Patent Application
20090199755
The present invention relates to a system for loading and unloading of hydrocarbons in waters with changing conditions, varying from periods with extreme ice conditions, such as unbroken ice or packed ice and/or drifting ice which quickly may change direction of flow; to open waters exposed to large waves and very strong wind, wherein a vessel having icebreaking properties is moored to a sea bed, and wherein a vessel by means of at least one hawser is moored with its bow to the aft of the vessel with the icebreaking properties, either at a distance from the vessel with the icebreaking properties during conditions with no influence from the ice or in physical contact with the vessel with the icebreaking properties during conditions where ice is present.
Offshore loading of oil and hydrocarbon products, including gas, in ice covered waters has up to present only been performed to a limited extent. The need for this type of operations is expected to increase to a substantial degree in the years to come, amongst other in respect to increased petroleum activities in the arctic waters.
Characteristics for such operation will be that the equipment and systems to a degree must withstand extreme ice and temperature conditions during the winter season. At the same time the equipment must during periods without the presence of ice be able to operate under “open sea” state often characterized by wind and wave conditions, for example corresponding to the ones experienced in the North Sea. Such changing operational conditions between what may be characterized as the boundaries of climatic conditions, imposes particularly strict requirements for the facilities. The ability of quickly adapting to the changing modus from ice operations to “open sea” operations represents great challenges. Correspondingly, the safety aspects are of great importance, and it is imperative and of great importance that the operations may be performed with a very low probability of “no-planned” environmental spillage.
During the winter season temperatures down to −50° C. may be expected together with very challenging ice conditions characterized by, amongst other:
During “open sea” operation the facility will typically have to perform loading operations at up to 5.5 m significant wave heights, corresponding to a wave height of up to 10 m. During operation in ice, the impact from the waves will be substantially less.
The real sea regions have in addition often very challenging current conditions which must be catered for when designing and engineering the operations to be performed. It should for example be appreciated that the tidal water generated currents may turn 180 degrees up to four times during a 24 hour period, while at other sites less predictable current conditions may exist.
The real sea areas are often shallow, meaning that the loading installations must be installed relatively far away from shore, so that the water depth may be sufficient. Use of large pipelines may produce high costs.
US 2006/0037757 A1, which is filed by the applicant, describes a protective system for protection of risers from drifting ice, where the riser is suspended from a turret buoy, connected to the vessel, and where the upper end of the riser is protected from influence and impact from drifting ice.
US 2005/0235897 A1 and EP 1 533 224 A1 show a system for transfer of hydrocarbons, where an icebreaker and a shuttle tanker, moored to the aft end of the icebreaker is used for transferring hydrocarbons to a tank vessel. The icebreaker is moored to the sea bed by means of four mooring lines and the bow of the tanker is moored to the aft of the icebreaker by means of a hawser, which also forms suspension of the hose for transfer of hydrocarbons from the sea bed to the vessel via the icebreaker. The tanker is moored either at a distance from the icebreaker in case of situations without ice, or in physical contact with the icebreaker in situations with ice appearance.
US 2004/0106339 A1 relates to offshore loading of hydrocarbons where a production vessel is pivotably moored to a submerged buoy and where a shuttle tanker is moored to the aft of the production vessel by means of a hawser.
An object of the invention is to provide a loading and unloading system with large inherent flexibility and large robustness against the appearing outer environmental forces, such as the possibilities of unintentional oil pollution to the environments are prevented.
Another object of the invention is to provide that loading operations may be performed with high efficiency, even under demanding and changing weather and ice conditions.
A further object is to be able to combine “open sea” and ice operations in an effective and safe manner.
A still further object is to be able to perform loading operations during the course of six hours and where the loading operations in an effective and safe manner may be employed in shallow waters, possibly down to depths about 20 m.
A still further object is to provide a loading system designed for loading rates typically up to 15000-18000 m3 per hour.
Another object is to provide a system which in a safe manner may handle appearances of drifting ice from abaft without creating any safety hazard for the loading or unloading operations.
The objects are achieved by a system for loading and unloading of hydrocarbons which is further defined by the characterizing part of the independent claims.
Preferred embodiments of the invention are defined by the dependent claims.
