FPSO BALLAST SYSTEM DESIGN A CONCEPT · 2018-11-02 · Trading vessels are built to transport various cargoes or passengers’, but the design criteria of an FPSO ballast capacity

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International Journal of Advanced Research in Engineering and Technology (IJARET) Volume 9, Issue 5, September - October 2018, pp. 95–107, Article ID: IJARET_09_05_010 Available online at http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=9&IType=5 ISSN Print: 0976-6480 and ISSN Online: 0976-6499

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FPSO BALLAST SYSTEM DESIGN A CONCEPT

Anup Srivastava

Sr. Hull & Systems Engineer, Chevron Shipping Asia Pacific Pte. Ltd.

ABSTRACT

Trading vessels are built to transport various cargoes or passengers’, but the design

criteria of an FPSO ballast capacity and pumping system could be based on many

different governing criteria. In this paper the drivers for an FPSO ballast system design

are discussed. A case study is presented to demonstrate the ballast system design

process. The proposed ballast system in the case study is compared with possible

alternatives. The ballast tanks architecture design for an FPSO is also discussed and

stability criteria explained. The paper concludes with a summary explaining the

outcome of the study.

Keyword Head: FPSO Floating Production Storage and Offloading, Ballast water, Loading, Off-loading, Stability, Ballast water tank architecture.

Cite this Article: Anup Srivastava, Fpso Ballast System Design A Concept. International Journal of Advanced Research in Engineering and Technology, 9(5), 2018, pp 95–107. http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=9&IType=5

1. INTRODUCTION

By the definition, ballast is a heavy material, such as gravel, sand, iron, or lead, placed low in a vessel to improve its stability. However, out at sea the water is available in abundance, hence most time seawater is used as ballast.

Trading vessels are built to transport various cargoes or passengers by the sea or inland waterways. When a vessel is not fully laden, additional weight is required to compensate for the increased buoyancy that can result in:

• lack of propeller immersion,

• inadequate transversal inclination, i.e., heeling,

• inadequate longitudinal inclination, i.e., trim,

• static and dynamic stresses on the vessel’s hull including shear and torsion forces, bending moments and slamming, and

• static and dynamic transversal and longitudinal instability,

The design criteria of an FPSO ballast capacity and pumping system could be based on quite different governing criteria.

A floating production storage and offloading (FPSO) unit is a floating vessel used by the offshore oil and gas industry for the production and processing of hydrocarbons, and for the

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storage of oil. An FPSO vessel is designed to receive hydrocarbons produced by itself or from nearby platforms or subsea template, (process them), and store oil until it can be offloaded onto a tanker or, less frequently, transported through a pipeline. FPSOs are preferred in frontier offshore regions as they are easy to install, and do not require a local pipeline infrastructure to export oil. A vessel used only to store oil (without processing it) is referred to as a floating storage and offloading (FSO) vessel.

There are various configurations of FPSO connections to subsea and off-loading used mainly based on geographical location. The figure below shows a typical configuration.

Figure 1 Typical FPSO Connections Diagram

The ballast system on an FPSO is used to manage the draft, trim, list, stability and longitudinal static bending moment for all loading/off-loading conditions and sea states.

Effective use of the ballast system minimizes stresses in the hull due to shearing forces and bending moments induced during the FPSO towing phase and during on-station operation.

The ballast system design on an FPSO could be very different to a trading vessel as the real estate on an FPSO is crucial and critical.

After comparing the various alternatives, a design is proposed to locate the ballast pumps and associated piping within the ballast tanks.

2. DRIVERS OF BALLAST SYSTEM

For trading vessels, the main criteria for the ballast system design is to compensate draft for off-loaded cargo when the vessel is not fully laden to provide for the vessel’s seaworthiness, to compensate for the increased buoyancy which can result in the lack of propeller immersion, inadequate transversal and longitudinal inclination, minimize stresses on the vessel’s hull, and to keep the vessel stable within the allowable stresses.

Unlike trading vessels, loading on an FPSO is a continuous process, though at much slower rate.

Fpso Ballast System Design – A Concept


Major ballast operation is undertaken when cargo is being discharged to an export tanker either by tandem off-loading or side by side off-loading or through a single buoy mooring (SBM). De-ballast is also done periodically to adjust the draft, trim and heel within the operating window and reduce the free-surface effect. The ballast system may also be used to manage the stability and ship inclination post collision.

