You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a multiple element seal, such as a seal for use in a downhole environment. The seal may include swellable elements. 
         [0003]    2. Description of the Relevant Art 
         [0004]    Applicant supplies packers, in the form of full joint seal sections, under the E-ZIP trade mark. These packers comprise a number of discrete spaced-apart annular seal elements mounted to a base pipe. A packer is incorporated into a pipe or tubing string and run into a drilled bore, such as are used to access subsurface hydrocarbon-bearing formations. The bore may be lined with casing or may be unlined. The seal elements include swellable elastomer which will swell when exposed to an activating substance, typically water or oil. The seal elements may thus increase in diameter to occupy and seal the annulus between the base pipe and a surrounding bore wall. 
         [0005]    The E-ZIP packer thus provides a series of individual seal locations and is thus capable of holding an elevated pressure differential significantly higher than the pressure holding capabilities of packer having only a single seal element. 
       SUMMARY OF THE INVENTION 
       [0006]    According to an embodiment there is provided a seal arrangement including a plurality of seal elements arranged in series, at least one seal element having a predetermined rated pressure and including a relief arrangement permitting relief of pressure across the seal element at a predetermined level lower than said rated pressure without harming the seal&#39;s integrity. 
         [0007]    According to another embodiment there is provided a seal method that includes providing a series of seal elements and permitting relief of pressure across at least one of said seal elements at a pressure differential below a predetermined rated pressure. 
         [0008]    The arrangement allows pressure relief across the at least one seal element before the pressure acting across the element reaches the predetermined rated pressure, which may be the maximum safe or fail pressure of the seal element. Thus, by controlled relief of pressure, the pressure across the seal element may be maintained at a level below the rated pressure, minimizing the risk of seal failure. The pressure bleeding across the seal element may be held by an adjacent seal element. 
         [0009]    Two or more seal elements of the seal arrangement may include respective pressure relief arrangements, facilitating equalization of the pressure drop across the individual seal elements of a multiple seal element sealing arrangement. Some or all of the seal elements may include pressure relief arrangements. 
         [0010]    Detailed analysis and testing of conventional seal arrangements having multiple seal elements has revealed that the first seal element, looking from the high pressure side, tends to experience the largest pressure drop, with subsequent seal elements experiencing progressively lower pressure drops. When the first seal element experiences a pressure differential above its pressure limit, the seal element may fail suddenly, exposing the second element to a sudden pressure increase. The ability of the second seal element to hold the increase in pressure appears to depend to some extent on the shock absorbing characteristics of the element, and in such circumstances the pressure limit of the element is likely to be lower than if the element had been exposed to a gradual pressure increase. Also, a sudden failure of a seal element may result in damage to the seal element, reducing subsequent sealing ability. In the worst case, failure of the first element may lead to a domino-like failure of subsequent seal elements and irreparable damage to the seal elements. 
         [0011]    In embodiments described herein, the likelihood of a sudden failure of a seal element is reduced, and even in the unlikely event of an element failure subsequent seal elements are more likely to be pre-loaded and thus less vulnerable to failure resulting from shock-loading. 
         [0012]    The arrangement may be adapted for use in downhole applications, such as within bores drilled to access subsurface hydrocarbon-bearing formations and may be in the form of a packer. However, the arrangement may have equal utility in other applications. 
         [0013]    One or more of the seal elements may include a swellable material, such as a swellable elastomer. Such a material may swell when exposed to a suitable activator. The activator may be a substance, for example water or a hydrocarbon, or may be a condition, for example a particular pressure, temperature, or electro-magnetic radiation at a particular wavelength. 
         [0014]    The seal arrangement may take any appropriate form. In one embodiment the seal arrangement includes a base pipe providing mounting for a series of annular seal elements, which may be axially spaced. Activation or energizing the seal elements results in the radial extension or expansion of the seal elements to engage and seal with a surrounding bore wall, which bore wall may be formed by installed tubing, such as casing or liner, or by unlined drilled bore. 
         [0015]    The relief arrangement may take any appropriate form, and may include one or more valves, which may be one-way valves. The valves may permit a controlled flow or bleed of fluid when exposed to a predetermined pressure differential. The valves may be mounted within a seal element including a swellable material. 
         [0016]    Alternatively, the relief arrangement may be provided by selecting an appropriate configuration or material for the seal element. For example, the seal element may include a relatively hard surface-defining portion which will permit low flow rate fluid passage between the seal element and an opposing surface at a pressure differential lower than the normal rated pressure of the element. The provision of a harder material also minimizes erosion to the element by the passage of the bleed fluid. 
         [0017]    The relief arrangement may be arranged to operate only in a single direction, or may be arranged to provide for relief in two opposite directions across the seal arrangement. The latter configuration has the advantage that the orientation of the seal arrangement is immaterial to the operation of the arrangement, such that it is not possible to install the seal arrangement the wrong way round. Also, this offers greater flexibility in operation of the arrangement, allowing the seal arrangement to be utilized in applications where the arrangement may experience pressure in different directions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    These and other aspects will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0019]      FIG. 1  is a view of an a sealing arrangement in accordance with a first embodiment; 
           [0020]      FIG. 2  is an enlarged sectional view of area  2  of  FIG. 1  in an initial configuration; 
           [0021]      FIG. 3  is a sectional view corresponding to  FIG. 2 , but illustrating the sealing arrangement in an activated configuration in an unlined borehole; 
           [0022]      FIG. 4  is a sectional view of a second embodiment in an initial configuration; and 
           [0023]      FIG. 5  is a sectional view corresponding to  FIG. 4 , but illustrating the sealing arrangement in an activated configuration in an unlined borehole. 
       
