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1 Copyright © 2013 by ASME
ASSESSMENT OF EFFECTIVE STIFFNESS FORMULATION OF CONCRETE COATED RIGID PIPELINE
Joe-Joe Chellakat Satyaraj
McDermott Middle East, Inc. Dubai, United Arab Emirates
Manoj John
McDermott Middle East, Inc. Dubai, United Arab Emirates
Vinod Puthiyaparambath
McDermott Middle East, Inc. Dubai, United Arab Emirates
Senthilkumar Durairaj
McDermott Middle East, Inc. Dubai, United Arab Emirates
ABSTRACT
Application of concrete coating on rigid pipelines alters the stiffness of the pipeline. Estimation of the effective stiffness of the concrete coated pipe is challenging as it depends on the diameter and thickness of the pipeline, thickness of the concrete coating, the corrosion coating and the cut back lengths. Realistic estimation of the stiffness is essential in order to define the pipelay profile and for the estimation of pipeline stresses, which in turn governs the limiting weather window for the installation. This paper describes the effect of concrete stiffness of the rigid pipeline using three different methods of stiffness calculation, namely Mogbo, Lund and DNV methods. Comparison of results from each of the method is presented with the aim of choosing the appropriate method for installation. The objective of this paper is to provide a better insight into the significant operational and economic influences these alternative approaches can have during the installation campaign.
1 INTRODUCTION
Utilization of oil and gas continues to rise unabated, resulting in a growing demand for the installation of larger and heavier pipelines. To comply with this requirement, unexplored ocean neighborhoods with complex and challenging geology are often being looked upon as viable solutions. With challenges aplenty the requirement for adequate stability forms one of the main aspects of subsea pipeline design. Application of concrete coating is one of the traditional methods whereby it nourishes the bare pipe with the much required stability and protection from mechanical damage. Besides stability, the addition of reinforced concrete to the steel pipe alters certain key properties of the line pipe – the major contribution being evident through the supplementary stiffness of the concrete coating. From a general outlook, realistic estimation of effective stiffness of the concrete coated pipeline forms a critical part of the installation analysis. In an installation analysis standpoint, calculation of effective stiffness assumes a factor of importance since arriving at appropriate realization of stiffness may lead to different predictions of the installation weather window, which would also produce tangible effects in terms of timely execution of planned operations. This paper presents a dynamic case study of a 20” rigid pipeline from one of McDermott’s past projects. Concrete coatings of 40 mm, 100 mm and 140 mm are considered for the study. Effective stiffness of the coated pipeline is derived using three methods, namely Mogbo, Lund and DNV methods. Each of these methods employs ideas different to one another resulting in different realizations of effective stiffness. The case study presented in this paper could serve as reference for future projects as it captures many influential factors in the analysis including barge motions, environmental loadings, pipelay profile variations etc. Results obtained through this case study forms the main topic of discussion in this paper. The analysis outputs predicted by these methods are compared and their associated impacts in the installation campaign are discussed. Results are plotted against the actual data that were recorded on-site. Key findings of which are presented under relevant sections of this paper.
Proceedings of the ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering OMAE2013 June 9-14, 2013, Nantes, France
OMAE2013-10523
2 Copyright © 2013 by ASME
2 NOMENCLATURE
ASME American Society of Mechanical Engineers CC Corrosion Coating CWC Concrete Weight Coating DNV Det Norske Veritas FBE Fusion Bonded Epoxy JONSWAP Joint North Sea Wave Energy Project MOI Moment of Inertia OD Outer Diameter OMAE Ocean, Mechanics & Arctic Engineering RP Recommended Practice SMYS Specified Minimum Yield Strength WT Wall Thickness
3 ESTIMATION OF EFFECTIVE STIFFNESS
The evaluation of effective stiffness has been well documented in literature and the associated inherent issues are explored at great lengths in numerous publications by engineers, professors and various research teams. However, the findings by each method do not stand in unison to one another when it comes to the assessment of the final effective stiffness formulation, thus leaving the analysts the need to conduct sensitivity studies. Succinct descriptions of three such methods by Mogbo, Lund and DNV are presented below to aid in the completeness of this paper. Each of which has a well balanced consideration by the pipeline industry experts. It is of interest to note the difference in approach these methods hold which in turn results in some interesting findings in the case study as presented in the later part of this paper. Below-mentioned methods are referenced based on the srcinal works by the respective author(s) with special emphasis on the key equations relevant to the case study.
