T 0842/12 16-04-2015

Reasons for the Decision

1. Main request - Articles 100(c) and 123(2) EPC

1.1 The respondent contends that the feature of claim 1 that "the bypass conduit ... (is) provided between the pump and the longitudinal drilling fluid passage" constitutes added subject-matter in the sense of Articles 100(c) and 123(2) EPC.

1.2 This feature already existed in claim 1 as granted.

1.3 In the communication pursuant to Article 15(1) RPBA, the Board pointed out that the ground of Article 100(c) EPC is a "fresh ground for opposition" within the meaning of decision G 10/91 (OJ 1993, 420), as it was neither raised or substantiated in the notice of opposition of the respondent nor introduced into the proceedings by the Opposition Division, and that it may be considered in the appeal proceedings only with the approval of the appellant as patent proprietor (point 5.2 of the communication).

1.4 In response, the appellant did not consent to the admission of this fresh ground for opposition. Thus, it must be disregarded.

1.5 The Board also indicated in its communication that the objection under Article 123(2) EPC would not be considered because it did not arise from the amendments made to claim 1 during the opposition or appeal proceedings, but concerned amendments made before grant (point 5.3 of the communication).

1.6 In response, the respondent submitted that it is necessary to consider the issue of added subject-matter because it is highly relevant, it does not add any significant procedural complication and it has an impact on how the claims should be interpreted. The Board disagrees. The mere fact that the claims have been amended after grant does not allow the above objection to be raised under Article 123(2) EPC. Since the disputed feature had been introduced in claim 1 before grant, an objection under Article 123(2) EPC cannot be taken into consideration.

2. Main request - Article 100(b) EPC

2.1 In its reply to the statement of grounds of appeal, the respondent raised an objection under Article 100(b) EPC, arguing that the feature of claim 1 that "the bypass conduit ... (is) provided between the pump and the longitudinal drilling fluid passage" is not described in the patent in a manner sufficiently clear and complete for it to be carried out by a person skilled in the art.

2.2 This issue also concerned claim 1 as granted.

2.3 Consideration of the objection in the appeal proceedings

The ground for opposition under Article 100(b) EPC had been raised and substantiated in the opposition notice. In the communication pursuant to Article 15(1) RPBA, the Board pointed to the fact that the minutes of the oral proceedings before the Opposition Division indicate that the respondent had abandoned all its objections under Article 100(b) EPC (see the minutes, page 2, paragraph 4, page 5, paragraph 2, page 7, paragraph 8 and page 8, paragraph 1).

In response, the respondent contested for the first time the correctness of the minutes and the interpretation by the Opposition Division of its declarations. The respondent submitted that, during the oral proceedings, it never renounced its challenge to the patent on the ground of Article 100(b) EPC. Rather, the respondent did not wish to make further oral submissions during the oral proceedings before the Opposition Division on this issue.

If the respondent was of the view that the minutes did not correspond to what has happened during the oral proceedings before the Opposition Division, a request for correction of the minutes should have been duly filed. In the absence of such a request and a subsequent decision of the Opposition Division to correct its minutes, the Board has to assume that they accurately reflect the course of the oral proceedings.

However, the Board takes the view that the withdrawal of a ground for opposition has to be explicit, unambiguous. In the present case, the Board agrees with the respondent that it cannot be inferred with certainty from the minutes that it did withdraw its ground for opposition under Article 100(b) EPC. Moreover, as acknowledged by the appellant, the re-introduction of the above objection under Article 100(b) EPC in the appeal proceedings does not give rise to any new or complex issue. The Board has also no reason to assume an abuse of procedure by the respondent.

Thus, the Board decided to take the respondent's objection under Article 100(b) EPC into consideration, under Article 114(2) EPC and Article 12(4) RPBA.

