Subsea well intervention vessel.

SUBSEA WELL INTERVENTION VESSEL

The present invention relates to a subsea well intervention vessel.

Hydrocarbon production wells are established by using a rotating drill assembly. A rotating drill assembly is driven from the surface, generally in the case of a subsea well from a rig mounted on a platform positioned over the well. The platform can be mounted on the seabed or may be a semi-submersible assembly the location of which can be maintained in all but the most extreme conditions. After completion of drilling, the well is lined with tubing to enable hydrocarbon liquids to flow through the tubing from any hydrocarbon reserve into which the tubing extends. In some formations, hydrocarbon fluids and water occupy the same reservoir, the hydrocarbon fluids forming a layer on top of the water. If the production tubing of a well penetrates the formation initially occupied by the hydrocarbon fluids, as fluid flows to the well tubing the phenomenon known as “water coning” can occur, that is the interface between the hydrocarbon liquids and water slopes upwards towards the well. This effect results from pressure gradients established within the reservoir formation as a result of fluid flow through the formation to the well tubing. If the tip of the cone-shaped interface reaches the well tubing, large volumes of water will enter the well tubing, reducing the rate of hydrocarbon liquid production and increasing the costs of separating the produced hydrocarbon fluids from the water.

In wells where water coning has become a problem, it is known to conduct further drilling operations so as to prevent or minimise water cone generation. For example, a bottom hole drilling assembly can be used to drill lateral passageways into the hydrocarbon liquid-bearing formation. This can be achieved by using conventional drilling techniques, but such techniques demand the shutting down of the well and often require the removal of the tubing lining the well. This involves substantial costs and risks. In addition, the hydrocarbon liquid bearing formation can be damaged by drilling fluids required for the additional drilling operations.

In order to avoid the possibility of loss or damage to a well resulting from drilling interventions, an advanced drilling technology has been developed which allows technically difficult drilling to be achieved without substantial risk of damage to the formation. The technique is referred to as “underbaianeed” drilling. With

ΑΡ/Γ7 0 2 7 0 2 3 v «Ρ ϋ01 3 7 0 underbalanced drilling, the ·.> . live (positive pressure at the sir d all times. This can be achieved by either using a lightweight drilling fluid or reiving upon gas lift controi using a puipos out preventer assembly. A ck . u hi.J is pumped down the well, and this mixes with the formation fluids that are allowed to flow up the well, that flow transporting the rock cuttings to the The five phases (gas. oil , formation water, drilling fluid and drilling solids ι · n s, pamed. On land this is a straightforward process as space is not at a premiu- , . an pnent however is large and has not been thought suited for offshore operat.

Underbalanced drilling can be conducted using either cm · mouai rotary drilling or coiled tubing drilling. In the UK sector of the North S·... ' weii- h,wy been drilled using underbalanced rotary drilling but this has only '··. smoh using relatively large fixed (seabed-supported) platforms. On land, con n nt drill, ig has been used. In these known applications, a long seamless pipe nh. · .' '•pored on a drum is. pushed into the well by an injector against the live well pn. - A turbine drill is mounted on the bobm J of the pipe and hydraulic pressure is delivered to the turbine drill through the pipe This drives the turbine and perp',! y ,|,ng to lake place. The small diameter of the pipe (typically 2.5 cm to 7.3 cm -ml ρ Sc fo; the pipe to pass through existing well-lining tubing (norma·! co cP i. as completions) so that it is not necessary to incur the substantial , md -fix of removing such tubing.

.Light intervention . are available which make π p·.- ‘ conduct operations such as well sen > ’ well logging and general n, ·, .· wtv Si, fib vessels however cannot be considered appropriate platforms l.n ·’ »·“· requiring drilling as they are not sufficiently stable for sm > mu ¹ - < id furthermore could not ope· ·; /balanced drilling as they are ί · fo r> candle the volumes of material that result in such drilling. Furthermore. , intervention , requite large capital investments as compared with the m , fir an be generated, particularly as fo ie highly vulnerable to bad , . .η ί ,a

I. :./ ..umi ..'Si ·· are relatn . ..n and utilisation time is relatively low. It would of . ν i. .foe to u,.v - . mi-submersible for well inter’ > but semi submersibles cannot be nsfo for underbalanced drilling. I \ w m cl*.

.- , · up.·. ..u)port vessels to receive the produced liquids and solids

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Accordingly no attempts have been made to use underbalanced coiled tubing drilling from floating units.

It is an object of the present invention to provide a subsea well intervention vessel capable of re-entering existing production wells in a manner which allows well interventions to be performed without removing the well from its production mode and without polluting the subsea production system with well intervention effluent, e.g. drilling solids.

