Load measurement

A device for measuring externally applied loads on a structural member, such as a support leg of an off-shore installation, comprises one or more flexible fluid-filled cells constrained between the surface of the structural member and a casing capable of transmitting externally applied loads to the structural member through the fluid within the cell(s), the loads applied to the member being derived by monitoring the fluid pressure within the cell(s). Preferably the cell(s) and casing are comprised in a sleeve-like assembly removably mounted upon the structural member and retained in position by friction.

The present invention relates to the measurement of externally applied 
loads on structural members. Particularly, though not exclusively, the 
invention is concerned with the measurement of loads on structural members 
arising from the exposure of such members to fluid flows. An important 
example of this is the measurement of the hydrodynamic loading of 
submerged support structures for off-shore installations, although the 
invention may also find application in the measurement of loads arising 
from air flows, e.g. in the measurement of the wind loading of buildings 
or in wind-tunnel testing. 
In accordance with a first aspect of the invention a device for measuring 
an externally applied load on a structural member comprises one or more 
flexible fluid-filled cells, means to sense the fluid pressure within the 
or each cell or the relative fluid pressures within different cells, and a 
casing for constraining the or each cell between itself and the surface of 
the structural member the load on which is to be measured, said casing 
being capable of transmitting an externally applied load to said surface 
through the fluid within the or each cell. 
In a second aspect, the invention resides in a method of measuring an 
externally applied load on a structural member which comprises the steps 
of: constraining one or more flexible fluid-filled cells between a casing 
and the surface of the structural member the load on which is to be 
measured, said casing transmitting an externally applied load to said 
surface through the fluid within the or each said cell; and sensing the 
fluid pressure within the or each said cell or the relative fluid 
pressures within different cells. 
By monitoring the pressure of the fluid, (typically air), within a said 
cell the magnitude of the load applied to the portion of the surface of 
the structural member over which that cell extends can be derived, and by 
suitably disposing a plurality of such cells over the surface of the 
structural member the load distribution over the entire surface in 
question can be derived, as also can the resultant magnitude and direction 
of such loading. If it is required to know only the relative magnitudes of 
the loads applied to the respective portions of the surface of the member 
over which a plurality of cells extend it is sufficient only to monitor 
the relative fluid pressures within the cells. 
In a preferred embodiment of a device according to the invention the casing 
and cells are comprised in a sleeve-like assembly adapted to embrace and 
be frictionally retained upon a structural member, for an example an 
elongate member such as one of the support legs of an off-shore drilling 
rig, production platform or the like. The frictional retention of the 
assembly is particularly advantageous as it requires the provision of no 
special attachment means on the structural member nor any other 
modification to the structure of that member. In order to facilitate the 
application of the sleeve-like assembly to the structural member the 
assembly may comprise two circumferential portions adapted to clamp the 
member between them. For example, the two portions of the assembly may be 
hinged together at one each of their circumferential ends, their other 
circumferential ends being adapted to be detachably fastened together.

