the invention will be described using examples of embodiments . it should however be realized that the embodiments are included in order to explain principles of the invention and not to limit the scope of the invention , defined by the appended claims . fig1 illustrates a hull 10 which is suitable to be used in the construction method of the present invention . the hull 10 comprises a first and a second longitudinally extending float 12 , 14 . each one of the first 12 and second 14 floats is adapted to extend through a still water level ( swl ) of a body of water . the first and second floats 12 , 14 are connected to one another by means of a substantially transversally extending bridging arrangement , which bridging arrangement in the implementation of the hull 10 illustrated in fig1 is constituted by two transversally extending bracings , namely a first 16 and a second bracing 18 , wherein each one of the bracings 16 , 18 connects a lower portion of the first float 12 to a lower portion of the second float 14 . the floats 12 , 14 and the bracings 16 , 18 are preferably made out of steel . purely by way of example , the hull 10 may have a displacement within the range of 50 000 - 500 000 metric tonnes when in an operational draught . furthermore , as may be gleaned from fig1 , the first and the second floats 12 , 14 in combination with each one of the bracings 16 , 18 form an opening in the hull . as such , the first and second floats 12 , 14 and the first bracing 16 form a first opening 20 whereas the first and second floats 12 , 14 and the second bracing 18 form a second opening 22 . in the presentation of the construction method of the present invention hereinbelow , reference will be made to the first opening 20 only . however , a person skilled in the art will directly realize that each step in the construction method is equally applicable for the second opening 22 . at least one , but preferably both , of the floats 12 , 14 are preferably furnished with tanks for storing hydrocarbons , such as gas or oil . more preferred , the floats are furnished with tanks for storing liquefied natural gas ( lng ). moreover , the floats 12 , 14 — and possibly also the bracings 16 , 18 — are provided with ballast water tanks and a ballast water system including inter alia water pumps for controlling the water level in the aforementioned ballast water tanks such that the load and hence the floating condition of the hull 10 may be controlled . fig2 presents a side view of the fig1 hull 10 , wherein deflections of the hull 10 have been exaggerated . as may be realized from fig2 , when the hull 10 is floating in water , there is a risk that the hull may be subjected to initial deflections due to e . g . difference in the internal pressure and the external pressure — i . e . the water pressure — of the hull 10 . moreover , fig2 illustrates that it is not unlikely that the bracings 16 , 18 may be subjected to a bending deflection resulting in that the first and second floats 12 , 14 will be inclined away from one another . as such , if an end piece ( not shown in fig2 ), adapted to be fitted into the first opening 20 was to be attached to the hull 10 in the floating condition illustrated in fig1 and fig2 , there is a risk that the hull 10 thus obtained — i . e . comprising the original hull 10 and the end piece — would contain residual stresses which may impair the strength of the hull 10 . this problem is at least reduced by the construction method of the present invention , as will be explained hereinbelow . fig3 illustrates a further step of the construction method of the present invention , wherein the hull 10 has been lowered into the water such that the freeboard , i . e . the distance from the still water level to the uppermost portion of the first and second floats 12 , 14 , is so small that a deck structure 24 may be floated over the hull 10 . preferably , the hull 10 is lowered by introducing ballast water into the hull 10 , e . g . by using the aforementioned ballast water tanks ( not shown ). since the water plane area of the hull 10 is only constituted by the water plane areas of the first 12 and second 14 floats , respectively , the hull 10 may be lowered to a desired draught without subjecting the hull 10 to excessive loads . furthermore , the water plane areas of the first 12 and second 14 floats , respectively , preferably are adapted to provide an appropriate stability to the hull 10 in the floating condition illustrated in fig3 . when the hull 10 is in the position illustrated in fig3 , the deck structure 24 is floated over the hull 10 . to this end , the deck structure 24 may be loaded onto one or several barges ( not shown ) wherein at least one of the barges may be guided through the second opening 22 of the hull 10 . optionally , the deck structure 24 may be buoyant — for instance a centre portion of the deck structure 24 may be adapted to be immersed in water and thus carry the rest of the deck structure 