Three Years of Operations With the Montrose SBM's
Authors: Fairbrother N.J. 
Published: December 2007

Fairbrother, N.J.,* SPE, Amoco (U.K.) Exploration Co.

Crude oil from Amoco (U.K.) Exploration Co.'s Montrose field in the North Sea is transported to shore by means of an offshore loading system using two single-buoy moorings (SBM) and two shuttle tankers. The field has no storage facilities, hence a tanker must be moored and loading for the field to produce. The buoys were installed in Summer 1976. This paper outlines the 3 years of operating experience with the system, illustrating how weather conditions and repairs have ted to an average of 24% downtime for the Montrose field.

Buoy Description

The two buoys are of the catenary anchor leg mooring (CALM) type. They are situated approximately 1 mile to the southwest and to the northwest of the Montrose production platform in approximately 300 ft of water (Fig. 1). Each buoy is positioned over the production pipeline end manifold (PLEM) by six 1,140-ft lengths of 4-in. chain. On the end of each of these are two 360-ft lengths of 3-in. chain connected to 25-ton anchors as shown in Fig. 2. A 10-in. diameter "steep S" configuration subsea hose string joins the PLEM to a universal joint below the buoy. Oil is piped to the tanker through a single floating hosestring consisting of 11- x 35-ft sections, which tapers from 12 in. in diameter to 6 in. at the third section off the buoy.The mooting hawser (Fig. 3) is a 350-ft-long, 21-in.-circumference double-braided nylon rope. This is spliced into 400 ft of 12-in.-circumference polypropylene messenger rope. The end of the messenger rope has a knot turned in it (to facilitate grapple hookup for mooring) and a large marker buoy.

Tanker Equipment

The Montrose field is using two dedicated 72,000 dead-weight ton tankers for offshore loading.The tankers moor to the buoy with a hydraulic double drum rope traction winch mounted on the forecastle deck in the peak of the bow, with the rope running through a U-shaped, rounded fairlead. The winch can develop a holding capacity of 310,000 lbf. The hawser tension is measured by hydraulic load cells attached to the winch brake arms.The floating loading hose is secured in a hydraulic door on the port rear forepeak, and then is connected to the ship's piping by means of a quick-makeup flange on the end of a flexible hose. A small crane is mounted on the deck to aid these operations.

Loading Downtime

Original projections assumed that use of a buoy-based offshore loading system would result in 25 % downtime. The average total downtime to date has been 24%, although in the first 6 months of operation it was 27% and currently averages 22%. The major causes of downtime are the tankers' inability to remain moored in rough weather and unavailability of the buoys due to necessary repairs or waiting on weather to perform those repairs. Monthly figures for the field downtime to date show cumulative averages of: waiting on weather 19%; waiting on repairs, 1%; waiting on weather for repairs, 3.5%; and miscellaneous, 0.5 %.

Weather Downtime

The adverse weather conditions on the North Sea are the major cause of loading downtime (Fig. 4). Weather has two effects on the tanker-it affects ability to moor to the buoy, and ability to stay on the buoy once moored.

JPT

P. 733^