Subsea drilling and production system for use at a multiwell site

A subsea drilling and production system where the production unit is removably connected onto a drilling template and operatively connected to one or more Christmas trees so that the production unit may be disconnected from the trees and removed from the template without first removing any of the trees and so that any of the trees may be disconnected from the production unit and removed from the underwater site without removing the production unit.

FIELD OF THE INVENTION 
The present invention relates to subsea drilling and production operations, 
and more particularly, to a system for drilling, completing and producing 
oil and gas wells at a multi-well site. 
BACKGROUND OF THE INVENTION 
Much of today's search for new oil and gas supplies is being conducted 
offshore. Offshore operations, however, are usually more expensive than 
onshore operations and require a much larger commitment of capital with no 
greater guarantee that oil or gas will be found at any particular site. In 
a typical operation, an exploratory well is drilled from a floating vessel 
at a site where geological and geophysical data indicate the possibility 
of finding oil and/or gas in commercially producible amounts. If 
significant reserves are located, an offshore platform may be erected from 
which a number of wells may be drilled and completed. In conducting 
operations from the platform, the wellhead may be located above the 
surface of the water with the production and completion equipment 
available on the platform. In this case, the operation of the well is not 
greatly different from the operation of a well at an onshore site. 
However, in very deep waters or where local conditions and expected returns 
make bottom-supported platforms uneconomical, it is desirable, if not 
necessary, to provide a subsea system for drilling, completing and 
producing wells. Although much effort has gone into the design of subsea 
drilling and production installations, the various designs proposed 
heretofore have been found to have certain shortcomings. In some subsea 
production installations, the necessary production equipment is made an 
integral part of the drilling template. Thus, production equipment is 
committed to the underwater site prior to verifying the actual extent of 
the field to be developed. In other subsea production installations, the 
equipment is arranged on the template in such a manner that maintenance 
and repair of the system is very costly and difficult. This results from 
the fact that certain production equipment cannot be removed from the 
template without first removing other associated equipment. 
For instance, U.S. Pat. No. 3,618,661 discloses a subsea production 
installation having a multiwell drilling template, a manifold module 
releasably secured on the template, and a plurality of production wellhead 
assemblies or Christmas trees operatively connected on top of the manifold 
module. In this arrangement, the Christmas trees are connected on top of 
the manifold module such that the manifold module cannot be removed 
without first removing the Christmas trees. This, of course, is very 
inconvenient and costly should the manifold module have to be removed, to 
be repaired or replaced. 
Another approach is disclosed in U.S. Pat. No. 3,778,812 in which the 
subsea production system includes a multiwell drilling template completely 
equipped with all the necessary manifold piping and equipment for handling 
well production fluids, gas injection, and well maintenance. In this 
system, the production wells are drilled through the fully-equipped 
template; thus, production equipment is committed to a particular site 
without first verifying the extent of the commercially producible 
reserves. Therefore, should the particular drilling site turn out to have 
fewer commercial wells than anticipated, an unnecessary and expensive 
investment in production equipment has been made. 
From the above, it can be seen that there exists a need in the art for a 
subsea drilling and production system that is relatively inexpensive to 
maintain and that does not commit production equipment to a particular 
drilling site prior to verifying the actual extent of the reserves at the 
site. 
SUMMARY OF THE INVENTION 
Broadly speaking, the present invention is for a subsea system for 
drilling, completing and producing wells. This system limits production 
equipment committed to a particular site to that which is actually 
necessary in terms of the size of the field to be produced. This system 
also provides that production equipment be arranged on the template in a 
manner which reduces maintenance costs. 
To accomplish the above, a drilling template is first positioned on the 
underwater bottom for the purpose of drilling a well or a number of wells. 
After the extent of the field is determined, an appropriate production 
unit is removably positioned on the template. Christmas trees are then 
lowered to the template and disconnectably connected to the casingheads at 
the various wells. Flowline and control-line means are provided to 
respectively connect the Christmas trees to the manifold piping and 
control lines of the production unit. The Christmas trees are connected to 
the production unit in a manner which permits the production unit to be 
disconnected from the Christmas trees so that it may be removed from the 
template without first having to remove any of the Christmas trees. 
Likewise, any of the Christmas trees may be disconnected from the 
production unit so that any tree may be removed from its casinghead 
without first removing the production unit or any other tree. 
