Development Company
SUCTION PILE DESIGN SPECIFICATION EMDC-BRE-C-JS-0332.1001
This cover page is a record of all revisions of the standard/specification identified above by number and title. All previous cover pages are hereby superseded and are to be destroyed.
0 REV
9/12/00 DATE
BY
CHKD
ENG APPV
GLH EMDC APPV
Issued for Reference Project DESCRIPTION
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EXXONM OBIL DEVELOPMENT COMPANY — HOUSTON, TX
EXXONMOBIL DEVELOPMENT COMPANY GENERAL SPECIFICATIONS
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
Page i of ii Rev. 0
Sept. 12, 2000
Table of Contents 1.
2.
3.
Scope...............................................................................................................................1 1.1.
Introduction............................................................................................................1
1.2.
Design Assumptions..............................................................................................1
References......................................................................................................................3 2.1.
Project Specifications............................................................................................3
2.2.
Industry Codes and Standards.............................................................................3
2.3.
Design Reports......................................................................................................4
Definitions.......................................................................................................................5 3.1.
General..................................................................................................................5
3.2.
Terms.....................................................................................................................5
3.3.
Acronyms...............................................................................................................7
4.
Pre-Design Deliverables................................................................................................8
5.
Pile Loads.......................................................................................................................9
6.
5.1.
Floating Systems...................................................................................................9
5.1.1.
Mooring.........................................................................................................................................9
5.2.
Subsea Systems..................................................................................................11
5.3.
Suction Pile Installation Analysis.........................................................................11
5.4.
Transportation......................................................................................................11
5.5.
Lifting...................................................................................................................11
Geotechnical Suction Pile Design..............................................................................12 6.1.
Suction Pile Design Basis...................................................................................12
6.1.1. 6.1.2.
Floating Systems.........................................................................................................................12 Subsea Manifold..........................................................................................................................14
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7.
8.
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
Page ii of ii Rev. 0
Sept. 12, 2000
6.2.
Design.................................................................................................................14
6.2.1. 6.2.2.
General........................................................................................................................................14 Suction-Installed Anchor Pile Sizing (Geotechnical Design)........................................................15
6.3.
Suction Pile Installation.......................................................................................19
6.3.1. 6.3.2. 6.3.3.
Embedment..................................................................................................................................19 Removal......................................................................................................................................19 Pile Retrieval and Reinstallation..................................................................................................20
6.4.
Geotechnical Design Deliverables......................................................................20
Structural Suction Pile Design....................................................................................22 7.1.
General Design Requirements............................................................................22
7.1.1. 7.1.2.
Floating Systems.........................................................................................................................22 Subsea Systems...........................................................................................................................22
7.2.
Design and Analysis............................................................................................22
7.2.1. 7.2.2. 7.2.3. 7.2.4. 7.2.5. 7.2.6. 7.2.7. 7.2.8. 7.2.9. 7.2.10.
In-Place Finite Element Analysis..................................................................................................22 Allowable Stresses and Usage Factors.........................................................................................23 Buckling Checks..........................................................................................................................23 Fatigue Analysis..........................................................................................................................25 Mooring Chain Padeye Connection..............................................................................................26 Transportation and Lift................................................................................................................26 Materials, Welding and Fabrication.............................................................................................27 Classification of Structural Steel.................................................................................................27 Tolerances....................................................................................................................................29 Metocean Criteria........................................................................................................................29
7.3.
Corrosion.............................................................................................................29
7.4.
Paint....................................................................................................................30
7.5.
Appurtenances....................................................................................................30
Company Intervention..................................................................................................31
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EXXONMOBIL DEVELOPMENT COMPANY GENERAL SPECIFICATIONS
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
Page 1 of 30 Rev. 0
Sept. 12, 2000
1. Scope 1.1.
Introduction This document describes the general geotechnical and structural design requirements for the suction installed mooring piles (hereafter referred to as suction piles) for floating systems (Surface Wellhead Platform (SHWP), Floating Production Storage Offloading vessel (FPSO), oil offloading system, and riser supports), and subsea systems (subsea manifolds). This specification assumes the suction piles will be designed and installed in normally to slightly over-consolidated clayey soils. The primary objective of this document is to provide a basis and methodology for the design of the suction piles. This document summarizes the design requirements and specifies the various analyses that shall be performed in the design and verification of the suction piles. This document will also serve as a reference guide for third party verification efforts and classification society review. The CONTRACTOR shall use the most recent issue of the codes and standards listed in Section 2.0 as part of this specification. If any conflicts exist between this specification and the codes and standards referenced, the requirements of this specification shall govern. Any conflict in Specifications shall be submitted to the COMPANY for resolution.
1.2.
Design Assumptions Functional requirements are:
Must withstand the long-term static and dynamic loading.
Capacities shall be degraded as appropriate for (a) cyclic degradation of soil strength, (b) creep, and (c) pile-soil setup for the initial loading history of the piles. Analysis shall reflect positioning tolerances for installation (refer to installation specifications). Suction-installed anchor piles shall be designed for the same in-place suite of global load conditions as the component that it supports. The suction piles supporting floating system moorings shall be designed to withstand maximum intact and one line damaged mooring loads.
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EXXONMOBIL DEVELOPMENT COMPANY GENERAL SPECIFICATIONS
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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Sept. 12, 2000
The design process shall comprise the following engineering activities: 1. Global sizing of the suction pile based on soil strength properties; 2. Global sizing of the suction pile to ensure that the pump-in suction pressure is acceptable for the available soil strength (i.e., soil plug stability check); 3. Global sizing of the suction pile to ensure that the pump-out pressure is acceptable for the available soil strength, in case that anchor removal is necessary; 4. Structural design for maximum installed loads and soil reactions, including detailed design of the padeye area for the local stresses due to the mooring line loads and the padeye castings; 5. Structural design for pump-in operation and, if applicable, for pump-out/pull-out operation; 6. Design of appurtenances and pile top configuration for installation and for recovery. The following design assumptions have been adopted in developing these specifications. These assumptions shall be honored by the structural designer, suction pile fabricator and the foundation installation contractors in developing their respective designs.
