1 INTRODUCTION
This document reported the finite element analysis (FEA) of deck’s plate under Booster Compressor Tie-Down Pads to ensure that plate due to induce the impact of equipment support and its skid during operation and critical conditions (earthquake).
1.1 General
The Maleo Field is located in Indonesian waters about 40 km south east of Madura Island and approximately 25 km south of Puteran Island (Figure 1.1) within the Madura Offshore Production Sharing Contract (PSC) area. The water depth at location is approximately 60m.
Figure 1.1 Current Madura Offshore PSC Area
Santos Maleo facility is MOPU with natural gas processing facilities and sales gas pipeline which is connect to existing East Java Gas (refer to figure 1.2). The Natural Gas processing facilities will consist of free water separation, reservoir solids removal, gas compression, gas processing and custody transfer metering.
Gas Production, which commenced in September 2006 is currently between 100 and 130 MMscfd. The Maleo facilities consist of a minimum facility tripod wellhead platform with four gas wells tied back to the adjacent Maleo Offshore Production Unit (MOPU). The MOPU is a jack up gas process and compression facility which exports sales quality gas via am 8 km 14” diameter spur line into the Pertamina owned East Java Gas Pipeline.
Declining Reservoir Pressure will require the installation of an additional suction booster compressor in 3Q 2012 to maintain gas production.
Figure 1.2 Maleo MOPU Development Concept
1.2 Purpose and Scope of Work
The purpose of this document is to present Finite Element Analysis (FEA) of deck’s plate under Booster Compressor Tie-Down Pads to ensure element nodal stress due to induce the impact of equipment support.
Modeling and analysis of the structure are performed by using Abaqus 6.7.1 software. The FEA analysis result (and Abaqus output) presented consist nodal stress code check and displacement.
The following FEA are carried out (and presented in this document) to ensure that the Codes and COMPANY general Specifications requirements are satisfied.
1.3 Structure Description
1.3.1 Structures Geometry
The design booster compressor skid framing refer to Dwg. No.9701-90-DWG-005 (see Appendix B) Booster Compressor Skid Framing Support Plan & Elevation.
Figure 1.3 Booster Compressor Skid Framing Support
Figure 1.4 Booster Compressor Tie-Down Pads
2 SUMMARY AND CONCLUSION
The FEA of deck’s plate under Booster Compressor Tie-Down Pads are carried out using FEA program, Abaqus 6.7.1. The computer program used to check Von Misses Stress and evaluated them with allowable stress and safety factor (FOS) for ultimate strength assessment.
The outcome of the present analysis note demonstrates the von mises stress check of the deck’s plate under Booster Compressor Tie-Down Pads to sustain the most severe combinations of operating, storm, and earthquake loads design.
2.1 Summary
This FEA evaluates several load configurations of plate under Booster Compressor Tie-Down Pads. Table 2.1 are showing the summary of maximum value von mises stress every case, where UC at all element less than 1. The further details of the output listing are provided in appendix B.
Table 2.1 Summary Von mises stress
Element
|
Case
|
Load Combination
|
Load (N)
|
Von Misses Stress (MPa)
|
Allowable Stress (MPa)
|
UC
|
Check
|
832
|
Dead Load
|
Deadload
|
148,257.33
|
147.400
|
155.100
|
0.950
|
OK
|
21169
|
DL + W op (y)
|
Deadload + Wind Operation
|
149,967.33
|
149.100
|
155.100
|
0.961
|
OK
|
832
|
DL + W e (y)
|
Deadload + Wind Extreem
|
156,797.33
|
155.900
|
186.120
|
0.838
|
OK
|
832
|
DL + E (y)
|
Deadload + Earthquake
|
180,606.33
|
179.600
|
232.650
|
0.772
|
OK
|
Table 2.2 are showing the summary of safety factor (FOS) for ultimate strength assessment every case, where FOS element at all case more than 1. The further details of the output listing are provided in appendix B.
Table 2.2 Summary FOS
Element
|
Case
|
Load Combination
|
Load (N)
|
Von Misses Stress (MPa)
|
Allowable Stress (MPa)
|
FOS
|
Check
|
832
|
Dead Load
|
Deadload
|
148,257.33
|
147.400
|
155.100
|
1.052
|
OK
|
21169
|
DL + W op (y)
|
Deadload + Wind Operation
|
149,967.33
|
149.100
|
155.100
|
1.040
|
OK
|
832
|
DL + W e (y)
|
Deadload + Wind Extreem
|
156,797.33
|
155.900
|
186.120
|
1.194
|
OK
|
832
|
DL + E (y)
|
Deadload + Earthquake
|
180,606.33
|
179.600
|
232.650
|
1.295
|
OK
|
2.2 Conclusion
Base on the FEA summary of deck’s plate under Booster Compressor Tie-Down Pads at section 2.1 almost all elements every case in safe condition. These designs can be used for construction with assumption Booster Compressor and Tie-Down Pads position not change.
