Title: Semi-submersible vessel without braces

Field of the invention

The present invention relates to semi-submersible vessels, which are known in the field of the art. Semi-submersible vessels are utilized for various kinds of operations at sea including laying of pipeline on a seabed and other operations.

Prior art

Semi-submersible vessels for use in offshore work are known in the art. This type of vessel is characterized in that they have at least one pontoon (or floater), which is connected to a deck structure (or deck box) via columns. Typically, a semi-submersible has a left pontoon and a right pontoon. During operation, the vessel is ballasted so that the pontoons are located below the water line and the deck structure is above the water surface. The open spaces between the columns allow waves to partially pass between the columns and underneath the deck structure which is positioned above the wave zone. The form and position of the columns are designed such that this type of vessel has an improved dynamic behaviour in comparison to ship shaped vessels, also called monohulls.

Originally, semi-submersibles had both transverse bracings extending between the pontoons and/or transverse bracings extending between the columns. Also, semi- submersibles were provided with diagonal bracings connecting the columns and/or the floaters with the deck structure. The transverse and diagonal bracings were used to absorb the forces working on the vessel, such as splitting forces between the floaters. Also, the forces from loads carried on deck and the weight of the deck itself are directed to the floaters via the diagonal braces .

The Heerema vessels Balder and Hermod were the first semi-submersibles that were built without diagonal braces. This was possible since the structural strength of the deck structure was taken into account during the design of the vessels.

Later, designs for semi-submersibles without diagonal and transverse bracings became known in the art. Reference is made to EP1598264.

An advantage of a semi-submersible without bracings is that loads due to wave slamming are reduced. The bracings form a considerable surface against which - in use - the waves slam, and the absence of this surface advantageously reduces the slamming forces.

Also, less hydrodynamic drag will occur when the vessel is sailing, essentially for the same reason, i.e. less water hits the braces. This allows a higher transit speed as well as improved station keeping characteristics.

ier advantage of a braceless semi-submersible is an accessibility of the space between the columns and the deck structure. This accessibility for example creates the possibility to move the vessel over a specific target or to position another vessel between the floaters of the semi-submersible. EP1598264 mentions moving a cargo barge between the floaters in order to offload cargo from the barge when positioned between the floaters. A problem which occurs with a semi-submersible without bracings is that large bending moments and forces need to be absorbed or transformed by the connection between the deck structure vessel and the columns.

In comparison with semi-submersibles with bracings, the bending moments occurring in the connection between the column and the deck are significantly larger in magnitude. Further, the bending moments at the connection of the deck structure and the columns are directed such that they 'open the corner', i.e. the angle of approximately 90 degrees which is defined by the substantially horizontal deck and the substantially vertical columns tends to be increased. Typically, the pontoons tend to move away from one another. One way of resolving this challenge is to create a semi-submersible having a high deck structure (or deck box) and a requirement to use thick plating. However, these are weight inefficient measures and lead to a heavy deck structure. A heavy deck structure in turn result in a relatively unstable vessel.

Further, the bending moments tend to be more dynamic than the bending moments of semi-submersibles with braces. This creates a serious fatigue challenge. It has proven to be difficult to resolve this fatigue challenge. Damage due to fatigue is a major issue in the connection between the deck and the columns.

EP1598264 focuses on this fatigue issue and discloses a construction method using truss members, i.e. diagonal connections between the floater and the underside of the deck structure. See for example fig. 2 items 25 and 26 of EP1598264.

A disadvantage of this method is that the truss members are located underneath the deck, at the upper end of the columns. This is a position that renders the truss members difficult for access for inspection and maintenance. Since the truss members are dynamically loaded and are critical parts in the overall structural integrity of the vessel, inspection and maintenance are especially important for this part of the construction. The bad accessibility is therefore a serious disadvantage.

A further disadvantage is that the truss members are located below the main deck on the outside of the vessel construction. This leaves the truss member exposed to the saline environmental conditions at sea and increases the chance of corrosion of the truss member. A need for a higher frequency of inspection and maintenance is a result.

sr, the truss members of EP1598264 cause an increased water plane area of the vessel at the level of the truss members. Under certain operating conditions, this may be disadvantageous, because the dynamic properties are affected.

