METHOD OF FORMING COVERING PROTECTING LAYER ON JOINT BETWEEN COVERED STEEL PIPES

Abstract: A covering protecting layer is formed on a joint between covered steel pipes (33) each having a protecting concrete layer formed on the periphery thereof by a method wherein a heat-shrinkable sheet of the low temperature shrinking type is wrapped around the periphery of the joint; a fused mastic is applied onto the periphery of the heat-shrinkable sheet; and then, the applied mastic is cooling whereby the mastic is solidified to form a covering protecting layer.

Description: METHOD OF FORMING COVERING PROTECTING LAYER ON JOINT BETWEEN COVERED STEEL PIPES BACKGROUND OF THE INVENTION (1) Field of the Invention This invention relates to a method of forming a covering protecting layer on a joint between covered steel pipes. More particularly, it relates to a method of forming a covering protecting layer on a joint between covered steel pipes each having a protecting concrete layer formed on the periphery thereof. (2) Description of the Prior Art Covered steel pipes, that is, steel pipes having a synthetic resin covering layer formed on the periphery thereof, have a high resistance to corrosion caused by environmental factors, and therefore, they are widely used for construction of crude oil transportation pipelines, gas pipelines and city water pipelines. Of these pipelines, petroleum transportation pipelines are ordinarily laid out in districts of severe environmental conditions, and therefore, they are usually constructed from covered steel pipes. The resistance of ordinary covered steel pipes is insufficient for the use as the pipelines laid out in districts of environmental conditions more corrosive than in districts of ordinary environmental conditions, for example, pipelines placed in environments where the pipes always fall in contact with sea water. Accordingly, under such severe conditions, for example, in the sea bottom, ordinary covered steel pipes are not used, but covered steel pipes having a protecting concrete layer formed on the covering synthetic resin layer (hereinafter referred to as protecting concrete covered steel pipes") are used. The protecting concrete layer of a protecting concrete covered steel pipe of this type exerts various functions. For example, the protecting concrete layer protects the steel pipe from corrosion by sea water, prevents the steel pipe from deterioration or break age due to collision against a rock or reef on the sea bottom and acts as the weight for the pipeline. In the case where a pipeline is laid out on the sea bottom by using such protecting concrete covered steel pipes, there is ordinarily adopted a method in which units of protecting concrete covered steel pipes are connected one by one on a ship on the sea to form a continuous pipeline and the pipeline is sunk on the sea bottom. A protecting concrete covered steel pipe of the type to be used for this purpose ordinarily comprises a covering synthetic resin layer formed on the periphery of a steel pipe while both the end portions of the steel pipe are left exposed ad a protecting concrete layer formed on the periphery of the covering synthetic resin layer while both the end portions, in the longitudinal direction, of the covering synthetic resin layer are left exposed. Connection of protecting concrete covered steel pipes having the above-mentioned structure is accomplished by welding or the like, and the steel pipe should naturally be exposed in the vicinity of the joint. Accordingly, a covering synthetic resin layer and a protecting concrete layer should also be formed in the exposed portion close to the joint. The operation of forming a covering synthetic resin layer and a protecting concrete layer on the exposed portion in the vicinity of the joint of a protecting concrete covered steel pipe is ordinarily performed according to the following procedures At first, a synthetic resin tape or sheet is wrapped around the exposed portion (i.e., the portion which is not covered with a concrete layer) of steel pipes in the vicinity of the joint, a flame of a gas burner or the like is applied to the tape or sheet to elevate the surface temperature of the tape or sheet, and the tape or sheet is manually pressed and elongated by hands with shrinkage of the heat-shrinkable tape or sheet while intermittently applying the flame of the gas to the tape or sheet, if necessary, so that the exposed portion of the steel pipe is completely covered with the heat-shrinkable tape or sheet and both the end portions of the tape or sheet are lapped on the ends of the covering synthetic resin layer formed in advance on the steel pipe. Then, a cover (such as a protecting metallic tube) for connecting the ends of the protecting concrete layers on both the sides of the joint is attached so that a closed space defined by the protecting concrete layers on both the sides of the joint is formed, and a molten mastic such as molten asphalt is forced into the space through a mastic pouring opening formed on the cover. The poured asphalt is thus filled in the space within the cover. After covering and protection have thus been completed, the pipe line is laid out on the sea bottom or buried in the ground. The above-mentioned method of forming a covering protecting