EP0424225A1 - Riser for deep waters - Google Patents

Abstract

The invention relates to a riser (6) for deep water, comprising a main tube (13) or extension tube and peripheral lines (12). This riser has at least two of the following characteristics: &cirf& use of an internal lining (15) of the extension tube, &cirf& use of a float (8) on only the upper part of the extension tube, &cirf& use of an extension tube and/or of at least one peripheral line of which the mass is lower than that of an equivalent steel tube and/or of an equivalent steel peripheral line respectively. <IMAGE>

Description

The present invention relates to a riser for great depths of water. This column can be used either in the field of drilling or in the field of production.

The drilling of very deep sea, for example beyond 1,000 m and in particular 2,000 m, requires that the architecture of the current columns be reviewed.

The difficulty raised by the transport, handling, storage and implementation of a very large number of elements making up this column, the resistance of the connector tubes and the floats subjected to very high static and dynamic stresses and sometimes difficult to know, the manufacture, use and maintenance of a very large volume of mud, would indeed greatly reduce the efficiency, reliability and safety of the drilling system and operations.

By the terms of riser, extension tube or risers, is meant a column allowing the transfer, in particular of fluids, between the bottom of the water and an installation which is situated at a higher level, that is to say which may be located substantially on the surface of the water, or be submerged.

The present invention provides a column capable of working at great depths by overcoming the difficulties indicated above, without greatly penalizing the efficiency or the reliability of the column.

We can observe, for a few years, the constant tendency of operators to increase the diameter of the laughter (16 "- 18 5/8" - 21 ") and the operating pressure (series 10.000 - 15.000) of BOP, this going from combined with the use of high density mud (maximum density considered in this study: 17ppg or 2.03) This development, justified by reasons of safety and efficiency, is particularly penalizing for the holding of the extension tube.

The problems associated with the static dimensioning of the riser appear when dimensioning a column exceeding 1,000 m according to the prior art.

It can be seen that the interior volume of the riser is greater than the volume of the well itself, as soon as the water depth exceeds 1,000 m (for example, the maximum volume of the well, which under 1,246 m of water, is 164 m3, while the riser volume reaches nearly 200 m3). It exceeds double or even triple this volume at 3,000 m of water depth. This is obviously not without raising serious technical and economic difficulties for the manufacture, storage, maintenance, control and treatment of the mud and, finally, nor without asking questions about the safety of such drilling.

As the apparent weight of this sludge must be entirely supported by the tensioners, this results in a head tension of riser the greater the greater the water depth, even if the self weight of the riser is zero (in the 'water).

This has the consequence of requiring, on the one hand, the installation on board of the surface units of a sufficient tensioning capacity, taking into account the recommendations made in the standards in force to date (API RP 2Q standard) and , on the other hand, the use of thicker tubes in the upper part of the riser so that the stresses there remain limited to admissible values.

Such sizing shows that the ratio between the minimum tension required at the top and the available tensioning capacity would, in many cases, exceed the 71% authorized by API RP 2Q standard, even though the apparent weight of the riser would have been taken equal to 0, which is neither in accordance with reality, nor desirable, as we will see below.

Finally, the different pressure generated in the lower part of the riser by the difference between the maximum density of the mud (d _mud = 2.03) and that of seawater (d _sw = 1.03) requires , here too, the use of very resistant tubes.

These observations illustrate, in fact, the difficulties raised by the simple static dimensioning of the riser due to the increase in weight and pressure of the sludge contained therein when the depth of water increases. One might think that it is enough, to overcome them, to use sufficiently thick tubes and fairly resistant connectors. Unfortunately, this is not correct due to the consequences that this would have on the floats, on the one hand, and the worsening of the dynamic behavior of the riser which would result from it, on the other hand.

• 1) Les tensions minimales en tête mentionnées ci-dessus ont été déterminées, comme on l'a dit, en supposant que le poids dans l'eau des éléments de riser était nul. Une telle hypothèse implique qu'une quantité suffisante de flotteurs leur soit attachée pour compenser complètement le poids des tubes et des connecteurs qui les constituent.
• 2) Or, on sait que la densité des flotteurs en mousse syntactique qui équipent habituellement les éléments de riser est d'autant plus élevée que la profondeur d'eau (et, par conséquent, la pression hydrostatique à laquelle ils doivent résister) est grande. Au-delà de 2.000 m (200 bar), la tenue de ces flotteurs n'est d'ailleurs pas prouvée et les coûts de développement, de qualication et d'approvisionnement correspondants risquent d'être très élevés.
• 3) D'autre part, le diamètre extérieur des flotteurs doit être inférieur à une certaine valeur, de l'ordre de 1,2 m (47 pouces), pour permettre leur passage dans la table de rotation 125,7 cm (49 1/2 pouces) pendant les manoeuvres du riser. Le volume des flotteurs et, par conséquent, la flottabilité induite sont donc physiquement limités. Ainsi, il existe dans la pratique une profondeur au-delà de laquelle les mousses syntactiques ne permettent plus de compenser complètement le poids du riser, le manque de flottabilité devant alors être compensé par une plus grande capacité installée de tensionnement. Cette profondeur peut être estimée à environ 2.000 m.

