US3580207A

US3580207A - Method and means for mooring

Info

Publication number: US3580207A
Application number: US813598A
Authority: US
Inventors: Frans V A Pangalila
Original assignee: John J Mcmullen Associates Inc
Current assignee: John J Mcmullen Associates Inc
Priority date: 1969-04-04
Filing date: 1969-04-04
Publication date: 1971-05-25
Anticipated expiration: 1988-05-25

Abstract

A method and means for mooring and particularly adapted for mooring semisubmersible platforms, such as drilling platforms used in the offshore drilling of oil wells. The invention contemplates providing the platform with a series of tensioned mooring lines, sensing means for sensing tension in the various mooring lines, and means for relieving the tensions on selected mooring lines responsive to the tension on a mooring line rising above its initially tensioned condition due to surge motions of the platform resulting from wind and waves.

Description

United States Patent Inventor Frans V. A. Pangalila Matawan Township Appl. No. 813,598

Filed Apr. 4, 1969 Patented May 25, 1971 Assignee John J. McMullen Associates, Inc.

New York, NY.

METHOD AND MEANS FOR MOORING 9 Claims, 3 Drawing Figs.

US. Cl 1 14/144, 1 l4/0.5 Int. Cl ..B63h25/00, B63b 35/44 Field of Search 114/230,

[5 6] References Cited UNITED STATES PATENTS 3,121,954 2/1964 Foster 114/144(B)X 3,422,783 1/1969 Moulin Primary Examiner-Trygve M. Blix AttorneyFleit, Gipple & Jacobson ABSTRACT: A method and means for mooring and particularly adapted for mooring semisubmersible platforms, such as drilling platforms used in the offshore drilling of oil wells. The invention contemplates providing the platform with a series of tensioned mooring lines, sensing means for sensing tension in the various mooring lines, and means for relieving the tensions on selected mooring lines responsive to the tension on a mooring line rising above its initially tensioned condition due to surge motions of the platform resulting from wind and waves.

METHOD AND MEANS FOR MOORING BACKGROUND OF THE INVENTION The semisubmersible platform, because of its superior characteristics with regard to motions, is becoming increasingly important to the offshore oil industry. As commonly used, a semisubmersible platform containing drilling equipment is positioned over a well site and a riser extends from the platform down to the well site. If the semisubmersible platform were to move horizontally (surge) more than about percent of the water depth, the riser would be subjected to such severe stress that it could be in danger of cracking or breaking.

To maintain the semisubmersible platform over the well site and within the 5 percent limit of surge movement, a moor is employed consisting of a series of mooring cables extending from the body of the platform down to the floor of the associated body of water, the cables having a preselected tension called pretension.

It has been found that as the pretension in the cables is increased, so too is the external force required to move the platform. Therefore, the platform may be stabilized by simply increasing the pretension in each of the mooring cables. However, by increasing the pretension in all of the cables, the cables and the platform must be extremely strong. Further, the effects of shock loading (a large increase in tension caused by a small movement) are pronounced.

In view of the above, other methods have been devised to ensure that the semisubmersible platform remains substantially above the drilling surface. One method which is extensively used in the industry consists of pretensioning the mooring cables an amount sufficient to stabilize the platform under normal conditions, and then to manipulate only the windward cables to counteract the external forces causing surge movement in the platform. However, the additional tension required on the windward side of the platform is often of such a magnitude that it is necessary to provide platforms and cables with extremely high strength capabilities. For example, if the wind force in -foot waves is 45,000 pounds and if the total horizontal component of the forces in the leeward cables is 50,000 pounds, it is necessary that the counteracting horizontal component of the forces generated by cables on the windward side be 95,000 pounds. This is the case since the windward cables must fight" both the external forces and the tension in the leeward cables.

A similar method is disclosed in US. Pat. No. 2,986,889, issued to Ludwig on June 6, 1961. This patent discloses a counterweight system wherein, in response to movement of the platform, the windward cables are increased in tension while the leeward cables are decreased in tension. This system shows some improvement over those systems described above. However, the windward cables are still increased in tension and must therefore be quite strong.

SUMMARY OF THE INVENTION The present invention relates to a method of mooring semisubmersible platforms and contemplates providing same with a plurality of pretensioned mooring cables spaced about its periphery, means for sensing, comparing and regulating the tension in each of these mooring cables, and manipulating only the mooring cables whose tensions are below pretension. More particularly, the tension in the leeward cables is responsively reduced to counteract external forces to maintain tension in respective windward cables substantially at pretension. The advantages achieved by the invention are that the cables are operated in a more elastic region, shock loadings are avoided, lower pretensions are possible, and a smaller chain size can be used to obtain the required results. In practicing the invention, below-pretension cables are manipulated to ensure that none of the cables substantially exceed pretension. When below-tension cables reach pretension, manipulation stops or shifts to other cables on the platform, and their tension is reduced a suitable amount to maintain new windward.

cables substantially at pretension. In this manner, the platform remains well within its 5 percent surge limit and is there maintained without subjecting any cable to excessive tensions.

