Note: Descriptions are shown in the official language in which they were submitted.
40~
SEWAGE DISPOSAL
The present invention relates to sewage di~posal. Currently,
treated sewage, e.g. digested sludge, is commonly dlsposed of by
6imple dumping in shallow inshore waters, usually from barges or by
direct pumping through pipellnes from onshore sites. Thls is
leading to sn unacceptable build-up in the i~shore environment of
toxic substance6 such as heavy metal compounds.
According to the present invention, treated sewage such as digested
sludge is loaded into the tanks of a large ocean going tanker [e.g.
an oil/bulk ore (OBO) vessel, or a large, very large or ultra large
crude carrier], transported therein to a deep water site, and then
deposited directly onto the seabed at that site through pipin~
deployed from the tanker and extending downwardly therefrom. The
piping may be a flexible hose (e.g. plastics tubin~ in continuous or
segmented form), or a string of steel pipes, or a combination of the
two - in which case a hose portion may descend from the vessel and
connect with a pipe strin~ extending the remainder of the way. The
pipin~ may be deployed over the side of the vesse], or from a moon
pool. The depth of the seabed for deposit might for example be one
thousand or fifteen hundred metres, or considerably deeper, e.g.70n~
m; in extremely deep water it may not be necessary for the pipin~ to
extend fully to the seabed, although it is recommended that it
extend at least below the depth at which the ~ajority of fish are
found (the "fish line") and below the depth where there are
significant thermal changes. At depth of about 4000 metres or more
it is preferred to release the sewage sludge close to the ocean bed.
The height of the point of release above the bed is a function of
the relative densities of the sewage sludge and the sea water at the
release depth, the release rate, and the buoyancy frequency of sea
water. ~t is currently considered that an optimal release heiRht in
deep water is about 350 m above the sbyssal plain.
Treated sewage as currently received for dispo6al is usually an
1;'.~4(~
aqueous di~ested slud~e hsving a dry solid6 content of 10 to 5 wt.
or less; this may be concentrated, e.g. by partial de-waterln~ by
centrifuge or the like before or after loading onto the disposal
tanker; Euch partially de-watered ~lurry may be re-diluted on board
the tanker before seabed deposit if thls should be necessary for
ease of p~mping. The trea~ed 6ewage tsken on board the disposal
tanker may if desired be further treated on board before seabed
dispo6al; the sewage may have ~dded thereto various microorganisms
to asfiist in breakdown and degradation of the deposited matter. The
disposal tanker may be provided with an onboard laboratory for
analysing sewage and also wster 6amples before and after disposal.
The inventlon not only provides a disposal procedure as described
above, but also the disposal tanker per se, provided with a hose
reel and/or pipe erection plant and/or equipment for handling piping
and for deploying the disposal piping from the tanker to the deep
seabed disposal re~ion. When piping is deployed over the side of
the vessel the tanker hull may be used to provide a weathershield by
allowing the tanker to be beam-on to wind and waves. Pipe erection
plant e.~. pipe handling derricks may be accommoda~ed within the
hold space of the tanker, or may be on deck and shielded from the
elements by some suitable superstructure or housing.
~mbodiments of the present invention will now be described in
detail, by way of example only, with reference to the drawings in
which :
FIGURE 1 is a schematic representation of a disposal tanker with
hose reel equipment;
FIGURE 2 is a schematic representation of a disposal tanker with
pipe erection plant; and
FIGIIRE 3 ls a schematic representatlon of a disposal tanker with
equipment for handlin~ plastics tubing in segmented form.
In the drawinRs, unless othervise stated, like numerals are used to
l~Y406f:3
refer to like psrts.
As shown in Fig,l the dlsposal tanker 1 hss holds 2 for carrying
sewage. The total stor~ge space within the tanker is e.g. ,,
500 000 m3~ of which about 80~ may be utilised for sewage storage.
The tanker is of generally conventional type, having holds 2 below
deck, a bridge 3 and accommodation units 4 above deck. Laboratory
facilities 5 for carryinR out microbiological and biochemical
procedures are also provided.
The piping used for sewa~e disposal comprises a continuous extruded
pIastics hose 6 of about 7000 m length wound on a hose reel 7. The
internal diameter of the hose is e.g. 0.5 m. The reels 7
illustrated have horizontal axes of rotation; equally they could be
laid flat for rotation about a vertical axis. The approximate
dimensions of a reel holding 7000 m of hose i5 25 m diameter and 7 m
width. One advanta~e of continuous extruded plastics hose is that
it can be produced at the dockside and wound directly onto the hose
reel while the tanker is in harbour. Conventional plastics
extruslon plant could be used to generate 6uitable continuous
plastics hose at a rate of about 1000 ~ per day.
A number of hose reels can be carried on a tanker; in the
embodiment illustrated one reel 7 is mounted below deck in hold 8
and another reel is mounted on deck. The deck mounted reel could,
of course, be weathershielded.
The hoses 6 are deployed throu~h moon pools 9 in the tanker hull.
At the submer~ed end of a hose there is carried a wei~ht 10 which
prevents the pipe end driftin~ up towards the sea surface 11. At
the hose end there is o~tionally provided a pump 12, powered via a
cable from onboard the tanker, which creates a demand in the hose
and so assists sewa~e flow throu~h the hose, thereby i~proving the
rate of disposal onto the fieabed 13. It is presently estimated that
a 0.5 m hose would allow an unassisted flow rate (i.e. by gravity
alone) if around 1000 tonne/h. The hose end may be directed towards
the ~eabed, or may steerable (e.~. by provision of a fiwivellable
~4()~
elbow 14) fiO as to allow some control over the wei~hted hose end by
utillsing the thrust of a directed ~et of 6ewage.