layer in the vicinity of the joint of a protecting concrete covered steel pipe has problems. For example, a long time is necessary for completion of the operation and a burner should be used for heating the surface of the steel pipe. If a flame of a gas burner or the like is applied to a heat-shrinkable tape or sheet, thermal deterioration is partially caused in the tape or sheet composed of a synthetic resin. Furthermore, the covering operation of manually pressing and elongating the heat-shrinkable tape or sheet wrapped around the surface of the steel pipe by hands requires high experiences and is very troublesome. SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a method of forming a covering protecting layer on a joint between protecting concrete covered steel pipes, in which the problems of the conventional method are solved or at least mitigated. In accordance with the present invention, there is provided a method of forming a covering protecting layer on a joint between covered steel pipes, which comprises wrapping a heat-shrinkable sheet of the low temperature shrinking type around the periphery of a joint between covered steel pipes each having a protecting concrete covering layer formed on the periphery thereof, applying a fused mastic onto the periphery of the heat-shrinkable sheet and cooling the applied mastic whereby the mastic is solidified to form a covering protecting layer. In the present invention, a heat-shrinkable sheet of the low temperature shrinking type is used instead of a heat-shrinkable tape or sheet used in the conventional method of forming a covering protecting layer on a joint of covered steel pipes, whereby the conventional heating operation using a gas burner can be omitted, and formation of a covering layer of the heat-shrinkable sheet is accomplished by the heat given at the step of applying a fused mastic such as fused asphalt, which is conducted after the heatshrinkable sheet has been wrapped around the periphery of the steel pipes. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in detail with reference to the accompanying drawings, in which: Fig. 1 is a diagram illustrating a joint between covered steel pipes; Fig. 2 is a diagram illustrating the state where the operation of wrapping a heat-shrinkable sheet around the pipe joint shown in Fig. 1 has been completed; Fig. 3 is a diagram illustrating the state where an asphalt-pouring cover is attached to the heat-shrinkable sheet-wrapped pipe joint shown in Fig. 2; and, Fig. 4 is a graph showing the heat shrinkability characteristics of heat-shrinkable sheets prepared in the Example and Comparative Example. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, reference numerals 11, 12 and 13 represent an exposed portion of the steel pipe, a covering synthetic resin layer formed on the steel pipe and a protecting concrete layer formed on the covering synthetic resin layer 12r respectively, and reference numeral 14 represents the joint between steel pipes. As is seen from Fig. 1, the protecting concrete covered steel pipe ordinarily comprises a covering synthetic resin layer which covers a steel pipe while both the end portions of the steel pipe are left exposed and a protecting concrete layer which is formed on the periphery of the covering layer of the steel pipe while both the end portions, in the longitudinal direction, of the covering synthetic resin layer are left exposed. According to the conventional method of forming a covering protecting layer on a joint between covered steel pipes, a heat-shrinkable tape or sheet is wrapped around the exposed portion 11 of the steel pipe, the tape or sheet is heated by a flame of a gas burner to elevate the temperature to a level higher than the shrinking temperature of the tape or sheet, the heating is stopped, and immediately thereafter, the tape or sheet is pressed and elongated by hands, whereby the tape or sheet is closely covered on the exposed portion of the steel pipe. In contrast, according to the present invention, the operation of heating the exposed portion of the steel pipe by a gas burner or the like is not performed at all, and a heat-shrinkable sheet is directly wrapped around the exposed portion of the steel pipe. This state where the heatshrinkable sheet is wrapped around the exposed portion of the steel pipe shown in Fig. 1 is illustrated in Fig. 2. The operation of wrapping the heat-shrinkable sheet 25 on the exposed portion of the steel pipe in the present invention may be the same as the wrapping operation in the conventional method. For example, there may be adopted a method in which the heat-shrinkable sheet 25 is wrapped so that parts near both the ends of the heat-shrinkable sheet are sufficiently lapped on the covering layer 12 formed on the steel pipe and both the ends are contacted with the side wall faces of the protecting concrete layer 23 or located slightly on the inner sides thereof, and both the end portions, in the longitudinal direction, of the heat shrinkable sheet are lapped or bonded with a margin enough to comply with the shrinkage of the sheet or the shrinkage stress due to the heat. The present invention is characterized in that a heatshrinkable sheet of the low temperature shrinking type is used as the heat-shrinkable sheet. By the