Improvements relating to cargo tanks

Abstract

A single lobe tank 1000 suitable for transportation in a tapered cargo space 1030 of a sea going vessel 1040. The tank includes a shell (1012, 1014, Fig 3a), an aft head (1016, Fig 3a) and a forward head 1018. The shell includes a cylindrical section (1012, Fig 3a) and a truncated conical section (1014, Fig 3a). One of the aft head and the forward head is attached to an end of the cylindrical section and the other of the aft head and forward head is attached to an end of the truncated conical section. The arrangement of the shell, the aft head and the forward head creating a closed tank. The truncated conical section is oriented such that the head attached to the truncated conical section is offset both vertically and horizontally relative to a longitudinal axis of the cylindrical section.

Description

FIELD OF THE INVENTION The present invention relates to improvements relating to cargo tanks, in particular improvements to single lobe tanks designed to carry liquefied gas. BACKGROUND A cargo tank contained in the hold space of a seagoing vessel is typically a closed cylindrical vessel with domed hemispherical ends or heads. In the situation where the tank is contained in the forward hold, to reduce wasted space the cargo tank can include a forward portion that is tapered to emulate the tapered shape of the forward hold. This improves the capacity of the tank compared with conventional cylindrical tanks, by allowing the tank to extend all the way to the forward bulkhead of the cargo hold. For single-lobe tanks (see Fig. 1), the tank may include a rearward cylindrical portion and a forward truncated conical portion, where the truncated conical portion emulates the shape of the forward most portion of the forward hold. Single lobe tanks may be used in pairs (see Fig. 2). Bi-lobe tanks are also known and these comprise two parallel cylindrical segments arranged side by side joined by a longitudinal partition, where the forward portion can be two angled cylindrical segments (part-angled bi-lobe). SUMMARY OF THE INVENTION The present invention provides a single lobe tank for transportation in a tapered cargo hold of a sea-going vessel, the tank comprises: a shell, an aft head and a forward head; wherein the shell includes a cylindrical section and a truncated conical section, wherein the cylindrical section and the truncated conical section are joined together at an interface and wherein one of the aft head and the forward head is attached to an end of the cylindrical section and the other of the aft head and forward head is attached to an end of the truncated conical section thereby creating a closed tank, wherein the truncated conical section is angled such that the centre of the head attached to the truncated conical section is offset both vertically and horizontally relative to a longitudinal axis of the cylindrical section. This contrasts with known designs, which have the centre of the head that is attached to the conical section offset either vertically (Fig. 1) or horizontally (Fig. 2) from the axis of the cylindrical portion, but not both vertically and horizontally. It will be appreciated that the orientation of the tank determines which head, i.e. the forward head or aft head, is offset both vertically and horizontally relative to a longitudinal axis of the cylindrical section. The forward head being offset for positioning in a tapered forward hold and the aft head being offset for positioning in a tapered rear / aft hold. The terms cylindrical and truncated conical refer respectively to the shape of the sections making up the shell. The terms cylindrical and truncated conical take their normal meaning, with all cross-sections of the cylinder and the cone, perpendicular to their respective axes, being circular. The configuration of the single lobe tank is such that the tank occupies as much space as possible in the tapered cargo hold (forward or rearward) of a seagoing vessel and such that the capacity of the tank is maximised. The forward cargo hold being the hold mainly below deck and located towards the bow of a ship / seagoing vessel and the rearward cargo hold being mainly below deck and towards the stern of a ship / seagoing vessel. The configuration of the single lobe tank according to the present invention is such that the volume of the truncated conical portion of the shell and the head attached thereto can be maximised. This is achieved by offsetting the head portion, both horizontally and vertically, to make the shape of the tank conform to the shape of the tapered hold. This allows the head to be as large as possible, whilst respecting the required clearances between the head and structure of the cargo space. A larger head makes the volume of the truncated cone larger, too. Therefore, the tank configured as above maximises the use of the available space in a tapered hold. The length of the cylindrical portion is adjusted to respect the required clearances between the