Pipeline Routing / Route Selection
Thursday, January 21, 2016
Route selection is a complex procedure, which can be governed by several variables. Clearly, the shortest distance between the terminal points is likely to be the most economic from a material standpoint, but possible overriding factors must be considered.
When layout the field
architecture, several considerations should be accounted for:
- Compliance with regulation authorities and design codes
- Future field development plan
- Environment, marine activities, and installation method (vessel availability)
- Overall project cost
- Seafloor topography
- Interface with existing subsea structures
According to Jaeyoung Lee in his
book, the pipeline route should be selected considering:
- Low cost (select the most direct and shortest P/L route)
- Seabed topography (faults, outcrops, slopes, etc.)
- Obstructions, debris, existing pipelines or structures
- Environmentally sensitive areas (beach, oyster field, etc.)
- Marine activity in the area such as fishing or shipping
- Installability (1st end initiation and 2nd end termination)
- Required pipeline route curvature radius
- Riser hang-off location at surface structure
- Riser corridor/clashing issues with existing risers
- Tie-in methods
Meanwhile, according to Yong Bai
and Qiang Bai, typically the route selection will be affected by:
- End point locations
- Water depths
- Presence of adverse environmental features such as high currents, shoaling waves
- Presence of other fields, pipelines, structures, prohibited zones (e.g. naval exercise areas, firing ranges)
- Presence of unfavorable shipping or fishing activity
- Suitability of landfall sites, where applicable.
The required minimum pipeline
route curve radius (Rs) should be determined to prevent slippage of the curved
pipeline on the sea floor while making a curve, in accordance with the
following formula [1]. If the pipeline-soil friction resistance is too small,
the pipeline will spring-back to straight line. The formula also can be used to
estimate the required minimum straight pipeline length (Ls), before making a
curve, to prevent slippage at initiation. If Ls is too short, the pipeline will
slip while the curve is being made.
If a 16” OD x 0.684” WT pipe is
installed in 3000 ft of water depth using a J-lay method (assuming a catenary
shape), the bottom tension and the Rs and Ls can be
estimated as follows:
The submerged pipe weight, Ws
= 22.6 lb/ft
Assuming the pipe departure angle
(α) at J-lay tower as 10 degrees
Top tension, T = Ws x
WD / (1-sinα) = 22.6 x 3000 / (1-sin10) = 82047 lb ≈ 82 kips
Bottom tension, TH = T
x sinα = 82 x sin10 = 14.2 kips
If the curvature angle (α) and
the pipe rigidity (elastic stiffness = elastic modulus (E) x pipe moment of
inertia (I)) are considered to do a big role on the Rs and Ls
estimates, the above formula can be modified as follows:
Fabrication, Installation and Operational Cost Considerations
A significant proportion of the
total cost to install a pipeline which is directly affected by the chosen route
is incurred during fabrication and installation. The associated activities are:
- Length of fabricated pipeline pipe (coated)
- Pre sweeping of route
- Pre lay installed freespan correction supports
- Post lay installed freespan correction supports
- Trenching, burying or rock dumping.
Some or all of these activities
will be present within the selected pipeline route. As a general rule the
design should be performed to:
- Minimize length of pipeline required
- Avoid requirement for presweeping
- Avoid pre-lay installed freespan correction supports
- Minimize post-lay freespan correction supports
- Minimize trenching, burying and rock dumping.
Route Optimization
Optimization of pipeline routing
is usually not performed as the route probably has no obstruction, is in an
accessible water depth and the seabed topography is flat: Hence a straight line
between the two termination points would suffice. However, on seabeds with
onerous terrain significant savings on fabrication and installation costs can
be made if route optimization is performed.
To perform a route optimization,
reasonably accurate costs for the following activities are required:
- Supply of additional pipeline pipe/unit length
- Presweeping a corridor/unit length, including cost of reduced lay rate due to a smaller lay corridor
- Prelay freespan correction supports (each), again including cost of reduced layrate due to smaller lay corridor
- Post lay freespan correction supports (each)
- Trenching, burying and rockdumping/unit length (for each).
Based on the derived costs, a
total cost for each route can be derived. It is worth noting that the
optimization cannot be completed until all the pipeline design parameters are
finalized (for instance the number of freespan correction supports will not be known
until the allowable freespan has been determined).
Source:
Bai, Yong and Bai, Qiang. Subsea
Pipelines And Risers. USA: Elsevier Inc. 2005.
Lee, Jaeyoung. Introduction to
Offshore Pipelines and Risers. 2007.
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