3.1 Mogbo method (1972):
Initiating the effort in the estimation of effective stiffness of the concrete coated pipe, Mogbo et al. compiled and published the results of their experimental studies in a comprehensive research paper [1]. Mogbo’s paper initially notes that the flexural stiffness of the bare (steel) pipe increases significantly with the application of a high density coating of reinforced concrete. However, the paper goes on to indicate that the slip between concrete coating and the interface material – wrapped over the steel pipe – would actually result in reduction of the concrete-induced additional stiffness. Full-scale bending tests of eight coated pipes along with theoretical studies of pipe bending formed part of Mogbo’s experimental studies. The increase in bare pipe stiffness due to concrete coating was measured. Comparison of experimental results for pipes with and without field joints was discussed toward the end. From the results, the measured increase in stiffness in the conducted experiments was observed to be 75% to 85% of the computed concrete stiffness,
K
c
. The effective stiffness in particular was predicted based on conditional equations that took into account the length of pipe influenced by the field joint (transition length). Key equations are as follows: for L
j
< L/2;
L LK K L
K
cc j j
e
2
…. 1 for L
j
= L/2;
2
cbe
K K K
…. 2 for L
j
> L/2;
2
'
cbe
K K K
…. 3 Where: L Total length of pipe (i.e.
L
= 2
L
j
+ L
c
) L
j
Pipe length influenced by field joints L
c
Effective length of pipe over which no slip occurs K
e
Effective stiffness of coated pipe K
j
Pipe stiffness at the field joint K
c
Computed initial stiffness of coated pipe K`
c
Reduced stiffness due to concrete slippage K
b
Initial stiffness of bare pipe
Figure 1: Distribution of stiffness along the pipe [1]
Since our present case study satisfies the criterion set by equation (1) (as shown in Table 2, Section 5.2), equations (2) and (3) will be ignored in upcoming discussions. In equation (1)
K
j
was obtained by averaging the stiffness of the bare pipe,
K
b
, and the coated pipe,
K
c
, whereas
K
c
was computed as a weighting function of steel and concrete cross-sections for the stiffness distribution as shown in Figure 1. It could be noticed that
L
j
is representative of the transition length and the cutback length both.
3 Copyright © 2013 by ASME
3.2 Lund method (1993):
This alternative method of evaluating the effective stiffness was introduced by S. Lund et al. in 1993 [2]. In order to identify the parameters responsible for affecting additional stiffness and strain concentration at field joints, Lund developed an experimental model to interpret the flexible behavior of concrete coated pipes for 20” and 40” pipelines, simulating the laying conditions. The obtained results were then modeled in a finite element tool for the purpose of verification. According to Lund [2], the assumption of full bondage between pipe and concrete on the compression side led to conservative values for the bending stiffness of the coated pipeline, and the strain concentration at field joints. Experimental evidences such as concrete cracking and slipping due to the shear transformation were pointed as possible causes resulting in significant reductions of the calculated stiffness of the composite section.
Figure 2: Distribution of stiffness along the pipe [2]
The elaborate discussions outlaid in Lund’s paper are cut short by the parent equation hereunder for the calculation of effective stiffness:
jcbcb p
pst eff
L L L L
L EI EI
122
…. 4 Where: E
Young’s modulus of elasticity I
comp
Composite moment of inertia I
eff
Effective moment of inertia I
st
Steel moment of inertia L
Total length of pipe (i.e.
