OE 5050

ANCHORS

Dr R Panneer SelvamDept of Ocean EngineeringAug-Nov 2010

MOORING SYSTEMS

• Mooring systems have been around just as long as man has felt the need for anchoring a vessel at sea.

• These systems were used, and are still used, on ships and consisted of one or more lines connected to the bow or stern of the ship.

MOORING SYSTEMS

• Generally the ships stayed moored for a short duration of time (days).

• When the exploration and production of oil and gas started offshore, a need for more permanent mooring systems became apparent.

• Numerous different mooring systems have been developed over the years, of which a short selection is presented here.

Semi-submersible drilling rig

• Semi-submersible drilling rig - generally the semi-submersibles are moored using an eight point mooring.

• Two mooring lines come together at each of the columns of the semi-submersible.

Catenary Anchor Leg Mooring

• CALM buoy - generally the buoy will be moored using four or more mooring lines at equally spaced angles.

• The mooring lines generally have a catenary shape.

• The vessel connects to the buoy with a single line and is free to weathervane around the buoy.

Single Anchor Leg Mooring

• SALM buoy - these types of buoys have a mooring that consists of a single mooring line attached to an anchor point on the seabed, underneath the buoy.

• The anchor point may be gravity based or piled

Turret mooring

• Turret mooring - this type of mooring is generally used on FPSOs and FSOs in more harsh environments.

• Multiple mooring lines are used, which come together at the turntable built into the FPSO or FSO.

• The FPSO or FSO is able to rotate around the turret to obtain an optimal orientation relative to the prevailing weather conditions.

SPREAD MOORING

• Spread mooring - generally used on FPSOs and FSOs in milder environments.

• The mooring lines are directly connected to the FPSO or FSO at both the stern and bow of the vessel.

Catenary Mooring System

• When oil and gas exploration and production was conducted in shallow to deep water, the most common mooring line configuration was the catenary mooring line consisting of chain or wire rope.

• For exploration and production in deep to ultra-deep water, the weight of the mooring line starts to become a limiting factor in the design of the floater.

Catenary Mooring System

Taut leg Mooring System

• To overcome this problem new solutions were developed consisting of synthetic ropes in the mooring line (less weight) and/or a taut leg mooring system

Taut leg Mooring System

Catenary vs Taut Leg Mooring• The major difference between a catenary

mooring and a taut leg mooring is that where the catenary mooring arrives at the seabed horizontally, the taut leg mooring arrives at the seabed at an angle.

• This means that in a taut leg mooring the anchor point has to be capable of resisting both horizontal and vertical forces, while in a catenarymooring the anchor point is only subjected to horizontal forces.

•

Catenary vs Taut Leg Mooring

• In a catenary mooring, most of the restoring forces are generated by the weight of the mooring line.

• In a taut leg mooring, the restoring forces are generated by the elasticity of the mooring line.

• An advantage of a taut leg mooring over the catenary mooring is that the footprint of the taut leg mooring is smaller than the footprint of the catenary mooring,

• i.e. the mooring radius of the taut leg mooring will be smaller than the mooring radius of a catenary mooring for a similar application.

Anchors

• Dead Weigh Anchor• Drag embedment Anchor• Pile Anchor• Suction Anchor• Vertical Load Anchor

Dead weight anchor

• Dead weight• The dead weight is probably the oldest

anchor in existence. • The holding capacity is generated by the

weight of the material used and partly by the friction between the dead weight and the seabed.

• Common materials in use today for dead weights are steel and concrete

Drag Embedment Anchor

• This is the most popular type of anchoring point available today.

• The drag embedment anchor has been designed to penetrate into the seabed, either partly of fully.

• The holding capacity of the drag embedment anchor is generated by the resistance of the soil in front of the anchor.

Drag Embedment Anchor

• The drag embedment anchor is very well suited for resisting large horizontal loads, but not for large vertical loads although there are some drag embedment anchors available on the market today that can resist significant vertical loads

PILE ANCHOR

• The pile is a hollow steel pipe that is installed into the seabed by means of a piling hammer or vibrator.

• The holding capacity of the pile is generated by the friction of the soil along the pile and lateral soil resistance.