According to the invention a robust system is provided, enabling loading under extreme conditions, both in open sea state and during situations of strong drifting ice.
Further, the sensitive parts of the loading and unloading system are protected against influence of the appearing ice, so that the possibilities of damaging impact of the sensitive parts of the system are reduced.
Further, the system according to the invention contributes to reductions of the forces in the hawser, since the size of the ice channel produced by the icebreaker is made larger by means of thrusters arranged in the hull of the icebreaker at the fore and/or aft end of the vessel.
The system according to the invention is based on thirty years of experience of North Sea buoy loading operations and is developed for mooring of tank vessels up to 100000 tdw. In offshore operations such sizes are twice as large as the vessels normally employed.
Further advantages of the system according to the invention will be apparent when reading the specifics of the invention, describing such system in respect to the accompanying drawings, disclosing several preferred embodiments of the invention, where:
a shows a side view of an icebreaking vessel according to the invention, with a tank vessel moored to the icebreaker at a distance from the former, where the mooring system shown is used for transferring hydrocarbons by means of hoses, stored on drums;
b shows a horizontal view of the vessels shown in
a and 2b show corresponding views, where hydrocarbons are transferred by means of hoses suspended from a hose boom;
a and 3b shows a view of the two vessels, where the tanker is moored in contact with the icebreaker vessel;
a-5c show in perspective, different views of the loading and unloading system according to the invention.
Firstly, it should be appreciated that common element shown in the different figures of the drawings will have the same reference numbers. Hence not every detail will be described in relation to each single FIGURE.
a shows a side view of an arctic production and tandem offshore terminal, while
The mooring system comprises a buoy 11 which at one end is fixed to the sea bed 12 by means of a plurality of mooring lines 13, extending between the buoy 11 and mooring points (not shown) on the sea bed 12. On the sea bed 12, in the vicinity of the icebreaking vessel 10, a template equipped with a so called <<Pipe Line End Manifold>> 14 is installed. A riser 15 extends from the manifold 14 to the icebreaking vessel 10 via the buoy 11. Both the buoy 11, the riser 15 and the connections with the icebreaking vessel are well known in the art and will not be described in further detail.
In order to protect the buoy 11, the riser 15 and the upper parts of the mooring system against impact from ice, a net 22 is installed, preferably attached to the lower end of the buoy 11 and further preferably with its lower end attached to the mooring lines 13, forming a protective surface.
A shuttle tanker 16 is moored to the ice breaker 10 by means of hawsers 17. The tanker 16 is moored at a distance, for example 50-60 m, away from the icebreaker 10. In order to be moored to the icebreaker, the shuttle tanker 16 is approaching the icebreaker 10 from aft. At a distance of about 50-60 m away from the icebreaker 10, the shuttle tanker 16 stops its approach. Hawsers 17 are transferred from the icebreaker 10 to the shuttle tanker 16 by means of a line (not shown), is connected to the mooring winches 18 on the bow part of the shuttle tanker 16. Correspondingly, two such mooring winches are arranged on each side of the aft deck of the icebreaking vessel 10. Two independent hawsers 17 are employed. The hawsers 17 are arranged symmetrical with respect to the centreline of the shuttle tanker 16, so that the bow of the shuttle tanker 16 will be stabilized in direction towards the icebreaker 10 when there is a tension in the hawsers 17. Optionally, two hawsers 17 on each side may be used in order to further securing that the tanker vessel 17 maintains its position even if a hawser 17 should break.
According to the invention an ice reinforced shuttle tanker 16 is employed, which normally also may be equipped with a dynamical positioning system (DP) 19; conventional bow thrusters 20 and offshore loading equipment 21 on the bow region of the tanker 16.
According to an embodiment shown in the
The arrow A in
a and 2b show an alternative embodiment of the invention shown in
a and 3b show another typical mooring modus, different from the one shown in
It should be appreciated that in connection with escorting a vessel in ice waters, the icebreakers used are often equipped with equipment having the described “V”-shaped arrangement at the aft end.
Hawser winches 18 on board the OIB 10 are designed with a rendering function, securing that the shuttle tanker 16 will not overstrain the hawsers in periods when the active hawser length is short, i.e. when there is little elasticity available in the mooring system. Such rendering functions will gradually be reduced when the active hawser length and consequently available elasticity is increased. It should be appreciated that such type of winch function with variable rendering function is not previously known or used in connection with offshore loading operations.
