The development of a twenty-four inch diameter marine loading arm is presented in detail. The size of the arm, field servicing techniques and other features all combine to influence its geometric evolution. Illustrations are includedto depict major details. All information pertains to a pilot model arm.
At the present time there are tankers of 326,000 DWT size in operation and berthing facilities are being designed to accomodate tankers up to the 500,000 DWT range. In order to prevent excessive "turn around" time of these super-tankers, cargo must be transferred at a proportionately increased rate. One method presently employed to load and unload liquid cargo is by use of marine loading arms. These arms, consisting of rigid pipe and swivel joints, form a mechanism between vessel and dock which compensates for ship';s movement while transferring the product. Currently, the largest all-metal, articulated marine loading arms in service are sixteen inches in diameter. With the existence of the super-tanker, there is ';a definite requirement for loading arms of larger diameter. The operational range of these arms must also be of increased magnitude resulting in relatively longer reaches. With arms of this size (and weight), field servicing procedures deserve special attention to minimize possible down-time. With the above in mind, the following criteria was established for such an arm:
Diameter to be twenty-four inches to permit higher flow rates.
Simplified swivel joint packing replacement to eliminate dismantling arm.
Minimize arm width to accomodate anticipated spacing of ship';s flanges
In addition, the following features would be incorporated:
Arm to be fully counterbalanced in dead weight condition to reduce loading on ship';s flange and eliminate exterior handling facilities.
Symetrical weight distribution to eliminate "built-in" base moment
Reduction in dead weight by having fluid-carrying components act as structural members where possible.
Layouts and studies were undertaken resulting in generations of concepts and ideas which eventually shaped the arm.
Figure 1 is a general outline of the pilot model and serves to illustrate some of its aspects.
Like many of its predecessors, the arm basically consists of a riser tower, inboard arm and outboard arm. The inboard arm is free to rotate about a vertical axis coincident with the riser tower centerline (riser axis) and about a horizontal axis near the top of the tower (trunnion axis). The outboard arm rotates through a vertical plane about a horizontal axis passing through the forward end of the inboard arm (apex axis). The above nomenclature is used hereafter to identify the swivel joints which permit this motion, i.e., riser joint, trunnion joint and apex joint. At the ship';s end of the outboard arm there is a combination of three swivel joints and elbows which permit freedom of movement of the ship in relation to the arm.
