Wellhead Fatigue Analysis : Surface casing cement boundary condition for subsea wellhead fatigue analytical models
Material fatigue is a failure mode that has been known to researchers and engineers since the 19th century. Catastrophic accidents have happened due to fatigue failures of structures, machinery and transport vehicles. The capsizing of the semisubmersible rig Alexander L. Kielland in Norwegians water...
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| description | Material fatigue is a failure mode that has been known to researchers
and engineers since the 19th century. Catastrophic accidents have
happened due to fatigue failures of structures, machinery and transport
vehicles. The capsizing of the semisubmersible rig Alexander L.
Kielland in Norwegians waters in 1980 killed 123 people, and
investigations pointed at the fatigue failure of a weld as one of the
direct causes. This accident led to a number of improvements to the
design of offshore structures. The noticeable safety principle ”No
single accident should lead to escalating consequences” has since been
adopted in a widespread manner. Since 1992 the Petroleum Safety
Authority in Norway has enforced a risk based safety regime.
Wells are designed to hold back reservoir pressures and avoid
uncontrolled escape of hydrocarbons. In other words a well is a
pressure containing vessel. Norwegian safety regulations require a dual
barrier construction of wells. This safety principle ensures that one
“barrier” is preventing an escalating situation should the other barrier
fail. A wellhead is a heavy walled pressure vessel placed at the top of
the well. The wellhead is part of the second well barrier envelope
during drilling.
The subsea wellheads are located at sea bottom and during subsea
drilling the Blow Out Preventer (BOP) is placed on top of the subsea
wellhead. The drilling riser is the connection between the BOP and the
floating drilling unit. Waves and current forces acting on the drilling
riser and drilling unit will cause dynamic movement. Flexible joints at
top and bottom of the drilling riser protects the drilling riser from
localised bending moments.
The subsea wellhead is both a pressure vessel and a structurally load
bearing component resisting external loads transmitted from a
connected riser. These external loads can be static and cyclic
combinations of bending and tension (compression). Cyclic loads will
cause fatigue damage to the well. The well can take a certain amount of
fatigue damage without failing. A fatigue failure of a WH system may
have serious consequences. Should the WH structurally fail its pressure
vessel function will be lost and for this reason WH fatigue is a potential
threat to well integrity. The structural load bearing function will also be
affected.
Wellhead fatigue analysis can be used as a tool to estimate the
accumulated fatigue damage. Analysis results then compares to a safe
fatigue limit. This thesis addresses selected aspect |
| format | Dissertation |
| fullrecord | <record><control><sourceid>cristin_3HK</sourceid><recordid>TN_cdi_cristin_nora_11250_191246</recordid><sourceformat>XML</sourceformat><sourcesystem>PC</sourcesystem><sourcerecordid>11250_191246</sourcerecordid><originalsourceid>FETCH-cristin_nora_11250_1912463</originalsourceid><addsrcrecordid>eNqNzL8KwjAQgPEuDqK-w72AYOof0E3E4q7gWK7JpR6kF8glSN9ehLo7fcvHb16lJ4XwInTQYOa-EJwFw6iscIJ7SR4tgUVl6cHSQJKhi0UcphFsFMeZo4CPCbR0Sgjvn-cnD79eZosBhugo6LKaeQxKq6mLCprr43Jb28SaWVqJCVtj6v2mNUdT7w7bP5YPNWNDOg</addsrcrecordid><sourcetype>Open Access Repository</sourcetype><iscdi>true</iscdi><recordtype>dissertation</recordtype></control><display><type>dissertation</type><title>Wellhead Fatigue Analysis : Surface casing cement boundary condition for subsea wellhead fatigue analytical models</title><source>NORA - Norwegian Open Research Archives</source><creator>Reinås, Lorents</creator><creatorcontrib>Reinås, Lorents</creatorcontrib><description>Material fatigue is a failure mode that has been known to researchers
and engineers since the 19th century. Catastrophic accidents have
happened due to fatigue failures of structures, machinery and transport
vehicles. The capsizing of the semisubmersible rig Alexander L.