2.4 Merit of the objection

A skilled reader of claim 1 using common general knowledge would recognise at once that the bypass conduit, which "bypasses the longitudinal fluid passage", cannot at the same time be "provided between the pump and the longitudinal drilling fluid passage". From the wording of method claim 5 (see the before last method step), as well as the teaching in the description and drawings of the patent specification (see bypass conduit 7 in paragraph [0020] and Figures 1 to 3), it is clear that the disputed wording in claim 1 "that the bypass conduit and the three way valve are provided between the pump and the longitudinal drilling fluid passage" should read "that [deleted: the bypass conduit and] the three way valve are provided between the pump and the longitudinal drilling fluid passage".

The patent comprises a detailed description of a number of ways to carrying out the invention (see Figures 1 to 3), such that a skilled person would be able to perform the invention. Hence, the disclosure of the patent is sufficiently clear and complete within the meaning of Article 100(b) EPC.

3. Claim 1 of main request - Novelty over D1

3.1 D1 discloses in Figure 5 (see below) a conventional drilling system for overbalanced drilling operations, with a drill string and an open fluid circulation system comprising a pump 161 for pumping drilling fluid through the drill string into the annulus 167 and a pipe 171 for discharging the fluid from the annulus to a pit 162. During drilling, when the pump 161 is circulating the fluid ("dynamic mode"), the annulus is kept open to the atmosphere and the circulated fluid exerts a pressure against the annulus that substantially equals the hydrostatic pressure of the fluid column plus the annular frictional pressure resulting from fluid circulation. When the pump 161 is shut down to make connections ("static mode"), the annular pressure reduces to the hydrostatic pressure. If it no longer balances the formation pressure, bleeder valve 181 is opened to bleed off. If that is not effective, the BOP 133 and valves 195 and 196 are closed, while valves 197 and 181 are opened; heavy fluid is then directly reverse circulated into the annulus 167 to balance formation pressure and kill any formation fluid inflow (see D1, column 2, lines 39 to 46; column 7, lines 33 to 43; column 8, lines 1 to 12). The three way fluid junction with valves 195 and 197 forms a three way valve in the sense of claim 1 (see e.g. dependent claim 4 of the disputed patent).

FORMULA/TABLE/GRAPHIC

3.2 The parties have disputed whether or not D1 discloses a "fluid backpressure system" as defined in claim 1, whereby "the pump is in fluid communication with the fluid discharge conduit via the three way valve and the bypass conduit".

3.3 "Fluid backpressure system"

3.3.1 In the relevant technical field, the term "backpressure" generally means a pressure added to the borehole to reduce or prevent the inflow of formation fluid. In the context of claim 1, a skilled reader thus understands the expression "fluid backpressure system" in the broad sense of defining equipment for imposing backpressure in the annulus to reduce or prevent formation fluid inflow.

3.3.2 Instead, the appellant submits that the "fluid backpressure system" of claim 1 is an automated control system which continuously provides the annulus with a backpressure that maintains the annular pressure at all times within a prescribed operational window, as established by the formation pressure and the fracture pressure.

3.3.3 However, such an interpretation cannot be derived from the wording of the claim. The claim does not specify that the circulation system is a closed circulation system with a rotating control device for sealing off and allowing pressurisation of the annulus, even though this is an essential feature for enabling dynamic annular pressure control (see e.g. column 4, lines 25 to 31 in the patent specification; pressure containment device 26 closed at all times in D3). Furthermore, claim 1 does not specify the circumstances when backpressure should be added by means of the backpressure system.

Hence, it follows from the wording of claim 1 that the circulation system can be a conventional open circulation system, and that the "fluid backpressure system" can be any equipment for imposing backpressure on the annulus during the well drilling and completing process, for instance to control the well when the circulation through the drill string is shut off to make drill string connections. This understanding accords with the teaching in method claim 8 and also in paragraphs [0020], [0027] and [0029] of the patent specification.

The claim itself thus imparts a clear and credible technical teaching to the skilled reader. Therefore there is no reason for consulting the description and the drawings of the patent to give the term "fluid backpressure system" a narrower meaning, such as defining a particular type of control system (see paragraphs [0001] and [0013]).

