Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Patent
1998-11-04
2000-09-19
Bagnell, David
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
405166, 4051683, 4051684, F16L 114, F16L 302, F16L 318
Patent
active
061202126
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to improvements in or relating to marine pipelaying methods and apparatus. The invention is particularly concerned with controlling the ovality of rigid pipeline during pipelaying operations in which the pipeline is plastically deformed during bending of the pipeline around an arcuate path and is subsequently straightened prior to laying.
The invention will be discussed herein with particular reference to rigid steel pipe, but is also applicable to rigid pipe formed from other materials.
Rigid steel pipe is manufactured to a nominal circular diameter. However, in practice the pipe will not be perfectly circular along its entire length, but will exhibit variations in ovality, within defined tolerances. Subsequent processing of the pipe, such as by bending, will cause further variations in ovality. In the context of marine pipelaying, ovality affects the ability of the pipe to resist hydrostatic pressure, particularly at extreme water depths, and it is important that the ovality of the pipe as finally laid does not exceed predetermined limits. Ovalisation of the pipe may become particularly significant where the pipe is being laid in relatively great water depths requiring unusually high tension to be applied to the pipeline, thereby increasing the forces exerted between the pipeline and an underlying pipe bearing surface, prior to the launch of the pipeline from the vessel.
Ovality may be defined as: diameter of the pipe. In a given length of pipe, the angle formed between the maximum diameter (or "major axis") and a reference plane extending through the longitudinal axis of the pipe may vary along the length of the pipe. Typically, the maximum diameter may rotate along the length of the pipe so that the ovality spirals along the pipe.
In the present discussion the following conventions will be employed: will be referred to as a positive ovality; in this case, the diameter along the reference plane is greater than the nominal circular diameter; ovality will be referred to as a negative ovality; in this case the diameter along the reference plane will be less than the nominal circular diameter; plane will be the plane of curvature of the pipe.
It can readily be seen that where the pipe exhibits positive ovality prior to bending of the pipe, the ovality of the pipe will be reduced by such bending, since the process of bending will tend to increase the diameter at right angles to the plane of bending and to reduce the diameter in the plane of bending. Conversely, where the pipe exhibits negative ovality prior to bending, the ovality will be increased by bending.
Where the pipe is bent elastically, it can be expected to return to its original ovality when the bending forces are removed. However, where the pipe is plastically deformed during bending and is subsequently straightened, the pipe will not fully recover its original ovality and there will be a net residual change in its final ovality as compared with its ovality prior to bending. Where the original ovality is positive, the net residual change will result in a reduced positive ovality. Where the original ovality is negative, the net residual change will result in an increased negative ovality. In the latter case it can be seen that there may be cases where a length of pipe which is within predetermined ovality tolerances prior to bending might exceed such tolerances after bending and straightening owing to the net increase in negative ovality. In the former case the net decrease in positive ovality will generally be desirable.
It will be understood that, where the pipe is bent against a supporting surface, there will also be a degree of flattening of the pipe. Herein, such flattening is considered to be a component of the overall ovalisation.
The present invention is primarily concerned with controlling pipeline ovalisation in marine pipelaying operations where the pipe is subject to plastic deformation during bending and subsequent straightening in the course of the laying operation. Such plas
REFERENCES:
patent: 3775987 (1973-12-01), Rochelle et al.
patent: 3950955 (1976-04-01), Meeres
patent: 4531391 (1985-07-01), Engman
patent: 5044825 (1991-09-01), Kaldenbach
patent: 5094340 (1992-03-01), Avakov
patent: 5893682 (1999-04-01), Oliveri
Bagnell David
Coflexip Stena Offshore Ltd.
Mayo Tara L.
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