An FPSO with Swivel
An FPSO with Swivel
 Berg AB and Azipod ABB
Berg AB and Azipod ABB
 
An Alternative to a Swivel is a series of  Thrusters  working in a controlled manner to position the floating hull. The Thrusters work in unison to maintain a fixed position which may be a target point  from transponders set on the sea bed or a reference provided from satellite positioning.
West Navion  thruster changeout
West Navion thruster changeout
Positioning using satellite, subsea and tight wire
Positioning using satellite, subsea and tight wire
Work Over Support Vessel, Draft Data Sheet
Work Over Support Vessel, Draft Data Sheet
The thrusters themselves can be various types, mechanically, electrically or hydraulically driven. The Diagram on the right shows two typical styles, the upper is a shrouded  thruster with a mechanical, or hydraulic  Drive and can be varipitch, The lower is an ABB Azipod which has an electric drive and the motor is immersed (assists cooling). The control systems for these thrusters needs to be precise and reliable. To ensure this codes and operational requirements exists, two typical examples are given see links below.
http://rules.dnvgl.com/docs/pdf/dnvgl/RP/2015-07/DNVGL-RP-E307.pdf
Dynamically Positioned Units (DPU's)have numerous codes and standards for classification operation and maintenance. The typical data sheet (above right) for a WIV, for Wire Line (Slick or Dry), Coiled Tubing Intervention would be Classified to a Recomended Practise such as RP 2013-005 OSS 101 Rules for Classified Offshore Drilling and  Support Unit (Vessel) reviewed issued 2014, an IMO Code would be DP3 3000 metres (now progressed to Guide Lines f0r 6000 metres) and Operational Guide Lines for such a craft would be DNV RP E307
The example on the left of a Swivel on an FPSO moored  in shallow waters shows the lower cost approach of using a conventional chain mooring array . Deeper situations 2 to 6000 m will favour dynamic positioning particularly for drilling and work over. The reasons for this are many and include the extreme weight of chain or fibre mooring, the cost of setting out the anchor pattern, floatation collars to support column weights etc
The diagram on the left shows a method to service or change out a thruster on West Navion a DP Drill Unit. Other ways for such operations involve hinge arm or unit drawn into an internal can within the vessel. Being used in remote locations away from dry docking these units must be self supporting. The West Navion FDU was in a mishap with its DPS assumed to be in Autocontrol when in fact it was in Manual, instead of Automaticallyfacing to windward it drifted from position and a helicopter on the landing pad in a hold mode flipped over. This is an inverse reason that demonstrates the ability of DPS to hold station.
Positioning using, subsea, Transponders Triangulation
Positioning using, subsea, Transponders Triangulation
C2V
C2V
C2V
C2V
C2V
C2V
Level 1
MODULEitus
a general understanding of Modules 
Level 2
Level 3