This invention relates to production systems for underwater wells/ particularly (but not exclusively) oil or gas wells.

It is known to drill a number of subsea wells using a drilling template to provide a subsea satellite production system, the oil or gas produced being fed to a platform terminal or shore terminal.

Such groups of satellite wells, which are serviced individually, have drawbacks in that there is considerable duplication of flowlines and umbilical lines between the wells and the terminal.

According to the present invention there is provided a production system for extracting material from an underwater deposit using a caisson, characterised in that there are a plurality of Christmas tree assemblies which can be accommodated simultaneously in the caisson, a drilling template for use within the caisson, firstly in the drilling of wells and subsequently for supporting the Christmas tree assemblies, manifold means for connecting each Christmas tree assembly to at least one flowline, which serves all of the Christmas tree assemblies, and distribution means for connecting each Christmas tree to a power and control umbilical which serves all of the Christmas tree assemblies.

The manifold means preferably comprise connections to a production flowline and a test flowline; and the distribution means preferably comprise connections to electrical and hydraulic lines in the umbilical.

The umbilical will normally include chemical injection

hoses for the transmission of chemicals (for example methanol), in which case the drilling template will also support a chemical injection manifold for distributing the chemicals to the Christmas tree assemblies.

The manifold means, distribution means and chemical injection manifold are preferably modularised units which may be fitted to the template before or after it is located within the caisson. Each unit preferably has its own lock-down system to facilitate its installation or removal.

The Christmas tree assemblies are preferably four in number. Whatever the number, preferably they are distributed around a vertical axis of the caisson, on which axis the manifold means lies.

To provide physical protection and minimise the ingress of sand and silt the caisson may have a lid which is preferably slightly domed, so as to minimise scouring effect to optimise hydrodynamic loading, and preferably has a removable cover above each Christmas tree and the manifold means to allow access by a diver.

Although the invention may be carried out in a variety of different ways one particular embodiment thereof will now be described, by way of example, with reference to the accompanying drawings in which

Figures 1 and 1A are perspective views of a subsea oaisson production system according to the invention. Figure IB being an exploded, cutaway and enlarged view of the caisson assembly;

Figure 2 is a vertical section through the installed caisson;

Figure 3 is a vertical section through the caisson showing its construction in more detail;

Figure 4 is a plan corresponding to Fig. 3 of the caisson;

Figure 5 is a plan of the caisson lid;

Figure 6 is a plan of a drilling template;

Figure 7 is an exploded side elevation of the components housed in the caisson;

Figure 8 is an exploded plan view, partly in section, of the caisson components;

Figure 9 is an elevation of a Christmas tree installed in the caisson;

Figure 10 is a plan of manifold piping, and Figs. 10A and 10B are corresponding elevations;

Figure 11 is a diagram showing the general arrangement of the production and control systems; and

Figure 12 is a plan of the caisson with lid removed showing the caisson control system in more detail, Figure 12A being a detail of an umbilical distribution unit and adjacent caisson wall.

As shown in Figs. 1 to 4 a subsea caisson production system comprises a caisson 10 supported in an oversized sloping-sided excavation in the seabed 12 on a stabilised pre-engineered concrete foundation layer 14 through which

project four conductors 16, and a central conductor 17 (see Fig. IB) these being surrounded by a protective rock dump 18. The caisson has a floor formed by a drilling template 22 on which are mounted four Christmas tree assemblies distributed around a central vertical axis (not shown) of the caisson and a central manifold module 26 which lies on the axis and from which a production flowline 28 and a test flowline 30 run to an on-shore plant 32 via a shore crossing 34 or to a platform. An umbilical 36 which comprises electric, hydraulic and chemical lines also runs to the plant 32 in an excavated trench. Pig receivers 31, a splitter box 33 and tie-in spools 35 are also shown.

The foundation layer 14 is laid on a fine rock dump 40 at the bottom of the excavation 11. The excavated area surrounding the sides of the caisson 10 is filled with selected fine rock dump material 42 covered by a top layer of coarse material 44 which provides a stabilised gently sloping surround extending from the top of the caisson 10 to the existing seabed 12 some 2m above.

While the whole of the caisson 10 and its lid 20 lie in a recess in the seabed in the illustrated installation, it is possible for part of the assembly of caisson and lid to project out of the recess so that the top of this assembly is at a higher level than that of the seabed.

The caisson 10 has a 12-sided wall 46 comprising twelve double skin wall sections 48 formed of welded plates 50 and interconnected by vertical tubular members 52; the sections 48 may be flooded with sea water or • -filled with other material when the caisson 10 has been lowered into position. If desired the caisson 10 may be supplied with levelling mechanisms (not shown).