For an FPSO main drivers for ballast system design are as listed below:

• to manage inadequate transversal inclination, i.e. heeling

• to manage inadequate longitudinal inclination, i.e. trim

• to manage static and dynamic stresses on the vessel’s hull including shear and torsion forces, bending moments and slamming

• to manage static and dynamic transversal and longitudinal instability

• to manage the draft during continuous low volume crude loading and bulk crude off-loading

• to manage the stress and fatigue on the mooring system, e.g. internal or external turret and mooring ropes

• to manage the F(P)SO motions when off-shore supply vessels coming alongside, especially in rough sea conditions

• to manage the F(P)SO motions during helicopter operations, especially in rough sea conditions

• to manage the F(P)SO motions for process conditions

• to manage the F(P)SO motions for crew comfort etc.

• to manage non-equal distribution of weights

• to address the weather and sea conditions

• to manage the consumption of fluids between two off-loadings, e.g. fuel, lube oil and freshwater

The ballast system should be typically capable of:

• filling / emptying ballast tanks from / to the sea (ballasting / de-ballasting) by pumping.

• discharging ballast water overboard by pumping.

• free flooding of ballast tanks by gravity flow in/out through the sea suction chest(s) to the FPSO draught level

• transfer of ballast water between ballast tanks by pumping

• simultaneously empty and fill ballast tanks on the port and starboard sides

Seawater is ballasted and de-ballasted in / from dedicated ballast water tanks using the ballast water system to adjust the required draft, vessel trim and list.

3. DESIGN PROCESS

For trading vessels, the ballast capacity is mainly determined by the vessel cargo capacity in terms of cargo weight and the speed at which the cargo operations may be conducted. Generally, the more weight of cargo a vessel is capable to carry, the more ballast may be needed when sailing without cargo on board. If the cargo operations on a vessel are very fast, then the ballast uptake or discharge has to be correspondingly fast. The ballast water capacity of a vessel is given in terms of volume of spaces that are available for ballasting expressed in m3 and in terms of the ballast pumps capacity expressed in m3/h.

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Ballast water is carried by many types of vessels and is held in a variety of tanks or holds. The relative complexity of ballast operations depends on the size, configuration, and requirements of the ship and on the complexity of its pumping and piping systems. Large tankers can carry in excess of 200,000 m3 of ballast. Ballasting rates can be as high as 15,000 to 20,000 m3/h.

For an FPSO, the cargo is continuously loaded though at much slower rate. Using all the criteria explained in Section 2.0, operating draft of the facility is decided. Once the operating draft of the facility is known, the next step is to carry out the ballast pump capacity calculations.

3.1 Case Study -

The ballast pump capacity calculation is explained by an example -

• Facility operating draft –

• Design max draft = 22 m (even keel).

• Design minimum mean Draft = 17 m

• Design offloading Parcel size = 850,000 Bbls

• Off-loading time = 16 h

• Production rate = 120,000 Bbls /day = 19,081 m3/day = 15799 MT/day = 658 MT/h.

Ballasting capacity shall be adequate to maintain minimum operating draft of an FPSO at all times throughout any export operation while minimizing any requirements for pre-off take ballasting.

The necessary ballast operations shall be concurrent and concluded within the off-take time.

Production in 16h which will assist in reducing the ballast -

Bbls/day m3/day MT/day MT/h

Production 120,000 19,081 15,799 658

Off-loading rate for 850,000Bbls parcel size -

Bbls/ 6h m3/16h MT/16h

850,000 135,178 111,927

3.2 Assumptions -

• Produced gas is consumed and exported so there is no accumulation of gas on an FPSO on routine basis.

• Produced water is injected back into the field with the same rate so that there is no accumulation onboard.

• Fresh water is produced and consumed at same rate

• There is no appreciable consumption of marine gas oil (MGO) which will bring any major changes in draft

• The ballast capacity shall handle at least 40% of the cargo capacity as a minimum and shall be capable of maintaining safe vessel stability, trim and draft for all possible loading and unloading conditions (including emergency or non-normal operations)

• The number of ballast tanks for forward the ring-main are four (4) following a N+1 philosophy. Hence, ballast pump configuration is 4X33%.

• Two cases were considered –



3.3 Case 1 The off-loading commences when the FPSO is at maximum operating draft of 22 m for parcel size of 850,000 Bbls.

Figure 2 Case 1 Pump Sizing Calculation

Each ballast pump capacity = 715 m3/h.

3.4 Case 2 The off-loading commences 2.5 days after the previous off-loading for same parcel size of 850,000 Bbls.

Cargo quantity added in 2.5 days = 300,000 Bbls

Draft at the end of 2.5 days of production (without ballasting) = 19.63 m

Cargo off-loading window = 16 h.