shell and the structure of the cargo space. The aft head and forward head may be a spherical-cap, hemispherical, a spherical-segment, torispherical or ellipsoidal shaped domes, where one head is joined to the cylindrical section and the other head is joined to the truncated conical section. The cylindrical and truncated conical sections may be joined by a mitre joint at the interface, which is defined by a mitre plane at which the cylinder and the truncated cone meet. The mitre plane cuts the cylinder and the truncated cone producing an identical elliptical cut on both parts. The angle of the mitre plane, the angle of the cone axis and the cone generator angle are adjusted to make the two elliptical cuts identical, ensuring the cylinder and truncated cone join correctly. The combination of both vertical and horizontal offsets of the head centre from the axis of the cylinder may cause the circumference of the head to protrude outside the circumference of the cylinder, as viewed in front and rear elevations of the tank (Fig. 3c and Fig. 3d). This differs from known designs, in which the head circumference is typically inside the circumference of the cylinder, as viewed in front and rear elevations of the tank (Fig. 1c and Fig. 2c). In the invention, it may be possible to draw a line touching both the forward head and the aft head such that said line lies outside the tank (Fig 6a). The tank in the invention may, by this principle, have a concave shape. This contrasts with known designs, in which typically all lines touching both heads lie inside the tank, meaning that known designs typically have a convex shape. In the invention, a concave shape of the tank is possible; it is therefore possible for the head attached to the truncated cone to be closer to the hull’s centreline than the cylindrical part is (Fig. 3d). The limitations to the position and size of the head may include (a) minimum allowed distance from tank to ship’s structure, (b) minimum allowed distance between two tanks in the same cargo hold and (c) avoidance of trapping vapour pockets in parts of the tank. The cylindrical section and the truncated conical section may be joined together at an interface, wherein the interface of the cylindrical section and the truncated section is angled such that interfacial end faces of both the cylindrical section and the truncated section include substantially identical ellipses. The shell may be circular in cross-section. The truncated conical section may be a regular cone-shape with circular cross section. The aft head and forward head may be spherical, hemispherical, spherical-segment, torispherical or ellipsoidal shaped domes, where one head is joined to the cylindrical section and the other head is joined to the truncated conical section. An interfacial face of the cylindrical section may be defined by an offset angular cut / slice of the cylindrical section, wherein the offset angular cut / slice creates an elliptical shaped sloping face and wherein the interfacial face of the truncated conical section is defined by a corresponding substantially identical elliptical shaped sloping face created by an angular cut / slice of the truncated conical head. In a further example, the shell may include a first and a second cylindrical section and a regular truncated cone section, wherein the first and second cylindrical sections are joined together at a first interface and the second cylindrical section and the truncated conical section are joined together at a second interface, wherein the first interface is angled such that interfacial end faces of each cylindrical section includes substantially identical ellipses and wherein interfacial end faces of the second cylindrical section and the truncated conical section are circular. The truncated conical section and the head attached thereto may be arranged such that, in use, a longitudinal axis of the truncated conical section slopes sideways and upwards towards a mid-section of the tapered hold and towards a highest point of the cylindrical section. The outer surface of the shell may have a concave 3D shape. A radius of the head attached to the truncated conical section may be more than 50% of a radius of the cylindrical section. For example the radius of the head may be between 50% and 100% of the radius of the cylindrical section. The volume of the cargo tank may be in the range 1000m3 to 10,000m3. The length of the cargo tank may be in the range of 10m to 50m. The cargo tank may be a Type-C tank made of materials qualified for pressure vessels. Such materials include, but are not limited to, aluminium, carbon-manganese steels, nickel steels and austenitic stainless steels. The cargo tank may be configured to contain liquid cargo or liquefied gas cargo, for example carbon dioxide, ammonia, hydrocarbons and other chemical substances, in pure or impure form or as commercial mixtures, such as liquefied natural gas (LNG), ethane and liquefied petroleum gas (LPG). A further aspect of the invention