Flow channels will not occur near the pile-soil interface
Full frictional resistance can be developed along the pile wall requiring minimal soil displacement during installation Reverse end-bearing can potentially be developed, requiring that the pile enter the soil with low displacement of soils (e.g. minimal heave of soil plug at the mudline) body.
The pile behaves with respect to foundation behavior mechanics as a rigid
These assumptions collectively result in the following restrictions in design:
Piles shall include no internal or external ring stiffeners
Soil plug heave and internal pressures will be monitored to ensure that rate of installation for self-weight and suction penetration occurs at a rate that will not cause plug heave. Refer to installation specifications.
Piles shall behave elastically for all design loading conditions.
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EXXONMOBIL DEVELOPMENT COMPANY GENERAL SPECIFICATIONS
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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Sept. 12, 2000
2. References The DDCV (hull and mooring system) is considered to be a site-specific, fixed offshore installation that will be unflagged. Det Norske Veritas (DNV) has been engaged by COMPANY to provide classification services to ensure the various elements of the DDCV are designed, constructed, installed and commissioned in compliance with a variety of classification society rules and guidelines, recommended practices, codes, etc. DNV’s classification scope of work is outlined in DNV letter RNA440/CRCO/DWT2940-J-11, titled SWHP Class Requirements.
2.1.
Project Specifications EMDC-EDE-G-NS-0230.2001, Design of Deck Structures for Floating Offshore Platforms EMDC-EDE-G-NS-0106.2001, Fabrication of Deck Structure for Offshore Platforms EMDC-EDE-G-MS-0260.2002, Structural Materials EMDC-EDE-G-KS-0213.2001, Structural Welding and Inspection of Offshore Platforms EMDC-EDE-G-MS-0262.4007, Painting General Requirements EMDC-BRE-C-NS-0217.1001, DDCV Hull Casting Specification EMDC-BRE-G-ZS-0121.1002, General Glossary Specification
2.2.
Industry Codes and Standards American Institute of Steel Construction AISC ASD
Specification for Structural Steel Buildings – Allowable Stress Design
American Petroleum Institute (API) API RP 2A
Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms
API-RP-2SK
Recommended Practice for Design and Analysis of Stationkeeping Systems for Floating Structure
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Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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GENERAL SPECIFICATIONS
Rev. 0
Sept. 12, 2000
American Welding Society (AWS) AWS D1.1
Structural Welding Code – Steel
Det Norske Veritas (DNV)
2.3.
DNV MOU Rules
Rules for Classification of Mobile Offshore Units
DNV CN 30.1
Classification Note No. 30.1, Buckling Strength Analysis
DNV CN 30.2
Classification Note No. 30.2, Fatigue Strength Analysis for Mobile Offshore Units
DNV Marine Ops
Rules for Planning and Executing Marine Operations
DNV RP B401
Recommended Practice Cathodic Protection System Design (1993)
Design Reports None referenced.
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3.
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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Sept. 12, 2000
Definitions 3.1.
General Unless noted below, terms and acronyms used in this specification shall be interpreted as defined in General Glossary Specification, EMDC-BRE-G-SZ-0121-1002, or by a standard English dictionary if not listed in the General Glossary Specification.
3.2.
Terms The following terms shall be interpreted in this Specification as meaning: Wave Frequency Range -
The range of frequencies containing wave energy. Wave frequencies are specified as cycles per second or radians per second. It is also used as a specific description. For example, wave frequency motions are responses with frequencies within the wave frequency range.
Low Frequency Range -
The range of frequencies lower than the wave frequency range. It is also used as a specific description. For example, low frequency motions are responses with frequencies within the low frequency range.
Cut-off Frequency
-
A specified frequency defining the division between wave frequency and low frequency ranges. A typical cut-off frequency for the DDCV global performance analysis is 0.03 Hz (33.3 seconds).
Sea-state
-
The environmental conditions (wind, waves, and current) existing at a given site over a finite period of time.
Wave Spectrum
-
A statistical description of regular wave components giving the distribution of energy used to analytically model an ocean wave environment.
Wind Spectrum
-
A statistical description of wind components giving the distribution of energy used to analytically model an ocean wind environment.
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Suction Pile Design Specifications
GENERAL SPECIFICATIONS
EMDC-BRE-C-JS-0332.1001 Page 6 of 30 Rev. 0 Sept. 12, 2000
Data Sheet:
-
A data sheet is a type of report used to facilitate data transfer between disciplines. Data sheets typically contain particular response summaries to be used as design input for riser, structural, etc. designs. Detailed technical descriptions of the assumption, models, etc., will be reserved for the main analysis report.
Anchor Points
-
Locations on the seabed where anchors are set.
Catenary
-
The shape assumed by a mooring line suspended between the mooring fairlead and seafloor. If buoys or clump weights are included, then a compound catenary shape results.
Dip Zone
-
This is the segment of mooring line between touchdown points at the seabed when the line is most loaded and most slack. It is the segment that incurs the most abrasion because of cyclic contact with the seabed. Generally, a length of chain is provided in the dip zone.
Dynamic Mooring Line Analysis
-
A time domain finite element analysis procedure where the line tensions due to fairlead wave frequency motions are explicitly computed. This may also be done in frequency domain, which is fast but involves simplifying assumptions that may impact accuracy. It may be done in time domain which is longer, more expensive but most accurate. API and Class Societies recognize both methods.
