One
important aspect in designing an offshore pipeline system is its stability for
being underwater, on the seabed for a life time service (operation). The
analysis of keeping the pipeline system remained on the seabed is known
as On-Bottom
Stability. There are few methods to maintain pipeline at the
seabed, such as pipe burial, trenching, as well as building a rock berm, and
thicken the concrete coating. On-bottom stability consists of vertical
stability and dynamic lateral stability.
Force for Subsea Pipeline On Bottom Stability
Source : http://www.piping-engineering.com/wp-content/uploads/2014/07/pipeline-on-bottom-stability-design.jpg
Vertical
Stability
Total
pipe weight is the weight of the pipe alloy steel material, anti-corrosion
coating, and field joint coating. A cross-section of an offshore pipeline can
be seen through the image below:
Pipeline Cross Section
Source : https://nonerieska.wordpress.com/2013/01/30/on-bottom-stability-of-offshore-pipeline/
In
order to avoid floatation in water, the submerged weight of the pipeline shall
meet the following criteria:
where:
ΥW
= safety factor
b
= pipe buoyancy per unit length : ρw • g • ∏ •
D2 / 4
ρw
= mass density of water
g =
acceleration of gravity
D
= pipe outer diameter (including all coating)
ws
= pipe submerged weight per unit length
sg
= pipe specific density : (ws+b)/b
If
a sufficiently low probability of negative buoyancy is not documented, the
safety factor ΥW = 1.1 can be applied.
Dynamic
Lateral Stability
The
objective of a dynamic lateral stability analysis is to calculate the lateral
displacement of a pipeline subjected to hydrodynamic loads from a given
combination of waves and current during a design sea state. On-bottom
stability is a highly non-linear phenomenon with a large degree of stick/slip
response. This is particularly important to keep in mind for large values
of current to wave ratios and large wave periods, and more so for stiff
clay and rock than for soft clay and sand where the build up of penetration and
passive resistance is more pronounced.
1.
Current Condition
The
steady current flow at the pipe level may have components from:
§ Tidal
current
§ Wind-induced
current
§ Storm
surge induced current
§ Density
driven current
2.
Short Term Wave Condition
The
wave induced oscillatory flow condition at the pipe level may be calculated
using numerical or analytical wave theories. The wave theory shall be
capable of describing the conditions at the pipe location,
including effects due to shallow water, if applicable. The
short-term, stationary, irregular sea states may be described by a wave
spectrum Shh(ω) i.e. the power spectral density function of the sea
surface elevation. Wave spectra may be given in table form, as
measured spectra, or in an analytical form.
For
instance, the JONSWAP spectrum, the spectral density function reads:
3.
Forces Affecting Pipeline On-Bottom Stability
§ Hydrodynamic
force, consists of drag force and inertia force (can be calculated using
morrison formula), as well as lift force. Lift force is a vertical hydrodynamic
force. This would happen with the concentration of streamline on the
pipe.
§ Soil
friction force, is a horizontal force influenced by friction coefficient
between pipe and seabed. Value of the friction coefficient depends on the
seabed soil characteristics. For example, friction coefficient for
clay-soil is 0.2 and friction coefficient for sandy-soil is 0.6.
Sumber
:
Recommended Practice DNV-RP-F109 October 2010
Dega Damara Aditramulyadi
Student ID : 15512046
Course : KL4220 Subsea Pipeline
Lecturer : Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D.
Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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