J. Marine Sci. Appl. (2011) 10: 321-324 DOI: 10.1007/s11804-011-1075-0

Finite Element Simplified Fatigue Analysis Method for a Non-tubular Joint of an Offshore Jacket Platform

Qinghua Bao* and Heng Feng

China National Petroleum Offshore Engineering Co. Ltd., Beijing 100176, China

Abstract: This paper proposes the finite element simplified fatigue analysis method for fatigue evaluation of the composite non-tubular joint structure of an offshore jacket subjected to wave loads. The skirt pile sleeve of the offshore jacket, which had been in service, was taken as an example of the non-tubular joint structure. SACS software was used for global analysis of multi-directional wave loads for the jacket platform, and ALGOR software was used to build a finite element model, perform finite element analysis, post-process stress results for acquiring the stress range, and perform fatigue evaluation. The analysis results indicate that the extreme stress range is within the allowable stress range and meets the requirements of DNV code. That means the simplified fatigue analysis method is effective and can be used in fatigue design for the non-tubular joint structure of an offshore jacket. Keywords: offshore jacket platform; skirt pile sleeve; finite element; fatigue evaluation Article ID: 1671-9433(2011)03-0321-04

1 Introduction1 An offshore jacket platform subjected to large numbers of cyclic oscillating loads caused by waves may have fatigue damage even at very low nominal stress levels (DNV, 2004). For typical tubular joints of the jacket platform such as T, Y, K, TK, double T, and X, relevant fatigue check codes provide stress concentrated factor (SCF) calculation formulas and fatigue evaluation methods of these typical tubular joints (Zhang et al., 2004). But for complicated non-tubular joints, the SCF formulas of codes are not suitable (Tan et al., 2005), and the fatigue program of SACS software which is normally used to evaluate the performance of the typical tubular joints of the jacket platform with respect to fatigue failure cannot be applied. In this paper, a method of finite element simplified fatigue analysis is proposed to calculate stress. After that, hot stress can be obtained, and finally fatigue property can be evaluated. In order to introduce this simplified fatigue analysis method in detail, the fatigue property of the offshore jacket skirt pile sleeve, which consists of the pile sleeve, grout connection, top Yoke plate, bottom Yoke plate, shear plate and brace, is evaluated. Those structure components on an offshore jacket are the most complicated parts of a non-tubular joint structure of an offshore jacket platform’s infrastructure. The analysis method and evaluation results of the jacket skirt pile sleeve approved by DNV indicate that the simplified fatigue analysis method is very effective and can be taken as a reference in a fatigue design for the non-tubular joint structure of an offshore jacket.

Received date: 2010-09-13. *Corresponding author Email: baoqh.cpoe@cnpc.com.cn

© Harbin Engineering University and Springer-Verlag Berlin Heidelberg 2011

2 Analysis procedure The finite element simplified fatigue analysis method includes the following analysis procedures. First, the SACS model of a jacket platform shown in Fig.1 is built to carry out global analysis for obtaining a critical fatigue load case and deflection boundary condition. Then ALGOR finite element analysis software is used to build the finite element model of the corresponding jacket skirt pile sleeve as shown in Fig.2. Loading is applied to perform finite element analysis. Finally through manually post-processing the stress output of ALGOR analysis for recording the stress range, the fatigue property of the jacket skirt pile sleeve can be evaluated according to the DNV code.

Fig.1 Jacket SACS global analysis model

Qinghua Bao, et al. Finite Element Simplified Fatigue Analysis Method for a Non-tubular Joint of an Offshore Jacket Platform

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Fig.2 Finite element model of the jacket skirt pile sleeve

2.1 Modeling The major structural details of the jacket skirt pile sleeve incorporated in the model include the jacket bracing, jacket leg, yoke plate, shear plate, and grouted pile sleeves as shown in Fig.2. The weld bead stiffeners are not included in the model. The model adopted a 4-noded structural shell element with six degrees of freedom at each node. The shell element is ideally suited to represent curved shells and a flat plate of variable thickness, and effectively model through-thickness properties. Mesh size at connection is close to component thickness in modeling, and verified with significantly less difference of stress at these connections through mesh dividing for some times. The pile sleeve grouted connection is modeled as an 8-noded structural solid element with three degrees of freedom at each node. The outside and inside pile sleeves are modeled by a 4-noded structural shell element. All nodes along boundaries between grout elements and shell elements are coupled ones. This means that the deflection of the boundary nodes is simultaneously satisfied with the shell and grout boundary deflection conditions. The element nodes at the ends of each of the jacket braces, legs, and piles are rigidly constrained to a single node, and located on the central line of the tubular pipes. Load or boundary conditions are applied to these central nodes (Liu et al., 2002). 2.2 Material properties The skirt pile sleeve assembly and the pile sleeve connection will be respectively constructed with high strength steel and grout. The relevant material properties are shown in Table 1.

