Means for relieving load on stacked casing hangers

Using additional load shoulders on wellhead casing hangers when multiple casing hangers are needed increases the capacity of the casing hangers to withstand loads. The shoulders of the casing hangers are not required to land simultaneously. Either the upper or lower set of shoulders may contact prior to the other, with the first set to make contact plastically or elastically deforming to allow the other set to land. Other tubular members, such as a test plug may be inserted to pressure test the shoulders of the casing hanger for a casing string that is supported by the casing hanger.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention deals with oil and gas wellheads, and particularly with a 
means for supporting load on stacked casings hangers within a wellhead. 
2. Description of the Prior Art: 
In an oil or gas well, concentric strings of casing extend into the well, 
each being cemented in place. In one type of wellhead, each casing is 
supported at its upper end by a casing hanger. The casing hanger for the 
first string of casing lands on a load shoulder contained in the bore of 
the wellhead housing. The next casing hanger lands on top of the neck of 
the first casing hanger. similarly, if a third string of casing is run, 
the third casing hanger will land and be supported on top of the second 
casing hanger. 
High loads are imposed on these casing hangers in high pressure wells. The 
load is due to internal pressure in the casing string exerting a downward 
force on the casing hanger. If adequate space exists in the bore, the 
casing hanger neck will be sized sufficiently thick to withstand the load 
imposed by the casing hanger or hangers supported on top. In some 
installations, however, inadequate room exists to provide an adequately 
thick casing hanger neck to withstand the desired loads. This occurs 
particularly where casing designed for use in a larger wellhead housing is 
supported within a smaller bore wellhead housing. 
SUMMARY OF THE INVENTION 
In this invention, the capacity of casing hangers to withstand desired 
loads is enhanced with the use of additional load shoulders on each casing 
hanger. A first casing hanger is supported within the bore of a wellhead 
housing by a coventional, exterior load shoulder. The casing hanger has a 
conventional or upper load shoulder to land a second casing hanger on top 
of its neck. A second lower shoulder protrudes radially inward from the 
first casing hanger inside its bore, thereby sharing the load when a 
second casing hanger is landed on it. In the preferred embodiment, the 
shoulders are conical. The second casing hanger has upper and lower 
exterior shoulders to land on the upper and lower shoulders of the first 
casing hanger. Similar to the first casing hanger, the second casing 
hanger has two shoulders upon which a third casing hanger may land, if 
needed. Multiple casing hangers may be landed in this manner to reach the 
desired depth in the well. 
The shoulders of the casing hangers are not required to land simultaneously 
and either the upper or lower set of shoulders may contact prior to the 
other. If the lower shoulders make first contact, the lower shoulder of 
the first casing hanger will plastically deform before the upper shoulders 
make contact. If the upper shoulders make first contact, the upper 
shoulder of the first casing hanger will elastically deform before the 
lower shoulders make contact. 
In a second embodiment, other tubular members may be landed in a manner 
similar to the second casing hanger. One example is a test plug. A test 
plug which is part of a test tool may be inserted to pressure test the 
casing hanger's seal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the wellhead shown includes an outer wellhead housing 
11 which will be connected to conductor pipe which extends into the well 
to a first depth. An inner wellhead housing 13 lands within outer wellhead 
housing 11. Wellhead housing 13 is a high pressure tubular member which 
will be secured to a first string of casing that extends to a second depth 
in the well. Wellhead housing 13 has a bore 15 that contains an upward 
facing conical load shoulder 17. In the embodiment shown, load shoulder 17 
is a separate ring of higher strength than the strength of the body of 
wellhead housing 11. 
A first or lower casing hanger 19 is secured to a string of casing (not 
shown) that extends into the well and is cemented in place. Casing hanger 
19 lands on load shoulder 17. Casing hanger 19 has a neck 21 that extends 
upward and is spaced radially inward from the wall of bore 15. A 
conventional casing hanger seal 23 locates in this space. Neck 21 has an 
upper end 22 which is a conical shoulder as shown in FIG. 2. 