3.3.4 It follows from the teaching of D1 that, in the static mode, after closing the BOP 133 and valves 195 and 196 and opening valves 197 and 181, the pump 161 can be used to supply heavy fluid directly to the annulus 167 via the pipe 171 to add backpressure to the hydrostatic pressure, and thus balance/kill the formation pressure before unscrewing the mud saver valve assembly 110. The bypass conduit and the three valve junction with valves 195 and 197 thus form a part of the backpressure system as is broadly defined in the claim.

3.4 "The pump is in fluid communication with the fluid discharge conduit"

3.4.1 In claim 1, the term "fluid discharge conduit" defines a conduit for "discharging said drilling fluid" during drilling. In D1, the pipe 171 fulfils this function when in the dynamic mode and thus forms a "fluid discharge conduit" in the sense of the claim. In the static mode, when reverse circulating as taught in D1, the pump 161 is in fluid communication with the pipe 171 via the three way valve and the bypass conduit, as required in the last feature of claim 1 of the disputed patent.

3.4.2 The appellant submits that the pipe 171 is no longer a "fluid discharge conduit" when reverse circulating, as the fluid is then discharged to the pit 162 only via pipe 164. This argument is not convincing. In claim 1, the term "fluid discharge conduit" can be read as a label used to refer to a specific pipe or conduit, irrespective of whether or not it is discharging drilling fluid at all times. Pipe 171 is a "fluid discharge conduit" during normal fluid circulation and within the definition given in claim 1.

3.5 In conclusion, the Board agrees with the Opposition Division that the subject-matter of claim 1 lacks novelty in the sense of Article 54 EPC in light of D1.

4. Auxiliary request 1 - Consideration of the request in the appeal proceedings

4.1 Auxiliary request 1 was filed for the first time in the appeal proceedings with the statement of grounds of appeal.

4.2 The amendments relate to the introduction of limiting features in the independent claims as granted, with the aim of overcoming the Opposition Division's objection that the subject-matter of these claims lacks novelty. Thus, the filing of this request does not amount to an abuse of procedure on the part of the appellant, and the Board decided to take it into consideration (Article 114(2) EPC and Article 12(4) RPBA).

5. Auxiliary request 1 - Article 123(3) EPC

5.1 Claim 1 as amended no longer requires that "the pump is in fluid communication with the fluid discharge conduit via the three way valve and the bypass conduit". It now defines the pump to be in fluid communication with the drilling fluid source via the three way valve, the bypass conduit and the fluid discharge conduit.

5.2 Hence, claim 1 covers an embodiment wherein the fluid discharge conduit is in parallel to the three way valve and the bypass conduit. Such an embodiment was not covered by claim 1 as granted, hence auxiliary request 1 contravenes Article 123(3) EPC.

6. Auxiliary request 2 - Article 123 EPC

6.1 Claims 1 and 5 differ from claims 1 and 5 of the main request only in that they both include the further limitation that "the drill string is provided with a check valve to prevent backflow of fluid from the annular space into the drill string".

6.2 This amendment meets the requirements of Articles 123(2) and (3) EPC. The added feature of the check valve is supported by the teaching in the application documents as originally filed, see page 6, lines 13 to 17 with Figures 1 to 3 (see check valve 118). Even though it follows from page 6, lines 13 to 17 that the check valve is part of a bottom hole assembly (BHA) supported by the drill string, together with a drill bit 120, a mud motor 118, a MWD/LWD sensor suite 119, a pressure transducer 116 and a telemetry package 122, a skilled reader of this passage would recognise that these further features are entirely optional to achieve the promised effect, namely to prevent backflow of fluid from the annulus into the drill string. Consequently, there is no unallowable intermediate generalisation, contrary to the respondent's view.