Figure 3 Case 2 Pump sizing calculation

Each ballast pump capacity = 1679 m3/h.

Note – Aft ring-main ballast pumps are not considered in design cases while deciding the ballast pump capacity as these pumps will rarely run.

The basis of the FPSO location cases are generated as these will dictate the conditions to avoid tank-top conditions. Considering the FPSO is located in harsh environment, case 2 is considered credible for deciding the ballast pump capacity.

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4. FPSO’S PROPOSED BALLAST SYSTEM

Traditionally the tanker vessels have separate ballast system for hazardous and non-classified areas. For hazardous areas the ballast pumps are located in pump room and for non-hazardous area, the ballast pumps are located in aft machinery room.

In the proposed FPSO design, the ballast system is two separate closed loop systems using ballast ring mains. The forward and aft ballast piping is configured as two large ring-mains, consisting of a port and starboard header with crossover on forward and aft.

The design is in compliance with class rules to have separate ballast systems for hazardous and non-hazardous areas.

The ring mains are installed in the wing ballast tanks with isolation valves at each bulkhead penetration and connected to the individual ballast tank from the adjacent tank with a fail closed bulkhead isolation valve. The ballast system is completely independent with no connection to machinery spaces, cargo tanks or any other piping systems.

The ballast ring main design allows the flexibility to use the bilge suction for applicable void spaces as per class rules.

In the presented examples of double hull construction FPSO design, the following ballast tanks are shown: –

• In the bow (Fore Peak Tank - forepeak tank)

• In the stern (Aft Peak Tanks – after peak tanks)

• Port and starboard, “L” shape tanks

• Selected void spaces which are connected to ballast system for bilging purpose.

Ballast tanks are connected with the in-tank ballast water pumps by a ballast water pipeline located inside the ballast tanks.

Sea water is loaded on the F(P)SO through the sea-chests via the ballast pipe in to the ballast tanks. Inside the ballast tanks sea water is loaded and discharged via the ballast water pipe suction bell mouth.

Forward ballast ring main is equipped with 4 ballast pumps. Whereas, the aft ballast ring is equipped with 2 ballast pumps.

The ballast system has been designed to handle the list and trim of the FPSO during the maximum loading and off-loading conditions and sea states.

The ballast piping material often selected is Glass Reinforced Epoxy (GRE) due to light weight in comparison to steel piping.



It is absolutely critical to know how much ballast is in each tank to be able to provide for the vessels seaworthiness. The ballast tanks are proposed to be equipped with instruments that automatically measure the quantity of sea water in ballast tanks. Ballast tanks will also be equipped with sounding pipes to allow direct measurements in the case of automatic system failure.

The ballast water is discharged through the overboard discharge, which are situated above the water level.

5. F(P)SOS BALLAST WATER TANKS ARCHITECTURE DESIGN

(CAPACITY AND NUMBERS DESIGN CRITERIA)

Generally, for the number of FPSO ballast tanks and their sizes are designed to meet the strictest safety standards and to comply with the damage criteria as per MARPOL rules. In addition, interpretation that the worst-case damage condition could involve flooding of two adjacent empty ballast tanks plus the two adjacent empty cargo tanks (two adjacent ballast tanks and adjacent cargo tanks all empty at the same time is not likely to occur in normal operations but may occur during tank inspection/maintenance activities that would only take place during the turnaround period).

Ballast tanks are equipped with hydrocarbon gas monitoring and with a temporary connection from inert gas main to replace the ullage spaces with inert gas.

In addition, a permanent connection is provided to transfer the contaminated water from ballast tanks to slop tanks in case of leakage of cargo into adjacent ballast tank by using any of the forward ring main pump.

Generally, following the criteria is considered to meet intact and damaged stability.

5.1 Intact criteria

• The International Code on Intact Stability, 2008

• Code for the Construction and Equipotent of Mobile Offshore Drilling Units (MODU CODE 2009 Ch. 3 Pt. 3) on Subdivision, Stability and Freeboard

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• Applicable Class requirements, e.g. for DNVGL Class (DNV-OS-C301)

• Local admiration rules where the FPSO will be finally placed, such as UK HSE Intact Stability criteria (RR387) for facilities placed in UK jurisdiction

5.2 Damaged criteria

• MARPOL Reg. 28 73/78

• Applicable Class requirements, e.g. for DNVGL Class (DNV-OS-C301)

• Local admiration rules where the FPSO will be finally moored, such as UK HSE Damaged Stability criteria (RR387) for facilities moored in UK jurisdiction

5.3 The International Code on Intact Stability, 2008

Chapter 2.3 requires: –

1. The ability of a ship to withstand the combined effects of beam wind and rolling shall be demonstrated with reference to Fig.1 as follows:

a. the ship is subjected to a steady wind pressure acting perpendicular to the ship's centerline which results in a steady wind heeling lever (lW1);

b. from the resultant angle of equilibrium (θ0), the ship is assumed to roll owing to wave action to an angle of roll (θ1) to windward.