Moreover, in this drilling and producting system, the production unit may 
be actually comprised of a number of different sections connected into 
operative association with each other.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now referring to the drawings, FIG. 1 represents a drilling template or 
template frame 100 being lowered to subsea bottom 200. The template may be 
lowered by any appropriate means, and as is shown, it may be lowered on a 
drill string 104 which is connected to the template by a hydraulic 
connector 150. 
After the template is set into position on the underwater bottom, as 
illustrated in FIG. 2, drilling is conducted through any of a plurality of 
apertures 101 extending through the template, the vertical axes of the 
apertures being generally perpendicular to underwater bottom 200. The 
template is held in position on the underwater bottom by its own weight, 
or if necessary, the template may be anchored to the underwater bottom by 
means of piles, which are not shown. The template of the present invention 
may have any appropriate form different from the open tubular arrangement 
illustrated in FIG. 1. For example, as shown in FIG. 13, the drilling 
template may be in the form of a pile supported base plate 155. 
After template 100 has been positioned on the underwater bottom, see FIG. 
2, drilling will begin with the drilling of a conductor-pipe hole 135. To 
this purpose, a drill bit 127 is lowered, which is shown in phantom, by 
means of drill string 143 to pass through an aperture in the template. The 
drill bit is guided through the appropriate aperture by any suitable 
guidance means. As shown, the guidance means may comprise guide posts 131 
which are fixedly connected to the template frame, guide lines 106 which 
are fixedly connected to the guide posts to extend to the surface, and 
guide arms 132. After drill bit 127 is positioned in the appropriate 
aperture, drill string 143 is rotated to form conductor-pipe hole 135. 
After hole 135 has been drilled to the desired depth, conductor pipe 124, 
see FIG. 3, is lowered, shown in phantom, to the sea floor by a pipe 
string 104, which is attached to conductor-pipe head 133 of pipe 124 by 
means of running tool 105. The conductor pipe 124 is guided through the 
appropriate aperture where it is set into hole 135 with conductor-pipe 
head 133 extending above the template frame. The conductor pipe is then 
cemented in place by conventional subsea cementing techniques. The next 
step in the drilling operation is illustrated in FIG. 4 wherein a rotary 
bit, which is not shown, is guided into conductor pipe 124 to drill a 
surface-casing hole 141. A surface casing 122 is then lowered into 
surface-casing hole 141 and cemented therein, casinghead 129 of surface 
casing 122 protruding through conductor-pipe head 133. 
With surface casing 122 cemented in place, a blowout preventer 103 is then 
lowered to template 100 by means of a marine riser 137. The blowout 
preventer is latched into position on casinghead 129, as shown in FIG. 5, 
by any suitable means, such as hydraulic connector 139. With the blowout 
preventer in position, drilling is completed with the drilling--by means 
of drill string 143 and drill bit 144--of a production casing hole 142. 
After drilling of the production casing hole is completed, a production 
casing, which is not illustrated, is lowered into and cemented in 
production casing hole 142. Thereafter, production tubing is run in the 
wellbore and landed in casinghead 129. Well plugs are then lowered and 
secured in the tubing to prevent crude from escaping while other wells are 
being drilled and completed. With this done, the blowout preventer may 
removed and a well cap, not shown, may be installed on casinghead 129. 
The heretofore-described drilling procedure is repeated until the number of 
wells that are to be drilled at the particular site are completed. Once it 
has been established which and the number of wells that are to be 
produced, a production unit for producing oil and/or gas is readied for 
installation at the site. In this respect, it is expected that more than 
one well will be produced at any particular site, but it is noted that the 
present invention may be used where only one well is found to be 
commercially productive. As illustrated in FIG. 6, production unit 111 
essentially comprises a flow manifold which includes manifold piping 112, 
protective piping 167 in which hydraulic control lines for the production 
system are run, production flow connector stab plate 148 that is part of 
the manifold piping, and control system stab plate 108A. Production unit 
111 is lowered to the template by means of running string 104 and guided 
thereto by guide lines 106 and guide posts 131. The production unit is 
latched into position on the template by production unit-hydraulic 
connector 134 and production-connector mandrel 136, which is located on 
the template to mate with connector 134. 
