FPSO Mooring System

MOORING SYSTEMS

10 INTRODUCTION

Mooring systems are common to all FPSOrsquos and FPVrsquos whatever their type mono-hulls semi-submersibles deep draught semi-submersibles or spars

Mooring systems for FPSOrsquos have evolved initially from the components and technology already available in the Marine Industry ie the conventional chains winches and anchors However the technology has evolved very rapidly to satisfy the specific requirements of offshore applications more severe environments larger vessels longer design lives deeper waters

20 MOORING SYSTEMS BASICS21 RESTORING FORCES

The primary purpose of a mooring system is to maintain a floating structure on station within a specified tolerance typically based on an offset limit determined from the configuration of the risers The mooring system provides a restoring force that acts against the environmental forces which want to push the unit off station In the following diagrams the main components of mooring system restoring force are explained

The connection between the mooring system and the body of the vessel is where the restoring force of the mooring system acts At this connection point there are two force components present horizontal and vertical The horizontal component of the mooring linersquos tension acts as a restoring force The vertical component acts as a vertical weight on the vessel In deep water the vertical force can be quite considerable For some designs of FPS with limited payload capacity the vertical mooring force can have significant design implications

It is informative to understand the significance of the mooring line angle as it departs the point of connection to the vessel A low angle to the vertical will generate a low restoring force with significant vertical load on the vessel If the angle here is large then the restoring force will be increased while the vertical load on the vessel will be reduced

The tensions in a mooring line are split into two components the restoring force that opposes the environmental loading and the lateral force which may balanced by another mooring line

22 ENVIRONMENTAL LOADING

When there is no external loading on the system the vessel will not move from its static equilibrium position When environmental loading does occur an imbalance in the system will occur To restore equilibrium the mooring system restoring force must become equal to that of the environmental load This is achieved through the vessel offsetting from its original position As this occurs the lsquowindwardrsquo lines will pick up tension and the lsquoleewardrsquo lines will shed tension

The vessel will offset until the lsquowindwardrsquo lines have generated a restoring force that balances the environmental loading This means that the distance between the anchor and fairlead will increase and thus the tension at the fairlead will also increase

30 MOORING SYSTEM TYPES

There are two main types of mooring systems Spread Mooring and Single Point Mooring (SPM)

31 SPREAD MOORING

This conventional mooring approach is widely adopted for semi-submersible production units For floating production applications spread moorings are used primarily with semi-submersibles and non-weathervaning FPSOs Since the wave loading on a semi-submersible is relatively insensitive to direction a spread mooring system can be designed to hold a semi on location regardless of the direction of the environment although there is probably an optimum heading However a spread system can also be applied to ship-shaped vessels which are more sensitive to environmental directions as long as the environmental conditions are relatively benign and the weather direction is fairly uniform without strong cross currents In a location such as the North Sea the forces which can

be generated on the beam of a spread moored FPSO plus the motions in such conditions effectively prohibit such a mooring arrangementThe mooring lines can be chain wire rope fibre rope or a combination of the three Either conventional drag anchors or anchor piles can be used to terminate the mooring lines

Spread moorings are typically cheaper than turret moorings since they are mechanically far less complicated However they are limited to where they can be used and they can make offloading operations by a shuttle tanker somewhat more involved

32 SINGLE POINT MOORING

Single point moorings (SPMs) such as internal or external turrets are used primarily for ship shaped units They allow the vessel to weathervane which is necessary to minimise environmental loads on the vessel by heading into the prevailing weather There is a wide variety in the design of SPMs but they all perform essentially the same function

The main types in increasing order of environment severity are

Fixed towerA fixed tower is suitable for shallow water depths (20 ndash 50 m) and small wave heights (about 5 m significant) It can be connected to the floating vessel by a simple hawser However to avoid the risk of extensive damage in the event of a minor collision between the tower and the FPSO the hawser is usually replaced by a yoke and pendulum system Fixed tower mooring systems are common in the shallow water locations of a number of Chinese FPSO Developments for example the Bohai Bay area with the new ALP ndash SYS system as described below

CALM buoy (Catenary Anchor Leg Mooring)A CALM buoy is suitable over a wider range of water depths (30 ndash 150 m) and larger wave heights (up to 8 m significant) It can be connected to the floating vessel by a yoke and pendulum system similar to that for the fixed tower or by a rigid arm that is hinged or rigidly connected to the buoy and hinged to the vessel