comprises twin single lobe tanks as defined above, wherein each tank is arranged such that the truncated conical section of each is orientated such that the longitudinal axis of each truncated conical section slopes inwards towards each other and towards a mid-section of the tapered hold and upwards towards a highest point of the cylindrical section. The combination of two single lobes oriented in this manner relative to each other and to the hold configuration maximises the space occupied by the tanks. Compared with conventional tanks, the tanks according to the present invention represents an increase in the volumetric capacity in the region of 5 to 10% by utilising and orientating a truncated conical section to adapt the tank to maximise utilisation of the hold space. DRAWINGS Examples of acknowledged prior art and the present invention will now be described with reference to the accompanying drawings in which: Fig. 1 (a), (b) and (c) illustrate a prior art single lobe cargo tank including a cylindrical section and a truncated conical section, which is angled upwards; Fig. 2 (a), (b) and (c) illustrate a prior art cargo tank arrangement including twin single lobe tanks arranged side-by side, where each tank includes a cylindrical section and a truncated conical section, which is angled sideways; Fig. 3a illustrates a birds-eye view of an arrangement of twin single lobe cargo tanks, where each tank is in accordance with the present invention; Fig. 3b illustrates a side view of the tanks illustrated in Fig. 3a; Fig. 3c illustrates a front elevation (viewed from the bow of a ship) of a forward cargo hold containing the tanks as illustrated in Fig. 3a and shows the forward head is aligned with the highest point and the widest point of the cylindrical section; Fig. 3d illustrates a front elevation (viewed from the bow of a ship) of a forward cargo hold containing tanks of Fig. 3a, where the forward head is at the optimum location for the illustrated hold; Fig. 4a shows the effect of an angular slice through a horizontal cylinder; Fig. 4b shows the effect of various slices through a horizontally oriented cone; Fig. 5 shows a 2-D representation of an angled mitre cut through a cylindrical section and conical section to achieve the offset placement of the forward head as illustrated in Fig. 3a; Fig. 6a and 6b show a view of a tank in which the offset of the forward head causes the uppermost points of the forward and aft heads to be at the same height (see Fig. 6b) and causes the forward head to be closer to the hull / cargo hold centreline than the aft head; Fig. 6a (View ‘A’) shows a projection with a line of sight angled to show the true angle between the axes of the cylindrical section and the conical part; the true angle between the mitre plane and the axis of the cylindrical part is apparent and a line tangent to both heads is shown; this line lies outside the tank, demonstrating that the tank in fig. 6a and 6b is concave; Fig. 6b is the front elevation of the tank of Fig. 6a, showing the vertical and horizontal offsets of the centre of the forward head from the axis of the cylindrical section; and Fig. 7 is a view of another example / arrangement of a tank including an offset forward head, including two cylindrical sections each including an angled cut and a truncated cone with regular circular ends. DESCRIPTION The following description relates to single-lobe tank configurations 10, 100, 1000 (see Fig. 1, 2 and 3) which in the illustrated examples are horizontally mounted / supported in a forward hold of a seagoing vessel e.g., a liquefied gas carrier. Each tank 10, 100, 1000 illustrated is a single lobe type-C tank. Each tank 10, 100, 1000 includes a cargo dome 50, 150, 1050. In the examples described, the tank 10, 100, 1000 is located below deck and the cargo dome 50, 150, 1050 extends above deck. The cargo dome 50, 150, 1050 serves as mounting point for cargo pumps, nozzles for cargo piping, instrumentation and electrical power (not illustrated). In each of the examples illustrated the tanks 10, 100, 1000 are each supported conventionally on twin saddles / cradles, where a fixed support is positioned towards the rear of a cylindrical section of the tank 10, 100, 1000 and a sliding saddle / cradle is located towards the front of the cylindrical section of the tank 10, 100, 1000. In each of the examples described and illustrated, a truncated conical section 14, 114, 1014 extends from the front cylindrical section of the tank 10, 100, 1000. The truncated conical section 14, 114, 1014 extends beyond the forward saddle / cradle support (see Fig. 3bThe location of the forward saddle depends on the saddle loads, hull architecture and cost considerations. Fig. 1 and Fig. 2 illustrate two examples of known single lobe tanks 10, 100. Fig. 3 illustrates twin single lobe tanks 1000 as an example according to the present invention. Fig. 1 shows an example of a singular, single lobe tank 10, which comprises a shell, that includes a cylindrical