Abstract:
A marine buoyancy unit which fits at least partly around a marine object such as a riser with auxiliary conduits is composed of syntactic foam with an external layer or skin of harder polymeric composite. To reinforce the unit slots or channels are cut in the exterior of the unit or borings are created which extend within the unit and reinforcement in the form of webs or strips are placed into the slots or channels or borings. The reinforcement is embedded into the slots, channels or borings by introducing a curable resin.

Description:
FIELD OF THE INVENTION 
     The present invention is concerned with units and modules used primarily to impart buoyancy to marine objects and secondarily to protect such objects. 
     BACKGROUND OF THE INVENTION 
     Known buoyancy modules are made of low-density syntactic foam with a skin or layer of harder polymeric composite. In harsh marine environments such units and modules tend to become damaged or to fracture from time to time. 
     An object of the invention is to provide a method of renovation upgrading and/or reinforcement for such units and modules. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a method of reinforcing a marine buoyancy unit designed to fit at least partially around a marine object; said method comprising cutting slots or channels in an exterior surface of the unit, placing reinforcement material in the channels and filling the channels with a curable resin to embed the reinforcement material into the channels. 
     In another aspect the invention provides a method of reinforcing a marine buoyancy unit designed to fit at least partially around a marine object; said method comprising producing borings extending within the unit, placing reinforcement in the borings and filing the borings with a curable resin to embed the reinforcement into the borings. 
     The unit may be of part-cylindrical, e.g. semi-cylindrical, shape with an exterior skin or layer of high strength polymeric composite and an interior formed of syntactic foam. The channels penetrate the skin and extend into the syntactic foam interior whereas the borings would extend through the syntactic foam closely adjacent the skin. A number of such units would be united to form a module surrounding part of the marine object. The channels or borings extend longitudinally of the unit. A glass fibre mat may be laminated onto the exterior after the channels or borings are filled. 
     The invention may be understood more readily, and various other features of the invention may become apparent, from consideration of the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of invention will now be described, by way of example only, with reference to the accompanying drawing in which FIG. 1 is a cross-section of a buoyancy module at a preliminary stage in carrying out a method of reinforcement in accordance with the invention and 
     FIG. 2 depicts part of a strip of reinforcement used in the method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, a buoyancy module  10  is composed of two complementary units  9  each of semi-cylindrical form with a continuous outer layer  11 . This is merely illustrative and there can be more than two units  9  making up the module  10 . The interior main bodies  15  of the units  9  within the layers  11  is composed of syntactic foam. The units  9  are designed to fit around a marine object and part of the layers  11  define an internal cavity  5  shaped to conform with the marine object. By way of example the marine object can be a riser or pipeline with auxiliary conduits. Semi-circular recesses  12  in the diametric plane  14  separating the units  9  are designed to fit around a pair of such conduits. The units  9  can be held on the marine object by means of flexible bands or bolts but it is possible to provide an interlocking connection between the units  9 . Normally a series of modules  10  each composed of a pair of the units  9  would be arranged end-to-end along the marine object and the ends of the modules  10 . 
     At any time during their service life in a marine environment, the modules  10  and the units  9  may need to be renovated, upgraded and/or at least reinforced to prevent fracture or separation into parts following fracture. A method of such reinforcement, in accordance with the invention, will now be described. As shown in FIG. 1, a number of channels, here three, are made in the exterior of each of the units  9 . Typically, the channels  13  are positioned at 10°, 90° and 170° from the diametric plane  14  in the anti-clockwise sense. Typically the width of each channel  13  is around one inch and the depth of each channel  13  is around two to two and a half inches. The depth of each channel  13  is such that the outer layer  11  is penetrated and the channel extends into the interior foam  15 . Each channel  13  extends along the length of the unit  9  but terminates inwardly of the ends. Typically around one inch separates the ends of the units  9  from the ends of the channels  13 . After the channels  13  have been produced any paint on the exterior peripheral surface  16  is removed in the vicinity of each channel  13  and the regions  17  of the surface  16  at the juncture with the channels  13  are buffed to provide a mechanical key. Any debris left from the cutting of the channels  13  and the other treatments is removed by brushing or by vacuum cleaning. The channels  13  are then filled with reinforcement such as strips  20  made of a polymeric fibre mesh. 
     Part of one of the strips  20  are shown in FIG.  2 . The strip  20  has warp bands  21  extending along its length and thinner weft bands  22  running in the transverse direction. As shown there are ten to twelve warp bands  21  over the width of the strip  20 . The overall width of the strip  20  is greater than the width of the channels  13 . For convenience, each strip  20  is bent and rolled into a tight cylinder held with tie wraps. After placing the rolled strips  20  in the channels  13 , the tie wraps are released and removed. The next stage in the preferred method is to apply a glass fibre mat over the filled channels  13  and to introduce a resin mix which fills the channels  13  to embed the strips  20 . Vents in the mat allow air to escape. The mat itself can be held in place with a pre-formed semi-rigid sheet which is removed once the resin has cured. After curing the exterior of the reinforced unit  9  is buffed or sanded and then painted. 
     The reinforcement need not be in the form of a mesh instead continuous webs or strips of reinforcement material can be used or individual polymeric fibres or strands can be used. In all cases the reinforcement is made from polymeric fibres or strands either separate or joined together. The number of channels  13  and their dimensions discussed above is suitable for the strips  20  of polymeric mesh fibre and the channels  13  may be adapted to suit other reinforcement materials. For example, single fibres or strands would need a channel depth of at least 15 mm. The most important characteristic is the tensile strength of the mesh reinforcement material. Ideally once the strips or other reinforcement  20  have been embedded in the channels  13  and the glass fibre mat has been bonded onto the exterior surface of the unit  9  the additional or supplementary tensile strength of the reinforced unit  9  or module  10  is between 25 and 200 kilo Newtons per metre circumference. The number of channels, the width or diameter of the reinforcement and the characteristics of the reinforcement can be adjusted to produce this tensile strength. 
     The fibres making up the reinforcement are made of tough rather than brittle material and have significant elongation at break (minimum 5%, ideally over 20%). This elongation performance may be inherent in the nature of the fibre itself, e.g. PE.PP Nylon, polycarbonate PET PE/PP, copolymers or imparted by twisting the fibres, e.g. “Kelvar” (RTM). 
     A material suitable for the reinforcement is the mesh system in the “Sympaforce” range made by Synteen Technical Fibres Inc of Lancaster, S.C., USA and Synteen GmbH of Klettgau-Erzingen, Germany. This is a high tenacity PET mesh bonded and encapsulated in PVC “plastisol” paste. Meshes with weight and tensile strength biased in the axial direction are preferred, with grades of axial strength of 50-200 kN/m and transverse/circumferential strength of 25-100 kN/m being particularly suitable. The mesh nature achieves the advantageous intermittent mechanical locking, whilst plastic surface finish gives the desired, only poor adhesive bonding to the epoxy resin encapsulating mix. 
     It is advantageous if the reinforcement is intermittently locked into the resin mix rather than continuously bonded, to allow the essential elongation of the reinforcement at the fracture location to be accommodated over a greater length of reinforcement. Reinforcement in the form of mesh systems provide the intermittent locking whilst the surface finish on the reinforcement can be selected to limit continuous bonding. 
     In an alternative method instead of creating the open channels  13  longitudinal borings can be made in the units  9  near the exterior surface. The reinforcements  20  are inserted into the borings which need to penetrate at least one end face of the unit  9  and the borings are filled with resin as before.