SALM (Single Anchor Leg Mooring)A SALM is generally a column hinged at the sea bed and connected to the floating vessel by a rigid arm or yoke hinged at both ends The buoyancy can be provided in the upper part of the column itself this is the conventional SALM

Internal turretAn external turret eliminates the CALM buoy and allows the turntable and swivels to be directly attached to the vessel bow or stern It is suitable for deep waters and large wave heights It can be used up to the point where the combined heave and pitch motions may cause slamming on the bottom of the turret (depending on vessel size and length up to approximately 12 m significant wave height)

External turretAn internal turret is convenient when a large number of risers are to be installed and therefore a large turret and swivel assembly are required An internal position also reduces the risk of slamming due to the reduction of the effect of pitch Consequently internal turrets can be used in deep waters and the most severe environments (up to 18 m significant wave height)

33 CATENARY AND TAUT MOORING SYSTEM

Two main types of mooring system can be used for either the Spread or Single Point system Taut-Leg and Catenary Both methods allow the system to withstand the applied forces but through different mechanisms

A lsquocatenaryrsquo system generates restoring force through the lifting and lowering of the line onto the seabed plus a limited amount of line stretch

A lsquotaut-legrsquo system makes use of the material properties of the mooring line namely its elasticity

34 PASSIVE ACTIVE SYSTEMS AND USE OF THRUSTERS

Mooring systems can also be classified as passive or active

In a passive system 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

In an active system Active winching can be undertaken on FPSOs and FPVs There are two basic options namely

1 Leeward line slackening2 All round length adjustment including windward lines so that the tensions are aswell balanced as possible at the limit of vessel surge

If the leeward lines are slackened down too much this can result in greater yawingsurging and reduced direction control which can lead to higher line tensions In other words if there is too much slack in the system there is an increased danger of high line snatch loadings

The disadvantage of an active system is that individual tensioners are required for each mooring line this involves additional equipment and therefore additional payload and cost

Thruster AssistanceA number of semi-submersibles and some FPSOs are equipped with thruster assistance A4163-01 It has been found that operation of the thrusters can be very effective in reducing peak line tensions even though the thrust delivered can be modest Typically in a mooring analysis the thrusters are considered to reduce the mean load applied to the mooring system However thrusters also seem to damp down the magnitude of the slow drift second order offsets They can also be helpful with respect to heading control This can be particularly useful on a production vessel if a small change in heading can result in reduced vessel motions thus improving the efficiency of the oilwater separation process

In practical terms when operating in manual thruster mode high line snatch loads can be avoided by applying thrust as the wave train approaches This will tend to push the vessel in the direction of the advancing sea As the wave passes it is necessary to ease down on the thrust to avoid over slackening the windward lines If these become too slack there is an increased danger of snatch loading when the next wave train passes through

40 MOORING SYSTEM DESIGN41 VESSEL DYNAMICS

Waves will cause a vessel to move in all six degrees of freedom surge sway heave roll pitch and yaw

The motion of the vessel to individual waves is called its wave frequency or first-order response As a mooring line moves through the water it will be subject to dynamic line drag and inertia loading and sometimes a whipping effect It is possible to take this into account by undertaking a dynamic mooring analysis but this does increase computing time significantly

The compliance of a mooring system is such that conventionally the presence of the mooring system is not considered to affect the wave frequency response The overall mooring system stiffness and associated natural frequency will influence its second order or low frequency slow drift response

In deep water for certain floating objects such as deep draft Spars the wave frequency motion is attenuated to a certain extent by the mooring system due to the higher system stiffness Hence a coupled analysis is sometime undertaken The general conclusion from this type of analysis appears to be that the mooring quasi-static tension has an impact on a floaters wave frequency response which in turn will affect the mooring dynamic tension On the other hand the effect of dynamic tension is less important to a floaters wave frequency response For deep water the effect of risers on the vessel response becomes increasingly important and this should be taken into account

