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Floating Production Technology
Mooring Systems
Specialist Diploma in Marine & Offshore Technology
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Mooring Systems vs DP Systems
text
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Mooring Systems
Keeps the offshore unit at station (for risers runningbetween the FPS and subsea manifold)
Need to work all year-round and is subject to hostile
weather conditions
Comply with wave motions rather than resist them
Normally installed by anchor-handling tugs prior to arrival of
FPS
This enables rapid hook-up and secure FPS once it arrives at
field location
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Types of Mooring Systems
Spread Mooring
Single Point Mooring
Fixed tower
CALM buoy (Catenary Anchor Leg Mooring)
SALM (Single Anchor Leg Mooring)
Internal turret
External turret
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Spread Mooring
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Spread Mooring
http://www.offshore-technology.com/projects/bonga/ -
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Mooring Pattern
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Spread Mooring
Consists of multiple mooring linesattached to four cornersof the production unit
Link production unit to anchoring points on the sea bed
Do not require to weather-vane since the unit has a fixedheading
Not suitable for production units which are sensitive to theincident wave direction
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Spread Mooring
Advantages of spread mooring:
Wide applicability in terms of vessel type and water depth
Use of traditional and therefore relatively inexpensiveshipboard equipment
Suitable for a wide range of mooring lines (chains, wires, etc)
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Single-Point Mooring (SPM)
Allow production unit to weathervane around the mooring
Suitable for all mono-hulls (tankers and barges) moored insevere environments
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Types of SPM
Fixed tower
CALM buoy (Catenary Anchor Leg Mooring)
SALM (Single Anchor Leg Mooring)
Internal turret
External turret
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Fixed Tower
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Fixed Tower
Suitable for shallow water of depths (20 to 50 m) and smallwave heights (about 5 m significant)
Can be connected to the floating vessel by a simple hawser
Hawser is usually replaced by a yoke to avoid the risk of
extensive damage in the event of a minor collision between
the tower and the tanker.
Hawser
Yoke
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CALM Buoy
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CALM Buoy
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CALM Buoy Soft and Rigid
CALM Buoy Soft Yoke and Rigid Arm
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CALM Buoy
Suitable over a wider range of water depths and larger waveheights
30 to 150 m
up to 8 m significant
Can be connected to the floating vessel by a yoke andpendulum system similar to that for the fixed tower, or by arigid arm that is hinged or rigidly connected to the buoy andhinged to the vessel
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SALM Single Anchor
Leg Mooring
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SALM Single Anchor Leg Mooring
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SALM Single Anchor Leg Mooring
Column hinged at the sea bed and connected to the floatingvessel by a rigid arm or yoke hinged at both ends
Buoyancy can be provided in the upper part of the column
itself, this is the conventional SALM
Alternatively the buoyancy can be provided by a buoyancy
chamber attached to the yoke; this system is known as SALS
(Single Anchor Leg Storage)
Both SALM and SALS are suitable for deeper waters and
large wave heights
up to 200 m
12 m significant
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SALM Single Anchor Leg Mooring
Cossack Pioneer (Woodside Energy)
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Internal Turret
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Internal Turret
An internal turret is convenient when a large number ofrisers are to be installed, and therefore a large turret and
swivel assembly are required.
An internal position also reduces the risk of slamming due tothe reduction of the effect of pitch.
Internal turrets can be used in deep waters and the most
severe environments
up to 18 m significant wave height
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Internal Turret
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Entry of risers and chains
at bottom of Internal Turret
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Various Deck Levels of Turret
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Swivel stack located
on top of the turret
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Swivel stack located on top of the turret
Swivel stack (Anasuria)
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External Turret
An external turret eliminates the CALM buoy and allows theturntable and swivels to be directly attached to the vesselbow or stern.
Suitable for deep waters and large wave heights
12 m significant wave heights
Can be used up to the point where the combined heave andpitch motions may cause slamming on the bottom of theturret (depending on vessel size and length, up toapproximately wave height).