Mean Offset
-
The amount of horizontal movement of the vessel from the initial position to its equilibrium position when the system restoring forces and moments balance mean environmental forces and moments.
Quasi-Static Analysis
-
A frequency domain analysis procedure where the maximum mooring line tension is obtained by simple horizontal translation of the fairlead point by the predicted maximum offset position. Line tensions due to low frequency motions will in general be accurately estimated but may be significantly underestimated for wave frequency motions.
Restoring Force
-
The longitudinal and transverse forces and moments created by the resultant of all of the mooring lines’ tensions as the
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Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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Sept. 12, 2000
vessel moves in response to environmental loads. A restoring force curve plots restoring force versus vessel offset.
3.3.
Break Strength Safety Factor
-
This is the ratio of the new catalog break strength of the mooring line component to its predicted maximum tension. The catalog break strength used will be for the nominal line diameter after deducting corrosion and abrasion allowances.
Seabed Slope
-
Defined from the bathymetry of the seafloor and linearized within the touchdown range for each anchor.
Alpha Factor
-
a measure of pile-soil setup versus time expressed as a percentage of the fully consolidated pile soil frictional resistance
-
Remotely Operated Vehicle
Acronyms ROV
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GENERAL SPECIFICATIONS
Rev. 0
Sept. 12, 2000
4. Pre-Design Deliverables The following deliverables shall be issued by CONTRACTOR to COMPANY during early project execution: Suction Pile Design & Analysis Methodology – A methodology document providing detailed discussions and descriptions of the specific software and other analytical tools that will be used to perform both preliminary pile sizing and final design. Modeling techniques, design constraints, assumptions, and methodologies will be described. Method of performing preliminary sizing to optimize the pile aspect ratio (L/D) versus steel weight prior to final suction pile design shall also be described. The analysis tools and methods to be used shall be approved in advance by COMPANY. Analysis Matrix Data Sheet – A data sheet identifying the controlling load cases, seastates, and environmental headings to be analyzed during the detailed suction pile design. This will include the mooring line tensions for the intact and damaged conditions. It will also include the build-up and decay data used to evaluate soil strength degradation due to cyclic loading. Anchor Design Data Sheet - A data sheet listing the installation assumptions for verticality, horizontal placement and padeye rotation (see Section 6.2.2.5).
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Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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Sept. 12, 2000
5. Pile Loads 5.1.
Floating Systems 5.1.1.
Mooring 5.1.1.1.
Environmentally Derived Loads (Current and Storm)
Environmentally derived loads applied to the suction pile by the mooring line will be obtained from dynamic mooring analyses approved by COMPANY. The environmentally derived loads will generally consist of a mean offset load with a cyclic component. Seasonal Environmental Conditions In offshore locations where extreme environmental conditions occur on a seasonal basis, a two-tier criterion will be employed in evaluating the suction pile response to the applied loading. This criterion includes: (1) seasonal environmental loads, generally less severe than maximum storm conditions, applied immediately after the hull is attached, and (2) extreme environmental loads that may occur several months after installation is complete. The CONTRACTOR shall evaluate the following loading conditions: Load Condition 1: The appropriate storm loading (minimum 10 year storm) or, if applicable, loop current loading will be applied, and mooring line loads calculated corresponding to the environmental conditions that prevail at the time the project schedule indicates the anchor lines are attached to the hull. This calculation of mooring line loads will consider both intact and one-line damaged conditions. Load Condition 2: The maximum storm load for the 100-year return period extreme environmental conditions will be applied, and mooring line loads calculated. This calculation will also consider both intact and oneline damaged conditions.
Non- Seasonal Environmental Conditions
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EMDC-BRE-C-JS-0332.1001
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In offshore locations where extreme environmental conditions occur on a nonseasonal basis, Load Conditions 1 and 2 shall be consolidated, and the design shall consider the most severe environmental loading applied at the time the anchor lines are attached to the hull. If it is necessary to commence suction pile design work prior to the completion of dynamic mooring analyses, then suitably factored quasi-static loads may be used for preliminary suction pile design.
5.1.1.2.
Long Term Sustained Loads
The CONTRACTOR shall evaluate the pretension loads in the anchor piles for the maximum pretension allowed in the operating manual.
5.1.1.3.
Build-up and Decay
Mooring tensions will be provided to the suction pile designer that will allow the impact of cyclic loading on the soil strength to be assessed. The mooring tensions will be evaluated during the build-up and decay of the 100 year design event. A rainflow or similar method shall be used to provide the geotechnical designer with the cyclic components of the loading. These cyclic components will be filtered into low frequency T>30 seconds and wave frequency T<30 seconds prior to the rainflow analysis is performed.
5.1.1.4.
Fatigue Loads
Mooring tensions will be provided to the suction pile structural designer that will allow the impact of fatigue loading to be assessed. The mooring tensions will be evaluated for the full scatter diagram. These cyclic components will be filtered into low frequency T>30 seconds and wave frequency T<30 seconds prior to the fatigue summary. It is assumed that a spectral fatigue analysis will be performed. The suction pile designer will be provided with both the wave frequency RMS tension and zero crossing period and the low frequency RMS tension and zero crossing period. The suction pile designer can calculate the fatigue damage from these two types of loadings in two ways as outlined in API RP 2SK. The fatigue damage can be calculated for these terms separately and then summed. A combined RMS tension and zero crossing period can be calculated and used to determine the total damage. The CONTRACTOR must verify the validity of the method chosen as described in API RP 2SK.
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5.2.
