SEAL ELEMENT

SEAL ELEMENT

Technical field of the invention The present invention concerns the field of elastomeric seals, more particularly elastomeric seals suitable for use in applications where the seals are subject to transient conditions of very low temperatures.

Background

In high pressure retaining systems, such as subsea production and workover equipment, there may be situations when the system pressure needs to be reduced to a required level, e.g. through choking. Rapid depressurization may also be encountered in some units, e.g. quick disconnect pressure control packages installed between Christmas Tree (XT) or Wellhead (WH) and riser string, when the emergency shutdown function is activated.

When gas flow is decreased by a choke valve, or it is released through a blow-down valve with a high pressure differential between up- and downstream the plugs, Joule-Thomson effect takes place manifesting itself in a sharp temperature drop. Although the cooling time may be rather short (typically less than an hour) it is very detrimental for elastomeric seals which are vulnerable to large temperature variations. In fact, any shock or undesirable movement could crack or rupture an elastomeric composite when it is cooled down to a brittle state.

Another point to consider here is regions with harsh weather conditions, in particular those of high alternating diurnal temperatures in mid seasons. In some areas of Arctic the maximum daily temperature difference may reach 30-40 °C. In the glassy or brittle state rubber seals may not necessarily be damaged, but, since the material stiffen too much, the seal may also lack sealability at low temperatures, and, as a consequence, fail to separate contained medium from the outer

environment. The consequences of spillage of hydrocarbons or aggressive chemicals may undeniably be harmful either for people or flora and fauna which are much more sensitive in cold areas. Hence, it is of utmost importance to maintain elastomeric seals above the material glass transition temperature. Elastomers with very low glass transition point, i.e. elastomers which tolerate low temperatures without becoming brittle, may be utilized in some instances. However, since in general their chemical resistance is inhibited, they are normally not applicable in high temperature wells or those producing extremely aggressive fluids, e.g. containing a significant share of ¾S in the gas phase. On the other hand there exist elastomeric composites with excellent high temperature stability exhibiting inertness to most of the production media as well as chemicals utilized in the offshore industry, but their low temperature behavior is quite inferior. Currently a compromise between these properties and failure probability is found.

The present idea aims to solve or alleviate the material selection challenge for sealing elements with high material glass transition points that might be temporarily subjected to influence of low temperatures.

Summary of the invention The present invention discloses a polymer seal element comprising a phase changing material (PCM) in order to prevent transient overcooling of the seal element caused by blow down (depressurization) or other temporary chilling conditions that may be faced in frozen regions with rapidly alternating weather like Arctic. This is especially important whenever elastomeric seals may reach a temperature corresponding to the glass transition or brittleness point at which they may be easily damaged by any impact load or result in a leak of contained fluid to the environment. PCMs represent a group of substances that undergoes a phase transformation at a certain temperature, or over a distinct range of temperatures, accompanied by release of heat. The seal element in the present invention may be formed from an elastomeric composite comprising a high temperature and fluid resistant elastomer, which by virtue of the added PCM and the inherently low thermal conductivity of polymers, can withstand a time period of exposure to cold environment while maintaining its sealing capability. The present invention is defined by the attached claims and in the following:

In one aspect, the present invention provides a seal element comprising an elastomeric composite, said composite comprising an elastomeric polymer and a phase change material (PCM), wherein the PCM have a heat of fusion larger than 100 kJ/kg, a phase transition point within the temperature range of 233-288 K and is able to provide heat to the elastomeric polymer upon cooling of the seal element to the phase transition point of the PCM.

In a further aspect of the invention, the amount of PCM is in the range of 1-25 % v/v, based on the total volume of the elastomeric composite, at 20 °C. The range may advantageously be in the range of 5-25 % v/v, 1-20 % v/v, 1-15 % v/v or 1-10 % v/v. Alternatively, the amount of PCM in the elastomeric composite may be defined as being within the range of 1-40 % w/w, 5-40 % w/w, 1-35 % w/w, 1-30 % w/w or 1-25 % w/w based on the total weight of the elastomeric composite.

In a further aspect of the seal element according to the invention, the elastomeric composite constitutes more than 20 % v/v, more than 50 % v/v, more than 90 % v/v, more than 95 % v/v or more than 99 % v/v of said seal element, based on the total volume of the seal element.

In a further aspect of the seal element according to the invention, the PCM have a heat of fusion larger than 150 kJ/kg, larger than 200 kJ/kg, larger than 300 kJ/kg, and preferably larger than 350 kJ/kg.

In a further aspect of the seal element according to the invention, the phase transition point is within the temperature range of 243-283 K, or 243-272 K.

In one aspect, the elastomeric polymer in the seal element of the invention is a synthetic rubber. The synthetic rubber may be chosen from the group of NBR (nitrile butadienes), HNBR (hydrogenated nitrile butadienes), FKM

(fluoroelastomers) or FFKM (perfluoroelastomers), or any combination thereof, and the synthetic rubber is preferably FKM or FFKM.

