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Ravenspurn Oil Platform
The Ravenspurn Constructionarium project presents a very clear answer to the question “how successful
were the students?”. The platform will either float or it won’t, and if it floats, does it end up in the right
position? The project has simple but time-critical tasks done on a large scale: it presents a real challenge.
The dry-build Concrete Gravity Substructure (CGS) concept was originally developed by Arup Energy in
1989 for Hamilton Brothers' Ravenspurn North CGS in the North Sea off Britain and Ravenspurn
demonstrated the viability of dry-build concrete structures for offshore application worldwide. As a result,
the oil and gas industry has seen an average of about one such substructure installed as part of a major
offshore development per year since 1989. CGS provides an alternative to piled steel jackets as oil platform
support structures.
The key features that lead to economical construction costs on CGS structures are:
• Simplicity and repetition of concrete structural elements
• Low reinforcement and pre-stress densities, which lead to simple, repetitive details
• Use of normal density concrete of a strength that can easily be batched by local civil engineering
contractors using readily available sources of aggregate, sand and cement and requiring no special
additives.
Ravenspurn North weighs over 28000 tonnes and was installed 80km off Flamborough Head, UK. It had
various design advantages including the ability to support two decks where previously only singles were
possible. It allowed a choice between building entirely in dry dock (followed by towout to sea) or as a
floating structure. The structure was designed for constructability and aimed to disassociate the concrete
caisson from the complex (and less enduring) pipework it supports.
Lessons that Arup report as a result of developing the Ravenspurn platform include:
(a) Installing decks offshore using a semi-submersible crane vessel allows the base caisson to be reduced in
size. One of the chief determinants of the base caisson size is the floating stability. “For every tonne of
structure at the top of the shafts, between 5 and 6 tonnes are required low down in the structure to
maintain the same stability characteristics (metacentric height).” (Arup Journal v75 n3)
(b) These smaller caissons allow construction in existing UK dry docks, avoiding the cost premium of doing
construction whilst floating at an inshore location.
(c) Dry dock work makes the caisson construction become a conventional matter of prestressed concrete
construction. Economic benefits from using familiar, repetitive processes using familiar materials that are
readily available made Ravenspurn economically feasible.
The design components of the Ravenspurn are simple: connections for the decks, concrete shafts,
concrete base caisson and steel foundation skirts. Caisson size depended on hydrodynamic loading,
structural considerations, geotechnical considerations, naval architecture and floatout draught. Some of
the requirements are conflicting, so Arup developed the base caisson having the lowest density (weight in
air divided by enclosed volume) and divided the caisson up into open and closed cells. Closed cells
provide buoyancy for towout. Open cells improve floatout draught and reduce the centre of gravity of the
structure (as they have no roof). Open cells also allow ballast to be installed once the platform is in place
(to improve slide resistance).
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Environmental conditions and key stats
Depth of water at field
Height of design wave
Associated period
Surface current
Re original oil platform
41.6m
18.7m
12.3 – 15.3 secs
2.41m/sec
Dimensions
Caisson size
Caisson height
Shaft height above caisson
Shaft diameter base/top
62 x 54.5m
16m
37.5
11m / 6m
Materials
Volume of concrete (C50)
Weight of reinforcement
Weight of prestressing (27km)
Weight of steel skirts
Weight of steel deck connections
9750 m3
2750 tons
450 tons
350 tons
410 tons
Weight and draughts
Final weight of CGS in air
Floatout draught (on 1.5m deep)
Towout draught
Main deck operating weight
Main deck lift weight
Compression deck operating weight
(note Constructionarium version is single deck only)
Compression deck lift weight (note Constructionarium
version is single deck only)
27850
9m
13.5m
8500 tons
6000 tons
4000 tons
3250 tons
Figure 1: Impression of Ravenspurn North Central Platform
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Brief : Ravenspurn Oil Platform
Ravenspurn is a North Sea gravity oil platform. The project involves making and sinking in position a concrete
base with steel superstructure in the middle of a lake.
Basic structure:
• The base of the platform is a concrete caisson 4m square and 1.5m high, with 150mm thick walls bottom
and top slabs
• The superstructure consists of a steel framework constructed using a proprietary concrete formwork system.