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
Page 11 of 30 Rev. 0
Sept. 12, 2000
Subsea Systems The pipeline manifold will apply mainly steady-state loads to the anchor piles. These loads include compression, lateral, moment, and torsion. Short-term loads that may be applied include those due to thermal expansion and contraction. For the subsea manifold foundation, the CONTRACTOR shall evaluate the suction pile response to the following loading conditions: Load Condition 1: The appropriate installation loading shall be applied. Such loading may include, but not limited to, loads due to the mating of the manifold and pile in addition to the manifold submerged weight, flowline/pipeline pull-in load while connecting to manifold, etc. Load Condition 2: The appropriate in-service loading shall be applied. Such loading may include, but not limited to, submerged weight of the manifold, loads due to fluid flow and thermal expansion/contraction, etc
5.3.
Suction Pile Installation Analysis Loads associated with the suction pile embedment and removal shall be developed based on installation scenarios. The installation CONTRACTOR shall be consulted to jointly develop the installation scenarios for analysis.
5.4.
Transportation The structure shall be analyzed for the forces imposed during transportation. The forces shall include the self weight of the suction anchor and inertial loads developed from a motion analysis.
5.5.
Lifting The suction pile and lift attachments (padeyes, trunnions, etc.) shall be analyzed for lift for the loads during installation and removal onto the transport barges, and during mooring installation
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Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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Sept. 12, 2000
6. Geotechnical Suction Pile Design 6.1.
Suction Pile Design Basis 6.1.1.
Floating Systems 6.1.1.1.
Dynamic Mooring Analyses
Final suction pile design shall be based on the pile loads obtained from dynamic mooring analysis, and the following load factors and soil resistance factors: For Intact Mooring System:
Load Factor =
Soil Resistance Factor = 0.80
1.30
For One-line Damaged Mooring:
Load Factor =
Soil Resistance Factor = 0.90
1.20
In the calculation of suction pile axial capacity (when subject to maximum dynamic loading), pile suction may be considered for environmentally derived loads such as peak storm loads, provided the pile is fitted with a closed top and means of closure for vent openings and the pump suction fitting are provided and it can be demonstrated that the suction will not degrade over the life of the structure (minimum 25 years). Pile suction shall not be considered for long duration loads, such as the pretension load, the maximum anchor load at the mean offset due to 100-year metocean events, or for loop current loads(if applicable), unless suitable data are available for demonstrating that suction can be safely relied upon.
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6.1.1.2.
Sept. 12, 2000
Pretension Load Analysis
The pretension load analysis shall evaluate the suction pile design to minimize the potential for long term pile displacements due to creep. The analysis shall evaluate the suction pile design for the condition of the vertical component of the pretension anchor load resisted by pile axial skin friction. For maximum allowable pretension in the anchor the following load and resistance factors shall be used:
Axial Load Factor =
1.30
Axial Soil Friction Resistance Factor =
0.30
6.1.1.3.
Sustained Tension Load Analyses (Riser Supports Only)
Suction piles may also support other component structures (e.g. risers) that apply vertical uplift and lateral loading to the foundation. For these structures, the CONTRACTOR shall evaluate the anchor pile design to minimize the potential for progressive pile failure and long term pile displacements due to creep. The analysis shall evaluate the anchor pile design for the tension load resisted by the pile axial skin friction and, if applicable, lateral loads and associated moments. For load cases that include lateral loading and/or moments, the soil skin friction resistance from the mudline to the first point of zero lateral deflection shall be ignored. The CONTRACTOR shall account for soil degradation due to cyclic loading, as appropriate. For maximum allowable pretension in the anchor the following load and resistance factors shall be used:
Axial Load Factor =
1.30
Axial Soil Friction Resistance Factor =
0.30
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6.1.2.
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
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Subsea Manifold The compressive load analysis shall evaluate the anchor pile design for adequate bearing capacity. The analysis shall consider dead loads - D, live loads - L (e.g. fluid flow and thermal expansion/contraction) and moments associated with both loading conditions. The following load and resistance factors shall be used:
6.2.
Load Factors =
1.3D+1.5L
Axial Soil Resistance Factor =
0.7
Design 6.2.1.
General Suction piles are typically used in conjunction with taut-leg type mooring lines, due to this pile design's capability of withstanding horizontal and vertical loads. The suction piles shall be designed to withstand maximum intact and one line damaged mooring loads. The design process shall comprise the following engineering activities: 1. Global sizing of the suction pile based on soil strength properties; 2. Global sizing of the suction pile to ensure that the pump-in suction pressure is acceptable for the available soil strength (i.e., soil plug stability check); 3. Global sizing of the suction pile to ensure that the pump-out pressure is acceptable for the available soil strength, in case that anchor removal is necessary.
6.2.2.
Suction-Installed Anchor Pile Sizing (Geotechnical Design) 6.2.2.1.
General
CONTRACTOR and the approved geotechnical design subcontractor shall size the suction pile (length, diameter, wall thickness and internal bracing) to
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provide adequate capacity for the mooring line loads (combined lateral and axial loading), and soil properties, in conjunction with the load and soil resistance factors defined in section 6.1.1 and 6.1.2, above. The CONTRACTOR shall calculate the potential configurations of the chain between the seabed and the chain attachment point at the anchor, and use the most severe conditions in design. COMPANY will provide the CONTRACTOR soil properties for design purposes.
6.2.2.2.
Floating Systems
In carrying-out the design, the CONTRACTOR shall consider, pile set-up (Section 6.2.2.4), soil creep, and cyclic degradation of soil strength in calculating pile capacity. The CONTRACTOR's design may be performed using limiting equilibrium, finite-element or other analysis methods acceptable to COMPANY. If design studies use analysis tools other than finite element methods, a finite element analysis shall be carried-out to verify the final recommended design. Finite element analyses of suction piles, with the padeye located significantly below the mud-line, indicate the load versus displacement (vertical) behavior of the pile is hyperbolic. Thus, the failure load can be somewhat arbitrarily defined. Consequently, displacement may become an issue and the CONTRACTOR shall verify that the pile displacements (vertical and horizontal) do not permit excessive deformations or potential loss of suction along the backside of the pile.
