Onboard the World's Largest and World's First Purpose Built LPG FPSO SANHA

The Sanha LPG FPSO is the World’s largest in terms of LPG storage capacity and first New Build LPG FPSO. ?I was very fortunate to be a part of this Project as the 1st Indian Chief Officer to be selected by Chevron, almost 10 years ago.

The Sanha LPG FPSO has some of the largest and first of its kind equipment and is truly a prototype.

The Sanha LPG FPSO has six (6) insulated SPB (Self-standing Prismatic IMO Type B) tanks for storage of both propane and butane products, stored separately after fractionation. The six tanks provide a total storage capacity of 135,000 m3 at a design temperature of – 50 C and a design pressure of 0.45 barg. The SPB tanks, a proprietary IHI Marine United design, are made of low temperature carbon steel. The cargo tank supports are made of reinforced plywood.?

Engineering, fabrication and procurement activities were carried out all over the world for the Sanha LPG FPSO. The vessel launching from the Kure Shipyard dry dock occurred in November 2003.

One paramount criterion for the hull design is the sea-keeping behaviour of the FPSO. The depropanizer column imposes very stringent motion limits to ensure the fractionation process delivers “on-spec” propane product. After detailed studies it was decided that the FPSO should not roll more than 5° and not pitch more than 1° up to the 10-year environmental conditions as predicted for the Sanha site. An extensive model test program in a basin confirmed the FPSO motions calculations.

?The FPSO is designed to stay on-station at the Sanha site for 20 years without drydocking. In order to achieve this goal, onerous design criteria are imposed on the hull and topsides structures:?

?????????The Hull and Topsides has a fatigue life of 20 years, based on North Atlantic environmental conditions;

?????????The Hull and Topsides has a fatigue life of 30 years, based on the environmental conditions of Sanha site;

?????????Not only the hull but also the topsides are designed for the North Atlantic 20-year return conditions;

?????????The use of high tensile steel is restricted;

?????????A high quality coating system is applied in the ballast tanks.

?Mooring & Fluid Transfer System

?The FPSO is permanently moored to the seabed by an external turret mooring system that was designed and fabricated by SBM. The external turret mooring system is cantilevered from the bow and comprises both a rotating and a fixed part. The two parts are connected via a main ‘slewing’ bearing, which allows the turntable to freely weathervane around the anchoring system so that the FPSO can take up the position of least resistance to the prevailing weather at all times.

?The anchoring system consists of an array of nine (9) chain legs and nine (9) drag anchors. The drag anchors are off SBM standard “MAG” design and each anchor weighs 28 tonnes and is among the largest ever built by SBM.

?Two 6” flexible risers lift the two incoming LPG feeds from the Pipeline End Manifold (PLEM) to the turret. A 4” flexible riser is provided to supply Fuel Gas to the FPSO, to be burnt in the steam boilers. Emergency Shutdown valves are provided on the PLEM to isolate the pipeline in case of emergency.

?The fluid transfer system from the fixed to the rotating part of the turret consists of swivel stack of two identical toro?dal swivels for the LPG feeds and one in-line pipe swivel for the Fuel Gas line. An electric swivel transmits electrical power and control signals to the fixed part of the turret.

?The Depropanizer Plant is the heart of the FPSO. It is located slightly aft of the FPSO midship

Mixed LPG streams consisting of propane and butane arrive at the FPSO via two 6-inch pipelines from the Sanha Processing/Compression Platform in Area C and the Fox-GIP Platform in Area A. On board the FPSO, these streams are metered. The LPG streams are treated to remove trace levels of H2S in the H2S Removal Vessels.?

The combined streams are then filtered and fractionated in the Depropanizer. The bottoms butane stream coming off the Depropanizer is partially cooled in two Depropanizer Inlet /Bottoms Heat Exchangers.

?The butane is then cooled to storage temperature in the Butane Refrigeration Plant and then routed to the FPSO cargo tanks for storage. Propane coming off the overhead of the Depropanizer is condensed in the Depropanizer Condensers and then flow to the Depropanizer Accumulator.

?Part of the propane stream returns to the column as reflux, while the remainder is partially cooled in the Depropanizer Inlet/Overhead Product Heat Exchanger. The propane is then cooled to storage temperature in the Propane Refrigeration Plant and routed to the cargo tanks for storage.

?Refrigeration and Reliquefaction Plants

The propane refrigeration consists of a 2-stage cooling plant, using a closed loop of commercial propane as refrigerant. There are 3 cooling units (numbered C1~C3) in total, all using oil-flooded screw compressors. The propane stream is cooled in two units arranged in series (C1 + C2 or C2

+C3). The propane refrigeration plant also supplies refrigerant to a trim cooler, which is used when the depropanizer plant is operating at low turn-down rate.

The propane refrigeration plant is installed in an enclosed deckhouse located on the topsides forward of the depropanizer plant, over the pipe racks. The deckhouse is split into a compressor room and a motor room. A gas-tight bulkhead separates the two subdivisions.

The butane refrigeration consists of a single stage cooling plant, using also a closed loop of commercial propane as refrigerant. It consists only of 2 cooling units (C4~C5), using also Aerzener oil-flooded screw compressors. The butane stream is cooled in either of the two units. The butane refrigeration plant also supplies refrigerant to the reliquefaction plant.

?All five refrigeration units C1~C5 are of the same design. Each skid consumes around 2 MW of electrical power.

?The reliquefaction plant is designed to separately handle the boil-off gas from the propane and butane storage tanks. It is different from the design commonly applied on LPG tankers since it is of the cascade type, using refrigerant from the butane refrigeration plant.