Kielland in Norwegians waters in 1980 killed 123 people, and
investigations pointed at the fatigue failure of a weld as one of the
direct causes. This accident led to a number of improvements to the
design of offshore structures. The noticeable safety principle ”No
single accident should lead to escalating consequences” has since been
adopted in a widespread manner. Since 1992 the Petroleum Safety
Authority in Norway has enforced a risk based safety regime.
Wells are designed to hold back reservoir pressures and avoid
uncontrolled escape of hydrocarbons. In other words a well is a
pressure containing vessel. Norwegian safety regulations require a dual
barrier construction of wells. This safety principle ensures that one
“barrier” is preventing an escalating situation should the other barrier
fail. A wellhead is a heavy walled pressure vessel placed at the top of
the well. The wellhead is part of the second well barrier envelope
during drilling.
The subsea wellheads are located at sea bottom and during subsea
drilling the Blow Out Preventer (BOP) is placed on top of the subsea
wellhead. The drilling riser is the connection between the BOP and the
floating drilling unit. Waves and current forces acting on the drilling
riser and drilling unit will cause dynamic movement. Flexible joints at
top and bottom of the drilling riser protects the drilling riser from
localised bending moments.
The subsea wellhead is both a pressure vessel and a structurally load
bearing component resisting external loads transmitted from a
connected riser. These external loads can be static and cyclic
combinations of bending and tension (compression). Cyclic loads will
cause fatigue damage to the well. The well can take a certain amount of
fatigue damage without failing. A fatigue failure of a WH system may
have serious consequences. Should the WH structurally fail its pressure
vessel function will be lost and for this reason WH fatigue is a potential
threat to well integrity. The structural load bearing function will also be
affected.
Wellhead fatigue analysis can be used as a tool to estimate the
accumulated fatigue damage. Analysis results then compares to a safe
fatigue limit. This thesis addresses selected aspects of fatigue damage
estimations of subsea wellheads and surface casings. The presented
work is a contribution to the fatigue analysis methodology currently
being developed within the industry. The well cement role as a
boundary condition for surface casings in analytical models is
particularly addressed.
The majority of research focuses on the casing shoe and formation
sealing, which is the primary objective of well cementing. Recent
research focus on the cement limits conditions e.g. elevated
temperatures. The “near-seabed” conditions of lead cements have seen
less scrutiny. Some researchers have shown interest in this issue related
to deep water cementing. Deep water bottom temperature is low all
year round regardless of location latitude.
Low sea water temperatures will depress the normal thermal gradient of
the upper parts of the soil. Subsea wells are typically cemented using a
lead and tail cement system, and the lead top casing cement will be
pumped all the way to seabed. This lead cement will then be left curing
in a low temperature environment. Hydration of cement is an
exothermic chemical reaction, and the reaction rate is dependent on
temperature. Laboratory measurements of low temperature early
compressive strength of typical lead cement slurries are presented
herein.
In the North Sea the duration between placement of surface casing lead
cement and installation of BOP/drilling riser will typical be around 24
hrs. Then dynamic riser loads will start acting on the upper part of a
subsea well. Bending of the well causes relative motions between the
conductor and surface casing. The cement around these casings will
experience these relative motions. The combination of delayed cement
setting due to low temperature and surface casing motions will cause
localized failure of cement bonding in the upper part of the well.
In subsea wellhead fatigue analysis finite element models are used.
Boundary conditions in analytical models are important in ensuring
similar behaviour of model and reality. One boundary condition in
wellhead models is the lateral cement support of the surface casing.
Modelling this cement support as infinitely stiff with a discrete vertical
transition is the existing solution. In this work a modified boundary
condition is presented based on low curing temperatures in combination
with “premature” loading of the supporting cement.