section 12, an angled truncated conical section 14, and two domed ends 16, 18 i.e., a hemispherical aft head 16 and a spherical-cap / spherical segment forward head 18. In this example, the tank 10 is horizontally oriented and is arranged centrally relative to the width of the tapered forward hold i.e. the longitudinal axes / centre lines 20, 22 of the cylindrical section 12 and the truncated conical section 14 of the tank 10 align, in plan view (a) with a longitudinal axis / centreline of the forward hold 30 of a seagoing vessel 40. In this example, in plan view (a), the longitudinal axis / centreline 20 of the cylindrical section 12 and the longitudinal axis / centreline 22 of the truncated conical section 14 appear aligned. However, a side elevation (b) shows the axis 22 of the truncated conical section 14 sloping upwards. The front elevation (c) shows the resulting position of the forward head 18 within the forward hold 30. In this configuration, the forward head 18 is centred about the vertical axis 24 and is offset relative to the horizontal axis 26 of the cylindrical section 12. The maximum height of the forward head 18 limited by the height of the top / uppermost part of the cylindrical section 12 and the base of the cargo dome 50. The top of the forward head is tangential with the highest point of the cylindrical section 12. In this example, the maximum height of the forward head coincides with the interface between the cylindrical section 12 and the base of the cargo dome 50. In this example and the further examples described below, the vertical offset of the forward head 18 relative to the cylindrical section is limited to the highest point of the forward head being level with the top of the cylindrical section such that trapped vapour bubbles / pockets are avoided. Fig. 2 illustrates an example of twin single lobe tanks 100, where each tank 100 is configured to be installed side-by-side in a forward hold 130 of a seagoing vessel 140 e.g. a liquefied gas carrier. In this example, each single lobe tank 100 comprises a shell that includes a cylindrical section 112, an angled truncated conical section 114, and two domed ends 116, 118 i.e., a hemispherical aft head 116 and a spherical-cap / spherical segment forward head 118. In this example, the tanks 100 are located one on each side of the longitudinal axis / centreline 130A of forward hold 130 of a seagoing vessel 140. A plan view (a) shows the axis 122 of the truncated conical section 114 sloping inwards i.e., towards the centreline of 130A the cargo hold 130. A side elevation (b) shows the longitudinal axis / centreline of the cylindrical section 112 and the longitudinal axis / centreline of the conical section 114 appear aligned. However, the front elevation (c) shows the resulting position of the forward heads 118 within the forward hold 130. In this configuration, the forward heads 118 are each centred about a horizontal axis 126 and offset relative to the vertical axis 124 of the cylindrical section 112. Fig. 3(a), (b) and (c) illustrate a further example of twin single lobe tanks 1000, where the tanks 1000 are located one on each side of a longitudinal axis / centreline 1030A of a forward hold 1030 of a seagoing vessel 1040. In this example, which is in accordance with the present invention, the volumetric capacity of each single lobe tank 1000 is maximised by utilising the volumetric space available in the tapered front end of the forward hold. Each tank 1000 comprises a shell that includes a cylindrical section 1012 and an angled truncated conical section 1014, and two domed ends 1016, 1018 i.e., a hemispherical aft head 1016 and a spherical-cap / spherical segment forward head 1018. In this example, each of the plan view (a), side elevation view (b) and front elevation view (c) illustrate the angular connection of the truncated conical section 1014 relative to the cylindrical section 1012. The arrangement is such that the truncated conical section 1014 is inclined sideways and upwards. The sideways incline of each tank being inwards towards the centreline / longitudinal axis 1030A of the tapered cargo hold and the upwards incline being that the truncated conical section is angled such that the spherical-cap / spherical segment 1018 is orientated towards the top of the space defined in the tapered cargo hold and towards the highest point of the cylindrical section 1012. As can be seen from the front elevation view (c), this arrangement results in the forward head 1018 being offset relative to both a vertical axis 1024 and a horizontal axis 1026 of the cylindrical section 1012. Front elevation views (3c and 3d) show the forward head 1018 extending outside the circumference of the cylindrical section 1012; when this is the case, the shape of the tank 1000 is concave. The meaning of concave can be understood with reference to Fig. 6a. The concavity 1060 of the tank 1000 is due to the combined horizontal and vertical offset (1022H, 1022V - see Fig. 6b) of the spherical