A plastics hose ln water is generally neutrally or slightly
po6itively buoyant. The combined weight of the deployed ho6e,
weights, pumps etcO is 6upported by one or more cables 15 controlled
by winches 16.
The tanker is provided with 6uitable pipework (not shohTn) for
feeding sewage to the hose for di6posal.
In the embodiment of Fig.2, the piping used for sewage disposal is
made up of sections 17. Thege are generally steel pipe sections,
althou~h other rigld materials may be used1 e.g. glass fibre in
order to reduce weight. Pipe sections 17 are stored on deck ready
for incorporation into the pipe string.
Pipe sections are ~oined together to form the string using a pipe
derrick 18 on a weather shielded pipe deck 19. The pipe string is
deployed through a moon pool 9 in the tanker hull.
The 6uhmerged pipe end is weighted 10 and the pipe weight is
supported by cables 15 controlled by winches 16. A steel pipe of
several thousand meters length has considerable weight, and
consequently buoyant collar6 20 may be used to reduce the load in
the pipe string and on the cables. As in the previous embodiment of
Fig.l, a subsea pump and/or 6teerable pipe opening may optionally be
provided.
In any case, while sewage is being deposited on or near the seabed~
the tanker will be makin~ some way; about half a knot is considered
to be satisfactory. The tanker may be allowed to drift, or it may
be propelled by its engines (e.g at a rate of 1¦2 knot in reverse).
The presently preferred embodiment is illustrated in Fig.,3. The
deep ocean discharRe ves6el i6 a tanker 1 of about 100,000 tonnes
dead wei~ht. Such a vessel is preferred for its combinstion of
~4()~
.
-- 6 --
stabillty in heavy weather, lts draft which is sufficlently shallow
to allow approach close to existlng sewage terminals, and its
economic efficiency. The tanker carries racks 30 ln which are
6tacked 6ectlons of plastic6 discharge piping 32. The piping 32
suitably comprises up to 150 metre lengths of flexible high density
polyethylene piping of about 0.45 m dlameter.
Individual pipe lengths may be 6electively taken from the pipe racks
hy means of a fully articulated stacker arm 34. As each plpe length
is taken from the stack it is ~oined - for example by clamping - to
the precedin~ pipe length and then passed over rollers 36 on a pipe
guide quadrant 38 and deployed through a moon pool 9. In order to
facilitate passage of the pipe over the quadrant some of the rollers
may be driven. As an alternative the pipe guide quadrant could be
replaced by a rotatable pipe guide wheel. The deployed piping is
supported by guide-line cables or ropes controlled by winches (not
shown). The guide-lines are also used to retrieve the deployed
piping when it is desired to restow the pipes.
At the outlet end of the discharge pipe there ~s provided a
dropweight of about 40 tonne, which i6 designed to overcome t~e dra~
effects of ocean currents, The deployed piping will not, of course,
hang directly below the moon pool opening but will instead adopt a
curved profile dependant on the currents at various depths and the
direction of movement of the tanker. The dropweight is equipped
with depth senslng intrumentation and transponders are fixed at
predetermined intervals along the deployed pipe, thereby allowing
the underwater aspect of the pipe to be determined on board the
tanker. If the outlet end of the pipe should rise, the tanker can
move in the direction of the dropwei~ht thereby reducing the guide-
line tension and allowing the dropweight to sink. Conversely,
should the dropweight sink too close to the seabed, the vessel can
sail slowly away causing the deployed piping to regain its correct
height from the bed.
A dynamic positioning sy6tem using a combination of azimuth and
tunnel thrusters 40,42 keeps the tanker head into the weather during
deployment of the piplng, snd maintain6 the po61tion of the
dropweight and plplng relatlve to the 6eabet during dl6charge by
utilisin~ data derlved from the depth 6en60r and transponders.
The operation of this deep ocean dlscharge vessel for sewage
disposal will now be descrlbed. The vessel can lle at anchor close
by a sewage termlnal. Small feeder barges (e.g. 1500 to 4000 dwt.3
can transport sewage sludge from the terminal to the vessel.
Desirably, a gas return system i6 employed in order to prevent the
escape of displaced gas from the tanker holds in order to avoid
atmospheri~ pollution. Once loaded, the tanker will proceed to a
designated deep water dump site. At the dump slte the required
number of pipe lengths is determlned and these are selected from the
plpe racks, ~olned and deployed through a moon pool. The ships
pumps are used to dlscharge the sewage sludge from the holds,
through the piping and onto the ocean bed. The rate of pumping may
be of the order of 1000 tonnes per hour. After the sewage has been
discharged the holds and pipeline may be thoroughly washed and the
washings discharged to the ocean bed.
When sewage d~sposal has been completed the piping is retrleved and
restacked ln the pipe racks. The tanker then proceeds under normal
navigation to lts loading anchorage close to the sewage terminal.
Durin~ the voyage to and from the loading terminal the outlet end of
the dischar~e pipe together with the dropweight may be locked into a
moon pool cursor 44 so as to close off the moon pool. The cursor is
a passive sliding guide capable of runnin~ freely on rails set In
the moon pool wall. The cursor ls raised and lowered by raising and
lowering of the dropweight.
Whilst the embodiments illustrated have deployed piping through one
or more moon pools in the tanker hull, it ls to be understood that
in any case plping could be deployed over the side of the tanker.