When the distance between the vessels 10,16 is adjusted, also the operative hose length must be adjusted.
The OIB 10 may preferably be equipped with one or two thrusters/propellers 27 in the bow region, the main purpose of which being to break up the ice and hence contribute to maintaining the required position of the vessel 10 without overstraining the mooring lines 13.
A main purpose of the two thrusters 27 abaft is to contribute during ice operation, making the ice channel as wide as possible. Ice operation experience shows that the ice channel may be made wider in an effective manner by tilting the thrusters 27 up to 90°. The efficiency may be increased further by using so called nozzle propellers, producing concentrated water jets in required direction. The method is applied on icebreaking vessel, but has not previously be dedicated as a function as described above. The width of the ice channel will be a function of amongst other, the ice thickness, the propeller effect and the thrust angle with respect to the centreline of the vessel 10. For ice thicknesses around 1 m, two thrusters will typically produce an ice channel with a width of 150 m. If the ice thickness is 0.5 m, the width of the ice channel will typically increase to about 300 m. In this connection it should also be appreciated that the width of the ice channel will be larger if the vessel does not move forward, which may be case for this particular concept, since the flow energy will be directed in required direction and will not be affected/reduced by the forward directed velocity component.
A comparison should also in this aspect be made to the alternative wherein the loading operation is performed from a platform resting on the sea bed. For such installations, the width of the ice channel may only correspond to the width of the platform, since no thrust energy is available for increasing the width of the ice channel. In most cases the ice channel will not exceed typically 50-70 m, thus a substantial deterioration of the operative conditions, compared with the proposed thrust propeller based solution.
a-5c show in perspective an embodiment of the invention, showing that the icebreaker 10 is provided with four thrusters 27, two of which being placed at the bow of the ice breaker 10, and two at the aft end of the icebreaker 10. The Figures show a modus where the shuttle tanker 16 is moored a distance apart from the icebreaker 10.
In the enclosed drawings the OIB 10 is disclosed with parallel hull sides. It should be appreciated, however, that the OIB 10 may be constructed in such way that the hull width may have its largest width at mid-ship, the hull sides forming an angle which is different from 90° with respect to the water line plane. Hence, the OIB 10 may in principle be characterized as something in between a vessel and a floating platform/buoy. The advantage of a solution as described above is that the ice channel behind the OIB will be wider. In addition, the inclined hull sides will be well suited for breaking up the ice, if the vessel 10 is exposed to compacted ice. Such solutions may however always be considered with respect to the capability of the vessel to operate in open sea state.
According to the invention double hoses are used in the loading operation between the OIB and the tanker. Such arrangement yields a high loading rate and short loading time, which is of great significance in waters where the water current directions frequently are changed. As described above, the tidal water dominated current may turn 180° during a six hour period. With two 20″-hoses it will be feasible to complete the loading operation of a 100.000 tdw tanker in the course of such six-hour period. If the loading operation is not completed prior to directional change or reversal of the current, it will otherwise be necessary to disconnect the tanker 16 and re-moor the vessel when the direction of the current again has been stabilized.
Subsequent to completed loading operation, the hose(s) are emptied by means of nitrogen and the hose(s) are then spooled back on the hose drum 8 on the aft deck of the OIB 10. The same type of operations is performed with the mooring hawser, stored on separate storing drums/winches 23 on the aft part of the OIB 10. Alternatively, a hose boom 25 may be used, swinging in above the aft deck of the OIB 10 subsequent to completed loading operation. The loading hose(s) 24 will then adopt a advantageous storing position onboard the OIB 10 as further illustrated in the accompanying drawings.
Onboard the OIB 10, the hoses 24 and the hawsers 17 may preferably be stored under controlled temperature conditions and maintenance may be performed as and when required.
The hose and pipe system may preferably be used in a manner as schematically shown in
The OIB 10 is equipped with a drainage tank 29 allowing the hose(s) 24 to be emptied and the pipe system onboard and down to PLEM 14, if required. The capacity of this tank 29 may be increased if required, so that the tank during periods where the shuttle tanker 16 is disconnected from the OIB, may function as a storage tank.