The above-described guidance system which uses guide lines and guide posts 
is but one means for guiding drilling and production equipment to the 
underwater bottom. Other guidance systems may be employed; for example, a 
guideline-less system that incorporates a television guidance system, a 
sonar system, or a transmitter-receiver system may be used. In FIG. 14, 
there is illustrated an arrangement for a transmitter-receiver system in 
which a transponder 152 is located on template 100 to receive a signal 
from a transmitter, which is not shown, to guide equipment onto the 
template. The other guidance systems that might be used are sonar and 
television systems. The television system, as is known in the art, 
essentially comprises a television camera which is suspended beneath the 
water's surface so that personnel at the surface may use the television 
image to guide equipment onto the template. The sonar system makes use of 
a sonar transponder that transmits and receives a signal reflected by a 
sonar reflector located on the template. Of course, it would also be 
possible to use one type of guidance system in combination with another; 
for example, the guidance system may consist of both television and sonar 
equipment. 
Now referring to FIGS. 7 and 11, after production unit 111 has been 
positioned on the template, a running tool 147 is used to lower a control 
system pod 108 to the template. Hydraulic installation lines 149 are 
attached to the running tool and extend from the running tool to the 
surface where they are connected to a hydraulic power unit. Once the 
control pod is guided into position on the template, the hydraulic power 
unit at the surface is activated to actuate latches within the control pod 
to connect the pod to a mandrel 158, see FIG. 11. A suitable hydraulic and 
electric connector, which is not illustrated, is located within pod 108 
for operatively connecting the pod to control system stab plate 108A on 
the production unit. This arrangement permits the production unit to be 
disconnected from the control pod in situations where the production unit 
or the control pod is to be removed from the template. 
The power source at the surface, which is not illustrated, for operating 
the control pod is in communication with the control pod through hoses or 
cables 183 and 184, see FIGS. 10 and 11. Hoses 183 and 184 are connected 
to the control pod through a control line connector 159, which is 
connected to the template by means of a mandrel 157. It is noted that 
hoses 183 and 184 may either be connected to the control line connector to 
be orientated along an essentially vertical axis as illustrated or, 
alternatively, to be orientated along an essentially horizontal axis. It 
is further noted that connector 159 is lowered to the template in a manner 
similar to that of the lowering of control pod 108. 
The control pod contains the necessary pilot valves, which are preferably 
hydraulically actuated but which may be pneumatically or electrically 
operated or some combination thereof, for controlling production, that is, 
for controlling the operation of production unit 111 and Christmas trees 
130, see FIG. 8. As discussed heretofore, control pod 108 is connected to 
control stab plate 108A. The stab plate in turn is connected to hydraulic 
control lines that are run in protective piping 167 of the production 
unit, see FIG. 11. Some of the control lines in piping 167 are also 
connected to the Christmas trees through a control line connector 119, 
which will be discussed in more detail below. In this manner, control pod 
108 is in communication with the respective Christmas trees to establish a 
control system for the trees. 
As illustrated in FIGS. 7-10 and 12, the present invention further includes 
production pipelines 178 and 179 in communication with manifold piping 
112. The production pipelines carry oil and/or gas production away from 
the production unit. The production pipelines are connected to production 
flow stab-plate 148 of manifold piping 112 through of a pipeline connector 
145. The pipeline connector is operatively connected to production flow 
stab-plate 148 and latched onto the template by means of mandrel 140. As 
in the case of control pod 108 and stab plate 108A, pipeline connector 145 
is connected to stab plate 148 in a manner that permits the production 
unit to be disconnected from connector 145 for the purpose of removing the 
production unit. The method of connecting pipelines 178 and 179, either 
exiting the flow manifold horizontally or vertically, to pipeline 
connector 145, as in the case of connecting hoses 183 and 184 to control 
line connector 159, may be any appropriate method. And since such methods 
are well known in the art, they are not described in any detail. 
With the production unit in place on the template, Christmas tree 130 may 
be lowered, see FIG. 8, onto casinghead 129 to be connected thereto. By 
way of explanation, the Christmas trees are an assemblage of control 
valves, pressure gauges and chokes located at the top of a well to control 
the flow of oil and gas from the well. The Christmas trees may also 
include fittings suitable for use in pumping and assisted recovery 
operations. 
