datasheet silo tank fpso concept usos

8/6/2019 Datasheet Silo Tank FPSO Concept USOS

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Ulstein Sea of Solutions B.V. Member of the Ulstein GroupChurchillsingel 432-446, 3137 XB Vlaardingen, the Netherlandstel. +31(0)10 475 00 11, fax +31(0)10 474 67 33E-mail: [email protected] , Internet: www.seaofsolutions.nl

SILO TANK FPSO CONCEPTIntroduction

All FPSOs currently operated in harsh weather environments have experienced either green water on deck,slamming of the hull or fatigue cracking of the hull. The results of these phenomena are limited utilization,downtime, or additional CAPEX and revenue loss if the vessel is taken off station for repairs. Limited utilizationcan be the result of reduced capacity during harsh weather periods, restrictions on offloading, or reduced accessfor personnel at certain areas of the vessel. Sea of Solutions BV therefore set of with the intention of creating anFPSO design that reduces the chances of these incidents taking place. In addition, conventional FPSO

technology has a problem with volatile organic compound (VOC) emissions. For certain types of crude this canyield significant loss of revenue, but also cause an environmental risk. The Norwegian government requires VOCsystems where the emissions are reduced by 78%. The proposed silo tank FPSO will have no VOC because of

the individual tanks that are always filled, thereby leaving no space for the formation of VOC.

Figure 1: 3-D Image of the Tanks Located in the Hull

New Concept

The main goal in developing a concept for a new type of FPSO hull and structure was to design an FPSO whichsolves the problems that are present in the current tanker-based designs.The main objectives for the concept development were to solve the following issues:

• Green water on deck

• Slamming of hull

• Stress cracks in the tanks

• VOC emission

• Reduce outflow with breach of hull

• Reduced motions for better process performance




Green water is caused by waves breaking over the bow or sides of the ship. This uncontrollable mass of water

can cause damage to instruments and structures on deck. It is therefore important to have enough clearancebetween still water draft and the deck. On some FPSOs, where green water has been a problem, extra walls orbulwarks have been installed along the side of the ship and higher bows were erected in order to reduce the

incidence of green water and possible damages that it may cause.

Slamming is caused by the hull being lifted out of the water and hitting a coming wave with a certain speed.Slamming can cause extreme load conditions that are difficult to predict and it is therefore important to minimizethe possibility of its occurrence.

During harsh weather, a tank that is not completely filled will experience sloshing. The uneven loading of thesloshing may create cyclic loads in the structure of the tanks, and therefore the vessel hull. Such cyclic loads can

lead to the formation and propagation of fatigue cracks. Cracks may create hazards and require costly repairs.An FPSO is normally meant to stay in the field for the lifetime of the project and interruption for repairs will

therefore incur high costs.

The proposed storage system on the new concept FPSO will avoid VOC releases into environment without anyadditional power requirements. By having storage tanks that are always filled, there will be no gas layer whereVOC can be formed. It should be noted that the UK sector does not require VOC recovery systems for FPSOsand shuttle tankers at this time, but this will likely change in the future.

In order to increase the environmental safety, FPSO’s are normally double hulled vessels. This is in order toreduce the chance for oil spill to the environment in case a vessel or similar hits the side of the FPSO and theimpact causes the hull to breach. However, in case the hull should breach, due to smaller tanks for the Silo TankFPSO much less oil enters the environment; 11,000m

3vs. 2,000m

3.

The motion characteristics of an FPSO affect both the process system and the well being of the people on board.An FPSO in harsh environments will experience large accelerations, which can both damage topsides equipment

and reduce its operability. It is therefore important to design the FPSO hull so as to reduce the motions on thevessel and to locate the most motion sensitive equipment where the motion is least. The proposed Silo TankFPSO will naturally be larger and therefore have a better motion behavior. In addition, there will be more surfacearea for the process facilities.

