Patent Application
20200216150
The present invention generally relates to offshore vessels used for the production and/or storage of petroleum products. More specifically, the present invention relates to offshore vessels for connection of a plurality of submarine risers and a deck structure to support topside modules, such as a Floating Production Storage and Offloading vessel (FPSO) or a Floating Liquefied Natural Gas vessel (FLNG). The hull of the vessels may also be used as a base for a drilling ship.
A Floating Production Storage and Offloading (FPSO) system is a floating facility above or close to an offshore oil and/or gas field to receive, process, store and export hydrocarbons.
The system consists of a floater, which may either be a purpose-built vessel or a converted tanker, moored at a selected site. The cargo capacity of the vessel is used as buffer storage for the produced oil. The process facilities (topsides) and accommodations are installed on the floater. The mooring configuration in FPSOs may be of a spread mooring type or a single point mooring (SPM) system such as a turret. A mooring configuration based on Dynamin Positioning (DP) is also possible, but not recommendable due to high complexity and cost.
The high-pressure mixture of produced fluids from the well is delivered to the process facilities on the deck of the vessel in which oil, gas and water are separated. The water may be reinjected in the reservoir or discharged overboard after treatment eliminating hydrocarbons. The stabilized crude oil is stored in the cargo tanks of the vessel and subsequently transferred to trading tankers, either directly, via a buoy, by laying side by side/in tandem to the FPSO vessel or by use of shuttle tankers/cargo transfer vessels (CTV).
The gas may be used for enhancing the liquid production through gas lift and/or for energy production onboard the vessel. The surplus of gas may be compressed and transported by pipeline or reinjected into the reservoir.
A Floating Liquefied Natural Gas vessel (FLNG) is conceptually similar to the FPSO. The difference being that the hydrocarbon mixture from the well is predominantly gas and that the process facility's purpose is to separate, clean and liquefy the gas for storage in dedicated cryogenic tanks within the hull. Offloading of the liquefied gas is done towards trading gas (LNG) vessels.
Conventional ship shaped FPSOs require weather vaning facilities such as a turret when located in harsh environmental areas. A characteristic for these types of FPSOs is however a very different pitch and roll behavior, allowing large waves in head sea conditions, but significantly smaller waves from beam and quartering seas. Weather vaning is hence needed for these type of ships.
Semi-submersible designs may provide favorable and uniform motions. The storage capacity is however limited, and the sensitivity with respect to topside weight is critical. A semi-submersible design is hence not considered advantageous when large storage capacity is an important design criteria as for an FPSO or FLNG unit. Further, structural details are more complex on semi-submersibles, resulting in higher steel-weight per ton topside payload, as well as higher fabrication cost. An early example of a semi-submersible platform is disclosed in WO 02/090177 A1.
Other designs such as box shaped units or cylindrical hulls may provide uniform motional behavior independent of wave direction. If equipped with motion suppressing elements favorable motions may also be achieved. The shape of such units does however not allow use of standard ship-shaped construction facilities. Automatic panel line facilities can not be used without significant modifications, and the shape/dimension gives further limitations. The critical measurements in this respect are breadth and depth. For use of cylindric designs having storage capacity greater than 1,000,000 bbl there will be significant limitation with respect to available dry docks and floating cranes. Another disadvantage of the cylindric design is the low deck-area-to-storage-volume ratio and maximum obtainable distance from safe to hazardous side which complicates the topside design. (1 bbl (oil barrel) is a unit of volume corresponding to 159 liters).
US 2004/0067109 A1 discloses a drilling vessel without storage capability having an elongated shape, preferably of rectangular shape, and moored to the sea bed in a substantially fixed orientation. The vessel comprises two transverse skirts near its keel level having such a width that the natural roll period of the vessel is above a predetermined period. US 2004/0067109 A1 states that length-to-width ratio of the vessel should be at least 1.5, preferably at least 2, since a length-to-width ratio of 1.5 or less may be subject to roll instability or Mathieu instability. The objective of the design of this prior art vessel is to control roll, hence not the combination of heave, roll and pitch. Similar elongated vessels without a pronounced bow and having length-to-breadth-ratios greater than 1.5 are disclosed in patent publications US 2011/0209655 A1, U.S. Pat. No. 4,015,552 and US 2002/0083877.