heat-shrinkable sheet of the low temperature shrinking type is meant a heat-shrinkable sheet of the type in which heat shrinkage of a relatively high degree is caused even at relatively low temperatures and the heat shrinkage percentage is not abruptly increased even if the temperature is elevated to a high level. As a preferred example of the heat-shrinkable sheet of the low temperature shrinking type, there can be mentioned a heat-shrinkable sheet having such shrinkage characteristics that the shrinkage (the percentage of shrinkage based on the size of the sheet at normal temperature) at 10000 is at least 7%, preferably at least 10%, and the shrinkage at 14000 is not more than 30%, preferably not more than 258. A heat-shrinkable tape or sheet of the high temperature shrinking type has been used for the conventional method of forming a covering protecting layer on a joint between covered steel pipes. Namely, according to the conventional method, as pointed out hereinbefore, after a heat-shrinkable tape or synthetic resin sheet has been wrapped around the exposed portion of a steel pipe, the surface temperature of the tape or sheet is elevated by applying a flame of a gas burner or the like for a certain time, and the heat-shrinkable tape or sheet is pressed or elongated by hands with shrinkage of the tape or sheet, whereby the exposed portion of the steel pipe is covered with the heat-shrinkable tape or sheet. Accordingly, at the step of covering the heat -shrinkable tape or sheet closely to the exposed portion of the steel pipe under heating, in order to form a closely adhering covering, it is necessary to complete the heat shrinkage within a short time under inspection by a worker. For this reason and in view of physical properties of synthetic resin sheets, in the conventional method, there have been used heat-shrinkable tapes or sheets of the high temperature shrinking type which exhibit no substantial shrinkage at low temperatures and in which, when the temperature is elevated to a certain level, prominent heat shrinkage is caused and the shrinkage is advanced almost to the shrinkage limit in a short time. In contrast, according to the present invention, by using a heat-shrinkable sheet of the low temperature shrinking type, the step of preheating the exposed portion of a steel pipe can be omitted, and shrinkage of the heatshrinkable sheet is accomplished by utilizing the heat possessed by a fused mastic, such as fused asphalt, applied afterwards to the periphery of the heat-shrinkable sheet. A material as used for a heat-shrinkable sheet used in the conventional method may be used for the heat-shrinkable sheet of the present invention, after it has been treated so that it is converted to the low temperature shrinking type. As the sheet material, there can be mentioned synthetic resins such as polyethylene, polypropylene, an ethylene/propylene copolymer, polyvinyl chloride, a polyester and a polyamide. High-density polyethylene is especially preferred, and also an ethylene/propylene copolymer is preferred. The technique of converting the sheet material to the low temperature shrinking type is known. This conversion can easily be accomplished by appropriately adjusting and controlling the drawing conditions such as the drawing temperature and drawing ratio during or after the sheetpreparing step according to the kind of the material and the desired low temperature shrinking degree. It is preferable that the heat resistance be given to the heat-shrinkable sheet by crosslinking by irradiation with electron beams or the like before or after the step of imparting the heat shrinkability. Furthermore, in order to ensure a good adhesion to the surface of the exposed portion of the steel pipe, it is preferable that an adhesive layer be formed on one surface of the heat-shrinkable sheet (the surface to be contacted with the exposed portion of the steel pipe). After the heat-shrinkable sheet has been wrapped around the joint between the steel pipes as shown in Fig. 2, as in the conventional method, a cover (such as a protecting metallic tube) 36 is attached to form a closed space defined by the joint having the covering layer of the heat-shrinkable sheet formed thereon and side walls of protecting concrete layers 33 on both the sides of the joint, as shown in Fig. 3, and then, a fused mastic is poured into the closed space through a mastic pouring opening 37 formed on the cover. As the mastic, asphalt is preferable. As in the conventional method, the mastic such as asphalt is heated at about 160 to 2500C and poured in the fused state. The poured mastic is applied to the periphery of the heat-shrinkable sheet wrapped around the exposed portion of the steel pipe, whereby heat sufficient for heat shrinkage is given to the heat-shrinkable sheet and the sheet begins to shrink and covers the surface of the exposed portion of the steel pipe closely. The mastic is filled in the space within the cover 36 and is naturally cooled and solidifed by standing. Formation of a covering protecting layer on the joint between covered steel pipes is thus completed. The above-mentioned method of forming a covering protecting layer according to the present invention is advantageous in various points. For example, a burner for heating the surface of a steel