Discussion. The Production of North Sea Gas.

Abstract

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 ...