Wall Thickness DesignNRG ENGINEERING
August 2 – August 28, by Mr. Eng Bin NG
Applicable Codes
Pipe Expansion Calculations
Flexibility Analysis Methodology
5
Pipeline Construction - Conventional & Unconventional
DNV OS F101 (2000) – Submarine Pipeline Systems
DNV RP E305 (1988) – On bottom Stability Design of Submarine
Pipelines
DNV 1981 – Rules for Submarine Pipelines
AGA Software (not a code but acceptable practice)
NRG ENGINEERING
The Power to Deliver™ #*
The on-bottom stability design focuses on determining the concrete
weight coating requirement for the pipeline so that it is stable
during it’s operating life
Typically, the stability analysis is performed for 2
conditions:
Installation condition – Pipeline Empty and subjected to 1-year
return period wave and current
Operating condition: Pipeline filled with Product (minimum density)
and subjected to 100-year return-period wave and 100- or 10-year
return-period current.
NRG ENGINEERING
The Power to Deliver™ #*
Wave approach to the pipeline axis can be utilized to derive
optimum concrete thickness in shallower waters.
The pipeline may be divided into many sections to account for water
depth variation, soil data and environmental loading.
The concrete density used in the analysis is adjusted to account
for field joint content.
Normally no corrosion allowance is considered for the lateral
stability calculations unless corrosion allowance is very
significant, as the corroded pipe can still contribute to the pipe
weight.
Water absorption is considered, e.g. 3% may be assumed during the
installation and hydrotest conditions, and 5% during operational
conditions.
NRG ENGINEERING
DNV 1981 - Lateral Stability Design
The design is based on no movement and no pipe soil
interaction.
Pipeline lateral stability refers to the stability of the pipeline
against lateral movements when subjected to hydrodynamic loadings
from wave and current.
NRG ENGINEERING
DNV 1981 - Lateral Stability Design (cont’d)
It is fundamentally based on static balance between applied
hydrodynamic forces and resisting soil forces as illustrated in
Figure below. The resisting soil forces is typically characterised
by frictional force at the pipe/soil interface.
FORCES ACTING ON SUBMARINE PIPELINE
Pipe Submerged Weight, WSub
Hydrodynamic Lift Force, FL
Pipe
Seabed
Appropriate hydrodynamic force coefficients are used in the
stability analysis of the pipeline.
The stability criterion is expressed by:
NRG ENGINEERING
Where:
FL = Hydrodynamic lift force per unit length (N/m)
FD = Hydrodynamic drag force per unit length (N/m)
FI = Hydrodynamic inertia force per unit length (N/m)
= Coefficient of friction between pipe and seabed
(Varies between 0.3 for dense clay to 0.7 for sand,
normal friction coefficient =0.5)
Fs = Safety factor = 1.1
DNV 1981 - Lateral Stability Design (cont’d)
The drag force per unit length (FD) of the pipeline is calculated
as follows:
NRG ENGINEERING
DNV 1981 - Lateral Stability Design (cont’d)
The lift force per unit length (FL) is calculated as follows:
NRG ENGINEERING
DNV 1981 - Lateral Stability Design (cont’d)
The inertia force per unit length (FI) is calculated as
follows:
NRG ENGINEERING
Where:
CI = Inertia coefficient (3.29)
Dt = Total outer diameter of pipeline including coatings (m)
a = Horizontal water particle acceleration normal to
the pipe axis (m/s²)
Ud = Horizontal water particle velocity normal to
the pipe axis
particle velocity normal to pipe (m/s)
Uc = Horizontal steady current velocity normal to
pipe axis (m/s)
DNV 1981 - Lateral Stability Design (cont’d)
The hydrodynamic coefficients will be reduced to care of trench
effects if applicable as per Jacobsen et al OTC paper “Fluid Loads
on Pipeline: Sheltered or Sliding” (what are the values?)
NRG ENGINEERING
The Power to Deliver™ #*
DNV RP E305, 1988
DNV RP E305 is based on the PIPESTAB Joint Industry Project
conducted in the North Sea in the mid-eighties. Three design
methods are described in this Code.
NRG ENGINEERING
Simplified Analysis
This analysis is based on quasi-static method with results
calibrated from the Generalised Stability analysis.
NRG ENGINEERING
2. Generalised Analysis
This analysis is based on a set of non-dimensional stability curves
which have been derived from a series of runs presenting pipe
movement and strain results with a dynamic response model. Net pipe
movement is permitted for pipe on sandy soil up to 40 pipe
diameters. Pipe on clay, however, does not allow net pipe
movement.
