4 CAPACIDAD DE COMERCIALIZACIÓN 9.9%

Observations of the LIOA/F pipeline scour hole revealed that it appeared to be more than 15 metres wide having gentle side slopes of the order 1:15. The alternating tidal current would probably have been able to follow the downward slope reasonably well, without separation occurring even after the pipeline sank lower than the average seabed level. Wide, open scour trenches of this form, leave the pipeline much more exposed to the oncoming flow in both directions. With an unseparated flow through the scour hole, the scouring process would have been able to continue, and it appears likely that maximum scour depths will increase in these circumstances leading in turn to an increase in self-burial depths. Scour holes formed by tunnel erosion, have been observed to be typically of 1:2 gradient on the upstream face and 1:5 gradient on the downstream face, as illustrated in figure 3.6(a). Scour holes formed under lee erosion, that is the wake erosion, have been observed to be of much shallower gradients than those formed under tunnel erosion, with gradients of the order of 1:15, this is shown in figure 3.6(b). Mao(9) has observed the wake induced erosion causing scour at the sediment bed to eight diameters downstream of the pipeline. These findings tend to indicate that lee erosion was responsible for the deep burial of the LIOA/F bundle.

A pipeline experiencing reversing, tidal flow at the ocean floor will be subjected to leeside erosion on both sides. A double scour pit will develop with the pipeline in a central elevated position, resulting in the pipeline being more exposed to the flow. It is believed that the self-burial of the LIOA/F pipeline is the result of the process of alternating leeside erosion and local pipe sagging. Erosion formed by reversing flow is illustrated in figure 3.6(c). Furthermore, the conflicting findings of the field observations in the Dutch sector of the North Sea, and the scour holes achieved under uni-directional flow being to the order of 1 diameter beneath the pipeline, indicates that there

(a) T u nnel Erosion

(b) L e e Erosion

E r o s i o n F o r m e d under Two-dimensional Flow

is some other factor operating in the offshore environment which produces larger scour depths. Laboratory scour tests have been carried out using oscillatory wave induced currents (8),(9),(10). Waves in isolation, tend to produce symmetrical scour holes with a maximum depth somewhat less than the corresponding uni-directional flow case, but now located directly under the pipeline. There is some evidence, however, that very large amplitude, high velocity wave induced motion can produce wide symmetrical scour holes with significantly increased d e p t h s (9).

In wave flows the depth of the scour hole will be affected by changing Keulegan-Carpenter number, KC, where.

u

K C = — ÜEL_ 3 . 2

D

where D = pipe d i a m e t e r , = The maximum value of in the two-directional flow and T' = The period of the wave as discussed by M a o (9).

As the KC number becomes greater, ie as the amplitude of the water particle motion past the pipeline increases, the vortex train downstream of the pipe become more fully developed and extended. An increased number of vortices are shed and hence the scour zone downstream of the pipe, becomes wider and deeper scour holes are allowed to form. M a o (9) experimentally confirmed that for all tests apart from the clear water case, the scour depth under a fixed pipe exposed to a reversing flow field is deeper than that in the uni-directional flow case. These findings confirmed the 'lee erosion' theory for deep self-burial. This was also confirmed by Littlejohns(2) who carried out some prototype field measurements utilising pipelines installed on a flat sand bed in the intertidal zone. Littlejohns stated that;

"The general upstream slope of the bed was fairly steep at the commencement of the tests but gradually developed into a wide gently sloping profile after a few days".

These observations of the potentially increased scour depths have great relevance to the present discussion since reversing tidal flow can be viewed as a wave induced oscillatory flow of extremely large amplitude. Potential flow can be used more formally to support these arguments. It can be shown, for example (59) , that when a uniform, inviscid flow, with velocity U, passes over a solid surface and encounters a semi-circular shaped trench in the boundary, the velocity at the bottom of the trench is reduced to 2U/9. If we generalise the trench shape to a semi-ellipse and increase the major to minor axis length ratio thus increasing the trench width, then once again potential flow theory indicates that the reduced velocity at the bottom of the trench progressively increases until it, sensibly, reaches the undisturbed magnitude, U, for infinite trench width.

The presence of the scour hole on the seabed thus reduces the local sediment transport capacity of the current whilst the cylinder enhances the bed gap velocities and hence increases the transport capacity. It is the balance between these two effects which eventually produces an equilibrium condition and hence a finite limit on the maximum depth of scour. The preceding arguments suggest that the wider the trench, the greater will be the equilibrium scour depth. Elaborate potential flow modelling of the combined situation of a pipeline positioned at various heights above a scour trench of variable width and depth confirm these qualitative predictions(9). Lee or wake scour provides a natural mechanism in reversing flow for generating very wide scour holes. The preceding arguments are thus potentially of central relevance and importance in explaining the deep self-burial of certain operational pipelines offshore. There clearly exists a lack of experimental results concerning the effect of reversing flows on a developing scour hole. Whilst Mao's tests were very important, the bulk of his tests were restricted to rapidly reversing, wave type motion. Mao also states that the height of the sand waves was of the same order as the pipe diameter; this makes any quantitative assessments very difficult. There is obviously much scope for further investigations into the

effect of tidal type reversing flows. 3.7 The Effect of Sagging

The sagging of a pipeline in uni-directional flow, can have two different effects on the formation of a scour hole. A scour hole, still in its early stages of development, will sustain an increase in scour rate under pipe sagging action. This enhanced erosion process has been confirmed in the simulated sagging tests carried out in a preceding s t u d y (5) and subsequently by other investigators(9). This is because as the pipe sags the gap between the pipe and the scour hole is decreased, causing a velocity enhancement and an enhancement of the sediment transport rate q^ and thus an increase in rate of scour. This is illustrated in figure 3.7(a) and (b). At a later stage, however, the pipe will have sagged sufficiently into its own scour hole to become less of an obstruction to the flow, reducing the velocity, q^ will then decrease until eventually the condition exists, and the scour hole will fill as shown in figure 3.7(c) and (d). The equilibrium scour depth will be increased by the sagging effect of the pipe in a uni-directional flow test, as the scour rate is enhanced by the decrease in gap ratio as discussed above. M a o (9) also investigated the reversing flow case under

simulated tidal type flow, and the effect of sagging. He found that sagging in this case decreased the scour depth, the opposite to that for the uni-directional flow case.

He postulated that this is due to the fact that in reversing flow there is always a deposition mound upstream of the pipe and this plays a key role in the sagging procedure of the pipeline exposed to reversing flow. Because the degree of protrusion of the pipe into the flow is less in the sagged case than the fixed case, more flow passes above the pipe. Therefore the scour depth is shallower. These mounds are very important in the self-burial process under reversing flows. If these mounds are removed by higher sediment transport rates, the sagging process will enhance the sediment transport rates

o\ o\ O a ) a n d b ) E a r l y i n t h e s c o u r p r o c è s a ) a n d b ) E a r l y i n t h e s c o u r p r o c e s s t h e s a g g i n g e n h a n c e s t h e v e l o c i t y , c ) a n d d ) I n t h e l a t e r s t a g e c ) a n d d ) I n t h e l a t e r s t a g e s s a g g i n g r e d u c e s t h e v e l o c i t y ,

sagging and scour rate enhancement to occur and hence deep self-burial.

The rate of sagging of a pipeline is dependent on the length of free span of the pipe, the weight of the pipe and also the pipe's structural properties. Whilst the sag rate of the pipe is vitally important to the self-burial of a pipeline, little information is available as to the rates which may be experienced in the field or indeed the mechanism of scour at the span ends. Section 3.3 below discusses the effect of different controlling parameters.