In a further aspect of the seal element according to the invention, the PCM is chosen from the group of paraffins, such as octadecane, fatty acids, fatty esters, alcohols, glycols, salt hydrates, ionic liquids, water, eutectic solutions, eutectic metals, metals with low melting points or any combination thereof.

In a further aspect of the seal element according to the invention, the PCM has a solid-liquid phase change at the transition point. In a further aspect of the seal element according to the invention, the PCM has a solid-solid phase change at the transition point.

In a further aspect of the seal element according to the invention, the PCM is linked to, or is an integrated part of, a polymer material.

In a further aspect of the seal element according to the invention, the PCM is encapsulated in macro-, micro- or nanosized capsules having a mean diameter size of from 0.1 run to 5 mm. The capsules are preferably made in a polymer material, but may also be made up of other materials suitable for encapsulating PCM.

In a further aspect of the seal element according to the invention, the PCM is in the form of granules or particles.

In a further aspect of the seal element according to the invention, the PCM is evenly distributed throughout the elastomeric composite. In yet a further aspect of the seal element according to the invention, at least one surface of the seal element is PCM-free or the PCM only occupies an internal volume of the seal element. The % v/v of PCM, based on the volume of the elastomeric composite, may vary in a gradual or stepwise manner through the internal volume of the seal element. It may for instance be advantageous to have the largest w/w % of PCM close to the surface of the seal element.

In yet a further aspect of the seal element according to the invention, the heat provided by the PCM is sufficient to increase the temperature of the elastomeric polymer. The temperature increase may be in the range of 5 to 50 K when the seal element is cooled to the phase transition point of the PCM.

In yet a further aspect, the invention provides a method for manufacturing a seal element comprising the following steps:

- blending monomers for an elastomeric polymer with PCM; and

- curing or vulcanizing the resulting blend.

In a further aspect of the method according to the invention, the resulting blend is added to a suitably seal shaped mold prior to the curing/vulcanization step.

In yet a further aspect, the invention provides the use of a PCM material having a heat of fusion larger than 100 kJ/kg, and a phase transition point within the temperature range of 233-288 K in an elastomeric composite of a seal element. Preferably, the amount of PCM in the elastomeric composite is sufficient to avoid that the seal element reach a temperature corresponding to the glass transition or brittleness point of said seal element.

In both the use and the method according to the invention, the heat of fusion of the PCM, the temperature ranges of the phase transition point, and the amount of PCM in the elastomeric composite, may advantageously be within the ranges used in the seal element according to the invention.

Short description of the figure

Fig. 1 illustrates a schematic workover riser system comprising a seal element according to the invention. Description of the invention

Fig. 1 illustrates a workover system, and is meant to help understand the advantages that the present invention provides regarding improvement of safety and reliability of the system. Furthermore, any other equipment experiencing transient blow down conditions could benefit from the proposed technology.

Referring to Fig. 1, an offshore oil and gas rig performing a well intervention operation is depicted. The rig is positioned over an underwater gas well on top of which permanent seabed equipment is installed, i.e. Wellhead (WH) and Christmas Tree (XT). The rig is connected to the XT by means of a completion/workover (CWO) riser, a subsea conduit generally extending from Surface Flow Tree (SFT) on the rig down to a temporary installed workover stack incorporating Emergency Disconnect Package (EDP) and Lower Riser Package (LRP). The latter represents an arrangement of valves that is directly placed on top of the XT with function of a well barrier during the operations.

The LRP shall essentially contain a production isolation valve (PIV) and, when necessary, it may shut down the flow of hydrocarbons by the PIV or cut any wireline and coiled tubing by virtue of shear rams to prepare for a quick disconnect of the riser. The EDP shall provide a fast remotely operated disconnect of the riser system and prevent hydrocarbons contained inside the riser from releasing to sea by the retainer valve. During a production test both valves are opened, and a produced gas at a high pressure and the reservoir temperature flows up to the SFT. In the event of closing PIV, when the gas pressure above the valve is drastically reduced if compared to the downstream, Joule-Thomson effect takes place leading to a considerable gas cooling possibly down to subzero temperatures. Hence, the surrounding equipment including elastomeric seals may be subjected to undesirable cooling.

Another typical case of Joule-Thomson effect could be gas bleed down of a riser through its SFT. The upper part of the riser system above the sea level is

particularly vulnerable as there is no seawater to keep the riser rather warm. The air being cold or warm does not possess sufficient heat transfer capability compared to the water, therefore with the other parameters being equal the effect of Joule- Thomson cooling above sea is stronger than subsea.

Elastomeric seal elements of different geometry and dimensions can be installed all the way throughout the system from WH to SFT and wetted with produced hydrocarbons. Some of them are primary seals and eventually may be exposed to the transient cooling in gas systems. According to the present invention such seal elements can be made of a hydrocarbon resistant elastomer comprising a PCM material embedded into the main body of the seal forming a composite. Upon cooling PCM liberates a certain amount of thermal energy or heat, when the temperature of retained medium or ambient air drops to that of the phase transition temperature. Conversely, PCM will absorb heat from the environment when/if the temperature rises.