The coupled wave frequency motion of a floater can be calculated in the time domain using the wave force wave frequency added mass and damping and mooring force at each time step Usually a convolution method needs to be adopted in the radiation force calculation Although the coupled wave

frequency motion calculation in the time domain is slower than the Response Amplitude Operator (RAO) based wave frequency motion calculation it is still acceptable 42 MOORING DESIGN

The development of the mooring system will require a number of inputs including -

FIELD LAYOUT DATA- Water Depth- Mooring Lines Layout Pattern- Location of Other Structures on the Seabed

ENVIRONMENTAL DATA- Wind Waves Current etc- (Operational and Survival Conditions)

VESSEL RESPONSE DATA- Response Amplitude Operators (RAOs)- Maximum Excursions from Static Offset

SEABED SOIL DATA- Soil Type and Performance Data- Seabed Friction

MOORING LINE DATA- Length of Line Segment- Axial Stiffness- Weight in Air and Water- Drag Coefficients- Breaking Load- Final attachment type

MOORING LAYOUTThe mooring layout should be designed to distribute the loads in the individual lines as equally as possible and also to give sufficient redundancy to the overall system The important factors are

The strength of each line 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 or 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 (eg well workover)

Note that mooring systems are often symmetrical but they donrsquot have to be For specific environmental conditions asymmetric systems may be more effective

ENVIRONMENTAL DATAMooring systems are normally designed for the 100-year storm conditions ie for the combination(s) of wave height wind and current velocities which are likely to occur once in a 100 years These conditions are established by extreme value analysis and extrapolation based on environmental data measured over a sufficient length of time

Typical values of waves in 100-year storms are

100-YearSignificant Wave Height

Associated WavePeriod

(seconds)West of Shetland 18 20Northern North Sea 16 17Gulf of Mexico 13 16Philippines 11 15Brazil 7 14

West Africa 4 17

43 ANALYSIS METHODS

The tensions experienced by a mooring system at any time are driven by the following

1048696 Static component from Wind Mean Wave Drift and Current

1048696 Wave frequency component caused by 1st order wave frequency motions anddraginertia effects on the line

1048696 Low frequency component due to 2nd order low frequency waves and wind dynamics

In addition in deep waters (beyond 300 m) the dynamic response of the lines themselves may also need to be taken into account

The essence of mooring design is to optimise the behaviour of the mooring system such that the excursions of the surface vessel do not exceed the allowable flexible riser offsets while at the same time ensuring that the line tensions are within their allowable values Thus the mooring system load offset curve should not be too hard or too soft Hence considerable iteration work may be required to optimise a system for a particular locationA4163-01The results obtained are the tensions in the mooring lines and the excursions of the FPSO vessel

The tensions are then checked against factors of safety specified by the regulatory authorities such as Lloyds Register (LR) American Bureau of Shipping (ABS) Det Norsk Veritas (DNV) etc The factors apply for different design conditions as shown below

Condition DNVAll intactOperational

All intactSurvival

One line damagedOperational

One line damagedSurvival

It is worth noting that spring buoys (mid water buoys) and clump weights can also be used to obtain an optimised mooring system stiffness by extending the resistive forces over greater distances hence allowing clearance over subsea features

44 METOCEAN PARAMETERS AND THEIR IMPACT ON MOORING INTEGRITY

For relatively benign environments such as off West Africa there is a much smaller difference between operational and survival sea states compared to say the North Sea This means that if the metocean parameters or the response of the vessel due to these parameters is underestimated there is significantly less of an in built safety margin compared to harsher climates particularly with regard to fatigue

The degree of spreading of the waves can also affect mooring analysis results The geographic area and fetch distance will influence the type of waves likely to be encountered in practice Conventionally short crested seas are considered to result in reduced wave frequency response and hence reduced mooring line tensions A4163-01 45 ROGUE STEEP BREAKING WAVES AND SHOCK LOADING

Mariners have used phrases such as ldquoFreak Waves Rogue Waves Walls of Water or even Holes in the Seardquo to describe some of the conditions they have experienced at sea Trading vessels are typically weather routed to avoid the worst of predicted weather conditions However permanently moored FPSOs and FPVs have to ride out whatever weather is thrown at them

From a statistical sense the longer the Floater is on station the more likely it is to experience 100 year + conditions If an elderly Vessel with a mooring system which has seen wear corrosion and has accumulated some hair line cracks is subject to such conditions the likelihood of single or even multiple line failure is increased