7. Claim 1 of auxiliary request 2 - Inventive step in light of D1

7.1 The drilling system of D1 shows all the features of claim 1 of the main request (see point 3 above) but it fails to disclose the added feature that "the drill string is provided with a check valve to prevent backflow of fluid from the annular space into the drill string". Hence, the system of claim 1 differs from that of D1 only in that it comprises such a check valve.

7.2 However, it is a standard safety measure to provide the drill string with a check valve, in order to prevent uncontrolled backflow up the drill string in the event of a sudden inflow of formation fluid in an unexpectedly high pressure area, see e.g. D2 (page 52), D4 (check valve 110), D5 (column 4, lines 40 to 44 and check valve 52) and D6 (column 1, lines 10 to 23). Hence, it would be an obvious safety measure for the skilled person to incorporate such a check valve in the drill string of D1.

7.3 The appellant submits that the skilled man would not provide the drill string of D1 with a conventional check valve because, as taught in column 8, lines 22 to 24 of D1, such a check valve would prevent reverse circulation. The Board is not convinced. The cited passage of D1 aims only to distinguish the mud saver valve assembly 110 as disclosed therein from an ordinary check valve ("It is only desired to distinguish the present valve assembly from an ordinary upwardly closing check valve which would prevent reverse circulation"). This passage would not hinder the skilled person from using a drill string check valve preventing unexpected backflow if it is considered necessary for safety reasons. In addition, it is common knowledge that check valves with reverse flow capabilities exist that allow regulation of the rate of reverse flow, see e.g. D6 (column 4, lines 9 to 14). If need be, the skilled person would, as a manner of routine design, incorporate such a check valve in the drill string of D1.

7.4 Thus, the Board agrees with the Opposition Division that the subject-matter of claim 1 lacks an inventive step in the sense of Article 56 EPC when starting from the conventional drilling system of D1.

8. Auxiliary request 3 - Articles 123 and 84 EPC

8.1 Claim 1 differs from claim 1 of auxiliary request 2 only in that it includes the further limitation that "a choke is provided in said fluid discharge conduit, whereby by maintaining pump action of said pump, sufficient flow through the choke to control backpressure is ensured". Method claim 5 differs from claim 5 of auxiliary request 2 in that it comprises the additional feature that "a choke ... is provided in said fluid discharge conduit" and the additional step of "maintaining pump action of said pump to ensure sufficient flow through the choke ... to control backpressure".

8.2 These amendments meet the requirements of Articles 123(2) and (3) EPC. The added features are supported by the teaching in the application documents as originally filed, see page 7, line 28 to page 8, line 21 and Figures 1 to 3 (see choke 130) and page 10, lines 13 and 27. In these passages, the function of the backpressure choke is described for the static mode, wherein the flow through the drill string is shut off. For a skilled reader of the application documents as originally filed, however, it is clear that these further features are not essential for the functioning of the backpressure choke. Consequently, the introduction of the isolated features does not amount to an unallowable intermediate generalisation, contrary to the respondent's view.

8.3 The amendments made in the claims also meet the requirements of Article 84 EPC. In claim 1, the wording "whereby by maintaining pump action of said pump, sufficient flow through the choke to control backpressure is ensured" is a functional feature, which further defines the choke.

9. Consideration of D7 in the appeal proceedings

9.1 D7 was filed after expiry of the opposition period and the Opposition Division decided to admit it into the proceedings, using its discretionary power under Article 114(2) EPC (see point 2.3.1 of the decision under appeal).

9.2 It is not the function of the Board to review all the facts and circumstances of the case as if it were in the place of the Opposition Division, in order to decide whether or not it would have exercised such discretion in the same way. Rather, the Board must confine its review as to whether the Opposition Division has exercised its discretion according to the wrong principles, without taking into account the right principles, or in an unreasonable way.

