US3394672A - Apparatus for mooring floating structures - Google Patents

Description

July 30. 1968 w. LEDGERWOOD, JR 3,394,672

APPARATUS FOR MOORING FLOATING STRUCTURES Filed Sept. 20, 1966 2 Sheets-Sheet 1 Vs 552, 57!} LBS- ciy.

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h H v I755 FT. BUOYANT CHAIN Hb 736,078 LBS. 77/

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Y ATTORNEY- July 30. 19.68

| w. LEDGERWOOD, R 3,394,672

APPARATUS FOR MOORING FLQATING STRUCTURES Filed Sept. 20, 1966 2 Sheets-Sheet 2 20b n55 I9 INVENTOR.

ATTORNEY.

United States Patent 3,394,672 APPARATUS FOR MOORING FLOATING STRUCTURES Leroy W. Ledgerwood, Jr., Houston, Tex., assignor to Esso Production Research Company Filed Sept. 20, 1966, Ser. No. 580,784 3 Claims. (Cl. 114-206) ABSTRACT OF THE DISCLOSURE A combination buoyant and nonbuoyant anchor line for mooring a floating vessel. The anchor line includes flexible means connected at one end thereof to the floating vessel and at the other end thereof to the land underlying the water, and buoyant means arranged on said flexible means in a manner such that one end portion of the anchor line has a net positive buoyancy, preferably a positive buoyancy per foot equal to the weight per foot of anchor line in water. When the floating vessel is acted upon by environmental forces, the buoyant portion of the anchor line assumes an inverted catenary form while the remaining nonbuoyant portion assumes a regular catenary form. The preferred embodiment of the anchor line is a stud link chain buoyed with spaced apart pairs of spheres.

The present invention concerns method and apparatus for mooring floating vessels to permit increasing mooring depth without increasing anchor line load. More specifically, the invention concerns a combination buoyant and nonbuoyant anchor line which permits mooring depth to increase without an increase in the anchor line load. In its more particular aspects, this invention concerns means for doubling the depth of water in which a given anchor line can be used with no increase in the stress in the line and with no change in the horizontal force available for resisting mooring loads. This invention also provides means for eliminating the vertical down load imposed by conventional anchor lines on moored vessels. The invention concerns primarily offshore structures used for drilling and producing wells in marine locations. Simple barge-type floating platforms are desirable for use in offshore drilling operations because they are cheap to build and are readily towed to sheltered areas in the event of severe storms. However, such barge-type platforms must be restrained from horizontal motion while being allowed vertical motion caused by waves and tides.

The utility of conventional anchor chains for mooring floating platforms is limited because when mooring in deep water, the weight of the chain necessary to give the chain adequate strength consumes all of the chains strength in supporting the weight of the chain before any mooring loads are added. Thus, in deep Water, e.g., water 600 or more feet deep, the weight of conventional mooring chains rapidly adds stresses that drastically reduce the ability of the anchor chain to transmit any effective lateral restraining force to the moored structure or vessel. In addition, the conventional chain has a downward force component that in no way contributes to the lateral restraint desired in mooring drilling platforms. In some instances an additional expenditure must be made to provide the moored structure with the necessary buoyancy to counteract this vertical load. In other cases, however, this vertical mooring component ofl ers a useful means for controlling the periods of the moored vessels motion.

A primary object of the present invention is to provide method and apparatus in which the depth of water in which a given anchor line could 'be used may be increased to double the water depth without increasing 3,394,672 Patented July 30, 1968 the stress in the anchor line and without changing the horizontal force available for resisting lateral mooring loads. Another object of the present invention is to provide method and apparatus in which lateral restraint for mooring a floatin'g vessel is provided which has no vertical component tending to pull the floating vessel under Water and which permits the moored floating vessel to move vertically with waves or tide with no appreciable change in the horizontal mooring force.

These objects are achieved by mooring offshore floating vessel with an anchor line, attached at one end to the vessel and at the other end to the land underlying the water (hereinafter referred to as sea bottom), provided with buoyant means attached to a section of the anchor line in a manner to make the anchor line have a net positive upward buoyancy, preferably a positive buoyancy equal to the weight per foot of the anchor line in the water without buoyancy. In the preferred mode of operation, buoyant means are provided such that onehalf of the anchor line forms a regular catenary and the other one-half of the anchor line forms an inverted catenary.