Very occasionally an unusually steep wave slam load could occur at the same time that a floating structure is around its maximum slow drift offset The resulting shock or spike load on the mooring might be quite considerable How much this shock loading is transferred to the mooring lines will depend to a significant extent on the degree of structural damping in the hull structure the vessel inertia how long the load acts and where the moorings are relative to where the wave impacts For a semi where you might get wave slamslap right into one of the corners the amount of structural damping might well be less than compared say to a FPSO with an internal turret Hence the loading could be higher

In November 1998 the Schiehallion FPSO was struck by a wave which was felt throughout the vessel The wave caused tears in the forward shell plating of the forecastle superstructure buckling of supporting stiffeners and permanent deformation of the forecastle lsquotween deck Production was shut down and non essential personnel were evacuated to a nearby drilling rig In this instance no damage was reported to the mooring system but it illustrates the danger presented by infrequent steep breaking waves 46 INTEGRATED MOORING AND RISER SYSTEMS DESIGN

Traditionally the design of mooring and riser systems for a FPSO assumes uncoupled behaviour of the two systems and each system is analysed independently The motion of the FPSO is calculated taking into account the mooring system only the motion obtained is then imposed as an input to the design of the riser system

However it is not uncommon for a FPSO to support as many as 40 or even 75 risers of different diameters (eg several applications offshore Brazil) With so many risers the stiffness and damping contribution from the risers can no longer be neglected and a fully integrated analysis of the mooring and riser systems becomes necessary The important difference between a mooring line and a riser is that a riser has bending stiffness whereas a mooring line does not

The effect of including the risers is primarily to increase the damping of the system The system stiffness is also increased with the water depth number and size of the risers

47 MOORING ANALYSIS CALIBRATION WITH FULL SCALE BEHAVIOUR

The determination of maximum tensions for a multiple line system requires application of specialist computer programmes which in many cases have been under continuous development for a number of years Despite this there are still uncertainties in estimating mooring loads using analysis software and model tests Hence it would be desirable to compare the behaviour of a full scale FPSO FPV in known weather conditions versus predictions A4163

50 MOORING SYSTEM COMPONENTS

The main components of mooring systems from the points of attachment of the mooring lines on the FPSO to the seabed are

Winches Fairleads Mooring lines (chains wire ropes or hybrid materials) Surface or submerged buoys Clump weights Anchor system (drag embedment gravity pile or suction anchors)

51 WINCHES AND FAIRLEADS

When reaching the FPSO the mooring lines are guided through fairleads which can be either sheaves (pulleys) or bending shoes The sheaves can handle both chain and wire rope whilst the bending shoe is designed for wire rope only and is coated with a special high density nylon bearing material to reduce friction

The payout haul-in and tensioning of a mooring chain are normally accomplished with a rotary winch (windlass) or a chain jack

The rotary winch is equipped with a gypsy wheel which meshes with the chain links As the gypsy wheel rotates the chain is drawn over the wheel and lowered into the chain lockerThe chain jack works by pulling a length of chain engaging a stop retracting and then repeating the process

For a wire rope a drum winch is commonly used for handling The drum has special grooves on it sized to match the specific rope diameter and the whole rope is progressively coiled on the drum Because the rope overlays after the first layers damage can be caused to the rope at high tension

There are two alternatives a linear winch and a traction winchThe linear winch works in an analogous manner to the chain jackThe traction winch consists of a drum on which the rope makes a few wraps and a take-up reel on which the rope is coiled This system can handle high tensions and long wire ropes without loss of pull capacity which is a problem with the drum winch

52 MOORING LINES

In floating production systems chain and wire are the most commonly used mooring line materials chain and wire are also often used in combination

Chain provides weight and therefore stiffness through the catenary effect whilst the wire rope provides greater elasticity and therefore compliance at high tension levels

Thus the combination of chain and wire rope provides optimal performance in a wide range of water depths In shallow waters (less than 100 m) the use of heavy chain through the water column provides the initial high catenary stiffness and the use of wire rope (sheathed in this case) on the seabed provides the compliance at high tensions In deeper waters the use of wire rope through the water column helps to reduce the vertical loads and the use of chain at the touchdown point provides the stiffness required At the fairleads on the FPSO chains are often preferred to avoid bending loads and also for easier handling

