Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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E~ERGENCY GANGWAY
_ .
This invention relates to an emergency escape gangway suitable
for installation on an emergency support vessel.
In recent years the number of offshore oil production platforms
has grown considerably, particularly in waters which are exposed to
severe climatic conditions such as the North Sea where gale force
winds and temperatures below free~ing are not infrequent. Provision
must always be made for emergencies which requtre the rapid
evacuation of these platfonms.
In extreme conditions, traditional means of escape such as
lifeboats or escape chutes are occasionally inoperative, often
hazardous and æometimes leave victims immersed ln the sea or under
exposed conditions for undesirably long periods of time, with
possibly fatal results.
For this reason an emergency support vessel (ESV) has been
built and is permanently on-station in the North Sea in the vicinity
of a group of production platforms. This vessel has both emergency
and support roles. One of its emergency roles is to provide a
rescue and lifesaving facility to a stricken offshore installation
by enabling direct evacuation of personnel by means of a gangway.
The vessel is designed to keep station bo~h in the vicinity of
a platform or close ~o it9 linked by ~he gangway, in poor weather
conditions without recourse to mooring. This is achieved by a
dynamic positioning system.
Under poor weather conditions, with the ESV subject to pitch,
roll, yaw, heave, sway and surge, the deployment of a conventional
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gangway and its positioning on the platform is a difficult and
sometimes impossible undertaking. Movement of the ESV due to
rolling is magnified at the end of the gangway and the possibility
of the gangway making a violent impact with the platform and
damaging itself and possibly the platform is a very real one.
We have now devised an emergency gangway capable of being
deployed from an ESV and being landed safely on an offshore platform
in gale force conditions, despite movements of the vessel relative
to the platform.
Thu~ according to the present invention there is provided a
telescopically extendable gangway comprising:
(a) a first section pivotally and rotatably mounted on a support
vessel,
(b) a second section telescoping within the first,
(c) a servo-mechanism for controlling the extension of the second
section of the gangway,
(d) a servo-mechanism for controlling the elevation of the gangway,
(e) a servo-mechanism for controlling the slew of the gangway and
~f) stabilising means for stabilising the end of the gangway in
~0 space.
The gangway may additionally comprise
(g) a relatively short third section connected to the remote end of
the second section in such manner that in use the third section
slopes downwardly from the second section.
The connection between the second and third sections may be
rigid, in which case the second and third sections are permanently
offset, or pivoted, in which case they may be aligned for better
storage when not in use, and more flexibly deployed when in use.
The third section reduces the magnitude of the impact load
during deployment since its mass is small.
The angle between ~he second and third sections in use ~s
governed by the maximum safe slope for carrying stretcher-borne
casualties. A typical value lies between 25 and 30.
Landing can be on an area of the platform which is above the
level of the base of the gangway. It is preferred, however, to
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employ the gangway in an approximately horizontal position from the
ESV to a specially designed and positioned landing area on the
platform.
Accelerometer based sensors and compensators provlde suitable
stabilising means. The sensors continuously monitor deviations from
the last known position and the compensators take restorative
action.
The landing tip of the gangway preferably comprises landing
wheels which on contact actuate the servo mechanism controlling the
telescoping of the gangway so that it may compensate for changes in
distance between the base of the gangway and the landing area on the
platform caused by relative motion between the ESV and the platform.
It is envisaged that the operation of the gangway will be
implemented manually by one operator with stabilising assistance
provided by the gangway control system. This allows maximum
flexibility of operation to cater for unforeseen circumstances
whilst also being well within the limitations of one man's
capabilities.
The elevation and slew servos are capable of opera~ing in the
following modes:-
(a) Unstabilised
(b) Space stabilised
(c) Idle.
The unstabilised mode has two functions. Firstly, when moving
the gangway from the stowed position on the ESV, the gangway should
be driven relative to deck. This will avoid any possible collisions
due to deck motions if the gangway were stabilised in space. When
the gangway is safely ou~board of the vessel structure, the
stabilised mode can be selected.
The principle of the space stabilised scheme means that over a
sensible limited period of time~ the elevation and slew motions of
the gangway will be space stabilised. The extension motlon of the
gangway during deployment remains a manual operation, since expected
short-term excursions in this direction are within ~ 1 ~etre.
The extension systèm i5 therefore capable of operating only in
two modes, namely:-
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(a) manual
- (b) automatic.
In "manual", the extension of the gangway is manually
controlled by the operator at the gangway control console. In the
"automatic" mode, the gangway extension is controlled automatically
once successfully deployed on the platform.
When the elevation and slew servos are in the "idle" mode, a
preset pressure will be maintained across the elevation ra~s or
motors thus countering any net out of balance whilst also ensuring
the hydraulics are free to move in sympathy with the gangway.
During gangway deployment, the operator will position the
gangway ln elevation and slew so that the gangway is pointing at the
platform. He will then leave this motion alone while the gangway
control system maintains the end point approximately stable in
space.
During this period, the operator is then free to concentrate on
manually extending the gangway to the re~uired position before
lowerlng the tip and finally locking it in the drooped position.
