mooring-systems

name	mooring-systems
description	Mooring systems — catenary vs. taut-leg, chain/wire/polyester properties, stiffness, pretension, watch circle, extreme load analysis, API RP 2SK, ISO 19901-7, MODU mooring.
metadata	{"priority":7,"promptSignals":{"phrases":["mooring system","catenary mooring","taut leg mooring","anchor chain","mooring line","FPSO mooring","API RP 2SK"],"minScore":3}}

Mooring Systems — Complete Skill

Mooring System Types

Catenary Mooring

Lines hang in catenary curve; restoring force from line weight and geometry

• Horizontal offset → line lifts off seabed → increasing tension → restoring force
• Typical: steel chain (studlink or studless) + wire rope or chain only
• Applications: FPSOs, MODUs (semisubs, drillships) in 50–2000 m water depth

Taut-Leg Mooring

Nearly straight lines from anchor to vessel; no seabed touchdown Restoring force from line elasticity (stretch)

• Requires high-holding capacity anchors (VLAs, suction caissors)
• Polyester rope essential: high elastic elongation (~10–15%), low weight
• Applications: deepwater FPSOs > 1000 m, TLPs, SPARs

Single Point Mooring (SPM)

CALM (Catenary Anchor Leg Mooring): disconnected tanker mooring; most common offshore oil transfer SALM (Single Anchor Leg Mooring): smaller vessels; coastal operations Turret mooring: built into FPSO; vessel weathervanes around fixed turret

Mooring Component Properties

Chain (Stud-Link / Studless)

Grade	Proof Load Coefficient	MBL Coefficient	Wet Weight [kg/m]
R3	0.0156 d²	0.0249 d²	0.1875 d²
R3S	0.0180 d²	0.0274 d²	0.1875 d²
R4	0.0216 d²	0.0320 d²	0.1875 d²
R4S	0.0240 d²	0.0356 d²	0.1875 d²
d = nominal chain diameter [mm]; loads in kN; mass in kg/m with d in mm

Studless chain (no studs): lighter, better fatigue, preferred for permanent mooring Stud-link chain: cheaper, less fatigue performance, used for temporary moorings

Axial stiffness: EA_chain = E_eff × A_steel = 860 × d² [kN, d in mm] approx

Wire Rope (6×36 IWRC)

MBL ≈ 0.70 × f_u × A_steel (fill factor 0.70 for 6×36) Specific weight (air): 40.1 d² [N/m, d in mm] EA: 100,000 d² [N, d in mm] (approximately)

Polyester Rope (deepwater)

Density: nearly neutrally buoyant in seawater (ρ = 1380 kg/m³) MBL: manufacturer specific; typically 1.5–15 MN depending on diameter Stiffness (post-installation): EA_static ≈ 10–15 × MBL (static stiffness for storm analysis) EA_dynamic ≈ 50–100 × MBL (dynamic stiffness for motion analysis) Elongation at MBL: ~12–15%

Catenary Line Static Analysis

Catenary equations (chain or wire on seabed): Horizontal tension H [constant along line]: H = T_top × cosα_top

Vertical tension at fairlead: V = T_top × sinα_top = w × L_suspended [w = submerged weight per unit length]

Profile: y(s) = H/w × (cosh(ws/H) - 1) x(s) = H/w × sinh(ws/H)

Restoring stiffness (horizontal offset δx): C_h = dH/dδx (compute numerically; stiffness increases with offset)

Quasi-static restoring force curve: nonlinear; fit polynomial for dynamics

Extreme Load Analysis (API RP 2SK)

Design Cases

Intact mooring: all lines present; survival storm (100-year for permanent, 10-year for MODUs) One line damaged: most loaded intact line limits Progressive failure: if one line fails, verify others don't also fail

Load Combination

T_design = T_mean + T_LF + T_WF (dynamic amplification) T_mean = mean environmental offset (current + wind + mean wave drift) T_LF = low-frequency slowly-varying tension (from swell, wind gust; dominant for catenary) T_WF = wave-frequency tension (from platform heave, pitch)

Combined: T_max = T_mean + √(T_LF² + T_WF²) [statistical combination]

Simplified: T_max = T_mean + T_LF + 0.6 T_WF (API RP 2SK method)

Safety Factors (API RP 2SK Table 4)

Condition	Design Factor on MBL
Intact	0.60 (FOS = 1.67)
One line removed	0.80 (FOS = 1.25)

ISO 19901-7 (2013): similar structure; partial safety factor format

Anchor Design

Drag Embedment Anchor (DEA): conventional; resists horizontal only; holding capacity = μ × W_fluke Suction Caisson: cylindrical; installed by pumping; resists uplift + horizontal; for taut-leg Vertically Loaded Anchor (VLA): rotates to align with load; efficient for taut-leg Pile anchor: driven or drilled; high capacity; expensive

Holding capacity: F_h = A_p × N_c × S_u (suction caisson in clay; N_c = 9 for deep embedment) S_u = undrained shear strength [kPa]

Anchor pile lateral capacity: API RP 2A method: p-y curves; lateral load from FEA

Watch Circle and Position Keeping

Watch circle: maximum expected horizontal offset of vessel from nominal position Typically < 5% water depth for FPSO with catenary mooring (100 m at 2000 m water depth)

Station-keeping efficiency: fraction of total time within prescribed watch circle

Intact mooring offset: often limits pipe flexjoint angles (< 10° for SCR connections) Check: SCR/umbilical fatigue damage at PLEM influenced by vessel offset

Mooring Fatigue

T-N curves (chain in seawater): DNV-OS-E301 / API RP 2SK provide T-N data for mooring components ΔT × N^k = Constant; k ≈ 3–4 for chain; k ≈ 5 for wire

Damage accumulation: D = Σ n_i / N_i < 1/3 (API) or 1/5 (DNV) for mooring

Critical location: chain links at fairlead (wear + fatigue); connectors; first length off anchor

Output

Provide: MBL [kN] required per line, number of lines and pattern, chain/wire/polyester selection and grade, pretension [kN], maximum design tension [kN] vs. allowable (MBL × CF), catenary profile (L_suspended, angle at fairlead), restoring stiffness [kN/m] at design offset, watch circle [m], anchor type and holding capacity [kN], applicable standard (API RP 2SK or ISO 19901-7).