3 DESIGN PARAMETERS
3.1 Design Life
The design life of the Maleo MoPU is 15 years starting from 2006. By the time this analysis is performed, the MoPU has operated in Maleo field for about 6 years.
3.2 Design Codes and Standards
The analysis is complied in accordance with the 21st Ed. API RP2A (2000), Ref [1], as the Maleo MoPU is categorized as fixed platform and the 9th Ed. of the American Institute of Steel Construction – WSD (AISC, 1989), Ref [2].
3.3 Units System
The following unit system shall be adopted in the analysis files:
· Great lengths : in meter (members’ length, joints co-ordinates, water depth)
· Small lengths : in mm or cm (tubular diameter, wall thickness, etc).
· Forces & moments : in kN and kNm.
· Masses : in metric tons.
· Stresses : in N/mm2 (MPa).
· Angles : in degrees (wave heading, phases’ angle).
3.4 Material Properties
Structural steel is considered as grade A36 that is deemed equivalent with ABS grade A, B, CS and DS for varying plate thicknesses. Table 3.1present summary of grade A36 material properties used in the analysis.
Table 3.1 Steel Properties
Property
|
Unit
|
Value
|
Remark
|
Young’s modulus (E)
|
kN/m2
|
2.06 × 108
|
-
|
Poison’s ratio (n)
|
-
|
0.3
|
-
|
Shear Modulus (G)
|
kN/m2
|
0.8 × 108
|
G = E/[2(1 + n)]
|
Steel Density (rs)
|
tons/m3
|
7.85
|
-
|
Min yield strength (Fy)
|
MPa
|
235
|
For ABS grades being used
|
Min tensile strength (Fu)
|
MPa
|
482
|
-
|
3.5 Allowable Stress
Since the Maleo MoPU is a fixed platform during its operation, several environment conditions in accordance to Ref [1] are considered in the analysis. These conditions are operating and extreme. In addition, earthquake condition is accounted to test the robustness of the platform during in-service. Allowable stress for Plate A36 shall be complying with equation (3.1) accordance to Ref [2] & Ref [1]. All stress components in the left-hand side of below inequalities are calculated as equivalent von Mises stress.For Bening 0.66 Fy
3.6 Von Mises Criterion
For an elastic body that is subject to a system of loads in 3 dimensions, an equivalent stress can be computed using von Mises criterion for ductile failure. This criterion can be formulated in terms of von Mises stress. Eq. (3.2) mathematically express von Mises stress (svm) as a product of principal stresses of any point within a body.
 |
(3.2)
|
Where s1, s2 and s3 are principal stresses. Eq. (3.3) presents von Mises stress expression in 3 dimensional Cartesian coordinate terms. Eq. (3.4) and (3.5) are von Misses expression in-plane and simple linear Cartesian, respectively. In all of these equations, t denotes shear stress acting on a Cartesian plane.
 |
(3.3)
|
 |
(3.4)
|
 |
(3.5)
|
Based on this criterion, a ductile material is predicted starts to yield when the Von Mises stress reach the yield strength (svm = Fy). Ref [5] specifies Fy = 235MPa for the above steel material. This reference requires that the safety factor (FOS) for ultimate strength assessment of the plate is to be taken not less than unity. Since the Maleo MoPU is classified as fixed platform, allowable stress as defined in Section 3.4 is used in FOS calculation. The following equations are used to calculate the FOS.
Reference:
[1] American Petroleum Institute, “Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Working Stress design.” API Recommended Practice 2A-WSD, 21ed, December 2000.
[2] American Institute of Steel Construction (AISC), “Manual of Steel Construction – Allowable Stress design”, 9th Ed, 1989.
[4] Santos (Madura Offshore) Pty Ltd., “Maleo MOPU Leg to MAT connection FEA”, 9701-90-RPT-0005, 2011.
[5] Santos (Madura Offshore) Pty Ltd., “Maleo MOPU Booster Compres Support Local Check”, 9701-90-CAL-0001, 2012.
[6] Santos (Madura Offshore) Pty Ltd., “Longitudinal & Transverses Non Tight Bulkheads MAT”, Dwg. No. 4914-4 Sht 1of 2.