WO 2015/038003 A1 discloses a platform comprising a hull with a main portion which is substantially axis-symmetrical about a center axis, without a pronounced bow and parallel mid-ship. The upper end of the platform is supporting a deck and the lower end of the platform, situated below a nominal water line, is provided with a non-circular stabilizing element which protrudes from the main portion.
WO 2012/104308 A1 discloses a cylindrical platform for production and storage of hydrocarbons. The substantially circular hull of the vessel is configured to allow suspension of risers on at least one frame arranged in a moonpool in the center of the hull. The frame is placed so that connection of risers may be performed above the water-line when the platform has its minimum draft. The moonpool may comprise a conical form at its lower end allowing static and dynamic angular deflections of the risers. The moonpool extends above the main deck wherein the extended vertical moonpool is narrowed down for increasing the space availability on the deck. The hull may further be equipped with a protrusion to reduce heave, pitch and roll motion.
WO 2014/167591 A1 discloses a drillship with a pronounced bow and parallel mid-ship, where its heave and pitch behavior has been improved by the addition of a protuberance having a flattened shape, either at the bow or at the stern. Due to the lack of roll damping devices this vessel will experience significant roll motions if exposed to waves from abreast. Further, no turrets are disclosed in WO 2014/167591 A1. It is hence assumed that the ship's positioning system is based on a DP system since a spread mooring system would not be able to sufficiently suppress the wave induced motions.
The above mentioned prior arts do not disclose vessels having a design that enables safe, easy and effective handling in harsh environment at the level offered by the FPSO of the present invention.
Consequently, the object of the present invention is to provide a vessel for production and/or storing of hydrocarbons arranged to float in a body of water, hereinafter abbreviated FPSO, offering beneficial properties concerning motional behavior, storage capacity and safety, relative to prior art FPSOs. The application is equally relevant for similar purpose vessels such as FSO or FLNG, but only the term FPSO is used in the following for simplicity.
A second object of the invention is to provide a vessel of non-cylindric design that has motional behavior that is independent of wave direction relative to vessel orientation. Heave, pitch and roll motions shall be favorable and uniform regardless of position on the vessel.
A third object of the invention is to provide an FPSO which is spread moored and does not require a turret, or equipment similar to a turret.
A fourth object of the invention is to provide an FPSO in which the number and/or dimension of mooring lines are less than the number and/or dimension used on conventional spread moored FPSOs of comparable storage capacity.
A fifth object of the invention is to provide an FPSO having a bow design that optimizes the orientation at the field, both with respect to green sea protection and with respect to mooring through reduced drag/wave forces on the hull.
A sixth object of the invention is to provide an FPSO design that is suitable both in benign and harsh environmental conditions.
A seventh object of the invention is to provide an FPSO that is scalable in size with respect to its oil storage capacity.
An eighth object of the invention is to provide an FPSO having a vessel design that enables a higher topside weight capacity compared to conventional FPSO designs.
A ninth object of the invention is to provide an FPSO having a vessel design that ensures a large deck area for placing topside modules and a simple interface, compared to rotational symmetric FPSO designs.
A tenth object of the invention is to provide an FPSO having a design and size such that fabrication can be carried out using standard ship building facilities including existing dry docks, thereby allowing flexibility in choice of fabrication yard.
An eleventh object of the invention is to provide an FPSO having favourable and uniform vertical motions, thereby allowing riser hang-off at any longitudinal and transverse positions.
A twelfth object of the invention is to provide an FPSO having a design that through adjustment of its suppressing element/bilge box allows for use of free hanging steel catenary risers (SCRs) in harsh environment for large water depths, for example between 1,500 meters and 3,000 meters. SCRs may also be applied for more shallow water in case of more benign environmental conditions.
A thirteenth object of the invention is to provide an FPSO design with significantly reduced fatigue compared to conventional ship shaped FPSOs.
In addition to fulfilling one or more of the above-mentioned objects, the particular vessel design of the FPSO should preferably comply with international regulations including class society, MARPOL (International convention for the prevention of pollution from ships), SOLAS (international convention for the safety of life at sea) and/or relevant site specific shelf state requirements. Further, the inventive FPSO should preferably fall within the rule regime associated with conventional ship-shaped vessels.
The above-mentioned objects are obtained by the invention as set forth and characterized in the main claims, while the dependent claims describe further embodiments of the invention.