Basics features of the proposed Crude Storage system

The proposed system for crude storage comprises a series of individual tanks that are completely filled at alltimes with both seawater and crude oil. The two liquids are naturally separated due to difference in theirrespective specific gravity where the upper layer will be crude and the lower layer will be seawater. Crude and

seawater will not naturally mix but in order to decrease the contact area between the crude and seawater and tominimize heat transfer; a floater with a specific gravity between crude and seawater will separate the two media.

When crude is being filled into a tank, seawater will be displaced at the same rate, ensuring that the tank isalways filled. It is envisaged that sets of 10 tanks will be filled and emptied at the same time. When the tanks arefull of crude, the floaters will be in their lowest position. During the offloading of crude to a shuttle tanker,seawater is pumped into the tanks and the crude will be displaced and flow out at the top of each tank to a headerand through pipes located under deck towards the stern of the ship where offloading to a shuttle tanker will take

place. Figure 1 presents a 3-D image of 20 tanks placed in the hull of the ship. Figure 2 presents the sequence offilling and emptying the crude in a tank.




Figure 2 Loading and Offloading Sequence of Crude

The conceptual silo tank design is that each tank will be cylindrical, with a diameter of 10 m and a total height of30 m. The main body will be 25 m high, while a cone shaped section on each end will be about 2.5 m high. The

tanks will have a smooth internal surface, which will be coated. Each tank will have a sacrificial anode placed inthe bottom of the tank where there will always be seawater.

The tanks will be supported by the hull structure bottom and with stiffeners. However, the tanks will not be part ofthe load carrying part of the hull structure, and will therefore not be experiencing cyclic loads that can lead to

fatigue failure. There will be sufficient gaps between the hull and the tanks for external tank inspections and theair will act as insulation to the tanks so that the crude will have limited temperature drop during storage.

The silo tank FPSO will have a draft differential of less than one meter between full condition and emptycondition. A conventional FPSO may have a draft differential of up to 10 m. The constant draft will allow for anoptimized hull design with respect to motions and bending moments. In addition there will be less vertical motionsacting on risers and mooring. A midship section of a Silo Tank FPSO and a conventional FPSO for the samestorage volume is presented in Figure 3. It can be seen that the Silo tank FPSO hull will be larger in alldimensions especially in the height above sea level. Taking into considerations that the elevation difference willbe nearly constant compared to the conventional FPSO, means that 100% utilization of the FPSO can be ensuredalso during harsh weather conditions.




Figure 3: Midship section of conventional and Silo Tank FPSO

Proposed future developments

The silo tank FPSO system is currently in its conceptual stage and additional design work is necessary to confirm

the technical superiority of the system. In order to provide the technical documentation that is needed for thisverification and to prepare the concept for further detail design the following main tasks should be performed in aninitial concept verification study.

The vessel hull shape should be optimized for the required amount of crude storage. Further hull design will alsorequire tank testing. Preliminary hull design is not optimized with respect to strength and fatigue analysis. Duringthe conceptual phase this should be further detailed in order to verify the steel weight requirements. The use ofan internal frame between the tanks may reduce the need for bulkheads. Although bulkheads are necessary for

the integrity of the vessel, fewer bulkheads may be beneficial with respect to cost.

The operational aspects of the silo tank system need to be detailed. Although the basic principles of the systemwith seawater and crude in the same tank is a design proven in concrete base platforms in the North Sea, theproposed method of pumping seawater into the tanks to displace crude oil needs to be verified. Also themeasurements and cleaning of the seawater that is displaced by incoming crude oil in the tanks needs to beverified.

Procedures for the initiation and start-up of the tank system need to be determined. These procedures will includeinitial filling of the tanks, concurrent filling and emptying of seawater and crude, and emptying of the crude andseawater before tank cleaning.

The lay-out and required process facility components to be placed on the FPSO are normally project specific andare determined based on the composition of oil and gas in the reservoir. The storage capacity will also largely be

based on the amount of oil that will be produced and the required offloading rate.

datasheet silo tank fpso concept usos

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