Mooring Design of Floating Platforms

CIVL 4171Pipeline, Platform and Subsea Technology

Mooring Design

1. Introduction

2. System Types

3. Mooring Components

4. Design Considerations

5. Design Criteria

6. Design Methods• Quasi – Static• Dynamic• Model Tests

Station Keeping System:

System Types:• Most floating facilities are designed to stay at a single location secured to

the sea floor by a purpose built mooring system• Some systems are designed to be disconnectable to allow escape from bad

weather such as cyclones (eg BHPB’s Griffin Venture)• DP & Thruster Assisted station keeping is also used, though much less

frequently

Hull type Vs. Wave period

Spar - Motions

• Spars are different from both Semis and TLPs in the mechanism of motion control.

• The centre of gravity (VCG) is lower then the centre of buoyancy (VCB) –unconditionally stable.

• The spar derives no stability from its mooring system.

• The deep draft is favourable for minimal heave motions esp. with heave plates.

• The hull natural period in heave & pitch is above the range of wave energy periods.

• The reduced heave & pitch motions permit the use of dry trees.

TLP Motions

• The vertical forces acting on the TLP must be in balance ie the fixed & variable loads + tendon tension equal its displacement.

• The hulls excessive buoyancy causes the tendons to always be in tension and restrains the platform in heave.

• The displacement of the hull and the tendon axial stiffness are chosen such that the vertical and angular natural periods are well below the wave excitation periods and the horizontal natural periods are well above the wave excitation periods.

• TLPs undergo ‘setdown’, as environmental forces cause an ‘offset’ displacement ie the draft increases as the platform is moved horizontally due to lateral loads thereby increasing the tendon tensions.

• The reduced heave & pitch motions permit the use of dry trees.

Semisubmersible -Motions

• Limited sensitivity to water depth• Trending to deeper draft to reduce

heave motions esp. in response to low wave periods (<8 seconds).

• Columns are sized to provide adequate waterplane area to support all anticipated loading conditions, spaced to support topsides modules, and tuned for a natural period of at least 20 seconds.

• These columns are supported by two parallel pontoons or a ring pontoon. Pontoons are sized to provide adequate buoyancy to support all weights and vertical loads, and proportioned to maximize heave damping.

• Taut or spread catenary mooring system.

Common FPS Configurations

Spread Moored FPSOSpar Platform CALM Buoy

Mooring

Tensioned Risers

Common FPS Configurations

Common FPS Configurations

Spread Moored Semi-Submersible

FPS Mooring Configurations

FPSO Turret Configurations

Bow Mounted External Turret Bow Mounted Internal Turret

Other Station Keeping Methods

Dynamic Positioning Single Anchor Leg Mooring (SALM)

Catenary Mooring Basics

• Loads on Floater:– Steady & fluctuating

wind– Wave & wave drift– Current

• Loads on Mooring lines:– Top end surge motions (small

heave)– Wave – Current– Sea-bed friction

Mooring Components

Basically the mooring system comprises of:• Chain• Wire• Synthetic line• Clump weights• Buoys• Hardware & Accessories• Anchor Point

Mooring Components -

Chain• Chain has proven durability offshore. • Several grades available (ORQ, K4, U3 etc.. depending upon

classification society)• Studlink & Studless (studless has greater strength & fatigue life, but

lower mass/m for a given size)• Corrosion & wear catered for by increasing diameter ~0.4mm/year

service allowance in splash zone & dip zone, ~0.2mm elsewhere.

Mooring Components -Wire

Wire• Greater restoring force for a given

pretension• Costs less per load capacity than chain but

doesn’t have the same restoring effect as weight is 40% or so.

• Wear issues due to abrasion• 6-strand, spiral strand, non-rotating

Spiral Strand – Advantages

•Higher Strength to Weight Ratio

•Higher Strength to Diameter Ratio

•Torsionally Balanced

•Higher Resistance to corrosion

Six Strand – Advantages

•Higher elasticity

•Greater Flexibility

•Lower Axial Stiffness

Mooring Components -Synthetic lines / Clump

Weights

• Synthetic lines:– Recent developments in ultra deep water used

them– Still in development phase for permanent

moorings

• Buoys– Reduces weight of mooring lines on system– reduced dynamics in deep water– increased hardware costs / complexity of

installation

• Clump Weights– sometimes used to improve performance or

reduce cost– used in ‘dip zone’ to increase restoring forces – added installation complexity

Mooring Components - Buoys / Connecting Hardware

• Connecting Hardware– shackles, swivels, link

plates

• Vessel Hardware

Mooring Components -Anchors

• Options:– Drag Embedment– Driven Piles– Suction Installed Piles– Gravity Anchors

• Choice based upon costs as well as system performance, soil conditions, reliability, installation & proof loading

Drag Anchor Types

Suction Anchors

Recap of the FPSO Design Overview

FUNCTIONAL REQUIREMENTS

-Mooring envelope-Allowable Motions -Allowable displacements.

