US3436920A - Protection of offshore structure from icebergs - Google Patents

Description

April 8, 1969 PROTECTION OF OFFSHORE STRUCTURE FROM ICEBERGS Filed Jan. 5, 196'! K. A. BLENKARN ET AL Sheet of 5 u cu o l KENNETH A. BLENKARN ATTORNEY April 1969 K. A. BLENKARN ET AL 3,436,920

PROTECTION OF OFFSHORE STRUCTURE FROM ICEBERGS Filed Jan. 3, 1967 Sheet 2 of 5 KENNETH Af BLENKARN ALPHIA E. KNAPP INVENTORS ATTORNEY PROTECTION OF OFFSHORE STRUCTURE FROM ICEBERGS April 1969 K. A. BLENKARN ET AL Sheet Filed Jan.

KENNETH A. BLENKARN ALPHIA E. KNAPP INVENTORS.

A T TORNE Y April 8, 1969 K. A. BLENKARN ET 3,435,920

PROTECTION OF OFFSHORE STRUCTURE FROM ICEBERGS Filed Jan. 1967 Sheet 4 of s KENNETH A. BLENKARN ALPHIA E. KNAPP INVENTORS.

ATTORNEY April 8, 1969 K. A. BLENKARN ET AL 3,436,920

PROTECTION OF OFFSHORE STRUCTURE FROM ICEBERGS Filed Jan. 5, 1967 Sheet Q of 5 T 94 l /95 l I i? in? ll m "i r; j so "7'62 --62 i 5 f eo j 64 :2 ie

FIG.6

KENNETH A.'BLENKARN BY 4M- ATTORNEY hired 3,436,920 PROTECTHON F OFFSHORE STRUCTURE FRGM ICEBERGS Kenneth A. Blenkarn and Alphia E. Knapp, Tulsa, ()ltla,

assignors to Pan American Petroleum Corporation,

Tulsa, 02th., a corporation of Delaware Filed Earn. 3, 1967, Ser. No. 607,050 Int. Cl. E02b 3/00; E02d 21/00; B63g 9/00 US. Cl. 61-1 3 Claims ABSTRACT OF THE DISCLQSURE Fending systems for protecting offshore structure from icebergs. A fence or structure surrounds the offshore structure. The fending system incorporates members such as crush tubes for absorbing large amounts of energy. These large energy absorbing means permit the changing of the course of an iceberg by supplying the required impulse over a long time period. Three embodiments are described: 1) a system of buoy-supported cables anchored to the ocean floor, (2) a spider-web system with the outer ends of the cable anchored to the ocean floor and the inner ends anchored to the offshore structure through I tubes and crush tubes, and (3) a protecting structure surrounding offshore platform and attached to it through crush tubes. All systems are below the Water surface sufiicient to permit boat passage.

This invention is related to means for protecting offshore structure from icebergs. It relates especially to fending systems for stopping or changing direction of a moving iceberg.

Background The search for oil and gas has, in recent years, caused many wells to be drilled in marine locations. Frequently, in these offshore locations, drilling or production platforms have been built which are in reality towers supported by the ocean floor and extending above the surface of the water. These platforms are used for drilling the well, or later for supporting roduction facilities. Some wells have been drilled in offshore water in areas where the hazards of icebergs are a definite problem, such as in the Grand Banks of Newfoundland. Due to an icebergs size and its velocity, which is usually about one knot but can be as high as 3 to 5, an iceberg possesses great amounts of kinetic energy. Many of the medium size icebergs may have kinetic energy of about 50,000 ft. kips. (This is an averaged value and some individual icebergs may be considerably higher.) If such icebergs strike a permanent offshore structure, extremely large forces will be generated within this structure. The exact size of these forces will depend upon the rigidity of the offshore structures and the potential energy of the iceberg. Offshore structures can be designed to withstand these forces. However, such designs would result in such platforms being designed to withstand up to ten to forty times the present desigm loads and would thus add a heavy financial burden to offshore development of oil and gas properties, which is already very costly. It is thus seen that there is a real need for a system of protecting offshore structure from icebergs without increasing the design of the structure itself to withstand the shock of being struck by an iceberg.

Brief description of invention This invention includes a first means enclosing, or at least partially surrounding, an offshore structure which is situated in an area which might be subject to the flow of icebergs. Means are provided for holding the enclosing means in a relatively fixed position until struck by an ice- 3,435,920 Patented Apr. 8, 1969 berg. It also includes energy absorbing means connected to, or forming a part of, the enclosing means for absorbmg great amounts of energy when such means is struck by an iceberg.