segment head 1018 relative to both the vertical axis 1024 and the horizontal axis 1026 of the cylindrical section 1012. In this example, the maximum vertical offset of the forward head 1018, as with the example shown in Fig. 1 and Fig. 6b, is where the top of the forward head 1018 corresponds with (is tangential to) the top of the cylindrical section 1012 such that the position of the forward head 1018 does not cause vapour to be trapped in the head 1018. Fig. 3d illustrate a front elevation view of a further example (similar to Fig. 3c) where twin single lobe tanks 1000 are arranged side-by side in the forward hold 1030. This is illustrative of achieving the maximum functional horizontal and vertical offset of the forward heads 1018 for this particular forward hold configuration. The maximum vertical offset 1022V (see Fig. 6b) being where the top of the forward head 1018 corresponds with the top of the cylindrical section 1012 and the maximum horizontal offset 1022H (see Fig. 6b) is where the widest / innermost point 1018A of the forward heads 1018 (see twin lobe arrangement of Fig. 3d) substantially touch each other on the midline 1030A of the forward hold 1030 of the seagoing vessel 1040. In practical applications, there will be a minimum distance between forward heads 1018, governed by the need for inspection; however, this will be less than the distance between cylindrical sections 1012, which is governed by saddle / cradle size. It will be appreciated that the forward hold 1030, which is located at the bow of a seagoing vessel, has an irregular shaped volume due to narrowing of the bow of the seagoing vessel. The configuration of each single-lobe tank 1000 i.e., the orientation of the angled truncated conical section 1014 relative to the cylindrical section 1012 and the shape of the cargo hold 1030 is such that volumetric capacity of the tank 1000 is maximised by adapting the configuration of the tank and the orientation of the truncated conical section 1014 to maximise the utilisation of the reducing volumetric space towards the front of the forward cargo hold. In the example illustrated in Fig. 3 (a) to (d), the cylindrical section 1012 and the truncated conical shaped section 1014 meet at a mitre joint 1015. The mitre joint 1015 is at the interface of the cylindrical section 1012 and the truncated conical section 1014. The interface is the coming together of two flat faces, which are arranged at an angular plane relative to the normal orientation of the cylindrical section 1012. The longitudinal position and angle of the interface are dependent primarily on the cross-sectional dimensions of the cylindrical section 1012 of the tank 1000 and the shape and volumetric space within the forward hold. The longitudinal position of the mitre joint is determined by the length of the cargo hold / hull structure to ensure that the length of the cylindrical section 1012 is shorter than the length that would be close to contacting the hull structure. For clarity, Fig. 4a illustrates a regular cylinder 200 arranged horizontally showing a circle 210 in a vertical plane and an ellipse 220 in an angular plane. Fig. 4b shows a regular cone 300 arranged horizontally. In the horizontal orientation, as illustrated, the regular cone 300 produces a circle 310if vertically sliced and an ellipse 320 if sliced at an angle between vertical and horizontal. Referring again to Fig. 3a to 3d, in common, each tank 1000 includes (in plan view) a continuous surface from the cylindrical section to the conical section. This arrangement, and the different conformation of the cylindrical section 1012 and the truncated conical section 1014 (i.e., the truncated conical section includes a changing cross-sectional dimension along its length), determines the angle of slice required through each of the cylindrical section 1012 and the truncated conical section 1014 to achieve a mating surface, which includes an identical ellipse on each. Due to the conformation and required orientation of the truncated conical section 1014 the angle of the slice through the truncated conical section 1014 is not the same angle as the angle of the slice through the cylindrical section 1012. However, for each cut surface to be joined by a mitre joint the ellipses produced at the interface of the cut cylindrical section 1012 and the cut conical section 1014 must be identical, such that the cylindrical section 1012 and the conical section 1014 can be welded together to form a closed tank 1000 capable of containing pressurised liquefied gas. In the arrangement illustrated by Fig. 3a to 3d, the volumetric capacity of each tank is maximised and the size of the forward head 1018 is maximised because optimising the orientation of the truncated conical section means the forward spherical segment head 1018 can have a larger diameter and therefore have greater volumetric capacity than the examples illustrated in Fig. 1 and 2, due to the position and size of the forward head being optimised relative to