Once the gangway tip is lowered and locked in position, the
operator may switch the elevation and slew systems to "idle" and the
extension system to "auto".
From an operational viewpoint, it is preferred that droop snoot
can be locked in any position with respect to the gangway. This
allows for easier and more flexible operation than if discrete
locking positions are used.
In this mode the ESV gangway wlll automatically extend and
shorten, due to ESV motions9 to maintain the end point stationary on
the platform.
If the operator wishes to modify the position of the end point
on the platform, he can switch the extension system to manual,
reposition the end point with the ~oystick and then revert to
"auto". The gangway end point will then be maintained at this new
position. This ad~usting operation can be performed with the slew
and elevation systems remaining in idlen.
To return the gangway from the deployed to the stowed position,
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the elevation and slew servos will be returned to the "stabilised"
mode, the extension system returned to manual, and the tlp will be
lifted off the platform. The operator will then move the gangway
towards the stowed position and final stowage will be implemented
with the elevation and slew servo in the "unstabilised" mode.
The previous operation assumes that, after deployment, the gangway
rests on the rig with both elevation and slew servos in the idle
mode. This concept results in inertial forces due to the ESV base
motion being transmitted to the platform. These forces can be
reduced by incorporating a suspension system on the end of the
gangway which results in relative motion between the gangway and
platform as energy is absorbed.
An alternative configuration is a stabilised gangway wrist with
the angled tip being allowed to rest freely on the platform.
This configuration reduces the loads on the landing.
The invention is illustrated with reference to Figures 1-5 of
the accompanying drawings wherein Figure l is an elevation of the
gangway with the telescopic section fully retracted, Figure 2 is an
elevation with the telescopic section fully extended9 Figure 3 is a
plan view with the telescopic section partially extended, Figure 4
is an elevat~on showing the gangway in two different positions and
Figure 5 is a schematic diagram of the control systems.
With reference to Figures 1-4, the gangway is rotatably and
pivotably mounted on a slewing base l. The gangway comprises an
inboard section 2, a telescopic section 3 and a "droop snoot" 4.
Operation is controlled from a cabin 5.
Section 3 is extended by means of a drive winch 6 and
assoclated wires and sheaves.
The droop snoot 4 is pivoted to the telescopic section 3 at 7
and hydraulic rams 8 control the desired configuration. A landing
wheel assembly 9 is fitted to the tip of the snoot.
The gangway extends from a stowed length of 25 m to a maximum
of 43 m and is approximately 0.6 m wide. It is constructed from
lightweight hollow steel sections.
Figure 4 shows the gangway at rest in the horizontal posi~ion.
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The Figure also shows the inboard section 2 in the elevated
position, the other sections being omitted for reasons of clarity.
A short flight of steps lO leads down from the gangway to the
deck 11 of the ESV.
With reference to Figure 5.
Additionally shown are a slew motor and gearbox assembly 12 for
slewing the base 1, and hydraulic cylinders 13 for elevating the
gangway.
A position stabilising accelerometer pack 14 is fitted to the
inboard section 2 of the gangway.
TV cameras 15 for viewing tip motion are fitted at the end oE
the droop snoot 4.
A hydraulic powerpack 16 supplies hydraulic fluid to the
various units and signals a.e relayed to a control console 17
containing servo processing and power supplies 18 and video
screens 19 as follows:
Signal Description Function
-
21 Gangway Wheel Rig Position Measures rotation of wheels
Sensor which indicates the need to
telescope and actuate tele-
scoping servo
22 Systems Landed Interlock Detects gangway landing from
axle load on wheels
23 Accelerometer Signals Velocity feedback on elevation
' 25 24 DP Transducer Telescopic extension limiting
- hydraullc pressure feedback
signal
25 Servo Valve Spool Posi~ion Feedback of telescoping
velocity
26 Ser~o Valve Signal Telescoping control command
27 Blocking Valve Signal Stops gangway telescoping on
loss of hydraulic pressureO
Locks hydraulic supply line
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Signal Description Function
28 Tacho Feedback on velocity of
telescoping
29 Elevation Inertial Signals Feedback on angle o~ elevation
Tacho Feedback on rate of change of
aDgle of elevation
31 Tacho Feedback on rate of change of
angle of slew
32 Blocking Valve Signal Stops gangway slewing on loss
of hydraulic pressure. Locks
hydraulic supply line
33 Servo Valve Signal Slew control command
34 DP Transducer Slew limiting hydraulic
pressure feedback signal
35 Servo Valve Spool Position Slew velocity feedback
36 Servo Valve Signal Elevation control command
37 Blocking Valve Signal Stops gangway elevation on 108s
of hydraulic pressure. Locks
hydraulic supply line
38 DP Transducer Elevation limiting hydraulic
pressure feedback signal
39 Servo Valve Signal Elevation control command
Slew Reference Data Inertial slew reference in
space
41 Powerpack Tellbacks Warning in case of malfunction
42 Powerpack Control Signals Powerpack control