In particular, the present invention relates to a spread moored vessel suitable for production and/or storage of hydrocarbons. The vessel comprises a laterally extending main deck, a mooring arrangement suitable for mooring the vessel to a seabed when the vessel is floating in water, and a longitudinal hull. The mooring arrangement is preferably arranged symmetrically relative to the main deck, i.e. mirroring at least one central plane of the hull directed perpendicular to the main deck. The longitudinal hull further comprises a bow, a midbody, a stern and at least one motion suppressing element protruding out from the longitudinal hull, below the vessel's maximum draught, preferably from each of the hull sections. The motion suppressing element(s) causes a significant reduction of undesired motion of the vessel, especially heave, pitch and roll. The ratio between a maximum length and a maximum breadth of the longitudinal hull, at the vessel's maximum draught, is between 1.1 and 1.7, more preferably between 1.1 and 1.7, even more preferably between 1.2 and 1.4. The particular ratios, in combination with the motion suppressing element(s), have the advantage that the effect the waves has on the movements on the vessel relative to longer vessels is reduced, thereby making the vessel more stable during operation. The longitudinal hull as seen from above may have a shape of a rectangle with a rounded triangle at the forward end.
As a consequence of the above features, the vessel motion will be almost independent of wave direction and the requirement of mooring systems other than a spread mooring system may be eliminated. Further, the total number of mooring lines may be reduced as compared to conventional ship shaped spread moored FPSOs, thus reducing complexity and cost for the vessel's mooring arrangement compared to prior art vessels having the same or similar function. It should be noted that spread moored arrangement can only be applied to conventional FPSO designs for areas with relatively benign wave climate.
The term ‘laterally extending main deck’ signifies a deck having a surface that extends parallel to the water surface when the vessel is floating in a body of motionless water. Further, the hull is hereinafter defined as the area of the longitudinal vessel situated below the main deck area of the vessel.
In an advantageous example the motion suppressing element(s) protrude(s) laterally from the hull along at least 70% of the hull's lateral extending circumference, more preferably at least 80%, for example along the entire circumference.
In another advantageous example the motion suppressing element(s) protrude(s) laterally from a lowermost part of the hull. The lowermost part may be flat, i.e. parallel to the deck.
In yet another advantageous example, the lateral protrusion length of the motion suppressing element(s) is between 5% and 30% of the hull's maximum breadth at the vessel's maximum draught.
In yet another advantageous example, the midbody comprises a port side portion and a starboard side portion, where at least 30% of the longitudinal length of the midbody are flat, i.e. without kinks and/or curves, and oriented parallel to a center plane of the hull. The center plane is hereinafter defined as the plane intersecting the hull midway between midbody, i.e. midway between the port and starboard side portions, and aligned perpendicular to the laterally extending main deck.
In yet another advantageous example, the transition region between the bow and the midbody forms abrupt change of angle at the vessel's maximum draught, relative to the tangent plane of the midbody directed parallel to the center plane, preferably at least 20 degrees.
In yet another advantageous example, the longitudinal length of the bow at the vessel's maximum draught is at least 25% of the maximum length of the hull.
In yet another advantageous example, the mooring arrangement comprising a plurality of mooring lines, wherein at least one mooring line is moorable from a location at or near the center of the bow relative to the hull's breadth, at least one mooring line is moorable from a location adjacent the stern at the port hull side and at least one mooring line is moorable from a location adjacent the stern at the starboard hull side. However, in this particular embodiment additional mooring lines may be arranged at other locations around the lateral periphery of the hull in order to obtain the required positioning/stability. At the locations of the plurality of mooring lines the at least one motion suppressing element has preferably a suitable recess, or is omitted totally, for allowing the mooring lines to be guided into the body of water closer to the lateral center of the vessel. These recesses may also provide additional control of the vessel's motion.
In yet another advantageous example, the longitudinal length of the vessel is separated into a cargo zone and at least one non-cargo zone, for example by a wall and/or a safety distance. Further, the longitudinal hull displays at least one cargo tank for containing cargo, wherein the cargo tank, or all cargo tanks in case of a plurality of cargo tanks, are confined within the cargo zone of the vessel. No cargo tanks are thus located outside the cargo zone. The non-cargo zone is preferably situated at the bow of the vessel. However, such a non-cargo zone may also be situated at the stern for specific topside layouts. Further, the hull may be double side around its circumference of the vessel, having one or more ballast tanks in between the hull walls.
