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

Tree connector threadedonto bottom of Composite Valve Block

Horizontal subsea tree (HXT)

Tubing hanger

Wellhead (HP) housing

Conductor (LP) housing

Tree connector

Subsea Tree TerminologySubsea Tree Terminology

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 13628-7 Table B.6 Principal parameters

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

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 Tree Connector / Wellhead Fatigue Design PracticeDesign 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

Wellhead Integrity in 13628-1• ISO 13628-1• 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• Fatigue

– Analyses Input Data– Local Response Analyses– Global Load Analyses

• Design Criteria (general, no new factors)– Extreme/Survival (different i ISO 13628-4/-7/13624-1)– Fatigue

• Reporting– Objective

• 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