Reducing Tanker-SPM Loading Downtime

Abstract

Introduction Since July 1976, crude oil from the Montrose Field in the North Sea has been offloaded via two single point moorings (SPM's) and two shuttle tankers. The production platform has no storage capacity and must therefore shut in production whenever a tanker is not moored and loading. Adverse weather conditions in the North Sea affect the tankers' ability to stay moored to the SPM and their ability to remoor, and are the major cause of offloading downtime. In September 1978, an in-house project was initiated to investigate ways to reduce the downtime without instituting major changes in the system. This paper discusses some of the major operational factors leading to downtime and describes steps taken in attempting to reduce it. These include periodic technical discussions with operations personnel and tanker Captains, which resulted in minor hardware modifications and computer simulation studies for predicting tanker responses in severe weather. 2. Offloading System Description The two buoys are of the catenary anchor leg mooring (CALM) type, and are situated approximately one mile from the Montrose production platform, in approximately 91 m (300 ft) of water. The field uses two dedicated 72,000 dwt tankers for offshore loading. The tankers self-moor to the buoy using a hydraulic double drum ropetraction winch mounted on the forecastle deck in the peak of the bow, with the rope running through a U-shaped rounded fair-lead. The winch can develop a holding capacity of r.38 × 106 Newtons (310,000 lbs). The hawser tension is measured by hydraulic load cells attached to the winch brakearms. The mooring hawser is a 107 m (350 ft) long, 170 mm diameter, double braidednylon rope. It is spliced into 122 m (400 ft) of size 12, eight-strand, polypropylene messenger rope. The end of the messenger rope has a knot turned in it to facilitate grapnel hook-up for mooring. A more complete description of the Montrose offloading system has been reported previously in Ref. 8.