The Christmas tree is lowered and positioned on the template by means of a 
completion riser 146 and the above-discussed guidance system. The 
Christmas tree is connected to the completion riser by a 
remotely-operated, hydraulic connector 172. Connector 172 includes a 
stab-receiver plate assembly having a female portion 163 and a male 
portion 164. The female portion is connected to hydraulic lines 166 of 
hose bundle 168 and mates with the male portion. The male portion is 
mounted on the Christmas tree where it is connected to a hydraulic valve 
operator on the tree. After the Christmas tree has been latched into 
position on casinghead 129, the valve operator on the tree may be actuated 
to disconnect female portion 163 from male portion 164. This permits 
connector 172, hose bundle 168, and female portion 163 to be raised from 
the subsea bottom. 
After the Christmas tree has been lowered onto the appropriate casinghead 
129, it is disconnectably connected thereto by a connector 170, which is 
affixed to the lower end of the Christmas tree. The Christmas tree may 
then be placed in communication with manifold piping 112 through a 
flowline connector 109, which is described in greater detail below. The 
flowline connector and block valves 110, which prevent the flow of 
production out of manifold piping 112, see FIG. 10, are operated by 
control pod 108 to establish a flow path between the Christmas tree and 
manifold piping 112. A control-line connector 119, which is also operated 
by control pod 108 and which is described in greater detail below, is 
provided to establish a control circuit between the Christmas trees and 
the control pod through the lines in piping 167. It is understood that for 
operation of valves 110 and connectors 109 and 119 by control pod 108, 
appropriate control lines are run from the control pod and through piping 
167 and associated piping to connect to the aforementioned valves and 
connectors. 
In the present invention, a Christmas tree is provided at each well from 
which hydrocarbons are to be produced, and it is understood that any 
Christmas tree may be disconnected from the production unit by means of 
flowline connector 109 and control-line connector 119. That is to say, 
flowline connector 109 may be operated to disconnect any Christmas tree 
from manifold piping 112, and control-line connector 119 may be operated 
to disconnect the tree from the control lines in piping 167. When this is 
done, production from the disconnected well into the production unit is 
stopped, and the control circuit between control pod 108 and the Christmas 
tree is broken. When the flowline and control-line connectors are operated 
to disconnect any Christmas tree from the production unit, that tree--or 
trees--may be removed from its casinghead without removing the production 
unit. Likewise, the arrangement of the flow-line and control-line 
connectors permits the production unit to be disconnected from each of the 
Christmas trees so that the production unit may be removed from the 
template without first removing any of the Christmas trees. As is known in 
the art, appropriate means is provided at each casinghead 129 to prevent 
the flow of production out of the well when a tree is removed from its 
casinghead. Further, it is understood that when the production unit is 
disconnected from the Christmas trees, the valving on the trees is 
operated to stop the flow of production from the wells. 
With reference to FIGS. 15, 16, and in particular 18, a detailed 
description of the operation and construction of flowline connector 109 is 
provided. The flowline connector is located on the production unit and 
essentially includes an inner sleeve 114 coaxially positioned and slidably 
movable within an outer sleeve 113. A fluid channel 126 extends through 
the inner sleeve and a piston 117 is formed at that end of the inner 
sleeve located in the outer sleeve. The opposite end of the inner sleeve 
is adapted to slidably engage a flow-line connector receiver 123 which is 
located on each Christmas tree. Outer sleeve 113 has ports 115 and 116 
located at opposite ends thereof. These ports are connected by suitable 
lines of communication through piping 167 to control pod 108. The control 
pod thus controls the operation of the flowline connector by regulating 
the flow of fluid to ports 115 and 116. To explain more fully, when 
control pod 108 is operated to flow hydraulic fluid through port 115, a 
fluid pressure will act on one face of piston 117. This will cause the 
inner sleeve to move within the outer sleeve in a direction away from port 
115 and towards receiver 123, exhausting any fluid on the opposite side of 
piston 117 through port 116. Conversely, when control pod 108 is operated 
to flow fluid through port 116, a fluid pressure will act on the opposite 
face of piston 117. This causes the inner sleeve to travel in a direction 
towards port 115 and away from receiver 123, exhausting fluid through port 
115. 
