GB2066191A

GB2066191A - Mooring system

Info

Publication number: GB2066191A
Application number: GB8037645A
Authority: GB
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 1979-11-30
Filing date: 1980-11-24
Publication date: 1981-07-08
Also published as: NO803608L; JPS5677415A; GB2066191B

Abstract

The invention concerns a mooring system for a marine structure of the so-called tension leg type, which structure includes a working deck, a plurality of posts for supporting the working deck, connecting members for the posts as well as buoyancy means for urging and supporting the working deck above sea level, in which system the structure is moored by means of a plurality of mooring lines strained in the water. A disadvantage with such a system is that there can be an excessive variation in the tensions generated in the mooring lines as well as oscillations of the structure due to wave motion, wind force, etc. An object of the invention is to reduce this tension variation and structure oscillations. The invention is characterised in that damping devices (11, 11) are incorporated in the system; said damping means may be suspended from the working deck and have clamping means to clamp respective mooring lines. Alternatively, said damping devices may provide mountings for pulleys associated with respective mooring lines. <IMAGE>

Description

SPECIFICATION Mooring system The present invention relates to mooring systems for a marine structure of the so-called tension leg type.

One known type of floating marine structure which is especially aimed at providing improvements in the oscillation characteristics of the structure in waves, is the so-called tension leg (or strained mooring) marine structure.

As shown in Figure 1, a tension leg marine structure is basically composed of a platform or working deck 1 supported above sea level 6 by a plurality of posts 2 (in most cases these posts also serve as buoyancy bodies), horizontal members 3 connecting the posts 2, strained mooring lines 4, and sinkers 5 anchored to the sea bottom. The structure is given a buoyancy B which is large as compared to a weight W of the structure (that is the structure is made to have a spare buoyancy); a sum F of initial tensions of the mooring lines 4 is made to be equal to B-W to maintain a balance, and thereby oscillations (especially vertical oscillation, pitching and rolling) of the structure due to wave movement can be reduced.

In the case of a marine structure is which oscillations are reduced by the above-described means, however, the period of the natural frequency of the oscillation normally falls within the range of about 3 to 10 seconds, and it often coincides with the period of waves generated in an ocean, which is in the range of 3 to 30 seconds. Therefore, such marine structures have a disadvantage in that the tensions generated in the mooring lines upon resonance becomes excessively large.In order to obviate such a disadvantage, various structures have been proposed such as the cross-section configuration of the horizontal members being made elliptic to reduce resistance and inertial force, the ratio of the displacement of the horizontal members; to the total displacement being selected in the range of 0.3 - 0.6 to reduce the tension in the mooring lines, and the ranges of various parameters being limited - such as by limiting the ratio of the initial tension to the displacement of the structure to within 0.05 - 0.3, the ratio of the displacement of a sinker to its weight to within 0.1 - 0.45, the ratio of the weight of the sinker to the displacement of the structure to within 0.1 - 0.6, and the ratio of the displacement of the sinker to that of the structure to within 1.05 - 1.30.

However, no prior art example of a mooring system has been found for a marine structure in which appropriate damping means is incorporated for the purpose of reducing a variation of the tension generated in the mooring lines and oscillations of the structure in the waves.

One object of the present invention is to provide a mooring system for a tension leg type marine structure in which tensions generated in mooring lines due to wave movement can be reduced.

According to the present invention, there is provided a mooring system for a so-called tension leg type marine structure which includes a working deck, a plurality of posts for supporting said working deck, connecting members for the posts, and buoyancy means for urging and maintaining the working deck above sea level, in which system said structure is moored by means of a plurality of mooring lines strained against the buoyancy provided by the buoyancy means, characterised in that damping devices are incorporated in said mooring system.