Object of the invention

It is an object of the invention to provide a semi-submersible which suffers less from at least one of the above mentioned disadvantages.

It is a further object of the invention to provide a braceless semi-submersible which is strong enough to operate in rough conditions. It is yet another object of the present invention to provide a semi-submersible without braces which is stronger than known braceless semi-submersibles.

It is another object of the invention to provide a braceless semi-submersible which has a relatively thin deck structure.

Description of the invention

At least one of the objectives is achieved with a semi-submersible vessel comprising at least:

- a deck structure comprising a lower deck and an upper deck;

- at least two spaced apart pontoons which are connected to the deck structure via at least one column per pontoon, wherein the deck structure comprises at least one reinforcement means constructed for absorbing a bending moment exerted by the columns on the deck structure or vice versa, wherein said at least one reinforcement means comprises:

- at least one bulkhead or similar structural element, which bulkhead extends substantially vertically and extends over at least a part of the width of the deck structure, the bulkhead extending between the upper and lower deck in such a way that a beam is created comprising a part of the lower deck as a lower flange, a part of the upper deck as an upper flange and the bulkhead or similar structural element as a body of the beam and/or - at least one plate, beam or other structural element which, when viewed from the front of the vessel, extends diagonally between the lower deck and the upper deck and connects the upper deck to the lower deck in such a way that a truss beam is created comprising a part of the lower deck as a lower flange, a part of the upper deck as an upper flange and the at least one plate, beam or other structural element as a truss part of the beam.

lkhead may also be referred to as a reinforcement wall or a reinforcement partition. A similar structural element can be, amongst others a truss beam, a framework beam or a large l-profile or H-profile.

In a suitable embodiment, the reinforcement means provide a connection between the upper deck and lower deck and allow the full height of the deck structure to cooperate as a single structural element in bearing the bending moments. The lower deck of the vessel will typically be subject to tensile forces while the upper deck will typically be subject to compressive forces.

In a suitable embodiment, a profile of the vessel , when viewed from the front of the vessel, is determined by the deck structure, the columns and the pontoons, and wherein the reinforcement means are located within said profile such that the reinforcement means do not extend outside said profile.

In contrast with the Q4000, the profile of the vessel, when viewed from the front, need not be changed by the reinforcement means, or is less changed. This advantageously provides a greater space under the vessel.

In a suitable embodiment, the at least one bulkhead or similar structural element extends within the deck structure in the area above the columns but does not extend into the columns. The bulkhead takes on bending moments where necessary and prevents the deck structure from deforming. In a suitable embodiment, said reinforcement for absorbing bending moments are only provided in the deck structure in an area above the columns. In some situations, the rest of the deck structure can be free of the reinforcement means.

In a suitable embodiment, the reinforcement means extend over the entire width of the deck structure from a left side of the vessel to a right side of the vessel. This may apply to the diagonal beam which extends from a left bottom corner of the deck structure to a right top corner of the deck structure. Also, a bulkhead or similar structure may extend over the whole width of the deck structure.

In a suitable embodiment, the semi-submersible vessel further comprises a plurality of bulkheads which extend longitudinally. These bulkheads provide additional strengthening and stiffening capacity for the deck structure.

In a suitable embodiment, a reinforcement means extends from an inside corner wherein the column meets the deck structure to an outer upper corner of the deck structure.

The reinforcement means result in a relatively strong vessel with a relatively thin deck structure. The fatigue issue has been adequately addressed by the invention. In a suitable embodiment, the reinforcement means in particular substantially prevent slanting of the deck structure, i.e. a movement of the upper deck relative to the

< in a substantially horizontal direction which is transverse to the longitudinal direction of the vessel.

In a suitable embodiment, the vessel will comprise special connection means for the deck with the columns to transfer the entire moment between deck structure and column without the assistance of a truss member to alleviate that moment.

The current invention further provides a solution that can be used to construct a vessel that is significantly larger than braceless semi-submersibles known from the art.

The structural elements that are responsible for absorbing moments and forces are advantageously located within the deck structure of the vessel, making them very good accessible for maintenance and inspection. Also maximum protection against negative influences of the environment is obtained.

The structural element can comprise metal plates positioned transverse to the vessel, as a shear panel.