According to the present invention, there is provided a subsea well intervention vessel comprising a dynamically positionable tanker and direct well intervention equipment mounted on the deck of the tanker, the direct well intervention equipment including equipment for underbalanced non-rotating drilling and hydrocarbon liquid separation coupled to storage tanks of the tanker such that separated hydrocarbon liquids can be stored in the tanker.

The invention also provides a method for conducting off-shore underbalanced drilling, wherein a tanker having direct well intervention equipment mounted on its deck is dynamically positioned over a riser extending from a subsea well, the well intervention equipment is coupled to the riser, and underbalanced non-rotating drilling is performed, the resultant multi-phase mixture being separated on the tanker and separated hydrocarbon liquids being stored in storage tanks of the tanker.

The term “non-rotating drilling” is used herein to include any drilling in which there is no rotation of the drill string including but not limited to underbalanced drilling using a rotary drill head powered through a non-rotating drill string.

The well intervention equipment may be mounted on a superstructure above the main deck of a conventional shuttle tanker. Coiled tubing equipment may be mounted adjacent a skid deck which may be displaced to an outboard position over a well riser to which the coiled tubing equipment is to be connected. Thus a well intervention can be achieved by dynamically positioning the shuttle tanker adjacent a well riser, moving the skid deck to the outboard position, coupling the coiled tubing equipment to the riser, and performing the necessary' interventions in the well to which the riser is connected, fluids and solids produced during the coiled tubing drilling process being separated by equipment mounted on the superstructure and

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APu01370 hydrocarbon liquids being traxislerred from the separation equipment to the shuttle tun.. Term.·'· mid.

As an alternative to providing a skid deck displaceable position, tne drilling equipment could be mounted adjacent a moon pool extending Lr&lt; .- fr u m.. r deck.

Embodiments of the present invention will now be describee, by way of example, with reference fe the accompanying drawings, in which:

figure 1 is a schematic representation taken from an avr.!o · · iucumenr showing the phenomenon of water coning;

Figure 2 ’is a further illustration taken from a published docuir · · ι ig i·. results of coded tubing drilim c structure of Figure 1 so as to improve the rate of production of hydro carbon ii···-. .

Figure 3 is a side view of a known North Sea shuttle tanker mcotporating direct well intervention equipment in accordance with the present in\c t

Figure 4 is a schematic layout diagram of the direct well oucrvsntion equipment shown in side view in Figure 3; and

Figure 5 is a schematic frustration of a tanker which deim· t. p. m, through which coiled tubing drilling can be performed;

Referring to Figure o. this illustrates a series of strata : rating a hydrocarbon bearing stratum I which lies over a water bearing stratum A A well 3 is drilled through the strata 1 and 2, Pressure within the hydrocarbon iiei · wife. such that flow is established to the well 3. As a result of that flow a' u is defined around the well 3 and as a result a conical interface 5 is estol.. ί ? , the hydrocarbon liquid and water if the well 3 is lined with steel tt b · n top of the strata 1, and the water cone reaches to adjacent the lined portion of the well, large volumes of water will be produced. Clearly this is highly disau wou· uw therefore it is known to mte·⁴ .. wells which suffer from the w atm

Figure 2 illustrates the resuh &gt;. r an intervention.

Referring to Figure 2, a branch well 6 is shown as being &gt;w :..

stratum 1. Drilling such r m ι 6 can substantially improve the mwoortioi· of [r&gt;:. o, · tiade up by ^: trbon liquids. It is well known to -i .: ^r such as ihe branch 6 o¹' I-ig using coiled tubing drilling tschovoasr It is

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necessary however when using such techniques to maintain underbalanced conditions (that is maintain a positive pressure at the top of the well 3) in order to avoid drilling solids damaging the well. Such techniques have never been used offshore because the volume of material generated can only be handled in large installations.

Figure 3 illustrates a shuttle tanker embodying the present invention. Figure 3 is based on a drawing extracted from “First Olsen Tankers” and shows a shuttle tanker of the type widely used in the North Sea. The only modification made to the standard shuttle tanker is the mounting of a superstructure 7 above the main deck of the tanker, for example at a height of approximately 3m so as to clear the installed deck pipes and vents. On that superstructure all the equipment necessary for direct well intervention is mounted, including a crane 8. The detailed layout of the equipment mounted on the superstructure 7 of Figure 3 is shovwi in Figure 4.

Referring to Figure 4, a skid deck 9 is centrally mounted on the superstructure 7 adjacent a gantry crane 10. Coiled tubing drilling equipment 11 of conventional form is mounted adjacent the gantry' crane 10. A separator assembly 12 and ancillary drilling support equipment assembly 13 are also mounted on the superstructure 7. All other equipment relied upon to achieve the required direct well intervention is also mounted on the superstructure 7. The separator assembly 12 is coupled to an appropriately positioned flare stack, for example at the stem of the vessel (not shown) and to the storage tanks of the tanker so as to enable produced hydrocarbon fluids to be stored for subsequent transport.