An overall analysis methodology approach has been suggested. Using a
detailed local model of the well to define the lower boundary condition
for the global riser load analytical model is one of its features. The
implementation of a modified cement boundary condition will change
the global stiffness of the local well model. The possible effect on
global riser load from variations to the lower boundary condition has
been studied. The conclusion supports the suggested analysis approach.
Overall well ultimate structural strength will be reduced by the
presence of a fatigue crack in a non pressurised load bearing part of a
subsea well. An analysis methodology with case results are presented
and indicate that the location of a fatigue crack affects the reduction in
ultimate strength. Cases of significant reduction are expected to impact
normal operating limitations.
To be able to include the wellhead fatigue failure mode in an overall
risk management system, the failure probability needs to be estimated.
This can be done by applying a structural reliability analysis
methodology to the problem. A suggested structural analysis
methodology approach is suggested and notational failure probabilities
are presented.
Future improvements to wellhead fatigue analysis may emerge from
calibrations from measurements of the reality. A comparison between
analytical fatigue loading and measured fatigue loading has been
presented and results indicate that the analysis results are conservative.
This is evidence that analytical estimate on acceptable fatigue limits
can be trusted from a safety point of view. It also indicates the
monetary potential that measurements can present to the well.</description><language>eng</language><publisher>University of Stavanger, Norway</publisher><subject>Berg‑ og petroleumsfag: 510 ; brønnhode ; materialtretthet ; Petroleumsteknologi: 512 ; Teknologi: 500 ; VDP</subject><creationdate>2012</creationdate><rights>info:eu-repo/semantics/openAccess</rights><oa>free_for_read</oa><woscitedreferencessubscribed>false</woscitedreferencessubscribed></display><links><openurl>$$Topenurl_article</openurl><openurlfulltext>$$Topenurlfull_article</openurlfulltext><thumbnail>$$Tsyndetics_thumb_exl</thumbnail><link.rule.ids>230,311,778,883,4040,26550</link.rule.ids><linktorsrc>$$Uhttp://hdl.handle.net/11250/191246$$EView_record_in_NORA$$FView_record_in_$$GNORA$$Hfree_for_read</linktorsrc></links><search><creatorcontrib>Reinås, Lorents</creatorcontrib><title>Wellhead Fatigue Analysis : Surface casing cement boundary condition for subsea wellhead fatigue analytical models</title><description>Material fatigue is a failure mode that has been known to researchers
and engineers since the 19th century. Catastrophic accidents have
happened due to fatigue failures of structures, machinery and transport
vehicles. The capsizing of the semisubmersible rig Alexander L.
Kielland in Norwegians waters in 1980 killed 123 people, and
investigations pointed at the fatigue failure of a weld as one of the
direct causes. This accident led to a number of improvements to the
design of offshore structures. The noticeable safety principle ”No
single accident should lead to escalating consequences” has since been
adopted in a widespread manner. Since 1992 the Petroleum Safety
Authority in Norway has enforced a risk based safety regime.
Wells are designed to hold back reservoir pressures and avoid
uncontrolled escape of hydrocarbons. In other words a well is a
pressure containing vessel. Norwegian safety regulations require a dual
barrier construction of wells. This safety principle ensures that one
“barrier” is preventing an escalating situation should the other barrier
fail. A wellhead is a heavy walled pressure vessel placed at the top of
the well. The wellhead is part of the second well barrier envelope
during drilling.
The subsea wellheads are located at sea bottom and during subsea
drilling the Blow Out Preventer (BOP) is placed on top of the subsea
wellhead. The drilling riser is the connection between the BOP and the
floating drilling unit. Waves and current forces acting on the drilling
riser and drilling unit will cause dynamic movement. Flexible joints at
top and bottom of the drilling riser protects the drilling riser from
localised bending moments.