2. The angle of heel under action of steady wind (c0) should not exceed 16° or 80% of the angle of deck edge immersion, whichever is less;

3. the ship is then subjected to a gust wind pressure which results in a gust wind heeling lever (lW2); and

4. under these circumstances, area "b" shall be equal to or greater than area a, as indicated in figure 5 below;

Figure 5 Severe Wind and Rolling

Where the angles in figure 1 are defined as follows;



θ0: Angle of heel, in degrees, under action of steady wind

θ1: Angle of roll to windward due to wave action

θ2: Angle of down flooding (θ f) in degrees, or 50˚ or θ C, whichever is less,

where:

θf: Angle of heel in degrees, at which openings in the hull, superstructures or deckhouses which cannot be closed weathertight immerse. In applying this criterion, small openings through which progressive flooding cannot take place need not be considered as open;

θC: Angle in degrees, of second intercept between wind heeling lever lW2 and GZ curves

2) The wind heeling levers lw1 and lw2 are constant values at all angles of inclination and shall be calculated as follows:

where:

P = wind pressure of 504 Pa. The value of P used for ships in restricted service may be reduced subject

to the approval of the Administration

A = projected lateral area of the portion of the ship and deck cargo above the waterline (m2)

Z = vertical distance from the center of A to the center of the underwater lateral area or approximately to a point at one half the mean draught (m)

Δ = displacement (t)

g = gravitational acceleration of 9.81 m/s2.

5.4 HSE Intact Stability Criteria (RR387)

The FPSO is a surface unit; it is therefore required that surface stability criteria are adopted for the assessment. Detail intact stability criteria are tabulated as below in Figure 6 and Table.1. HSE intact stability is checked to calculate the allowable VCG curves and all loading conditions are comparing with this allowable VCG curve.

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Figure 6 Intact Stability Criteria

Table 1- HSE Intact Stability Criteria

5.5 MODU CODE (2009 Ch. 3 Part 3)

Under 70/100 knots wind, the requirements to be fulfilled consist of:

1. The area under the righting arm curve, from 0° to flood or second intercept, shall not be less than 40% in excess of the area under the wind heeling curve



2. The righting arm curve shall be positive over the entire range of angles, from 0° to flood or second intercept (of wind heeling and righting arm curves).

A typical curve is illustrated below;

Figure 7- MODU Intact Criteria

5.6 UK HSE Damaged stability criteria (RR387)

Consideration should be given to providing the unit with sufficient reserve stability in damaged conditions to withstand the wind heeling moment based on a wind velocity of at least 25.8 m/s (50 knots) imposed from any direction. In this condition the final waterplane after flooding, taking into account sinkage, trim and heel, will need to be below the lower edge of any opening through the external watertight boundary.

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Figure 8 The damaged stability criteria.

5. CONCLUSION

To effectively use the real estate on an FPSO, improve ballasting, de-ballasting and transferring functionality, the effort led which resulted two separate closed loop ballast ring main system for hazardous and non-hazardous areas design. Since the pumps and ballast piping were located inside the ballast tanks, lots of useful area in aft machinery space and separate pump room for forward ballast pump room was saved.

The design provides adequate redundancy for ballast pumps in system.

Among the several advantages of this design, the most important advantage was to free up space occupied by pumps and piping laid in the aft machinery room for the aft ballast tanks.



Also, the requirement for a pump room was eliminated for the forward ballast tanks due to a forward ballast ring main for the hazardous area.

REFERENCES

[1] The International Code on Intact Stability, 2008

[2] MODU CODE

[3] DNVGL Class Rules for ships

[4] UK HSE Intact stability criteria (RR387)

[5] Global Maritime Transport and Ballast Water Management: Issues and Solutions - by Matej David, Stephan Gollasch

[6] FPSODiagram

https://en.wikipedia.org/wiki/Floating_production_storage_and_offloading

[7] Ballast Water and Ships https://www.nap.edu/read/5294/chapter/4

Documents

FPSO BALLAST SYSTEM DESIGN A CONCEPT · 2018-11-02 · Trading vessels are built to transport various cargoes or passengers’, but the design criteria of an FPSO ballast capacity