This will be attached to the bottom of the caisson, and the platform will be about 4.5m above the bottom
of the concrete base
• The final location for the platform is in the centre of the lake, in approximately 2m of water
• Once sunk the platform will sit approximately 2.5m above the water level
• The concrete box has been designed to float with a draught of approximately 1.2m
Site layout at the start of the week:
• A drydock has been prepared with a 6m square flat base. When flooded the dock has a depth of 1.5m of
water. There is a small sump in one corner of the drydock – the site staff will indicate where this is
• The drydock will be flooded when you arrive
• The lake is deep enough to float the rig from the drydock to the sinking location
• The sinking location will be identified for you by the site staff when you arrive. The bed is not necessarily
level at this point
What you will have to do:
• You will have to build the concrete base, sides and roof of the caisson in the drydock
• The superstructure must be erected, and the upper platform decked out in plywood and railing installed
• The bed of the lake needs to be prepared and levelled to receive the rig
• To move the completed rig you must flood the drydock and tow it to the sinking site.
• You must sink the rig in a controlled manner in the correct location before climbing to the top to bore your
well.
Friday feature
•
You will flood the dock, tow the oil rig out into the lake and sink it into position. You must then test the
drilling rig.
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P2 -RAVENSPURN DESIGNER'S RISK REGISTER
Ref
Activity element
Construction adjacent to
water
Significant potential hazards
Population at risk
Design action to mitigate risk
Residual action required/information for the Students
Drowning
Site personnel
Construction to be carried out within dry dock.
1. Provision of barriers to all deep water excavations
2. Provision of life belts adjacent to deep water excavations
3. Briefing of site personnel.
4. Students to prepare safe method of working.
5. Sinking of platform will require works to be carried out in centre of
lake from a boat.
Cuts of diseased water leading to
Dysentery, Gastro Enteritis,
contaminated water in contact with
broken skin causing infections or Weils
disease, Cryptosporidium or similar
Site personnel
1. Works in or over water have been limited to the minimum
practicable.
Briefing to site operatives on risks.
1
Moving plant
Being run over / harmed through activity of Site personnel
plant by others
Hand tools
Injury through misuse
Site personnel
Works designed to minimise use of handtools.
Handtools to be used only be trained personnel (Students shall be
supervised at all times when using hazardous equipment)
Working at height
Falls of materials and people
Site personnel
Kicker rail and handrailing provided to minimise falls from height.
Appropriate PPE to be worn at all times.
Working in dry dock
Dock gates fail or sides collapse causing
flooding leading to drowning
Site personnel working in
dock
1. Steel gated dry dock can only open against the hydrostatic pressure Inspection of dry dock prior to construction. Supervision of Students at
of the lake the other side.
all times while working in the dock. Appropriate briefing and provision
2. Sloped sides of 1:4 to the dry dock for added stability and allow for
of buoyancy aids.
emergency egress, staircase provided for more controlled egress1.
3. Dry dock designed with ties and concrete base for added robustness
4. Large working area reduces impact of collapse of one side
5. Pump provided to remove any incoming water through leaks etc.
Overall stability
Rig overturns with personnel at top,
drowning
Individuals overseeing the
installation of the dock in
final position
1. Design is self stabilising - a tilt will tend to restore the rig
2. Large margin of error allowable before rig susceptible to overturning
3. Rig designed with cable tie points to control movement whilst
floating.
Rig sinking
Rig sinks in an uncontrolled manner and
causes harm to personnel
Site personnel
1. Rig designed with a lid so that can be sunk in controlled manner and Construction to be supervised so that finalised design will not deviate
largely from intended design. A safe method of working statement
will not get a sudden loss of buoyancy.
2. Rig has cast in tie points for tethering.
shall be provided for carrying out flooding of dry dock, floating out of
rig and sinking of rig. No one shall be located on rig during sinking.
Caisson access
Confined space
Site personnel
1. No permanent means of access provided.
2. Temporary fixed lid specified on drawings
2
All plant to be operated only be trained personnel. Access routes to
be clearly marked out on site and all visitors to site and site personnel
to be briefed prior to entering site.
3
4
5
6
7
8
Construction to be supervised so that finalised design will not deviate
largely from intended design. A safe method of working statement
shall be provided for carrying out flooding of dry dock, floating out of
rig and sinking of rig.
Access onto caisson rig deck to be controlled. Temporary fixed lid to
be installed immediately after concrete has set. Noa ccess into the
caisson through the square hole shall be permitted during construction
and installation - to be controlled as a confined space and accessed
for demolition only. Briefing of site personnel.