Mooring System – Offshore O&G Project：Mooring Line

Overview
A Mooring System – Mooring Line is a critical component in offshore oil and gas projects. The mooring system secures floating offshore units to the seabed, maintaining position stability and enabling safe offshore operations. The mooring line, as the main load path within the mooring system, connects the floating unit to the anchoring system while integrating key line segments and hardware. Proper selection and maintenance of mooring lines are essential for operational safety and longevity.

Mooring System Definition & Functions

• FPSO (Floating Production, Storage and Offloading Unit)
  • FPSO units process and store crude oil offshore, enabling production in remote locations without pipelines.
• FLNG (Floating Liquefied Natural Gas Vessel)
  • FLNG vessels liquefy natural gas offshore for easier transportation and storage, supporting remote gas fields.
• FPU (Floating Production Unit)
  • FPUs serve as floating production units that handle various processing activities for oil and gas fields.
• FSO (Floating Storage and Offloading Vessel)
  • FSOs provide oil storage and offloading.
• FSRU (Floating Storage and Regasification Unit)
  • FSRUs focus on regasification of liquefied natural gas at sea.

Purpose and Importance of Mooring System

• Mooring System Function: Mooring systems secure floating units to the seabed to maintain position stability, with the mooring line as the core element transmitting loads.
• Environmental Forces: Mooring lines in the mooring system counter environmental forces like wind, waves, and currents for safe operations in harsh offshore O&G environments.

General – Types of Mooring Systems

Type	Description	Operational Note
Spread Mooring	Spread mooring uses multiple anchor points to secure a vessel firmly in place, providing stability in various sea conditions.	Used when fixed heading is acceptable and multi-leg stability is required.
Turret Mooring	Turret mooring allows the vessel to rotate around a fixed turret, enabling better alignment with wind and currents.	Common where weathervaning is beneficial for metocean alignment.
Dynamic Positioning-assisted mooring	Dynamic positioning systems use thrusters to maintain vessel position without anchors, providing precise station-keeping. Typically used for offshore supporting vessels, drilling ships etc.	Useful where anchoring is restricted or high precision is needed.

Mooring System – Offshore O&G Project: Mooring Components

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Mooring System – Offshore O&G Project: A – Mooring Line

A1. Classification

Classification	Key Points
All wire rope system	Much lighter than chain Greater restoring force for a given pre-tension Much longer line length because of anchor uplift Wear situation due to long term abrasion where it contacts the seabed Seldom use for mobile/permanent mooring
All Chain	Better resistance to bottom abrasion Better contributes significantly to anchor holding capacity In deep water, increasing weight penalty on the vessel’s load carrying capacity by own self weight and high initial tension requirements
Combination	Combination of chain, wire rope, fiber rope. A length of chain is connected to the anchor The choice of chain or wire rope at the vessel end and type of termination also depends on the requirements for adjustment of line tensions during operations. Reduced pre-tension requirements with higher restoring force; improved anchor holding capacity and good resistance to bottom abrasion

A2. Mooring Line Type

A2.1 Anchor Chain

A2.2 Steel Wire Rope

• Spin-resistant
  • Spin-resistant strand type constructions are attractive for use with permanent mooring since not generate significant torque with tension change.
  • Spiral strand and multistrand are spin-resistant.
• Corrosion resistance in permanent
  • Polyethylene/polyurethane jacketing (High density type material) typically
  • Galvanized to all wires
  • Zinc filler wires sometimes
• Terminal
  • Open/close socket
  • For permanent, socket shall be with flex relieving boot (bend stiffener), which can seal out the ingress of water and limit free benching fatigue.
  • Resin material for pouring
  • Zinc anode
  • Isolation washings
• Lifecycle

Steel Wire Rope Description	Lifecycle (Years)
Galvanized 6/8 strands	6-8
Galvanized unjacketed spiral strand	10-12
Galvanized unjacketed spiral strand with Zinc filler wires	15-17
Galvanized jacketed spiral strand	20-25
Galvanized unjacketed spiral strand with Zinc filler wires	30-35

