines in particular, although there exists a number of international
treaties that govern the disposal of general waste at sea.

The decommissioning of subsea pipelines is
governed by the relevant national legislations and whatever good practice which
is done before successfully.

The following methods are available for
decommissioning of subsea pipelines:

§ 
Reverse Reeling,

§ 
Reverse S-Lay,

§ 
Cut and Lift,

§ 
Leave in Situ with Minimal
Interventions,

§ 
Rock Dumping,

§ 
Burial or Trenching.

Step by step procedures shall be followed,
when proceeding with a pipeline decommissioning. In general cases, following
list should take into account before starting to the pipeline decommissioning
for all kind of methods:

 

Pre-Decommissioning Steps Available for All
Methods:

§ 
Carry out desk top study and
gather data regarding production history, operational philosophy and relevant
geometrical, burial and crossing data

§ 
Stakeholder Engagement and
Consultations

§ 
Review recent survey(s) and
IRMs

§ 
Review recent pigging results
including intelligent pigging

§ 
Study the relevant regulations
and governing legislations in detail

§ 
Where possible, carry out a
site visit to verify the information obtained during the desktop study

§ 
Carry out a detailed EIA (Energy Information Administration)

§ 
Examine the availability of the
waste disposal facilities and methodology to be used

§ 
Study and document any
potential effect on the marine environment, inter alia, water quality,
geological and hydrographical characteristics, presence of endangered species,
existing habitat types, local fishery resources, contamination potential

§ 
Prepare a detailed cost
assessment for all potential decommissioning scenarios

§ 
Carry out CA for all possible
decommissioning scenarios

§ 
Prepare methodology for
pipeline pigging, cleaning and flushing

§ 
If necessary, carry out a
dedicated survey

§ 
Inform the regulatory authorities
of the planned decommissioning and date for CoP (Cessation of Production)

§ 
Prepare offshore pipeline
decommissioning study and hand over to the regulatory authorities

§ 
Prepare detailed offshore
action plan including the disconnecting method, cutting method, plugging, and
type of vessels to be used, etc and regulatory approval of the decomm?ss?on?ng
plan

 

If the best decision is removal of the
pipelines from the sea bed, we will consider the following table (Table 4) which
shows pipelines types corresponding to removal alternatives.

Table 4 Pipelines Removal Options

                                 REMOVAL OPTIONS  PIPELINES

Reverse Reeling

Reverse S-Lay

Cut and Lift

Trunklines

 

R

R

Rigid
Flowlines

R

 

R

Flexible
Flowlines

R

 

R

Umbilical
Systems

R

 

R

Power
Cables

R

 

R

 

 

1.1.1.     
Reverse S-Lay

 

Larger diameter and concrete coated
trunklines are typically installed using the S-lay method. This method is useful
for decommissioning pipelines in excess of 16 inches diameters.

This method would involve recovering a
pipeline end to the deck of a specialist S-lay vessel. The vessel moves along
the way of the pipeline and stopping at most appropriate points where a cut
would be made to remove a section of pipe from the recovered pipeline string on
the deck of the vessel. These sections would then be transferred to a suitable
transportation barge for onshore recycling.

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