24 when it is introduced through the second opening 22 . the deck structure 24 may have various designs , depending inter alia on the purpose of the semi - submersible unit to be constructed . purely by way of example , the deck structure 24 may be a buoyant member or at least have a water tight bottom . optionally , the deck structure 24 may comprise , or even be constituted by , a truss structure ( not shown ). moreover , although the deck structure 24 in fig3 is illustrated as being constituted by one single unit , the deck structure 24 may instead be split up into several sub - structures ( not shown ) wherein each one of the sub - structures are floated into position and connected to the hull 10 . additionally , the deck structure 24 may comprise additional equipment , e . g . for drilling , producing or storing hydrocarbons ( not shown ). purely by way of example , a deck structure 24 without the additional equipment as presented hereinabove may weigh approximately 10 000 to 20 000 metric tonnes . in a case wherein a deck structure 24 is provided with arrangements for e . g . producing lng , the deck structure 24 , including such arrangements , may weigh 60 000 to 80 000 metric tonnes . when the deck structure 24 has been placed in position over the hull 10 , the two components may be connected to one another to form a semi - submersible unit 26 . to this end , the deck structure 24 may be fixedly attached to the hull 10 , e . g . by means of welding . optionally , the deck structure 24 may simply be resting on the hull 10 wherein the deck structure 24 will be held in place in relation to the hull 10 by means of friction forces developed between the two structures . fig4 illustrates a side view of the fig3 semi - submersible unit 26 . as may be realized when comparing fig4 and fig2 , since the fig4 semi - submersible unit 26 comprises the deck structure 24 which is connected to the hull 10 and since the deck structure 24 often is relatively stiff , the deflections of the hull 10 in the fig4 configuration are generally substantially less pronounced than in the case illustrated in fig2 . thus , the configuration as proposed in fig4 provides for that an end piece may be inserted in the first opening 20 in the hull 10 and may also be attached to the hull 10 when the hull 10 subjected to small initial deflections , resulting in a semi - submersible unit with low residual stresses . fig5 illustrates a perspective view of the semi - submersible unit 26 obtained from the construction step illustrated in fig3 . fig5 further illustrates that an end piece 28 may be floated into the opening 20 and connected to the semi - submersible unit 26 . in this respect , the draught — i . e . the distance from the bottom of the hull 10 to the still water level — of the semi - submersible unit 26 in the condition illustrated in fig5 is preferably substantially lower than the draught of the hull 10 during the deck structure 24 mating step as illustrated in fig3 . this reduction of the draught is carried out order to facilitate the floating in of the end piece 28 . however , in some implementations of the construction method of the present invention , it may not be necessary to reduce the draught between the steps of the construction method of the present invention as illustrated in fig3 and fig5 . as further may be realized from fig5 , the end piece 28 preferably constitutes a substantial part of the side wall . purely by way of example , the end piece 28 may have a height which is in the interval of 0 . 2 - 0 . 8 of the height of the hull 10 and the width of the end piece 28 may also range from 0 . 2 to 0 . 8 of the width or length , whichever is the largest , of the hull 10 . in a preferred implementation of the end piece 28 floating in step of the method of the present invention , the end piece 28 is buoyant such that it may be self - floating into the first opening 20 . to this end , the end piece 28 may preferably be connected to an arrangement ( not shown ) adapted to propel the end piece 28 . purely by way of example , such an arrangement may comprise one or more tugs ( not shown ). optionally , if the end piece 28 is not buoyant or if it by some reasons is so desired , the end piece may be loaded on a floating arrangement ( not shown ) such as a barge ( not shown ) which floating arrangement in turn may be floated into the first opening 20 . the end piece 28 is preferably made out of steel and preferably comprises structural tanks , such as ballast water tanks ( not shown ). such ballast water tanks may be used to control the floating condition of the end piece 28 , in the case of a buoyant end piece 28 . moreover , once the end piece 28 has been connected to the semi - submersible unit 26 , the ballast water tanks of the end piece 28 may preferably be connected to a ballast water system ( not shown ) of the semi - submersible unit 26 such that the ballast water tanks of the end piece 28 may be used for controlling the