Propulsion system options for FSO vessel conversions

Crude oil is produced and processed from inland or subsea wellheads and loaded onto a moored floating storage and offloading (FSO) vessel via a subsea pipeline, pipeline-end-manifold (PLEM) assembly, and underwater hoses.

Trade-offs in retaining or removing propulsion system

Bill C.M. Chen
Crude oil is produced and processed from inland or subsea wellheads and loaded onto a moored floating storage and offloading (FSO) vessel via a subsea pipeline, pipeline-end-manifold (PLEM) assembly, and underwater hoses.

The vessel, and those associated installations, constitute an offshore exporting facility for product crude. The vessel can be moored through the bow or stern turret by a single-point mooring (SPM) system or other facilities. Cargo stored in the vessel is exported through the offloading hoses at the stern (or bow) to a shuttle tanker berthed in-tandem or another configuration.

The facility presented here is one installed off the coast of West Africa, far from major world shipyards. The facility's design life is presumed to be at least 20 years. The FSO vessel, as studied, is a converted ultra-large crude carrier (ULCC) built in the 1970's. During this era, ULCCs were equipped with a steam power plant with steam turbines for propulsion.

The steam power plant, associated systems, and propulsion system were investigated for vessel conversion engineering in this study. The converted ULCC presented here has the cargo-carrying capacity of 400,000+ DWT. The methodology of the investigation can be applied to any similar vessel.

The study deals with the option of retaining or removing the existing propulsion system during the vessel conversion. The propulsion system is not needed for the vessel's daily operation at the export facility. However, the retained system fulfills certain functions such as propelling the vessel to and from the export facility. The removed system simplifies certain operations, but requires that an alternate means be provided to transport the vessel to and from the facility.

The final decision can be made during the conceptual study and is based on an overall economic assessment of the propulsion system for the vessel design life as well as the field production life. There are industry examples of either option. The purpose of the study was to lay out the planning philosophy and cost basis, and to support the later design effort for the FSO vessel. Detailed engineering or examination of operations and maintenance were not undertaken in the study.

For the propulsion system modification design, the study's objectives were to:

  • Analyze the option of retaining or removing the propulsion system
  • Estimate the number of crew, distance, number and cost of voyages for the vessel transporting between the West Africa coast and various shipyards under either option
  • Consider the possible operational, maintenance, and design implications on both options
  • Compare the cost of both options.
The FSO vessel inherits the propulsion system if it is converted from a ULCC. The propulsion steam power plant includes the steam turbines, reduction gears, shafts, propeller, steering gears, rudder, associated piping systems, and supporting machineries. Retaining or removing the system will be implemented during the vessel's conversion. A different design, conversion work, operation, and maintenance will be required for the option adopted.

System conversion

Removal of the propulsion system does not imply that all system machinery, piping, and equipment must be physically removed, but rather that they are disconnected. Some system modifications are required to fulfill the classification requirements as well as the budgetary constraints.

For economic reasons, a minimal effort to disable the propulsion system is sufficient for this option. This includes sealing off permanently all steam inlets to the turbines, stopping air leakage to main condensers, and dismantling the shafts, propeller, and rudder. A consultation with the classification society can be made for the other tasks. Shipyards can also be consulted for the detailed engineering and tasks. The removal tasks are permanent processes which are intended to be irreversible.

Retaining the propulsion system also involves system modifications to fulfill the classification requirements as well as the budgetary constraints. Blanking off steam inlets to the turbines and stopping air leakage to condensers are the typical main tasks. The propeller shafts and rudder should be secured based on demands such as during severe weather. The deactivation tasks are temporary processes and are required to be reversible to the original status of the system. The deactivated propulsion system is required to be reactivated for propelling the vessel again with minimum outside assistance at the facility.

Vessel voyages

The FSO vessel with the removed propulsion system needs to be towed by a tug(s) to the export facility, whereas the one with the retained system can self-propel to the facility. The types of voyages which require the vessel to be towed or self-propelled are summarized as follows:

  • The first voyage to the facility from a conversion yard is assumed to be 4,000 miles from southwestern Europe, 9,300 miles from Southeast Asia via the Cape of Good Hope, and 13,500 miles from east Asia.
  • The voyage for scheduled shipyard maintenance should be undertaken about every 10 years after the vessel is converted with repair and life extension. The distance to a shipyard is the same as above.
  • The voyage for unscheduled shipyard maintenance should be taken into account.
  • The voyage to a safe haven due to adverse weather rarely happens and depends on the weather at the facility.
  • The voyage to a new production site or a salvage yard may happen, but should not be included in this study and depends on the design life of the facility.
Considering the facility's operational life of more than 20 years and the vessel's age, the number of one-way trips for the FSO vessel to be towed or to self-propel is estimated to be five voyages (first trip to the facility and four possible one-way trips to and from shipyards for maintenance). The extent of repair and life extension on the vessel during conversion plays the key role for determining scheduled or unscheduled shipyard maintenance. Another factor such as the operational environment also affects this determination.


