Introduction

Mooring System:
The mooring system consists of freely hanging lines connecting the surface
platform to anchors, or piles, on the seabed, positioned at some distance from
the platform.
“Often laid out symmetrically in plan view around the object in question”
Types of Mooring Lines:
1. Steel-Linked chain
2. Wire rope
3. Synthetic fiber rope

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Dept. of Chemical Engineering,VIT University. December 1
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1. Steel-Linked chain
2. Wire rope

The above two types of catenary lines are conventionally used

for mooring floating platforms.

Each of the lines forms a catenary shape, depending on an

increase or decrease in line tension as it lifts off or settles on
the seabed, to produce a restoring force as the surface
platform is displaced by the environment.

Thus a spread of lines generates a nonlinear restoring force to

provide the station-keeping function.
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This force increases with vessel horizontal offset and balances
quasi-steady environmental loads on the surface platform.

The equivalent restoring stiffness provided by the mooring is

generally too small to influence wave frequency motions of
the vessel significantly, although excitation by low-frequency
drift forces can induce dynamic magnification in the platform
horizontal motions and lead to high peak line tensions.

The longitudinal and transverse motions of the mooring lines

themselves can also influence the vessel response through
line dynamics.

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Dept. of Chemical Engineering,VIT University. December 3
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3. Synthetic rope

To operate in more water depths, the suspended weight of

mooring lines becomes a prohibitive factor. In particular, steel
chains become less attractive at great water depths.

Recently, advances in taut synthetic fibre rope technology have

been achieved offering alternatives for deep-water mooring.

Mooring systems using taut fibre ropes have been designed and
installed to reduce mooring line length, mean- and low-frequency
platform offsets, fairlead tension and thus the total mooring cost.
(Still a lot of R&D in progress)
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Dept. of Chemical Engineering,VIT University. December 4
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Mooring system design philosophy:

Mooring system design is a trade-off between making the system

compliant enough to avoid excessive forces on the platform, and
making it stiff enough to avoid difficulties, such as damage to
drilling or production risers, caused by excessive offsets.

Easier to achieve for moderate water depths, but becomes more

difficult as the water depth increases.

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Dept. of Chemical Engineering,VIT University. December 5
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Single Point Mooring (SPM):

Excessive offsets are often observed due to the environmental

factors on the mooring system.

SPM have been developed to overcome this disadvantage.

In this the lines are attached to the vessel at a single point.

This connection point is located on the longitudinal centre line of
the vessel.

The vessel is then free to weathervane and hence reduce

environmental loading caused by wind, current and waves.
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Dept. of Chemical Engineering,VIT University. December 6
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Single Buoy Mooring (SBM):

A typical early facility consisted of a buoy that serves as a mooring

terminal. It is attached to the sea floor either by catenary lines, taut
mooring lines or a rigid column.

The vessel is moored to the buoy either by synthetic hawsers or by

a rigid A-frame yoke.

Turntable and fluid swivels on the buoy allow the vessel to

weathervane, reducing the mooring loads.

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Dept. of Chemical Engineering,VIT University. December 7
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In order to further reduce the environmental loading on the
mooring system from the surface vessel in extreme conditions, dis-
connectable turret mooring systems have also been developed.

Here the connected system is designed to withstand a less harsh

ocean environment, and to be disconnected whenever the sea state
becomes too severe such as in typhoon areas.

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Dept. of Chemical Engineering,VIT University. December 8
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Functional requirements for the mooring system:

1. Offset limitations
2. Lifetime before replacement
3. Install-ability
4. Positioning ability

These requirements are determined by the function of the

floater.

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Dept. of Chemical Engineering,VIT University. December 9
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 Comparison of typical MODU and FPS mooring requirements:
MODU Floating Production
Design for 50-yr return period event. Designed for 100 yr return period
Anchors may fail in larger events. events.

Risers disconnected in storms Risers remain connected in storm

Slack moorings in storm events to Moorings are usually not slacked
reduce line tensions because of risk to the risers, and lack
of marine operators on board

Components designed for < 10 yr life Components designed for > 10 yr life
Fatigue analysis not required Fatigue analysis required
Life dynamics analysis not required Life dynamics analysis required
Missing line load case not required Missing line load case required
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Dept. of Chemical Engineering,VIT University. December 10
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Steel Chain or Wire Catenary lines:

In the figure: Catenary

mooring is deployed from
point A on the submerged
hull of a floating vessel to
an anchor at B on the
seabed.

Some part between AB is

resting on the seabed, &
horizontal distance “a” is 5-
20 times larger than the
vertical dimension “b”.
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Dept. of Chemical Engineering,VIT University. December 11
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As we shift the mounting point from A1to A4 the catenary line
laying/resting varies from a significant length at A1 to none at A4.