?The propane reliquefaction plant consists of three units (numbered R1~R3). In normal operation, two units are required to re-liquefy the propane vapors. R3 unit acts as stand-by unit both for the propane and the butane reliquefaction. To achieve that dual function, the R3 compressor is fitted with a two-speed electric motor.

?The butane reliquefaction plant consists of a small direct type fresh water-cooled unit (R4). All four reliquefaction skids are using oil-free screw compressors. The reliquefaction plant is also installed in an elevated deckhouse located aft of the depropanizer plant. It also houses the two butane refrigeration units. The building is of the same design as that of the propane refrigeration plant.

?Marine Systems

The Sanha LPG FPSO includes both a forward and an aft machinery room. The equipment fitted in these two spaces supplies all of the utilities necessary for the accommodations and topsides operations. FPSO utilities that are provided include approximately thirty (30) auxiliary systems such as control air, utility air, inert gas, nitrogen, cooling water, fire water, potable water, sewage treatment, hydraulic oil, fuel oil, etc. In this space alone, there are over 60 pumps, 1,200 valves, and 10,000 spool pieces of piping.

?The key equipment installed in the forward machinery space includes three (3) dual fuel, “D”-type, 90 ton/hr. The boilers, the largest ever built of that type, provide superheated steam for power generation in three (3) 9MW steam turbines and saturated steam for the topsides process heating demand. Under all normal operating conditions, only two boilers shall be operating with the third boiler on warm standby.

?In addition to the key equipment above, a number of other auxiliary equipment are installed including items such as purifiers, condensers, distilling plants, sewage treatment plants, incinerators, etc.

In the aft machinery space, a 4,000-HP tunnel thruster with variable pitch is installed. The function of this thruster is to increase the fractionation system uptime and throughput by minimizing vessel rolling and the associated negative impact this would have on production. Furthermore, it can also adjust the FPSO heading before and during cargo export operations.

The aft machinery space also houses two diesel engine driven emergency fire and deluge pumps.

?Offloading

The Sanha LPG FPSO has a much greater offloading flexibility than any other turret-moored FPSO. This outstanding flexibility includes:

·???????The capability of exporting cargo to LPG tankers moored either in tandem at the stern of the unit or in side-by-side (SBS), on both sides of the FPSO.

·???????The capability of accommodating all sizes of export tankers, from small pressurized coastal vessels of 2,000m3 capacity to fully refrigerated Very Large Gas Carriers (VLGC’s) of 85,000m3 capacity. The tandem mooring is even designed for future VLGC’s of 125,000m3 capacity.

·???????The capability of exporting to side-by-side moored refrigerated propane and butane export tankers; with vapor return from the export tankers (starboard side only) back to the FPSO.

The stern tandem offloading system consists of the following main components:

·????????A cryogenic floating hose assembly of 12” diameter, approximately 200m long, connected from one of the two the FPSO stern manifolds to the export tanker midship manifold;

·????????A mooring hawser that will connect the export tankers bow to a chain stopper located at the stern of the FPSO.

For SBS offloading, export tankers will be moored with multiples lines, some of them to quick release hooks (QRH) installed on the deck of the FPSO, the others to mooring winches suitably located on the main deck. Due to the variety of tankers likely to moor alongside, not less than 48 QRHs and 7 winches are provided, offering an infinite number of mooring combinations. For VLGCs, which will be of the same length as the FPSO, up to 16 mooring lines might be deployed at the same time.

The other main features of the SBS offloading system are as follows:

·????????Five (5) export manifolds on starboard side midship, respectively for refrigerated propane and butane, propane and butane vapour return and pressurized butane.

·????????Three (3) export manifolds on portside midship, respectively for refrigerated propane, refrigerated butane and pressurized butane.

·????????Two (2) 12” corrugated stainless hose strings for refrigerated products export.

·????????Two (2) 8” corrugated stainless hose strings for propane and butane vapour return.

·????????One (1) 6” rubber hose string for pressurized butane export.

·????????Two knuckle boom type hose-handling cranes on starboard side, used to hold the export hoses between the FPSO and the export tankers. Up to four hose strings might be deployed at the same time.

·????????One knuckle boom type hose handling crane on portside, for the same function.

?Safety in Design

Various engineering safety studies were carried out at an early stage of the design to validate the main technical choices. The most important ones included:

·????????A Fire & Explosion Analysis (FEA), aiming at simulating various Fire an Explosion scenarios and their consequences on the asset. Some modifications of the topsides layout were made as a result of that study, in order to mitigate the consequences of explosions overpressure.

·????????A Smoke & Gas Dispersion Analysis (SGDA) that confirmed that gas or smoke clouds resulting respectively from accidental gas leaks and fires would disperse into the atmosphere without any risk escalation.

An Emergency Systems Survivability Analysis (ESSA) that evaluated qualitatively the robustness of all safety systems

·????????An Emergency Escape & Rescue Analysis that studied qualitatively means of egress and rescue.

·????????Hazard & Operability Studies (HAZOP) and Hazard Identification Studies (HAZID) were conducted during the design phase. All essential FPSO systems were extensively analysed by such HAZID and HAZOP Techniques.

In addition, a pro-active safety behavior was adopted during design both by the designer and the approval parties to detect and correct unsafe design features at the drawing board level. Special attention was paid to avoiding tripping hazards, designing safe ladders, platforms, staircases etc.

Some Memorable Pics from my time onboard the LPG FPSO Sanha