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Passive and Active Systems
Passive systems
once the individual mooring lines have been installed and
pre-tensioned, they are locked off and they are not
modified over the FPSO life at the site.
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Passive and Active Systems
Active systems
Lengths and the tensions of the mooring lines can be
modified over the FPSO life at the site.
FPSO can be moved over a short distance from its original
position in order to allow another vessel to come inposition to carry out workover operations.
Pre-tensions in the lines can be adjustedfor specific
environmental conditions, e.g. to better resist an
oncoming storm. Individual tensioners for each mooring line additional
equipment, payload and cost.
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Mooring System Design
comparison between ships and FPSOs
Conventional sea-going vessels
Intermittent useof mooring system
Regular inspectionof individual lines for wear and fatiguecan be carried out using the onboard equipment
Damaged mooring line components can be replacedwhilethe vessels are in transit or moored at docks.
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Mooring System Design
comparison between ships and FPSOs
FPSOs Mooring Systems
Subject to permanent useat exposed locations
Any breakage and replacement in the field is extremelyexpensive.
Design requires a much more detailed knowledge of:
environmental conditions
motion response of vessel
fatigue and wear properties of the materials used for themooring lines.
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Mooring Loads
Loads are due to wind, waves and current.
Wind load
Caused by the action of the wind on the part of FPSO
above waterline, i.e. on the accommodation block, theprocess facilities on deck, etc.
Current load
Caused by the action of the current on the immersed partof the FPSO
Wave load (see next page)
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Mooring Loads
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Mooring Loads
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Mooring Loads
Wave LoadWaves have two effects, they impose:
first order loads at wave frequencies,
second order loads: these loads are known as slow drift
forces, which is a second order effect in the interactionbetween waves and FPSO motion
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Mooring Design
To distribute the loadsin the individual lines as equally aspossible
To give sufficient redundancy to the overall system.
Mooring Pattern
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Mooring Layout
The important factors are:
Strength of each mooring line
Breakage tension of 3,130 to 5,160 kN (320 to 525
tonnes)
Seabed topography and soil friction
Prevailing directions of wind, waves and current
Proximity of other fixed structures on the seabed such as
templates and pipelines
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Mooring Layout
The important factors are (continued):
Proximity of other fixed structures in the water column, such
as risers and riser mid-water arches, etc.
Other storage or drilling vessels moored in the vicinity
Future operational activities in the field (e.g. well workover).
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Mooring Analysis
Design criteria
Normally designed for the 100-year storm conditions, i.e. for
the combination(s) of wave height, wind and current
velocities which are likely to occur once in a 100 years.
Conditions are established by extreme value analysis and
extrapolation based on environmental data measured over a
sufficient length of time.
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Mooring Analysis
Typical values of waves in 100-year storms are:
100-Year Significant
Wave Height (m)
Associated Wave Period
(seconds)
West of Shetland 18 20
Northern North Sea 16 17
Gulf of Mexico 13 16
Philippines 11 15
Brazil 7 14
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Mooring Analysis
Tensions and excursions
Design of the mooring system should allow floater remains
within acceptable limits of horizontal distance (or excursion)
whilst subject to the worst environmental loads.
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Regions of Vessel Offsets vs Line Tension
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Mooring Analysis
Traditional mooring and riser systems design
Traditionally, uncoupled behaviour of the two systems, and
each system is analysed independently.
The motion of FPSO is calculated taking into account the
mooring system only; the motion obtained is then imposed
as an input to design of riser system.
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Mooring Analysis
Integrated mooring and riser systems design
System stiffness increases with:
Increase in riser number
Size of the risers Water depth
Nowadays, stiffness and damping contribution from the
risers are included in fully integrated analysis of the mooringand riser systems.
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Mooring Analysis
Broadly what does mooring analysis involve?
design criteria (e.g. 100-year storm)
tensions in lines, excursions anticipated/allowed
integrated mooring and riser design