Golder Associates’ Ken Been explains the best approach to geotechnical engineering and pipeline construction when facing scours and pits in the Arctic ice.
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Fig. 1: Ice scouring. |
Over the coming decades, the Arctic will become an immensely important region for petroleum extraction. As the region opens up for exploration and production, more and more engineers will face the particular challenges of the High North. To succeed in the region, companies will need detailed specialist knowledge of the Arctic environment and how this differs from the rest of the world.
One of the main differences between operations in the Arctic and elsewhere is the different types of geohazards a frozen climate creates. This means that engineers in the Arctic have to deal with issues such as ice scours, ice pits and strudel scours to manage their surroundings and run efficient and safe operations. Scours and pits are furrows or indentations on the seabed created by the ice, and of all the geohazards in the Arctic, they are the most challenging. Success north of the Arctic Circle could hinge on methodical insights into the effects of the ice on the seabed.
Avoid it, use it or fight it
We have three simple ground rules for dealing with ice. These are, in order of priority, (i) Avoid it; (ii) Use it; (iii) Fight it. This may sound like an oversimplified approach to a whole range of complicated problems; but it is in fact a way of framing the solutions so that they are clearly defined and ordered. From this starting point, we can more readily anticipate the various challenges facing us in the Arctic.
The easiest way of avoiding ice is simply to drill south of the ice edge or out of the ice season. This, however, will not always be possible – and, besides, the Arctic is by nature changeable, so an ice free drilling site one year will not necessarily remain that way the next. When operations have to be undertaken in areas with ice, or risk of ice, excavated drill centers and burying pipelines below the deepest ice scours will be the methods to avoid the ice.
As for using ice, if it’s robust enough, it can be used as a building material or foundation for roads, landing sites or to support drilling platforms. This can be a very beneficial addition when building infrastructure. But when the ice can’t be used to your advantage, the remaining alternative is fighting through it with ice-breaking vessels or platforms that can resist the force of the surrounding ice.
Ice scours and ice pits
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Fig. 2: Strudel scour. |
Pipelines, however, can’t readily resist the forces of the ice, as they are vulnerable to any external impact. Even if today’s pipelines are designed to have some plasticity, they can only handle a limited amount of strain. They will therefore have to be buried deep enough that they are shielded from any contact with ice. Because deeper pipelines mean significant added costs, however, research into optimal pipeline depths is a key priority for many operators.
The main danger for pipelines is ice scours, which are grooves in the seabed created by masses of ice that drift across shallow water (Figure 1). The scours are roughly linear and often not deeper than about a meter, but scours up to four meters deep have been mapped in soft clay soils. Pipelines have to be placed below the extreme scour levels, and the difficult and important challenge is to determine the optimal burial depth. The pipes must not only avoid direct impact from ice keels, but also impact from soil displacement in the ground below the scours.
In addition to scouring from moving ice, large stationary masses of ice – stamukhas – form pits on the seabed. Such pits are usually considered as separate from scours – but, in practice, pits will often appear at the end, or beginning, of a scour. It can therefore be useful to consider the two as a continuum of ice loadings, only differentiated by the degrees of movement and other variables.
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Marine seismic data gathering in Arctic waters. Images from Golder Associates. |
Optimal pipeline depth
In calculating the optimal depth for pipelines, the main headache for engineers is not the scours and pits already on the seabed. Instead, it’s the scours and pits that may occur after a pipeline is in place. The potential for future ice impact necessitates a probabilistic analysis of the ice conditions and the scouring and pit data in order to determine the extreme events that define the design depth and likelihood of soil displacement above and around pipelines. Additionally, the various soil conditions will have to be mapped and analyzed to determine their propensity for soil displacement under the action of ice loads.
It’s furthermore important to emphasize that designs need to consider extreme or rare ice scours and pits, and so even decade long studies of ice movements and soil displacements may not register all the relevant variables. Nonetheless, not planning for these exceptional events can be a recipe for disaster in the fragile Arctic environment.
In the Kashagan field in the northern Caspian Sea – where the water is ice-covered for about three months of the year – the team at Golder worked with the operators over ten years to survey, test and develop advanced numerical models. Based on this, we mapped a range of various load cases that had to be considered in the design of the Kashagan pipelines (see table overleaf). It’s such considerations that are essential for determining the optimal pipeline depth – the depth that does not expose pipelines to unnecessary risk and at the same time isn’t overly conservative.
Strudel scours
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Sampling with a boxcore in the Canadian Beaufort Sea. |
A different type of scouring that can be a particularly difficult Arctic geohazard is strudel scouring. Strudel scours occur when flood water from melting rivers during spring makes its way through the ice and produces strudel shaped depressions in the seabed (Figure 2). These scours can be deeper than ice scours and more difficult to predict, but are only found in in the vicinity of river deltas.
Since pipelines heat up their immediate surroundings, they may even attract strudels if the pipes are routed through a strudel risk area. The way to avoid this is to re-route pipelines away from locations where strudels are found to occur, or to dig the pipes even deeper into the soil around river deltas.
Although extreme depth strudels are a rare occurrence, as with ice scours operators must nonetheless plan their pipelines for the exceptional cases. To analyze the probability of various strudel scours, seabed surveys to map strudels could be conducted over several years to investigate their frequency over time. On the basis of this, it’s possible to calculate the statistical probabilities for strudels along various potential pipeline routes. Such an analysis can be vital for determining the optimal pipeline depth.
Looking north to the future
Ice scours, ice pits and strudel scours are not the only geohazards facing geotechnical engineers in the Arctic. But for designing and placing pipelines they are pressing concerns. Another issue that can affect northern pipeline construction is subsea permafrost. The essential task for avoiding permafrost affecting pipelines is to ensure that the soil surrounding the pipes remains in its current frozen, or unfrozen, state. But the engineering challenges here, although in no way negligible, are normally less challenging than those for scours pits and strudels.
Unique and demanding geohazards lie ahead of engineers in the Arctic. This remote and harsh-climate region is our time’s most exciting frontier for petroleum exploration. In the coming decades we will see activity in High North increase considerably and geotechnical engineers should prepare accordingly. Discovering the optimal route and land access point for a pipeline is an indispensable risk mitigation effort in Arctic environments. It can mean the difference between one Arctic oil spill and zero Arctic oil spills – which is all the difference in the world.
Ken Been is a principal with Golder Associates. He holds a PhD from the University of Oxford. He played a leading role in supporting offshore exploration in the Beaufort Sea in the 1980s and has been involved in designing offshore pipelines in ice environments since 1989.
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