L
= 2
L
j
+
2
L
cb
+
L
c
) L
cb
Bare pipe length L
j
Pipe length influenced by field joints L
c
Effective length of pipe over which no slip occurs
β
Ratio of steel pipe stiffness to the maximum stiffness of the composite section
3.3 DNV method (2006):
DNV-RP-F105 provides yet a different approach for calculating the stiffness of concrete coating. As per section 6.2.5 in DNV [3], the expression for the calculation of CSF takes the following form:
75.0
steelconcc
EI EI k CSF
…. 5 Where: CSF Concrete stiffness factor k
c
Empirical constant for slippage / deformation EI
conc
Bending stiffness of concrete EI
steel
Bending stiffness of steel The empirical constant,
k
c
was assumed as 0.25 for the Fusion Bonded Epoxy (FBE) coating. DNV, in its description, also points out that the equation (5) is valid only when the following criteria hold true: -
Pipe joint length exceeds 12 m; -
Field joint length in the range of 0.5-1.0 m; -
Concrete coating thickness does not exceed 150 mm. Upon satisfying the criteria as stated, the effective moment of inertia of the concrete coated pipe is calculated using the expression below:
SCF I I
st eff
…. 6 Where: I
eff
Effective moment of inertia I
st
Steel moment of inertia SCFStress concentration factor (
SCF = 1 + CSF
) However, DNV code does not provide adequate guidance notes on the consideration of bond strength between concrete and the interface layer.
4 CASE STUDY
The theory of added stiffness of reinforced concrete to the bare pipe is backed unequivocally by all three methods as discussed in Section 3. While noting that, however, each of the above-mentioned approach gives rise to different realization of the effective stiffness owing to subtle theoretical variations based on apprehensions from a number of experimental tests. The details of which will be discussed toward the end.
4 Copyright © 2013 by ASME To better illustrate the variation in effective stiffness, a case study was performed using a 20” pipeline to analyze the influence of each concept. Below table details the material properties of the 20” pipeline considered for the case study: Table 1: Properties of Coated Pipe Pipe OD (mm) 508 Pipe WT (mm) 15.9 22.2 CWC Thickness (mm) 40 / 100 140 CWC Density (kg.m
-3
) 3045
3400 CC thickness (mm) 0.75 (FBE coating) CC density (kg.m
-3
) 1400 Steel Grade X65 Steel density (kg.m
-3
) 7850 Design properties described above form part of a project that was undertaken by McDermott in the past. For the purpose of our analytical comparison, concrete thicknesses of 40 mm, 100 mm and 140 mm were considered. The effective moment of inertia of the concrete coated pipeline was estimated using MathCAD for all three methods based on the foregoing methodologies in Section 3. Prior to running the analysis, a graph outlining the effective MOI for concrete thicknesses up to 140 mm was plotted. The idea behind this check was to provide us with a panoramic view over the influence each discussed method had in the estimation of effective stiffness. This supplementary check was in fact useful in understanding the trend line of calculated MOIs as shown in Figure 3.
Figure 3: Comparison of MOIs
Figure 3 shows the behavior of the effective moment of inertia for each of the methods as discussed earlier. For smaller thickness of concrete (30-40 mm), the formulation of MOI by Lund and DNV appeared to be in good agreement. However, the MOI predicted by Mogbo was higher by 8%. Halfway through the graph, the uniformity in comparison collapsed amongst the three methods demonstrating noticeable gap in the predicted moment of inertia. For instance, at 140 mm thickness the MOI predicted by the Lund method appeared to be lesser than DNV and Mogbo methods by a substantial margin of 16% and 33% respectively.
5 ANALYSIS 5.1 Methodology and Results
During installation where the pipeline catenary (in the sagbend region) is constantly exposed to the combination of hydrodynamic loads (under the combined effect of axial tension and bending moment) and the coupled barge-stinger induced motions, sensitivity checks for the dynamic conditions appeared reasonable for our study rather than general static checks. Full-fledged dynamic analyses (with duration of 3 hours) were performed using OFFPIPE software. S-lay method of installation was considered for this project and investigations both. The pipelay was setup with McDermott’s Derrick Barge and a few sections of articulated floating stinger. Analyses in this case study were performed in water depths whose ranges varied between 15 m and 200 m. Jonswap spectrum was used for simulating the on-site environmental conditions in line to the metocean data.
Figure 4: Comparison of Seastates (H
s
)
Keeping the pipe stress levels within the allowable limits of SMYS, the pipeline profile was optimized to determine the maximum significant wave height,
H
s
. Figure 4 shows the graphical comparison of limiting seastates obtained using all three methods.
5.2 Discussions
Generally strain based analysis offers relaxed installation criteria as opposed to stress based analysis. However, the case study in this paper follows the stress based analysis. The decision to adopt the load controlled criteria (LCC) in place of the displacement controlled criteria (DCC) is chiefly attributed

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