9.3 In the present case, in view of the reasons given in the decision under appeal and the summary provided in the minutes, the Board has no reason to overrule the way in which the Opposition Division exercised its discretionary power. In particular, the Board does not share the appellant's view that this decision was incorrect, since the disclosure of pages 1 and 2 of D7 was not "more weighty" than D3. Instead, the Board considers the principles applied by the Opposition Division (i.e. development of the case, complexity of the subject-matter, length of the document, prima facie relevance for the case) as being appropriate. Moreover, the respondent relied upon D7 in its reply to the statement of appeal grounds. Consequently, D7 is taken into consideration.

10. Claim 1 of auxiliary request 3 - Inventive step in light of D1 and D7

10.1 In addition to the feature of the check valve (see point 7 above), D1 fails to disclose a choke in the fluid discharge conduit. Hence, the drilling system of claim 1 differs from that of D1 in that:

- "the drill string is provided with a check valve to prevent backflow of fluid from the annular space into the drill string"; and

- "a choke is provided in said fluid discharge conduit, whereby by maintaining pump action of said pump, sufficient flow through the choke to control backpressure is ensured".

10.2 The Board agrees with the respondent that there is no functional reciprocal relationship between these distinguishing features. The check valve is not defined as being part of the backpressure system, neither in claim 1 nor in the patent specification (see paragraph [0015]), and it does not necessarily interact with the choke to control backpressure. The check valve is an independent safety device against uncontrolled backflow in the drill string. The two distinguishing features thus solve different partial problems, namely how to prevent uncontrolled backflow up the drill string in an unexpectedly high pressure area (check valve) and how to apply additional backpressure in the annulus (choke), and they can be considered independently for the purpose of inventive step analysis.

10.3 For the reasons already given for claim 1 of auxiliary request 2 (see point 7 above), it would be obvious for a skilled person to solve the first partial problem by providing the drill string with a safety check valve.

10.4 With respect to the second partial problem, it is common general knowledge to provide the annulus discharge conduit of an open circulation system with a choke, in order to create an additional backpressure in the annulus in the event that the weight of the circulated fluid is insufficient to balance the formation pressure and control kicks, see for instance D7, page 1, lines 6 to 18 and also page 1, lines 2 to 4 and page 2, lines 12 to 13. The skilled person would recognise the advantage of using such a choke to control the well before possibly reverse circulating heavy fluid. He would have no practical difficulties in incorporating the choke of D7 in the open circulation system of D1 by installing it in the annulus discharge conduit 171, for example in place of annulus wing valve 170, to add the required backpressure. Thus, the skilled person would arrive at the second distinguishing feature in an obvious manner.

10.5 Hence, the Board agrees with the Opposition Division that the subject-matter of claim 1 lacks an inventive step in the sense of Article 56 EPC when starting from D1.

11. Auxiliary request 4 - Consideration of the request in the appeal proceedings

11.1 Auxiliary request 4 was filed for the first time in the appeal proceedings, together with the statement of grounds of appeal.

11.2 The amendments relate to the introduction of limiting features in the independent claims of auxiliary request 3, with the aim of overcoming the Opposition Division's objection that the subject-matter of these claims lacks inventive step. Thus, the filing of this request does not amount to an abuse of procedure on the part of the appellant, and the Board decided to take it into consideration (Article 114(2) EPC and Article 12(4) RPBA).

12. Auxiliary request 4 - Articles 123 and 84 EPC

12.1 Claim 1 differs from claim 1 of auxiliary request 3 in that it includes the additional limitation that the three way valve is "controllable variable". Method claim 4 differs from claim 5 of auxiliary request 3 in that it comprises this additional feature and also the additional method step of "controlling delivery of the drilling fluid from the pump via the bypass conduit into the discharge conduit by controlling the controllable variable three way valve".

12.2 These amendments meet the requirements of Articles 123(2) and (3) and 84 EPC. In particular, the added features are supported by the teaching in the application documents as originally filed, see page 9, lines 5 to 29, in particular lines 6, 18, 24 and 27, and page 10, lines 13 to 15.