6.2.2.3.
Subsea Systems
In carrying out the suction pile design for subsea manifold, the CONTRACTOR shall consider pile set-up (Section 6.2.2.4) and, if applicable, soil creep and cyclic degradation of soil strength in calculating pile capacity. The CONTRACTOR shall perform the design using analysis methods appropriate for the principal modes of loading being considered and resistance mechanisms of the suction pile.
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GENERAL SPECIFICATIONS
6.2.2.4.
Rev. 0
Sept. 12, 2000
Pile-Soil Set-up
The CONTRACTOR shall provide suitable data or perform an effective stress analysis to determine the buildup and dissipation of excess pore pressures in the soil as a result of installing the pile. The data or study shall address both the soil outside the pile, and the soil plug. These results shall be used to estimate pile set-up (gain in pile capacity with time) for both internal and external axial friction resistance. CONTRACTOR shall document results graphically by plotting "pile-soil set-up (%)" versus "time after pile installation" for the selected pile geometry. The CONTRACTOR shall confer with COMPANY during the development of the pile soil set-up curves prior to their application for design of the suction pile. The CONTRACTOR's suction pile design will incorporate pile-soil set-up as described in the following load conditions. In order to perform these analyses, it will be necessary for the CONTRACTOR to obtain the pile installation schedule from the installation CONTRACTOR. The pile design drawings shall state the minimum time between pile installation and attachment to the hull. 6.2.2.4.1.
Floating Systems Seasonal Environmental Conditions Load Condition 1: The CONTRACTOR shall use a pile-soil set-up factor(s) for both internal and external skin friction corresponding to the project schedule's time interval between the pile last installed, and the attachment of the hull. If the sequence of pile installation is known, then different set-up factors may be applied to individual piles. Load Condition 2: The CONTRACTOR shall use a pile-soil set-up factor(s) for both internal and external skin friction corresponding to the project schedule's time interval between the pile last installed and the first potential occurrence of the extreme environmental load. If less than full pile set-up has occurred, then different set-up factors may be applied to individual piles. Non-Seasonal Environmental Conditions The design procedures described under Load Condition 2, above, shall apply. Pretension Load Case The design procedures described under Load Condition 1, above, shall apply.
6.2.2.4.2.
Subsea Systems Compressive Load Case
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Sept. 12, 2000
The CONTRACTOR shall use a pile-soil set-up factor for both external and internal skin friction corresponding to the project schedule's time interval between pile installation and application of the structure. If the pile is designed to mobilize the end bearing capacity, then full end bearing resistance shall be used to calculate pile capacity at the time of installation.
6.2.2.5.
Pile Sensitivity Studies
The following applies only to suction piles for surface wellhead platform, floating production storage offloading and oil offloading systems. The suction pile design calculations shall include sensitivity studies to demonstrate that the solution has been optimized and is robust. These studies shall include the following: 1. Calculation of the change in pile capacity for the following three scenarios: a) Pile penetration of 1.5 and 3 m less than design penetration. b) Full penetration but assuming the top 1.5 and 3m are disturbed during installation (or later eroded) and, therefore, provide negligible axial and lateral resistance. c) Padeye is 1.5m higher and lower than the selected optimum depth 2. Develop a suite of pile capacity curves, for cases shown in the figure below in which the pile is not in vertical alignment and for misorientation of the padeye. The analysis will consider: a) the pile out of vertical alignment + 2.5º; +5.0º; -2.5º; -5.0º in directions both parallel and perpendicular to the mooring line direction (no torsional misorientation) b) a "misorientation" analysis considering the padeye torsionally rotated 5º and 10º out of alignment. c) Integration of these analyses (a and b) will be necessary to account for combined effects.
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EMDC-BRE-C-JS-0332.1001
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Sept. 12, 2000
.
Installation tolerance governing for lateral resistance
In s ta lla tio n to le r a n c e g o v e r n in g fo r v e r tic a l r e s is ta n c e
Seabed
Seabed
Mooring line
Minimum penetration
M o o r in g lin e
M in im u m p e n e tr a t io n
Target penetration
T a rg e t p e n e tr a tio n
M o o r in g lin e
.
.
O r ie n t a tio n to le r a n c e
3. Investigate the influence on pile capacity of relocating one or more piles a distance of 60m from the original design location, in directions both inboard and outboard relative to the DDCV. Calculate required changes in pile length to satisfy design load changes and/or estimated soil variability for this movement (if soil variability is not explicit in the geotechnical report, modified load and resistance factors should be developed in consultation with COMPANY). 4. Establish the minimum relative x-y spacing of the piles within pile clusters, should the original pile configuration need to be modified during installation.
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GENERAL SPECIFICATIONS
6.3.
Rev. 0
Sept. 12, 2000
Suction Pile Installation 6.3.1.
Embedment The CONTRACTOR shall calculate the allowable differential pressure inside the pile during self-weight penetration, in order to prevent the channeling of water through the soil plug and along the outside wall of the pile. The allowable differential pressure shall not exceed 50% (F.S. = 2.0) of the value to cause incipient flow (maximum differential pressure). The CONTRACTOR shall report the results as a graph of "allowable differential pressure and maximum differential pressure" versus "penetration below the mudline". The pile geometry will also be selected to ensure that the maximum vacuum required for pump-in embedment will not exceed 50% of the critical vacuum at which soil plug heave inside the pile can occur. This will provide a safety factor against soil plug uplift of 2.0 during all phases of the pump-in embedment. The pump-in suction required at increasing pile penetration will be plotted against the suction causing soil plug uplift in order to check that the required safety factor will be met anywhere between self penetration and maximum penetration. For the purpose of sizing the suction pumps, an upper bound, static undrained soil shear strength profile will also be developed for the stratigraphy. For pile installation, alpha values less than the reciprocal of the soil sensitivity shall not be used. The design and the upper bound undrained soil shear strength profiles will be provided to the installation CONTRACTOR for pump design.
