CA1222383A - Method of protecting offshore structures against iceberg impacts - Google Patents
Method of protecting offshore structures against iceberg impactsInfo
- Publication number
- CA1222383A CA1222383A CA000460496A CA460496A CA1222383A CA 1222383 A CA1222383 A CA 1222383A CA 000460496 A CA000460496 A CA 000460496A CA 460496 A CA460496 A CA 460496A CA 1222383 A CA1222383 A CA 1222383A
- Authority
- CA
- Canada
- Prior art keywords
- iceberg
- energy
- seabed
- tension
- tension member
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02B—HYDRAULIC ENGINEERING
- E02B17/00—Artificial islands mounted on piles or like supports, e.g. platforms on raisable legs or offshore constructions; Construction methods therefor
- E02B17/0017—Means for protecting offshore constructions
- E02B17/0021—Means for protecting offshore constructions against ice-loads
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
- B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING
- B63B2211/00—Applications
- B63B2211/06—Operation in ice-infested waters
Landscapes
- Engineering & Computer Science (AREA)
- General Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Civil Engineering (AREA)
- Structural Engineering (AREA)
- Earth Drilling (AREA)
Abstract
Title: METHOD OF PROTECTING OFFSHORE
STRUCTURES AGAINST ICEBERG IMPACTS
Inventor: STIG BERNANDER
ABSTRACT OF THE DISCLOSURE
The invention relates to a method for protec-ting offshore structures such as oil platforms against impacts from icebergs by attenuating the impact energy of the icebergs. Attentuation of the iceberg's energy is achieved through the use of cables, chains or other flexible tension members which are attached to energy absorbing devices on the structure. The cables are disposed around the structure to be protected, with one end anchored on the seabed and the other end attached to the structure via the energy absorbing device. The impact of an iceberg against the cables causes movement in the energy absorption device and, consequently, elongation of the cables under tension. The energy absorbed in this manner brings the iceberg to a stop considerably more slowly than would be possible if one were relying on crushing of the ice mass alone. The stopping force is thereby reduced and so, consequently, is the structural strength required of the offshore installation.
STRUCTURES AGAINST ICEBERG IMPACTS
Inventor: STIG BERNANDER
ABSTRACT OF THE DISCLOSURE
The invention relates to a method for protec-ting offshore structures such as oil platforms against impacts from icebergs by attenuating the impact energy of the icebergs. Attentuation of the iceberg's energy is achieved through the use of cables, chains or other flexible tension members which are attached to energy absorbing devices on the structure. The cables are disposed around the structure to be protected, with one end anchored on the seabed and the other end attached to the structure via the energy absorbing device. The impact of an iceberg against the cables causes movement in the energy absorption device and, consequently, elongation of the cables under tension. The energy absorbed in this manner brings the iceberg to a stop considerably more slowly than would be possible if one were relying on crushing of the ice mass alone. The stopping force is thereby reduced and so, consequently, is the structural strength required of the offshore installation.
Description
3$3 FIELD OF THE INVENTION
This invention relates to a method of protec-ting offshore structures such as oil drilling and produc-tion platforms against iceberg impacts, and to structures protected or intended to be protected in accordance with the method.
BACKGROUND OF THE INVENI'ION
Development of offshore hydro-carbon reserves, particularly off the eastern coast of Canada, requires the installation of various structures in water where icebergs frequently occur. The fact that these icebergs may be of immense mass will govern the structural design of the offshore installation.
The characteristics of iceberg occurrence (size, frequency and speed of movement) are poorly under-stood at this time since data gathering has not been going on long. Furthennore, certain physical character-istics of ice masses which are critical to the accurate quantification of forces involved in stopping icebergs are imperfectly understood at this time.
As a consequence of these uncertainties a rational design of an iceberg-resistant structure that relies on the properties of the ice itself (for deter-mining the required stopping capabilities of the struc-ture) must tend to significantly overstate the forces tobe accommodated in the interests of safety and security.
~r ~
hi~ 3 Even so, there exists -the possibility that the design assumptions have significantly underestimated the magnitude and nature of an extreme event such as irnpact from an iceberg of a size statistically likely to be encountered only once in several decades.
An object of the present invention is to provide a method of protecting an offshore structure against iceberg impacts while reducing the collision forces that occur when an iceberg contacts the structure.
BRIEF SUMMARY OF THE INVENTION
The method provided by the invention involves providing the structure with a protective barrier formed by a series of flexible tension members each of which extends downwardly from a point on the structure above water level to a point below water level on the structure or on the seabed.
The tension members are arranged so that an approaching iceherg contacts at least one of the members before the structure and each member is adapted to thereupon deflect towards the structure and absorb kinetic energy from the iceberg, retarding movement of the iceberg towards the structure, whereby the impact energy of the iceberg against the structure is attenuated. At least some of the tension members are each provided with an energy absorbing device adapted to absorb and dissipate at least most of the said kinetic energy.
By the method of the invention, the collision forces that occur when an iceberg contacts a fixed offshore platform are reduced. This is achieved by dissipating the kinetic energy of the moving ice mass over a longer time than is possible when a direct impact is permitted. By reducing the collision forces, a reduc-tion in the structural strength, and therefore the cost oE such an offshore platform can be achieved without reducing its security and safety.
It is believed that the invention provides a method of controlling the magnitude of the iceberg stop-ping forces for which an offshore structure should be designed, which method is not sensitive in the least to the ice properties and is relatively insensitive to the precise shape and size etc. of the iceberg impacting on the structure.
A large proportion of the cost of any offshore platform that may be subjected to iceberg impact lies in the provision of the strength required to resist this impact. This strength is required both locally, near the water surface level, to resist smaller, wave propelled icebergs, and globally to resist the large impacts of extremely large ice masses. It is believed that the method provided by the invention will reduce the possible ice-to-structure forces very significantly in both the above described cases. It is therefore possible to reduce the structural strength of the offshore platfcrm, and hence the cost of this platform , very significantly by employing the method provided by the present inven-tion.
3~3 Other aspects of the invention provide an offshore structure having a protective barrier of the form defined above and a marine structure which is intended to be located offshore in a region subject to iceberg activity, and which is adapted to be provided with a protective barrier of the form defined above. In either case, the structure may be a floating s~ructure or may be designed to rest on ~he seabed.
The tension members may be adapted to absorb kinetic energy Erom the iceberg by elonga~ion of the effective length of the member between its attachment points. This may be achieved in a number of ways -- for example: a) by providing each member with an energy absorbing device disposed on ~he structure and adapted to permit increase in the effective length of the member while absorbing kinetic energy from the iceberg; b) by providing a drag anchor embedded in the seabed and arranging for energy to be absorbed at least partly by dragging of the anchor along the seabed. Yet another possibility is to allow a portion of the tension member to normally lie on the seabed and to arrange for that portion to be lifted when the member is contacted by an iceberg; in that case, some of the kinetic energy from the iceberg is dissipated in lifting the tension member.
The member may be weighted as required to provide the required energy absorption characteristics.
3~3 BRIE DESCRIPTION OF THE_DRAWINGS
In order that the invention may be more clearly understood, reference will now be made ~o the accompany-ing drawings which illustrate a number of preferred embodiments of the invention by way of example, and in which:
Fig. 1 is a diagrammatic side elèvation of an offshore structure in the form of a oil drilling platform in accordance with one aspect of the invention;
Fig. la is a detail ele~ational view of the part of Fig. 1 indicated at A;
Fig. 2 is a view similar to Fig. 1 showing a second aspect of the invention;
Fig. 3 is a detail view of an energy absorbing device for use on the structure of Fig. 1 or 2;
Figs. 4a to e are sectional views taken on the correspondingly marked section lines of Fig. 3 Fig. 5 is a view similar to Fig~ 3 showing a second embodiment of the invention, Fig. 6 i3 a sectional view on line 6-6 of Fig.
5; and, Fig. 7 is a sectional view on line 7-7 of Fig.
5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
Referring first to Fig. 1, the oil drilling platform is generally indicated by reference numeral 20 and includes a base 22 which is shown resting on the ~2~:23~3 -- 7 ~
seabed 24, a superstructure 26 on the base and a topside 28 which is supported above sea level (30). This part of the platform 20 is essentially of conventional construc-tion and includes living quarters, pumping equipment, storage and other features conventionally found in such structures. An iceberg is indicated at 32 and is shown approaching the structure in the direction of arrow 34.
Structure 20 differs from conventional struc-tures in that it is provided with a protective barrier formed by a series of flexible tension members, two of which are shown at 36. The tension members extend radially outwardly from the platform in substantially equi-spaced positions and form a substantially complete barrier around the structure. The angular spacing be-tween adjacent members will be selected to ensure that the members will trap icebergs of a de~ermined size.
That size will be determined based on the criterion of trapping icebergs larger than those which the structure itself has been designed to resist in direct collision.
The tension member themselves are shown as chains in the embodiment of Fig. 1 and, for convenience of description, reference will hereinafter be confined to chain-form tension members. However, it should be understood that tension members of other forms could be used. For example, the members could be ropes or cables or the members could be interconnected to form a mesh.
Fig. 1a shows in ghost outline at 38 transverse members interconnecting the chains 36 to form a mesh.
3~3;3 In Fig. 1, each chain 36 extends downwardly from a point denoted 40 which is above sea level on the strucure itself, to a point denoted 41 below sea level.
The tension members are arranged so that an approaching iceberg will contact the members before the structure and each member is adapted to thereupon deflect towards the structure and absorb kinetic energy from the iceberg, retarding movement of the iceberg towards the structure so that the impact energy of the iceberg against the structure will be attentuated. In this embodiment~ each tension member includes an energy absorbing device gener-ally indicated at 42 disposed on the structure adjacent the point oE attachment 40. This device may take a variety of forms, some of which are indicated below but basically is designed to allow elongation of the tension member while absorbing kinetic energy from the iceberg.
Examples of suitable forms of energy absorbing device are shown in Figs. 3 to 7 and will be described later.
In Fig. 1, the iceberg 32 and one of the ten-sion members 36 are both shown in a full line position atthe moment the iceberg contacts the tension member. In moving towards the structure, ~he iceberg deflects the tension member to the position indicated in ghost outline at 36a and the iceberg itself moves to the ghost outlined position 32a. This is the position just before the iceberg contacts the structure. The progress of the iceberg towards the structure is thus slowed by deflec--\
~;223l~3 _ 9 _ tion of the chain in tension. In this manner r an iceberg is brought to a halt in a time that can be significantly longer than the time which would be taken if the iceberg were to directly collide with the structure.
Proper design and regulation of the energy absorbing device will permit the maximum force that can be exerted on the structure to be accurately determined.
Fig. 2 shows an alternative embodiment of the invention in which primed reference numerals have been used to denote parts corresponding to the parts shown in Fig. 1. In this case, the structure 20' is essentially the same as the structure shown in Fig. 1 b~t in this case the chains (tension elements) 36' extend from attachment points 40' on the structure to anchors on the seabed, one of which is shown at 44. The anchors them-selves are conventional and the chains 36' are connected to energy absorbing devices 42' on the structure. In this casel the approaching iceberg 32' deforms the chain 36' in essentially the same way was described above in connection with Fig. 1. Energy absorbtion is provided by the devices 42'; in addition, there will be some tendency for the chain 36' to stretch and absorb additional energy. The amount of stretch will be greater in the embodiment of Fig. 2 because the potential (i.e. avail-able) elongation of ~he chain 36' will be greater than that of chain 36 of Fig. 1.
Anchor 44 will he essentially a conventional 3~3 anchor and may be designed to remain stationary ir, the seabed or drag towards the structure when the iceberg hits. In the case of a drag a~chor, additional energy will of course be absorbed in causing the anchor to S drag. Where a drag anchor is employed, the tension element may, but need not, be coupled to an energy ab !Jrbing device.
Another possible modification of the arrange-ment shown in Fig. 2 would be to allow a substantial portion of the chain 36 7 adjacent to anchor 44 to normally lie on the seabed as in the case of chain portion 36a' in Fig. 2 (although in practice, the length of portion 36a' could be substantially greater than shown). That portion of the chain would then tend to be lifted off the seabed when the chain is tensioned by an iceberg. The energy required to effect this lifting would absorb further kinetic energy from the iceberg.
Weights such a~ the weight 46 shown on chain 36' can be provided ~o absorb still further energy. These weights would normally be disposed along the portion of the chain which lies on the seabed. Again, ~his feature of lifting a portion of the chain from the seabed can be used with or without an energy absorbing device on the structue.
Figs. 3 and 4a to e on the one hand and Figs~
5, 6 and 7 on the other illustrate alternative forms of energy absorbing device that may be used as the devices 42 and 42' of Figs. 1 and 2. It should be understood 3~i3 that these devices are examples only and are not intended to be exhaustive.
The device shown in Figs. 3 and 4a to 4e is generally denoted by reference numeral 48 and essentially S comprises a hydraulic energy absorber. The energy absorber is denoted by reference numeral 50 and is shown mounted on a base support 52 carried by the structure 20. Chain 36 extends through a guide 54 in advance of the hydraulic energy absorber 50. The energy absorber itself comprises a piston rod 56 carrying a piston 58 within a cylinder 60 and chain 36 is connected to piston rod 56. Cylinder 60 bears against guide 54 which is anchored to the structure 20 by anchorages 90 ~e.g.
anchor bolts). A short chain 92 extends between guide 54 and chain 36 and serves to prevent chain 36 fro~ slacken-ing, without the need for the energy absorber to be continuously pressurized. Chain 92 is designed to break when chain 36 is subjected to significant iceberg impact.
Piston 58 is shown at the end of cylinder 60 remote from the chain 36. This is the position of the piston when the shock absorber i5 primed ready for use.
The space within cylinder 60 is filled with hydraulic fluid which is expelled through a cascading vent sys~em 62 (see below) when the piston 58 is drawn down cylinder 60 as shown. The work done in expelling the fluid from 3~3 cylinder 60 absorbs kinetic energy from the iceberg. The expelled fluid will be collected and returned to the cylinder so that the energy absorber can be reused, An important fea~ure of the energy absorber is that, by contro$1ing the maximum possible fluid pressure in cylinder 60, the possibility oE overloading the flexible tension members (36 or 36') can be wholly eliminated.
The action of the hydraulic energy absorber is further elaborated below:
The cascading vent system, generally indicated as 62, consists of several components as follows:
(a) A series of spring (pressure relieE) valves designated 98. The function of these valves is to permit hydraulic fluid to escape from cylinder 60 when the system is required to absorb energy but to prevent escape of the ~luid until a pre-determined threshold pressure is reached. A number of relief valves, 98, are shown in parallel arrangement. Each valve 98 has a different orifice diameter and pressure threshold such ~hat the threshold pressure increases with increasing orifice diameter. This arrangement permits the rate of discharge of hydraulic fluid to be automatically adapted to the different energy absorption requirements of various sizes of icebergs travelling at various speeds while preventing any overstressing of the chains. For example, the absorber can accommodate high instantaneous internal pressures resulting from impacts with small icebergs travelling at high speeds, as well as sustained pressures produced by impacts with large slow moving icebergs.
Overstress could occur if only a single size of fluid escape orifice were provided since under a fast impact the fluid could not escape quickly enough to keep the pressure in the cylinder within allowable limits.
(b) A one-way valve, 96, permitting quick reEilling of the cylinder after an energy absorbing stroke has occurred.
(c) A gate valve, lOO, which may be opened as required to permit slackening of the chain, 36, by exten sion of piston 58 in cylinder 60.
Also provided are a hydraulic reservoir 92, a pump 94, a selector valve 102 by means of which the energy absorber 50, may be returned to its primed posi-tion after use or in anticipation of use or extended to slacken chain 36, and a discharge line 104 including a valve 106. ~ine 104 permits fluid to flow to and from the right hand end of cylinder 60 (as drawn). Valve 106 is closed when piston 58 is to be driven to the left by fluid delivered from pump 92, e.g. to slacken chain 36 to permit the close approach of supply vessels etc.
Figs. S, 6 and 7 show an energy ahsorbing device generally denoted 64 in the form of a friction 23~3 .
brake. In this case, the brake itself is generally denoted ~6 and is mounted on the structure of the plat-orm 20. Chain 36 passes through a guide 68 to the brake. The brake incorporates two brake shoes 70 and 72 disposed on respectively opposite sides of a bar 74 connected to chain 36. The lower brake shoe 70 is stationary while the upper brake shoe 72 is movable towards and away from the bar to apply a frictional clamping force thereto. Thus~ brake shoe 72 is carried by a backing plate 76 which can be urged downwardly by hydraulic jacks 78 (see particularly Fig. 7) acting between backing plate 76 and across head 80.
Referring back to Fig. 5, bar 74 is connected at its end remote from chain 36 to a cable 82 connected to a conventional winch 84 on the structure. Winch 84 is of the type typically used on oil drilling platforms for controlling anchor chains and may be used to control the tension in, or length of, chain 36 in similar fashion.
A number of advantages of the met~od of the invention have been discussed previously and others are listed below:
The forces available for stopping an iceberg will~ to a large degree, be proportional to the size of the iceberg colliding with the structure. A larger iceberg will eng~ge more chains than will a smaller iceberg and will therefore be able to mobilize a larger stopping force.
23~3 During the process of collision with the chains, an iceberg wîll cause the chain to take a new shape which will result in a significant downward compo-nent of load from the chain to the structure on the impact sideO This downward component of the chain load will serve to counter~act the overturning effect of the ice mass contacting the structure proper.
An iceberg in collision with the chain system will tend to slide upwards along these chains leading to further energy absorption over and above the work per-formed by the energy absorption system attached to the chain.
An ice mass colliding eccentrically with the offshore structure will tend to rotate around the point of collision and will not therefore have to be brought to a full stop. The system described by this invention will bring the point of impact on the ice mass to stop over a larger distance than would otherwise be the case and will ~herefore allow a greater proportion of the icebergs kinetic energy to be transformed into rotational energy.
The force exerted by the ice mass on the struc-ture will be much more clearly and accurately definable with the invention described than it would be in the usual case where direct collision is permitted. This is because the chain retardation system can be designed to produce an accurately predictable force while the state of knowledge regarding ice characteristics etc. does not %~23~
permi~ an accurate prediction of the force ~hat will be generated in a direct collision.
The system described is particularly effective against ice masses of small size that may attain a speed equal to the particle velocity of the waves transporting them. Such ice masses are relatively small in mass and high in kinetic energy. They therefore tend to govern the design of the upper portion of any such offshore platform. The system proposed will catch any such wave-drive-ice masses of significant size.
The system described can be very quickly made ready for reuse after a collision. All that is required is that the energy absorption system (energy absorber~
friction device, etc.) be returned to its original position, for example by a pump automatically controlled by the fluid pressure in the eneryy absorber. The system is then fully operational against renewed impact.
The tension members of the protection system described herein may also be slackened to permit the approach of supply ships etc. right up to the offshore platform.
I~ should of course be noted that the preceding description relates to particular preferred embodiments of the invention only and tha~ many modifications are possible within the broad scope of the claims. Specifi-cally, it should be noted that the method of the inven-tion may be applied to floating structures and to structures intended to operate resting on the seabed.
This invention relates to a method of protec-ting offshore structures such as oil drilling and produc-tion platforms against iceberg impacts, and to structures protected or intended to be protected in accordance with the method.
BACKGROUND OF THE INVENI'ION
Development of offshore hydro-carbon reserves, particularly off the eastern coast of Canada, requires the installation of various structures in water where icebergs frequently occur. The fact that these icebergs may be of immense mass will govern the structural design of the offshore installation.
The characteristics of iceberg occurrence (size, frequency and speed of movement) are poorly under-stood at this time since data gathering has not been going on long. Furthennore, certain physical character-istics of ice masses which are critical to the accurate quantification of forces involved in stopping icebergs are imperfectly understood at this time.
As a consequence of these uncertainties a rational design of an iceberg-resistant structure that relies on the properties of the ice itself (for deter-mining the required stopping capabilities of the struc-ture) must tend to significantly overstate the forces tobe accommodated in the interests of safety and security.
~r ~
hi~ 3 Even so, there exists -the possibility that the design assumptions have significantly underestimated the magnitude and nature of an extreme event such as irnpact from an iceberg of a size statistically likely to be encountered only once in several decades.
An object of the present invention is to provide a method of protecting an offshore structure against iceberg impacts while reducing the collision forces that occur when an iceberg contacts the structure.
BRIEF SUMMARY OF THE INVENTION
The method provided by the invention involves providing the structure with a protective barrier formed by a series of flexible tension members each of which extends downwardly from a point on the structure above water level to a point below water level on the structure or on the seabed.
The tension members are arranged so that an approaching iceherg contacts at least one of the members before the structure and each member is adapted to thereupon deflect towards the structure and absorb kinetic energy from the iceberg, retarding movement of the iceberg towards the structure, whereby the impact energy of the iceberg against the structure is attenuated. At least some of the tension members are each provided with an energy absorbing device adapted to absorb and dissipate at least most of the said kinetic energy.
By the method of the invention, the collision forces that occur when an iceberg contacts a fixed offshore platform are reduced. This is achieved by dissipating the kinetic energy of the moving ice mass over a longer time than is possible when a direct impact is permitted. By reducing the collision forces, a reduc-tion in the structural strength, and therefore the cost oE such an offshore platform can be achieved without reducing its security and safety.
It is believed that the invention provides a method of controlling the magnitude of the iceberg stop-ping forces for which an offshore structure should be designed, which method is not sensitive in the least to the ice properties and is relatively insensitive to the precise shape and size etc. of the iceberg impacting on the structure.
A large proportion of the cost of any offshore platform that may be subjected to iceberg impact lies in the provision of the strength required to resist this impact. This strength is required both locally, near the water surface level, to resist smaller, wave propelled icebergs, and globally to resist the large impacts of extremely large ice masses. It is believed that the method provided by the invention will reduce the possible ice-to-structure forces very significantly in both the above described cases. It is therefore possible to reduce the structural strength of the offshore platfcrm, and hence the cost of this platform , very significantly by employing the method provided by the present inven-tion.
3~3 Other aspects of the invention provide an offshore structure having a protective barrier of the form defined above and a marine structure which is intended to be located offshore in a region subject to iceberg activity, and which is adapted to be provided with a protective barrier of the form defined above. In either case, the structure may be a floating s~ructure or may be designed to rest on ~he seabed.
The tension members may be adapted to absorb kinetic energy Erom the iceberg by elonga~ion of the effective length of the member between its attachment points. This may be achieved in a number of ways -- for example: a) by providing each member with an energy absorbing device disposed on ~he structure and adapted to permit increase in the effective length of the member while absorbing kinetic energy from the iceberg; b) by providing a drag anchor embedded in the seabed and arranging for energy to be absorbed at least partly by dragging of the anchor along the seabed. Yet another possibility is to allow a portion of the tension member to normally lie on the seabed and to arrange for that portion to be lifted when the member is contacted by an iceberg; in that case, some of the kinetic energy from the iceberg is dissipated in lifting the tension member.
The member may be weighted as required to provide the required energy absorption characteristics.
3~3 BRIE DESCRIPTION OF THE_DRAWINGS
In order that the invention may be more clearly understood, reference will now be made ~o the accompany-ing drawings which illustrate a number of preferred embodiments of the invention by way of example, and in which:
Fig. 1 is a diagrammatic side elèvation of an offshore structure in the form of a oil drilling platform in accordance with one aspect of the invention;
Fig. la is a detail ele~ational view of the part of Fig. 1 indicated at A;
Fig. 2 is a view similar to Fig. 1 showing a second aspect of the invention;
Fig. 3 is a detail view of an energy absorbing device for use on the structure of Fig. 1 or 2;
Figs. 4a to e are sectional views taken on the correspondingly marked section lines of Fig. 3 Fig. 5 is a view similar to Fig~ 3 showing a second embodiment of the invention, Fig. 6 i3 a sectional view on line 6-6 of Fig.
5; and, Fig. 7 is a sectional view on line 7-7 of Fig.
5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
Referring first to Fig. 1, the oil drilling platform is generally indicated by reference numeral 20 and includes a base 22 which is shown resting on the ~2~:23~3 -- 7 ~
seabed 24, a superstructure 26 on the base and a topside 28 which is supported above sea level (30). This part of the platform 20 is essentially of conventional construc-tion and includes living quarters, pumping equipment, storage and other features conventionally found in such structures. An iceberg is indicated at 32 and is shown approaching the structure in the direction of arrow 34.
Structure 20 differs from conventional struc-tures in that it is provided with a protective barrier formed by a series of flexible tension members, two of which are shown at 36. The tension members extend radially outwardly from the platform in substantially equi-spaced positions and form a substantially complete barrier around the structure. The angular spacing be-tween adjacent members will be selected to ensure that the members will trap icebergs of a de~ermined size.
That size will be determined based on the criterion of trapping icebergs larger than those which the structure itself has been designed to resist in direct collision.
The tension member themselves are shown as chains in the embodiment of Fig. 1 and, for convenience of description, reference will hereinafter be confined to chain-form tension members. However, it should be understood that tension members of other forms could be used. For example, the members could be ropes or cables or the members could be interconnected to form a mesh.
Fig. 1a shows in ghost outline at 38 transverse members interconnecting the chains 36 to form a mesh.
3~3;3 In Fig. 1, each chain 36 extends downwardly from a point denoted 40 which is above sea level on the strucure itself, to a point denoted 41 below sea level.
The tension members are arranged so that an approaching iceberg will contact the members before the structure and each member is adapted to thereupon deflect towards the structure and absorb kinetic energy from the iceberg, retarding movement of the iceberg towards the structure so that the impact energy of the iceberg against the structure will be attentuated. In this embodiment~ each tension member includes an energy absorbing device gener-ally indicated at 42 disposed on the structure adjacent the point oE attachment 40. This device may take a variety of forms, some of which are indicated below but basically is designed to allow elongation of the tension member while absorbing kinetic energy from the iceberg.
Examples of suitable forms of energy absorbing device are shown in Figs. 3 to 7 and will be described later.
In Fig. 1, the iceberg 32 and one of the ten-sion members 36 are both shown in a full line position atthe moment the iceberg contacts the tension member. In moving towards the structure, ~he iceberg deflects the tension member to the position indicated in ghost outline at 36a and the iceberg itself moves to the ghost outlined position 32a. This is the position just before the iceberg contacts the structure. The progress of the iceberg towards the structure is thus slowed by deflec--\
~;223l~3 _ 9 _ tion of the chain in tension. In this manner r an iceberg is brought to a halt in a time that can be significantly longer than the time which would be taken if the iceberg were to directly collide with the structure.
Proper design and regulation of the energy absorbing device will permit the maximum force that can be exerted on the structure to be accurately determined.
Fig. 2 shows an alternative embodiment of the invention in which primed reference numerals have been used to denote parts corresponding to the parts shown in Fig. 1. In this case, the structure 20' is essentially the same as the structure shown in Fig. 1 b~t in this case the chains (tension elements) 36' extend from attachment points 40' on the structure to anchors on the seabed, one of which is shown at 44. The anchors them-selves are conventional and the chains 36' are connected to energy absorbing devices 42' on the structure. In this casel the approaching iceberg 32' deforms the chain 36' in essentially the same way was described above in connection with Fig. 1. Energy absorbtion is provided by the devices 42'; in addition, there will be some tendency for the chain 36' to stretch and absorb additional energy. The amount of stretch will be greater in the embodiment of Fig. 2 because the potential (i.e. avail-able) elongation of ~he chain 36' will be greater than that of chain 36 of Fig. 1.
Anchor 44 will he essentially a conventional 3~3 anchor and may be designed to remain stationary ir, the seabed or drag towards the structure when the iceberg hits. In the case of a drag a~chor, additional energy will of course be absorbed in causing the anchor to S drag. Where a drag anchor is employed, the tension element may, but need not, be coupled to an energy ab !Jrbing device.
Another possible modification of the arrange-ment shown in Fig. 2 would be to allow a substantial portion of the chain 36 7 adjacent to anchor 44 to normally lie on the seabed as in the case of chain portion 36a' in Fig. 2 (although in practice, the length of portion 36a' could be substantially greater than shown). That portion of the chain would then tend to be lifted off the seabed when the chain is tensioned by an iceberg. The energy required to effect this lifting would absorb further kinetic energy from the iceberg.
Weights such a~ the weight 46 shown on chain 36' can be provided ~o absorb still further energy. These weights would normally be disposed along the portion of the chain which lies on the seabed. Again, ~his feature of lifting a portion of the chain from the seabed can be used with or without an energy absorbing device on the structue.
Figs. 3 and 4a to e on the one hand and Figs~
5, 6 and 7 on the other illustrate alternative forms of energy absorbing device that may be used as the devices 42 and 42' of Figs. 1 and 2. It should be understood 3~i3 that these devices are examples only and are not intended to be exhaustive.
The device shown in Figs. 3 and 4a to 4e is generally denoted by reference numeral 48 and essentially S comprises a hydraulic energy absorber. The energy absorber is denoted by reference numeral 50 and is shown mounted on a base support 52 carried by the structure 20. Chain 36 extends through a guide 54 in advance of the hydraulic energy absorber 50. The energy absorber itself comprises a piston rod 56 carrying a piston 58 within a cylinder 60 and chain 36 is connected to piston rod 56. Cylinder 60 bears against guide 54 which is anchored to the structure 20 by anchorages 90 ~e.g.
anchor bolts). A short chain 92 extends between guide 54 and chain 36 and serves to prevent chain 36 fro~ slacken-ing, without the need for the energy absorber to be continuously pressurized. Chain 92 is designed to break when chain 36 is subjected to significant iceberg impact.
Piston 58 is shown at the end of cylinder 60 remote from the chain 36. This is the position of the piston when the shock absorber i5 primed ready for use.
The space within cylinder 60 is filled with hydraulic fluid which is expelled through a cascading vent sys~em 62 (see below) when the piston 58 is drawn down cylinder 60 as shown. The work done in expelling the fluid from 3~3 cylinder 60 absorbs kinetic energy from the iceberg. The expelled fluid will be collected and returned to the cylinder so that the energy absorber can be reused, An important fea~ure of the energy absorber is that, by contro$1ing the maximum possible fluid pressure in cylinder 60, the possibility oE overloading the flexible tension members (36 or 36') can be wholly eliminated.
The action of the hydraulic energy absorber is further elaborated below:
The cascading vent system, generally indicated as 62, consists of several components as follows:
(a) A series of spring (pressure relieE) valves designated 98. The function of these valves is to permit hydraulic fluid to escape from cylinder 60 when the system is required to absorb energy but to prevent escape of the ~luid until a pre-determined threshold pressure is reached. A number of relief valves, 98, are shown in parallel arrangement. Each valve 98 has a different orifice diameter and pressure threshold such ~hat the threshold pressure increases with increasing orifice diameter. This arrangement permits the rate of discharge of hydraulic fluid to be automatically adapted to the different energy absorption requirements of various sizes of icebergs travelling at various speeds while preventing any overstressing of the chains. For example, the absorber can accommodate high instantaneous internal pressures resulting from impacts with small icebergs travelling at high speeds, as well as sustained pressures produced by impacts with large slow moving icebergs.
Overstress could occur if only a single size of fluid escape orifice were provided since under a fast impact the fluid could not escape quickly enough to keep the pressure in the cylinder within allowable limits.
(b) A one-way valve, 96, permitting quick reEilling of the cylinder after an energy absorbing stroke has occurred.
(c) A gate valve, lOO, which may be opened as required to permit slackening of the chain, 36, by exten sion of piston 58 in cylinder 60.
Also provided are a hydraulic reservoir 92, a pump 94, a selector valve 102 by means of which the energy absorber 50, may be returned to its primed posi-tion after use or in anticipation of use or extended to slacken chain 36, and a discharge line 104 including a valve 106. ~ine 104 permits fluid to flow to and from the right hand end of cylinder 60 (as drawn). Valve 106 is closed when piston 58 is to be driven to the left by fluid delivered from pump 92, e.g. to slacken chain 36 to permit the close approach of supply vessels etc.
Figs. S, 6 and 7 show an energy ahsorbing device generally denoted 64 in the form of a friction 23~3 .
brake. In this case, the brake itself is generally denoted ~6 and is mounted on the structure of the plat-orm 20. Chain 36 passes through a guide 68 to the brake. The brake incorporates two brake shoes 70 and 72 disposed on respectively opposite sides of a bar 74 connected to chain 36. The lower brake shoe 70 is stationary while the upper brake shoe 72 is movable towards and away from the bar to apply a frictional clamping force thereto. Thus~ brake shoe 72 is carried by a backing plate 76 which can be urged downwardly by hydraulic jacks 78 (see particularly Fig. 7) acting between backing plate 76 and across head 80.
Referring back to Fig. 5, bar 74 is connected at its end remote from chain 36 to a cable 82 connected to a conventional winch 84 on the structure. Winch 84 is of the type typically used on oil drilling platforms for controlling anchor chains and may be used to control the tension in, or length of, chain 36 in similar fashion.
A number of advantages of the met~od of the invention have been discussed previously and others are listed below:
The forces available for stopping an iceberg will~ to a large degree, be proportional to the size of the iceberg colliding with the structure. A larger iceberg will eng~ge more chains than will a smaller iceberg and will therefore be able to mobilize a larger stopping force.
23~3 During the process of collision with the chains, an iceberg wîll cause the chain to take a new shape which will result in a significant downward compo-nent of load from the chain to the structure on the impact sideO This downward component of the chain load will serve to counter~act the overturning effect of the ice mass contacting the structure proper.
An iceberg in collision with the chain system will tend to slide upwards along these chains leading to further energy absorption over and above the work per-formed by the energy absorption system attached to the chain.
An ice mass colliding eccentrically with the offshore structure will tend to rotate around the point of collision and will not therefore have to be brought to a full stop. The system described by this invention will bring the point of impact on the ice mass to stop over a larger distance than would otherwise be the case and will ~herefore allow a greater proportion of the icebergs kinetic energy to be transformed into rotational energy.
The force exerted by the ice mass on the struc-ture will be much more clearly and accurately definable with the invention described than it would be in the usual case where direct collision is permitted. This is because the chain retardation system can be designed to produce an accurately predictable force while the state of knowledge regarding ice characteristics etc. does not %~23~
permi~ an accurate prediction of the force ~hat will be generated in a direct collision.
The system described is particularly effective against ice masses of small size that may attain a speed equal to the particle velocity of the waves transporting them. Such ice masses are relatively small in mass and high in kinetic energy. They therefore tend to govern the design of the upper portion of any such offshore platform. The system proposed will catch any such wave-drive-ice masses of significant size.
The system described can be very quickly made ready for reuse after a collision. All that is required is that the energy absorption system (energy absorber~
friction device, etc.) be returned to its original position, for example by a pump automatically controlled by the fluid pressure in the eneryy absorber. The system is then fully operational against renewed impact.
The tension members of the protection system described herein may also be slackened to permit the approach of supply ships etc. right up to the offshore platform.
I~ should of course be noted that the preceding description relates to particular preferred embodiments of the invention only and tha~ many modifications are possible within the broad scope of the claims. Specifi-cally, it should be noted that the method of the inven-tion may be applied to floating structures and to structures intended to operate resting on the seabed.
Claims (28)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method protecting a fixed offshore structure against iceberg impacts, comprising the steps of providing the structure with a protective barrier formed by a series of flexible tension members each extending downwardly from a point on the structure above water level to a point below water level on the structure or on the seabed, the tension members being arranged so that an approaching iceberg contacts at least one of said members before contacting the structure and each member being adapted to thereupon deflect towards the structure and in the process absorb kinetic energy from the iceberg, each of at least some of said tension members being provided with an energy absorbing device adapted to absorb and dissipate at least most of said kinetic energy, whereby movement of the iceberg towards the structure is retarded and the impact energy of the iceberg against the structure is attenuated.
2. A method as claimed in claim 1, wherein each said tension member is adapted to absorb kinetic energy from the iceberg by elongation of the effective length of the member between its said attachment points.
3. A method as claimed in claim 1, wherein each said energy absorbing device is disposed on the structure between said attachment points of the associated tension member, said device being adapted to permit elongation of the effective length of the member while absorbing kinetic energy from the iceberg.
4. A method as claimed in claim 1, wherein each said energy absorbing device comprises a drag anchor embedded in the seabed, whereby energy is absorbed at least partly by dragging of the anchor along the seabed.
5. A method as claimed in claim 1, wherein each tension member has an attachment point defined by an anchor on the seabed positioned a substantial distance from the structure, wherein the tension member lies on the seabed over a significant distance from said anchor towards the structure and forms a said energy absorbing device, whereby energy is absorbed and dissipated by lifting of said portion of the tension member as a result of deflection of the member by an iceberg.
6. A method as claimed in claim 5, wherein said portion of the anchor member on the seabed is provided with weights to increase the energy absorbing capacity of the system .
7. A method as claimed in claim 5, wherein said tension members are chains or cables.
8. A method as claimed in claim 5, wherein said energy absorbing device is a hydraulic piston coupled to a said tension member and arranged to absorb and dissipate energy by expelling fluid from said cylinder.
9. A method as claimed in claim 8, further compri-sing the step of providing a hydraulic fluid outlet of said cylinder with a cascading set of pressure relief valves arranged in parallel to permit the escape of hydraulic fluid from a hydraulic cylinder to be automa-tically adjusted to the energy absorption needs of a particular iceberg impact and to thereby function as a means of energy absorption for the tension member.
10. A method as claimed in claim 9, wherein the cascading set of pressure relief valves are calibrated to prevent the energy absorption device from being subject to an internal pressure high enough to cause failure of the tension member while still retaining the required energy absorbing characteristics.
11. A fixed offshore structure having a protective barrier formed by a series of flexible tension members, each extending downwardly from a point on the structure above water level to a point below water level on the structure or on the seabed, the tension members being arranged so that an approaching iceberg contacts at least one of said members before the structure, and each member being adapted to thereupon deflect towards the structure and absorb kinetic energy from the iceberg, each of at least some of said tension members being provided with an energy absorbing device adapted to absorb and dissipate at least most of said kinetic energy, whereby movement of the iceberg towards the structure is retarded and the impact energy of the iceberg against the structure is attenuated.
12. A structure as claimed in claim 11, wherein each said tension member is adapted to absorb kinetic energy from the iceberg by elongation of the effective length of the member between its said attachment points.
13. A structure as claimed in claim 11, wherein each said energy absorbing device is disposed on the structure between said attachment points of the member, said device being adapted to permit elongation of the effective length of the member while absorbing kinetic energy from the iceberg.
14. A structure as claimed in claim 11, wherein each said energy absorbing device comprises a drag anchor embedded in the seabed, whereby energy is absorbed and dissipated at least partly by dragging of the anchor along the seabed.
15. A structure as claimed in claim 11, wherein each tension member has an attachment point defined by an anchor on the seabed positioned a substantial distance from the structure, wherein the tension member lies on the seabed over a significant distance from said anchor towards the structure and forms a said energy absorbing device, whereby energy is absorbed and dissipated by lifting of said portion of the tension member as a result of deflection of the member by an iceberg.
16. A structure as claimed in claim 11, wherein said portion of the anchor member on the seabed is pro-vided with weights.
17. A structure as claimed in claim 11, wherein said tension members are chains or cables.
18. A structure as claimed in claim 13, wherein said energy absorbing device is a hydraulic piston and cylinder unit arranged with the piston coupled to a said tension member and arranged to absorb and dissipate energy by expelling fluid from said cylinder.
19. A structure as claimed in claim 18, wherein said cylinder has a hydraulic fluid outlet communicating with a cascading set of pressure relief valves arranged in parallel and adapted to permit the escape of hydraulic fluid from a hydraulic cylinder to automatically accom-modate the energy absorption needs of a particular iceberg impact and to thereby function as a means of energy absorption for the tension member.
20. A structure as claimed in claim 19, wherein the cascading of pressure relief valves are calibrated to prevent the energy absorption device from being subject to an internal pressure high enough to cause failure of the tension member while still retaining the required energy absorbing characteristics.
21. A structure as claimed in claim 13, wherein said energy absorbing device further comprises means for slackening the aforesaid tension members to permit the close approach of supply vessels.
22. A structure as claimed in claim 11, wherein said protective barrier is formed by a series of flexible tension member arranged radially about the structure and forming a substantially complete barrier around the structure.
23. A structure as claimed in claim 17, wherein at least some of said flexible tension members are connected with transverse members to form a mesh.
24. A marine structure which is intended to be disposed in a fixed location offshore in a region subject to iceberg activity, wherein the structure is adapted to be provided with a protective barrier formed by a series of flexible tension members adapted to extend downwardly from the point on the structure above water level to a point below water level on the structure or on the seabed, the tension members being intended to be arranged so that an approaching iceberg contacts at least one said member before the structure, and each member being adapted to thereupon deflect towards the structure and absorb kinetic energy from the iceberg, each of at least some of said tension members being provided with an energy absorbing device adapted to absorb and dissipate at least most of said kinetic energy, whereby movement of the iceberg towards the structure is retarded and the impact energy of the iceberg against the structure is attenuated.
25. A structure as claimed in claim 24, wherein each said tension member is adapted to absorb kinetic energy from the iceberg by elongation of the effective length of the member between its said attachment points.
26. A structure as claimed in claim 24, wherein a plurality of said energy absorbing devices are disposed on the structure, each said device being adapted to permit coupling thereto of a said tension member and to permit elongation of the effective length of the member while absorbing kinetic energy from an iceberg contacting the member in use.
27. A structure as claimed in claim 24, wherein each said energy absorbing device comprises a drag anchor embedded in the seabed, whereby, in use, energy is absorbed at least partly by dragging of the anchor along the seabed.
28. A structure as claimed in claim 24, wherein each tension member has an attachment point defined by an anchor adapted to engage the seabed in use, at a position a substantial distance from the structure, whereby the tension member can be arranged to lie on the seabed over a significant distance from said anchor towards the structure and forms an energy absorbing device so that energy is absorbed and dissipated by lifting of said portion of the tension member as a result of deflection of the member by an iceberg in use.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000460496A CA1222383A (en) | 1984-08-07 | 1984-08-07 | Method of protecting offshore structures against iceberg impacts |
| NO843227A NO843227L (en) | 1984-08-07 | 1984-08-14 | OFFSHORE CONSTRUCTION AND PROCEDURE FOR PROTECTING IT AGAINST THE ASSOCIATION WITH ISFJELL |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000460496A CA1222383A (en) | 1984-08-07 | 1984-08-07 | Method of protecting offshore structures against iceberg impacts |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1222383A true CA1222383A (en) | 1987-06-02 |
Family
ID=4128478
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000460496A Expired CA1222383A (en) | 1984-08-07 | 1984-08-07 | Method of protecting offshore structures against iceberg impacts |
Country Status (2)
| Country | Link |
|---|---|
| CA (1) | CA1222383A (en) |
| NO (1) | NO843227L (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5224800A (en) * | 1990-12-12 | 1993-07-06 | National Research Council Of Canada | Protective system against icebergs or floating objects |
-
1984
- 1984-08-07 CA CA000460496A patent/CA1222383A/en not_active Expired
- 1984-08-14 NO NO843227A patent/NO843227L/en unknown
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5224800A (en) * | 1990-12-12 | 1993-07-06 | National Research Council Of Canada | Protective system against icebergs or floating objects |
Also Published As
| Publication number | Publication date |
|---|---|
| NO843227L (en) | 1986-02-10 |
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