A second or intermediate casing hanger 25 is secured to another string of 
casing 27 which extends into the well and is cemented is place. Casing 
hanger 25 has an upper shoulder 29 which lands on shoulder 22 of first 
casing hanger 19. Second casing hanger 25 has flow channels 31 that extend 
along its exterior to join flow passages 33 that extend to the annular 
space surrounding neck 35. Passages 33 and channels 31 allow the return of 
fluid during cementing. A lower downward facing shoulder 37 is located on 
a protruding annular band 38 which is located on the exterior of casing 
hanger 25 at the lower end of flow channels 31. Lower shoulder 37 lands on 
an internal shoulder 39, shown in FIG. 2, which is formed in the bore 41 
of lower casing hanger 19. A conventional casing hanger seal 43 is set 
between neck 35 and wellhead housing bore 15. In the preferred embodiment, 
shoulders 22, 39 are both conical. Lower shoulder 39 is shown to be at 
about a 45.degree. angle while shoulder 22 is shown to be at about a 
75.degree. angle relative to the longitudinal axis. However, both angles 
may be the same. 
Because of space requirements in this instance, the thickness of neck 21 is 
insufficient to provide the desired strength against the specified load 
from high pressure applied on it from intermediate casing hanger 25. Lower 
shoulder 39, however, provides additional strength to upper shoulder 22 so 
that when combined, shoulders 22, 39 will support the load. Generally, the 
ability to withstand the stress created by a downward acting axial load is 
proportional to the cross-sectional area A1 of neck 21, taken 
perpendicular to the axis, plus the area A2 of shoulder 39 taken 
perpendicular to the longitudinal axis. Shoulder 39 protrudes radially 
inward farther than neck 21 and is in an area of casing hanger 19 which 
has greater cross-sectional thickness than neck 21. The additional area A2 
thus adds to the ability of casing hanger 19 to withstand the load. 
Because of manufacturing tolerances, it is not practically possible for 
both shoulders 29, 37 to land simultaneously on their respective shoulders 
22, 39. One of the shoulders 29, 37 will land prior to any contact of the 
other one. Then the casing hanger shoulder 22, 39 that is first in 
engagement with one of the shoulders 29, 37 must deflect axially downward 
before the second shoulder 29, 37 lands. The downward deflection can be 
either through plastic deformation or elastic deformation. In elastic 
deformation, the yield strength of the member is not exceeded. In plastic 
or permanent deformation, the yield strength is exceeded. 
In one example of plastic deformation, the tolerances will be selected so 
that lower shoulder 37 contacts internal shoulder 39 before upper shoulder 
29 contacts shoulder 22. Upon initial contact of shoulders 37, 39, a 
slight gap will exist between between shoulders 22, 29. The gap is small, 
such as 0.005 inches+/-0.005 inches. This small gap is selected so that 
the structural integrity of shoulder 39 is not compromised by axial 
plastic deformation of shoulder 39, rather the deflection will be a slight 
coining due to the bearing stress. The shear stress is kept low enough to 
avoid shearing of load shoulder 39. Once the deflection has occurred, 
shoulder 29 lands on shoulder 22 and continued deflection will cease. 
In the case of elastic deflection, one way in which this could be achieved 
would be to size the tolerances so that shoulder 29 lands on shoulder 22 
before shoulder 37 lands on shoulder 39. A slight gap would exist between 
shoulders 37 and 39 when shoulder 29 lands on shoulder 22. On an 
application of sufficient load, neck 21 flexes, causing shoulder 22 to 
move downward axially a slight amount until shoulders 37, 39 come into 
contact with each other. At this point, deflection ceases even though the 
same load is applied. The gap between shoulders 37, 39 must be small 
enough so that the yield strength of neck 21 is not exceeded before 
shoulders 37, 39 contact each other. Preferably, it is sized so that a 
safety factor which is a fraction of the yield is employed. 
