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I should like to compliment the Authors on covering the various aspects of a topic which poses an increasing number of problems. It strikes me as a pity that, in this case, the most economic solution proved to be final sludge disposal at sea, rather than further use on the land, unless the value as a fertilizer is not very great.65. This project provides a good illustration of the desirability of carrying out experimental work before embarking on a complex design. I should like to give a few more details concerning the operation of the viscometer developed by the British Hydromechanics Research Association (BHRA) specially for this job. MC A. Linford was responsible for the viscometer design. The most notable feature was that the three tubes were arranged horizontally, rather than vertically, as for most normal slurry viscometers. This resulted from consideration of the space available on site, as the relatively large diameter tubes necessary for use on sludge meant additional length for a given pressure drop. The three tube sizes were intended to cover the range of expected viscosities. It was not expected that settlement of solids would occur with this type of sludge, even in laminar flow, in a horizontal tube.66. Proving of this device was carried out at the BHRA on a chalk slurry with a similar range of viscosity as the actual sludge. Operation is relatively simple, and viscosity may be calculated from measurement of shear stress (PDI4L) and shear rate (8 V / D ) for laminar flow.67. Departure from ideal Bingham plastic behaviour at low shear rates may be explained by the central core or plug of fluid starting to move before that adjacent to the pipe wall.68. The Paper mentions marked differences in viscosities of sludges having similar solids concentrations. Was there any marked difference in the particle size distribution? Did the reported gas content of the sludge have any adverse effect on the accuracy of pressure measurements in the full-scale trial? 69. I was interested to note the reasons given for the choice of positive displacement primary pumps rather than centrifugal pumps. With the general trend to larger single pumping units, would the Authors envisage centrifugal pumps being considered for the primary pumping duty if the abrasive wear and screening problems could be overcome? Presumably these problems also exist to some extent with positive displacement pumps. Abrasive wear in both pumps and pipelines is currently being investigated at the BHRA. M r W. T. Greer, Depute City Engineer, GlasgowThe disposal of sewage sludge can be a considerable undertaking and the Paper deals with an alternative solution to that put forward by Dainty et a1.l 71. Without doubt, the disposal of sewage sludge to sea is a very attractive proposition to those authorities able to obtain access to shipping facilities. Sludge dumping has been going on for a long time and within my experience no cause for alarm has been substantiated.

The two submarine pipelines described by the Authors in 5 75 represent a total capital investment of about L14 000 000 and they appear to be the largest examples of this class of work yet undertaken in British waters.95. Increasing use is now being made of submarine pipeline techniques in other fields and it would be useful if the Authors could give some more information about the design of the natural gas pipelines in relation to sea bed conditions. In 5 78 trenching depths of from 4 to 10 ft, according to location, were mentioned and it would be interesting to know what minimum depths of cover were specified. From 5 81 it appears that the pipelines may remain exposed for several months before becoming buried by natural sand movements. Were the pipelines in operation during this period and was the risk of damage by ships' anchors considered to be negligible?96. Finally, it appears from 5 77 that the thickness of the weight coat has been increased where the pipelines are within territorial waters. The limit of territorial waters is a notional line, not connected directly with either depth or sea conditions, and it would be helpful if the Authors would explain the reasons for the adoption of a more expensive form of construction within this line. M r A. M. Muir Wood, Sir William Halcrow and PartnersAn unsuspecting reader of $5 45-49 might suppose that a fully sufficient theory was available for the calculation of wave force against a drilling platform. I suggest to the Authors that the following factors are among those that require considerably more study before there can be any complacency among engineers responsible for designing such structures.98. While the higher order wave theories (or Cnoidal wave theory in shallow water) may appear better to represent the particle motions of steep regular symmetrical waves, they take no account of important modifying effects such as eddy viscosity. Moreover, in many marine applications, the irregular, short-crested and asymmetric wave of the open sea is found to have significantly different effects on structures than the long regular wave of the wave theories.99. Considerable doubt is attached to the evaluation of the inertia and drag coefficients and it is highly improbable, from fundamental considerations, that these are each constant through a wave cycle as frequently assumed for marine structures.100. Knowledge of the motions of the incipiently breaking wave relies more on experiment than on theory; this is often the most critical type of wave for an offshore platform but is such a rare event at the situation of a platform that few full-scale recdrds are available.101. Where, on account of steepness of the wave or the slope of the structural member, the motion of the water is inclined obliquely to the axis of the member, the inertial and drag coefficients will be varied in consequence.102. The vortex street that forms in the wake of a cylinder in a steady current causes a cyclical variation in drag and in lift (i.e. the lateral force perpendicular to the direction of ...