The subsea wellhead is both a pressure vessel and a structurally load
bearing component resisting external loads transmitted from a
connected riser. These external loads can be static and cyclic
combinations of bending and tension (compression). Cyclic loads will
cause fatigue damage to the well. The well can take a certain amount of
fatigue damage without failing. A fatigue failure of a WH system may
have serious consequences. Should the WH structurally fail its pressure
vessel function will be lost and for this reason WH fatigue is a potential
threat to well integrity. The structural load bearing function will also be
affected.
Wellhead fatigue analysis can be used as a tool to estimate the
accumulated fatigue damage. Analysis results then compares to a safe
fatigue limit. This thesis addresses selected aspects of fatigue damage
estimations of subsea wellheads and surface casings. The presented
work is a contribution to the fatigue analysis methodology currently
being developed within the industry. The well cement role as a
boundary condition for surface casings in analytical models is
particularly addressed.
The majority of research focuses on the casing shoe and formation
sealing, which is the primary objective of well cementing. Recent
research focus on the cement limits conditions e.g. elevated
temperatures. The “near-seabed” conditions of lead cements have seen
less scrutiny. Some researchers have shown interest in this issue related
to deep water cementing. Deep water bottom temperature is low all
year round regardless of location latitude.
Low sea water temperatures will depress the normal thermal gradient of
the upper parts of the soil. Subsea wells are typically cemented using a
lead and tail cement system, and the lead top casing cement will be
pumped all the way to seabed. This lead cement will then be left curing
in a low temperature environment. Hydration of cement is an
exothermic chemical reaction, and the reaction rate is dependent on
temperature. Laboratory measurements of low temperature early
compressive strength of typical lead cement slurries are presented
herein.
In the North Sea the duration between placement of surface casing lead
cement and installation of BOP/drilling riser will typical be around 24
hrs. Then dynamic riser loads will start acting on the upper part of a
subsea well. Bending of the well causes relative motions between the
conductor and surface casing. The cement around these casings will
experience these relative motions. The combination of delayed cement
setting due to low temperature and surface casing motions will cause
localized failure of cement bonding in the upper part of the well.
In subsea wellhead fatigue analysis finite element models are used.
Boundary conditions in analytical models are important in ensuring
similar behaviour of model and reality. One boundary condition in
wellhead models is the lateral cement support of the surface casing.
Modelling this cement support as infinitely stiff with a discrete vertical
transition is the existing solution. In this work a modified boundary
condition is presented based on low curing temperatures in combination
with “premature” loading of the supporting cement.
An overall analysis methodology approach has been suggested. Using a
detailed local model of the well to define the lower boundary condition
for the global riser load analytical model is one of its features. The
implementation of a modified cement boundary condition will change
the global stiffness of the local well model. The possible effect on
global riser load from variations to the lower boundary condition has
been studied. The conclusion supports the suggested analysis approach.
Overall well ultimate structural strength will be reduced by the
presence of a fatigue crack in a non pressurised load bearing part of a
subsea well. An analysis methodology with case results are presented
and indicate that the location of a fatigue crack affects the reduction in
ultimate strength. Cases of significant reduction are expected to impact
normal operating limitations.
To be able to include the wellhead fatigue failure mode in an overall
risk management system, the failure probability needs to be estimated.
This can be done by applying a structural reliability analysis
methodology to the problem. A suggested structural analysis
methodology approach is suggested and notational failure probabilities
are presented.
Future improvements to wellhead fatigue analysis may emerge from
calibrations from measurements of the reality. A comparison between
analytical fatigue loading and measured fatigue loading has been
presented and results indicate that the analysis results are conservative.