6.3.2.
Removal The suction pile design shall account for removal of the pile within a seven (7) day period after embedment proceeds. Pump-out pressure shall be based on pile resistance corresponding to seven (7) days of set-up. This pressure shall not exceed 75% of the pressure at which plug failure occurs. It shall be assumed that the recovery vessel will be capable of applying an uplift load at the top of the pile, which will be well in excess of the submerged pile weight. Since the pump-out pressure is typically higher than the maximum embedment vacuum, the pump-out pressure may govern the structural design of the pile top. This shall be taken into account when defining the suction pile geometry.
6.3.3.
Pile Retrieval and Reinstallation It may be assumed that the vessel used for the suction pile recovery will be capable of applying a lifting load in excess of the submerged pile weight at the top of the pile. The lifting load shall be calculated based on the capacity of
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GENERAL SPECIFICATIONS
Rev. 0
Sept. 12, 2000
the type of winches used on large anchor handling vessels, and on the strength of the wire rope and spreader bar used for installation of the suction anchor. With regard to reinstallation of the pile, analyses will be performed to determine the minimum x-y distances that the pile can be reinstalled relative to the original pile location. In the calculations, the following plug conditions will be assumed: (1) the soil plug is extruded and remains intact when the pile is retrieved; and (2) the soil plug is removed with the pile.
6.4.
Geotechnical Design Deliverables The CONTRACTOR shall submit a final report and drawings as final documentation of their work. The final report shall contain the following information: 1. Introduction - Describes the project and the work performed. 2. Summary and Recommendations – Documents main findings, design assumptions (including those in Section 4, above) and recommendations. 3. Background Information – Containing sections including:
Detailed discussion of analysis procedure.
Validation of design procedure
Soil parameter selection.
Design loads.
4. Design and Analysis – Contains design analyses, results and recommendations for:
Pile capacity and pile geometry.
Soil reactions for structural design.
Chain configuration.
Results of sensitivity studies (see Section 6.2.2.5)
Pile installation by self-weight and suction, and pile retrieval/removal.
Pile installation procedures and recommended instrumentation and data acquisition systems to monitor and record real time during installation: (a) pile position; (b) tilt in both the north-south and east-west directions; (c) pile orientation (torsion) relative to the desired orientation; (d) penetration rate; (e) internal differential pressure; (f) total penetration; (g) internal soil plug heave; and (h) pump speed
5. Suction Pile Structural Design Report – a technical report providing a detailed description of the structural design of the suction piles, including drawings showing the particulars and dimensions of the mooring piles.
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Sept. 12, 2000
6. Appendices - Contains details of each of the sections in the main body of the report. The CONTRACTOR shall submit a draft written final report for review by COMPANY prior to submitting the final report. The CONTRACTOR shall incorporate in the final written report all reasonable modifications to the draft final report requested by COMPANY.
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Sept. 12, 2000
7. Structural Suction Pile Design 7.1.
General Design Requirements 7.1.1.
Floating Systems The suction pile structure shall be designed to withstand the maximum loads (and corresponding soil reactions) applied by the mooring line, the maximum vacuum required for pile embedment, and the maximum internal pressure required for pile pump-out (Sections 5.1, 5.3, 5.4 and 5.5).
7.1.2.
Subsea Systems The suction pile structure shall be designed to withstand loads applied by the following: maximum operational loads, transportation, lifting, lowering, the maximum vacuum required for pile embedment, and the internal pressure required for pile pump-out (Section 5.1, 5.3, 5.4 and 5.5) and recovery.
7.2.
Design and Analysis 7.2.1.
In-Place Finite Element Analysis Stresses in suction piles used for the mooring of floating systems shall be determined by detailed shell element finite element analysis. Finite element stresses should be obtained for the main body (cylinder), pile top, internal stiffening and padeye. Gravity and mooring loads (Section 5.0) should be considered. The effect of the surrounding soil shall be accounted for either by applying the soil reactions to the model or as part of the stiffness model. Detailed shell element finite element analysis is generally not required for subsea system foundations.
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GENERAL SPECIFICATIONS
7.2.2.
Rev. 0
Sept. 12, 2000
Allowable Stresses and Usage Factors The nominal Von-Mises (equivalent) stress shall not exceed the maximum permissible stress as calculated below:
p = 0 Fy Where: 0 = basic usage factor Fy = material yield strength The basic usage factor shall be 0.8 for the maximum in-place loading condition and 0.6 for normal operating, transportation, lifting, lowering and recovery It should be noted that the permissible stresses are based on the fiber stresses for simple beam analyses, and the membrane, or mid-thickness, stresses for finite element analyses using plate elements. For laterally loaded plates also exposed to inplane (e.g. membrane) stresses, the surface Von Mises stress computed at the middle of the plate field (e.g. midway between stiffeners and/or girders) shall not exceed the following:
p = (0 + 0.1) Fy However, the nominal elastic stress calculated in the middle of the plate field due to lateral pressure acting alone shall not exceed 0Fy.
7.2.3.