Table 1 Material property

Item Steel Grout Young’s modulus/Pa 2.1×1011 2.4×1010

Poisson’s ratio 0.3 0.167 Density/(kg/m3) 7.85×103 3.45×103

Yielding strength/MPa 345 —

3 Fatigue strength analysis 3.1 Loading The corresponding part of global jacket fatigue analysis model is shown in Fig.3. In order to attain a critical fatigue load case, the SACS global analysis including multi-directional wave loads is performed (API, 2007), and Morison formula is used to calculate wave load including drag and inertia loads. The wave fatigue load case pair extracted from the SACS global analysis is used for evaluating the approximate stress range and is listed in Table 2. The load case corresponds to the wave of the 100 year return period in accordance with API code. The wave phases which produce maximum/minimum base shear are the same as those that produce the maximum/minimum overturning moment. So, only the maximum/minimum base shear load case pair is used in the fatigue analysis (API, 2007).

(a) Plane defined by 205 201 207

(b) Plane defined by 101 103 125

(c) Plane defined by 105 101 205

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(d) Plane defined by 107 103 207

Fig.3 Joint numbers of SACS global analysis

Table 2 Loadcase definition

SACS Loadcase Description

Maxbaseshear-Max The wave position producing the maximum base shear

Minbaseshear-Min The wave position producing the minimum base shear

Deflections and rotations of every jacket joint are outputted by SACS global analysis as shown in Table 3 and Table 4. The joint numbers in the table refer to the SACS model shown in Fig.1, and a brief description of the members framing into the point in question is shown in Fig.3. All these joint deflections and rotations are considered as the boundary conditions and applied to the corresponding joints during the local fatigue strength analysis.

Table 3 Joint deflections and rotations under loadcase Max from SACS output

Joint number Load case Deflection(X)

cm Deflection(Y)/

cm Deflection(Z)/

cm Rotation (X)/

rad Rotation (Y)/

rad Rotation (Z)/

rad 37 MAX −2.863 166 8 2.776 659 3 −1.930 273 7 −0.000 707 2 −0.000 748 6 0.000 047 2 124 MAX −3.666 150 3 2.847 753 5 −1.402 404 8 0.000 089 9 −0.000 589 9 −0.000 946 6125 MAX −2.923 683 9 3.682 750 5 −1.406 427 5 −0.000 618 8 0.000 119 4 0.000 981 7 203 MAX −3.707 204 6 3.775 317 2 −0.942 111 7 −0.000 208 1 −0.000 641 4 −0.001 847 7

Table 4 Joint deflections and rotations under loadcase Min from SACS output

Joint number Load case Deflection(X)/

cm Deflection(Y)/

cm Deflection(Z)/

cm Rotation (X)/

rad Rotation (Y)/

rad Rotation (Z)/

rad 37 MIN 1.545 483 1 −1.563 599 5 −0.418 820 6 0.000 534 2 0.000 527 8 −0.000 095 3124 MIN 2.068 869 1 −1.746 377 1 −0.768 034 6 −0.000 233 1 0.000 500 6 0.000 644 5 125 MIN 1.679 374 3 −2.167 774 4 −0.765 119 4 0.000 524 1 −0.000 231 4 −0.000 568 7203 MIN 2.065 717 7 −2.129 249 6 −0.903 987 9 0.000 415 4 0.000 026 9 0.001 173 8

3.2 Fatigue evaluation method According to DNV standard, the finite element simplified fatigue analysis method described above is used in fatigue assessment. The long-term stress range distribution is a fundamental requirement for fatigue analysis which is based on fatigue tests (S-N curves) (DNV, 2008). In lieu of calculating the long-term distribution directly, a form of Weibull distribution is used to represent the long-term distribution. When the Weibull distributed stress range and bilinear S-N curves are used, the fatigue damage expression is given by (DNV, 2008):

1 21 1 2 1

01 2

1 ; 1 ;h hm m

dq m S q m SD Ta h q a h q

ν Γ γ η⎡ ⎤⎛ ⎞ ⎛ ⎞⎛ ⎞ ⎛ ⎞⎢ ⎥⎜ ⎟ ⎜ ⎟= + + + ≤⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎢ ⎥⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠⎣ ⎦

(1)

Based on the assumption of the allowable fatigue damage 1.0η = , 52 years fatigue design life including 2 times of 25

years service life and 2 years transportation fatigue life (API, 2007), with an average wave period of 9.312 sec and

1.1h = , the largest stress ranges of different joint classifications are shown in Table 5.