PILE ANCHOR

• Generally the pile has to be installed at great depth below seabed to obtain the required holding capacity.

• The pile is capable of resisting both horizontal and vertical loads

SUCTION ANCHORS

• Like the pile, the suction anchor is a hollow steel pipe, although the diameter of the pipe is much larger than that of the pile.

• The suction anchor is forced into the seabed by means of a pump connected to the top of the pipe, creating a pressure difference.

• When pressure inside the pipe is lower than outside, the pipe is sucked into the seabed.

SUCTION ANCHORS

• After installation the pump is removed. The holding capacity of the suction anchor is generated by the friction of the soil along the suction anchor and lateral soil resistance.

• The suction anchor is capable of withstanding both horizontal and vertical loads

Vertical load anchor

• A new development is the vertical load anchor (VLA).

• The vertical load anchor is installed like a conventional drag embedment anchor, but penetrates much deeper.

• When the anchor mode is changed from the installation mode to the vertical (normal) loading mode, the anchor can withstand both horizontal and vertical loads

ANCHOR

• An anchor is a heavy object, often made out of metal, that is used to attach ships to the bottom of a body of water at a specific point.

• There are two primary classes of anchors—temporary and permanent.

ANCHOR – Permanent

• A permanent anchor is often called a mooring, and is rarely moved;

• the vessel may not hoist it aboard • must hire a service (AHS – Anchor

handling ship) to move or maintain it

ANCHOR – Temporary

• A temporary anchor is usually carried by the vessel, and hoisted aboard whenever the vessel is under way; it is what most non-sailors mean when they refer to an anchor.

Anchors• Anchors have been used in water depths exceeding 1500m

Anchors

• Anchors works by resisting the movement force of the vessel which is attached to it• Two primary ways to do this• via Sheer Mass• via “Hooking” into sea bed

Anchors

• Loads due Wind and currents are not the largest• Actually the vertical movement of waves develop the largest loads, • modern anchors are designed to use a combination of technique and shape to resist all these forces.

Anchors

• An anchor is described as aweigh when it has been broken out of the bottom and is being hauled up to be stowed. (keep it on board, ship sides)• Aweigh - which describes the anchor when it is hanging on the rope, not on the bottom;

Anchors

FLUKE

SHANK

CROWN

Anchors

• A modern temporary anchor usually consists of • a central bar called the shank, • an armature with some form of flat surface (fluke or palm) to grip the bottom • the position at which the armature is attached to the shank is called the crown, •and the shank is usually fitted with a ring or shackle to attach it to the cable. •There are many variations and additions to these

Mushroom Anchors

• shaped like an inverted mushroom, the head becoming buried in the silt.

• A counterweight is often provided at the other end of the shank to lay it down before it becomes buried.

•A mushroom anchor will normally sink in the silt to the point where it has displaced its own weight in bottom material.

Mushroom Anchors

• These anchors are only suitable for a silt or mud bottom, since they rely upon suction and cohesion of the bottom material,

•Cannot be used in rocky or coarse sand bottoms

•The holding power of this anchor is at best about twice its weight unless it becomes buried, when it can be as much as ten times its weight.

•They are available in sizes from about few kgs up to several tons.

Dead weight Anchors

• relies solely on being a heavy weight.

•It is usually just a large block of concrete or stone at the end of the chain.

•Its holding power is equal to its weight underwater (i.e. taking its buoyancy into account) regardless of the type of seabed, although suction can increase this if it becomes buried.

Dead weight Anchors

• Consequently deadweight anchors are used where mushroom anchors are unsuitable, for example in rock, gravel or coarse sand.

• An advantage of a deadweight anchor over a mushroom is that if it does become dragged, then it continues to provide its original holding force.

•The disadvantage of using deadweight anchors in conditions where a mushroom anchor could be used is that it needs to be around ten times the weight of the equivalent mushroom anchor..

AnchoringFour primary questions to be considered before actually anchoring:

Is the anchorage protected? Is the seabed good holding ground? What is the depth, tidal range, and the current tide state? Is there enough room?

Anchors

• Is the anchorage protected?•A good anchorage offers protection from the current weather conditions, and will also offer protection from the expected weather.