Regarding float issues, the following observations can be made:

• 1) The minimum head voltages mentioned above were determined, as said, by assuming that the weight in water of the riser elements was zero. Such an assumption implies that a sufficient quantity of floats is attached to them to completely compensate for the weight of the tubes and connectors which constitute them.
• 2) However, it is known that the density of the syntactic foam floats which usually equip riser elements is higher the greater the depth of water (and, consequently, the hydrostatic pressure which they must resist) . Beyond 2,000 m (200 bar), the behavior of these floats is also unproven and the development, qualification and corresponding supply costs may be very high.
• 3) On the other hand, the outside diameter of the floats must be less than a certain value, of the order of 1.2 m (47 inches), to allow their passage through the rotation table 125.7 cm (49 1/2 inches) during riser maneuvers. The volume of the floats and, consequently, the induced buoyancy are therefore physically limited. Thus, in practice there is a depth beyond which the syntactic foams no longer make it possible to completely compensate for the weight of the riser, the lack of buoyancy then having to be compensated for by a greater installed tensioning capacity. This depth can be estimated at around 2,000 m.

• 1) Lorsque le riser, connecté à la tête de puits, est soutenu par les câbles à tension constante d'un système d'anti-pillonnement, on constate que la réponse dynamique de celui-ci est d'autant moins bonne que la tension appliquée est plus grande. En d'autres termes, pour une densité de boue donnée, la raideur du système croît avec la profondeur d'eau. Il s'ensuit, à pilonnement constant, une augmentation de l'amplitude des oscillations de la tension (autour de la tension moyenne) dans toutes les sections du riser qui doit être prise en compte dans les calculs de tenue en fatigue des tubes et des connecteurs.
• 2) Mais c'est lorsque le riser, non connecté à la tête de puits, est suspendu sous le support flottant sans découplage des mouvements de pilonnement relatifs de ce riser et du support en cas de tempête ou de manoeuvre, que la situation est la plus préoccupante. L'accroissement de la longueur du riser provoque, en effet, une augmentation au moins proportionnelle de sa masse. Si, de plus, on considère que, d'une part, les tubes et les connecteurs doivent, pour les raisons exprimées plus haut, être plus résistants et donc plus lourds et que, d'autre part, les flotteurs qui leurs sont adjoints ont une densité d'autant plus élevée que la pression hydrostatique ambiante à laquelle ils doivent résister est grande, on conçoit que le facteur de proportionnalité est, en fait, supérieure à 1.
  Cette augmentation rapide de la masse du riser quand la profondeur d'eau augmente provoque l'émergence de deux phénomènes, peu importants et souvent négligés pour les profondeurs d'eau faibles et moyennes. Ces phénomènes peuvent alors conditionner le dimensionnement et leurs caractéristiques, leurs causes et leurs effets doivent être étudiés avec soin :
  . L'accroissement des surtensions dues à l'inertie du riser au cours des grandes tempêtes peut provoquer le détensionnement, partiel ou total, de la partie supérieure du riser et y induire, en corrélation avec les autres mouvements (cavalement, embardée) et l'action directe de la houle, des contraintes de flexion rédhibitoires.
  . L'élévation de la période propre en vibrations longitudinales au-delà des valeurs pour lesquelles l'amplitude du pilonnement est nulle, pourrait limiter considérablement, même par temps relativement calme, les opérations de manoeuvre du riser en raison des risques qu'elles présenteraient.

With regard to the problems linked to the dynamic behavior of the riser, several factors contribute to the deterioration of the dynamic behavior of the risers when the water depth increases:

• 1) When the riser, connected to the wellhead, is supported by the constant voltage cables of an anti-pillaging system, we note that the dynamic response of the latter is all the less good as the voltage applied is greater. In other words, for a given mud density, the stiffness of the system increases with the depth of water. It follows, with constant heaving, an increase in the amplitude of the oscillations of the tension (around the average tension) in all the sections of the riser which must be taken into account in the calculations of fatigue resistance of the tubes and connectors.
• 2) But it is when the riser, not connected to the wellhead, is suspended under the floating support without decoupling the relative pounding movements of this riser and the support in the event of a storm or maneuver, that the situation is the more worrying. The increase in the length of the riser in fact causes an at least proportional increase in its mass. If, in addition, it is considered that, on the one hand, the tubes and connectors must, for the reasons expressed above, be more resistant and therefore heavier and that, on the other hand, the floats which are attached to them have the higher the density, the greater the ambient hydrostatic pressure which they have to withstand, we can see that the proportionality factor is, in fact, greater than 1.
  This rapid increase in the mass of the riser when the water depth increases causes the emergence of two phenomena, unimportant and often overlooked for shallow and medium water depths. These phenomena can then condition the dimensioning and their characteristics, their causes and their effects must be carefully studied:
  . The increase in overvoltages due to the inertia of the riser during major storms can cause the stress relieving, partial or total, of the upper part of the riser and induce there, in correlation with the other movements (cavally, swerve) and the direct action of the swell, unacceptable bending stresses.
  . The elevation of the natural period in longitudinal vibrations beyond the values for which the amplitude of the heaving is zero, could considerably limit, even in relatively calm weather, the operations of maneuvering the riser because of the risks that they would present.