As offshore applications go into deeper waters (beyond 1000 m) man-made fibres (eg Kevlar polyester and polyethylene) become beneficial because of their superior strength to weight ratios

ChainsThere are two types of chains studded and studless

Studded chains are commonly used in the marine industry because they are easy to handle and can be stored in chain compartments without risk of becoming tangled up However during use the studs tend to get loose and the seat of the studs is often the initiation point for fatigue cracking

For long term application of chains in floating production systems the studless chain has been developed It features smaller end diameters to reduce the bending loads consequently it is less susceptible to fatigue

As a chain is a series system it is only as strong as its weakest link Thus there is a need for a high degree of uniformity of chain link in order to equate link strength with chain strength

A number of different grades of chain are currently in use in the offshore industry The grades are distinguished by the different yield strengths of their steel which are themselves dependent on the steel quality and the heat treatment The most commonly used grades are ORQ (Oil Rig Quality Grade 3 and Grade 4) The main mechanical properties of these grades are given below

Chain Grade ORQ 3 3S 4Yield Strength(N mm2)

- 410 490 580

Ultimate Tensile Strength (N mm2)

641 690 770 860

Chains are currently made in sizes up to 7rdquo diameter cross section in grade 4

Wire RopesThere are three types of wire ropes spiral strand six-strand and multi-strand

Spiral strand consists of concentric helical layers of wire The layers have alternately opposite helical directions such that the wire rope is more or less torque balanced to minimise any axial torque generated by tensile load

Six-strand consists of six (or eight strand) ropes manufactured individually then twisted together over a core to make the rope In general the strands themselves are not torque balanced but the helical direction of the wires in the strand are different from that of the strands in the rope in order to achieve some torque balance

Multi-strand consists of two or more layers of strands the direction and lay of which are selected to achieve a maximum degree of torque balance

Spiral strand ropes are more commonly employed for floating production systems where their greater longitudinal stiffness torque balance and ability to be coated in a polyethylene sheath makes them more suitable for long term installation

Six-strand ropes with independent wire rope core (IWRC) are most commonly used for mobile drilling units due to their lateral flexibility and relative cheapness Multi-strand ropes are not common offshore although they are used for crane ropes and diving bell hoist ropes

The breaking strength is dependent upon the construction of the rope as well as the grade of steel used It is generally better known than that of a chain because a segment of the wire rope is usually tested to destruction and the load at failure is recorded in the certificate of the wire rope

Hybrid Systems (combinations of Chain and Wire Rope)In support of cost efficiency it is common to use a combination of chain and wire rope A typical arrangement for a shallow water FPSO (ie 120m water depth) might be -

1st section from Turret = 275m chain Portion through Water Column = 100m wire rope Touch Down section = 300m heavy weight chain Final Touch Down Section = 270m light weight chain Pile end

Man-made Fibre RopesThe technology of man-made fibre ropes is at the development stage although it is advancing rapidlyThe materials mostly used for fibre ropes are polyesters aramids high modulus polyethylene (HMPE) and polyesters

The densities of these materials are close to unity (098-099 for HMPE 138-140 for polyester and aramid) and therefore fibre ropes are almost neutrally buoyant

The axial stiffness of fibre ropes is a more critical parameter than that of either chain or wire rope because the stiffness is mostly contributed by axial stretch The stiffness is not constant and is influenced by load level range frequency and history For example there is a factor of two or more between stiffness at installation and stiffness of a worked rope

The fatigue life of fibre ropes is influenced by creep abrasion and external wear Under constant load conditions the fibres will creep (ie load backs off) This means the lines must be re-tensioned periodically

53 BUOYS AND CLUMP WEIGHTS

Submerged buoys or clump weights attached to the mooring lines can modify the stiffness characteristics of a mooring system

In deep waters the use of submerged buoys reduces the vertical load component and increases the horizontal restoring force It also reduces the footprint of the mooring pattern on the seabed

Conversely in shallow waters the use of clump weights increases the stiffness of the mooring system and may help avoiding the use of heavy chains

54 ANCHOR SYSTEMS Detailed site information is required for anchor system design This should include geophysical data (bathymetry) and geotechnical borehole data (soil conditions)