As specified above, the OIB 10 may, in addition to the propellers 27 installed fore and aft, be provided with a turret mooring 13 which is so configured that disconnecting of the OIB 10 may be performed typically in the course of one hour under normal situations and within minutes in case of an emergency situation. Correspondingly, it will be possible to connect the OIB 10 to the mooring system within typically one to two hours, dependent upon the existing ice and weather conditions. When connecting, the OIB 10 is positioned above the buoy centre and a subsea means is employed for establishing contact between the OIB 10 and the submerged buoy 11. It should be appreciated that this type of subsea means is of well known technology which is commercially available in the industry.
The mooring system may be of the type <<Submerged Turret Loading>> (STL) or corresponding technology available in industry.
When the OIB 10 operates in iced waters and is connected to the mooring system, ice and ice blocks crushed by the propellers may cause damage to the risers 12 and may also build up between the mooring lines directly below the buoy 13. In order to prevent/reduce such type of accumulation with consequential damages and disturbances in the operations, a protective net 15 or corresponding means is arranged just below the buoy 13 and around the mooring lines 14. The net may typically be made of a flexible material able to resist the motions and the ice impacts which the net is exposed to.
When the OIB is disconnected from the mooring system, it will be naturally to let the buoy rest on the sea bed in shallow waters. Optionally, it may be necessary to excavate a ditch in the sea bed, into which the buoy wholly or partially may be lowered. Hence, it may feasible to operate in waters with a depth typically about 20 m.
The loading system may, however, also in a flexible manner be designed for use at different depths, varying from typically 20 m up to several hundred meters.
Between the OIB 10 and the PLM 14, it may preferably be arranged two flexible risers 15 which are further connected to the pipe system 15, including the required stop valves 28. This arrangement renders it possible to circulate the oil between the OIB 10 and the PLEM 14 when the shuttle tanker is disconnected. Hence, the oil will be prevented from becoming thicker due to low temperature.
The given arrangement will also allow the risers 15 to be emptied of oil for example by forcing the oil to the drainage tank 29 by use of nitrogen. Drainage of the risers 15 will for example be actual when the OIB 10 is to be disconnected in order to avoid pollution and/or undesired drop in temperature in the oil. It may also be possible to prevent the oil inside the risers 15 from solidifying by injecting an appropriate additive liquid.
From the PLEM 14 to shore double pipelines 31 may be arranged, enabling circulation of oil during periods with no loading activities.
So called pressure relieving valves or <<surge>> valves 30 may also be installed on the OIB. If the pressure in the pipe system will increase rapidly, for example as a consequence of operational fault, the pressure relief valves 30 will quickly open and drain oil to the drainage tank 29. Unacceptable pressure chocks in the pipe system are thus avoided. Further, dependent upon requirements, it may be actual to install one or more booster pumps 32 onboard the OIB 10 in order to maintain the high loading rate, even with long pipe lines 31 causing large pressure drops.
A manifold (not shown) may preferably be placed on the fore deck of the shuttle tanker 16, where a bow loading coupling 34 attached for each hose 24. The hoses 24 are for this purpose provided with, in corresponding manner, a hose valve 35. Correspondingly, the opposite ends of the hoses 24 are provided with couplings 36 for the hose valves. Drainage valves 37, by-passes 38, pivot connections 39 and QD/DC 40 are also forming a part of the system.
The OIB 10 may in a simple manner, as described above, be connected to and disconnected from the mooring system. In addition, the OIB may be equipped and manned for several other functions at the oil field. Such functions may be icebreaking, ice management, stand-by services, oil recovery and fire fighting, inspection and maintenance, field related transport, etc.
At many oil fields, it may probably be of commercial interests to consider such multi-purpose operations.
Finally, it should be appreciated that the described offshore concept also may be combined and/or prepared for vessels performing offshore production of oil and gas. It has recently been filed a patent application by the applicant, with the title “Means for positioning vessels in ice prone waters”. The positioning strategy described in the referenced application will also be possible for an OIB 10, ref. amongst other use of ice screws for breaking up the consolidated ice zone.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20062287 | May 2006 | NO | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO07/00129 | 4/18/2007 | WO | 00 | 1/8/2009 |