A2.3 Synthetic Fiber Rope

• Detailed design refer to API RP 2SM
• Use as segment in steel catenary or taut leg
• Differences from steel wire rope/chain mooring sys: Non-linear stiffness, minimum tension requirements, location to be away from fairlead/seafloor, creep phenomenon, and different handling procedures

A2.3.1 Synthetic Fiber Rope Type

4 Type Fibers Currently for Using in Deepwater Mooring

• Polyester (polyethylene terephthalate)
• Aramid (aromatic polyamide)
• HMPE (High Modulus polyethylene)
• Nylon (polyamide)

Fiber	Key Notes
Polyester	Polyester: good candidate for mooring Lower cost Low stiffness Induce less dynamic tension Good resistance to axial compression fatigue Good fatigue Good strength to weight ratio Good creep resistance Only fiber rope that has been installed in permanent
HMPE & Aramid	Smaller diameter/ frequent handling Ultra-deep water
Nylon	Hawser in CALM High Elasticity Inspection frequently and replaced In Shallow water, inserted in mooring line Absorb the energy from vessel dynamics

A2.3.2 Synthetic Fiber Rope Construction

A2.3.2 Synthetic Fiber Rope Properties

Properties	Stiffness	Creep	Factors for life
Description	Increase with mean load Decrease with cyclic load/relaxation A linear function of mean load and load range, after tension to allow bedding-in, cyclic load and relaxation for some time	Little data currently General indication by yarn data Continuing elongation under load may lead to a need to retension of line HMPE significant creep, lead to creep rupture failure Polyester/aramid, much lower than HMPE	Hydrolysis Heating and internal abrasion Tension-tension fatigue Axial compressive fatigue Creep rupture Checked

A3. Clump Weight

• Sometimes incorporation in mooring legs
  • Improve performance
  • or reduce cost
• At a point close to seabed
  • Replace a portion of chain
  • Increase restoring force of a mooring leg
• Carefully consideration of potentially adverse effects
  • Increase use of connectors
  • Installation complexity
  • Undesirable dynamic response of line
  • Embedment of CW in the seabed
• Construction
  • Concentrated CW
  • Casting Piece
  • Heavy chain segments

A4. Spring Buoy

Surface or subsurface buoys to connection to line
Benefits	Adverse effects	Material	Design	Shape	Arrangement
Reduced weight of lines that must be supported by vessel hull; particularly advantageous in deep water Reduced vessel offsets for a given line and pretension Increased vertical clearance between the mooring line and the equipment below	Increased use of connectors and installation complexity Potential increased design load on the mooring lines due to dynamic response of buoy in heavy seas	Typically steel or combination of synthetic surrounding a steel structure High density foam successfully use Steel buoys have been found to provide a cost competitive solution	adequate strength for maximum depth All welding: Correction protectio	spherical, using unstiffened dished ends welded together With ring stiffened cylindrical bodies and ends	Placed in line with mooring (with a strength member through buoy) Attached separately to mooring through a tri-plate In-line buoy, rotation in the end connection

A5. Connecting Hardware

Connecting Hardware: Used to connect together the main mooring line components.

Permanent mooring

• Inspection and replacement are difficult
• Fatigue life and corrosion protection very importance
• Design evaluation
  • Stress concentration factors
  • Fatigue life
  • Corrosion protection
• Manufacturing shall be subject to an appropriate level of quality assurance.

Mobile Mooring

• Connection links such as Kenter/Baldt
• Pass through fairlead/windlass
• Periodically inspection and replacement

Subsea connectors

• Connect and disconnect of two segments under water
• Typically a male part and a female receptacle
• By an ROV, the under water operation

Engineering Checklist for Mooring System – Mooring line (Optional, Non-destructive Additions)

• Confirm the mooring line classification choice (all wire rope, all chain, or combination) matches water depth, seabed interaction, and operational requirements.
• Review abrasion exposure where the line contacts the seabed, especially for wire rope systems.
• In deep water, account for the weight penalty and initial tension requirements of all-chain solutions.
• For permanent mooring, treat termination and connecting hardware as integrity-critical due to fatigue and corrosion exposure.
• For synthetic fiber rope segments, plan for non-linear stiffness, minimum tension requirements, creep behavior, and handling procedures.