Soil conditions at the installation site can greatly affect the selection of an anchor system and soil investigations should include both the nature and the depth of the seabed material

A large variety of anchor systems is available they include Gravity anchors Conventional drag anchors Drag embedment anchors Suction anchors Pile anchors

Gravity anchors or dead-weight anchors are large gravity blocks which rely on their weight and some friction with the seabed to remain in position They come in various sizes and shapes and have been used successfully in different water depths By adding skirts or pin piles the friction force with the seabed can be increased substantially

Conventional drag anchors are commonly used in the marine industry worldwide They come in a wide range of designs and sizes applicable to a wide range of conditions

They are generally designed to remain near the sea bottom or lightly embedded into it One significant disadvantage of drag anchors is the limited resistance to uplift forces To avoid uplifting long lengths of mooring lines have to be deployed on the seabed thus increasing the overall costsMany are used in offshore applications

Drag Embedment anchors in contrast to conventional drag anchors are designed to penetrate deep into the soil This capability increases the holding capacity of anchors by factors up to 8 times A drag embedment anchor penetrates the soil when pulled during installation The distance of forward movement is the drag length When in use it may move again and penetrate further if the load applied is larger than a certain threshold

Two types of embedment anchors may be defined High holding power anchors Vertical loaded anchors

High holding power anchors are characterised by a plated body (the shank) and two plated wings (the fluke) The angle between the fluke and the shank can be adjusted to control the depth of penetration a small angle in sand and medium clay and a large angle in very soft clay and mud

These anchors normally resist horizontal forces at the seabed but can also take a substantial amount of uplift of the order of 20 degrees

Vertical loaded anchors (VLA) have been specifically designed for application with taut-leg mooring systems for which the mooring points have to accept both horizontal and vertical loads They are characterised by a large flat plate (replacing the fluke) and a system of wires (replacing the shank) They are installed with a horizontal load with respect to the mudline to obtain the deepest penetration possible By changing the point of pulling at the anchor a loading perpendicular to the plate is obtained thus mobilising the maximum possible soil resistance this load is of the order of two times the penetration load

High holding power anchors are widely used (nearly 50 of all FPSOs are moored by means of such anchors) Vertical loaded anchors are progressively introduced in deep water applications

Suction anchors are specifically designed to resist vertical loads in soft soils A typical suction anchor consists of a steel cylinder open ended at the bottom and closed at the top but incorporating a

cap fitted with ports for connection of a temporary pump The mooring line is pre-attached to a padeye on the side of the cylinder

Installation after contact with the seabed is initially carried out by self-penetration of the cylinder thereby creating a seal around its periphery Water is then pumped out from inside creating a hydrostatic under-pressure forcing the cylinder to penetrate further The cylinder can be demobilised by reversing the process

Such systems are common in West Africa where the soil types are light They have the advantage of being simple constructions (ldquogiving local contentrdquo) and can be deployed by low cost vessels

Pile anchors usually consist of pipe piles installed by either impact vibratory driving or by drilling and grouting The resistance of the pile depends primarily on pile dimensions soil strength and pile installation technique

Pile anchors are suitable for resisting horizontal and vertical forces in various soil conditions Installation by impact driving (hydraulic hammer) is generally feasible in clays and sands Vibratory driving is generally more suitable for sands than clays

The costs of installation are high because of the specific installation equipment required

60 MOORING SYSTEM INSTALLATION

Mooring lines and their anchor system are usually pre-laid before the arrival on site of the FPSO The vessels used for this operation are Anchor Handling Vessels (AHV) or workboats The equipment on the vessels will be specific to each application but will generally include winches rigging spreader bars storage reels for wire ropes etc

In the case of suction anchor installation two AHVs will be used one for launching the suction anchors the other for installing the chains and wire ropes

In the case of drag embedment anchors two AHVs may also be used one for lowering the anchors on the seabed one at a time the other for pre-tensioning them

Once the mooring lines have been installed each of their free ends is temporarily connected to a tethered buoy for easy retrieval

When the FPSO arrives on site the buoys are picked up one at a time and the mooring lines are connected to the winches on the FPSO Tensioning of the lines can then be effected according to a sequence established in advance

