Over the years the means used to transfer petroleum cargo between ship and shore have evolved from all-hose assemblies to the present all-metal, double counterweighted marine arm.
With recent trends toward larger tankers, higher cargo transfer rates, and more demanding operating specifications, arms have become bigger (10-inch to 24-inch diameters and 100 feet long) and far more structurally complex. Stress analysis of these arms is more demanding because of the spatial and frequently indeterminate nature of the arm structure. Except for the simpler, early 1960 vintage arms, stress calculations are tedious, time consuming, and often, of necessity, imprecise.
Consequently, Esso Research has developed a computer program capable of rapid and exact arm analysis. In a single run, the program automatically applies a range of loading combinations and utilizes a stiffness matrix technique to calculate stresses in all members of the arm. The analysis can be made with the arm in a variety of attitudes including: stored maneuvering, operating, maintenance, washing, and hydrostatic testing.
The following areas are covered: the criticality and complexity of loading arm design, the three-dimensional stress analysis required, and the computer program developed to examine arm stresses.
One of the most common means for transfer of petroleum cargoes at conventional tanker berths is the double-counterweighted, all-metal marine loading arm. It is a three-dimensional articulated structure consisting of a fluid carrying piping system stiffened with pinframed or rigidly connected structural members on one or more of its basic components. At least six swivel joints in the fluid-carrying piping allow the arm to be maneuvered from a stored position to and within an operating envelope.
The all-metal loading arm has evolved over the last 25 years from the simplistic manual and semi-automatic hose systems shown in Illustration 1. Some of the shortcomings of these early systems, which spurred development of the double-counterweighted arm, are:
All-hose structures
limited loading rate
difficult handling
continual adjustment to maintain proper hose bend radius
Half-hose/half-pipe arms
continual adjustment to maintain proper hose bend radius
All-metal, freestanding arms
large reaction imposed on tanker's Manifold
For the most part, however, these systems presented few difficulties for the design engineer. While stress analysis of the prototype arm shown is complicated by its three-dimensional nature, the loads can be hand calculated throughout the arm by using the equations of statics.
Over the past seven years loading arms have steadily increased in size. Arm reach (total length of inboard and outboard tubes) has extended from 46 feet to 100 feet to keep pace with the growth of tankers from 50M dwt to 300M dwt. Another factor contributing to longer arm reach is the wider range of vessels loading arms are required to service.