This is evidence that analytical estimate on acceptable fatigue limits
can be trusted from a safety point of view. It also indicates the
monetary potential that measurements can present to the well.</description><subject>Berg‑ og petroleumsfag: 510</subject><subject>brønnhode</subject><subject>materialtretthet</subject><subject>Petroleumsteknologi: 512</subject><subject>Teknologi: 500</subject><subject>VDP</subject><fulltext>true</fulltext><rsrctype>dissertation</rsrctype><creationdate>2012</creationdate><recordtype>dissertation</recordtype><sourceid>3HK</sourceid><recordid>eNqNzL8KwjAQgPEuDqK-w72AYOof0E3E4q7gWK7JpR6kF8glSN9ehLo7fcvHb16lJ4XwInTQYOa-EJwFw6iscIJ7SR4tgUVl6cHSQJKhi0UcphFsFMeZo4CPCbR0Sgjvn-cnD79eZosBhugo6LKaeQxKq6mLCprr43Jb28SaWVqJCVtj6v2mNUdT7w7bP5YPNWNDOg</recordid><startdate>2012</startdate><enddate>2012</enddate><creator>Reinås, Lorents</creator><general>University of Stavanger, Norway</general><scope>3HK</scope></search><sort><creationdate>2012</creationdate><title>Wellhead Fatigue Analysis : Surface casing cement boundary condition for subsea wellhead fatigue analytical models</title><author>Reinås, Lorents</author></sort><facets><frbrtype>5</frbrtype><frbrgroupid>cdi_FETCH-cristin_nora_11250_1912463</frbrgroupid><rsrctype>dissertations</rsrctype><prefilter>dissertations</prefilter><language>eng</language><creationdate>2012</creationdate><topic>Berg‑ og petroleumsfag: 510</topic><topic>brønnhode</topic><topic>materialtretthet</topic><topic>Petroleumsteknologi: 512</topic><topic>Teknologi: 500</topic><topic>VDP</topic><toplevel>online_resources</toplevel><creatorcontrib>Reinås, Lorents</creatorcontrib><collection>NORA - Norwegian Open Research Archives</collection></facets><delivery><delcategory>Remote Search Resource</delcategory><fulltext>fulltext_linktorsrc</fulltext></delivery><addata><au>Reinås, Lorents</au><format>dissertation</format><genre>dissertation</genre><ristype>THES</ristype><btitle>Wellhead Fatigue Analysis : Surface casing cement boundary condition for subsea wellhead fatigue analytical models</btitle><date>2012</date><risdate>2012</risdate><abstract>Material fatigue is a failure mode that has been known to researchers
and engineers since the 19th century. Catastrophic accidents have
happened due to fatigue failures of structures, machinery and transport
vehicles. The capsizing of the semisubmersible rig Alexander L.
Kielland in Norwegians waters in 1980 killed 123 people, and
investigations pointed at the fatigue failure of a weld as one of the
direct causes. This accident led to a number of improvements to the
design of offshore structures. The noticeable safety principle ”No
single accident should lead to escalating consequences” has since been
adopted in a widespread manner. Since 1992 the Petroleum Safety
Authority in Norway has enforced a risk based safety regime.
Wells are designed to hold back reservoir pressures and avoid
uncontrolled escape of hydrocarbons. In other words a well is a
pressure containing vessel. Norwegian safety regulations require a dual
barrier construction of wells. This safety principle ensures that one
“barrier” is preventing an escalating situation should the other barrier
fail. A wellhead is a heavy walled pressure vessel placed at the top of
the well. The wellhead is part of the second well barrier envelope
during drilling.
The subsea wellheads are located at sea bottom and during subsea
drilling the Blow Out Preventer (BOP) is placed on top of the subsea
wellhead. The drilling riser is the connection between the BOP and the
floating drilling unit. Waves and current forces acting on the drilling
riser and drilling unit will cause dynamic movement. Flexible joints at
top and bottom of the drilling riser protects the drilling riser from
localised bending moments.
The subsea wellhead is both a pressure vessel and a structurally load
bearing component resisting external loads transmitted from a
connected riser. These external loads can be static and cyclic
combinations of bending and tension (compression). Cyclic loads will
cause fatigue damage to the well. The well can take a certain amount of
fatigue damage without failing. A fatigue failure of a WH system may
have serious consequences. Should the WH structurally fail its pressure
vessel function will be lost and for this reason WH fatigue is a potential
threat to well integrity. The structural load bearing function will also be
affected.