Further objects and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings wherein:

FIGS. 1 to 4 are diagrammatic representations of various mooring arrangements for floating structures;

FIG. 5 is a diagrammatic representation of the mooring arrangement illustrated in FIG. 1 with a coordinate system imposed thereon;

FIG. 6 is an enlarged plan view of an anchor chain to which buoyancy means are attached according to a preferred embodiment of the present invention; and

FIG. 7 is a view taken along lines 7-7 of FIG. 6.

-In FIG. 1 is shown a conventional nonbuoyant an: chor line 10 attached at one end to a vessel designated 11 floating on the surface of the body of water 12 and at the other end thereof to an anchor 13 secured to the sea bottom 14. In FIG. 2 a similar mooring arrangement is shown except a buoyant anchor line 15 is utilized in place of the nonbuoyant line 10. In FIG. 3 one embodiment of a mooring arrangement according to the invention is shown in which an anchor line 16 having an upper buoyant section 16a and a lower buoyant section 16b anchors a floating vessel 17 to a sea bottom anchor 18 in a body of water 19. In FIG. 4 another embodiment of a mooring arrangement in accordance with the present invention is shown which is similar to that illustrated in FIG. 4 except an anchor line 20 having an upper nonbuoyant section 20a and a lower buoyant section 20b is used in place of anchor line 16.

The advantages of the mooring arrangements illustrated in FIGS. 3 and 4 are made evident by the following calculations of the load forces to which the various anchor chains are subjected.

By imposing a coordinate system on a vertical elevation through the sea and sea bottom with the origin (0,0) of such coordinate system 0 units below the sea floor, as illustrated in FIG. 5, it can be shown from the equation describing a catenary that the following equations apply to various aspects of the catenary:

Equation 1 T=Ey Equation 2 D=yc Equation 3 H=Ec Equation 4 y =s +c Equation 5 V=5Es in which T=Axial tension in chain at any point (x,y) lbs. W=Weight in water of chain per unit length, 1bs./ ft.

y=Ordinate of point on catenary.

D=Water depth, ft.

c=Constant indicating the vertical distance (ft.) that the origin of the coordinate system is placed below the sea bottom.

H=Horizontal force in chain at point (xy), lbs.

s=Length of catenary curve from point (0,0) to point (x,y).

V=Vertical force in chain at point (x,y), lbs.

The following factors are assumed in order that the end reactions on the chain can be calculated for illustrative purposes:

The chain is a 3% in. stud link chain having a proof test of 922,000 lbs.; weight of the chain in water (w) is 106 lbs./ft.; water depth (D) is 1735 ft.; the axial load in the chain (T) is equal to the proof test of 922,000 l-bs.; and the chain is tangent to the sea bottom at its point of contact with the sea bottom.

H(mmu)=iic= (106) (6963) =738,078

The horizontal reaction at the ocean bottom, :Hwottom), is Oqufll t huriac e) 738,0781'05.

Equation 4:

y2=32+c2 =v (1/+ (0- s (8698+6963) (8698- 6963) s=5213 Equation V=ZE8= (106) (5213) =552,578

The numerical results of the above illustrative calculations are shown in FIG. 1.

Referring to FIG. 2, a similar chain 15 is buoyed to have a net positive buoyancy per foot equal to the weight of the chain in water; i.e., a net positive buoyancy of 106 lbs. per foot. The mooring characteristics of this chain and its loads in 1735 ft. of water give the exact inverse of the chain illustrated in FIG. 1 because the distributed weight causing the catenary is exactly the same but reversed in direction. Therefore, the resultant and component forces shown on the buoyant chain 15 of FIG. 2 are deducible by analogy from the example given for FIG. 1.

Since the force vectors at the upper end of chain 10 in FIG. 1 are opposite but otherwise identical to those at the lower end of chain in FIG. 2, these two chains can be joined at these two points to give a chain 16 with superior qualities as illustrated in FIG. 3. In this figure, the upper section 16a of chain 16 is made buoyant while the lower section 16b of chain 16 remains nonbuoyant.

In FIG. 4, an arrangement of chains is illustrated opposite to that illustrated in FIG. 3. The upper section 20a of chain 20 is the nonbuoyant portion thereof and the lower section 20b of chain 20 is made buoyant.