floating condition of the semi - submersible unit 26 . in order to facilitate the floating in of the end piece 28 into the first opening 20 ; the hull 10 and / or the end piece 28 is preferably provided with guide means for guiding the end piece 28 into position in the first opening 20 . purely by way of example , such a guide means may comprise a rod ( not shown ) of the hull 10 which rod extends substantially vertically in the first opening 20 as well as two frames ( not shown ) located on the bottom of the end piece 28 , which frames form a tapering gap . moreover , the hull 10 may be provided with supports ( not shown ) located below the opening on which supports the end piece 28 may rest when it has been inserted into the first opening 20 . when the end piece 28 has been inserted into the first opening 20 , the end piece 28 and the semi - submersible unit 26 are attached to one another . preferably , the end piece 28 is fixedly attached to the semi - submersible unit 26 , e . g . by means of welding . moreover , the end piece 28 is preferably fixedly attached to the hull 10 and , if possible , also to the deck structure 24 . the end piece 28 may be attached to the semi - submersible unit 26 when the unit 26 is in the floating condition as illustrated in fig5 , e . g . immediately after the end piece 28 has been inserted into the first opening 20 . however , in a preferred embodiment of the present invention , the semi - submersible unit 26 is raised — i . e . the draught of the unit 26 is reduced — before the end piece 28 is attached to the unit 26 . this implementation of the preferred embodiment is illustrated in fig6 . if the end piece 28 is attached to the semi - submersible unit 26 when the unit is in a floating condition as illustrated in fig6 , attachment procedures , such as welding , may be performed in a dry environment which generally provides for a more rapid and efficient attachment procedure . fig7 illustrates a side view of another implementation of a hull 10 which may be used in the construction method of the present invention . as may be gleaned from fig7 , the hull 10 comprises a truss assembly 30 at least partially delimiting the first opening 20 . the truss assembly 30 adds additional structural stiffness without adding any substantial hydrostatic stiffness to the hull 10 . as may be realized by a person skilled in the art , the aforementioned changes in the characteristics of the hull 10 obtained by the introduction of the truss assembly 30 may be useful when e . g . performing the construction method of the present invention , for instance as regards deflections of the hull 10 and / or the capability of raising and lowering the hull . with a semi - submersible unit 26 comprising a hull 10 as illustrated in fig7 , an end piece ( not shown in fig7 ) is preferably attached to the inner portion of the truss assembly 30 . in order to obtain a buoyant — or at least weather proof — side wall extending from the first float 12 to the second float 14 of the hull 10 as illustrated in fig7 , the truss assembly 30 is preferably sealed from ambient water after the end piece 28 has been inserted in the first opening 20 and attached to the truss assembly 30 . preferably , such a sealing is obtained by attaching a sealing arrangement to the truss assembly 30 . an example of such a sealing arrangement is illustrated in fig8 , wherein the sealing arrangement comprises four steel plates 32 ′, 32 ″, 34 ′ and 34 ″. each one of the steel plates 32 ′, 32 ″, 34 ′ and 34 ″ is placed in an appropriate position on or over the truss assembly and attached to the hull 10 and the end piece 28 . preferably , the sealing arrangement illustrated in fig8 further comprises a two more plates ( not shown in fig8 ) adapted to be located at each side of the end piece 28 and adapted to cover the portions of the truss assembly 30 located below the end piece 28 . finally , fig9 illustrates another hull 10 for use in the construction method of the present invention may be performed . as may be gleaned from fig9 , the hull 10 illustrated therein comprises a ring pontoon 36 and four substantially vertically extending columns 38 , 40 , 42 and 44 wherein each one of the columns is connected to the ring pontoon 36 . as may be realized from fig9 , the hull 10 has four openings 46 , 48 , 50 and 52 and each one of the openings is adapted to receive and end piece ( not shown in fig9 ). preferably , when all the four openings of a semi - submersible unit , based on the hull 10 illustrated in fig9 , have been provided with an end piece , a semi - submersible unit is obtained which has a continuous outer wall , i . e . the unit is of a so called ring wall type . it should be realized that the present invention is not limited to the embodiments described hereinabove and illustrated in the drawings . rather , a person skilled in the art will realize that many changes and modifications may be performed within the scope of the appended claims .