ISO 13628-1
Wellhead Integrity
A systems approach needed
requering information from several
traditional disciplines
Introduction: System overview
• Rig and riser
loads on well
• Complex
interaction
between drilling
and well
components
• Soil interface
Subsea Tree Terminology
Horizontal subsea tree (HXT)
Tubing hanger
Tree connector threaded
onto bottom of Composite Valve Block
Tree connector
Wellhead (HP) housing
Conductor (LP) housing
History
• Fatigue damage to wellhead West of
Shetland in the 80’ies. Assumed
damage caused by vortex induced
vibrations VIV
• Observed movements in several
wellhead in the North Sea in 2005,
2006 and 2010
– One wellhead was retrieved and
inspected without findings
• Fatigue calculations from different
parties with differences in results of
several magnitudes.
• “Wellhead Fatigue – Method Statement
Update” in 2009, as a joint effort, Statoil
and DNV, with input from WH suppliers.
ISO 13628-7 Table B.6 Principal parameters
• Further updated and issued by the WH
JIP
Background
• Increased re-entry on existing wells on the
Norwegian continental shelf.
• Complex well designs and operations including
multilateral- and smart wells increases drilling
time. Increased amount on intervention and workover operations on sub sea wells.
• Life time extension of wells, Specified total drilling
time for new complex wells can be up to 300 days
• Increased size of drilling rigs and weight of
BOP’s, on new rigs up to 400 ton
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Wellhead and connectors – current status
Dry wellheads and XT’s moved offshore and subsea
Designed for pressure containment
Designed with little consideration to fatigue i.e.
– Fatigue life not specified in design
– Complex geometry, difficult to calculate
Inspection not possible, no logging of fatigue damage
rates normally available and operational history records
are not “too” detailed.
Advanced Wellhead Calculation methods in place, but:
– By nature, sensitive to variations in input
parameters.
• The data as cement level etc. need to be logged for as built
doc’s of the well.
– Calculation models not calibrated with offshore field
measurements
• some info from instrumentation, but more needed for
calibration of analytical tool and identification of effects like rig
trim etc.
Tree Connector / Wellhead Fatigue
Design Practice
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Design Practice Main Points
• The fatigue assessment of subsea tree connectors
and wellheads requires analyses and
understanding the interactions of:
• The floating vessel and drilling and marine
riser
• Subsea well stack-up
• Wellhead and downhole casing assemblies
• Cement and soils
• The methodology reflects industry experience and
offers a balance among analysis efficiency
compatible with project and operational constraints
• The methodology is based upon dynamic analysis
of models to obtain global response loads of the
floating drilling unit, riser, and wellhead
• Finite element analyses are then used to translate
local stresses and to define stress concentration
factors for structural details of interest.
• Fatigue lives will be based upon local stresses and
fatigue resistance defined by S-N curves
Wellhead Integrity in 13628-1
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ISO 13628-1
Appendix XX Wellhead Integrity TOC
(based on Wellhead Fatigue Analyses Method and NORSOK U001)
Introduction
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Objective
Scope
Safety Philosophy
System Description
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Well
Drilling
CWO
Loads
Load Conditions
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General - Operational phases
Extreme/Survival (NORSOK U001)
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Installation, drilling, CWO
Fatigue
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Drilling, CWO
Analyses Methodology
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Modelling Philosophy
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Extreme/Survival
Fatigue
Analyses Input Data
Local Response Analyses
Global Load Analyses
Design Criteria (general, no new factors)
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Extreme/Survival (different i ISO 13628-4/-7/13624-1)
Fatigue
Reporting
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Objective
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Interaction with other stakeholders
Reporting format
What to report
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Stress concentration factors
Component capacities for pressure, tension, moment (combined loads)
Component/connector stiffnesses
Cumulative fatigue damage