SCHEME CONFIGURATION

-TLP, FPSO, Spar etc

HYDRODYNAMIC ANALYSIS

STRUCTURAL DESIGN-Strength -Fatigue

TURRET / MOORING INTERFACES

PRELIMINARY LAYOUT

METOCEAN DATA

MOORING ANALYSIS

REDESIGN & RERUN MOORING &

STRUCTURAL ANALYSIS

MOORING CONFIGURATION

MODEL TESTING

DESIGN & PERFORMANCE SATISFACTORY

FINAL DESIGN

ANCHOR POINT-Drag Anchors-Driven Pile-Suction Pile-Gravity Anchor

MOORING TYPE-Spread moored-Single point mooring-All Chain-Wire/ Chain/ Wire-Buoys / clump weights in-line

Topside Layout and structural support configuration

Functional Loads: Egcrude oil storage, production equipment etc.

N Y

This Section

Floating System Analysis

Main Methods of Analysis:

1. Simplified Quasi-Static Methods as per API RP 2SK-Suitable for preliminary design

2. Rigorous Analysis-Frequency Domain, Time Domain numerical solutions

Simplified Analysis

Simplified analysis is Quasi-Static – What does this mean?

• Dynamic wave loads are taken into account by statically offsetting the vessel by an appropriately defined induced wave motion

• Vertical fairlead motions and dynamic effects associated with mass, damping and fluid accelerations are neglected

• Research has shown this to be affected significantly by vessel, water depth, line configuration

• Simplicity has proven it useful & practical for preliminary studies

Rigorous Analysis – What’s the difference? • Vessel motions affect the dynamics of the mooring tensions. Eg acceleration

effects and loads as the mooring lines pass through the water• Typical analysis simplified in so much as the vessel motions are assumed to be

unaffected by the mooring lines – OK for water depths up to 500m or so.• Ultra deep mooring analysis requires that mooring effect on vessel motions

considered. In some cases even riser systems affect motions considerably

Floating System Analysis

Floating Structure Analysis

Prior to starting Mooring or Structure design, we need to work out how the vessel reacts to the environment.

How do we predict the response characteristics of the vessel?

Typically this work is performed by specialist engineers / naval architects.Work is performed using analysis or obtained from scale model tests

Design Criteria and Load Cases

(Environment, allow offsets etc)

Determine Environmental Effects1. Steady State Environmental

Forces2. Determine Low Frequency

Motions3. Determine Wave

Frequency Motions

Initial Mooring Pattern

Determine Mooring Tensions / Offsets

Criteria

Forces & O

ffsetsLoads &

FoS

Design Criteria / Arrangement

Primary Considerations:• Operations considerations

– Mooring / Riser interface = offset limitations (eg 10% - 20% water depth)

– Directional Offsets– Number of Risers / Heading

• Wire / Chain combinations depending upon mooring depth, loads etc…

• Pretension affected by allowable offsets

8 Leg Equispaced

3 x 3 System

Risers

Mooring

Design Cases

Basic Load Cases– Intact (all lines intact)– Damaged (one line broken)– Transient (motions after 1 line breaks)

Load cases have different Factors of Safety

Environmental Criteria

Environment = Principally wind, wave, current & tideKey aspects for mooring design are Extreme and Operating Environments

Extreme Environment:These conditions have a low probability of being exceeded within the design lifetime of the structure. Extreme environmental responses are likely to govern the design of a floating unit.Eg a 20 year design life system typically uses 100 year Return Period conditions. These have a probability of occurrence during the 20 year design life of about 20%

Environmental Criteria

Normal Environment:These conditions are those that are expected to occur frequently during the construction and service life.

Since different parameters and combinations affect various responses and limit operations differently (eg crane usage, installation etc) the designer should consider appropriate combinations for each situation.