• 1) La pression de service des lignes de sécurité ("Kill and choke lines"), portée à 1.050 bar (15.000 psi), nécessite des tubes plus épais qui contribuent à augmenter le poids du riser et, par conséquent, à accroître les problèmes liés à son comportement dynamique (voir ci-dessus).
• 2) L'augmentation des pertes de charges induites dans ces lignes au cours des opérations de contrôle des éruptions rendent ces dernières plus délicates et périlleuses à effectuer.
• 3) L'éloignement de la tête de puits et la pression intense qui y règne rendent plus difficile à déceler la présence éventuelle de gaz dans le puits et à agir en conséquence, en fermant à temps les vannes des obturateurs. La présence de quantités notables de gaz dans le tube central du riser devient donc plus probable.
• 4) Cela se traduit par la nécessité renforcée d'utiliser des matériaux parfaitement résistants à la corrosion provoquée par les effluents pétroliers, en particulier par l'hydrogène sulfuré H₂S.
• 5) Il en résulte aussi le risque de voir ce gaz, une fois décomprimé, emplir partiellement ou complètement l'intérieur du tube principal dont la paroi, insuffisamment épaisse pour résister à la comprssion hydrostatique qui s'ensuivrait, ne permettrait pas d'éviter l'écrasement (collapse).

The problems related to blowout control and drilling safety are, ultimately, of less importance on the design and sizing of the riser compared to the existing situation for shallower water depths. The main constraints in this area, when the water depth increases, are as follows:

• 1) The operating pressure of the safety lines ("Kill and choke lines"), brought to 1.050 bar (15,000 psi), requires thicker tubes which contribute to increase the weight of the riser and, consequently, to increase the problems related to its dynamic behavior (see above).
• 2) The increase in pressure losses induced in these lines during eruption control operations make them more difficult and dangerous to perform.
• 3) The distance from the well head and the intense pressure which reigns there make it more difficult to detect the possible presence of gas into the well and act accordingly, closing the shutter valves in time. The presence of significant quantities of gas in the central tube of the riser therefore becomes more likely.
• 4) This results in the reinforced need to use materials which are perfectly resistant to corrosion caused by petroleum effluents, in particular by hydrogen sulphide H₂S.
• 5) It also results in the risk of seeing this gas, once decompressed, partially or completely fill the interior of the main tube whose wall, insufficiently thick to withstand the hydrostatic compression which would ensue, would not make it possible to avoid the crash.

• 1) La résistance de la structure de la plate-forme, sous les aires de stockage, doit être suffisante pour supporter la masse totale de ces éléments (voir ci-dessus).
• 2) Le volume occupé par les éléments, compte-tenu de la dimension des flotteurs, doit être compatible avec les emplacements disponibles et avec la stabilité navale de l'appareil.
• 3) Le rangement et la manutention du riser doivent permettre la mise en oeuvre, selon un ordre précisément établi, d'éléments de caractéristiques différentes (épaisseur du tube central, densité des flotteurs...) d'autant plus nombreuses (4 ou 5) que la profondeur d'eau est plus grande. L'utilisation des moyens de manutention automatique des éléments de riser, de plus en plus répandus sur les appareils modernes, doit aussi être considérée.

The last type of problem mentioned concerns the difficulty of storing the large number of elements making up the riser on the deck of floating supports when its length increases. Several factors must be considered:

• 1) The strength of the platform structure, under the storage areas, must be sufficient to support the total mass of these elements (see above).
• 2) The volume occupied by the elements, taking into account the size of the floats, must be compatible with the available locations and with the naval stability of the aircraft.
• 3) The storage and handling of the riser must allow the implementation, in a precisely established order, of elements of different characteristics (thickness of the central tube, density of the floats ...) all the more numerous (4 or 5 ) that the water depth is greater. The use of means for automatic handling of riser elements, which are more and more widespread on modern devices, must also be considered.

The riser for great depth of water according to the invention comprises a main tube or extension tube, peripheral lines. This line is characterized in particular in that it has at least two of the following characteristics:
- use of an internal lining of the extension tube,
- use of float only on the upper part of the tube extender,
- use of extension tube and / or at least one peripheral line whose mass is lower than that of an equivalent steel tube and / or respectively of an equivalent peripheral steel line.

Of course, the column according to the invention can reproduce the three characteristics.

The extension tube and / or at least one of said peripheral lines may be made in part at least from a low density metallic material, such as a titanium alloy or / and comprises a composite material.

The reduction in mass may be obtained by strengthening by hooping at least part of the extension tube or at least part of at least one of the peripheral lines.

Reinforcement by shrinking can be obtained by winding a reinforcing tape around said at least part of said extension tube or said part at least of at least one of said peripheral lines.

The upper part may include the floats over a length or depth at most equal to 2,000 m.

The upper part may include the floats at a length or depth less than or equal to 2/3 of the total length of said column.

The extension tube may be constituted by the assembly of several elements comprising bayonet type connectors.

The extension tube may consist of the assembly of several elements and the column may include a lining of the extension tube, said lining being produced by the assembly of several lining elements, each of said lining elements being suspended from a corresponding element of the extension tube.