Buckling Checks The buckling strength of the piles shall be checked in accordance with the DNV Rules for Classification of Mobile Offshore Units, Part 3, Chapter 1, Section 5 and DNV Classification Note 30.1 Buckling Strength Analysis. The maximum permissible value of the usage factor, p, shall be calculated as follows:
p = 0 Where: = coefficient depending on the type of structure and reduced slenderness, , as per Table 7.1 0 = basic usage factor, as per Section 7.2.2
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Table 7.1 – Coefficient Type of Structure Unstiffened flat plate panels Girders, beams, stiffeners on plates and shells Columns, beam-columns Shells of single curvature Shells of double curvature
Sept. 12, 2000
0.2 1.10 1.00
0.2 < < 1.0 1.10 1.00
1.0 1.10 1.00
1.00 1.00 0.80
1.025 – 0.125 1.050 – 0.250 0.840 – 0.200
0.90 0.80 0.64
Buckling checks for piles used to moor floating systems shall be based on the results of the finite element analysis.
7.2.4.
Fatigue Analysis Fatigue damage is to be considered for the in-place condition. The total fatigue damage times a safety factor of 10 shall be less than unity for the design life of the platform. Fatigue analysis of suction piles used to support subsea systems is required only for piles subjected to significant dynamic loading. Analysis methodology should follow DNV CN 30.2. The fatigue analysis of suction piles used for mooring floating systems shall be based on a spectral analysis using appropriate S-N data and stresses from highly refined FE models. The detailed fatigue analysis for stiffened plate structural details shall be consistent with the fatigue analysis principles set forth in "FPSO Fatigue Methodology Specification", COMPANY Report No. 99-3053 (herein referred to as FMS). The general methodology for the fatigue analysis of suction piles used for mooring shall be consistent with Section 8.4 of the DDCV Hull Structural Design Specification, EMDC-BRE-C-NS-0201.1001. The following guidance can be found in this document:
General Methodology
Selection of Fatigue Details
Selection of Fatigue Control Points and S-N Curves
Computation of Stress RAO’s
Fatigue Damage Calculations
Misalignment
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7.2.5.
Suction Pile Design
Sept. 12, 2000
Treatment of Welds to Improve Fatigue Life
Mooring Chain Padeye Connection Mooring chain padeye connections shall be designed to satisfy both strength and fatigue requirements. The padeye shall be designed for a maximum load equal to the break strength of the chain multiplied by a load factor of 1.2. The orientation of the design load shall be the calculated orientation plus a +/- 5 degree allowance for vertical misalignment and a +/- 10 degree allowance for torsional misalignment. A usage factor of 0.8 shall be applied in conjunction with this load. Design calculations for padeyes shall include bearing, pull out, shear, axial, bending, and combined stress checks for the critical sections of the main plate, cheek plate and all weldments. Plate thickness, plate size, and pin hole diameter shall be proportioned to accommodate the selected shackle without excessive strain to the shackle and pin. The mooring chain padeye connection shall also be analyzed using a local finite element model that includes the primary structural elements framing into the padeye,. The finite element model shall be used to verify strength for the maximum design load and to determine hot spot stresses for fatigue analysis. The finite element analysis shall be perfomed in accordance with the general guidelines given in Section A5.9 of EMDC-EDE-G-NS-0230.2001, Design of Deck Structures for Floating Offshore Platforms. In many cases, a cast padeye arrangement will be utilized. For cast padeyes, a detailed finite element analysis using three-dimensional “brick” elements shall be performed.
7.2.6.
Transportation and Lift The analysis and design of the suction anchor and the lift attachments for transportation and lift shall be in accordance with Section 5.6 and Section 5.7 of EMDC-EDE-G-NS-0230.2001, Design of Deck Structures for Floating Offshore Platforms.
7.2.7.
Materials, Welding and Fabrication Requirements for materials, welding, welding inspection and fabrication for the suction pile structure are given in project specifications as noted in Section 2.1.
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7.2.8.
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
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Sept. 12, 2000
Classification of Structural Steel All steel in the suction piles shall be classified as shown in the table below. The design requirements for a given structure are a function of this classification. Primary structure is defined as those structural elements essential to the overall integrity of the suction pile. The following table summarizes the classification suction pile components:
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GENERAL SPECIFICATIONS
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Table 7.2 – Classification of Suction Pile Structural Steel
Structura l Category A
B
C
E
Item Description
Yield Type
Min. Yield Strength
Toughness Requirements
(Mpa)
T < 0.75" (19mm)
0.75"(19mm) T 2.0"(50 mm)
T > 2.0" (50mm)
I
248
NA
NA
NA
II
344
CV2
CV2Z4
CV2ZX
III
412
CV2
CV2ZX
CV2ZX
PRIMARY MEMBERS
I
248
NT
CV1
CV2
Pile Shell, Pile Shell Stiffeners, Diaphragm plates, Diaphragm brackets, Pile Top, Pile Top Stiffener
II
344
CV1
CV2
CV2X
III
412
CV2
CV2Z5
CV2X
SECONDARY MEMBERS
I
248
NT
NT
CV1
Chain Hang-Off padeye
II
344
CV1
CV1
CV1
III
412
CV2
CV2
CV2Z5
I
248
NA
NA
NA
II
344
CV2Z4
CV2ZX
CV2ZX
III
412
CV2Z4
CV2ZX
CV2ZX
PRIMARY MEMBERS w/ Through thickness loading
MOORING CONNECTION PADEYE AND LIFTING POINTS Padeye main plates and attachment points SPECIAL
I
Cast Padeyes or other cast components
II III
See Casting Specification DDCV Hull Casting Specification EMDC-BRE-C-NS-0217.1001,
Notes: NT – No impact testing required CV1 – Charp V-Notch category 1 as defined below CV2 – Charp V-Notch category 2 as defined below Z4 – Through thickness testing required - API S4 Z5 – Sulfur limit to improve through thickness properties - API S5 X – API RP2Z Prequalified steel - API S11 ZX – API S4+S11 Table 7.3 Structural Steels Minimum Performance Requirements Toughness Classes
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EXXONMOBIL DEVELOPMENT COMPANY GENERAL SPECIFICATIONS Specified Minimum Yield Strength Mpa (ksi) 240/248
Yield Type I II III
Suction Pile Design
EMDC-BRE-C-JS-0332.1001
Specifications
Page 28 of 30 Rev. 0
Sept. 12, 2000
NT(1)
CV1(1)
CV2 (1)
(No testing)
Test @ LAST
Test @ 30C (54F) below LAST
20 J
20 J
20 J
(35/36)
(15 ft lbs)
(15 ft lbs)
(15 ft lbs)
344
35 J
35 J
35 J
(50)
(25 ft lbs)
(25 ft lbs)
(25 ft lbs)
412
45 J
45 J
45 J
(60)
(35 ft lbs)
(35 ft lbs)
(35 ft lbs)
Notes: For some grades of steels the steel standard requirements for impact testing may be more or less stringent than required in the specification. Where there is a conflict between the specification and any steel standard requirements, the more stringent requirements have been specified in the steel specifications.