As indicated, the production flowline connector is located on the 
production unit, see FIG. 10, where one end of outer sleeve 113 is 
connected to block valve 110. The block valve prevents the flow of 
hydrocarbons out of piping 112 where the manifold is not connected to a 
Christmas tree. Alternatively, it is noted that the block valve may be a 
valve means arranged in the flowline connector itself. By operating a 
flowline connector at a particular Christmas tree, a flow path may be 
established between the tree and the manifold piping. This occurs when 
inner sleeve 114 engages receiver 123 located on the Christmas tree. Block 
valve 110 may then be opened to place the Christmas tree in productive 
association with the production unit. Production is thus free to flow from 
the well and through outlet 126 of the flowline connector and into 
manifold piping 112. 
If it becomes necessary to remove one or all of the Christmas trees from 
the well site, block valve 110 associated with the tree or trees to be 
removed is closed. The flowline connector is then operated to retract 
sleeve 114 from receiver 123. The Christmas tree or trees to be removed 
are now disconnected from the manifold piping. Each Christmas tree may 
then be disconnected from its respective casingheads to be raised to the 
surface. From the above, it can be seen that any Christmas tree may be 
retrieved from the underwater bottom without having to remove the 
production unit or any other Christmas tree. In a like manner, all the 
Christmas trees may be disconnected from the production unit, so that the 
production unit may be removed from the template without first having to 
remove any of the Christmas trees. 
In removing any Christmas tree or the production unit from the template, it 
would also be necessary to disconnect the control circuitry in the 
Christmas trees from the control system of the production unit. To this 
purpose, a control-line connector 119, see FIGS. 10, 15, 16, and 19, is 
provided. The control-line connector functions to establish a means of 
communication between control pod 108 and the control circuitry of the 
respective Christmas trees, and it operates in a manner similar to that of 
the flow-line connector. As discussed, the control pod is connected to 
stab-plate 108A from which control lines are run in piping 167 and 167A to 
the control-line connectors 119 located at each tree. The control-line 
connector is located on the template and has an inner sleeve 173 coaxially 
positioned and slidably movable within an outer sleeve 199. The 
control-line connector is positioned so that its inner sleeve may engage a 
control line receiver 174 located at the Christmas tree. The control line 
connector is operated by control pod 108 by regulating the flow of 
hydraulic fluid through ports 169 and 177 which extend through and are 
located at opposite ends of outer sleeve 199. A fluid flowing through port 
169 will exert a fluid pressure on piston 180 of inner sleeve 173 to cause 
the inner sleeve to move towards and engage receiver 174. To retract 
sleeve 173 from receiver 174, a fluid will be flowed through port 177 to 
exert a fluid pressure on the piston that causes the inner sleeve to move 
in a direction away from receiver 173. 
Control lines 181, 182, 183, and 184, see FIG. 19, are run in piping 167 
and 167A for operation of the respective Christmas trees. The control 
lines terminate at the control-line connector where they are each in 
communication with a respective port that extends through outer sleeve 
199. When inner sleeve 173 operatively engages receiver 174, control lines 
181 through 184 are aligned with channels 191, 192, 193, and 194, 
respectively, in inner sleeve 173. In turn, channels 191 through 194 are 
aligned with control lines 181a, 182a, 183a and 184a, respectively, of 
receiver 174. In this manner, a control circuit is established between 
control pod 108 and the Christmas trees. To remove any Christmas tree from 
its casing-head or the production unit from the template, the control 
circuitry of the Christmas tree must be disconnected from the control 
lines in the production unit. To accomplish this, control pod 108 is 
operated to flow fluid through port 177 which, as discussed, causes sleeve 
173 to be withdrawn from receiver 174. Thus, the control circuitry in the 
Christmas trees may be disconnected from the production unit so that any 
tree or trees may be removed without removing the production unit. 
Likewise, the control circuitry in the production unit may be disconnected 
from each of the Christmas trees so that the production unit may be 
removed from the template without first removing any of the trees. 
It is noted that should the control circuitry in control pod 108 be 
something other than the above-described hydraulic system--for example, a 
pneumatic or electrical system--connector 119 could be modified to 
establish the appropriate electric or pneumatic lines of communication. 
For instance, if an electric control circuit is used, electrical wiring 
rather than hydraulic lines would be run from control pod 108 and through 
piping 167 and 167A to the control-line connector. Further, it is noted 
that it would be possible to construct the control-line and flowline 
connectors as a single unit. Such a unit is illustrated in FIG. 17 wherein 
hydraulic control lines 120 and flowlines 121 from the production unit are 
connected to a single flowline-control-line connector 138. 