• 1) Fiabilité du montage mécanique et de l'étanchéité entre deux tubes consécutifs qui s'obtient par simple emboîtement.
• 2) De par cette conception, le chemisage n'est dimensionné que pour une différence de pression entre l'intérieur du tube et l'espace annulaire. Il ne travaille pratiquement pas en traction et est peu sollicité dynamiquement. Son poids est donc réduit, et celà d'autant plus que des matériaux de flottabilité (mousses syntactiques par exemple) peuvent être placés dans l'espace annulaire.
• 3) Ce type de chemisage ne nécessite aucun tensionnement, ni vissage ou serrage de connecteurs lors de sa mise en oeuvre dans le riser qui devient très simple et rapide.
• 4) Le comportement mécanique de l'ensemble "riser/chemisage" peut, à tout moment, être optimisé en modifiant la nature et la pression du fluide emplissant l'espace annulaire, grâce aux lignes de communication haute et basse prévues à cet effet.
• 5) En cas d'intervention sur les obturateurs de sécurité BOP (Blow Out Preventers) nécessitant la remontée du riser alors que le chemisage est en place à l'intérieur, il est possible de remonter, de stocker et de redescendre simultanément le riser et le chemisage, ces opérations pouvant donc être effectuées sans perte de temps supplémentaire.

Thus, the internal lining of the extension tube is no longer treated as casing lowered into the riser, but as an internal line of each element of the riser, put in place as it descends, and allows resolve most of these difficulties as described in the French application filed on behalf of the Institut Français du Pétrole on August 7, 1989 and which is part of this application.

• 1) Reliability of the mechanical assembly and of the seal between two consecutive tubes which is obtained by simple interlocking.
• 2) By this design, the liner is only dimensioned for a pressure difference between the inside of the tube and the annular space. It practically does not work in traction and is little dynamically stressed. Its weight is therefore reduced, and all the more so since buoyancy materials (syntactic foams for example) can be placed in the annular space.
• 3) This type of liner does not require any tensioning, nor screwing or tightening of connectors during its implementation in the riser which becomes very simple and rapid.
• 4) The mechanical behavior of the "riser / liner" assembly can, at any time, be optimized by modifying the nature and the pressure of the fluid filling the annular space, thanks to the high and low communication lines provided for this purpose.
• 5) In the event of intervention on the BOP (Blow Out Preventers) safety shutters requiring the riser to be raised while the lining is in place inside, it is possible to simultaneously raise, store and lower the riser and the lining, these operations can therefore be carried out without loss of additional time.

In practice, the use of a jacket becomes necessary only from a certain depth of water, depending on the particular data of the case considered, but which is of the order of 2,000 m. It would be taken into account in the following way in the static dimensioning of the riser:
- dimensioning of the central tube to the extension tube for an operation without lining and the head tension available, taking into account a mud density compatible with the realization of the first drilling phases (up to 44.5 cm, that is 17 1 / 2 "),
- dimensioning of the central tube for operation with a liner and the heaviest mud density required for carrying out small diameter drilling phases,
- the dimensioning finally selected would be the more efficient of the two thus obtained.

• 1) Non-emploi des flotteurs dans les profondeurs d'eau où ils sont les moins performants, les plus coûteux et où leur diamètre serait supérieur au diamètre de passage dans la table de rotation.
• 2) Diminution substantielle de la masse totale du riser contribuant à résoudre les problèmes liés au comportement dynamique (diminution des surtensions) et au stockage.
• 3) Augmentation du poids dans l'eau du riser, complétant l'effet précédent pour résoudre efficacement et de manière entièrement fiable, puisque passive, les difficultés soulevées par le comportement dynamique (augmentation de la tension moyenne).

One of the advantages induced by the use of the lining is, as we have seen, to release a tensioning capacity, hitherto used to support the weight of the mud, which is added to that resulting from the planned increase the number of tensioners to equip the drilling rig when its depth of intervention water increases. This additional tensioning capacity can be used to remove the floats in the lower part of the riser, which has many advantages:

• 1) Non-use of floats in water depths where they are the least efficient, the most expensive and where their diameter would be greater than the diameter of passage in the rotation table.
• 2) Substantial reduction in the total mass of the riser helping to solve the problems linked to dynamic behavior (reduction of overvoltages) and storage.
• 3) Increase in the weight in the riser water, supplementing the previous effect to solve effectively and in a completely reliable manner, since passive, the difficulties raised by the dynamic behavior (increase in the average tension).

In practice, the objective has been set to avoid the use of foam floats below about 2,000 m of water depth, depth to which their effectiveness has been demonstrated under operational conditions.

A further reduction in the mass of the riser can be obtained according to the present invention, increasing when it is possible, the resistance of the tubes vis-à-vis the internal pressure, by means of a hooping obtained by winding under tension around these tubes strips of composite material (glass, Keylar or carbon fibers coated in a resin thermoplastic), according to a technique described in US Pat. No. 4,514,245. This technique makes it possible to multiply by a factor at least equal to 2 the circumferential resistance of the tubes, without increasing their weight or, conversely, reducing their weight for a given resistance.

• 1) Sous pression nulle, la précontrainte induite dans le tube en acier par les rubans, est d'autant plus grande (en valeur absolue) que la tension d'enroulement et le nombre de couches sont eux-mêmes grands. Ceci permet d'ajuster, au stade de la fabrication, la valeur de la précontrainte au strict besoin justifié par les conditions de service des tubes.
• 2) Lorsque la pression augmente, les contraintes dans les deux constituants du tube s'accroissent linéairement, mais en général beaucoup plus rapidement dans l'acier que dans le composite en raison des raideurs différentes des deux matériaux. Cette phase se poursuit jusqu'à ce que la contrainte dans le tube en acier atteigne la limite élastique.
• 3) Au-delà du franchissement de la limite élastique dans l'acier, c'est le frettage composite qui reprend la plus grande partie des efforts supplémentaires et qui "retient" le tube en acier, empèchant son éclatement qui interviendrait très rapidement en son absence. Cette phase se poursuit jusqu'à la rupture des rubans qui, eux, ne possèdent pas de domaine plastique.
• 4) L'éclatement final intervient dès la rupture des rubans, le tube en acier étant bien entendu incapable de supporter la pression à lui tout seul.