The FSO vessel which has the propulsion system removed must be towed to the facility after the conversion. There are no specific requirements or guidelines from the towing company, marine insurance company, and classification society about the manning level aboard the vessel under towage and the number of tugs to tow the vessel. They can accommodate the owner's

towing specifications, which are based on safety and financial security reasons. Thus, the manning level and the required number of tugs depend entirely on the owner and his construction contractor.

In this study, the FSO vessel is manned for safety, stability, and training reasons, with a crew of 20 persons aboard during the towing voyage. The crew includes about 10 persons for engine and pump room operations and the remainder for the on-deck operations. The crew can be used for such activities as adjusting the vessel's draft and trim due to weather change, operating the power plant for ballast pumps and generators for the navigation lighting, and becoming familiar with system operations.

Two tug boats are required for the ocean tow, simply for safeguarding the initial huge investment. One powerful tug provides the main bollard pull for the vessel. The other tug provides backup force or emergency service.

The system-retained vessel is self-propelled to the facility. The manning level is estimated also to be a crew of 20 persons on board. There is no cargo aboard and a standard full marine crew is unnecessary. The operation of the vessel during the voyage is less laborious as a trading voyage made by the ULCC.

Other considerations

Removal of the propulsion system of the FSO vessel will be completed during the vessel conversion. No additional operation and routine maintenance on the system is required after the vessel leaves the conversion yard. The system does not require any upkeep during the vessel's scheduled maintenance in a shipyard after a lengthy service at the facility.

For the vessel retaining the propulsion system, the system deactivation is performed after the vessel has arrived at the facility. The required equipment and parts for the deactivation are prefabricated at the conversion yard. Deactivation work may require temporary and additional assistance from the yard.

The deactivated propulsion system requires routine maintenance to prepare the system to be functional if it is re-activated. The system maintenance work, such as periodically rotating the propulsion turbines with the turning gears and operating the lube oil pumps, are necessary. Auxiliary systems, such as the stern tube seal and the turbine shaft gland seals, require constant operation. The stern tube seal system serves as the shaft lubrication and keeps the engine room from flooding. The gland seal system helps maintain the vacuum in the main condenser. The steering gears and the propeller shaft are required to be locked in adverse weather. Other operations and maintenance are included in the vessel's detailed operating manuals. The system requires some upkeep during the vessel's scheduled maintenance in a shipyard.

Cost comparisons

The conversion cost for removing the propulsion system is slightly higher than that of retaining the system for the FSO vessel. The difference is small in contrast to the overall conversion cost. Higher cost is mainly due to the dismantling and removal of shafts, propeller, and rudder, which totals about $30,000. The vessel with the removed system could be stern-moored, which results in some cost saving to the SPM support structure at stern.

Operation and maintenance costs for the retained propulsion system are slightly higher than that of the system removed for the vessel at the facility. The difference is small in contrast to the overall operation cost. Higher cost is mainly due to the upkeep of turbines and auxiliary machinery as stated in the previous section. The retained system incurs cost for maintenance during the vessel's scheduled maintenance in a drydock and shipyard. The maintenance cost is for surface painting the rudder and polishing the propeller, and amounts to about $40,000.

For the system-retained vessel, the upkeep may actually amount to no extra labor cost since it is performed by the crew during the routine watch hours in their daily shifts within the engine room. The cost of consumption of fuel oil and lube oil for the deactivated propulsion system is very small. Re-activation of the system may result in no extra cost for labor.

The engine room is assumed to be manned with a regular crew of operators for boilers and machines. This crew should be adequate to maintain the deactivated propulsion system. However, one extra crew member may be included for the cost estimate of the retained system. The extra cost is less than $50,000/year. The system is capable of being reactivated within 24 hours with proper planning and execution. The operation cost estimate for the vessel's entire steam power plant is out of the scope of this study.

For a conservative estimate, five one-way voyages are required for the FSO vessel to be towed or self-propelled during its entire operating life of 20 years. The first voyage's cost from various conversion yards or shipyards around the world to the coast of West Africa was determined.

The daily operating cost of the FSO vessel with crew aboard contributes significantly to the expense. The towed vessel, manned with or without crew aboard, makes the major difference in cost comparisons. The vessel can be towed by 1-2 tugs, providing their brake horsepower (BHP) is sufficient for that purpose. The first voyage costs using various locations of conversion yards or shipyards were determined (Figure 1) in constant dollars. The costs and shipyard location for the 2nd to 5th voyages are assumed to be the same as the first. The five one-way voyage costs were also determined (Figure 2).