From a static point of view, the cable tension in the vicinity of

point A is due to the total weight in sea water of the suspended
line length.

The progressive effect of line lift-off from the seabed

due to the horizontal vessel movement from Al to A4 increases
line tension in the vicinity of point A.

This feature, coupled with the simultaneous decrease in line angle

to the horizontal, causes the horizontal restoring force on the vessel
to increase with vessel offset in a non-linear manner.
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Dept. of Chemical Engineering,VIT University. December 12
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Synthetic Lines:

For deep-water applications, synthetic fibre lines can

have significant advantages over a catenary chain or wire
because they are considerably lighter, very flexible and
can absorb imposed dynamic motions through extension
without causing an excessive dynamic tension.

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Dept. of Chemical Engineering,VIT University. December 13
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Additional advantages include the
fact that there is reduced line
length and seabed footprint, as
depicted in the adjacent figure

This, causes reduced mean- and

low-frequency platform offsets,
lower line tensions at the fairlead and smaller vertical load on the
vessel. This reduction in vertical load can be important as it effectively
increases the vessel useful payload.
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Dept. of Chemical Engineering,VIT University. December 14
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The disadvantages in using synthetics are that their material
and mechanical properties are more complex and not as well
understood as the traditional rope.

This leads to over conservative designs that strip them of

some of their advantages. Furthermore, there is little in-
service experience of these lines.

In marine applications this has led to synthetic ropes subject

to dynamic loads being designed with very large factors of
safety.

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Dept. of Chemical Engineering,VIT University. December 15
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Important properties of synthetic lines to considered in design:

▪ Stiffness
▪ Hysteresis and heat build up
▪ Fatigue
▪ Other issues

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Dept. of Chemical Engineering,VIT University. December 16
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Stiffness:

In a taut mooring system the restoring forces in surge, sway and

heave are derived primarily from the line stretch.

This mechanism of developing restoring forces mostly differs from

the conventional steel catenary systems that develop restoring forces
primarily through changes in the line catenary shape. This is made
possible by the much lower modulus of elasticity of polyester
compared to steel.

The stretch characteristics of fibre ropes can extend from 1.2 to 20

times as much as steel, reducing induced wave and drift frequency
forces. (Stiffness of line is a function of load & age)
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Dept. of Chemical Engineering,VIT University. December 17
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Hysteresis and heat build up:

The energy induced by cyclic loading is dissipated (hysteresis) in the

form of heat. In addition, the chaffing of rope components against
each other also produces heat.

Cases are known in which the rope has become so hot that the
polyester fibers have melted. This effect is of greater concern with
larger diameters or with certain lay types because dissipation of the
heat to the environment becomes more difficult.

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Dept. of Chemical Engineering,VIT University. December 18
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Fatigue:

The fatigue behavior of a rope at its termination is not good. In a

termination, the rope is twisted (spliced) or compressed in the radial
direction (barrel and spike or resin socket).

The main reason for this decreased fatigue life is local axial
compression. Although the rope as a whole is under tension, some
components may go into compression, resulting in buckling and
damage of the fibres.

In a slack line this mechanism is more likely to be a problem than in

a rope under tension. The phenomenon can appear at any position
along the rope.
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Dept. of Chemical Engineering,VIT University. December 19
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Other relevant issues:

Issues to consider are that the strength of a polyester rope is about

half that of a steel wire rope of equal diameter.

Additionally the creep behavior is good but not negligible (about

1.5% elongation over 20 years). Furthermore, synthetic fibre ropes
are sensitive to cutting by sharp objects and there have been reports
of damage by fish bite. A number of rope types such as high
modulus polyethylene (HMPE) are buoyant in sea water; other types
weigh up to 10% of a steel wire rope of equal strength.

Synthetic fibre lines used within taut moorings require the use of
anchors that are designed to allow uplift at the seabed.
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Dept. of Chemical Engineering,VIT University. December 20
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 Loading Mechanism on Mooring System:
There are various loading mechanisms acting on a moored floating vessel as depicted in the
previous figure are:

For a specific weather condition, the excitation forces caused by current are usually
assumed temporally constant, with spatial variation depending on the current profile and
direction with depth.

Wind loading is often taken as constant, at least, in initial design calculations, though
gusting can produce slowly varying responses.

Wave forces result in time-varying vessel motions in the 6 rigid body degrees of freedom of
surge, sway, heave, roll, pitch and yaw.

Wind gust forces can contribute to some of these motions as well.

