Artikel 26 Pipa Bawah Laut
Global Buckling and Upheaval Buckling
Global Buckling
Global buckling of a pipeline can be compared to a bar in compression or buckling of axially constrained railroad tracks at high temperatures. Before production starts trough a pipeline, the internal temperature is about the same as the ambient seawater temperature. When the pipeline is put into service the temperature and pressure in the pipe will increase. As a result of this, the pipe will expand. A constrained pipeline will not allow the expansion to occur which will result in axial compressive forces in the pipe wall. The pipeline will try to relieve the stresses by buckling. A buried pipeline will have sufficient resistance sideways provided by the soil. The pipe will buckle in the direction of least resistance which then will be upwards. For trenched pipelines the buckle will follow the side wall, while buried pipelines will buckle vertically. The red arrows on the cross section in figure 1, indicates the axial forces in the pipe caused by temperature and pressure expansion.
Figure 1: Buried pipe in compression
Typical candidates for global buckling are High Pressure and High Temperature (HP/HT) pipelines. Light pipelines with thin wall thicknesses may still be exposed to buckling at moderate temperature and pressure. There are several failure modes for a pipe exposed to global buckling. Global buckling is a load response and not a failure mode alone, but global buckling may lead to failures such as fracture, fatigue, local buckling, bending moments and large plastic deformations. For pipelines lying exposed on the seabed, global buckling may be allowed as long as it is displacement controlled. This means that the pipeline integrity must be maintained in post buckling configurations, and that the displacement of the pipelines is within acceptable limits. It shall not be able to interfere with surrounding structures or other pipelines. For buried pipelines, global buckling in the vertical plane shall be avoided. If a buried pipeline is exposed to upheaval buckling and the pipeline breaks through the cover, there are additional failure modes for such a scenario. The protection provided from the cover is lost, and if the curvature of the buckle leaves a gap between the pipe and seabed, a free span is formed. The pipeline may then be vulnerable to fatigue due to vortex induced vibrations at this region. If a buckle leads a pipeline into exposure on the seabed, the simplest solution would be to stabilize the pipeline at its new position. This can be done by covering the exposed part, for example by rock dumping. However if the integrity of the pipeline is reduced and the pipe wall is overstressed, this may lead to rupture. Then the damaged part will have to be replaced, before stabilizing it again
Failures caused by pipe being exposed are;
- interference with fishing gear
- damage due to dropped objects
- damage due to anchoring
- temperature drop leading to hydrate formation in the pipe
- instability due to currents and buoyancy
- vortex induced vibrations
- fatigue
Global buckling is a response to compressive force generated by high temperature and high pressure (HP/HT), which will generally reduce the axial capacity of the pipeline. Pipelines exposed to high temperature and high pressure or pipeline with a low buckling capacity will be governed by global buckling. In DNV’s RP-F110, three global buckling scenarios resulted from HT/HP are introduced:
- Exposed pipelines on even seabed. Global buckling occurs in the horizontal plane, post buckling configuration may be allowed.
- Exposed pipelines in uneven seabed. Global buckling occurs first in the vertical plane (cause feed-in and uplift) and subsequently in the horizontal plane, or combined scenarios with scenario I, post buckling configuration may be allowed.
- Buried/covered pipelines, global buckling in the vertical plane, so called upheaval buckling.
Global buckling is a load response, not a failure mode. However, global buckling will imply some
failure modes such as:
failure modes such as:
- Local buckling, for pipeline subjected to combined pressure. Longitudinal force and bending, local buckling may occur. The failure mode may be yielding of the cross section or buckling on the compressive side of the pipe.
- Fracture, which is caused by tensile strain, generally includes brittle fracture and plastic collapse.
- Fatigue, pipeline components such as riser, unsupported free spans, welding should be assessed for fatigue. Potential cyclic loading fatigue damages, which may include vortexinduced-vibrations (VIV), wave induced hydrodynamic loads, cyclic pressure and thermal expansion loads.
- Ratcheting, ratcheting generally describes the accumulated plastic deformations under cyclic loads in pipelines that exposed to high temperature and high pressure.
- Bursting, it is governed by tensile hoop stress, which may occur in the tensile part of pipeline.
Upheaval Buckling
The source of upheaval buckling is an interaction between the axial compressive force and overbend "hill" imperfections in the pipeline profile.
Figure 1 illustrates a sequence of events which initiates buckling in a buried pipeline. The pipeline is laid across an uneven seabed (Figure la), and later trenched and buried (lb). The trenching and burial operations modify the profile of the foundation on which the pipe is resting, so that it is not precisely the same as the original profile. Trenching may smooth the profile overbends, but may also introduce additional imperfections, if, for instance, a lump of bottom soil falls under the pipe. When the pipe goes into operation, its internal pressure and temperature are higher than when it was installed and trenched, and the axial force becomes compressive. The effective axial force in a constrained pipeline has two components, both of which contribute towards buckling (Palmer 1). The axial force in the wall is the resultant of a compressive constrained thermal expansion component and a tensile Poisson component, while in addition there is a compressive force component in the contained fluid.
On an overbend, the axial compressive force reduces the upward reaction between the foundation and the pipeline (Figure lc).
Figure 1 illustrates a sequence of events which initiates buckling in a buried pipeline. The pipeline is laid across an uneven seabed (Figure la), and later trenched and buried (lb). The trenching and burial operations modify the profile of the foundation on which the pipe is resting, so that it is not precisely the same as the original profile. Trenching may smooth the profile overbends, but may also introduce additional imperfections, if, for instance, a lump of bottom soil falls under the pipe. When the pipe goes into operation, its internal pressure and temperature are higher than when it was installed and trenched, and the axial force becomes compressive. The effective axial force in a constrained pipeline has two components, both of which contribute towards buckling (Palmer 1). The axial force in the wall is the resultant of a compressive constrained thermal expansion component and a tensile Poisson component, while in addition there is a compressive force component in the contained fluid.
On an overbend, the axial compressive force reduces the upward reaction between the foundation and the pipeline (Figure lc).
A further increase of operating temperature and pressure may reduce the reaction to zero. The pipe then lifts on the overbend, moves towards the surface of the cover, and may break out through the surface.
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