EG On the Banff FPSO in the North Sea, the novel design exhibited significant roll in moderate seas. Basically the crew were getting seasick. Solution → add bilge keels to stabilise roll = £10m in expenses and lost revenue

Other ConditionsPhenomenon such as tsunamis, icebergs, solitons etc.. May also need consideration for a particular project

Forces and Motions

• Steady Forces: wind, current and wave drift are constant in magnitude for the duration of interest

• Low-Frequency cyclic loads can excite the platform at its natural periods in surge, sway and yaw. Typical natural periods are 60 to 180 seconds

• Wave-Frequency cyclic loads are large in magnitude and are a major contributor to member forces. Typical periods are between 5 and 20 seconds

Environmental forces / motions should be calculated at the following 3 distinct frequency bands to evaluate their effects on the system

Steady Forces : Wind

Steady Wind ForcesCalculated on each part of the FPSO by summing the contribution of different areas :

SHAPE Cs

Large Flat Surface (hull, deckhouse) 1.00

Exposed beams, girders 1.30

Isolated shapes (cranes, booms etc) 1.50

Clustered deck houses 1.10

Cylindrical 0.50

F = 0.5 ρair.A.Vz2. Cs (kN)

where,

Vz = 1 hour mean wind velocity at specified height z

Vz = Vh (z/H)0.125

Vh = reference wind speed at 10m height

A= projected area (m2)

Cs = Shape coefficient

ρair = 0.00125 Tonnes/m3

Area 2

Area 1

Area 3

OR - use method of API RP2SK

Steady Forces : Current

Steady Current ForcesForces on the hull of an FPSO can be estimated by the following equations:

WETTED SURFACE AREA = S

Force on Bow or Stern of FPSO’s:

Fcx = Ccx .S.Vc2 (kN)

where,

Vc = Design Current Speed (m/s)

S= Wetted surface area of the hull (m2)

Ccx = current force coefficient on the bow

= 0.00289 kNsec2/m4

Force on Beam of FPSO’s:Fcy = Ccy .S.Vc

2 (kN)where,Ccy = current force coefficient on the beam

= 0.07237 kNsec2/m4

Low Frequency Wave Forces & Motions

2.0 Chose Wave Height

3.0 Read off values of

# Low Freq. Single Amplitude Motion

1.0 Choose Vessel Length

4.0 Read off values

# Mean drift Force

SIMPLE METHOD → Calculate Wave Loading using tables in API RP 2SK.

Other Methods: Analytical Software & Model Tests

Adjust Low frequency motions based upon factoring mooring stiffness value from the graph by the ratio of : (nominal stiffness / actual stiffness)1/2

5.0 Adjust Values for

•Mooring Stiffness

•Significant & Maxima

Wave Frequency Forces & Motions

Wave-Induced Vessel Motion Responses

1st Order: Motions at wave frequencies (periods approx 5secs to 20 secs) that are obtained by computer analysis or model tests. These are the motions that we are all familiar with (eg roll, pitch, heave, surge, sway, yaw).

Vessel : Wave Frequency Response

Predicting 1st Order Response (1)

How do we find vessel response?

•The vessels response functions are called Response Amplitude Operators (RAO’s) and are different for all 6 degrees of freedom (surge, sway, heave, roll, pitch and yaw)

•That is, in simplified form:-Vessel Response = Fn( Seastate , RAO’s)

•Typical RAO’s for a 100m long vessel with heading 30 degrees to waves for roll, heave, pitch and surge are shown:

•Heave & Surge : metres motion/metre wave height•Pitch and Roll : degrees per metre wave height•(ie for 2m regular waves at 10 second period, roll is approx 5 degrees and heave is 1.8m)

RAO's : 30 degree heading

0

1

2

3

4

0 5 10 15 20 25 30

Period (seconds)

Am

plitu

de

Surge

Heave

Roll

Pitch

Vessel : Wave Frequency Response

Predicting 1st Order Response (2)3 main calculation methods

– Time domain– Frequency Domain – Model tests

FREQUENCY DOMAINThese methods are much simpler and less computationally intensive. Most of these methods use STRIP THEORY in which the vessels motions are treated as forced, damped, low amplitude sinusoidal motions.

– Vessel is divided into a number of transverse sections (or ‘strips’)– Hydrodynamic properties are computed assuming 2D inviscid flow with no interference

from upstream sections– Coefficients of the equations of motions may be found

TIME DOMAINTime Domain methods model the wave passing a hull. At small incremental steps the net force on the hull is calculated by integrating the water pressure and frictional forces on each part of the hull. Using Newton’s Second Law the acceleration on the hull is computed, then this is integrated over the time step to compute the new vessel velocity and position

>> Although procedure is relatively straight forward, these methods are not routinely used.– Software / Hardware advances are making this method more common:– Used for “non-standard” vessels such as Semi-submersibles & Spars

Examples of Software: AQWA , MOSES (Aquamarine),WAMIT (DnV)

AQWA Model

Diffracted Water Surface Contours

Vessel : Wave Frequency Response

MODEL TESTSStill used today – why?