The operating principle of hooped tubes can be illustrated by considering the evolution of the circumferential stresses in the steel tube and in the composite hooping when the internal pressure increases from 0 to bursting. Four main phases can be described:

• 1) At zero pressure, the prestress induced in the steel tube by the ribbons, is all the greater (in absolute value) the greater the winding tension and the number of layers. This makes it possible to adjust, at the manufacturing stage, the value of the prestressing to the strict need justified by the conditions of service of the tubes.
• 2) When the pressure increases, the stresses in the two components of the tube increase linearly, but in general much faster in steel than in composite due to the different stiffnesses of the two materials. This phase continues until the stress in the steel tube reaches the elastic limit.
• 3) Beyond the crossing of the elastic limit in steel, it is the composite hooping which takes up most of the additional efforts and which "retains" the steel tube, preventing its bursting which would intervene very quickly in its absence. This phase continues until the ribbons which do not have a plastic domain break.
• 4) The final burst occurs as soon as the ribbons break, the steel tube of course being incapable of withstanding the pressure on its own.

Compared to a conventional steel tube, the field of use of the material in terms of constraints is therefore widened in both directions:
- Towards negative values (prestressing effect), the original zero being shifted (by - 322 MPa in the previous example). In fact, this amounts to artificially increasing the elastic limit of the steel without raising the usual problems of high strength steels (poor weldability, poor resistance to corrosion and fatigue, delicate processing, high cost). In the previous example, the apparent elastic limit would be greater than 760 MPa, while the real value is only 437 MPa (X65 steel).
- Beyond the elastic limit of the steel, we saw that there was a large margin before the tubes burst (autofrettage effect) which can be used to optimize their dimensioning. Thus, contrary to the criteria usually used, it is possible to use almost all of the elastic range of steel for operation in service or in test tubes, the necessary safety factor, sometimes imposed by regulations, being induced by the auto-frettage effect. Thus, in the previous example, we managed to ensure that the stress at the test pressure (1,575 bar) was only slightly below the elastic limit, a factor of 1.5 still existing before bursting (2,350 bar), while without hooping, it would not be possible, under identical conditions, to exceed 70% of this same elastic limit.

• 1) Les lignes de sécurité ("kill and choke lines") dont la pression de service de 1.050 bar nécessiterait l'emploi de tubes en acier de 1" (25,4 mm) d'épaisseur. Celle-ci peut être ramenée à 10 mm (du même acier) grâce au frettage composite. Leur poids total (embouts compris) est ainsi divisé par un facteur 2 en même temps que leur diamètre intérieur des tubes est sensiblement augmenté (94 mm au lieu de 76 mm) afin de faciliter le contrôle des éruptions et de contribuer à améliorer la sécurité des forages.
• 2) Le tube principal dans la partie inférieure du riser, de façon à lui donner une résistance suffisante pour que, sans augmenter l'épaisseur d'acier, les contraintes circonférentielles n'y dépassent pas la limite fixée par API RP 2Q. La méthodologie employée pour le dimensionnement de ce tube dans la suite de l'étude sera la suivante :
  . détermination de l'épaisseur du tube principal en acier du seul point de vue des contraintes longitudinales, statiques et dynamiques, de traction et de flexion,
  . détermination des caractéristiques du frettage composite pour donner à ces tubes une résistance suffisante vis-à-vis de la pression interne,
  . vérification du niveau admissible des contraintes combinées de Von Mises.
• 3) La méthode sera appliquée à la partie inférieure des tubes de chemisage, à la différence près que, dans le concept proposé, ceux-ci n'ont pas à être dimensionnés pour résister à la traction qui y est très faible. L'épaisseur du tube en acier peut donc être réduite au minimum compatible avec les impératifs opérationnels et industriels.

This reinforcement technique by composite hooping of tubes working at internal pressure will be widely used for the architecture of risers. It is planned, in particular, to use it for the constitution of the following tubes:

• 1) Safety lines ("kill and choke lines") whose operating pressure of 1.050 bar would require the use of 1 "(25.4 mm) thick steel tubes. This can be reduced to 10 mm (of the same steel) thanks to the composite hooping. Their total weight (end caps included) is thus divided by a factor 2 at the same time as their inner diameter of the tubes is significantly increased (94 mm instead of 76 mm) in order to facilitate the control of blowouts and to contribute to improving the safety of drilling.
• 2) The main tube in the lower part of the riser, so as to give it sufficient strength so that, without increasing the steel thickness, the circumferential stresses do not exceed the limit fixed by API RP 2Q. The methodology used for the dimensioning of this tube in the rest of the study will be as follows:
  . determination of the thickness of the main steel tube solely from the point of view of longitudinal, static and dynamic, tensile and bending stresses,
  . determination of the characteristics of the composite hooping in order to give these tubes sufficient resistance to internal pressure,
  . verification of the admissible level of the combined constraints of Von Mises.
• 3) The method will be applied to the lower part of the lining tubes, with the difference that, in the proposed concept, they do not have to be dimensioned to resist the traction which is very weak there. The thickness of the steel tube can therefore be reduced to the minimum compatible with operational and industrial requirements.

