3 - NRG - On Bottom Stability Presentation

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)
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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.
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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.
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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.
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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:
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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
The drag force per unit length (FD) of the pipeline is calculated as follows:
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The lift force per unit length (FL) is calculated as follows:
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The inertia force per unit length (FI) is calculated as follows:
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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)
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?)
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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.
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Simplified Analysis
This analysis is based on quasi-static method with results calibrated from the Generalised Stability analysis.
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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.
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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.
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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:
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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.
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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:
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The inertia force per unit length (FI) is calculated as follow:
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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)
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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:
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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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.
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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)
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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:
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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
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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.
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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.
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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.
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3 - NRG - On Bottom Stability Presentation