the conformation of the forward hold 1030. An angled cut / slice of the cylindrical section 1012 can determine the direction in which the conical section 1014 should extend from the cut end of the cylindrical section 1012 such that the forward spherical segment head 1018 is placed in the predetermined optimised location in the forward hold. With reference to Fig. 4a, an angled cut / slice can be achieved by tilting the regular angled cut of Fig. 4a about both the vertical and the horizontal axes of the cross section of the cylinder. The angled cut / slice provides an interface that directs the conical section 1014 (when attached) at an angle that offsets the forward spherical segment head 1018 to the highest and widest point of the tank 1000. The intersection of the common axis 1022 of the cone 1014 and forward head 1018 and the axis 1020 of the cylindrical section 1012 can be used to determine the true angle of the cut / slice through the cylindrical section 1012. Fig. 6a illustrates a true view (in direction A - see Fig. 6B) of the plane on which the axes of the cylinder section 1026 and the truncated conical section 1022 lie for the arrangement illustrated in Fig. 3d in accordance with an example of the present invention. The position of the forward spherical segment head 1018 and therefore the orientation of the common axis 1022 of the cone 1014 and of the forward spherical segment head 1018 depends on many factors, such as: • the required space for the supporting saddle / cradle structures; • the required air gap between the tank and hull to allow for hull deformation during collisions (that gap is stipulated in the ICG Code); and • the maximum height of the forward head 1018 that will avoid trapping vapour pockets that cannot be vented from the cargo dome 1050 (as noted above the maximum height of the forward head is limited to the highest point of the cylindrical section). When the factors above are known the information can be used to determine the true angle of the angled cut / slice through the cylindrical section 1012. This value can then be used to determine the true angle of the corresponding angular cut / slice through the conical section 1014 or the form that the conical section should take. In the present example, the shape and dimensions of the ellipse created by cutting the cylindrical section 1012 were determined by calculation, which involved considering a true view of the common plane, i.e. the plane containing the axes of the cylindrical section 1012 and the conical section 1014. An alternative method of obtaining the shape and dimensions of the ellipse is by iterative adjustments of all the dimensions in a 3D drawing. As noted above, the ellipse of the cut face of the cylindrical section 1012 and the cut face of the truncated conical section 1014 must be identical to facilitate joining the two sections together by welding. The angled cut / slice through the cylindrical section 1012 is determined by the desired angular direction (i.e., horizontal offset 1022H and the vertical offset 1022V - see Fig. 6b) of the forward spherical segment head 1018, as determined by the shape and form of the cargo hold 1030, the uppermost dimension of the cylindrical section 1012, the innermost dimension of the cylindrical section 1012 or the “midpoint” position of the cargo hold 1030. The angled cuts of the cylindrical section 1012 and the truncated conical section 1014 of the example illustrated in Fig. 3a to 3d (see also Fig. 6a and 6b) were determined using parameters set by the cargo hold dimensions and shape and the largest allowed diameter of the cylindrical section 1012. The diameter of the cylindrical section 1012 may be limited by dimensions of cargo hold and / or maximum allowed shell thickness and stress. From the known information, the following values were determined: • the length of the cylindrical section • the angle the cut plane forms with the cylinder axis • the angle the cone axis makes with the cylinder axis • the cone angle (the angle the cone generator makes with the cone axis) • the distance between the two points at which the axes of the cylindrical and conical sections intersect the cut face. From this information and with reference to Fig. 5 the angled cut / slice 1500 through the cylindrical section 1012 and the truncated conical section 1014 can be determined. The example described and illustrated above relates to single lobe type-C tanks, which are suitable for high-pressure containment of dense cargos. Such cargos are well suited to smaller radius tanks. Therefore, for such cargos twin single lobe tanks are preferred over a bi-lobe or a larger radius single tank (see Fig. 1), as demonstrated above. Twin single-lobe tanks 1000, as described and illustrated above with reference to Fig. 3a to 3d, optimise and maximise the capacity of the tanks 1000 within the confined volume of a tapered forward hold. In this example, the cylindrical section 1012 has a radius in the range 2.5m to 12m. In this example, the