The above-mentioned and other features and objects of the present invention will become more apparent by reference to the following description of the operation principle of a tension leg type marine structure and preferred embodiments of the invention taken in conjunction with the accompanying drawings, in which:: Figure 7 is a general schematic view for explaining the principle of a tension leg type marine structure and a mooring system therefor, Figure 2 is a diagrammatic view of the marine structure and mooring system shown in Figure 1 as replaced by a single mass point system model, Figure 3 is a mechanically equivalent model for the system shown in Figure 2, Figure 4 is a diagram showing the range of parameters a and ss for limiting a dynamic response magnification DAFF to 0.8 or less and a ratio of a dynamic displacement vs. a static displacement DAFX to 1.3 or less, Figure 5 is a schematic perspective view of a marine structure showing a practical example of the numerical values, Figures 6(A), 6(8), 6(C) and 6(D) are diagrams showing oscillation characteristics of the structure shown in Figure 5 in the case of incorporating damping devices (solid line) and in the case of not incorporating any damping device (dash line), Figures 7to 1 7 are partial side views showing first, second, third, fourth and fifth preferred embodiments, respectively, of the present invention, and Figures 12(A) and 12(8) are longitudinal cross-sectional viess showing two different examples of the damping device to be incorporated according to the present invention.

With reference to Figures 1 to 3, the operation principle of the mooring system for a tension leg type marine structure will now be described.

Referring now to Figure 1, as described hereinbefore, reference numeral 1 designates a working deck supported above sea level 6 by four posts 2 which also serve as buoyancy bodies, numeral 3 designates horizontal members connecting the lower ends of the respective posts 2, and numeral 4 designates four mooring lines. Each mooring line is associated with a corresponding post 2, and its upper end is wound around a winch 12 on the working deck 1, its lower end being connected to a sinker 5 anchored to the sea bottom 7. Numeral 11 designates damping devices fixed on the working deck 1 for damping variations in tensions in the respective mooring lines 4.

The tensions generated in the mooring lines for a tension leg type marine structure, due to wave movement, are generally determined by vertical oscillation under large constraint, pitching and rolling; these oscillations are represented by an equation of motion (1) for the equivalent model shown in Figure 3, and their natural period Th is represented by Equation (4) below: (W+W) Z+ Ce#Z + (Ke + γ#Aw)Z = Fw (1) 9 Ke = K (Cw) 2 (2) K + (Cw) K2 Ce = K 2 + (Cw) 2 (3)

where the respective symbols denote the following items: weight of a structure W': added weight acceleration of gravity a: specific gravity of sea water Aw: area of waterplane of posts 2, d: draft, K: spring constant of mooring lines 4, C: damping constant of damping devices 11, Ke: equivalent spring constant, Ce: equivalent damping constant, #: angularfrequencyofwaves (= 2z/T), period of waves, Z, Z, Z: longitudinal acceleration, velocity and displacement of a structure, Fw: wave force, F: initial tension in a mooring line, Now the following parameters are defined: a= K/(y.Aw), ..... (5)

b = F/W, ..... (7) # = W'/W, ..... (8) where the respective symbols denote the following items: a: ratio of spring constant/displacement per meter, damping ratio, ratio of initial tension/weight, added mass coefficient.

A solution Zd of the equation of motion (1) is given by Equation (9) below:

where parameters n and Cc are given by the following equations: # = Th/T

On the other hand, a solution Zd' of a motion of equation for the conventional structure not incorporating damping devices is derived by substituting Ce = 0 and Ke = K in Equation (9), and it is represented by Equation (10) below:

Here, representing the tension in the mooring lines in the case of incorporation the damping devices and in the case of not incorporating any damping device by Fd and Fd', respectively, then a ratio of Fd/Fd' (dynamic response magnification DAFF) is given by Equation (11) below, and a ratio DAFX of dynamic displacement vs. static displacement is given by Equation (12) below::

where parameter t is represented by the following equations: #= α(1+#)g (13) 4#ss(1+#)(1+α)d s)(1 +a Zd In a normal tension leg type marine structure, parameters difined in Equations (7), (8), etc. fall within the following ranges

0.1 < 6 < 1.0 0.0 < # < 1.0 # (14) d = 20m - 30m T - 3 seconds # 30seconds Within the ranges indicated by Inequalities and Equations (14), calcuiating and illustrating and conditions to be fulfilled by the parameters a and ss for limiting the dynamic response magnification DAFF to 0.8, 0.6, 0.4 or less and also limiting the ratio DAFX of dynamic displacement vs. static displacement to 1.3 or less, the parameters a and (3should fall in the hatched region in the diagram shown in Figure 4.