The option of retaining or removing the vessel's propulsion system was based on economic merits. Other factors were considered, such as vessel mobility in a politically uncertain region and the field production life.

Some options were clearly superior to others. The trade-offs and cost comparisons between the options were not marginal. The decision of retaining or removing the system is subjective sometimes and should be determined case-by-case. The approaches detailed in this study can provide the basis for decision-making about the propulsion system of any future FSO vessel conversion engineering.


Bill C. M. Chen is a Houston-based independent consultant. The author wishes to thank Judy F. Chen for assistance in preparing this technical paper.

Cost Estimates for transporting an FSO vessel

Towing by tugs

A tugboat of 6,000 BHP is required to tow an FSO vessel ballasted to 30 ft draft (assumed) across the open sea. For safety reasons, it is recommended to tow the vessel with two tugs. One main tug of 6,000 BHP and a second one of 3,000 BHP are proposed for towage from a conversion yard to the export facility off West Africa. An alternative, two 4,000 BHP tugs, will serve the same purpose.

The towing speed is about 6 knots in open water with modest weather. The main tug's daily rate is $5,500. Fuel consumption is 200 gallons/hr. The secondary tug's daily rate is $3,000. Fuel consumption is 120 gallons/hr. The fuel cost is $0.60/gallon FOB in the Gulf of Mexico. The fuel rate is $120/hr for the main tug, and $72/hr for the secondary one.

The crew should stay with the FSO vessel during towage. Crew on board allows training, familiarization with the vessel, changing vessel draft and trim due to the changing weather, and practicing fire drills. The daily vessel operation cost is estimated to be $9,500/day. Wages and food for a crew of 20 persons amount to about $7,500/day. The remainder is the fuel and lube oil consumption for the vessel's power plant being operated partially for the accommodations. Marine insurance and contingency are excluded in this cost estimate. After conversion, the vessel is towed to the West African coast from shipyards at various parts of world. The cost for towing of the FSO vessel is itemized (Table 3).

Self-propelled vessel
The FSO vessel is ballasted to an assumed 30 ft draft and self-propels across the open sea to the facility on the coast of West Africa. Vessel speed is about 10 knots in open water with modest weather. Fuel consumption for the vessel is about 101 tons/day. Boiler heavy fuel oil cost is estimated to be $ 82/short ton bunkered at the Gulf of Mexico.

Daily vessel operation cost is about $8,500/day. The cost is mainly for wages, overhead, and food for a crew of 20 persons. There is no need for a full marine crew. A 20-member crew is average operations in the engine/pump rooms and on the deck of the FSO vessel. Marine insurance and contingency are excluded in this cost estimate. After conversion, the vessel sails to the West African coast from shipyards at various parts of world. The cost for self-propelling of the FSO vessel is itemized (Table 4).

Table 1 - Cost estimates (US$) for first one-way voyage to West Africa
AlternativesSE AsiaSW EuropeNW Asia
Towed (2 tugs), manned1,271,000565,0001,854,000
Towed (2 tugs), unmanned737,000328,0001,075,000
Towed (1tug), unmanned471,000210,000687,000

Table 2 - Cost estimates (US$) for five 1-way voyages to/from West Africa
AlternativesSE AsiaSW EuropeNW Asia
Towed (2 tugs), manned6,544,0002,826,0009,269,000
Towed (2 tugs), unmanned3,884,0001,639,0005,375,000
Towed (1tug), unmanned2,556,0001,048,0003,436,000

Table 3 - Towing costs (US$) of an FSO vessel
FactorSE AsiaSW EuropeNW Asia
Distance (miles)9,3004,00013,500
Voyage time (days)56.22582
Towage cost (main tug)309,100137,500451,000
Fuel cost (main tug)161,76072,000236,160
Towage cost (2nd tug)168,60075,000246,000
Fuel cost (2nd tug)97,05643,200141,696
Vessel ops (crew of 20)533,900237,500779,000
Total cost - manned vessel, 2 tugs1,270,416556,2001,853,856
Total cost - unmanned vessel, 2 tugs736,212327,7001,074,856
Total cost - unmanned vessel, 1 tug470,860209,500687,160

Table 4 - Self-propelling costs (US$) for an FSO vessel
FactorSE AsiaSW EuropeNW Asia
Voyage time (days)33.614.548.8
Fuel cost278,600119,800404,200
Operation cost285,900123,000414,800
Estimated total cost564,500242,800819,000

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