A horizontal internal ring 54 is positioned near the lower edge of the caisson wall 46 to support the drilling template 22 and is fitted with a series of drilling template installation guides 56 and drilling template lockdown brackets 58. At its upper edge the wall 46 is capped by a box-section top chord 60 surmounted by a bumper frame 62 which is interrupted for a set of four lifting eyes 64, and of flowline clamp support inserts 66.

The caisson 10 is fitted with a removable lid 20 (Fig. 5) which comprises a tubular framework 68 covered by a serrated bar grating 70, and having five removable hatches 72 to provide access to the Christmas tree assemblies 24 and manifold module 26. Four lifting eyes 74 for connection to a sling 76 (Fig. 7) are also provided.

The drilling template 22 (see Fig. 6) comprises a tubular framework 78 including two intersecting parallel pairs of tubular members 80, each terminating in a buffer 82 which on lowering of the template 22 engages the respective guides 56 and is then secured by lock down brackets 58 when it has been levelled (to within 5 cm), which may be accomplished independently of the caisson 10 which is also alignable. Four frusto-conical, gimball-type guide funnels 84 are peripherally disposed and provide a facility to land drilling guide bases 86 (Fig. 7) which, after the drilling of four wells, support the Christmas tree assemblies 24. A similar guide funnel 84 is provided centrally to receive a similar guide base 86 for the manifold 26. Between each adjacent pair of guide funnels 84 there is positioned a pair of umbilical distribution socket inserts 90 incorporating port sockets which provide a choice of locations for mounting an umbilical distribution unit 92 (See Figs. ]] and 12). Two sets of lifting eyes 94 and 96 are welded to the frame-work 78.

Figure 9 shows one of the Christmas tree assemblies 24 located in the drilling template 22 by means of the drilling guide base 86 having upstanding guide posts 98 over which guide funnels 100 of a guide frame 102 engage. A tree cap 104 fits over the top of the frame 102 (see Figs. 7 and 8). The Christmas tree itself (106) has a minimum of four fail-safe barriers, which are all arranged to be automatically shut in the case of alarm or emergency, and leak detection, system monitoring and data acquisition facilities; and, as its construction is generally conventional, it will not be described in detail. Exploded views of the Christmas tree assembly 24 and a tubing hanger 108 are shown in Figs. 7 and 8.

Centrally disposed on the template 22 is the manifold module 26 shown in detail in Fig. 10, and generally in the exploded diagrams of Figs. 7 and 8. The manifold module 26 is mounted on the central guide base 86 by means of a framework 110, including four guide funnels 112, and is surmounted by a protective cap 114. The functional components of the module include a pair of tree tie-in spools 116 for each well which are all connected to a production flowline 120 and a test flowline 118 which communicate with the flowlines 28 and 30, respectively; a pigging loop 122; and valve actuators 124. Besides allowing test flow, production flow and well-killing, the manifold module also includes piping headers which allow a chemical treatment facility such as ethanol dosing to inhibit 'hydrate formation, pour-point depressant to prevent wax formation, corrosion inhibitor dosing and intermittent scale inhibitor. The chemicals are fed along the umbilical 36 to the umbilical distribution unit 92 and reach a chemical injection manifold 125 on the central manifold module 26 via a chemical injection jumper 126 (Fig. 11). From the

manifold 125 the chemicals pass to the Christmas tree assemblies 24.

The umbilical distribution unit 92 is located on a desired one of the umbilical distribution socket inserts 90, being mounted on two guide posts 127 which are engaged by guide funnels 128 flanking the unit 92. The unit 92 is connected to the umbilical 36 by a jumper 130, the connection being made through a pullhead 132 received within a pulling cone 134 mounted on an insert 90. Hydraulic and electric jumpers 136 lead to a control pod 138 located by each Christmas tree assembly 24 to provide power and control therefor; and further jumpers 140 lead from each pod 138 to the central manifold module 26 to supply the valve actuators 124.

The general arrangement of the control system is shown in block form in Fig. 11 and will not be described in detail.

It will be noted that the test flowline 118 and the production flowline 120 are connected by the pigging loop 122 so that "round trip" commissioning may be undertaken by passing the appropriate pi train through the lines. The manifold module 26 allows the flow of production hydrocarbons to either the production flowline 120 or, on an individual basis, to the test flowline 118 for well-testing purposes. In normal operation, the oil or gas from two wells passes down the production flowline 120 and that from the other two wells passes down the test flowline 118.

The two unused umbilical distribution socket inserts 90 may be used for further umbilicals and/or flowlines, for example if it is subsequently desirable ^{to use} 9 ^as lift or water injection techniques.