Global Strength Assessment Requirement of Ageing Fixed Offshore Structure with Joint Anomalies

Global Strength Assessment Requirement of Ageing Fixed Offshore Structure with Joint Anomalies

Global Strength Assessment Requirement of Ageing Fixed Offshore

Structure with Joint Anomalies

M S Ayob, Technology & Engineering Division, PETRONAS, MALAYSIA

B S Wong, Technology & Engineering Division, PETRONAS, MALAYSIA

ABSTRACT

There are increasing numbers of ageing structures in the water of South China Sea (SCS) which come along with various types structural integrity issue, such as visual crack, member flooding, lamination joint, shallow gas and excessive marine growth Advanced structural integrity analyses are needed to effectively quantify and assess the integrity status of the ageing structure to ensure continuing serviceability of the structure An integrity assessment methodology on lamination joints, cracked joints and flooded members are presented in the paper Non-linear plastic collapse analysis is performed by introducing local joint flexibility

in the laminated tubular joint, together with incorporation of cracked joints and flooded members modeling to check for the structure’s ultimate strength, the Collapse Reserve Strength Ratio (CRSR) as well as the joint capacity The requirement of the CRSR is validated through the target reliability requirement for SCS The structure’s Reserve Strength Ratio (RSR) is then identified when the joint capacity is exceeded and compared to the CRSR

of the structure The comparison will give indication of the necessary remedial and mitigation action need to be taken to strengthen the joint A case study is presented to give an insight on the methodology and findings

Keywords: Ageing structure, Integrity, Non-linear plastic collapse analysis, Reserve strength

ratio

1 INTRODUCTION

To date, there are increasing numbers of ageing fixed offshore structure worldwide as well as those installed in Malaysia water Generally, there are various degradation mechanisms that can cause the structural failure, fire and explosion events The degradation mechanisms can

be classified into time/age driven degradation mechanism, and event (non-age) driven degradation mechanism The time/age driven degradation mechanism is associated with damage process which can progress over a period of time, i.e corrosion, fatigue and marine growth The event (non-age) driven degradation mechanism is not associated with time and it can occur any particular point of time without prior indication, i.e increase in deck load, wave-in-deck and damaged members These degradation mechanisms trigger the need of re-assessment of the structures to determine their fitness for purpose Kallaby et al., (1994) [2] ISO 19902 [4] and NORSOK-006 [6] have listed similar structural assessment initiators According to API RP2A [1], Kallaby et al., (1994) [2], Krieger et al., (1994) [3], ISO 19902 [4] and O’Connor et al., (2005) [7], the structural assessment is generally divided into (1) Screening Level; (2) Design Level; and (3) Ultimate Strength Level The objective of these 3 (three) levels of assessment is to ensure the structure has adequate capacity to withstand the damage mechanism However, the complexity of the analysis increases as move from Screening Level to Ultimate Strength Level As complexity increase, conservatism incorporated in the analysis reduces to enable the engineer to gain better prediction of structural responses

Ultimate strength analysis shall be adopted as structure fail to comply Design Level requirement By conducting Ultimate Strength analysis, the system strength of the structure is obtained instead of its individual component, where its redundancy and damage tolerance can

be well-defined The outcome from the analysis can efficiently assist in understanding the structure failure mechanism and correctly defining type of mitigation measure Non-linear Plastic Collapse Analysis is often used to quantify the ultimate strength by introducing the Reserve Strength Ratio (RSR), which can be thought of factor of safety for the structure as a whole against its 100-years design load According to MARINTEK [5] and PRSB [8], the basic procedures of used to conduct non-linear plastic collapse analysis are: Basic load cases for dead loads are progressively increased to unity (1.0); Environmental load cases incremental till the specified relative load level is reached, or until a defined characteristic displacement is attained; Each load is applied in predefined steps; The deformed nodal coordinates are updated after each load step to record the changes in the geometry; Elements stiffnesses are then computed according to the updated geometry With such, the stiffness values can be assembled in each load step; At each load level, plastic capacity of the element (ends and mid) is checked, whether the applied force exceeding the capacity If such event happens, the load level will automatically scaled to make the applied force is ‘equal’ to the yield condition; As element forces reached to their yield surfaces, a plastic hinge is incorporated to demonstrate the material non-linearity The plastic hinge will be removed if the member later is unloaded and becomes elastic; Load increment is automatically reversed if global instability is detected; RSR of the structure is retrieved at the structure’s collapse point

In this paper, a 3-legged fixed offshore structure with joint anomalies is selected for ultimate strength assessment The purpose of this study is to demonstrate the methodology used to

quantified the ultimate strength of a structure with anomalies and propose cost efficient mitigation measure

2 STRUCTURAL MODEL

A 3-legged wellhead platform, located in water depth of 75.3m is taken as the subject of study It has been in service for more than 34-years and has exceeded its design life of 25-years The platform contains several anomalies as identified in the latest underwater inspection campaign There are as summarized as follow:

• Three (3) numbers of visual cracks

WN21, conductor frame guard at EL (-) 5m

WN25, conductor frame guard at EL (-) 5m

• Three (3) lamination leg node cans:

WN35, Leg A at EL (-) 14m

WN69, Leg A at EL (-) 34m

WN103, Leg B at EL (-) 59m

• Nine (9) flooded members detected:

Figure 1 shows the structural model and location of the identified anomalies The detail description of the structure is given in Table 1

Figure 1 Structural Model and its Associated Anomalies

Table 1 Descriptions of the Structure

3.1 General

The Non-linear plastic collapse analysis is used in the reassessment of ageing 3-legged structure to demonstrate that the platform has adequate strength and stability to withstand the specified loading criteria with the identified joint anomalies, but without collapse Failure of the platform is caused either by failure of critical members and/or joints and/or piles, foundation failure or a combination of these

The strength of the platform is quantified by Reserve Strength Ratio (RSR) In other word, RSR measures the reserve strength in a platform beyond the 100-years environmental load The RSR is defined as the ratio between base shear / overturning moment value at platform collapse and 100-years environmental load: Ecollapse / E100 The governing inequality for the reassessment can be written as:

E F D F F

R

F

R

F

F

S

F D = Partial load factor on non-environmental load = 1.0

F E = Partial load factor on environmental loads =1.0

F F = Partial material factor for foundation = 1.0

To suit the format of plastic collapse analyses, the inequality is rewritten as follows:

S

F

Substituting the factors, the inequality will reduce to:

E D

R

R

0

1

,

The analyses are based on the updated structural model, including the most recent information about the structure The analyses include pile analyses and non-linear soil-pile-structure interaction Gravity loads, buoyancy and functional loads are incremented to a load factor of 1.0 Environmental loads for the loading direction in question are subsequently incremented until the collapse of the platform

3.2 Environmental Load Modelling

The wave and current forces for the non-linear collapse analysis are based on Morrison’s equation with account of hydrodynamic coefficients, wave kinematics factor, current blockage factor, using Stokes 5th order kinematic theory Directionality and joint probability

of occurrence of the wave and current are to be considered For joint probability, 100-year directional wave criteria are combined with the coincident 100-years directional current criteria and 100-years with 1-hour mean omni-directional wind criteria Marine growth profile

is included and modeled to capture the additional drag force exerted on the structure The detailed description of each parameter used which accordance to API RP2A [1] and PRSB [8]

is shown below:

• Wave Theory: Stoke 5th Order wave theory Maximum load and overturning moment are identified by running 360 steps of wave cycle at 1o interval

• Hydrodynamic Coefficients

Drag coefficient, Cd = 1.06 (increase by 6% to account for anodes and other jacket appurtenances)

Inertia coefficient, Cm = 2.00

• Wave kinematics factor of 0.90 is adopted

• Current blockage factor for 3-legged structure

• Current stretching effect is considered as linear

• Wake encounter effect is turned off

• Doppler effect is neglected, thus the coefficient and loads are computed using the actual wave periods

• Marine growth profile

3.2.1 Metocean Criteria

Wave and current are assumed to be inline in the wave propagation directions Mean Sea level

is 75.3m The highest astronomical tide is 0.90 m; lowest astronomical tide is 0.00 m The 100-years storm surge is 0.60 m Total twelve loading directions are analysed, with an equal spacing of 30o from each other (Figure 2)

Figure 2 100-year Storm and Current Directionality

3.3 Member Modelling

Primary members are assigned initial imperfections and plasticity parameters to produce member buckling in accordance with the Chen column curve Initial imperfections are assigned in the direction of the environmental loading on each member Thus, different initial imperfection patterns are used for each of the loading directions

3.4 Foundation Modelling

The foundation (i.e piles and soil parameters) is modeled as integral part of the structure, based on the information supplied by soil investigation report and pile make-up drawing Each pile is modeled by a series of beam elements, one for each soil layer Nodes are positioned in the centre of each soil layer Non-linear springs are attached to each node to model the properties of each soil layer Soil characteristic for each soil layer is referred to the soil investigation report The effect of the scour which obtained from underwater inspection report is included in the model

3.5 Anomalies Modelling

To accounts for the effect the lamination, revised wall thickness of the joint-cans are modeled For joints with crack, the connecting members are modeled as non-structural member Flooded members are modeled as flooded and no longer buoyant in the water Local joint flexibility is incorporated in the analysis to further examine the behavior of the laminated joint-can before the collapse of the platform and to extract the RSR of the joint at its point of failure to compare with platform’s Collapse RSR (CRSR)

60.0 o

TN

PN

3.6 Acceptance Safety Criteria

The adopted minimum acceptance safety criterion is:

• Reserve Strength Ratio

1.50 manned structures

1.32 unmanned structures

The above criteria is derived based on a JIP project, Further Structural Integrity Assessment (FSIA) conducted in 1997 for 9 (nine) Malaysia fixed offshore structures

3.7 Analysis Procedures

A finite element model is generated for the non-linear plastic collapse analyses Initial imperfections are introduced on all primary members Joint capacity check is included for all laminated joints Piles are included as structural elements with soil springs defined at each soil layer The collapse mechanism for each loading direction (12 directions) is reviewed in detail This comprises identification of critical components, validation of the critical components’ behaviour, and validation of the numerical accuracy of the analyses