    
    
       [0024]    While the invention may be susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Reference is first made to  FIG. 1  of the drawings, which illustrates a sealing arrangement in accordance with a first embodiment in the form of a packer  10 . The general configuration of the packer  10  is similar to that of the packer supplied by the applicant under the E-ZIP trade mark, in that the packer  10  includes a base pipe  12 , for incorporation in a pipe or tubing string, and a series of thirteen seal elements  14 . Each seal element  14  is of similar construction and includes an annular band of swelling elastomer. In use, the packer  10  is incorporated in a pipe or tubing string and run into a bore to a desired location, with the elements in an initial, smaller diameter configuration ( FIG. 2 ). Exposure to the ambient fluid in the bore results in the seal elements  14  expanding to fill and seal the annulus between the base pipe  12  and the surrounding bore wall  15  ( FIG. 3 ). 
         [0026]    Reference is now made in particular to  FIGS. 2 and 3  of the drawings, which illustrate two of the seal elements  14   a ,  14   b  in section. Each element defines two bypass passages  16   a,b ,  18   a,b , each passage being provided with an oppositely directed spring-loaded one-way valve  20   a,b ,  22   a,b  arranged to open at 80% of the seal element rated pressure. 
         [0027]    The passages  16   a,b ,  18   a,b  and valves  20   a,b ,  22   a,b  are formed of an appropriate rigid material, such as stainless steel, such that they are not adversely affected by the swelling of the elastomer. 
         [0028]    On exposure of an activated first seal element  14   a  to a pressure differential at or above 80% of the seal element failure pressure, the relief valve  20   a  opens, as illustrated in  FIG. 3 , allowing pressure to bleed through the element  14   a  into the annulus  24  between the elements  14   a ,  14   b . Similarly, if the pressure differential across the second element  14   b  then rises to 80% of the seal element failure pressure, the relief valve  20   b  will open, allowing pressure to bleed through the element  14   b . This process may continue along the length of the packer  10 , ultimately resulting in an equalization of the pressure differentials across all of the individual elements  14 . The provision of the relief valves  20   a,b ,  22   a,b  also protects the elements  14  against overpressures, minimizing that possibility that any element  14  will be exposed to an overpressure, and damaged or subject to sudden failure—this would only occur in the event of a sudden increase in the pressure differential across an element which could not be accommodated by the flow rate through the relief valves  20   a,b ,  22   a,b.    
         [0029]    The maximum pressure differential that may be withstood by the illustrated packer  10  without leaking is thirteen times 80% of the maximum pressure capability of each individual seal element  14 . This is significantly higher than a similar packer without the pressure relief feature. Also, in the event that the pressure differential rises above this level the packer will likely not fail completely or be subject to damage but will permit a controlled degree of leakage or bleed-through, and return to a sealing configuration when the pressure differential falls below the maximum level. 
         [0030]    The elements  14   a,b  are provided with valves  20   a,b ,  22   a,b  oriented in opposite directions such that the packer  10  will operate in either orientation. 
         [0031]    In a downhole environment the ambient fluid will tend to carry particulates which could impact on the operation of the valves  20   a,b ,  22   a,b . To avoid such difficulties the valves  20   a,b ,  22   a,b  and passages  16   a,b ,  18   a,b  may be protected by filters or screens. Alternatively, or in addition, the passages  16   a,b ,  18   a,b  and valves  20   a,b ,  22   a,b  may be initially filled with clean fluid such as a high temperature grease. The volume of fluid which moves through the valves to provide pressure relief is relatively small, and thus the clean fluid is unlikely to be displaced from the valves by the ambient fluid, thus protecting the valves from contamination. 
         [0032]    Reference is now made to  FIGS. 4 and 5  of the drawings, these drawings illustrating part of a packer including an alternative form of seal element  30 . Each element  30  includes an inner band of swellable material  32  and an outer band of conventional elastomer  34 . The outer band  34  has an external surface featuring sealing lips  36  which are configured to deflect a small amount in response to a pressure differential of 80% or more of the seal element fail pressure. This deflection, as illustrated in exaggerated form in  FIG. 5 , permits a limited and controlled degree of leakage past the activated element  30   a , as illustrated in  FIG. 5 , and into the annular chamber  38  between the element  30   a  and the next element  30   b.    
         [0033]    Those of skill in the art will recognize that the principle of controlled relief of a seal element, and in particular relief of seal elements in a multiple seal element apparatus, is not restricted to use with seal elements including swellable material, and may be utilized in a wide variety of seal forms and arrangements. 
         [0034]    In other embodiments a packer or other sealing arrangement may feature a variety of sealing element forms, and the characteristics of individual sealing elements, or individual relief arrangements, may vary within a sealing arrangement. 
         [0035]    Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as examples of embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.

Summary:
A seal arrangement includes a plurality of seal elements arranged in series. At least one seal element has a predetermined rated pressure and includes a relief arrangement permitting relief of pressure across the seal element at a predetermined level lower than said rated pressure.