13. Claim 1 of auxiliary request 4 - Inventive step

13.1 The Board agrees with the parties that the drilling system in Figure 5 of D3 is the most promising starting point for the assessment of inventive step of the claimed drilling system, as it comprises a closed circulation system with automated means to continuously control the backpressure in the annulus and maintain the annular pressure within the desired operational window at all times.

13.2 This drilling system of D3 comprises (see Figure 5 below): a drill string 1, a pump 6 for pumping a drilling fluid from a drilling fluid source through the longitudinal drilling fluid passage of the drill string into the annulus 3, and a conduit 27 in fluid communication with the annulus 3 for discharging the drilling fluid. The annulus 3 is closed at all times by a pressure containment device in the form of a rotating BOP 26. The backpressure in the annulus is continuously adjusted by means of, among other things, a separate backpressure pump 23, a bypass conduit in the form of an injection line 22 and a choke in the form of a flow/pressure control device 12, which is provided in the discharge conduit 27 to decrease or increase the backpressure.

FORMULA/TABLE/GRAPHIC

13.3 The drilling system of claim 1 differs from that of D3 in that:

- the drill string is provided with a check valve to prevent backflow of fluid from the annular space into the drill string; and

- the backpressure system comprises a controllable variable three way valve that is provided between the longitudinal drilling fluid passage of the drill string and the pump for pumping drilling fluid through this passage into the annulus, whereby this pump is in fluid communication with the fluid discharge conduit via the three way valve and the bypass conduit.

13.4 The second group of distinguishing features has the effect that one and the same pump can be used to supply drilling fluid through the drill string and to add backpressure directly to the annulus via the annulus discharge conduit during well drilling, completion and intervention operations (see paragraphs [0011], [0023], [0029] in the patent specification). This makes the drilling system simpler and cheaper than that of D3.

13.5 The parties dispute which technical problem is solved by the second group of distinguishing features. The respondent submits that it solves the problem of using a single pump instead of two separate pumps (6 and 23 in D3). This formulation, however, includes some of the distinguishing features, and hence has an element of hindsight. The technical problem more objectively solved starting from D3 is how to simplify the drilling system, while still allowing additional backpressure in the annulus at any time, in particular when the drilling fluid is being circulated through the drill string.

13.6 The claimed solution to this objective problem is not rendered obvious by the teaching of D3 alone, or in combination with common general knowledge, as documented in D2.

13.7 The respondent submits that the claimed solution is suggested in the sentence bridging pages 46 and 47 of D3, which teaches that the injection line 22, i.e. the bypass conduit in the sense of claim 1, "may be incorporated as part of the standard circulation system, or embodied in other ways, the purpose being to provide an independent, of normal drilling circulation, means of flow into wellbore". However, this specific teaching must be read in context, in particular in the light of the teaching on page 20, line 1 to page 21, line 6, on page 29, lines 1 to 11 and on page 46, lines 24 to 26. When doing so, the only information which can be gleaned from D3 is that the system as described may comprise a standard well control means to inject fluid directly into the annulus and so add backpressure to compensate for the loss in annular frictional pressure when the normal circulation is stopped and that, in order to allow to decrease or increase the backpressure in the annulus at any time, in particular when the normal circulation is on, the well control means preferably comprises at least one circulation bypass consisting of a separate pump and a dedicated injection line. This does not provide any hint towards the claimed solution of using a single pump to impose additional backpressure on the annulus while circulating the fluid through the drill pipe.

13.8 The respondent also argues that the second group of distinguishing features is an obvious modification in the light of the teaching on pages 7 and 8 of the US patent application No. 10/368,128, one of the applications from which the disputed patent claims priority. However, no evidence has been submitted to show that this teaching was made public before the effective date of the patent.

13.9 The respondent further contends that, in light of common general knowledge, it would be obvious for the skilled person to use a single pump in place of the pumps 6 and 23. In support of this argument, the respondent refers to Figure 2.1 of D2 wherein a single pump 1 is used to circulate fluid through the drill string in the dynamic mode and to inject fluid in a kill line in the static mode. However, D2 only discloses conventional well control means which are used to inject drilling fluid directly into the annulus only in the static mode, i.e. when the normal circulation is stopped. Therefore, this conventional well control means leads the skilled person away from the claimed solution. The same would apply if he were to consider D1, since it discloses a similar conventional well control means.

13.10 In conclusion, when starting from D3, the provision of the second group of distinguishing features involves an inventive step.

13.11 The respondent has also disputed whether the subject-matter of claim 1 involves an inventive step when taking D1 or D2 as starting point, even though the conventional drilling system of either D1 or D2 is a less promising starting point than that of D3. For the sake of completeness, this issue is discussed hereafter:

13.12 Inventive step with respect to D1

In addition to the features of the check valve and the choke (see point 10 above), D1 fails to disclose the feature that the three way valve is "controllable variable".

The respondent contends that this feature is implicitly disclosed in D1 because the three way valve of D1 can be controlled to take at least two different states: the valves 195 and 197 can be operated so that the total pump output is delivered to the longitudinal drilling fluid passage, or alternatively to the discharge conduit 171. In addition, the respondent submits that the valve 197 of D1 could be partially opened during reverse circulation to allow the amount of reverse circulation to be regulated. The Board is not convinced by these arguments. It is common general knowledge that some valves are adjustable while others are not. The introduced limitation "controllable variable" makes it clear that the three way valve must allow variation or adjustment of the flow. This feature is neither disclosed nor suggested in D1. The valves 195 and 197 of D1 are closed and opened respectively in an emergency well control operation and there is no reason for them to be adjustable.

The distinguishing feature of the three way valve being "controllable variable" allows use of one and the same pump to create a backpressure on the annulus via the discharge conduit while continuing to circulate the fluid through the drill string. Neither D1, nor any of the cited prior art documents, discloses or suggests that this effect could be achieved by using a controllable variable three way valve (see points 13.7 to 13.9 above).

Thus, the claimed invention involves an inventive step over the conventional drilling system of D1.

13.13 Inventive step with respect to D2

D2 discloses in Figure 2.1 a conventional drilling system similar to that disclosed in D1. It comprises an open fluid circulation system with a pump 1 for pumping drilling fluid through the drill string and a flow line 11 for discharging the fluid from the annulus to a fluid tank 13. The flow line 11, not the choke line D, thus forms the fluid discharge conduit in the sense of claim 1. Should a kick occur, heavy fluid is injected directly into the annulus through the kill line B or C to kill formation pressure; the fluid is then discharged through the choke line D or E. When killing the formation pressure, the kill line B is used as a bypass conduit in the sense of claim 1. A choke G is provided in the choke line D, not in the discharge conduit 11. Hence, as with the system of D1, the drilling system of D2 fails to disclose a drill string check valve, a backpressure choke and a controllable variable three way valve as defined in claim 1. In addition, when killing the formation pressure by injecting heavy fluid in the annulus via bypass conduit B, the pump 1 is no longer be in fluid communication with the discharge conduit 11 via a three way valve and the bypass conduit B, because the conduit 11 is disposed above the then closed BOP.

The skilled person starting from D2 and using common general knowledge gains no motivation, at least for the reasons already given above when starting from D1, to apply the distinguishing features

Thus, the claimed invention involves an inventive step over the conventional drilling system of D2.

14. The above reasoning applies to the subject-matter of independent method claim 4 of auxiliary request 4 as well as that of the dependent claims of this request.

15. The Board comes to the conclusion that the claims of auxiliary request 4 meet the requirements of the EPC.

16. The description is not yet adapted to the wording of these amended claims. The Board exercises its discretion under Article 111(1) EPC to remit the case to the Opposition Division in order for the description to be adapted to the claims.