Alternatively the structural element can extend in a longitudinal direction, in an inclined orientation.

Since for a large vessel the dimensions of the structural elements can become large, one embodiment of the invention provides the application of multiple shear panels. The panels are placed perpendicular to the side of the vessel. This allows the panels to be integrated in the design of the vessel and to create passage ways between them. The layout of the deck area is less restricted than for the case where the panels are in the longitudinal direction of the vessel. Depending on the size of the vessel and thus the forces and moments being transferred, the shear panel may extend over the full width of the vessel, or over just a part of the width, or a combination of both.

In a suitable embodiment of the invention the semi-submersible is braceless. The vessel is sufficiently strong for harsh conditions. The reinforcement means are located within the structure of the vessel and not exposed to saline conditions. Further, the reinforcement means are easily accessible for maintenance and repair.

The present invention allows braceless larger semi-submersibles to be constructed than was previously possible. In the following, the aspects, features and advantages of the innovative vessel will be elucidated further by reference to the annexed figures illustrating exemplary embodiments. In the figures, the same parts or parts having the same function have been identified with the same reference numeral.

The following figures illustrate the invention in more detail.

schematically shows the design of a traditional semi-submersible with cross bracings and diagonal bracings;

Fig. 2 shows a front view of a braceless semi-submersible known from the art; Fig. 3A shows a schematic isometric view of the vessel according to the invention; Fig. 3B shows a schematic cross section of the vessel according to the invention;

Fig. 3C shows a schematic side view of the vessel according to the invention; Fig. 3D shows a schematic top view of the vessel according to the invention; Fig. 4 shows a cross section of an alternative embodiment of the invention; Fig. 5 shows a schematic top view of the bow of the vessel according to the current invention;

Fig. 6 shows a schematic side view of the different decks within the deck structure at the bow; and

Figure 7 shows a cross section of a variant of the invention Figure 8 shows a diagrammatic cross section showing forces which act in the construction; and

Figure 9 shows a diagrammatic cross sectional view from above showing forces in the construction .

Detailed description of the invention Figure 1 shows a view on the stern of an existing vessel of a semi-submersible type.

Vessel 1 comprises a deck structure 2 and pontoons 3 which are connected by columns 4 to the deck structure 2. The vessel has both horizontal bracings 6 between the pontoons 3 as well as diagonal (or inclined) bracings 7 which connect the columns with the deck structure 2. The combined structure allows heavy loads to be carried by the vessel, for instance heavy lift offshore operations or heavy pipeline installation. The foundations 8 of heavy lift cranes are shown in figure 1. In use, the vessel 1 generally carries a crane (not shown) on each foundation 8.

Figure 2 shows a view of a braceless semi-submersible known as disclosed in EP1598264, also known as the Q4000. It can be seen that no bracings are used, and that the forces between the deck structure and the columns are supported by reinforcements 25, 26 between the deck structure 27 and the respective columns 23 and 24. The reinforcements 25, 26 are positioned at an underside of the deck structure 22, where inspection and maintenance is difficult. The reinforcements further cause a decrease of a float-under profile 29 defined by the floater 20 , column 23 and truss 25 on one side, the underside 21 of the deck structure 27, and floater 21 , column 24 and truss 26 on the other side of the vessel.

Figures 3A, 3B, 3C, 3D show a vessel 40 according to the invention. The vessel has a bow 17 and a stern 16. The vessel 40 has a deck structure 41, columns 42 and pontoons

40 has four columns 42A, 42B, 42C and 42D on each side connecting the pontoons 43 with the deck structure 41. Between the columns 42, openings 48 are defined for allowing waves to pass through. The openings 48 determine the favourable dynamic behaviour of the vessel in a normal operating draft 95. Two cranes 9 are positioned on the deck structure 41.

The columns 42 comprise an inner wall 70 and an outer wall 72. The inner walls 70 and outer walls 72 extend substantially vertically. It is also possible that the columns 42 widen somewhat towards the upper ends 66, in order provide additional buoyancy at a deep operating draft 96. The vessel 40 has a transit draft 101 in which the pontoons 43 intersect the water line 19. The vessel 40 has a survival draft 99 which is slightly greater than the operating draft and in which in which the pontoons 43 also intersect the water line 19.