In a preferred embodiment, the enclosing means takes the form of a many-sided fence. Each side includes at least tWo spaced-apart, buoyant members connected together by a flexible line. Anchor pilings are set below each such buoyant member and the buoyant member is fastened to its associated anchor piling by a flexible line. The line is of such length as to hold the buoyant member a fixed distance below the surface of the Water, e.g., 30 to 50 feet. This permits ships to pass over the enclosing fence and reach the offshore structure, yet the fending system is sufficiently close to the surface so as to intercept any iceberg which might be flowing in the direction of the offshore structure.

Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawings in which:

FIGURE 1 illustrates a many-sided or segmented fence enclosing an offshore platform comprising a system of submerged buoys anchored to the ocean floor and connected by cables;

FIGURE 2 illustrates another embodiment and shows a structure surrounding the platform and attached to it by connecting members containing crush tubes;

FIGURE 3 illustrates another fending system and can be characterized as a spider-Web system with the outer ends of the cable anchored to the ocean floor and the inner end anchored to the platform through I tubes and energy absorbing means such as crush tubes;

FIGURE 4 illustrates a spacing arrangement of the legs of the offshore structure and the upright protecting columns surrounding the offshore structure of FIGURE 2;

FIGURE 5 illustrates a crush tube for insertion in rigid member;

FIGURE 6 illustrates a crush tube for insertion in a cable; and

FIGURE 6A is a perspective view of a portion of the guide mandrel of the device shown in FIGURE 6.

Description of preferred embodiments Attention is first directed toward FIGURE 1 which illustrates a buoy-cable system. Shown thereon is a platform or offshore structure 10 which is supported from the ocean fioor 12 by legs or columns 13 supporting a deck above a surface 14 of a body of Water 16. The en closing means shown here for protecting platform 10 includes four sides or segments; however, any practical number of sides might be used. If platform 10 is located in an area in which icebergs always come from a certain direction and never from another, the fending system would only partially surround platform 10. Each segment of the fence includes at least two primary floats or buoys 18, located at each end of a segment of the enclosure. Anchor pilings 20 are set in the ocean floor 12 beneath the point at which it is desired that its associated primary buoy 18 be located. Buoy 18 is connected to its associated anchor piling 20 by cable 22. These cables 22 hold the primary buoys 18 a desired distance beneath the surface of the water so that ships may approach platform 10.

Primary buoys 18 form the ends or extremes of one segment of the fence. Buoys 18 are connected together by an upper cable 24. It will ordinarily be preferred that there are a plurality of cables 26 which extend from upright connecting cables 22 and parallel to cable 24. Cable 24 is supported intermediate the buoys 18 by auxiliary buoys 28. Anchor weights 30 are connected by lines 32 to buoys 28 for aid in holding them inv position. These cables are made of material, such as high tensile steel, which when stretched absorbs tremendous energy. If desired, crush tubes 34 can be connected into lines 24 which connect buoys 18. One form of crush tube is illustrated in FIGURE and will be discussed later. This will provide additional energy absorbing means and also minimize the possibility of the cable parting by limiting the resisting forces in the cables. If an iceberg strikes the fending system of FIGURE 1, potential energy of the iceberg can be absorbed by downward displacement of the primary buoys, stretching of the cables, and the crush tubes 34. Additionally, if desired, drag anchors 36 can be connected to buoy 18 by cable 37. Horizontal displacement of buoy 18 will drag anchor 36 along the ocean floor, thus absorbing more energy.

Assume that an iceberg strikes fence or cables 24 and 26 headon; this will be the maximum force which the fence will have to protect against. The momentum of a moving object is given by the Equation mv where m is mass and v is velocity. In order to change the momentum, an impulse must act. This may be defined as:

where fimv is the change of momentum and F is an averaged force over the time interval At: t -t Since the iceberg is to be stopped, the original momentum is also the change in momentum and as an approximation then, (2) for F (force) follows:

mt) At From this rather simplified equation, the force which the protecting system will have to withstand for an iceberg of a given size and volume is dependent largely upon the contact time or the distance over which the force is absorbed. In our invention we take advantage of this principle and thus reduce the magnitude of the impact force of the iceberg which would otherwise be imparted to the protecting structure.