NRG ENGINEERING
3. Dynamic Analysis
The analysis described involves a full dynamic simulation of a
pipeline resting on seabed with soil resistance, hydrodynamic
forces, boundary conditions and dynamic response modelled for. It
forms the basis of reference for the Generalised analysis.
NRG ENGINEERING
DNV RP E305, 1988 (cont’d)
The hydrodynamic coefficients allowing pipeline to move a maximum
of 20 m in sand and no movement in clay.
Normally, a Pierson Moskovitz (PM) wave spectrum is assumed in the
analysis.
The stability criteria is expressed as:
NRG ENGINEERING
accordance with DNV RP E305.
(Friction coefficient varies between 0.15 to 1.3 for clayey
soil
depending soil shear strength and K C Number. The frictional factor
for sand is 0.7 regards of flow parameters)
Wsub = Submerged weight of pipeline (N/m)
FL = Hydrodynamic lift force per unit length (N/m)
FD = Hydrodynamic drag force per unit length (N/m)
FI = Hydrodynamic inertia force per unit length (N/m)
FW = Calibration factor depending on Keulegan Carpenter number and
velocity ratio. A safety factor of 1.1 is inherent in the
calibration factor.
NRG ENGINEERING
The Power to Deliver™ #*
The drag force per unit length (FD) of the pipeline is calculated
as follows:
The lift force per unit length (FL) is calculated as follow:
NRG ENGINEERING
The Power to Deliver™ #*
The inertia force per unit length (FI) is calculated as
follow:
NRG ENGINEERING
CM = Inertia coefficient
= 0.7 ??
D = Total outer diameter of pipeline including external coatings
(m)
a = Horizontal water particle acceleration normal to the pipe
axis
(m/s²)
Us = Horizontal water particle velocity normal to the pipe axis due
to wave
(m/s)
Uc = Horizontal steady current velocity normal to the pipe axis due
to wave
(m/s)
NRG ENGINEERING
The PRCI/AGA Stability software was developed based on analytical
research and large-scale test model sponsored by Pipeline Research
Council International, Inc. (PRCI).
The software represents the state-of-the-art design in pipeline
stability and models the complex behaviour of pipe/soil interaction
which includes:
Hydrodynamic forces which account for the effect of wake (generated
by flow over pipe) washing back and forth over the pipe in
oscillatory flow;
Embedment (digging) into clay or sand which occurs as the pipe
resting on the seabed is exposed to oscillatory loading and small
oscillatory deflections.
Three levels of analysis are provided by PRCI (AGA) Stability
software, namely Levels 1, 2 and 3. The general characteristics of
each level of analysis is summarised as follows:
NRG ENGINEERING
Level 2
Simplified Quasi-Static
Performs a static analysis based on: Realistic hydrodynamic forces
Realistic pipe embedment calculated by quasi-static simulation of
wave induced pipe oscillations.
Level 3
Dynamic Time Domain with Wave Kinematics for 3-D Random Seas
Consists of 3-program suite, WinWave, WinForce and WinDynamics.
WinWave generates wave kinematics for 3-D random seas. WinForce
generates wave forces based on time history of wave kinematics
WinDynamics analyses pipe dynamics with external forces and a
history dependent soil model.
NRG ENGINEERING
Level 1 analysis:
This approach is based on traditional stability analysis methods
where the Morrison type hydrodynamic forces and frictional soil
resistance are considered. Its design methodology corresponds to
that described in DNV 1976 and DNV 1981 Codes. A ‘no movement’
pipeline stability design criteria is assumed.
NRG ENGINEERING
Level 2 analysis:
With the similar ‘no movement’ stability criteria, it is based on
quasi-static analysis where it simulates pipeline embedment process
as in the Level 3 analysis.
NRG ENGINEERING
Level 3 analysis:
This level of analysis is most detailed where pipeline is simulated
in a finite element time domain software. Detailed information on
pipeline movement and stresses obtained are basis of pipeline
safety assessment.
NRG ENGINEERING
AGA (PRCI) Method (cont’d)
Normally, a Level 2 analysis is adequate. Level 3 analysis is
carried out only if further optimisation of concrete weight coating
thickness is required.
NRG ENGINEERING
The Power to Deliver™ #*
Analytical Methodology (Level 2)
According PRCI (AGA) a pipeline exposed to wave flow will
experience a hydrodynamic force, which is expressed by two
components: the in-line drag force and the lift force.