•Is the seabed good holding ground?•You should have charts to indicate the kind of bottom, as well as a tool to collect a sample from the bottom.

Anchors• most anchors will hold well in sandy mud, mud and clay, or firm sand.

• Loose sand and soft mud are not desirable bottoms, and especially soft mud which should be avoided if at all possible.

• Rock, coral, and shale prevent anchors from digging in, although some anchors are designed to hook into such a bottom.

• Grassy bottoms may be good holding, but only if the anchor can penetrate the bottom.

Anchors• What is the depth, tidal range, and the current tide state?•If your anchorage is affected by tide, you need to know the tide range and the times of high and low water.

• You need enough depth for your vessel throughout the range it might swing, at low tide, not just where you drop the anchor.•

Anchors• Is there enough room?•If your anchorage is affected by tide, you should keep in mind that the swing range will be larger at low tide than at high tide.

•No matter where you anchor you need to consider what the largest possible swing range will be, and what obstacles and hazards might be within that range.

•Keep in mind that other vessels in the anchorage may have a swing range which can overlap yours.

Anchors

• Boats on permanent moorings, or shorter scope, may not swing as far as you expect them to, or may swing either more rapidly or more slowly than your vessel (all-chain cables tend to swing more slowly than all-rope or chain-and-rope cables.)

•There are techniques of anchoring to limit the swing of a vessel if the anchorage has limited room.•

Anchors• To limit the vertical and lateral movements of• floating structures• submerged structures – positively buoyant, neutrally buoyant• For small instrument packages (buoys) which are used to monitor wave kinematics, water properties –salinity, density, temp or any other aspects at air-sea interface• For large buoys – data collection, navigational purposes• For restraining the movement of oil and gas pipe lines laid on the sea-floor

AnchorsMODU – Mobile offshore drilling unit

FPSO- Floating production storage/ offloading

UPTO 1000 mChain and wire ropeWeight of rope is a problem for > 1000m

> 1000 m , synthetic ropes

Anchors - Requirements• Should provide enough holding power• Minimum – size and weight – easy handling• Predominant forces influencing design

nature of sea bottom (clay or sand)the bottom slopedirection and intensity of mooring

line tension

Anchors - Requirements• Length of the cable should be aptToo short – intermittent submergence or

pull out of anchor

Too long – permit excessive movement , possible kinking and fouling, subsequently weakening the cable between the anchor and structure

Anchors – Holding power

The pulling force that the anchor can resist depends on• Depth of embedment• Submerged weight of the anchor•Angle that the cable make with the sea-bottom•Soil properties• For fluke anchor, the angle subtended by the anchor fluke , sea floor and the anchor shank

Anchors – Capability index• Holding power of an anchor : HP• Weight in air: W

Capability Index = HP / W

Mass anchors or Dead weight anchors

• Relatively inexpensive• Old car motors• Cast iron clumps• Welded (worn-out/ rejected) rails• Old rail-road car wheels (trains)• Bundles of surplus anchor chain• Concrete boxes filled with tremieconcrete and covered with rip-rap•

• Excellent resistance to vertical pull•Used in subsurface buoys set in small or medium currents•Whenever horizontal pull is expected to be small•Vertical holding power < weight in air• Because, holding power depends only on submerged weight

Mass anchors or Dead weight anchors

Mass Anchors – Holding power• Depends on

Cohesive soil -clay – adhesion between the contact of the anchor and the sea floor

Cohesionless soil-sand – friction between the contact of the anchor and the sea floor

Anchors - Mass anchors • Anchors weight required to resist the anticipated mooring tension(T) is inversely proportional to coefficient of friction•For same T, more the friction less the anchor wt•For same T, less the friction more the anchor wt

Mass Anchors• Less holding capacity on steeper slopes•Possibility of soft and weak soils• These soil fail under massive anchor weight •This in turn can fail the total anchorage system due to excessive tension

Mass Anchors – Application• soils which are not very weak and soft•Tension to be essentially vertical

Mushroom Anchors• Anchors get embedded by their own weight•Sometimes jetted to the desired depth•Used for permanent mooring in muddy bottoms