conical section 1014 is a regular truncated cone with an angled cut at one end. The length of the truncated conical section 1014 is determined by the space available in the tapered forward hold as determined by the hull structure. The volume of each tank 1000 is in the range 1000m3 to 10,000m3. The length of each tank 1000 is in the range 10m to 50m. The tanks 1000 are formed of materials qualified for pressure vessels. Such materials include, but are not limited to, aluminium, carbon-manganese steels, nickel steels and austenitic stainless steels. The tanks WOO are designed to carry liquid cargo or liquefied-gas cargo, for example, carbon dioxide, ammonia, hydrocarbons and other chemical substances, in pure or impure form or as commercial mixtures, such as liquefied natural gas (LNG), and liquefied petroleum gas (LPG). Fig. 7 illustrates an example of an alternative arrangement of welded components to form a tank 2000 including a horizontally and vertically offset forward head 2018. This example includes two cylindrical sections 2012A, 2012B, where a first cylindrical section 2012A forms the main volume of the tank structure 2000 and a second cylindrical section 2012B forms an adaptor that allows a regular circular cross-section truncated cone 2022 to be attached to the second cylindrical section 2012B. The angular position of the truncated cone 2022 is determined by the angle defined by the connection of the first and second cylindrical sections 2012A, 2012B. The arrangement is such that the forward domed head 2018 is horizontally and vertically offset relative to a longitudinal axis 2026 of the first cylindrical section 2012A. In the illustrated example, the first and second cylindrical sections 2012A, 2012B join together with a mitre joint 2015. The end / face of each cylindrical section at the mitre joint 2015 include an angled cut such that when the cylindrical sections are joined, by welding, the longitudinal axis of the second cylindrical section 2012B (in this example) is angled at an obtuse angle (e.g., 170°) a relative to the longitudinal axis 2026 of the first cylindrical section 2012A. This arrangement means that the free end 2012F of the second cylindrical section 2012B is circular and as such a regular truncated cone with circular cross-section (square cut ends) of corresponding size can joined, by welding, to the free end 2012F of the second cylindrical section 2012B. Similarly to the examples above (see Fig. 3a to 3d, 5 and 6a to 6b), the arrangement (Fig. 7) defines a horizontal and vertical offset of the forward head 2018 relative to the longitudinal axis of the first cylindrical section 2012A. Domed ends 2016, 2018 and a cargo dome (not illustrated) complete the cargo tank 2000 of this example. Referring to earlier figures e.g. Fig. 3c and 3d, it will be appreciated that this example of cargo tank 2000 can be orientated such that the forward head is offset both horizontally and vertically relative to the longitudinal axis of the first cylindrical section. As with the examples illustrated in Fig. 3c, 3d, 6a and 6b the arrangement illustrated in Fig. 7, and described above, maximises the volumetric capacity of the tank 2000 relative to the cargo hold volume and hull structure to maximise utilisation of the reducing volumetric space towards the front of the forward cargo hold. As with the examples illustrated in Fig. 3c, 3d, 6a and 6b, the arrangement illustrated in Fig. 7, and described above, relates to a single lobe type-C tank, which is suitable for high-pressure containment of dense cargos. Such cargos are well suited to smaller radius tanks. Therefore, for such cargos twin single lobe tanks are used i.e., two tanks of Fig. 7 arranged side by side in the cargo hold. Twin single-lobe tanks 2000, as described and illustrated with reference to Fig. 7, optimise and maximise the capacity of the tanks 2000 within the confined volume of a tapered forward hold. In this example, the cylindrical sections 2012A and 2012B have a radius in the range 2.5m to 12m. In this example, the conical section 2014 is a regular truncated cone with circular (square cut) ends. The length of the truncated conical section 2014 is determined by the space available in the tapered forward hold as determined by the hull structure. The volume of the tank 2000 is in the range 1000m3 to 10,000m3. The length of the tank 2000 is in the range 10m to 50m. The tank 2000 is formed of materials qualified for pressure vessels. Such materials include, but are not limited to, aluminium, carbon-manganese steels, nickel steels and austenitic stainless steels. The tank 2000 is designed to carry liquid cargo or liquefied-gas cargo, for example, carbon dioxide, ammonia, hydrocarbons and other chemical substances, in pure or impure form or as commercial mixtures, such as liquefied natural gas (LNG), and liquefied petroleum gas (LPG). Whilst specific embodiments of the present invention have been described 5 above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