In yet another advantageous example, the longitudinal hull further displays at least one slop tank situated adjacent to the at least one cargo tank, for collecting drainings, tank washings and other fluid mixtures. The at least one slop tank is preferably arranged in or adjacent to the center plane of the hull.
In yet another advantageous example, at least one of the at least one non-cargo zone is located within the bow.
In yet another advantageous example, the longitudinal hull comprises at least two walls having a space therebetween, into which at least one ballast tank is located.
In yet another advantageous example, the vessel is configured to allow hang off of a multiple riser arrangement at the midbody, the bow and/or the stern.
In yet another advantageous example, a plurality of riser guide pipes is arranged along at least part of the lateral circumference of the longitudinal hull. Each of the plurality of riser guide pipes is configured to allow at least one riser to be guided therethrough.
In yet another advantageous example, the projected lateral surface area of the hull at the vertical position of the main deck is larger than the projected lateral surface area of the hull at the vertical position of the vessel's maximum draught, preferably at least 10% larger, for example 20% larger. The onset of the increase preferably commences at or above the vessel's maximum draught. The full increase from the onset may take place abruptly. However, it is preferable that the increase is continuous, for example a linear increase with a ratio of 1:2, or a comparable parabolic increase. Such a vessel design increases the deck area available for placing topside modules, while enabling a simple interface. This, in combination with a stern and a midbody of rectangular shape forms a large available space for topside modules on the deck compared to conventional ship shaped FPSOs.
In yet another advantageous example, the ratio between the maximum length of the longitudinal hull and a maximum depth of the longitudinal hull, where the maximum depth is defined as a distance from the vertical position of the main deck to the lowermost part of the hull, is between 2 and 6, more preferably between around 2 and around 3. These ratios are considerably smaller than conventional ship shaped FPSOs, typically between 10 and 12. The small length to depth ratio of the inventive FPSO will result in significantly reduced hull girder bending stress and/or deflection compared to conventional ship shaped FPSOs, resulting in a simplified topside interface without need for sliding supports. Considering that the hull girder bending moment is proportional to the length squared (Lw12), and the capacity is a function of the depth squared (Dw12), it is clear that a reduction in Lw1/Dw1 causes a corresponding reduction in hull girder stress. The comparison may also be illustrated in terms of longitudinal hull girder stress at main deck level. Whereas the conventional ship-shaped FPSO designs experience about 75% of material yield in the deck plating, the inventive design will see less than 25% of material yield.
In yet another advantageous example the hull of the vessel is dimensioned in size/shape and with tank arrangement such that the hull may support a total weight above the main deck that is larger than the total weight of the hull including main deck.
Static loads dominate the load picture for the inventive FPSO design, meaning that fatigue in general is not governing. Hence, the number of critical details will be significantly less with the above mentioned inventive vessel relative to conventional ship shaped FPSO designs.
The governing static loading of the FSO/FPSO design also allows use of manufacturing materials such as high tensile steel (typically 355 MPa grade) to a greater extent than for conventional vessel designs with length>>breadth, giving not only lower weight (due to inter alia use of thinner plates), but may also result in lower cost since material such as high tensile steel has lower strength/cost ratio compared to normal strength steel.
The combination of reduced motion, large deck area, large topside load capacity and a large storage volume are all important characteristics for FPSOs, storage vessels and units for floating production, cooling and storage of natural gas (FLNG). The combination of a longitudinal vessel with (Lw1/Bw1) ratio of less than 1.7, preferably equal or less than 1.5, and with motion suppressing element(s) protruding from the hull have positive contributions to these characteristics.
As explained above, the particular design of the hull of the vessel also enables the use of SCR risers. This is a great advantage over use of traditional flexible risers since the latter solution is in general more expensive, gives a more complex installation and requires more maintenance compared to a solution with steel risers. Furthermore, flexible risers are more sensitive to irregularities during operation and have a shorter lifetime than steel risers. Since repair of a flexible riser has proved difficult, they are often exchanged with new ones, thereby increasing cost further. SCRs may be hung of at side, at the stern or through a moonpool within the hull.