One segment (i.e., two buoys 18 and their connecting line) of the protective system is capable of absorbing tremendous amounts of energy. For example, assume that two buoys 18 are about 24 feet in diameter and about 2,000 feet apart and that cables 22 holding buoys 18 are about 200 feet in length. It will further be assumed that the cables are 2%" steel cable having an elastic modulus of 20,000 k.s.i. The potential energy in the system is given by Equations 3, 4 and 5 below in which V is the potential energy in one horizontal cable 24, and V is the potential energy in two vertical cables 22. V is the potential energy in one horizontal cable 24, and

(3 V (in ft. kips)= /2'(T1)(S in which T =cable tension S cable extension 4 V (in a. kips)= /2-(T )(S 5 V (in a. kips)=250AZ in which Z is the vertical deflections of buoys 18.

For a 2,000 ft. buoy cable segment given in the example above, to deflect cable 24 a horizontal distance of 600 feet, requires over 50,000 ft. kips. This is as much energy as many individual icebergs possess.

The energy absorbing capacity of one segment can be increased greatly by different modifications such as by making support buoys 28 larger, adding additional horizontal cables between upright cables 22 such as cable 26. An average size iceberg weighing about 400,000 tons and traveling about one knot will have kinetic energy of about 35,000 ft. kips. As illustrated in the above example, the energy absorbing capacity of the protective segment shown in FIGURE 1 can readily absorb this, or be designed to absorb even greater energy for large icebergs. Thus, by absorbing the energy in an iceberg, it is effectively stopped.

A further modification mentioned above can increase the energy absorbing capacity by the addition of drag anchor 36 which is connected to buoy 18 by cable 37. The tension in anchor connecting cable 37 will cause the anchor to be dragged along the bottom and this could increase the energy absorbed by l030,000 ft. kips per anchor. The exact amount depends upon the size and type of anchor used and the type of sail on the ocean floor.

If one segment of the enclosure of the embodiment of FIGURE 1 is destroyed by an iceberg, only that segment, or segments, which is damaged need be replaced. It is entirely conceivable that no segment of the system might be destroyed at all in stopping an iceberg. For example, if no drag anchor 36 or crush tube 34 were used when the iceberg floated away or melted, the system sas a tendency to reposition itself due to the buoyant effect of buoyant or primary floats 18 being tied by vertical lines 22 a position above piling 20. If crush tubes 34 are used, cables 24 and 26 and 22 and floats 18 can readily be salvaged and a new crush tube inserted. Then the device is again operational. If the drag anchor 36 is used, then after once stopping an iceberg, the anchor 36 would be reset to be useful in again absorbing energy.

Another advantage of the system in FIGURE 1 is that the protecting system is located a considerable distance, i.e., 2,000 feet or more, from the platform. If the protecting system should not be adequate to stop an iceberg, such as one of unusual size or with a high velocity, then one would have adequate warning so that platform 10 could be evacuated.

FIGURE 2 illustrates another embodiment of an energy absorbing mechanism useful in stopping icebergs and preventing them from destroying the protected structure. The structure to be protected is illustrated as an offshore platform 40 supported well above the surface 42 of the body of water 44 by a plurality of columns 46. These columns are supported from piling drilled, or otherwise placed, into the ocean floor 48. As mentioned above, offshore drilling and producing platforms for oil and gas are well known in the art. The depth of the water may be 200600 feet, or even deeper as technology improves, and the platform is normally supported about 40-100 feet or so above the surface of the water. These platform are very costly to construct. When one much structure is destroyed, in additon to the cost of replacing the structure, there is also the economic loss in not being able to produce oil from the well during the time it takes to replace the destroyed platform.

The iceberg-protection structure surrounds the supporting columns or legs 46 of platform 40. It includes a plurality of vertical columns 50 which are preferably equally spaced from the legs of the platform. The vertical columns 50 are connected together by horizontal crossmembers 51. The upper end of vertical protection columns 50 are a substantial distance below the surface of the water so that ships can clock at platform 40. The lower ends of protecting columns 50 are supported in the ocean floor 48. However, the lower end need not be embedded deeply into the floor because the structure only has to support itself, and no large platform or the like such as columns 46 must support.

We shall next consider the connection between the outer protection structure and the legs 46 of the platform. As shown in the drawing, for illustration purposes there are 4 legs 46 of the platform and eight protecting columns 50. This relationship is illustrated in FIGURE 4. The protecting columns 50 are connected to legs 46 by horizontal members 52 which are provided with crush tubes 54. The lateral braces 54 are arranged to resist both a direct hit by an iceberg or a glancing blow which would cause torsional stresses in the protecting structure. The lateral members are provided with crush tubes 54 for absorbing large amounts of energy to prevent the platform legs 46 from feeling the full instantaneous impact of the blow from an iceberg.