In use, the tanker is dynamically positioned adjacent a subsea well riser. The skid deck 9 is then moved to an outboard position (not shown) over the riser to enable the coiled tubing equipment 11 to be coupled to the riser. Appropriate interventions can then be made via the riser and in particular coiled tubing drilling can be conducted in a manner which produces a multiphase mixture that is subsequently separated into its different phases in the separator assembly 12.

The system described with reference to Figures 3 and 4 represents a breakthrough in offshore drilling, testing, waste disposal and well maintenance. The tanker cargo holds can be used for the collection of produced oil during underbalanced drilling. The system can give direct access to test subsea wells for extended durations. The system can be used for an extended water injection test and

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APv.0 1 37 0 also allows for the disposal fo waste into a subsea well. Existing sy,,. as.,! m, cannot perform coiled tubing drilling and cannot collect produced * j π. i &gt; a separate shuttle tanker ’in the event that oil is being produced during d-¹: ·,.

Furthermore the original features of the shuttle tanker are · . r a tied and therefore the vessel can still be employed in the charter market when not being used for direct well interventions. As a result the invention offers a solution to the problem of achieving direct well interventions with coiled tubing drilling w.;i ,i for _{rrj }l}costs associated with building and operating specialist vessels.

A standard North Sea specified shuttle tanker with dynamic positioning can be readily chartered and fitted with a new deck above the installed deck p vfo vents. On that deck appropriate equipment can be installed such as:

A skid mounted derrick riser handling unit with subsea control

Stumps for the subsea well intervention equipment;

A pipe rack;

Coiled tubing reels, control unit and power pack;

Cementing unit and blender;

Production test equipment including choke manifold, heater trt a &lt; - marators.

degassing boot and gas flare;

Tanks for kill mud;

A dosed circulation system for handling drilling mud and drill· . ¹ during underbaianeed drilling;

Storage tanks for chemical and solid wastes;

Craneage for subsea equipment and supplies;

Remote controlled vehicles tor working and observation tasks;

Water supplies for cooling and fire fighting services;

ft is probably the case that there are of the order of 2000 subsea completions currently operative. With tic ,··. .-nt invention, such completions &gt; · -. made accessible for of the order - « &lt;. o0 US dollars per day in contrast i ’« ' quotes; costs of the order &lt; : r)0 to 300,000 US dollars per day. Thus the invention dramatically affects the technical capability of the offshore in the context of the financial constraints which face that industry.

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Coiled tubing drilling solutions include a cost-effective bottom assembly for standard mud systems and a wireline-based bottom hole assembly that fully exploits the benefits of through-tubing drilling, including use of foam and air systems. The present invention allows onshore underbalanced drilling technology to be transferred offshore without requiring extended equipment development. It also permits the production of significant volumes of hydrocarbons without requiring additional storage vessels, thereby reducing demands on cash flow whilst simultaneously avoiding damage to a well as a result of drilling operations. The motion characteristics of a relatively large shuttle tanker are more suited for delicate underbalanced drilling operations then the available relatively smaller and more buoyant alternative vessels. This extends the amount of time that weather permits operation and reduces fatigue stress on the coiled tubing where it is fed from the tanker to the subsea well riser. The invention also allows wells to be properly cleaned after interventions, thereby avoiding polluting the sometimes sensitive production system. Drilling waste can be managed in an optimal fashion, and all this can be achieved in relative safety given the large deck space available. All of these advantages are unavailable if using either a conventional semi-submersible vessel or a conventional purpose-built well intervention vessel.

In the embodiment of the invention described with reference to Figures 3 and 4, components necessary for the operation of the invention are mounted on a skid deck which can be moved to an outboard position. In an alternative arrangement illustrated in Figure 5, such components are mounted adjacent moon pools extending through the structure of an otherwise conventional tanker.

Referring to Figure 5, two moon pools 13 and 14 extend vertically through the structure of a modified shuttle tanker. Three cranes 15, 16 and 17 can extend over the moon pools and areas indicating cargo manifolds 18, a derrick module 19, and a lay down area 20. Area 21 houses gas compression and process units, area 22 a flare boom, area 23 a flare knock-out drum skid, and area 24 a further lay down area served by a crane 25.

Taking a standard double hull shuttle tanker, the modifications required to produce the vessel schematically illustrated in Figure 5 which can function in accordance with the present invention would be an upgrade of the dynamic

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AP C 013 7 Ο positioning capability, installation of a first moon pool (8m²) for nr o nok, installation of a second moon pool (4m²) for remotely operated ’ m work, mounting of cranes, process equipment and lay down areas for .-mutinted equipment, and the mounting of flare facilities and associated utilities.