Claims

1. A single lobe tank for transportation in a tapered cargo space of a seagoing vessel, the tank comprises: a shell, an aft head and a forward head; wherein the shell includes a cylindrical section and a truncated conical section, wherein one of the aft head and the forward head is attached to an end of the cylindrical section and the other of the aft head and forward head is attached to an end of the truncated conical section thereby creating a closed tank, wherein the truncated conical section is oriented such that the head attached to the truncated conical section is offset both vertically and horizontally relative to a longitudinal axis of the cylindrical section.

2. A single lobe tank as claimed in claim 1, wherein the cylindrical section and the truncated conical section are joined together at an interface, wherein the interface of the cylindrical section and the truncated section is angled such that interfacial end faces of both the cylindrical section and the truncated section include substantially identical ellipses.

3. A single lobe tank as claimed in claim 1 or 2, wherein the shell is circular in cross-section.

4. A single lobe tank as claimed in claims 1, 2 or 3, wherein the truncated conical section is a regular cone-shape.

5. A single lobe tank as claimed in any preceding claim, wherein the aft head and forward head are spherical-cap, hemispherical, spherical-segment, torispherical or ellipsoidal shaped domes, where one head is joined to the cylindrical section and the other head is joined to the truncated conical section.

6. A single lobe tank as claimed in claims 2 to 5, wherein the cylindrical and truncated conical sections are joined by a mitre joint at the interface defined by mating interfacial faces of the cylindrical section and the truncated conical section, wherein the mating faces are sloping and elliptical.

7. A single lobe tank as claimed in claims 2 to 6, wherein the interfacial face of the cylindrical section is defined by an offset angular cut / slice of the cylindrical section, wherein the offset angular cut / slice creates an elliptical shaped sloping face and wherein the interfacial face of the truncated conical section is defined by a corresponding substantially identical elliptical shaped sloping face created by an angular cut / slice of the truncated conical head.

8. A single lobe tank as claimed in any of claims 1 to 5 , wherein the shell includes a first and a second cylindrical section and a regular truncated cone section, wherein the first and second cylindrical sections are joined together at a first interface and the second cylindrical section and the truncated conical section are joined together at a second interface, wherein the first interface is angled such that interfacial end faces of each cylindrical section includessubstantially identical ellipses and wherein interfacial end faces of the second cylindrical section and the truncated conical section are circular.

9. A single lobe tank as claimed in any preceding claim, wherein the truncated conical section and the head attached thereto are arranged such that, in use, a longitudinal axis of the truncated conical section slopes sideways and upwards towards a mid-section of the tapered hold and towards a highest point of the cylindrical section.

10. A single lobe tank as claimed in any preceding claim, wherein the outer surface of the shell has a concave 3D shape.

11. A single lobe tank as claimed in any preceding claim, wherein a radius of the head attached to the truncated conical section is more than 50% of a radius of the shell.

12. A single lobe tank as claimed in any preceding claim, having a volume in the range 1000m3 to 10,000m3.

13. A single lobe tank as claimed in any preceding claim, having a length in the range of 10m to 50m.

14. A single lobe tank as claimed in any preceding claim, wherein the tank is a Type-C tank made of material qualified for pressure vessels.

15. A single lobe tank as claimed in claim 14, wherein the material is aluminium, carbon-manganese steel, nickel steel or austenitic stainless steel.5 16. A single lobe tank as claimed in claims 14 or 15, configured to containliquid cargo or liquefied gas cargo, including one or more of the following: carbon dioxide, ammonia, hydrocarbons and other chemical substances, in pure or impure form or as commercial mixtures, such as liquefied natural gas (LNG), ethane and liquefied petroleum gas LPG.1017. A kit comprising two single lobe tanks as claimed in any preceding claim, wherein each tank is arranged such that each head attached to the truncated conical section is offset towards the other and each is offset relative to both a horizontal and a vertical cross-sectional axis of the relevant 15 cylindrical section.

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