4 NUMERICAL RESULTS AND DISCUSSIONS

4.1 Non-Linear Plastic Collapse Analysis

The analysis is carried out for twelve selected loading direction (30o from each sector) to establish the ultimate strength of the structure, which measure in term of Collapse Reserve Strength Ratio (CRSR) The applied loading condition is combination of:

• Computed 100-years directional environmental condition

In all directions, the peak load associated with the first peak where there is a noticeable deviation in the global P-δ behaviour upon the unloading-reloading process is retrieved as CRSR value Collapse base shear for each direction is calculated to demonstrate the total allowance of the structure in resisting the base shear at the point of collapse Table 2 tabulates the summary of the CRSR and Collapse Base Shear for each direction The combination contour plot of total base shear, CRSR and collapse for the twelve principal loading directions are presented in Figure 3 As indicated, lowest CRSR, 1.72 is found on direction West (60o)

An interpretation of the analysis results is given in Table 3 Plot of collapse mechanisms and load deflection curve for direction West (60o) is given in Figure 4

Table 2: CRSR and Collapse Base Shear

Figure 3 Combined Contour Plot

Figure 4 P-δ Curve

Base Shear (MN)

Collapse Base Shear (MN)

Table 3 Failure Mechanism

4.2 Joint Capacity Check

The joint capacity check is carried out to examine the capacity of the lamination joint prior to the structure total collapse Three (3) lamination joints have been identified from latest underwater inspection report Their corresponding node in finite element model is 3110, 5110 and 7130, respectively Capacities of all these nodes are checked by introducing local joint flexibility Table 4 tabulates detailed of the nodes that under the joint check

Table 4 Failure Mechanism

A comparison is conducted between the RSR when joint capacity is exceeded and the collapse RSR Most of the joint are failed before the global collapse of the structure The results are presented in Table 5, 6, 7, 8 and 9 for five (5) selected directions

Table 5 Direction 30o – CRSR=3.30 Table 6 Direction 60o – CRSR=1.72

Node Brace RSR Node Brace RSR Node Brace RSR

3110

304 1.13

5110

310 1.58

7130

315 OK!

688 <1.00* 709 <1.00* 735 OK!

WNW(30o) 3.30 Buckling of leg failure at EL.+3 till EL.-14 followed by lateral soil failure

SSW(120o) 2.08 Buckling of leg failure at EL.+5 till EL.-14 followed by lateral soil failure

S (150o) 4.59 Buckling of leg failure at EL.+5 till EL.-14 followed by lateral soil failure

SSE(180o) 6.38 Buckling of leg failure at EL.+5 till EL.-14 followed by lateral soil failure ESE(210o) 5.45 Buckling of leg failure at EL.+3 till EL.-14 followed by lateral soil failure E(240o) 7.00 Buckling of leg failure at EL.+3 till EL.-23 followed by lateral soil failure ENE(270o) 2.67 Buckling of leg failure at EL.+5 till EL.-14 followed by lateral soil failure

Node Brace RSR Node Brace RSR Node Brace RSR

3110

304 1.00

5110

310 1.57

7130

315 OK!

Table 7 Direction 180o – CRSR=6.38 Table 8 Direction 210o – CRSR=5.45

Node Brace RSR Node Brace RSR Node Brace RSR

3110

304 2.46

5110

310 1.43

7130

315 OK!

688 3.78 709 <1.00* 735 OK!

Table 9 Direction 240o – CRSR=6.99

Note * exceeding joint moment capacity

5 CONCLUSION

The 3-legged structure has identified with serious anomalies, i.e 3 (three) visual cracks, 9 (nine) flooded members and 3 (three) lamination joints The lowest RSR is 1.72 for West direction (60˚); satisfied the stipulated unmanned platform requirement The collapse mechanisms identified for most of the direction are buckling of leg followed by lateral soil failure

Through the analysis, it is concluded that under current anomalies, the structure still having adequate strength to fulfill global integrity requirement for service As such, the existing cracks and laminated issue do not impact overall structure strength even most of the joints have fail before the collapse of the structure The structure has adequate redundancy to redistribute the load to other load path (adjacent member)

Based on the assessment, it is expected that grouting or member strengthening may not necessary as the failure mechanism is eventually governed by soil failure Any mitigation or enhancement on members does not give significant impact to RSR value or improvement in the system strength

For reassessment of ageing offshore structures, it is always recommended to perform non-linear plastic collapse analysis to quantify the ultimate strength of the structure before resorting to any mitigation measure It is because sometime costly change/strengthening work may only improve the platform’s ‘design level’ performance, but bring insignificant impact to improve its ultimate strength as demonstrated in this paper for the 3-legged structure

Node Brace RSR Node Brace RSR Node Brace RSR

3110

304 2.18

5110

310 1.25

7130

315 OK!

688 6.06 709 <1.00* 735 OK!

Node Brace RSR Node Brace RSR Node Brace RSR

3110

304 4.19

5110

310 2.33

7130

315 OK!

688 6.35 709 <1.00* 735 OK!