S13 ORME 7 2016 Penspen_Layout 1 26/10/2016 11:10 Page 60
significantly. A longer movement zone, and two bend apexes, causes the strain concentration to be shared between two points, as well as the greater area of soil resisting lateral movement of the pipe. This reduces the maximum displacement. Greater lateral restraint develops for longer bends because the soil resists movement over a larger area, providing a higher reaction force and restricting movements. The single peak (Figure 2) causes closure of the bend, leading to high local bending and compressive axial strains in excess of the local buckling limit. Separation of the bend is a useful method to reduce the strains and displacements to within acceptable levels, but not always suitable if space in the pipeline right of way is limited. Other methods for preventing high strains occurring at bends include: • Increasing the axial friction using special coating; • Higher wall thickness in the bend area; • Increasing the installation temperature; • Increasing the lateral restraint of the soil around the bend (usually requires importing of materials); • Increasing the bend radius - although this usually leads to a requirement for more pipe lengths in the bend, for geometrical reasons.
Expansion to an above ground installation The knowledge of movement of buried horizontal bends can be used beneficially in design for locations where it is desirable to limit the amount of expansion. If the bend is allowed to move, while remaining within acceptable strain limits, then it will prevent axial movement towards nearby features. This can be used at above ground
installations (AGIs). It is customary to use a buried horizontal bend, usually of 90º on the approach to an AGI. This limits expansion displacements at the AGI and reduces the development of stresses on the pipework, shown for example in Figure 3. The buried horizontal bend is often placed adjacent to the transition under-bend, however, it is possible to find an optimum distance (X, Figure 4) between the horizontal bend and the under-bend to minimise the expansion seen at the AGI.
There are two methods of calculating the optimum distance of the buried bend. The first is by establishing the point on the pipe section between the AGI and the buried bend where the onset of axial displacement towards the bend starts to develop. The second method is by finding the point at which the axial force profile starts to develop a separate peak at the side bend. It has been found that finding the optimum distance for the location of the buried bend can reduce the axial displacement at the AGI by up to 50 per cent, leading to potential cost savings on the AGI design.
A single 90° bend, up to 100m from the AGI may not be possible due to routing constraints. However, back-to-back 90° bends can be used in a similar way to reduce the axial expansion at the AGI.
Ratcheting Another consideration on the design of horizontal bends under high thermal loads is ratcheting behaviour of the bend. Ratcheting is the progressive movement of the bend through the soil during cyclic loading. When a pipeline experiences cyclic operational loads involving temperature change, a check for ratcheting must be carried out. The development of strain and additional movement can also be assessed using finite element analysis but it is difficult to predict how the soil and pipe will interact during cyclic loading of the pipeline. A good estimate can be made based on the lateral movement of offshore pipelines, although no information has been identified on the behavior of soils under large displacement cyclic loading of fully buried pipelines. The soil is heavily disturbed behind the displacing pipe during the load cycle and this makes it difficult to predict its behaviour during the subsequent unload cycle. An investigation into the displacement of the bend following the initial load and unload cycle indicates that there is residual displacement of the bend following the unload phase which modifies the subsequent displacement in the next load sequence. The lateral displacement of the bend may continue to increase with continued load cycling. Strain ratcheting can occur due to the progressive movement of the bend, which can possibly develop beyond the performance limit after numerous operational cycles.
Conclusions Strain-based design can be applied to onshore high temperature pipelines, allowing plastic deformation of the pipe material. This can lead to considerable financial savings for the pipeline construction compared to traditional stress-based approaches. High thermal loading leads to expansion at points of lower axial restraint such as bends. During strain-based design it must be ensured that the movement and strain developed at these bends do not exceed defined performance limits, and that significant movement due to ratcheting does not occur during cyclic operation. The lateral movement of buried horizontal bends can be used to reduce axial displacement towards above ground installations or other pipeline features. Finding the optimum position of the bend can reduce the displacement loading by up to 50 per cent. n
Issue 7 2016