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Turrets - Moorings - Risers overview

Published on 09 Apr 2019 by Brysen Lin

 

1. Introduction

1.1 Overview

This document describes the development of turret and swivel systems which allow 'n x 360 degrees' rotation have been the key technologies which have allowed FPSOs to be used in all met-ocean environments. Turret allows vessel to weathervane - that is, to take up a position which aligns it with the dominant forces of wave, wind and current. Weathervaning has the effect of minimizing environmental loads on the station keeping system, and minimizes the most sensitive vessel motions - generally for a mono-hulls roll. Turret provides connection to mooring lines, transfer of mooring loads to the hull and mechanical support for the risers and umbilicals.

Swivel systems allow the transfer of:

  • Production fluids from the sub-sea equipment to the vessel
  • Export fluids from the vessel to the sub-sea infrastructure
  • Well/subsea management/control fluids (e.g. methanol)
  • Electrical signals for control and monitoring
  • Electrical power for driving subsea/down hole equipment e.g. electric submersible pump (ESP)

1.2 Weathervaning FPSO

  • Weathervaning rotates around turret and fluid swivel
  • Mooring and risers fixed to sea bed and are “geostationary”

1.3 Turret Systems

  • Internal Turret 
  • External Turret 
  • External Disconnectable Turret
  • Internal Disconnectable Turret 
  • Yoke Tower

1.4 Turret Applications

  • Internal Turrets
    • Harsh environment
    • Large number of risers (up to 75)
    • Large amount of equipment
    • Water depth approximately above 70m
  • External Turrets
    • Moderate environment
    • Smaller number of risers (up to 25)
    • Small amount of equipment
    • Water depth approximately above 50m
  • Yoke System
    • Moderate environment
    • Large or small number of risers
    • Small water depth less than 50m

2. Main Turret Components

2.1 Turntable


One of the primary functions of the turntable is Manifolding: “Combining multiple flow lines to one or more swivel paths, and create the flexibility to add or interconnect other flow paths”

  

2.2 Turntable Typical Equipment

  • Subsea control hydraulic power pack
  • Subsea chemical injection  
  • Pipe headers
  • Control valves and safety valves
  • Riser maintenance equipment (pig launchers/receivers)
  • Riser and mooring pull in Winch

3. Different Turret Types

3.1 Internal Turret


3.2 External Turret

External turret on Cuu Long MV9 FPSO:

  • 46m water depth
  • 151,000 dwt tanker conversion
  • 1.0 million bbls storage


3.3 External/Internal Disconnectable Turret

Why select a disconnectable turret?

  • Weather driven: Disconnect for Cyclones, Hurricanes or Ice
  • Pre-installation of mooring lines and risers attached to the Buoy
    • Schedules of subsea installation and FPSO installation disconnected yields less risk
    • Smaller weather window required since installation time is reduced

3.4 Submerged Turret Production (STP)

  • Compact system
  • Swivel located in hull or on deck

  

3.5 Yoke Tower


4. Interface with Subsea Equipment

Turret and swivel provide the main interface between the FPSO and the subsea system.

Subsea system consists of flow lines, umbilical and mooring lines.

Risers are provided for:

  • Reservoir fluids   
  • Water injection
  • Gas injection
  • Gas lift
  • Gas export/import
  • Oil export

Umbilical are provided for:

  • Signal transfer from the well head
  • Well head control
  • Chemical injection into well

5. Moorings and Risers

5.1 FPSO Mooring Line Construction


5.2 Flexible Riser Configurations


5.3 Flexible Riser Cross Section


5.4 Rigid - Steel Catenary Risers (SCR)


5.5 SCR General Components


5.6 Hybrid Tower


6. CALM Buoys


7. Swivel System

Swivel systems allow the transfer of:

  • Production fluids from the subsea equipment to the vessel
  • Export fluids from the vessel to the subsea infrastructure
  • Well/subsea management/control fluids e.g. methanol
  • Electrical signals for control and monitoring
  • Electrical power for driving subsea/down hole equipment e.g. ESPs

Swivel systems allow N times 360 degrees rotation.

Swivel manufacturers are pushing back the boundaries of pressure and temperature to around:

  • Approximately 525 barg
  • Approximately 1300C

7.1 Simplified Swivel (single path)


7.2 Multiple paths can be stacked


7.3 Seal Materials

  • Generally seal material is polytetrafluoroethylene (PTFE)
  • High melting point (327°C) and an excellent chemical resistance
  • The main reason to use PTFE for dynamic seals is its low coefficient of friction and the absence of stick-slip phenomenon
  • Disadvantage of PTFE is its tendency to creep, particularly at high pressures and temperatures
  • The design of swivel seal arrangements is dominated by this tendency to creep, i.e. minimization of extrusion gap  

7.4 Ancillary Support Systems

  • Silt Barrier / Seal Energization System
  • Leakage Recovery System
  • Torque Drive System
  • Condition Monitoring System



Tags: Turret Mooring Riser

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