Mushroom Anchors• As the anchor oscillates in strain, mud fills the cup of the mushroom•Thus weight increases, and gets buried itself deeper•Holding power depends on

massresistance of soil above it

Drag Anchors

• These are primarily meant for ships• Designed to dig into the sea-floor as they are dragged along by a horizontal force• In this way they get embedded deeply and firmly in the bottom soil

Drag Anchors

• PartsFlukeStockCrownShankShackle

Drag Anchors - Fluke•Flukes are the plates which plough into the sea bottom and create resistance to lateral pull•Crown trips and maintain the fluke at the proper fluke angle (angle between fluke and shank)• Shank transmits the pull of the mooring line to the anchor•Stock prevents the anchor from rotating on itself

Fluke Anchors

• Typical fluke angle•30 to 35 degrees for sand bottoms•50 degrees for mud bottoms

Drag/Embedment Anchors

• These combinelight weightlarge resistance to lateral pull

Drag/Embedment Anchors -Disadvantages

• Necessary to drag the anchor along the bottom before it can dig and hold•Tendency for the anchor to pull out of the bottom if a vertical force is applied at the end of the shank•Susceptibility to pullout if torque is transmitted through the cable to the shank

Anchors

Pile Anchors

• Pre cast concrete piles•Drilled cast-in-place concrete piles•Steel piles

Suction Embedded Anchors

• To anchor floating exploration and production platforms•Soft cohesive soil•Used in deep water•Installed in water depths of 40m to 2500m•Diameter 3.5m to 7 m•Penetration upto 20m

Suction embedded Anchors

•

Suction embedded Anchors• A suction caisson anchor is a large diameter, cylinder (either steel or concrete) open-ended at the bottom and closed at the top.• Mooring loads are applied by an anchor line attached to the side of the caisson.• The length to diameter ratio of the caisson is typically six or less.• Once installed, the caisson acts much like a short rigid pile and is capable of resisting both lateral and axial load.

Suction embedded Anchors•The suction caisson gets its name from the fact that it is usually installed by applying under-pressure (“suction”) to its interior after it is allowed to penetrate under its own weight. •Since the caisson’s interior is sealed from the seafloor by the soil, vertical loading creates an internal draw-down pressure which in turn mobilizes the end bearing resistance of the soil at the caisson tip. •Of particular interest to operators and contractors is an assessment of the bias and uncertainty in predicting the anchor’s installation performance and holding capacity.

SEA BED ANCHORS

Sea bed anchors are required for the following

• To limit the vertical and lateral movements of floating structures and of positively or neutrally buoyant submerged structures.

• For small instrument packages which are used to monitor various aspects of the water body or the air-sea surface

• For large buoys which are used for navigational purposes or for data collection

• For restraining the movement of oil and gas pile lines laid on the sea floor

• For resisting uplift or overturning movements of offshore structures. (Ex: Tension leg platform requires very high uplift resistance anchors, in the order of 50,000 tonnes, for its stability).

Requirements of Sea Bed Anchors

(i) enough holding power and at the same time(ii) their size and weight must be kept to the

minimum for handling purposes. (iii) The nature of sea bottom, whether clayey or

sandy, the bottom slope, and the direction and intensity of mooring line tension at the anchor of anchors.

(iv) The cable length is very important too short cable can cause intermittent submergence of the structures or pull out of the anchor.

(v) Too long a cable might permit excessive movement of the structure and possible kinking and fouling, with subsequent weakening of the cable between the structure and the anchor.

TYPES OF SEABED ANCHORS

• Mass anchor or dead weight anchor• Mushroom anchor • Drag or Fluke anchor• Pile anchor

Holding Power of an Anchor

The holding power of an anchor with respect to horizontal and vertical tractions is the pulling force that an anchor can resist.It depends on the followingDepth of embedmentSubmerged weight of the anchorAngle that the cable makes with the sea bottomFor fluked anchors, the angle subtended by the anchor fluke, the sea floor and the anchor shankSoil properties at the particular site

MASS ANCHORS OR DEAD WEIGHT ANCHORS

Relatively inexpensive masses such as those given below are used as Mass Anchors.