An impact of an iceberg upon the protective structure of FIGURE 2 would result primarily in crushing and shortening of the crush tubes. Repair of the system then could require only the replacement of the connecting tubes. Of course, in the event of massive destruction of the ice-protecting structure, the entire set could be replaced from time to time to provide adequate protection of the entire platform.

FIGURE 5 illustrates an energy absorbing device which is commonly called a crush tube. Shown thereon is a frangible outer tube and an inner mandrel 62. The mandrel is spaced within a frangible tube by frangible centralizers 64. The lower end of frangible tube 60 is placed over lip 66 of die 68. A circular groove 70 is provided in die 68. The upper end of frangible tube 60 and mandrel 62 are connected by a cap member 72. The crush tube of FIG- URE 5 is inserted in the lateral supports, for example, by welding cap 72 to one end of the lateral member and die 68 to the other portion of the lateral member. When force is applied to move cap 72 toward die 68, one end of frangible tube 60 is shoved into groove 70 which is semi-circular in cross section. That end of tube 60 is flared outwardly causing it to become fractured by what may be called a fragmenting process. This absorbs tremendous energy through the force developed when the frangible tube is pressed over the die. There is practically no limit upon the energy these tubes can be designed and constructed to absorb.

Attention is next directed toward FIGURE 3 which shows still another embodiment of means for protecting an offshore platform from destruction by an iceberg. This system is appropriately termed the spider-web system. It includes a plurality of anchor pilings which surround legs 84 of platform 82 which is to be protected. Radial lines 96 extend from anchor pilings 80 upwardly through I tube 86 to be attached to platform or leg 84. A crush tube 88 is inserted in the line just above the J tube. The crush tube of FIGURE 5 can be modified as shown in FIGURE 6. Cap 72 is connected through interim line 91 to line segment 90, and die 68 is connected through internal lines to upper line segment 94 which is attached to the oflshore structure to be protected, e.g., line segment 94 may be attached to leg 84. Lines 95 are connected to eyelets 93 which extend into the bore of die 68. Mandrel 62 is provided with a central bore 97 for the passage of cable 91 and grooves 99 for the passage of cables 95 and also eyelets 93 as mandrel 62 moves toward die 68. Tension on line 96 forces cap 72 and frangible tube 60 toward die 68. It is to be noted that the J tubes are a suflicient depth below the surface of the water so that lines 96 will not interfere with the ships coming to dock against platform 82. As shown, there can be several cables 96 going toward one I tube 86. Several cables 96 can be fastened to one crush tube through interim line 91. Between cables 96 are a plurality of fending cables 98. In this embodiment there is seen that the extension of the various cables 96 and 98, as well as the crushing of crush tubes 88 when an iceberg encounters a protecting system, provides for the absorption of a tremendous amount of energy.

An advantage of the spider-web system is that repair of this system subsequent to iceberg damage is relatively easy. For example, the energy absorption devices are on the platform itself and readily accessible for replacement.

While the above inventions have been described with only a limited number of embodiments, it is to be understood that various modifications can be made without departing from the spirit or scope of the invention.

We claim:

1. A device for protecting an offshore structure which comprises:

two spaced-apart buoyant members;

a flexible line connecting said two buoyant members;

including energy absorbing means comprising a crush tube connected within said flexible line;

flexible means anchored to the ocean floor for holding said buoyant members a substantial distance below the surface of a body of water.

2. A device as defined in claim 1 including support buoys intermediate said buoyant members for supporting said flexible line connecting said buoyant members.

3. A device as defined in claim 2 including an anchor weight for each said support buoy and including means connecting said anchor weights to said support buoys.

References Cited UNITED STATES PATENTS 391,362 10/1888 Favarger 114-241 1,274,624 8/1918 Steinmetz 114-240 3,200,598 8/1965 Krepack 615 3,283,515 11/ 1966 Pottorf 61-46 JACOB SHAPIRO, Primary Examiner.

US. Cl. X.R.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.6. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,436 ,920 April 8 196! Kenneth A. Blenkarn et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 lines 20 to 2 1, in the equation, "FAt" should I EAt line 49 "in one horizontal cable 24 and" should I of the buoy. Column 4, line 8, "sail" should read s line 16, "sas" should read has line 43, "platform" should read platforms line 44, "much" should read 5L Signed and sealed this 7th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JI Attesting Officer Commissioner of Patents

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