A detailed pipeline design was completed for BP Exploration's Northstar oilfield development, located six miles offshore in the Alaskan Beaufort Sea (Lanan et al, 1999). The design was undertaken by Intec Engineering.
Unique aspects of this design include the pipeline environmental loading and the effective use of limit states design for extreme loading conditions. The unique Arctic environmental loading conditions, including ice gouging, offshore permafrost, upheaval buckling, and strudel scour.
Ice gouging
Ice gouging of the seabed is a feature of most of the coastal regions of northern continents. Sea ice, which is driven onto itself, will tend to pile up, creating a pressure ridge. This pressure ridge has a keel extending below the water surface. The ridge is driven primarily by ocean currents, and secondarily by wind and loading from other ice. Occasionally, these ice keels intrude into waters with depths less than their draft. Gouging then occurs when the ice moves while the keel is in contact with the seabed.
The gouge may take the form of a long linear furrow following a relatively straight line or it may be erratic in its path, covering a few tens of feet or many tens of miles. These gouges may be only a slight indentation on the seabed or they may be several feet deep. In northern oceans, such as the Beaufort Sea and the Barents Sea, most of the ice scours are caused by pressure ridges. However, they also can be caused by multi-year ice, ice islands, or icebergs in these regions.
As an ice keel passes over any point in the seabed, vertical and lateral stresses are applied to the soil at the keel base, resulting in some distribution of vertical and lateral soil displacements with depth beneath the ice keel depth.
The soil deformation at pipeline depth is a function of the soil type and the ice gouge depth and width (Woodworth-Lynas et al, 1996; Nixon et al, 1996). The effect of this soil displacement and resulting loading on the pipeline must be accounted for in the design of pipelines where these conditions exist.
This is typically done by non-linear finite element analysis with plastic steel behavior and large displacement capabilities. The soil-pipe interaction is simulated by non-linear spring elements. The bending is applied in a displacement-controlled manner by imposing the soil displacement field at the pipeline depth to spring nodes.
In turn, the springs pull (or push) the pipeline elements and the resulting pipeline strains are calculated. Analyses indicate that for a pipeline buried twice the gouge depth below the seabed, bending strains due to the ice keel could be on the order of about 1%.
Offshore permafrost
Any offshore pipeline transporting oil or gas to shore will need to transition from offshore through a shore crossing. In most Arctic applications, the offshore section of the pipeline would be installed in a trench, which would then be backfilled. In shallow water depths and at the shore crossing, the soil underlying the trench could contain ice-bonded permafrost. When the pipeline becomes operational, if it operates at a temperature higher than ambient, the temperature of the pipeline will gradually increase the temperature of the surrounding soil. The volume of soil that is affected by the pipeline's temperature, otherwise known as the thaw bulb, will increase over the operational life of the pipeline. This increase in temperature will change the load carrying properties of the ice-bonded permafrost.
Initially, the loads in the soil are carried by a combination of the soil skeleton material strength and frozen water in the pores. As the temperature of the soil increases, the ice in the soil pores melts and the soil material must carry the majority of the load that was previously shared by the two components. This results in settlement of the soil. In areas, the pipeline may be no longer supported vertically and may be, in effect, supporting the soil cover above and keeping it from settling.
If this area of high thaw settlement is adjacent to an area of low thaw settlement, the result is a uniformly loaded, unsupported pipeline span. This differential settlement can induce strain in the pipe and must be accounted for in the design. The analysis is conducted in a similar manner to that described above for ice keel gouging and could also yield bending strains on the order of 1% for typical pipeline burial depths.
Upheaval buckling
When a buried steel pipeline is operated at a temperature higher than that during installation, it will try to expand longitudinally (thermal expansion). A long buried pipeline is not free to expand due to the restraint provided by the surrounding soil, and thus it will develop an axial compressive force. If the buried pipeline has some residual vertical curvature, possibly due to trench bottom irregularities during installation, the effect of the axial force near the high points of the pipeline will attempt to move the pipe upward at these locations.
This results in an upward force from the pipe into the soil cover. If the upward force exceeds the downward force due to the combination of the resistance of the soil cover, the pipeline stiffness, and the pipeline self-weight, then the pipeline will move upward and may be exposed out of the trench. This phenomenon is known as upheaval buckling.
The immediate effect is that the pipeline becomes exposed at the mudline level, which increases the risk of impact by ice keels, anchors, or fishing gear. Although significant plastic deformation occurs, unstable fracture is not expected to take place due to the high deformation capability of steel pipe. Still, upheaval buckling is a limit state, that is an undesired condition or state, which must be designed against. Palmer et al. (1990) describes the upheaval-buckling phenomenon in detail and present an analytical model to evaluate upheaval-buckling potential.
Strudel scour
Strudel scour craters in the sea floor are formed during river breakup in the spring, when the river water overfloods the bottomfast sea ice in the nearshore coastal zone. The overflood water (typically 2-4 ft deep) spreads offshore and drains through discontinuities in the ice sheet. These discontinuities typically consist of tidal cracks, thermal cracks, and seal breathing holes. In those instances where the drainage rate is high and the water relatively shallow, scouring of the seafloor can occur from the water action on the seabed. The majority of strudel scours are circular in plan form, but linear scours can be created by drainage through lineal cracks in the ice. Strudel scours are only a hazard if certain conditions occur.