Calculating the deflection is handled by application of finite elements 
analysis or by standard calculations with a formula being following: 
EQU de=F/K 
with K=A1E/L where de=elastic deflection 
F=applied load, K=stiffness of the load path, 
A1=cross sectional area of neck 21, E=Young's modulas of elasticity, 
L=length of the load path 
Motion of seal 23 due to axial deflection of neck 21 relative to wellhead 
housing 13 is of concern. Such motion imposed on seal 23 could adversely 
affect the performance of seal 23 for long term production applications 
because seal 23 is designed for static applications. Multiple load 
shoulders 22, 29 and 37, 39 reduce the deflection of neck 21 relative to 
housing 13. With a portion of the load bypassing neck 21 and passing 
through shoulders 37, 39 to housing 13, axial deflection on neck 21 is 
significantly reduced. 
The same principles can be applied to additional casing hangers and other 
structures which are subjected to high pressure within a wellhead. For 
example, in FIG. 1, the additional structures include a third casing 
hanger 45 which lands on the intermediate casing hanger 25. Casing hanger 
45 is secured to an inner string of casing 47 which extends to the maximum 
depth within the well. Casing hanger 45 has an upper shoulder 49 that 
lands on the upper end 50 of neck 35 of intermediate casing hanger 25. 
Casing hanger 45 has a lower shoulder 51 that lands on an internal 
shoulder 53 located within the bore of intermediate casing hanger 25. In 
this instance, because of the smaller diameter of casing 47 than casing 
27, shoulder 53 is quite large in radial extent. Shoulder 51 need not 
extend the full extent of shoulder 53. The same analysis may be applied as 
previously discussed. 
Because of the large radial dimension of shoulder 53, however, it is 
preferable to utilize elastic deformation of neck 35 rather than plastic 
deformation of shoulder 53. That is, the dimensions are selected so that 
shoulders, 49, 50 contact each other before shoulders 51, 53. A slight gap 
will exist. Upon the application of load, the gap closes due to axial 
deflection of neck 35. When the gap closes, load is shared through 
shoulders 51 and 53. A wellhead having three casing hangers may utilize 
permanent deformation in the instance of the engagement of intermediate 
casing hanger 25 with lower casing hanger 19, and elastic deformation is 
the case of the engagement of upper casing hanger 45 with intermediate 
casing hanger 25. 
FIG. 3 illustrates the same principles applied to a device other than a 
casing hanger. In this example, prior to installing intermediate casing 
hanger 25 and upper casing hanger 45, a test plug 57 is inserted to 
pressure test the string supported by lower casing hanger 19. This test is 
of the blowout preventer stack using a test plug 57 which is part of a 
test tool. In the prior art, a test tool similar to test plug 57 would 
land only on a single shoulder such as on shoulder 22 of neck 21. Because 
extreme pressures would cause too much axial deflection of the thin neck 
21, test plug 57 is provided not only with an upper shoulder 59, but also 
a lower shoulder 61. Lower shoulder 61 lands on internal shoulder 39 in 
the bore of casing hanger 19. 
Again, either axial or plastic deformation will be employed as explained. 
Preferably, shoulder 61 will land on internal shoulder 39 first. Test 
pressure will cause plastic deformation of shoulder 39, bringing shoulder 
59 into load supporting relationship with shoulder 22. Test plugs such as 
test plug 57 may also test multiple casing hangers such as the assembly 
shown in FIG. 1. 
The invention has significant advantages. The use of multiple load 
shoulders results in a very high load bearing capacity wellhead system 
which avoids complicated mechanisms and expensive or exotic materials 
while controling the axial displacement of one component relative to the 
other. The invention is particularly advantageous where casing designed 
for use in a larger wellhead housing is supported within a smaller bore 
wellhead housing. 
While the invention has been shown in only one of its forms, it should be 
apparent to those skilled in the art that it is not so limited, but is 
susceptible to various changes without departing from the scope of the 
invention.