Installation activities generally involve several vessels working together over periods of several days or weeks Installation costs are therefore high and represent a large proportion of the overall mooring costs For this reason it is important to plan the various marine and installation activities in great detail and well in advance

Attachment of the chain to the seabed structure could be by the BALLGRAB system ldquoPlug in Systemrdquo

70 DEVELOPMENTS FOR DEEP WATER

In deep water the weight of the mooring lines can become excessive Not only the FPSO has to be designed to carry the extra weight of the lines but also the restoring forces become inefficient because the lines hang steeply down and provide little horizontal restraint

One solution is to attach submerged buoys to the lines to take part of the weight However in very deep water such as 1000 m to 3000 m this solution too becomes inefficient

Another solution is to use Taut Leg Moorings (TLMs) in which the lines are taut between the FPSO and the seabed and arrive at the seabed with angles of approximately 45 degrees

The requirements of TLMs are twofold Light weight lines Anchor systems which can resist vertical loads

Light weight lines can be achieved by using wire ropes andor man-made fibre ropesSteel wire ropes can be used to approximately 2000 m beyond this depth the tensions become excessive and they are difficult to deploy Man-made fibre ropes provide the means to moor up to 3000 m deep and beyondPolyester Ropes have been tested in both Brazil and the Gulf of Mexico in their respective Research Programmes Having been fully accepted they are being more widely used In Brazil more that twenty systems are in operation In the Gulf of Mexico the Mad Dog Spar is the first polyester rope mooring system to receive MMS approval and to de utilised there The use of Polyester rope involves the understanding of the Creep property of the material which requires the active re-tensioning of the mooring line in use

Anchor systems which can resist vertical loads are the vertical loaded anchors suction anchors and pile anchors

Suction anchors and pile anchors can be installed in deep waters Both suction pumps and hammers can be driven hydraulically from the surface down to approximately 900 m Beyond 900 m they need to be driven by subsea power pack or by ROV (Remotely Operated vehicle)

Pertobras has developed and tested a new type of pile system using a ldquoFree-Fallrdquo torpedo pile dropped from the surface The self penetration reduces the anchoring and installation costs and improves shot precision

71 THE MOORSPAR MOORING SYSTEM SBM Atlantia has proposed a new MoorSpar system a disconnectable mooring system that allows lower-cost higher-efficiency steel catenary risers (SCRrsquos) to be used in the development of deep and ultradeepwater fields

The MoorSpar unit consists of a truss-like structure set atop a long submerged slender buoy which is moored to the seafloor by a combination of vertical tethers and lateral polyester lines The FPSO and buoy are connected through an articulated yoke system terminated by female (on the yoke) and male (on the buoy) conical sections which match in dimension Situated at the top of the MoorSpar unit the male conical section is the structural link between a main roller bearing and a gimbal table (a heavy duty uni-joint at the tip of the yoke) This arrangement reportedly accommodates the vesselrsquos roll and pitch motions and also allows the FPSO to weathervane

SCRrsquos connect to the MoorSpar unit at riser porches located along the keel of the buoy The risers are then linked to internal piping which is routed up through the central column and then across hard piping before swiveling to the FPSO The high pressure rating can be reduced upstream of the swivel stack arrangement within the piping manifold located at the MoorSpar buoy top above the extreme wave crests Connecting the FPSO to the buoy with an articulated yoke allows us to filter out the FPSO heave motions The truss limits the wave loads and the wave-induced motions of the MoorSpar buoy and consequently the motions of the SCRrsquos porches are minimized The MoorSpar system can be used in water depths up to 3000m

In a severe weather event when the FPSO must be moved the yoke system is easily disconnected and goes with the vessel The buoy stays at the field location Once the weather event passes the FPSO can return to the field location and reconnect to the MoorSpar within a 4-6 hr period

Mooring systems are normally designed for the 100-year storm conditions ie for the combination(s) of wave height wind and current velocities which are likely to occur once in a 100 years These conditions are established by extreme value analysis and extrapolation based on environmental data measured over a sufficient length of time

Condition

All intact

One line damaged

46 INTEGRATED MOORING AND RISER SYSTEMS DESIGN

Chains

Wire Ropes

Man-made Fibre Ropes

be generated on the beam of a spread moored FPSO plus the motions in such conditions effectively prohibit such a mooring arrangementThe mooring lines can be chain wire rope fibre rope or a combination of the three Either conventional drag anchors or anchor piles can be used to terminate the mooring lines