Mooring System Design

▪Static design
▪Quasi Static Design
▪Dynamic Design 30
Dept. of Chemical Engineering,VIT University. December 21
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 Mooring Hardware Components

The principle components of a mooring system may consists of:

▪ Chain, wire or rope or their combination

▪ Anchors or piles
▪ Fairleads, bending shoes or pad-eyes
▪ Winches, chain jacks or windlasses
▪ Power supplies
▪ Rigging (e.g. stoppers, blocks, shackles)

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Dept. of Chemical Engineering,VIT University. December 22
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Chain, wire or rope or their combination:

Properties are given by “Det Norske Veritas OS-E301” codes.

Chain and wire make up the strength members for the mooring system.

There are primary 2 chain constructions:

a) Stud-Link Chain (studs provide stability to the link and
facilitate laying down of chain while handling.)

b) Stud-less Chain (removing stud reduces the weight per unit of

strength and increases the chain fatigue life, at the expense of
making the chain less convenient to handle.)

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Dept. of Chemical Engineering,VIT University. December 23
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Wire rope:
Wire rope consists of individual wires wound in a helical pattern
to form a “strand”.
The pitch of the helix determines the flexibility and axial stiffness
of the strand.

Wire rope used for mooring can be multi-strand or single-strand

construction.

Stud-link chain and six-strand wire rope are the most common
mooring components for MODUS and other “temporary” moorings.

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Dept. of Chemical Engineering,VIT University. December 24
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Wire rope:

Multi-strand ropes are favored for temporary applications because of

their ease of handling.

Six-strand rope is the most common type of

multi-strand rope used offshore.

Mooring line ropes typically consist of 12, 24, 37 or more wires

per strand.

The wires have staggered sizes to achieve higher strength

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Dept. of Chemical Engineering,VIT University. December 25
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Common “classes” of multi-strand rope include (Myers, 1969):

6x7 class: 7 wires per strand, usually used for standing rigging.
Poor flexibility and fatigue life, excellent abrasion resistance.
Minimum drum diameter/rope diameter (D/d) = 42.

6x9 Class: 16 to 27 wires per strand. Good flexibility and fatigue

life and abrasion resistance. Common in lifting and dredging.
Minimum D/d = 26-33.

6x37 Class: 27 to 49 wires per strand. Excellent fatigue life and

flexibility, but poor abrasion resistance. Minimum D/d = 16-26.

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Dept. of Chemical Engineering,VIT University. December 26
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Multi-strand wire ropes may contain either a fibre or a metallic core.

The core is important for support of the outer wires, especially on a

drum, and in some applications to absorb shock loading.

Fibre core (FC) ropes are not generally used for heavy duty marine
applications.

Metallic core ropes may be one of the two types:

a) Independent Wire Rope Core (IWRC)
b) Wire-Strand Core (WSC).

IWRC is the most common core filling for heavy marine

applications.
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Dept. of Chemical Engineering,VIT University. December 27
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Anchors or piles:

Anchors are basically of two types, relying either on self-weight or

suction forces.

The traditional embedment anchors, as shown in figure, are not

normally designed for vertical force components. Holding power is
related to anchor weight and type of seabed.

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Dept. of Chemical Engineering,VIT University. December 28
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Turrets:

The design of mono-hull turret structures used for single-point moorings in

floating production systems must allow for large static and dynamic loading
caused by the vessel motions in waves together with forces transmitted by the
mooring system.

The hull design in the turret region must reflect the fact that the amount of primary
steel is reduced here with an appropriate increase in the stress concentration.
Careful selection of turret position is important because of its influence on:

➢ Mooring line tension and riser loading.

➢ Vessel yaw

➢ Rigid body oscillation in the horizontal plane

Dept. of Chemical Engineering,VIT University.
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December 29
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 Mooring System Analysis:

The mooring system is assessed in terms of three limit states based

on the following criteria:

➢ Ensuring that individual mooring lines have suitable strength

when subjected to forces caused by extreme environmental loads -
ultimate limit state (ULS).

➢ Ensuring that the mooring system has suitable reserve capacity

when one mooring line or one thruster has failed - accidental limit
state (ALS).

➢ Ensuring that each mooring line has suitable reserve capacity

when subject to cyclic loading - fatigue limit state (FLS).
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Dept. of Chemical Engineering,VIT University. December 30
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 Potential failure modes as given in standards:

➢ Hysteresis heating:lubricants and fillers can be included to reduce

hotspots, creep rupture in particular this is relevant to HMPE yarns,
-

and the risks need careful evaluation.

➢ Tension: Tension fatigue-only limited data exist, indications being

that fatigue resistance is higher than for steel wire ropes.

➢ Axial compression fatigue - on leeward lines during storms for

example, prevented by maintaining a minimum tension on the rope.

➢ Particle ingress - causes strength loss by abrasion from water-

borne material such as sand, prevented by using a suitable sheath
and not allowing contact between the rope and seabed.
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Dept. of Chemical Engineering,VIT University. December 31
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