This implies that on the contrary if a set of parameters a and p falling in the hatched region in Figure 4 are selected by appropriately selecting the damping constant C, then the dynamic response magnification DAFF of the mooring lines will be maintained at 0.8 or less and the DAFX will be maintained at 1.3 or less, so that variations of the tensions in the mooring lines and thus oscillations of the structure can be widely reduced.

By way of example, enumerating a number of practical numerical values, various factors of the structure shown in Figure 5 are as follows: weight W = 25,500 t area of waterplane Aw = 530.9 m2 initial tension F = 4,500 t spring constant of mooringtlines K = 668 Vm damping constant C = 400 draftd = 30 m = 0.534 (according to a result of a water pool test) Then calculating the parameters a and p on the basis of Equations (5) and (6), the following values are obtained: 0t = x W .o2s X 530.9) - 1 .23

Therefore, in this instance it is seen from Figure 4 that DAFF < 0.4 can be fulfilled.

In addition, the results of calculations based on a response-in-water calculation program whose precision in analysis has been verified by a water pool test for the structure shown in Figure 5, have revealed that, as shown in Figures 6(A), 6(B), 6(C) and 6(D), a variation of tensions in the mooring lines due to wave motion (as represented by solid lines) can be substantially reduced, as compared with the case where no damping device is incorporated (as represented by dash lines).

Preferred embodiments of the present invention will now be described in greater detail with reference to Figures 7 to 12 of the accompanying drawings.

In a first preferred embodiment shown in Figure 7, two damping devices 11, 11 are suspended in parallel from the underneath surface of the working deck 1 adjacent each post 2 and the respective mooring line 4 is disposed therebetween. At the bottom of the damping devices 11, 11 a clamping device 14 is provided for releasably clamping the mooring line 4. After a predetermined initial tension F has been given to the mooring line 4 through wind-up/pay-out operations by means of the respective winch 12, the mooring line 4 is fixedly clamped by the clamping device 14. Thereafter, by a pay-out operation of its winch 12, a section 4' of the mooring line between the winch 12 and the clamping device 14 is somewhat relaxed.Thereby, the initial tension F is exerted steadily upon the damping device 11,so that even if the structure should oscillate due to wave motion or a wind force, a variation of the tension produced in the mooring line would be greatly reduced, and thereby the oscillations of the structure per se could be suppressed to minimum.

In a second preferred embodiment illustrated in Figure 8, a damping device 11 provides a mounting for a pulley 15, said device being disposed obliquely on an edge of the working deck 1, and the upper end of a mooring line 4 being guided around the pulley 15, before being wound around its winch 12. In this case, a steady force that is about < times as large as the initial tension F is exerted upon the damping device 11.

In a third preferred embodiment illustrated in Figure 9(A) or 9(B), a lever fixedly supporting a mooring line winch 12 at one end and a counter-weight 16 at the other end, is pivotably mounted on the working deck 1 via a pin 17 at its fulcrum. The arrangement is such that a left end of the lever (see Figure 9(A)) or a right end of the lever (see Figure 9(B)) may strike against an upper surface if a damping device 11 fixedly secured to the working deck 1. In either case, by appropriately selecting the weight and/or the fixing position of the counter-weight 16, the force exerted steadily upon the damping device 11 can be made nearly zero.

In a fourth preferred embodiment illustrated in Figure 10, two similar lever structures, each of which is shown in Figure 9(A), are provided for a mooring line 4. The lever structures are integrally joined so as to be symmetrical about the fulcrum pin 17. In this arrangement, owing to the balance of the moments produced by the initial tensions F and F', the counter-weight becomes unnecessary, and yet the forces exerted steadily upon the damping devices 11 and 11' can be made nearly zero.