It is therefore the main object of the'present invention to provide a method and means for maintaining a moored floating body near its rest position in such a manner that none of its mooring lines are subjected to tensions substantially above the initial tension of the lines. In this way, the strength capabilities of the mooring lines and the platform itself may be minimized, thereby effecting a savings in cost.

It is another object of the present invention to provide a method and a means for stabilizing a floating body in a manner which is readily automated.

These and other objects of the invention, as well as many of the attendant advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a semisubmersible platform hav ing mooring cables associated therewith;

FIG. 2 is a side elevation of the semisubmersible platform shown in FIG. 1; and

FIG. 3 is a schematic view of the cable control system forming a part of the present invention.

DETAILED DESCRIPTION OF THE INVENTION With reference first to FIGS. 1 and 2, there is shown a semisubmersible floating platform of the type useful in offshore oil drilling. The platform is shown generally at 10 and comprises, basically, a drilling platform 12, four legs 14, four pontoon members 16, appropriate structural members 18 and a riser l9. Attached to the body of the platform 10 and extending from their point of attachment downwardly to the floor of the associated body of water are a plurality of mooring cables. Fixedly securing the mooring cables to the floor of the water body are a plurality of anchors 20.

For convenience of description, the wind direction is taken to be that indicated by the arrow 22. Also, for ease of description, the cables on the windward side of the platform 10 are indicated by the numeral 24 and the cables on the leeward side of the platform 10 are indicated by the numeral 26. The remainder of the cables, those whose horizontal components lie transverse to the direction of the wind, are indicated by the numeral 28.

With reference now, to FIG. 3, a system for stabilizing a floating platform in accordance with the present invention is schematically illustrated. One windward cable is shown at 24. The windward cable 24, which is fixedly anchored to the floor of the water body, passes over a roller 30 and onto a spool 32 driven by a motor 34. Similarly, the corresponding leeward cable 26 is shown to pass over a roller 36 and wind onto a spool 38 driven by a motor 40. Windward cable 24 and leeward cable 26 are associated with

tension sensors

42 and 44, respectively, which sensors continuously monitor the tension in said cables. The tension sensed by each

tension sensor

42 and 44 is transmitted to an information station 46. Within this information station 46 are means 48 for receiving and comparing tension sensed by the tension sensors with the value of pretension, direction-sensing means 50 for sensing the direction in which each tensionis changing and control means 52 for issuing a control signal to

appropriate motors

34 or 40 in response to the value of tension and the direction of change of this tension.

In operation, assuming that the platform starts at its rest position and the wind is blowing in the direction of the arrow 22, the tension in cable 24 starts to rise while the tension in cable 26 starts to fall.

Tension sensors

42 and 44 transmit this information to the respective comparing means 48 and direction-sensing means 50. Each comparing means 48 is programmed so as to issue a response only if the tension in its associated cable is below pretension. Therefore, only the comparing means 48 associated with the line 26 is active. A signal from the comparing means 48 serves to gate on the respective direction-sensing means 50. As noted above, each direction-sensing means 50 receives a signal from a respective tension sensor. This signal, representative of the tension in a cable, is steady, increasing or decreasing. The direction sensors 50 are set to issue a signal of a first sense, positive for example, if the tension is increasing and of a second and opposite sense, negative for example, if the tension is decreasing. Assuming that the wind is increasing in velocity, the tension in the leeward cable 26 falls. Therefore, gated on" by its associated comparing means 48, the leeward direction sensing means 50 issues a negative signal, this signal being fed to the control means 52. With the wind blowing as noted above, the control means 52 dictates that the spool 33 reel out the cable 26. If the wind were decreasing in velocity, the leeward direction-sensing means 50 would issue a positive signal to the control means 52; the control means 52, in turn, would dictate that the spool 38 reel in the cable 26. Each direction-sensing means 50 is further equipped to issue a disabling signal which maintains the status quo of the system if the tension in its associated cable remains at pretension, this disabling signal being fed to the control means 52.

By means of the system described above, platform movement is held to a minimum. This system operates well regardless of the particular external forces acting on the platform. To illustrate the versatility of the systems operation, there follows a summary of the stabilizing method in general terms, not being limited to the wind direction shown in the figures.

The pretension in each of the cables is noted. When any cable tends to surpass its pretension, as is the case with a windward cable, a cable on the opposite side of the platform, serving as its counterpart, is decreased in tension. The decrease in tension of the leeward cable is just that necessary to maintain the windward cable substantially at pretension. This action stabilizes the platform since the tension reduction in the leeward cable substantially counteracts the force of the wind. No cable goes above pretension by a significant amount.

if, after the operation described in the preceding paragraph is conducted, the wind changes direction or subsides, further operations become necessary-each of these operations involving the manipulation of a cable which is at or below pretension. If the wind changes direction or subsides, the condition of the cables (after the initial operation is practiced) tends to cause the platform to move toward the source of the initial gust of wind. When the platform moves, the initially windward cable decreases in tension and the initially leeward cable increases in tension. In response to the above-described force imbalance, the tension in the initially leeward cable, presently below pretension, is increased so that the tension in the initially windward cable remains substantially at pretension. This stabilizes the platform, and does so without requiring a cable to go over pretension by a significant amount.

If, after the wind subsides or changes direction, and in order to stabilize the platform, it appears necessary to increase the tension in the initially leeward cable above pretension, a further operationis necessary. The initially leeward cable is increased in tension, as described in the preceding paragraph, until it reaches pretension. Then the place of manipulation automatically shifts and the initially windward cable is manipulated so as to maintain the initially leeward cable substantially at pretension. Thus, the initially windward cable becomes the new leeward cable, the initially leeward cable becomes the new windward cable and the operation is repeated.

It should be noted that while the present invention has been described with reference to a wind direction parallel to two sets of mooring cables and perpendicular to the remaining two sets, the system of the present invention works equally well with obliquely directed winds. Further, while the invention has been described with each cable having a separate control mechanism, a given control mechanism may be employed to simultaneouslypontrol the tensions in two or more cables.

The present invention has been described with reference to a specific method and a specific means for practicing this method. it is evident, however, that numerous alterations, modifications, and substitutions may be made to the abovedescribed embodiments without departing from the spirit and the scope of the invention. Accordingly, it is the intent that the present invention not be limited to the above but be limited only as defined in the appended claims.

I claim:

1. A method for stabilizing about a preselected position a floating platform of the type having anchored mooring lines and comprising the steps of: pretensioning each mooring line so that the horizontal components of the pretension add to zero when the platform is at said preselected position; sensing the tension in each mooring line; and manipulating only those mooring lines whose tensions are below pretension in response to the tension sensed so that platform motion is minimized and in such a manner that no line is subjected to a tension substantially above its pretension.

2. The method of claim 1 wherein said select lines are below pretension and are manipulated in such a manner that they are actively increased in tension when the external environment causes an increase in tension, and they are actively reduced in tension when the external environment causes a decrease in tension.

. 3. The method of claim 1 and further comprising the steps of: comparing the tension sensed in each line with the pretension in that line; and sensing the direction in which the tension changes.

4. The method of claim ll wherein each mooring line is provided with a counterpart mooring line and wherein only the mooring lines whose tensions are below pretension are manipulated, and further comprising the step of: manipulating the below-pretension lines so that their counterpart mooring lines are maintained substantially at pretension.

5. The method of claim 1 and further comprising the step of: discontinuing the manipulation of the mooring lines when the tension in said select lines remains at pretension.

6. A control unit useful in stabilizing a floating platform of the type having anchored and pretensioned mooring lines and comprising: first means for controlling the tension is one set of mooring lines; second means for controlling the tension in a further set of mooring lines, said further set of mooring lines being a counterpart of said first set of mooring lines; and means for sensing and controlling the tension in only those mooring lines whose tensions are below pretension so that platform movement is minimized and so that no line substantially increases above its pretension.

7. The invention as recited in claim 6 and further comprising: means for sensing the tension in each line; means for comparing the tension sensed with the pretension of each line; means for detecting the direction in which the tension of each line changes; and means for controlling tee tension in each line in response to the level of the tension and to the direction of tension change.

8. The invention as recited in claim 6 and further comprising: means for regulating the tension in predetermined mooring lines so as to maintain counterpart mooring lines substantially at pretension.

9. The invention as described in claim 6 and further comprising: means for actively decreasing the tension in a mooring line when environmental forces cause said line to decrease in tension; and means for actively increasing the tension in a mooring line when environmental forces cause said line to increase in tension.

Claims

3. The method of claim 1 and further comprising the steps of: comparing the tension sensed in each line with the pretension in that line; and sensing the direction in which the tension changes.

4. The method of claim 1 wherein each mooring line is provided with a counterpart mooring line and wherein only the mooring lines whose tensions are below pretension are manipulated, and further comprising the step of: manipulating the below-pretension lines so that their counterpart mooring lines are maintained substantially at pretension.