7.2.9.
Tolerances Requirements for construction tolerances of the suction piles are given in project specifications noted in Section 2.1.
7.2.10.
Metocean Criteria Design conditions for the transportation analysis will be determined by COMPANY as required by the project on a case by case basis.
7.3.
Corrosion The suction piles shall have a cathodic protection system design to provide the required design life for the piles. All internal and external surfaces (submerged or buried) shall be protected by galvanic anodes. The design basis and calculations shall be conducted in accordance with DnV RP B401 Recommended Practice Cathodic Protection System Design (1993). Due to the stagnant conditions, the design current density for internal surfaces of the pile shall be 1.0 mA/ft2. The anodes shall be placed above the mudline. The CONTRACTOR shall provide COMPANY, for review and approval, a Cathodic Protection System Design Report for the suction piles.
7.4.
Paint The CONTRACTOR shall have the piles painted as follows:
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GENERAL SPECIFICATIONS
7.5.
Rev. 0
Sept. 12, 2000
1.
The section of the pile that will be designed to be above the mudline including the pile top will be painted a high visibility yellow.
2.
The pile will be marked in 0.25 meter increments on the side of the pile in order to gauge penetration during self embedment and embedment. The distance from the pile bottom will be marked numerically every meter in 1/3-meter high numbers.
3.
The pile top will be marked with a pile number and lift weight.
Appurtenances The structural design CONTRACTOR will be required to interface with the installation CONTRACTOR to determine the requirements for installation interfaces. These should include but are not limited to the following: 1. Suction Port – Typically a 20” API flange. This flange must be set high enough to allow the suction skid to land and maneuver with out interfering with anodes or other appurtenances on the pile top. 2. Lowering padeyes or trunions – The configuration of these appurtenances shall properly interface with the lowering equipment used by the installation CONTRACTOR 3. Vent ports – the top of the pile shall be fitted with closable vents used to vent water during self-penetration and closed for suction embedment. These ports shall be large enough to allow the pile to be self penetrated at a sufficient speed without risking pile instability or damage to the soil around the pile due to “blow-by”. 4. Bullseye – Used to view the pile verticality using the ROV. 5. Transponder frames used to house the transponder used to locate the piles during installation. These frames shall be placed such that the transponders do not interfere with the installation operations and such that they can easily be recovered. 6. Handling Padeyes – The CONTRACTOR shall install such padeyes as required by the installation contractor for the handling of the suction piles.
8. Company Intervention The following table highlights all of the areas in this specification that require COMPANY intervention or approval in the design process suction piles. SECTION 1.1 Scope
4.0 Deliverables
INTERVENTION Any conflict in Specifications shall be submitted to COMPANY for resolution. Pre-design deliverables that CONTRACTOR
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GENERAL SPECIFICATIONS
Rev. 0
SECTION
5.0 Pile Loads 5.1 Floating Systems 5.1.1 Mooring 5.1.1.1 Environmentally Derived Loads 6.0 Geotechnical Suction Pile Design 6.2 Design 6.2.2 Suction Installed Anchor Pile Sizing 6.2.2.1 General 6.0 Geotechnical Suction Pile Design 6.2 Design 6.2.2 Suction Installed Anchor Pile Sizing 6.2.2.2 Floating Systems 6.0 Geotechnical Suction Pile Design 6.2 Design 6.2.2 Suction Installed Anchor Pile Sizing 6.2.2.4 Pile Soil Set-Up 6.0 Geotechnical Suction Pile Design 6.2 Design 6.2.2 Suction Installed Anchor Pile Sizing 6.2.2.5 Pile Sensitivity Studies
6.0 Geotechnical Suction Pile Design 6.4 Geotechnical Design Deliverables
7.0 Structural Suction Pile Design 7.2 Design and Analysis 7.2.10 Metocean Criteria
Sept. 12, 2000
INTERVENTION must clear with COMPANY are : (1) Analysis methodology (software and analysis tools); (2) Data sheet identifying load cases; (3) Pile design data sheet listing assumptions for verticality, placement tolerance, rotation. Mean and offset storm loads derived from mooring analyses shall be approved by COMPANY
COMPANY will provide the CONTRACTOR soil properties for design purposes
Suction pile capacity analysis methods shall be approved in advance by COMPANY.
CONTRACTOR shall confer with COMPANY during the development of the pile soil set-up curves prior to their application to design
When checking the effect of shifting 60m from the target location, if soil variability is not explicit in the geotechnical report, modified load and resistance factors should be developed in consultation with COMPANY CONTRACTOR shall submit draft written final design report for review by COMPANY prior to submitting the final report. Design conditions for the transportation analysis will be determined by COMPANY as required on the project on a case by case basis.
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