These two forces are calculated based on the physics of the
water-pipeline interaction.
NRG ENGINEERING
The AGA hydrodynamic force model is expressed as:
(1)
(2)
(3)
= Density of sea water (kg/m3)
= Pipe outer diameter (m)
= Fourier coefficient
= Fourier phases
= Inertia coefficient
NRG ENGINEERING
Analytical Methodology (Level 2) (cont’d)
The Fourier coefficient and phases are determined from extensive
model test programme, which includes the effects of steady current,
waves, pipe roughness, and seabed roughness. These values have been
stored as a database, which is implemented in the AGA Level 2
stability analysis.
NRG ENGINEERING
Analytical Procedures
The AGA Level 2 is a quasi-static analysis program, which is
designed to take advantage of the results from the AGA’s
hydrodynamic and pipe/soil interaction tests.
The procedure of the program analysis is as follows:
Based on user inputs, the program calculates the significant bottom
velocity, Us
Maximum and minimum in-line hydrodynamic forces for the Largest 200
waves contained in an assumed 4-hour long build-up sea state are
calculated.
NRG ENGINEERING
The Power to Deliver™ #*
Maximum and minimum inline forces for the largest 50 waves during a
subsequent 3-hour long design sea state are calculated.
Based on the forces calculated, a conservative estimate of pipe
embedment at the end of the 4-hr storm build-up period is
calculated.
Based on the forces calculated and the pipe embedment calculated,
the amount of additional pipe embedment that can be produced by the
50 largest waves in the design sea state is calculated in a similar
fashion similar. This embedment and the associated soil resistance
force are saved for future processing.
NRG ENGINEERING
The Power to Deliver™ #*
Hydrodynamic forces for a complete wave cycle are calculated for
four statistically meaningful wave induced bottom velocities which
are expected in a 3-hr long design sea state.
The four bottom velocities, and, the most likely number of wave
induced velocities exceeding each are:
U1/3 = 1.00 Us (135 exceedances)
U1/10 = 1.27 Us (40 exceedances)
U1/100 = 1.66 Us (4 exceedances)
U1/1000 = 1.86 Us (0 exceedances)
NRG ENGINEERING
The Power to Deliver™ #*
Using the soil resistance values obtained and the hydrodynamic
forces calculated, the minimum factor of safety against lateral
sliding is calculated for the pipe embedment at the end of the 4-hr
long build up period, and at the end of the 3-hr long design sea
state.
The factor of safety is calculated at one-degree intervals of wave
passage for a complete 360-degree from:
NRG ENGINEERING
The Power to Deliver™ #*
The recommended Level 2 stability criteria should satisfy the
following aspects. At the end of the 4-hour storm build up, the
pipeline should be stable in the U1/100 condition, i.e. FOS ≥ 1.0
At the end of the additional 3-hour storm period, the pipeline
should be stable in the U1/1000 condition, i.e. FOS ≥ 1.0
NRG ENGINEERING
COMPARISON BETWEEN AGA AND DNV RP E305
In general, DNV RP E305 designs are more conservative than the AGA
designs. This is true for most designs where pipeline is laid on
clay and all designs where the soil is sand. No net pipe movement
criterion is assumed.
For cases in DNV RP E 305 design where movement is allowed for pipe
on sand, the concrete requirements is significantly less, and often
similar to the AGA designs which movement is not permitted.
NRG ENGINEERING
COMPARISON BETWEEN AGA AND DNV RP E305 (cont’d)
Compared to the traditional method (DNV 1976 / 1981), both AGA
Level 2 analysis and DNV RP E305 designs result in concrete weight
coatings that are more sensitive to soil strength / density.
With no pipe movement criteria, this sensitivity is similar in both
AGA Level 2 analysis and DNV RP E305. However, concrete weight
coating is less sensitive to soil density when pipe net movement is
allowed for in DNV RP E305 design.
AGA designs produce concrete weight coating less sensitive to water
depth than DNV RP E305 designs. This is due to the reduction in
pipe embedment in the AGA Level 2 analysis at deeper waters.
NRG ENGINEERING
PIPELINE STABILITY IN OPEN TRENCH
Both AGA and DNV RP E305 are not applicable for pipeline resting in
an open trench. Thus, to analyse pipeline stability in an open
trench, an alternative method would have to be used.
NRG ENGINEERING