pipe need not be used at all, and no experience or skill is necessary for the operation of wrapping a heat-shrinkable sheet because the sheet is wrapped around the surface of a steel pipe maintained at normal temperatures. Furthermore, this wrapping operation can easily be accomplished in a very short time. Since the heat-shrinkable sheet is not heated at a temperature higher than the temperature of the fused mastic, little or no thermal deterioration of the heat-shrinkable sheet is caused. Moreover, the covering synthetic resin layer and protecting mastic layer formed according to the present invention are comparable or superior to those formed according to the conventional method. The present invention will now be described in detail with reference to the following Example and Comparative Example. Example and Comparative Example A polyethylene sheet having a thickness of 0.8 mm was treated under conditions shown in Table 1 to render it heat-shrinkable. Table 1 Specimen No. Drawing Temperature (OC) Drawing Ratio 1 90 1.25 2 85 1.25 3 115 1.25 4 ¯ 115 1.40 5 120 1.40 The heat shrinkage characteristics of the obtained heat-shrinkable sheets are shown in the graph of Fig. 4, wherein curves a, b, c, d and e correspond to specimen No. 1, 2, 3, 4 and 5, respectively. Specimens 1 and 2 are heat-shrinkable sheets of the low temperature shrinking type, and specimens 3 through 5 are heat-shrinkable sheets of the high temperature shrinking type. Each of these heat-shrinkable sheets was independently wrapped around the exposed portion of a joint between covered steel pipes without preheating the exposed portion, and then, a cover was attached to the outside as shown in Fig. 3 and fused asphalt (190 C) was poured through a pouring opening of the cover and applied to the periphery of the heat-shrinkable sheet. After the asphalt had been cooled and solidified, the cover was removed. Thus, a covering protecting layer was formed. Then, the protecting asphalt layer was removed and the state of the covering layer composed of the heat-shrinkable sheet was observed. The observation results are shown in Table 2. Table 2 Specimen No. State of Formed Covering Layer 1 Good appearance, high resistance to peeling from the surface of the steel pipe, very small amount of air bubbles present on the inner side of the covering layer 2 Same as above 3 Many concavities and convexities present on the entire surface, many wrinkles formed, large amount of air bubbles present on the inner side of the covering layer, small groove-like spaces communicated with the outside through both the sleeve portions of the covering layer 4 Irregular wrinkles formed on the entire surface, small amount of air bubbles present on the inner side of the covering layer 5 Many convexities and concavities on the entire surface, many wrinkles formed, large amount of air bubbles present on the inner side of the covering layer, small groove-like spaces communicated with the outside through both the sleeve portions of the covering layer

Claims: CLAIMS 1. A method of forming a covering protecting layer on a joint between covered steel pipes, which comprises wrapping a heat-shrinkable sheet of the low temperature shrinking type around the periphery of a joint between covered steel pipes each having a protecting concrete covering layer formed on the periphery thereof, applying a fused mastic onto the periphery of the heat-shrinkable sheet and cooling the applied mastic whereby the mastic is solidified to form a covering protecting layer. 2. A method according to claim 1, wherein the heatshrinkable sheet of the low temperature shrinking type exhibits a shrinkage of at least 7% at a temperature of l000C and a shrinkage of not more than 30% at a temperature of 140 C, as expressed in terms of the percentage shrinkage based on the size of the sheet at normal temperature. 3. A method according to claim 1, wherein the heatshrinkable sheet of the low temperature shrinking type exhibits a shrinkage of at least 10% at a temperature of l000C and a shrinkage of not more than 25% at a temperature of l400C, as expressed in terms of the percentage shrinkage based on the size of the sheet at normal temperature. 4. A method according to claim 1, wherein the heat -shrinkable sheet of the low temperature shrinking type is made of a thermoplastic synthetic resin selected from the group consisting of polyethylene, polypropylene, an ethylene/- propylene copolymer, polyvinyl chloride, polyester and polyamide. 5. A method according to claim 1, wherein the application of the fused mastic onto the periphery of the heat -shrinkable sheet is carried out by attaching a cover so that it forms a closed space defined by the joint having the covering layer of the heat-shrinkable sheet formed thereon and by side walls of the protecting concrete layers on both the sides of the joint; and then, pouring the fused mastic into the closed space through an opening formed on the cover. 6. A method according to claim 1, wherein the fused mastic is asphalt maintained at a temperature of about 1600 to 2500C.

Inventors: ITAKURA, AKIO C/O CHIBA SEKIYUKAGAKUKOJO, KATAOKA, HIDETO C/O CHIBA SEKIYUKAGAKUKOJO
Applicants: UBE INDUSTRIES, LTD.
Application year: 1982
Main classification: F16L58
IPC codes: B29C61, F16L58, B29C63, B29C51