In order for the pipeline to experience a loading event, the strudel scour must be located on top of the pipeline alignment, exceed the distance to the bottom of the pipe (otherwise the pipe remains supported by the soil), and have a horizontal dimension at the pipeline depth equal to or exceeding the design span length. Given site specific field data, the probability of each of these conditions can be calculated.
Limit state design
The limit state design concept can be expressed in relatively simple terms. A limit state condition is a non-conformity that interferes with the functional aspects of the pipeline system. A general list of pipeline limit states is as follows: burst, axial tensile failure, excessive deflection, excessive vibration, excessive corrosion, ovalization and local buckling, wrinkling or local buckling (under displacement controlled bending), upheaval buckling, hydrostatic collapse and buckle propagation, unstable crack propagation, and excessive expansion.
Applicable limit states for an aboveground Arctic onshore pipeline are burst, deflection (due to vertical loads), vibrations (due to wind), corrosion, and expansion (due to warm contents). Applicable limit states for offshore buried Arctic pipelines are burst, ovalization (due to displacement controlled bending), and unstable weld flaw propagation due to tensile bending strains. As two of these limits are dependent on pipe behavior during extreme bending, a full-scale research program was conducted on behalf of BP to quantify these effects and validate the design.
It is recognized (Murphy and Langner 1986; DNV 1996) that pipelines with D/t = 20 or less can safely sustain strain levels up to 1% without local buckling or excessive ovalization. An Engineering Critical Assessment (ECA) is usually required for such a strain level. The ECA will specify the allowable flaw size that can be accepted during welding of the pipeline. Allowable flaw sizes are increased by specifying a combination of pipe with a low SMYS (X-52, for example), which typically provides good ductility, a special pipe chemistry, and a slight over-match of the weld electrode material. The latter can produce good CTODs (typically 0.0001 in. or higher).
Experimental testing may be necessary to validate the limit state capacity of the pipeline. For the Northstar development, a dedicated bend test program was performed in order to check the collapse and fracture limit state (Nogueira et al, 1999).
One test involved a pipeline with the maximum allowable flaw (artificially implanted at a welded joint) at the extreme tensile fiber was successfully bent to a bending strain of 3.6%. This test was conducted at a temperature of 30°F and with maximum misalignment during welding. Ultrasonic inspection before and after bending as well as cross sectioning of the flaw by an independent laboratory confirmed that no flaw growth occurred. This validated the very high quality line pipe and welding process to be used Arctic offshore and subjected to bending limit state conditions.
Summary
For Arctic offshore service, the behavior of the line pipe under bending must be fully understood. Load conditions, which are of concern, include ice gouging and differential thaw settlement, both of which may induce bending of the pipeline.
Strain levels up to 1% or more are possible from these load conditions. In order to validate the theoretical limits of bending, a test program was performed which involved bending pipe to strains up to 3.6% under simulated field conditions.
The results showed excellent correlation with the theory of weld flaw propagation and confirmed the pipe and weld to be sufficiently robust for Arctic application.
References
DNV (1996), "Rules for Submarine Pipeline Systems".
Lanan, G., Nogueira, A, Even, T., Ennis, J. (1999). "Pipeline Limit State Design for Bending in support of the BPXA Northstar Project," ICAWT ྟ - Pipeline Welding and Technology, Galveston, Texas, scheduled for October, 1999.
McKeehan, D. (1990), "Construction Strategies for Arctic Offshore Pipelines," Workshop on Ice Scouring and the Design of Offshore Pipelines, Calgary, Alberta.
Murphy, C., Langner, C., (1986). "Ultimate Pipe Strength Under Bending, Collapse, and Fatigue," Proceedings, Offshore Mechanics and Arctic Engineering, Volume 1, ASME, Dallas, Texas.
Nixon, J., Palmer, A., Phillips, R. (1996). "Simulations for Buried Pipeline Deformations Beneath Ice Scour," Proceedings, Offshore Mechanics and Arctic Engineering, Volume 5, ASME, Florence.
Nogueira, A., Lanan, G., Even, T., Fowler, J., and Hormberg, B. (1999). "Experimental Validation of Limit Strain Criteria for the BPXA Northstar Project," ICAWT ྟ - Pipeline Welding and Technology, Galveston, Texas, scheduled for October, 1999.
Palmer, A., Ellinas, C., Richards, D., Guijt, J. (1990), "Design of submarine pipelines against upheaval buckling," Proceedings, Offshore Technology Conference, Houston, Texas.
Timmermans, W. (1982). "Design, Installation, and Operation Described for Beaufort Sea Pipelines," Oil and Gas Journal, May.
Woodworth-Lynas, C., Nixon, J., Phillips, R., Palmer, A. (1996). "Subgouge Deformations and the Security of Arctic Marine Pipelines," Proceedings, 1996 Offshore Technology Conference, Houston, Texas.
