1. Structural verification and certification of two existing platforms
Presenter:
Tarun Rewari
President
Aryatech Marine & Offshore Services Pvt. Ltd. India

2. Platform Status

3. PROJECT SCOPE
ARYATECH India has been awarded a contract from an overseas client to carry out a structural verification and certification of two existing platforms operated by Company. The platforms are in shallow water depth of 18.5m, approximately km West offshore
To this aim ARYATECH will carry out Structural Re-Assessment Analyses including Inplace, Seismic and Push-over (for storm loading and seismic) analyses in order to CERTIFY the platforms against the requirements of ISO (Ref 3.1-1) and ISO ( Ref 3.1-2).
The structural model was provided by Company and will be verified and rectified according to the relevant information provided by the Company such as Geotechnical & Metocean data, subsea inspection reports and as-built equipment/facilities layout and structural drawings,
Software used for design/analysis was SACS, from Bentley USA.

4. PLATFORM DESCRIPTION
The Platforms were installed in 1987 and consists of a production platform of 100m x 50m in plan dimension and an accommodation platform (LQ) of 20m x 30m size. The accommodation area is connected to the North-West of the production area through a bridge. The production platform is made of 12 mini tubular 6-legged jackets and LQ is made of 2 mini jackets each having a size of 10m x 20m in plan. The jackets/modules are joined at their top by latticed Module Support Frames (MSF).
This platform is combination of 2 jacket (6 legs) connected by a truss arrangement, which is different from conventional jacket which is a single jacket structure. Generally Topside is on one jacket but in this case topside is on truss platform created by connecting all jackets.

5. PLATFORM RE-ANALYSIS PROCEDURE
Input
Crude SACS model and PSI
Drawings and Survey Reports
Soil Report and Company Design Basis
Platform pictures
BOC
Prepared Basis of Certification stating design data, code and procedure
Review and Approval by Client
Review and Approval by Class
SACS Model
Prepared Detailed SACS model
In-place Analysis
Perform In-place Strength Analysis
Perform In-place Collapse Analysis
Seismic Analysis
Perform Seismic SLE strength Analysis
Perform Seismic ALE/SLE Collapse Analysis
Assessment
Any Strengthening / Repair is suggested to Client
In-place and Seismic Analysis is Re-analysed with updated model
PLATFORM RE-ANALYSIS PROCEDURE

6. AREAS OF CONCERN
Latest environment data showed 13.0 M wave which will have huge impact force on jacket as it is above deck level , hence after discussion with client and Certification authority, the wave height was reduced to 12.2 m.
Model was based on re-created drawings by Client based on on-site inspection.
Fatigue is not performed in this platform as per Client Scope.
One Leg was damaged in Impact. As no data was available, hence assumption is made. It was modeled with approx. 300mm bend and 40% dent using SACS dent cross-section feature.
Soil foundation (P-Y, T-Z and Q-Z) is based on foundation report provided by Client.
All these assumptions, procedures and design data were provided in Basis of Certification.

7. MORE INFORMATION ON BASIS OF ANALYSIS
The design re-assessment was performed in compliance with the requirements of ISO
For certification agency a quality check list was prepared and submitted in each re-analysis report.
SACS model was prepared in the reassessment and certification process to reflect the main jacket configuration during the operating lifetime including- the strengthening measures already carried out or required by the analysis for certification.
Following reinforcements and upgrades are introduced in the SACS model, in particular:
All leg-pile annuli grouted;
Additional members foreseen for reinforcement.
Platform has old truss-work arrangement which connects the 2 jackets. As it has been severely corroded and broken from few places it is simulated in SACS models by reducing it stiffness by 40%.

8. DETAILED SACS MODEL PREPRATION
Inputs
Basis of Certification
Crude SACS Model and PSI
Soil Report
Drawings and survey reports
Platform pictures
SACS Modification
SACS model refining ( joint numbers , group numbers, effective lengths, X brace definitions, material definition joint offsets)
Member Outer Diameter and Thickness modification w.r.t corrosion as per Survey report.
Deck dead and Live load verification and modifications.
Member releases for Gusset Connections
SACS hydrodynamic modelling (Waves, current, wind, Marine Growth)
Future Splash zone corrosion for 10 year life time.

9. INPLACE ANALYSIS Inputs Met ocean Data ISO 19902 Code
Basis of Certification
Output
Platform weight is 705 tonnes including Dead and Live loads.
Maximum Storm Base Shear is 320 tonnes for 12.2m wave.

10. Inplace Analysis Summary of Results
Item
Description
Values
Jacket Members
No. of Member with UC >1
Highest UC
0.89
Jacket Tubular Joints
No. of Joint with UC >1 1
1.06
Maximum Pile Forces (MN)
Axial Tension
1.0
Axial Compression
2.0
Pile Displacement (CM)
Maximum Displacement
10.8
Factor of Safety for the Piles Axial Load Bearing Capacity for Storm Condition
Compression
Min. Factor of Safety in Piles
1.18
Min. Required Factor of Safety
1.25
Tension
1.09
Factor of Safety for the Piles Axial Load Bearing Capacity for Operating Condition
1.64
1.50
1.68

11. Notes
Maximum punching UC of 1.03 is found in an X brace joint 1X02 on Frame 1. This member was checked for redundancy and jacket was found adequate in strength. 
Maximum lateral displacement of found to be more than allowable of 3.77 cm. In general all Piles show lateral deflection greater than allowable for storm condition.

12. INPLACE ANALYSIS- PUSHOVER RESULTS
The RSR is defined as:
RSR = Base Shear at Structure Failure
100 year return wave Base Shear
The maximum wave height of 13.2m for 100 year wave is reduced to 12.2m , due to deck clearance issue. Therefore pushover analysis is based on 12.2m wave height instead of New met-ocean data 100 year maximum wave height of 13.2 m.
The pushover analysis shows that structure is able to withstand only 1.20 times the environment load ( Base Shear = 2.9MN) of maximum base shear due to 12.2m wave height.

13. SEISMIC ANALYSIS Inputs Met ocean Data ISO 19902 Code
Basis of Certification
Output
Jacket Maximum Natural Period – 1.19
Seismic Base Shear is 400 tonnes for SLE condition (0.35g)
Minimum Pile FOS ( Allowable is 1.25)
All Member UC is less than 0.8 for SLE conditions except 1 Leg which has UC These legs are grouted and has been provided equivalent thickness.
All Joint punching UC are less than 0.9 with maximum UC of 0.89.
Pile-head Displacement is 10.3cm whereas allowable is 4.0cm. This is with new soil configuration provided by client.

14. SEISMIC ANALYSIS- PUSHOVER RESULTS
Pushover Analysis Results are summarised –
Structure is able to withstand only 1.38 times the earthquake load ( Base Shear = 5.5 MN) of ISO-ELE/API-SLE condition (0.35g) in positive X direction, see Figure 10.1.
Structure is able to withstand only 1.32 times the earthquake load ( Base Shear = 5.3 MN) of ISO-ELE/API-SLE condition (0.35g) in positive Y direction, see Figure 10.2.
As pushover analysis for ISO-ELE/API-SLE condition demonstrates that the structure has RSR < 1.5 with respect to SLE loads as specified in COMPANY SOW ( Ref 2.2-2).

15. ASSESSMENT
Output
No Strengthening / Modification is suggested for Living Quarter platform

16. AREAS OF CONCERN CLIENT No areas of concern. Certifying Authority
Agree on the Concept stated in Basis of Certification for platform.
Structure RSR is less than required , hence certification will be based on exclusions.
Need more detailed level documentation, we stated it will be provided in Report not in Basis of Certification.