Wellhead fatigue analysis can be used as a tool to estimate the
accumulated fatigue damage. Analysis results then compares to a safe
fatigue limit. This thesis addresses selected aspects of fatigue damage
estimations of subsea wellheads and surface casings. The presented
work is a contribution to the fatigue analysis methodology currently
being developed within the industry. The well cement role as a
boundary condition for surface casings in analytical models is
particularly addressed.
The majority of research focuses on the casing shoe and formation
sealing, which is the primary objective of well cementing. Recent
research focus on the cement limits conditions e.g. elevated
temperatures. The “near-seabed” conditions of lead cements have seen
less scrutiny. Some researchers have shown interest in this issue related
to deep water cementing. Deep water bottom temperature is low all
year round regardless of location latitude.
Low sea water temperatures will depress the normal thermal gradient of
the upper parts of the soil. Subsea wells are typically cemented using a
lead and tail cement system, and the lead top casing cement will be
pumped all the way to seabed. This lead cement will then be left curing
in a low temperature environment. Hydration of cement is an
exothermic chemical reaction, and the reaction rate is dependent on
temperature. Laboratory measurements of low temperature early
compressive strength of typical lead cement slurries are presented
herein.
In the North Sea the duration between placement of surface casing lead
cement and installation of BOP/drilling riser will typical be around 24
hrs. Then dynamic riser loads will start acting on the upper part of a
subsea well. Bending of the well causes relative motions between the
conductor and surface casing. The cement around these casings will
experience these relative motions. The combination of delayed cement
setting due to low temperature and surface casing motions will cause
localized failure of cement bonding in the upper part of the well.
In subsea wellhead fatigue analysis finite element models are used.
Boundary conditions in analytical models are important in ensuring
similar behaviour of model and reality. One boundary condition in
wellhead models is the lateral cement support of the surface casing.
Modelling this cement support as infinitely stiff with a discrete vertical
transition is the existing solution. In this work a modified boundary
condition is presented based on low curing temperatures in combination
with “premature” loading of the supporting cement.
An overall analysis methodology approach has been suggested. Using a
detailed local model of the well to define the lower boundary condition
for the global riser load analytical model is one of its features. The
implementation of a modified cement boundary condition will change
the global stiffness of the local well model. The possible effect on
global riser load from variations to the lower boundary condition has
been studied. The conclusion supports the suggested analysis approach.
Overall well ultimate structural strength will be reduced by the
presence of a fatigue crack in a non pressurised load bearing part of a
subsea well. An analysis methodology with case results are presented
and indicate that the location of a fatigue crack affects the reduction in
ultimate strength. Cases of significant reduction are expected to impact
normal operating limitations.
To be able to include the wellhead fatigue failure mode in an overall
risk management system, the failure probability needs to be estimated.
This can be done by applying a structural reliability analysis
methodology to the problem. A suggested structural analysis
methodology approach is suggested and notational failure probabilities
are presented.
Future improvements to wellhead fatigue analysis may emerge from
calibrations from measurements of the reality. A comparison between
analytical fatigue loading and measured fatigue loading has been
presented and results indicate that the analysis results are conservative.
This is evidence that analytical estimate on acceptable fatigue limits
can be trusted from a safety point of view. It also indicates the
monetary potential that measurements can present to the well.</abstract><pub>University of Stavanger, Norway</pub><oa>free_for_read</oa></addata></record> |
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| subjects | Berg‑ og petroleumsfag: 510 brønnhode materialtretthet Petroleumsteknologi: 512 Teknologi: 500 VDP |
| title | Wellhead Fatigue Analysis : Surface casing cement boundary condition for subsea wellhead fatigue analytical models |
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