The necessary buoyancy for the sections of the anchor chain illustrated in FIGS. 3 and 4 may be provided by buoyant spheres attached to the anchor chain 15 as illustrated in FIGS. 6 and 7. As shown in these figures, a plurality of pairs of steel spheres 21a,b, 22a,b and 23a,b are each secured to a chain link 21c, 22c and 230, respectively, by means of the respective struts 21d, 22d and 23d attached to each pair of spheres and the chain link.

As shown in the following calculations, the 3 /2 in. link chain illustrated in the figures, which is the heaviest chain in general commercial use, may be adequately buoyed where each sphere is 3 ft. in diameter and provided with in. thick walls. Such buoyed pairs of spheres suitably spaced along the length of the chain will provide the desired buoyancy and still not interfere with the flexibility which is low enough to prevent any deformation of the steel sphere by the water pressure.

Where:

P=hydrostetlc pressure of water at 1735 it. (lbs/i112) D =s here diameter (in.) t=sp ere wall thickness (in.) To establish the net positive buoyancy:

Buoyant force on two spheres in sea water=2(920) 1840 lbs. Loss weight of two spheres in air =2(4'25) +850 lbs. 990 lbs. +495 lbs. -495 lbs.

Less weight of chain in sea water (4.66) (106) Then:

Buoyancy of 4.66 it. of chain and spheres in water=495 lbs.

and

Buoyancy oi chain and spheres in water per it. of chain (495/4.GG)

106 lbs/it.

The combination chain illustrated in FIGS. 3 and 4 is most useful where mooring loads are primarily due to wind and wave action. The sphere float members on the buoyant half of the chain will increase the current drag on the chain and thus decrease its utility in areas of high subsurface currents. In the event high subsurface currents are encountered, this limitation can be modified and partially removed by reversing the positions of the two halves of the chain, as illustrated in FIG. 4. With the buoyant half of the chain placed on the lower section thereof and the heavy portion on the upper section thereof, the buoyant half of the chain would be subjected to smaller current loads at the greater water depths. However, in the arrangement shown in FIG. 4, thicker spheres would be required to resist the higher pressures and an anchor to resist vertical loads and a vessel to resist vertical loads would be necessary.

Although only one method of providing net positive buoyancy for the buoyed portion of the chain is shown, other buoyancy arrangements are possible. Any type of floatation arrangement which will give the necessary net positive buoyancy and not impair the catenary action of the chains can be used in place of the buoyancy arrangement illustrated in FIGS. 6 and 7. Further, although a stud link chain is shown and described as the anchoring line, other types of lines may be used instead, as, for example, a cable. Also, a length of neutral weight pipe or cables inserted at the mid-point of the combination chain could be used to extend the depth range even further.

Having fully described the objects, advantages, apparatus and operation of my invention, I claim:

1. Apparatus for use in mooring a floating structure comprising a flexible anchor line having the upper end thereof connected to said floating structure and the lower end thereof connected to the sea bottom and including buoyant means distributed solely along the upper half of said anchor line, said buoyant means providing sufficient net positive buoyancy to said upper half of said anchor line to substantially eliminate vertical mooring loads on said floating structure, and the lower half of said anchor line being without buoyant means whereby said point of contact of said anchor line with said sea bottom being tangent to said sea bottom, and said point of contact of said anchor line with the water surface being tangent to said water surface.

2. Apparatus as recited in claim 1 in which said anchor line comprises a stud link chain and said buoyant means comprises pairs of buoyant spheres attached to and spaced 5 6 along the length of said upper end portion of said chain. upper half of said chain to substantially eliminate verti- 3. Apparatus for use in mooring a floating structure cal mooring loads on said floating structure. comprising a flexible anchor line having the upper end thereof connected to said floating structure and the lower References Cited end thereof COIlI16Ct6d t0 the S63 bottom and including 5 UNITED STATES PATENTS buoyant means distributed solely along the entire upper v half of said anchor line, said anchor line comprising a 11 i 2 Hamllton i gg stud link chain and said buoyant means comprising pairs 295,489 19 7 Bossa 1 4 0 of buoyant spheres attached to and spaced along the length of said upper half of said chain, said buoyant MILTON BUCHLER Primary Exammer means providing suflicient net positive buoyancy to said T. M. BLIX, Assistant Examiner.

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