Table 5 Extreme stress ranges

Joint classification Description Extreme stress

range/ MPa T Tubular joint 231

F Tube to shear plate/yoke plate 182

The stress derived above corresponds to the reference thickness. When the thickness is larger than the reference thickness, an allowable extreme stress range may be obtained as (DNV, 2008).

ref0, 0,tref

k

ttt

σ σ ⎛ ⎞= ⎜ ⎟⎝ ⎠

(2)

3.3 Stress range evaluation The six stress components at each load case with reference to element local coordinates are outputted from the finite element analysis results. For each stress component, the algebraic differences between the stresses are calculated as follows (Shu, 1998).

Qinghua Bao, et al. Finite Element Simplified Fatigue Analysis Method for a Non-tubular Joint of an Offshore Jacket Platform

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1 1 2 2

1 1 2 2

1 1 2 2

( ) ( )( ) ( )

( ) ( )

x xx xb xx xb

y yy yb yy yb

xy xy xyb xy xyb

σ σ σ σ σσ σ σ σ στ τ τ τ τ

Δ = + − +Δ = + − +

Δ = + − +

(3)

The principal stresses σ listed as follows are calculated from the above resulting stress differences. Stress range is numerically the greatest of these principal stresses (Shu, 1998).

2 2( )2 2

x y x yxy

σ σ σ σσ τ

Δ + Δ Δ − Δ= ± + Δ

(4)

3.4 Fatigue analysis results The extreme stress range of the jacket skirt pile sleeve under the pair of load cases is evaluated as described in Sec. 0 and summarized in Table 6.

Table 6 Extreme stress range summary of load case pair

Item Thickness/ mm

Extreme stress range/

MPa

Allowable extreme stress

range/MPa Top yoke plate 32 79.87 171.11 Bottom yoke

plate 32 16.26 171.11

Shear PL-32 32 29.21 171.11 Shear PL-38 38 40.60 163.91 Pile sleeve

(φ2314×32) 32 11.41 231.00

Pile sleeve (φ2326×38) 38 13.37 221.29

Jacket leg (φ1829×32) 32 26.17 231.00

Jacket leg (φ1829×60) 60 41.81 197.41

Obviously, the extreme stress ranges are all less than the allowable extreme stress ranges. Therefore, the jacket skirt pile sleeve is acceptable to the fatigue strength evaluation. 4 Conclusions Fatigue design for a complicated non-tubular joint of an offshore jacket is very difficult. This paper proposed that the finite element simplified fatigue analysis method can be

used to effectively evaluate the fatigue properties of the composite non-tubular joint structure of an offshore jacket. Fatigue design of the skirt pile sleeve, which is the most complicated non-tubular joint on the jacket, was taken as an example to introduce and verify the method in detail. The method was used successfully in fatigue design of a skirt pile sleeve which had been designed and subsequently fabricated and installed in the sea. In addition, the analysis results have been approved by DNV. Consequently, this analysis method is significant to the fatigue design for the non-tubular joint structure of other offshore jackets.

References API (2007). Recommended Practice 2A-WSD. RP 2A, America

Petroleum Institute. DNV (2004). Structural design of offshore ships. OS-C102, Det

Norsk Veritas. DNV (2008). Fatigue strength analysis of offshore steel structures.

RP-C203, Det Norsk Veritas. Liu Gang, Zheng Yunlong, Zhao Deyou (2002). Fatigue strength

analysis of BINGO9000 semisubmersible drilling rig. Joural of Ship Mechanics, 6(2), 54-62.

Shu Yilin (1998). Mechanics of materials. Higher Education Press, Beijing.

Tan Kairen, Xiao xi, Huang Xiaoping (2005). Analysis and calculation of fatigue life of unconventional tubular joints. Ocean Engineering, 23(3), 51-54.

ZhangYuling, Pan Jiyan Pan Jinuan (2004). Analysis of common fatigue details in steel truss structures. Tsinghua Science and Technology, 9(5), 583-588.

Qinghua Bao is an engineer at China National Petroleum Offshore Engineering Co. Ltd. His current research interests are focused on marine engineering.

Heng Feng is an engineer at China National Petroleum Offshore Engineering Co. Ltd. His current research interests are focused on marine engineering.