Because it works!!! – basically numerical computation is good, but still needs work to be suitable

Predicting 1st Order Response (3)

Test 138

-1.000

-0.500

0.000

0.500

1.000

1.500

0.00 10.00 20.00 30.00 40.00 50.00 60.00

Time (secs)

Hea

ve &

Wav

e H

eigh

t (m

)

Wave Probe at Wall (CoG)Average

Quasi – Static Analysis:Mooring Tensions & Vessel Offset

Mean offset is defined as the vessel displacement due to the combination of current, mean wave drift and mean wind forces.

Maximum Offset is defined as mean offset plus appropriately combined wave frequency and low frequency vessel motions.

Mean “static”Offset

Steady Forces

“dynamic offset” +/-

Maximum Offset

Quasi – Static Analysis:Offset Definition

How do we calculate Maximum Offset?Let Smean = mean vessel offset

Smax = max vessel offsetSwfmax = max wave frequency motionSwfsig = significant wave freq. motionSlfmax = maximum low freq. motionSlfsig = significant low freq. motion

If Slfmax>Swfmax , then: Smax = Smean+ Slfmax+Swfsig

If Swfmax>Slfmax , then: Smax = Smean+ Swfmax+Slfsig

Note : it has been shown statistically that this method of combining wave frequency and low frequency motions defined in this manner would be exceeded on average once in every 3 hr storm. An alternative to this approach is a time domain simulation, usually several simulations performed with statistical establishment of maximums

Quasi – Static Analysis:Statistics of Peak Values

• Significant Value = 2 (RMS Value)• Max Value = Sqrt [2(ln N)] (RMS value)

where N = number of waves during the storm = T / TaT= specified storm period in seconds (usually 3hrs)Ta = average zero crossing period in secondseg for 3 hr storm, Tz=10seconds, Maximum =1.86

• Low frequency components - Ta can be taken as the natural period of the vessel Tn which can be estimated by:Tn = 2 π Sqrt (m/k)m= vessel displacementk = mooring system stiffness at mean position

Quasi – Static Analysis:Line Tension Definition

Mean Tension is defined as the tension corresponding to the mean offset of the vessel.

Maximum Tension is defined as mean tension plus appropriately combined wave frequency and low frequency tensions.Let Tmean = mean tension

Tmax = maximum tensionTwfmax = maximum wave frequency tensionTwfsig = significant wave frequency tensionTlfmax = maximum low frequency tensionTlfsig = significant low frequency tension

If Tlfmax>Twfmax , then: Tmax = Tmean+ Tlfmax+Twfsig

If Twfmax>Tlfmax , then: Tmax = Tmean+ Twfmax+Tlfsig

Quasi – Static Analysis:Line Tension Definition

Where do we get Mooring Tensions and Anchor Load from?

•Need to calculate force verses offset curves for the mooring system as a whole as well as individual line tensions.

•For most highly loaded lines, need to determine the suspended catenary distance

•Catenary calculations normally performed by software. Can be done by hand (see over for catenary formulae)

PREPARE GRAPH OF TENSION Vs OFFSET (& SUSPENDED LINE LENGTH)

Line Tension

Notation:

T- line tension (N)

h – water depth (m)

w – line weight in water (N/m)

• Catenary equation

• Maximum tension

• Suspended (Minimum) length

cosh 1h

h

T wxz hw T⎛ ⎞⎛ ⎞

+ = −⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

max hT T wh= +

maxmin 2 1Tl g

wh= −

Line Tension Definition

1

4

32

1. From Total force & vessel restoring force curve determine…

2. Mean offset3. Determine Smax as a function

of Low frequency & Wave frequency offsets

4. From Smax & Most loaded line tension force curve determine Maximum Mooring force

Anchor Load Definition

Where do we get Anchor Load from?

Max. Anchor Load = Max Line Tension – (unit submerged weight of mooring line ) x (water depth) - friction between mooring line and seabed

Where:Friction between mooring and seabed = friction coefficient x unit

submerged weight of mooring line x Length on seabed

Mooring Line Design Criteria

Mooring Line Design Checks:

Anchor Point Criteria

Anchor Point Design Checks:Factors of safety for various anchors, conditions and analysis methods

RecapDesign Criteria(Environment,

allowable offsets etc)

Determine Environmental Effects1. Steady State Environmental Forces2. Determine Low Frequency

Motions3. Determine Wave Frequency

Motions

Initial Mooring Pattern

Determine Mooring Tensions / Offsets