In the fifth preferred embodiment illustrated in Figure 11(A), 11(B) or 11(C), a damping device 11 is incorporated in the mooring line 4, at a position along its length (Figure 11(A)), at a position immediately above the sinker 5 (Figure 11(B)), or within the sinker 5 (Figure 11(C)).

Figures 12(A) and 12(B) show two different examples of a suitable structure for a damping device 11. In Figure 1 2(A), the damping device is a rubber damper formed by fitting rubber cylinders ron the opposite sides of a piston P slidably inserted in a cylinder. In Figures 12(B), the damping device is a hydraulic damper in which oil o in the respective pressure chambers of a hydraulic cylinder is communicated via a valve v and also a restoring spring s, is provided in one pressure chamber of the hydraulic cylinder.

In the above-described second to fifth preferred embodiments, the same damping effects as that described in connection to the first preferred embodiments shown in Figure 7, can be achieved.

Furthermore, with regard to the mooring lines, besides steel wires and chains, ropes made of chemical fibers, tubular members (pipes) and a combination of springs and these items, can be used.

The present invention is widely applicable to tension leg type marine structures which form working and living spaces on the sea-surface, such as installations for trial boring, production and storage of submarine oil, offshore plants, marine airports, off-land cities, etc., regardless of the configurations of the marine structures such as semi-submarine type, ship type, pontoon type, etc.

It will be appreciated from the foregoing that, in accordance with the present invention, a mooring system is provided for a tension leg type marine structure, in which a variation of tensions generated in the mooring lines due to wave motion, wind force, etc. can be suppressed to minimum and also oscillations of the structure can be minimized.

Claims

1. A mooring system for a marine structure of the so-called tension leg type, which marine structure includes a working deck, a plurality of posts for supporting said working deck, connecting members for the posts, and buoyancy means for urging and maintaining the working deck above sea level, in which system said structure is moored by means of a plurality of mooring lines strained against the buoyancy provided by said buoyancy means; characterised in that damping devices are incorporated in said mooring system.

2. A mooring system as claimed in Claim 1, in which said damping devices are suspended from said working deck, and the respective mooring lines are fixedly clamped by clamping means mounted on said respective damping devices.

3. A mooring system as claimed in Claim 1, in which pulleys adapted to guide the respective mooring lines are provided on said working deck, and said damping devices provide mountings for said pulleys for attachment to said working deck.

4. A mooring system as claimed in Claim 1, in which a wind-up/pay-out device for each mooring line and an associated counter weight is pivotably mounted on said working deck, and said damping devices are incorporated between said working deck and said respective wind-up/pay-out devices of the mooring lines.

5. A mooring system as claimed in Claim 1, in which a set of two wind-up/pay-out devices are provided for each mooring line, the set being disposed so that the devices are symmetrical about a pivotal mounting to said working deck, and said damping devices are incorporated between said respective pivotal mountings and said working deck.

6. A mooring system is claimed in Claim 1, in which said damping devices are incorporated along the length of respective mooring lines, immediately above sinkers for anchoring the lower ends of said respective mooring lines, or within said respective sinkers.

7. A mooring system as claimed in any one of Claims 1 to 6, in which each said damping device has a damping constant C such that a ratio a of spring constant/displacement per meter and a damping ration ss fulfil the following relations: if o < as0.75,thenssS0.5; if 0.75 < a 1.25,thenss < 0.9; if 1.25 < a S 2.5, then ss S 1.0; and if 2.5 < a < 4.0,thenss < 0.40rO.66ssS1; where a = K/y-Aw,

K denotes a spring constant of the mooring line, y denotes a specific gravity of the sea water, Aw denotes an area of water plan, W denotes a weight of the marine structure, W' denotes an added weight, and g denotes a gravitational acceleration.

8. A marine structure of the tension leg type incorporating a mooring system as claimed in any one of Claims 1 to 7.

9. A tension leg type marine structure or a mooring system therefor substantially as hereinbefore described with reference to, and as shown in, Figure 7, 8, 9(A), 9(B), 10, 11(A), 11(B) or 11(C) of the accompanying drawings.