The upper ends 66 of the columns 42 are connected to an underside 68 of the deck structure 41. In the pontoons 43 and the columns 42, a ballast system is provided for varying the draft of the vessel. The water line during operations is indicated by numeral 19. The vessel 40 comprises thrusters 18 for propulsion and station keeping.

The deck structure 41 comprises a lower deck (or floor) 52, an upper deck (or floor) 54 and a space 56 defined between the lower deck 52 and the upper deck 54. The deck structure 41 comprises side walls 74.

It is possible that between the lower and upper deck, additional intermediate decks (not shown) are provided as can be seen in figure 6.

The vessel 40 comprises structural members 44, 45 (or bulkheads) which are constructed to allow greater bending moments to be exerted between the columns 42 and the deck structure 41. The structural members 44, 45 are also referred to as reinforcement means 44, 45. The structural members 44, 45 are located substantially within the deck structure 41, i.e. between the lower deck 52 and the upper deck 54. The structural members shown in Figures 3A-3D are bulkheads or similar plate-like structures which are oriented substantially vertically and extend substantially transversally to the longitudinal orientation of the vessel.

The structural members 44, 45 form a body of a beam. The lower deck 52 forms a lower flange and the upper deck 54 forms an upper flange of the beam. Together, the lower deck, the structural members and the upper deck act as one structural element. It will be apparent to a skilled person that only a part 120 of the upper and lower deck which is located close to the structural member will cooperate with a specific structural member 44, 45. Since a plurality of structural members are generally provided, the upper deck and lower deck are divided in zones 120 which cooperate with a structural member which is close to or in said zone as is indicated in figure 3D for one bulkhead 45 with dashed lines 122. A zone 120 will be created for each bulkhead 44". This aspect is further elucidated in figure 9.

tructural members 44, 45 are connected at a lower side or lower ridge 58 thereof to the upper side of the lower deck 52. The structural members 44, 45 are connected at an upper side or upper ridge 60 thereof to the lower side of the upper deck 54. The structural members 44, 45 thus extend within space 56. A width 80 of the structural members 44, 45 may vary from the width 80a of the column 42 as is schematically indicated by reference numeral 44 and diagonal lines 44A (which form an X), up to the entire width 80b of the deck structure 41 as is schematically indicated by diagonal lines 45A , and any width in between.

The structural members 44, 45 remain within the outer profile of the vessel, when viewed from the front. Thus, a float-under profile 27 of the vessel is not reduced by the structural members.

The structural members define spaces 88, which are shown in figures 3A and 3D for the rear columns 42c and 42D.

Longitudinal bulkheads 47 may be provided as well, but these do not restrict the division of available space too much.

The structural members 44, 45 substantially limit a deformation of the deck structure under influence of the bending moments between the columns and the deck structure. In particular, a sagging deformation of the deck structure is substantially limited. Also, a slanting deformation of the deck structure is limited. The rigidity of the deck structure 41 is increased by the structural members 44, 45. The total number of structural members 44, 45 over each column may be one or more, depending on amongst others the size of the vessel, the number of columns per pontoon and the load bearing capacity. Also the number of structural elements per column may vary per column.

The columns 42 generally comprise a forward wall 130 , a rearward wall 132, an outer wall 72 and an inner wall 70. In a suitable embodiment, one structural member 44, 45 will extend in a same vertical plane as a vertical plane in which the forward wall 130 of the column extends and one structural member 44, 45 will extend in a same vertical plane as a vertical plane in which the rearward 132 wall of the column extends. A longitudinal bulkhead 47 may extend in a same vertical plane as a vertical plane in which an inner wall 70 extends. A side wall 74 of the deck structure may extend in a same vertical plane as an outer wall 72 of the column.

Figure 4 schematically shows an alternative embodiment of the invention. Diagonal plates 46 are positioned in a longitudinal direction of the vessel. Instead of a diagonal plate 46, a plurality of diagonal beams 46 or other diagonally extending structural members may also be utilized. A diagonal member is suitable for transferring a moment and this embodiment also allows the structural reinforcements to be placed within the circumference

0. The diagonal plate 46 or beam(s) prevents the deck structure from slanting and/or bending when a large bending moment is exerted on the deck structure by a column.

