Underwater pipe laying vessel

An underwater pipe laying vessel which installs a pipeline using "J" shape installation, by progressively adding pipeline lengths to an existing pipeline by welding each pipeline length to said pipeline as installation progresses, the vessel having a main top deck and comprising:

• a pipe laying well which is formed substantially at the center of the vessel, and extending from said main deck to a bottom of said vessel;
• a pipe laying boom of hollow construction mounted within said well, said boom having a through passage which allows a pipe cross-section to go clear through, said boom having a lower end which penetrates and extends beyond the bottom of said vessel;
• a swivel mounting means which mounts the boom so as to project upwardly of said main deck, said swivel mounting means having a horizontal swivel axis about which the boom can be given controlled oscillatory movement;
• support means to control an oscillatory position of said boom about said swivel axis;
• welding means in the form of a flash butt welder disposed at least partly within said through passage in a region proximate to said swivel axis, said welding means including pressure applying means to hold under pressure a free end of said pipeline and a mating end of an assembled pipe length;
• guiding support means disposed inside said through passage of said boom for guiding and supporting said free end of said pipeline in use and also to relieve any undue stresses occurring on said free end of said pipeline caused because of vessel movements because of high seas during welding for pipe laying;
• swivel arm means swivelled about a horizontal axis to pick up said each pipeline length from said main deck and to assembly said each pipeline length into said through passage in the boom to abut against said pipeline free end;
• and
• controlled means to selectively hold and lower an upper portion of an addcd on welded pipe length from within said through passage into an outside region of the ship bottom through the well for pipe laying.

A method of underwater pipe laying using a pipe laying vessel for progressively welding and adding shorter lengths of pipe to a free end of a pipeline which is in progress, by using a hollow swivelled boom mounted in a well which is formed substantially in the center of the vessel which has at least a main deck, said hollow boom having a lower end penetrating and extending beyond the vessel bottom, the method comprising the steps of:

• receiving individual pipe lengths from a delivery ship onto the main deck such that the pipe lengths are always aligned longitudinally with respect to the vessel length;
• forming pipe assemblies by welding at least two said pipe lengths end to end and storing the pipe assemblies so that their lengths are always oriented along the length of the vessel;
• supporting and holding a free end of said pipeline in progress at substantially a mid-portion of said hollow boom, said pipeline in progress entering said hollow boom from its said lower end;
• assembling one of said pipe assemblies by insertion, from a top region of said hollow boom to abut against said supported free end of said pipeline;
• applying an axially compressive force to hold said free end of said pipeline and a mating end of an assembled pipe assembly;
• butt welding said free end of said pipeline and said mating end of a said pipe assembly;
• treating a welded pipe surface for corrosion prevention in service;
• and
• lowering said pipeline and moving the vessel forward for adding a second of said pipe assemblies.