Spread moorings are typically cheaper than turret moorings since they are mechanically far less complicated However they are limited to where they can be used and they can make offloading operations by a shuttle tanker somewhat more involved

32 SINGLE POINT MOORING

Single point moorings (SPMs) such as internal or external turrets are used primarily for ship shaped units They allow the vessel to weathervane which is necessary to minimise environmental loads on the vessel by heading into the prevailing weather There is a wide variety in the design of SPMs but they all perform essentially the same function

The main types in increasing order of environment severity are

Fixed towerA fixed tower is suitable for shallow water depths (20 ndash 50 m) and small wave heights (about 5 m significant) It can be connected to the floating vessel by a simple hawser However to avoid the risk of extensive damage in the event of a minor collision between the tower and the FPSO the hawser is usually replaced by a yoke and pendulum system Fixed tower mooring systems are common in the shallow water locations of a number of Chinese FPSO Developments for example the Bohai Bay area with the new ALP ndash SYS system as described below

CALM buoy (Catenary Anchor Leg Mooring)A CALM buoy is suitable over a wider range of water depths (30 ndash 150 m) and larger wave heights (up to 8 m significant) It can be connected to the floating vessel by a yoke and pendulum system similar to that for the fixed tower or by a rigid arm that is hinged or rigidly connected to the buoy and hinged to the vessel

SALM (Single Anchor Leg Mooring)A SALM is generally a column hinged at the sea bed and connected to the floating vessel by a rigid arm or yoke hinged at both ends The buoyancy can be provided in the upper part of the column itself this is the conventional SALM

Internal turretAn external turret eliminates the CALM buoy and allows the turntable and swivels to be directly attached to the vessel bow or stern It is suitable for deep waters and large wave heights It can be used up to the point where the combined heave and pitch motions may cause slamming on the bottom of the turret (depending on vessel size and length up to approximately 12 m significant wave height)

External turretAn internal turret is convenient when a large number of risers are to be installed and therefore a large turret and swivel assembly are required An internal position also reduces the risk of slamming due to the reduction of the effect of pitch Consequently internal turrets can be used in deep waters and the most severe environments (up to 18 m significant wave height)

33 CATENARY AND TAUT MOORING SYSTEM

Two main types of mooring system can be used for either the Spread or Single Point system Taut-Leg and Catenary Both methods allow the system to withstand the applied forces but through different mechanisms

A lsquocatenaryrsquo system generates restoring force through the lifting and lowering of the line onto the seabed plus a limited amount of line stretch

A lsquotaut-legrsquo system makes use of the material properties of the mooring line namely its elasticity

34 PASSIVE ACTIVE SYSTEMS AND USE OF THRUSTERS

West Africa 4 17

43 ANALYSIS METHODS

All intactSurvival

52 MOORING LINES

- 410 490 580

641 690 770 860

Condition

All intact

One line damaged

Chains

Wire Ropes

West Africa 4 17

43 ANALYSIS METHODS

All intactSurvival

52 MOORING LINES

- 410 490 580

641 690 770 860

Condition

All intact

One line damaged

Chains

Wire Ropes

West Africa 4 17

43 ANALYSIS METHODS

All intactSurvival

52 MOORING LINES

- 410 490 580

641 690 770 860

Condition

All intact

One line damaged

Chains

Wire Ropes

West Africa 4 17

43 ANALYSIS METHODS

All intactSurvival

52 MOORING LINES

- 410 490 580

641 690 770 860

Condition

All intact

One line damaged

Chains

Wire Ropes

52 MOORING LINES

- 410 490 580

641 690 770 860

Condition

All intact

One line damaged

Chains

Wire Ropes

52 MOORING LINES

- 410 490 580

641 690 770 860

Condition

All intact

One line damaged

Chains

Wire Ropes

Condition

All intact

One line damaged

Chains

Wire Ropes

Condition

All intact

One line damaged

Chains

Wire Ropes

Condition

All intact

One line damaged

Chains

Wire Ropes

Condition

All intact

One line damaged

Chains

Wire Ropes

Documents

FPSO Mooring System