Offshore Oil Structures and Tsunami Hazard

There is an extremely serious risk of damage to offshore oil structures due to extreme variation in the level of the ocean surface during the period of time immediately prior to the propagation of the first tsunami wave. It is well-known, e.g. Harbitz (1991) that sea floor disturbance, whether it be from a seabed fault or from an underwater slide, is accompanied by strong initial vertical water motion. In general, when a fault or an underwater slide takes place, water moves into the area from which seafloor material has been displaced. Thus in the case of the Second Storegga Slide, the initial water motion accompanying the landslide is into the area defined by the landslide scarp slope. In any tsunami these movements of water in the open ocean are accompanied by a draw-down of water at the coast. In general terms, the volume of ocean water moved into and over the area of seabed disturbance is approximately equal to the volume of water displaced seaward in response to lowering of the ocean surface and the draw-down of water at the coast.

Thus, when a tsunami takes place the primary effect is a movement of tremendous volumes of water into the ocean area located above the sea floor disturbance. Ocean water tends to pile-up in an ocean area circa several tens of square km and the level of the ocean surface in this area can increase its elevation dramatically. This accumulation of surplus water over a restricted ocean area is followed by collapse and it is the collapse of this water column that constitutes the primary mechanism by which tsunami waves are propagated out from a point source in the ocean (figure 3).

It should be noted from this discussion that the precise altitude above the ocean surface to which water may rise is dependent upon the rate and dimensions of seabed displacement. Thus, a landslide that is generated on the seabed that moves relatively slowly will produce smaller volumes of surplus water over the area of sea disturbance as compared with an extremely rapid event. In the case of the Second Storegga Slide we do not know how fast the landslide moved. The only way that crude attempts can be made to reconstruct the former landslide velocity is by comparing the geological observations of runup at the coast with those produced by a mathematical model. Harbitz (1991) tuned the landslide model to correspond with geological estimates of runup at the coastline and was able to estimate a likely average landslide velocity of 35 ms-1 but it was always an impossible task to estimate the speed of initial slope failure in the area now defined by the scarp slope. These two parameters, once defined, produce the dimensions of the tsunami waves, the rate of slope failure the first factor that comes in to play when the tsunami is initially generated, but the landslide velocity also being important since it defines the rate at which the landslide is moving at a time when the tsunami waves begin to be propagated outwards from the area of initial ocean disturbance.

It is virtually impossible to estimate the rise in the water level in the area of the ocean that is subject to the initial accumulation of water. We choose a value of +100m to be included in this report, but this value could be significantly higher or lower dependent upon the processes described above. We therefore identify an area of ocean which is at greatest risk from water level changes and it should be noted that water level changes described for this area are likely to be several times higher than the water level changes associated with propagation of individual tsunami waves. The question therefore in terms of safety is to identify the area of ocean most likely to be subject to such extreme water level changes should a future disaster occur. Although this is extremely difficult to accomplish with any degree of precision, it is logical to conclude that the possible areas of extreme water disturbance are likely to occur above any future scarp slope area on the seabed where a slide takes place. Since the greatest accumulations of soft sediments occur on the continental slope west of Norway, we identify this area (as opposed to the continental shelf or the deepest (abyssal) areas of the ocean floor as the area in which oil installations are at greatest risk. Since the upper level of the scarp slope associated with the Storegga Slides occurs at circa 500m water depth, we identify a zone of the Norwegian Sea beyond the continental shelf at a water depth range between circa 500-750m as the area at greatest risk.

It is difficult to imagine what the effects of such extreme water level changes would be at an offshore oil installation. The first effect following the triggering of an underwater slide in the future would be a draw-down of the ocean surface surrounding a central area into which water would move. We envisage that in the first few minutes of such an event the water levels would increase by several tens of metres thus exerting immense strains on the structures as well as causing their temporary submergence and possible collapse.

Once a tsunami has been generated the propagation of the tsunami wave train is most responsive to changes in water depth. For the most part tsunamis are largely invisible or can scarcely be detected in the open ocean due to the exceptionally long wave lengths of the wave train (many tens and sometimes hundreds of km) and long-wave periods (typically 10-20 minutes). In the open ocean the passage of a tsunami wave train may scarcely be noticed from a ship since the long wave length is also associated with an extremely low wave amplitude. However the velocity of the propagated waves is exceptionally high and is calculated as:

Where g = gravitational acceleration and d = water depth (Figure 3)

Thus a tsunami generated at a water depth of 4,000m is associated with a propagation velocity of 450km/hr. Whereas submarine motion (e.g. a fault or a landslide) that takes place in a water depth of 400m will generate tsunami waves with velocities in the order of 63m/sec (Figure 3).

Due to the relative long wave length of tsunami waves, the deformation of the water column begins to take place as soon as the seabed occurs within a water depth that corresponds to a value <½ the wave length. In the case of the North Atlantic most tsunami waves will begin to cause water deformation across the continental shelf. As the waves approach shallower waters the deformation increases and results in the construction of a breaking wave. The breaking wave may be many tens of kilometres in length and as it approaches the coastline it slows down. Within several hundred metres of the coastline the breaking wave may be several metres in height and the height of the wave increases as it travels across shallower water depths. However, the wave is also slowing down as this process takes place so that when a tsunami wave approaches the coastal edge it may only be travelling at velocities in the order of 20-30m/sec. The breaking wave however during this time has increased in its amplitude and the height of the breaking wave will vary largely depending on the nature on the original source of the tsunami.

Therefore, it is to be expected with the exception of the area of sea surface located directly over a submarine sediment failure, the remainder of the ocean area will largely be unaffected by the passage of tsunami waves until the waves reach shallow water. Since most oil installations are located in relatively deep water, it is extremely unlikely that the height of a propagated tsunami wave would cause any damage. This research has not investigated the effects of tsunami on moored systems.