CIVL 4171 Pipeline, Platform and Subsea Technology
Mooring Design of Floating Platforms
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
Spar Platform
Mooring
Tensioned Risers
Spread Moored FPSO
CALM Buoy
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
Six Strand – Advantages
•Higher Strength to Weight Ratio
•Higher elasticity
•Higher Strength to Diameter Ratio
•Greater Flexibility
•Torsionally Balanced •Higher Resistance to corrosion
•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 Drag Anchor Types
• 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
Suction Anchors
Recap of the FPSO Design Overview FUNCTIONAL REQUIREMENTS -Mooring envelope -Allowable Motions -Allowable displacements.
SCHEME CONFIGURATION -TLP, FPSO, Spar etc
METOCEAN DATA
Topside Layout and structural support configuration
HYDRODYNAMIC ANALYSIS
Functional Loads: Eg crude oil storage, production equipment etc.
STRUCTURAL DESIGN -Strength -Fatigue
MOORING ANALYSIS
ANCHOR POINT -Drag Anchors -Driven Pile -Suction Pile -Gravity Anchor
TURRET / MOORING INTERFACES
MOORING CONFIGURATION
PRELIMINARY LAYOUT
MOORING TYPE -Spread moored -Single point mooring -All Chain -Wire/ Chain/ Wire -Buoys / clump weights in-line
MODEL TESTING
N REDESIGN & RERUN MOORING & STRUCTURAL ANALYSIS
Y DESIGN & PERFORMANCE SATISFACTORY
FINAL DESIGN
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
Initial Mooring Pattern
Prior to starting Mooring or Structure design, we need to work out how the vessel reacts to the environment. Determine Environmental Effects 1. Steady State Environmental Forces
2. 3.
Determine Mooring Tensions / Offsets
Loads & FoS
Typically this work is performed by specialist engineers / naval architects. Work is performed using analysis or obtained from scale model tests
Determine Low Frequency Motions Determine Wave Frequency Motions
Forces & Offsets
How do we predict the response characteristics of the vessel?
Criteria
Design Criteria and Load Cases (Environment, allow offsets etc)
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
Mooring
3 x 3 System
Risers
Design Cases Basic Load Cases – Intact (all lines intact) – Damaged (one line broken) – Transient (motions after 1 line breaks) ÎLoad cases have different Factors of Safety
Environmental Criteria
Environment = Principally wind, wave, current & tide Key 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 Conditions Phenomenon such as tsunamis, icebergs, solitons etc.. May also need consideration for a particular project
Forces and Motions Environmental forces / motions should be calculated at the following 3 distinct frequency bands to evaluate their effects on the system •
•
•
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
Steady Forces : Wind Steady Wind Forces Calculated on each part of the FPSO by summing the contribution of different areas : F = 0.5
ρair.A.Vz2.
Area 3
Cs
(kN)
where,
Area 1
Vz = 1 hour mean wind velocity at specified height z
Area 2
SHAPE
Cs
Vz = Vh (z/H)0.125
Large Flat Surface (hull, deckhouse)
1.00
Vh = reference wind speed at 10m height
Exposed beams, girders
1.30
A= projected area (m2)
Isolated shapes (cranes, booms etc)
1.50
Cs = Shape coefficient
Clustered deck houses
1.10
ρair = 0.00125 Tonnes/m3
Cylindrical
0.50
OR - use method of API RP2SK
Steady Forces : Current Steady Current Forces Forces on the hull of an FPSO can be estimated by the following equations: 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.Vc2 (kN) where, Ccy = current force coefficient on the beam = 0.07237 kNsec2/m4
WETTED SURFACE AREA = S
Low Frequency Wave Forces & Motions 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 1.0 Choose Vessel Length
2.0 Chose Wave Height
3.0 Read off values of # Low Freq. Single Amplitude Motion
4.0 Read off values
5.0 Adjust Values for
# Mean drift Force
•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).
Predicting 1st Order Response (1)
Vessel : Wave Frequency Response
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) RAO's : 30 degree heading 4
Surge 3
Heave
Am plitude
•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)
2
Roll
1
Pitch
0 0
5
10
15
20
Period (seconds)
25
30
Vessel : Wave Frequency Response Predicting 1st Order Response (2) 3 main calculation methods – – –
AQWA Model
Time domain Frequency Domain Model tests
FREQUENCY DOMAIN These 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 DOMAIN Time 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)
Diffracted Water Surface Contours
Vessel : Wave Frequency Response Predicting 1st Order Response (3) MODEL TESTS Still used today – why? Because it works!!! – basically numerical computation is good, but still needs work to be suitable Test 138
1.500
Wave Probe at Wall (CoG) Average
Heave & Wave Height (m)
1.000
0.500
0.000 0.00
10.00
20.00
30.00
-0.500
-1.000
Time (secs)
40.00
50.00
60.00
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 Smax Swfmax Swfsig Slfmax Slfsig
If Slfmax>Swfmax , then: If Swfmax>Slfmax , then:
= mean vessel offset = max vessel offset = max wave frequency motion = significant wave freq. motion = maximum low freq. motion = significant low freq. motion Smax Smax
= Smean+ Slfmax+Swfsig = 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 / Ta T= specified storm period in seconds (usually 3hrs) Ta = average zero crossing period in seconds eg 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 displacement k = 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 tension Twfmax = maximum wave frequency tension Twfsig = significant wave frequency tension Tlfmax = maximum low frequency tension Tlfsig = 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) Î PREPARE GRAPH OF TENSION Vs OFFSET (& SUSPENDED LINE LENGTH)
Line Tension
•
Catenary equation z+h =
•
⎛ wx ⎞ ⎞ Th ⎛ cosh ⎜ ⎜ ⎟ − 1⎟ w⎝ ⎝ Th ⎠ ⎠
Maximum tension
Tmax = Th + wh •
Suspended (Minimum) length lmin
T = g 2 max − 1 wh
Notation: T- line tension (N) h – water depth (m) w – line weight in water (N/m)
Line Tension Definition 1. From Total force & vessel restoring force curve determine… 2. Mean offset 3. Determine Smax as a function of Low frequency & Wave frequency offsets 4. From Smax & Most loaded line tension force curve determine Maximum Mooring force 1 4
2
3
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
Recap Design Criteria (Environment, allowable offsets etc)
;
Initial Mooring Pattern
;
Determine Environmental Effects 1. Steady State Environmental Forces
2. 3.
Determine Low Frequency Motions Determine Wave Frequency Motions
Determine Mooring Tensions / Offsets
;
;
