A Long-Term Data Collection Study of the Hawser Mooring Forces on a Single-Point Mooring Utilized by ULCC's and VLCC's
Authors: Johnston Allan G. 
Published: October 2008

The paper discusses the design, manufacture, installation and results from a mooring load monitoring system which operated on a single point mooring (S.P.M.) of the cautionary anchor leg mooring (C.A.L.M.) type from June 1979-July 1981. The S.P.M. is operated by the Arabian American Oil Company at the Ju'aymah Crude Oil Export Terminal in Saudi Arabia, which is currently the largest S.P.M. terminal in operation. Typical results obtained from the measuring system illustrate the effect of different types of environmental and operational conditions on the mooring forces generated. The measures taken to minimize the risk of failure are discussed and conclusions .drawn.

Introduction

In late 1976 ARAMCO operated three G.A.L.M. buoys installed approximately 10 miles off Ju'aymah in the Arabian Gulf with a fourth C.A.L.M. being commissioned in late 1977. The terminal has two single anchor leg moorings (S.A.L.M.) located close-by in slightly deeper water. The terminal, which is the largest of its kind in the world, regularly receives the world's largest V.L.C.C.s and U;L.C.C.s to be moored at these facilities.

The initial hawser system used on the C.A.L.M.s (Fig 1) incorporated a single 120 mm dia (15 in cir) nylon 2-in-l double braid nylon grommet with a nominal length of 60 m (200 ft). The grommet was constructed of two 60 m (200 ft) lengths spliced together. The splices were contained in the mid-length of the grommet but not adjacent. A thimble was attached by chain to a triangular spreader-plate on the mooring bridle deck of the S.P.M. The ship end of the hawser was connected to approximately 6 m (20 ft) of chafing chain which was taken aboard the ship through a central bow fairlead. A polypropylene pick-up rope, approximately 150 m (500 ft) in length, was connected between the ship end of the hawser and the chafing chain.

Buoyancy was provided to the nylon hawser by means of foam rubber floats, and to the chafing chain by a large cylindrical chain support buoy which was connected between the ship end of the hawser and the chafing chain.

Hawser Failures

In April 1976 a mooring hawser failed while a VLCC was moored to a C.A.L.M. buoy in moderate weather The vessel had a displacement of about 285 000 D.W.T., was 81 per cent loaded and the environmental conditions at the time of failure were given as wind speed 48 kn, current 0.5 kn and wave height 1.5-1.8 m (5-6 ft). The hawser failure took place some 3 m (9l:; ft) from the shipboard end of the gorge t on one side only, both cover and core parted at the same time.

Part of the failed hawser was sent to the National Engineering Laboratory (NEL) Scotland for tests and examination. A section of the hawser was spliced at NEL, in the form of a grommet, and the rope was then tested to determine its residual breaking load. The nominal breaking load of the hawser was stated to be 3.0 MN (300 tonf) but it failed crt 1.66 MN (166 tonf) for the loop, thus the tensile strength remaining in a single rope was approximately 0.83 MN (83 tonf).