1. ISO 13628-1 Wellhead Integrity
A systems approach needed requering information from several traditional disciplines

2. Introduction: System overview
Rig and riser loads on well
Complex interaction between drilling and well components
Soil interface

3. 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

4. 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.
Further updated and issued by the WH JIP
ISO Table B.6 Principal parameters

5. 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 work-over 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

6. 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.

7. Tree Connector / Wellhead Fatigue Design Practice
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

8. Wellhead Integrity in 13628-1
ISO
Appendix XX Wellhead Integrity TOC
(based on Wellhead Fatigue Analyses Method and NORSOK U001)
Introduction
Objective
Scope
Safety Philosophy
System Description
Well
Drilling
CWO
Loads
Load Conditions
General - Operational phases
Extreme/Survival (NORSOK U001)
Installation, drilling, CWO
Fatigue
Drilling, CWO
Analyses Methodology
Modelling Philosophy
Extreme/Survival
Analyses Input Data
Local Response Analyses
Global Load Analyses
Design Criteria (general, no new factors)
Extreme/Survival (different i ISO /-7/ )
Reporting
Interaction with other stakeholders
Reporting format
What to report
Stress concentration factors
Component capacities for pressure, tension, moment (combined loads)
Component/connector stiffnesses
Cumulative fatigue damage