The diagonal member 46 extends between an inner lower corner 102 where the lower deck 52 and the inner wall 70 of the column 42 meet and an outer upper corner 104 where the side 74 of the deck structure 41 and the upper deck 54 meet.

A longitudinal bulkhead 47 is provided within the deck structure 41 at a position where the diagonal member 46, the lower deck 52 and the inner wall 70 of the columns 42 meet and are connected to one another.

The diagonal member 46 thus creates two triangular structures 105 and 106 on the left side of the vessel and two similar triangular structures 105, 106 on the right side of the vessel. Triangular structure 105 is defined by the diagonal member 46 itself, the side wall 74 and lower deck part 52A resp. 52B which are located above the columns 42. Triangular structure 106 is defined by the diagonal member 46, longitudinal bulkhead 47 and parts 54a, resp 54B of the upper deck 54 which are located above the columns 42. In this way, the deck structure 41 is turned into a truss beam, which can bear greater bending moments and deforms less.

Figure 5 shows a top view of the bow arrangement. A position for a J-lay tower is provided near the bow 17. For this end, a slot 94 is defined by the bow structure. A practical layout is provided for the different utilities 110, tower equipment 112 and personnel quarters 114. Passageways 116 are provided which extend transversally. Directly forward or directly aft of the passageway, the bulkheads 45 may be provided.

Figure 6 shows that the deck structure 41 may comprise more decks than only the lower deck 52 and upper deck 54. In this example, two additional decks are provided, i.e. a lower tweendeck 90 and an upper tweendeck 92. Other deck configurations are conceivable. The intermediate decks are linked to the structural members 44, 45.

Figure 7 shows an embodiment with l-profile beams 45 comprising openings 98 in the body of the beam. The openings 98 may be substantially circular or honeycomb shaped or have different forms. The openings save material in the body of the beam 45 and thus save weight. The side walls 74 form end flanges of the beam. Openings can advantageously be created in beam 45 at the level of the floor of any of the decks to create passageways.

Figure 8 shows how a bending moment 134 is transferred from the column 42 into the deck structure 41 or vice versa. In the shown example, the structural member 44 is a bulkhead which extends within the deck structure over a width 80a of the column 42. The bulkhead 44 forms a beam 138 having a length 80a. A part of the upper deck 54 which extends over this length 80a forms an upper flange of the beam 138. A part of the lower deck 52 which extends over this length 80a forms a lower flange of the beam 138. The longitudinal

id the side wall 74 form end flanges of the beam 138. The bulkhead 44 forms the body of said beam.

A bending moment 134 in the column 42 works as a combination of a normal force 136A acting in the outer wall 72 and a normal force 136B acting in the inner wall 70. The normal forces together form the bending moment 134. Normal force 136A is a compression force and normal force 136B is a tension force.

The forces 136A, 136B are transferred into the beam 138 at the upper end 66 of the column, wherein the normal forces 136A, 136B are transformed into shear forces 140 acting in an outer part of the body 44 of the beam 138 and in the flanges 52, 54, 47, 74. The stiffness of the bulkhead 44 substantially prevents the beam 138 from deforming and transforms the normal forces 136A ₁ 136B into shear forces 140.

The shear forces 140 are subsequently transferred from the beam 138 into the part of the deck structure 41 which extends between the columns 42. In the transfer, the shear forces 140 are transformed into normal forces 136A, 136B which act in the upper deck 54 and the lower deck 52 and which together form the bending moment 134.

Figure 9 shows a top sectional view of a single bulkhead 44 forming a single beam 138 in which lines 142 indicate how the shear force 140 acting in beam 138 are dispersed as normal forces 136A, 136B in the upper deck 54 and lower deck 52. The lines 142 indicate tension trajectories of a compression force 136A and a tension force 136B. The shear force 140 spreads out from the beam 138 and transforms into normal forces 136A, 136B in the upper deck 54 and lower deck 52, such that the normal forces 136A, 136B act in a zone 120 having borders 122.

The connection between the upper end 66 of the column 42 and the underside of the lower deck 52 is formed by a welding connection. Bolts or other types of connections are also possible, however.

It will be obvious to a person skilled in the art that numerous changes in the details and the arrangement of the parts may be varied over considerable range without departing from the spirit of the invention and the scope of the claims.