• Old car motors• Cast iron clumps• Welded (rejected) rails• Old rail road car wheels• Bundles of surplus anchor chain• Concrete boxes filled with tremie

concrete and covered with rip rap.

Advantages• Because these anchors are relatively inexpensive, they

are expendable. In addition, they are also easy to fabricate.

• This type of anchors provide excellent resistance to vertical pull and are often used for subsurface buoys set in small or moderate currents or whenever the horizontal pull at the anchor is expected to be small.

Limitations

• The vertical holding power is less than their weight in air because the holding power depends almost solely on its submerged weight.

• The horizontal holding power depends on the adhesion (in the case of cohesive soils like clays) and friction (in the case non-cohesive soils like sands and gravels) between the contact of the anchor and the sea floor

• The application of mass or dead weight anchors is generally restricted to situations where the tension is expected to be essentially vertical and for sea bottom soils which are not weak and very soft.

• Apart from their lesser holding power on sea bottoms of steeper slopes, there is a possibility of soft and weak marine soils failing under the weight of a massive anchor; this is turn may fail the total anchorage system due to excessive tension on the cable, placing the anchored system in jeopardy.

MUSHROOM ANCHORS• These anchors get embedded by the force of their

own weight. Sometimes he anchor is jetted to the desired depth.

• It is generally used for permanent mooring in muddy bottoms.

• As the anchor oscillates under strain, mud fills the cup of the mushroom and the anchor buries itself deeper.

• Its holding power depends upon its mass and the resistance of the soil above it.

• The flukes are the plates which plough into the sea bottom and create resistance to lateral pull.

• The crown trips and maintains the flukes at the proper fluke angle (angle between the flukes and the shank).

• The shank transmits the pull of the mooring line to the anchor.

• The stock prevents the anchors from rotating on itself.

Quick and firm anchor embedment depends on a proper selection of the fluke angle. Typical values of fluke angles are 30° to 35° for sand bottoms and upto 50° for mud bottoms.

The rugged stockless anchor is suitable for a variety of sea bottoms. Its design combines (i) weight for holding in hard bottom with (ii) ability of biting and digging in softer sea floors.

The light weight, the Danforth, the Stato and the Boss anchors different versions of light weight anchors with large flukes. These anchors perform well in sand, clay, and mud as long as the pull is horizontal.

The relative inability to resist vertical forces, a length of chain (several shots ∞ of heavy chain) is usually placed between the anchor shank and the cable to help keep the shank horizontal. In addition, the scope ∗ is made as large as possible (say about 7m or more) to keep the chain, and the shank of the anchor lying on the sea floor.

Average Anchor Holding Power

Bottom TypeAnchor type Sand `Mud

Stockless* 6 2 or less

Lightweight* 16 9

Stato* 20 15

Boss** 35

* Smith, 1965 ** Beck 1973

Drag anchors is advantageous as it has light weight and large resistance to lateral pull.They suffer from the following disadvantages:

• necessity to drag the anchor along the bottom before it can dig and hold.

• Tendency for anchor to pull out of the bottom if a vertical force is applied at the end of the shank.

• Susceptibility to pull out if torque is transmitted through the cable to the shank.

PILE ANCHORSPile Anchor uses precast concrete piles,

drilled cast-in-place concrete piles and steel piles.

SPECIAL ANCHORSEmbedment Anchor with Movable FlukesIt penetrates the sea floor by (a) free fall(b) by being driven

(c) by use of explosive propellants. The flukes are forced into an open position

by either an upward lift or by a downward drive on the anchor after penetration is attained.

The units are placed vertically with a swivel arrangement between the anchors and the chain which assists in the transfer of only vertical forces to the anchor.

Anchors to Resist Pull at a Large Angle form the Bottom

Mooring lines forced by strong currents or buoy system configuration exert their pull at a large angle from the bottom. In such cases, the anchor must resist both vertical and horizontal components of the pull. The anchor should therefore combine dead weight and lateral resistance.

• Clump and embedment anchor• Heavy anchors equipped with flukes (porcupine

design)• Heavy anchors that tilt and dig when pulled by

horizontal forces (Stimson anchor)• Lengths of heavy chain.