Referring to FIG. 1, the illustrated measuring device comprises a flexible 
casing 1 in the form of a generally cylindrical sleeve which acts to 
constrain eight flexible air-filled cells 2 between itself and the surface 
of a cylindrical tubular structural member 3. The cells 2 are disposed in 
two axially spaced circumferential series of four upon the member 3, which 
by way of example will be described as the submerged portion of a support 
leg for an off-shore drilling rig. At the two longitudinal ends of the 
casing there are disposed rigid or inflatable fairings 4 and 5 which act 
to smoothly blend the contour of the casing with that of member 3. Each 
cell 2 is provided with an electrical pressure transducer 6 sensitive to 
the pressure within the cell, output leads 7 from the transducers being 
gathered together into a cable 8 passing through fairing 5 and up to a 
monitoring station above sea level. 
The construction of the measuring device is more fully shown in FIGS. 2 and 
3. The assembly of casing 1 and cells 2 is in two circumferential portions 
9 and 10 each of which is adapted to extend around one half of the 
circumference of member 3. At one each of their circumferential ends the 
portions 9 and 10 are coupled together by hinges 11, while their other 
circumferential ends bear respective coacting portions of one or more 
locking devices 12 by which those ends can be detachably fastened together 
to clamp the member 3. Elastomeric circumferential and longitudinal 
spacers 13 and 14 are cemented to the interior surface of casing 1 to 
ensure the correct location of cells 2, the cells in turn being cemented 
to the rims 13A and 14A of the spacers. 
In use, the sleeve-like assembly of casing 1 and cells 2, with the cells in 
a deflated condition, is lowered from a location above sea level to the 
required position on the member 3 under test, being supported by lines 
such as 15 (FIG. 3) attached to lugs 16 on the exterior surface of casing 
1 towards the upper end of the assembly. At this point the assembly is in 
the open condition indicated in FIG. 4. The assembly is then manoeuvred to 
embrace the member 3 with the two portions 9 and 10 of the assembly being 
brought together about hinges 11 until the coacting portions of locking 
device(s) 12 engage; this can be achieved, e.g. by the use of lines 
attached to handling lugs 17 on the exterior surface of casing 1. The 
device(s) 12 are tightened and the cells 2 inflated to a suitable 
pressure, whereupon the member 3 is firmly clamped between portions 9 and 
10 of the assembly and the support lines 15 can be removed if desired. The 
fairings 4 and 5 may also be incorporated in the hinged assembly along 
with casing 1 and cells 2 if convenient, or else they may be separate 
elements applied separately to member 3 after the casing and cells are in 
position. 
With the casing 1 and inflated cells 2 in position on member 3 as shown in 
FIGS. 1 to 3, hydrodynamic loads applied to the casing will be transmitted 
through the air in the cells and reacted by the surface of member 3 
thereby causing changes in the pressure within the cells which are 
monitored by way of tranducers 6. As will be evident from FIG. 2, the 
whole of the externally applied load will be transmitted to the surface of 
member 3 solely through the air within the cells 2, because at no point in 
the periphery of member 3 does the casing 1 directly contact the member 3. 
When suitably calibrated, the output from each transducer can thereby be 
employed to give the magnitude of the load applied to the portion of the 
surface of member 3 over which the respective cell extends, and the load 
values obtained for various portions of the surface can be summed to 
obtain the overall magnitude and direction of the loading upon the total 
surface in question. Clearly, if greater resolution of the load 
distribution upon the same surface is required a greater number of smaller 
cells will be employed. 
It will be appreciated that by virtue of the closeness with which the 
contour of casing 1 conforms to that of member 3 over the portion under 
test, (with regard to both size and form), the loads which are applied to 
casing 1, transmitted to member 3 and measured via transducers 6 will be 
substantially those which would have been applied to member 3 by the same 
external flow conditions in the absence of the measuring device. The use 
of fairings 4 and 5 also helps to ensure that the presence of the 
measuring device does not significantly perturb the external flow in any 
way differently to member 3 alone. By virtue of the compressibility of the 
fluid within cells 2 the contour of the flexible casing 1 will in use 
depart from a true cylindrical form under varying conditions of loading 
but such departure will not generally be such as to significantly affect 
the validity of the readings obtained from the measuring device. However, 
if in any particular case it was desired that there be no deflection of 
casing 1, an incompressible fluid, such as water, could be used in place 
of the air in cells 2. 
It will also be appreciated that by virtue of the frictional retention of 
the casing and cells upon member 3 no special attachment means are 
required to be provided on the member, nor is any other modification to 
the structure of the member required. The method of application of the 
hinged sleeve to the member is relatively simple and quick and the sleeve 
can readily be applied in like manner at different axial locations on the 
same member or like members. Although the sleeve has been described in 
terms of its application to a cylindrical structural member it will be 
understood that by virtue of the inherent flexibility of its component 
parts the sleeve will be somewhat tolerant of departures from a true 
cylindrical form in the members to which it is applied. Sleeves 
specifically adapted to be applied to structural members of other 
cross-sections can be constructed in a similar fashion to the generally 
cylindrical sleeve herein described, and the number, size and relative 
disposition of the cells in all such sleeves is open to considerable 
variation.