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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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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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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

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V07a Mooring 1206