Due to the vessel design with a bow, parallel midbody and a stern which are familiar characteristics for shipyards, the inventive vessel provides flexibility with respect to fabrication yard and fabrication method.
The inclusion of a pronounced bow on the vessel results in several advantages compared to prior art box and cylindrical shaped vessels:
In the following description, numerous specific details are introduced to provide a thorough understanding of embodiments of the claimed longitudinal vessel. One skilled in the relevant art, however, will recognize that these embodiments can be practiced without one or more of the specific details, or with other components, systems, etc. In other instances, well-known structures or operations are not shown, or are not described in detail, to avoid obscuring aspects of the disclosed embodiments.
The invention will now be described with reference to the attached drawings, wherein:
The flared region FR typically starts about 1 meter above waterline (w(l)), and extends to the process deck P or above depending on the required deck space. The standard flare angle of the flared region is typically 1:2 in terms of horizontal versus vertical increment, but may be increased for areas in which wave slamming is not an issue. The flare angle may thus be varied around the circumference of the vessel 1.
The main deck elevation D in relation to the waterline w(l) is determined for each specific application, but is as a rule kept as low as possible within the limits given by international load line convention, stability and green sea. A distance (d) of the main deck elevation D of about 10-12 meters above the waterline w(l) is typical for harsh environment areas, and somewhat less in case of benign conditions. The process deck P is typically located 4-6 meters above the main deck D. For very severe wave conditions, the fore deck F, at which the living quarter and lifeboats will be located, may be raised another 4-6 meters.
The suppressing element 6 provides additional added mass that inter alia influences heave, pitch and roll motions of the vessel 1 caused by external forces such as waves. By tuning the size of the suppressing element, the vessel shape, including length to breadth ratio and waterline area, and the total mass of the vessel including added mass, it is thus possible to achieve a natural frequency outside the range of the critical wave excitation frequency. In selecting the actual shape and design of the vessel, coupling effects between inertia, damping and buoyancy forces need to be considered as these effects have significant influence on the heave, roll and pitch motions. It is the combination of the increased natural period and the mentioned coupling effects that gives the favorable motion characteristics of the present invention. This motion behavior has been documented and verified through calculations and model testing.
The double hull configuration with flared outer hull 2 gives a significant area around the circumference of the main deck D in which there are no hydrocarbon content underneath. With a double side of 3-4 meters, and the mentioned hull 2 with the flared region FR, the width of the outer deck area above ballast tanks will be more than 8 meters.
The midbody of the hull 2 comprises a port side portion 2a and a starboard side portion 2b oriented parallel to a center plane CP of the hull 2, the center plane CP being defined as the plane intersecting the hull 2 midway between the port side portion 2a and the starboard side portion 2b and aligned perpendicular to the main deck D (see stippled line in
The wave excitation forces are greatest in the waterline area, and hence the vessels 1 shape and dimensions in this area are decisive in achieving the favorable and wave-direction-independent responses. The bow part 3 shown in
As an alternative, the distribution of pump rooms 103 and fuel tanks 102 may be located in the aft part 4 of the vessel 1.
The arrangement of ballast tanks 101 around the circumference of the hull 2 provide protection of the ballast and slop tanks 100a,100b, the fuel tanks 102 and the pump room 103. A double bottom 10 as shown in
An example of a mooring arrangement M is shown in
With the above-mentioned design, and within the constrains of an existing/standard yard- and construction facility, the inventive FPSO may obtain a storage capacity in excess of 2,000,000 bbls.
a) and c) shows calculated heave RAO's (Response Amplitude Operator) and roll and pitch RAO's for the present invention, respectively, while
As clearly seen by comparing
The presented calculations are for a Suezmax tanker of about 1,000,000 bbl storage capacity, where 1 bbl equals about 159 litres. The following input values have been used in the calculations:
The calculations of the RAO curves are made for motion responses in regular waves using potential theory, including corrections for viscous forces using Morison elements. Computer program used for the analyses is WADAM from DNV-GL. Calculations for larger and smaller size vessels show the same behavioral pattern.
For the inventive vessel 1, the pitch and roll motions (
In the preceding description, various aspects of the vessel according to the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the vessel and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the vessel, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, are deemed to lie within the scope of the present invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/067271 | 7/10/2017 | WO | 00 |