The present invention may also be used in production operations in which 
the production unit is composed of a number of individual sections. For 
example, in FIG. 14 there is illustrated a production unit comprised of 
two different sections 111X and 111Y. Section 111X includes control 
stab-plate 108A and section 111Y includes flow stab-plate 148. After 
sections 111X and 111Y are positioned on the template, production piping 
112 of the two sections are disconnectably connected into operative 
assocation by the heretofore-described flowline connector 109. To this 
purpose, inner sleeve 114 of the flowline connector is located on one 
production unit section to engage receiver 123 located on another 
production unit section. Similarly, the control lines in piping 167 in the 
two different sections are operatively connected by control-line connector 
119. Here, inner sleeve 173 of the control-line connector is located on 
one production unit section to engage receiver 174 located on another 
production unit section. Once production unit sections 111X and 111Y are 
in place on the template and the two sections are connected in operative 
association, they will function as a single production unit. 
Another production system is illustrated in FIGS. 15 and 16. In this 
arrangement, a first production unit section 111A is positioned on the 
template to produce a number of wells. Production unit section 111A is 
complete as it includes all the means necessary for flowing and 
controlling production. That is to say, it includes control stab-plate 
108A and flow stab plate 148 of the manifold piping. Additional production 
unit sections 111B, 111C and 111D, shown in phantom, may subsequently be 
put into operation to produce additional wells. These additional sections 
need not include a control stab-plate or flow stab plate as these sections 
will be connected in operative association with section 111A. The 
production unit sections have block valves 118 located at each end of 
manifold piping 112 to close the piping off to the sea when the production 
unit section is not connected into operative association with another 
production unit section. Outwardly of block valves 118 are the appropriate 
flowline connectors 109, and in piping 167, the appropriate control-line 
connectors 119. As discussed, connectors 109 and 119 allow the various 
production unit sections to be connected into operative association. As 
illustrated, production is carried away from the production unit sections 
by production pipelines 179 and 178 which are connected to production unit 
section 111A. Likewise, control cables 183 and 184 for control of the 
production system are connected to production unit section 111A. With 
section 111A in place, other production unit sections may be added to or 
removed from the template without disrupting production flowing into 
section 111A and the other sections connected into operative association 
therewith. 
FIG. 17 illustrates a production operation in which a number of wells may 
be produced by production unit sections located on different templates. 
Here production unit section 111A includes all the means necessary for 
flowing and controlling production so that as additional wells are 
produced from production units located on different templates, they may be 
connected into operative association with unit section 111A. For example, 
where additional wells are drilled from template 100, production unit 
section 111E located on that template may be connected into operative 
association with section 111A by flowline connector 109 and control-line 
connector 119. As in the previously-described operation, block valves 118 
are provided on units 111A and 111E to close-off piping 112 to the sea 
prior to connecting production unit section 111A to production unit 
section 111E. 
SUMMARY OF THE ADVANTAGES 
A significant feature of the present invention is the ability to remove any 
Christmas tree from its casinghead without having to remove the production 
unit or any other Christmas tree. Thus, production is only disrupted at 
the well from which the tree has been removed. Similarly, the production 
unit may be disconnected from each of the trees to be moved from the 
template without first having to remove any of the Christmas trees. After 
the production unit is removed, it could be serviced at an above-water 
site while a replacement unit is put into operation. These advantages lead 
to reduced maintenance costs as well as a more efficient method of 
producing oil and gas. 
The use of this invention also reduces the initial expenditure for drilling 
operations as no unnecessary investment in production equipment is made. 
This is because the necessary production equipment is not committed to the 
particular drilling site until the actual extent the reservoir is known. 
Moreover, with the present invention, different production unit sections 
may be positioned on the same or different templates to be connected into 
operative association with each other. This offers additional flexibility 
in production operations. For as additional wells are developed, 
additional production unit sections may be committed to the operating site 
and connected into operative association with the previously committed 
production unit sections. 
The foregoing describes selected embodiments of the present invention in 
detail. The invention, however, is not to be limited to any specific 
embodiment, but rather only by the scope of the appended claims.