The lightening of the extension tube with low density materials makes it possible to better solve the problems posed previously. Such lightening can be obtained by replacing all or part of the riser, the steel constituting the main tube (tubes and connectors) by the titanium alloy, for example of the Ti-6Al-4V type, three times more efficient if we consider its specific resistance (ratio of the elastic limit to the density) and, also, more resistant to fatigue and to marine and petroleum corrosion.

Such tubes can be obtained by the following operations:
- stamping with sheet metal in a "die / punch" tool, so as to obtain semi-cylindrical shells,
- production of the elementary tubes by longitudinal welding, along two generatrices, of two half-shells,
- splicing, by circular welding, of elementary tubes and end pieces so as to obtain elements of extension tubes.

French patent application FR-2,620,956 proposes a process for manufacturing titanium tubes.

The titanium tube may be reinforced by the composite hooping in a similar manner to that described above and a steel liner, of identical design to that defined above, must be deployed over its entire length. This high-performance architecture should allow the extension of the extension tube concept to at least 12,000 feet (3,600 m) of water depth, with the exception that the problems raised by a specific period of longitudinal vibration equal to or greater than 7 seconds do not turn out to be unacceptable.

• - la figure 1 représente un schéma d'ensemble d'une colonne selon l'invention,
• - les figures 2 et 3 représentent en coupe cette colonne à deux côtés différents,
• - la figure 4 illustre un élément constitutif de la colonne, cet élément étant équipé de moyens réduisant son poids apparent dans l'eau, tels des flotteurs, et
• - la figure 5 est une coupe d'un élément de la colonne dont le tube prolongateur est renforcé par un frettage externe.

The present invention will be better understood and its advantages will appear more clearly on description which follows of particular examples, in no way limiting, illustrated by the attached figures among which:

• FIG. 1 represents an overall diagram of a column according to the invention,
• FIGS. 2 and 3 represent in section this column with two different sides,
• FIG. 4 illustrates an element constituting the column, this element being equipped with means reducing its apparent weight in water, such as floats, and
• - Figure 5 is a section of an element of the column whose extension tube is reinforced by an external hooping.

In FIG. 1, the reference 1 designates a surface installation, such as a ship.

Reference 2 designates the means of attachment to the bottom of the water 3 of this column to a well head 4. This connection is made by means of a flexible joint 5. The column according to the invention is designated as a whole by the reference 6. In this figure the peripheral lines have not been drawn so as not to weigh down the representation.

Column 6 has two parts. The first designated by the reference 7 is composed of elements 7a, 7b ... 7n equipped with floats 8. The second, referenced 9, does not include a float and is constituted by the assembly of elements 9a, 9b .. .9n.

Thus, the extension tube according to the present invention comprises floats only on the highest part of the column. For example, the presence of floats can be interrupted from depths greater than 2,000 m. The column according to the present invention, or a part of this column which does not include a float, may be made of light materials, for example by the use of titanium, of composite material, or by the use of hooping based on composite material, for example using a ribbon shrinking a tube.

The part of the extension tube fitted with a float may include floats, continuously or intermittently. Of course, the presence of floats may be interrupted in the vicinity of the surface installation.

Figure 2 is a section along line AA of the column according to the invention. It was considered in this section that the main tube of the column was not equipped with internal lining.

Reference 8 designates the float. This float has clearances 11 adapted to receive the peripheral lines 12. The float is fixed around the main tube 13. In this example, the peripheral lines are secured to the extension tube 13 by means of arms 14.

Figure 3 shows a section along line BB of the column at a level where the floats 8 have been removed. Only the peripheral lines 12 fixed to the extension tube 13 remain by means of arms 14.

Furthermore, in Figure 3, we have considered the case of the use of a liner 15 which makes it possible to reduce the quantity of mud circulating inside the column, for example during drilling of small diameter. In FIG. 3, the mud circulates in the internal cylindrical zone 100 and the annular space 101 between the liner 15 and the extension tube 13 would be free.

These internal linings can advantageously be produced in accordance with the request filed on August 7, 1989 under the EN. 89 / 10.755 on behalf of the French Petroleum Institute.

FIG. 4 illustrates an element of the column, the column being constituted by the assembly of several of these elements, whether or not they are fitted with floats.

In the example used in FIG. 4, the column is produced by assembling elements already equipped with their peripheral lines. Reference 16 designates the element as a whole. This element has two ends 17 and 18. The references 19, 20 and 21 denote peripheral lines which are assembled to the extension tube 22 by means of fixing means which may include flanges (not shown), plates 23 and collars 40.

The element represented in FIG. 4 comprises floats 8. The assembly of the two adjacent elements is done by the connectors which can be of the bayonet type 24 and 25. The assembly of two adjacent elements connects the peripheral lines between- them, by joining the ends 26, 27 and 28 to the adjacent ends of the peripheral lines corresponding to the ends 29, 30 and 31.

FIG. 5 represents a section of an element making up the column according to the invention.

The peripheral lines bear the references 32, 33 and 34. These peripheral lines are connected to the element of the extension tube 35 by means of arms 36, 37 and 38. In this embodiment, the element of the extension tube 35 is reinforced by a hooping 39 which can be produced according to the teachings of American patent N. 4,514,245.

The embodiment bringing together all the elements represented in FIGS. 1 to 5 makes it possible to carry out operations of drilling or production for water depths greater than 2,300 m.