From experience and simulation models Joule-Thomson effect may cause a temporary chill of some gases down to -70 or -80 Celsius degrees, and, as a consequence, temperature of gas retaining equipment including elastomeric sealing steadily decreases. The temperature reduction rate in a seal core is certainly more delayed than surrounding steel elements due to rubbers low thermal conductivity and thermal inertia effects. Furthermore, PCM can retard the cooling process in elastomers to an even higher extent (temperature stabilization). The latent heat from phase transition will keep the sealing element rather warm and resilient for a sufficient time period and, thus, prevent release of hydrocarbons or other fluids enclosed in the system to the outer environment. The use of PCM would also counteract shock cooling effects by letting the seal loose temperature and resilience more gently; this may be beneficial for the seal as it might get more time to readjust in its seal groove.

The shape of the seal element of the present invention may be designed following commonly used seal practice including, but not limited to, O-rings, S -seals, T-seals, U-seals, V-seals, X-seals, flat seals, lip seals, numerous polymer sealants, back-up rings and gaskets. Also RAM sealing profiles could be made with PCM fillers.

As the main component of the elastomeric composite various rubbers,

thermoplastics or other polymer groups might be selected depending on

compatibility with flowing media and actual operational conditions of a particular seal member. The most favourable material is perfluorinated elastomer (FFKM), though other chemical classes might be beneficial. In particular the following elastomers could be used for composite formulation: nitrile butadiene (NBR), hydrogenated acrylonitrile butadiene (HNBR) or also referred as highly saturated nitrile rubber (HSN), carboxylated acrylonitrile butadiene (XNBR), fluoroelastomer (FKM), fiuorosilicone (FMQ), chloroprene (CR), ethylene propylene (EPM), polyurethane (PU), ethylene propylene diene (EPDM), tetrafluoroethylene and polypropylene (FEPM), copolymers thereof and the like. The composite may also comprise or be formed from a thermoplastic including, but not limited to, polyether ether ketone (PEEK), polyether ketone (PEK), polyether ketone ketone (PEKK), polytetrafluoroethylene (PTFE), polyoxymethylene (POM). Thermosetting polymers such as different epoxies and phenolics may also be selected as a matrix material. PCM elements can be either in organic or inorganic form with solid-liquid or solid- solid phase transitions. They could include specifically tailored paraffins, fatty acids, alcohols, glycols, salt hydrates and mixtures thereof, eutectic metals and metals with low melting point, various eutectics and the like. A large number of PCM's are described in the literature and may, provided they have a suitable phase transition point, be used in the present invention, see for instance E. Oro et al.

"Review on phase change materials (PCMs) for cold thermal energy storage applications"; Applied Energy, 2012, Vol. 99, pp. 513-533. A preferred phase transition temperature would commonly be in the range of -30 to +10 °C.

PCM may be embedded into the body of the seal element by being contained in macro-, micro- or nanocapsules, or as particles or granules with effective

capsule/particle size starting from 0.1 tun, as well as specific molecular

arrangements linked to polymer chains (for instance, so-called molecular

encapsulation). The PCM, especially PCM having a solid-liquid phase transition, may preferably be contained in capsules made in a polymer material to provide a protective coating and increase adhesion to the main material of the seal member. Various types of encapsulation of PCM's are well known to the skilled person.

The required volume share of a specific PCM depends on its energy storage capacity, physical qualities of polymer matrix and required combination of properties together with expected cold exposure periods and may generally vary over a wide range of 1-25 % v/v, based on the total volume of the elastomeric composite, at 20 °C. Alternatively, the amount of PCM in the elastomeric composite may be defined as being within the range of 1-40 % w/w based on the total weight of the elastomeric composite.

Introduction of PCM into the elastomeric composite should preferably be done prior to curing or vulcanization, though alternative processes, such as spray deposition, may be envisioned. The elastomeric composite may further comprise any necessary components that enhance physical and mechanical properties, impart desired appearance and initiate or accelerate chemical reactions. These components include curing agents, reinforcement fillers, plasticizers, antioxidants, pigments and the like.

An example of a calculated composite formulation

Assume an elastomeric composite comprises a predefined amount of PCM with a volume fraction x and mass τη^. Upon cooling down to the phase transition point the PCM releases heat corresponding to latent heat of fusion, λ (J/kg). The general equation linking heat added to or removed from a material with temperature change ΔT is:

Q = cm ΔT

where c - specific heat capacity of material (J/kg-K), m - mass (kg).

Therefore thermal energy transferred from PCM to elastomer matrix lead to a growth of its temperature ΔT which may be calculated as follows:

or in terms of density p

Here subscript el denote elastomer matrix. The effect of PCM heat release may be further exemplified taking an elastomeric matrix of FFKM rubber compounded with diethylene glycol (the glycol is encapsulated) as a PCM (see E. Oro et al. "Review on phase change materials (PCMs) for cold thermal energy storage applications"; Applied Energy, 2012, Vol. 99, pp. 513-533). Diethylene glycol has a latent heat of fusion of 247 kJ/kg and density of 1200 kg/m³. Knowing the typical characteristics of a FFKM elastomer (c^™ 945 J/kgK and p = 2000 kg/m³) the following magnitudes of temperature stabilization can be predicted: