CA1064272A

CA1064272A - Passive seabed protection against ice scour

Info

Publication number: CA1064272A
Application number: CA270,229A
Authority: CA
Inventors: G. Roger Pilkington
Original assignee: Dome Petroleum Ltd
Current assignee: Dome Petroleum Ltd
Priority date: 1977-01-21
Filing date: 1977-01-21
Publication date: 1979-10-16

Abstract

ABSTRACT OF THE DISCLOSURE
Protection against sea floor scour is provided by generating an artificial pingo on the sea floor by injecting fresh water into the sea bed and freezing the water to form ice in situ.

Description

10~42~72 Thls invention relates to method and means for protecting permanent structures and pipelines on the sea floor against scour by ice or dragging anchors.
In Arctic areas, and particularly in the Beaufort Sea~ the sea bed is marked with scour due to moving ice forms scraping the sea floor. Any pipeline or other structures on the sea floor, e.g., wellhead completion devices~ are susceptible to damage by ice keels lmpinging on the pipeline or structure.
Accordingly, it is an ob~ect of one aspect of this invention to provide a method for providing for protection for any permanent structure at or near the sea floor against scour.
An ob~ect of another aspect of this invention is the provision of a pipeline bed on the sea floor which is protected against scour.
- By one aspect of this invention, a pipeline bed is provided for a pipeline structure which has been protected against scour on a sea bet in a sea bed trench where sea bed temperatures are below 0C., the pipeline bed comprising:
a pipeline ridge extending parallel to the sea bed trench on each side of the trench, the ridge providing an artificial pingo comprising earth impregnated with, and made cohesive ' by, ice frozen, in situ, in the earth.
By another aspect of this invention, an undersea pipeline structure which has been protected against scour comprising: a pipeline bed on a sea bed where sea bed temperatures are below 0C., the sea bed including an excavated pipeline sea bed trench to provide a bed for the pipeline and an artificial pingo extending parallel to the sea bed trench on each side of the trench~ the artificial pingo comprising earth impregnated with, and made cohesive by, ice frozen~
in situ, in the earth; and a pipeline laid in the pipe bed.

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By another aspect of thls lnventlon, a method ls provided for provldlng protectlon agalnst Jcour for a permanent structure at or near a sea floor where sea bed temperatures are below 0C., whlch method comprises: forming a cavlty below the level of the sea bed, wherein the permanent structure ls adapted to be placed; formlng mounds of earth around such cavity; impregnatlng such mounds wlth fresh water; and freezing the fresh water, whereby the mound comprises earth impregnated with, and made cohesive by, ice frozen~ ln sltu~

in the earth.
By yet another aspect of this invention, a method is provided for protecting a pipeline structure on a sea floor from scour, where the sea bed temeperatures are below 0C., which method comprises: digging a pipeline bed trench; mounding up excavated earth along both sides of the trench along the length of the trench; impregnating the mounds of earth with fresh water; freezing the fresh water to provide mounds of earth , impregnated with, and made cohesive by, ice frozen, in situ, in the earth, ant laying the pipeline in the pipeline bed.
By the present invention, method and means have been ~-provided to protect against sea floor scour. The system is passive and once incorporated, will last practically indefinitely.
The system is relevant to Arctic and Antarctic waters where the sea bed temperature is below 0C., for example, the - Beaufort Sea.
It will be seen that the inventive concept resides in the provision of artificial pingos. Pingos are ancient subsea lakes that are a relic of the ice age. They are mounds on the sea floor that sometime rise up within 20 to 30 feet of the water surface. They survive scouring ice keels as they are fresh water phenomena which remain in the frozen state as the sea bed temperature is below 0C. The resulting frozen fresh ~` .

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water ice and sant is much stronger than lmplnglng ice keel~.
Thus, the artlflcial pingoS provide the same such protection.
In the accompanying drawing, the single figure represents a schematic cross-sectional vlew of the sea bed modified by an aspect of this invention.
Turning to the Figure, it is seen that the sea 10 includes a surface 11 and a sea floor or bed 12. The salty sea ,:.
water is at a temperature of -1C. The sea bed 12 would be prepared for installation of the pipeline 14 or other structure bg excavating the necessary trench 15. The excavated sand around the excavation would then be mounded up as ridges 16 ;
and would be impregnated with fresh water and frozen by refrigeration units or left to freeze naturally. ~nce the articial pingos .. .

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are frozen, the pipellne 14 would then be put into place. Below zero sea bottom temperatures would maintain the fresh water zone in the fro en state and would provide the pingos as ridges of earth impregnated with, and made cohesive by, ice.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pipeline bed for a pipeline structure which has been protected against scour on a sea bed in a sea bed trench where sea bed temperatures are below 0°C., said pipeline bed comprising: a pipeline ridge extending parallel to said sea bed trench on each side of said trench, said ridge providing an artificial pingo comprising earth impregnated with, and made cohesive by, ice frozen, in situ, in the earth.

2. An undersea pipeline structure which has been protected against scour comprising: a pipeline bed on a sea bed where sea bed temperatures are below 0°C., said sea bed including an excavated pipeline sea bed trench to provide a bed for the pipeline and an artificial pingo extending parallel to said sea bed trench on each side of said trench, said artificial pingo comprising earth impregnated with, and made cohesive by, ice frozen, in situ, in the earth; and a pipeline laid in said pipeline bed.

3. A method of providing protection against scour for a permanent structure at or near a sea floor where sea bed temperatures are below 0°C., which method comprises: forming a cavity below the level of the sea bed, wherein said permanent structure is adapted to be placed; forming mounds of earth around said cavity; impregnating said mounds with fresh water; and freezing said fresh water, whereby said mound comprises earth impregnated with, and made cohesive by, ice frozen, in situ, in the earth.

4. The method for protecting a pipeline structure on a sea floor from scour, where the sea bed temperatures are below 0°C., which method comprises: digging a pipeline bed trench;
mounding up excavated earth along both sides of the trench along the length of the trench; impregnating said mounds of earth with fresh water; freezing said fresh water to provide mounds of earth impregnated with, and made cohesive by, ice frozen, in situ, in the earth; and laying the pipeline in said pipeline bed.