Such an embodiment combines the three characteristics which make it possible to reach such operating depths.

These characteristics are the use of floats only on the upper part of the column, the use of a liner 15 for part of the operations calling for the use of dense mud whose density is close to or greater than 2 and finally the use of an extension tube 35 or of lower mass than the equivalent elements made of steel. This can be obtained by the use of low density material, such as titanium, by the use of a tube made of composite material with organic matrix or other, as well as by the use of reinforcement by shrinking. The latter technique improves the mechanical strength of a tube without excessively increasing its weight.

According to the present invention, it is possible to hoop the extension tube and / or the peripheral lines 32, 33 and 34.

The hooping 39 makes it possible to have elements of the light extension tube having good mechanical performance, in particular to the pressure differences existing between the internal part of these elements and the ambient medium.

Of course, it will not depart from the scope of the invention if the elements of the extension tube are made of low density materials, such as titanium, or by making them using composite materials, in particular with an organic matrix reinforced by fiberglass, or Keylar, carbon wires.

Similarly, according to the present invention, only a part of the column may include an extension tube, or peripheral lines reinforced by hooping. Indeed, it is towards the greatest depths that such arrangements are necessary.

The beneficial effects of the architectural measures explained above will be better understood by the description of a particular application case, corresponding to a riser according to the invention, able to work in 3,000 m of water depth.

The main data used in this description are defined below:
- water depth: 3,000 m
- outer diameter of the riser: 533.4 mm (21 ")
- effective element length: 22.86 m (75ft)
- peripheral lines:
. kill and choke lines (operating pressure: 15,000 psi)
. booster and hydraulic lines (operating pressure: 5,000 psi)
- maximum mud density:
. 1.68 (14ppg) up to installation of 13 3/8 "tubing
. 2.03 (17ppg) beyond
- dimensioning criteria: API RP 2Q
- tensioning capacity: 908t (2,000kip) usable at 71% (645t)
- used materials :
. X80QT steel (yield strength: 560 MPa)
. titanium Ti-6Al-4V (elastic limit: 830 MPa)
. Keylar fibers and polyamide resin for shrinking
. syntactic foams for floats
- density of floats:
. 0.368 (for depths up to 650 m)
. 0.456 (for depths up to 1.350 m)
. 0.513 (for depths up to 2,000 m)

• 1) les lignes de sécurité ("kill and choke lines") ont un diamètre intérieur de 93 mm (3,7") et sont renforcées par un frettage composite,
• 2) une partie des éléments du riser possède un tube principal en alliage de titane, ce qui permet de réduire considérablement leur poids,
• 3) l'épaisseur du tube principal est calculée en considérant uniquement les efforts longitudinaux de traction et de flexion ; la résistance aux efforts circonférentiels exercés par la pression est ajustée par un renforcement optimisé du tube au moyen du même procédé de frettage composite,
• 4) un tube de chemisage est mis en place à l'intérieur du riser après la pose du tubage 340 mm (13 3/8") ; la résistance circonférentielle de ce tube est aussi augmentée, en fonction des besoins, par un frettage composite,
• 5) les contraintes principales (longitudinales, radiales et circonférenteilles) et la contrainte combinée de Von Mises sont limitées, en conditions normales de forage, à 1/3 de la limite élastique du matériau constitutif du tube principal,
• 6) une partie des éléments de riser sont équipés de flotteurs de façon à ce que leur poids apparent dans l'eau soit théoriquement nul. Toutefois, l'emploi de flotteurs en mousse syntactique n'est pas retenu au-delà de 2.000 m de profondeur d'eau ; trois éléments sans flotteurs sont aussi prévus en partie supérieure du riser.

The guidelines used to define the characteristics of the riser follow from the indications already provided in this application. The most important of them are recalled below:

• 1) the safety lines ("kill and choke lines") have an internal diameter of 93 mm (3.7 ") and are reinforced by a composite hooping,
• 2) a part of the riser elements has a main tube made of titanium alloy, which makes it possible to considerably reduce their weight,
• 3) the thickness of the main tube is calculated by considering only the longitudinal tensile and bending forces; the resistance to the circumferential stresses exerted by the pressure is adjusted by an optimized reinforcement of the tube by means of the same composite hooping process,
• 4) a lining tube is placed inside the riser after the installation of the 340 mm (13 3/8 ") casing; the circumferential resistance of this tube is also increased, as required, by a composite hooping ,
• 5) the main stresses (longitudinal, radial and circumferential) and the combined stress of Von Mises are limited, under normal drilling conditions, to 1/3 of the elastic limit of the material of the main tube,
• 6) part of the riser elements are fitted with floats so that their apparent weight in water is theoretically zero. However, the use of syntactic foam floats is not retained beyond 2,000 m of water depth; three elements without floats are also provided in the upper part of the riser.

The composition of the riser was defined after a general study, static and dynamic, of the stresses in the main tube. Its main characteristics are given in the table below.

The following additional details can be added to the previous data:
1) Titanium was only used for the lower 1,000 m of the riser, the steel being preserved over most of the length. Several considerations are at the basis of this choice:
- the cost of titanium tubes being much higher than that of steel tubes, it is important to limit their number as much as possible.
- the total mass of the riser resulting from this architecture is, as will be seen, sufficiently reduced to give it acceptable dynamic behavior under the most severe weather-oceanographic conditions considered. A riser whose main tube is made entirely of titanium and BOARD Number of elements Material used Tube thickness Density floats Diameter floats Effective length Weight floats Total mass Apparent weight 3 steel 19.1mm - - 69m - 30t 25t 26 steel 19.1mm 0.368 1.10 594m 126t 391t 0t 29 steel 19.1mm 0.456 1.16 663m 202t 497t 0t 29 steel 91.1mm 0.513 1.21 663m 252t 547t 0t 45 titanium 12.7mm - - 1029m - 248t 202t 132 mixed 3018m 580t 1713t 227t whose mass would therefore be less than 1,000 t, would be recommended for greater water depths (up to 3,500 or 4,000 m).
- The natural period of the riser thus defined is, in all cases, less than 7 seconds, whereas it would exceed this value in the hypothesis of a tube made entirely of titanium, which could present serious drawbacks.
The connection between the steel and titanium elements would be made, without any particular difficulty, by means of special short joints.
2) The diameter of the floats was calculated assuming that they covered almost the entire tube of each element and that the presence of the peripheral lines reduced to 85% the effective volume occupied by the foams. It is found that the diameter thus calculated is compatible with the passage of all the elements of the riser in the rotation table 49 1/2 "(1.26 m).
3) The ratio of the apparent weight to the mass of the riser is 13% which is, as will be seen, a value sufficient to give it excellent stability and prevent stress relieving.
4) Additional equipment has been taken into account in the calculations presented below. Those are :
- a telescopic seal, 30 m long and 20 t mass, constituting the connection between the riser itself and the floating support during drilling operations. While waiting, the telescopic seal is removed and the riser is suspended under the support, placed on a swiveling table (spider) or taken by other means,
- an elastic articulation constitutes the connection between the riser and the well head when the first is connected to the second,
- the wellhead has, under the joint, the connection gasket (mass 125 t) and the BOP 47.6 cm (18 3/4 ") - 15,000 proper (mass 225 t). two pieces of equipment or the first only that can be suspended under the riser when it is not connected (operation or waiting situation),
- short joints with various functions (length adjustment, steel / titanium connection, measuring joint, filling valve, etc.) may also be present without this modifying the results presented below.

Estimated calculations have shown that the column according to the invention has satisfactory mechanical behavior and is compatible with the safety requirements essential for its operational use.

The angle at the foot of the riser is kept within the limit of 2 ^o fixed by the API if the offset at the head did not exceed 1% of the water depth and if the current was not very violent. Otherwise, an additional tension of a few tens of tonnes should be applied.

These estimated calculations have shown that the mechanical behavior, both static and dynamic, of the riser is perfectly controlled. The stresses of all kinds remain below a third of the elastic limit, the maximum value that has been set, and the amplitude of the overvoltages is very much lower than the average tension, thus eliminating any risk of stress relieving and not requesting the material beyond its limits.

This is in addition to the direct advantages induced by the various measures explained in this paper: very significant reduction in the volume of mud in the drilling phases where it is the heaviest, elimination of the floats where they are the least efficient and the most expensive. , significant reduction in the weight of riser elements, facilitating their storage and handling on the deck of platforms, greater efficiency of safety lines, due to the increase in their passage diameter.

This set of qualities, made largely possible by the intensive use of new materials, undeniably gives the new riser architecture a remarkable ability to be used, in complete safety, in the deepest water depths. currently being considered for offshore drilling.

In addition, the nature of the solutions implemented, characterized by permanent availability and which, unlike competing devices such as air floats or active tensioning of the riser, do not require any human or material intervention to be effective at the time. timely, provides the drilling system with high reliability and high productivity, guarantees of profitability essential for such risky operations, both technically and economically.

Claims (10)

1. - Riser for large water depth comprising a main tube or extension tube, peripheral lines, characterized in that it has at least two of the following characteristics:
- use of an internal lining of the extension tube,
- use of float only on the upper part of the extension tube,
- use of extension tube or at least one peripheral line whose mass is lower than that of an equivalent steel tube or respectively of an equivalent peripheral steel line. 2. - Column according to claim 1, characterized in that it has all of said three characteristics. 3. - Column according to one of claims 1 to 2, characterized in that said extension tube or at least one of said peripheral lines is made at least in part from a low density metallic material, such as a titanium alloy. 4. - Column according to one of claims 1 to 3, characterized in that said reduction in mass is obtained by strengthening by hooping at least part of the extension tube or at least part of at least one peripheral lines. 5. - Column according to claim 4, characterized in that said reinforcing by hooping is obtained by winding a reinforcing tape around said at least part of said extension tube or said part at least of at least one said peripheral lines. 6. - Column according to one of claims 1 to 5 comprising floats on said upper part, characterized in that said upper part comprises the floats over a length at most equal to 2,000 m. 7. - Column according to one of claims 1 to 6 comprising floats on said upper part, characterized in that said upper part comprising the floats has a length less than or equal to 2/3 of the total length of said column. 8. - Column according to one of claims 1 to 7, characterized in that said extension tube is constituted by the assembly of several elements comprising connectors of the bayonet type. 9. - Column according to one of claims 1 to 8, characterized in that said extension tube is constituted by the assembly of several elements and in that said column comprises a lining of the extension tube, said lining being produced by assembly of several lining elements, each of said lining elements being suspended from a corresponding element of the extension tube. 10. - Column according to one of claims 1 to 3, characterized in that said extension tube or at least one of said peripheral lines comprises a composite material.

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