Explosive Technology Meets North Sea Challenge
Authors: De Frank Pete 
Published: December 2007

The North Sea challenges every phase of offshore operations. The combination of high waves, strong winds, strong tidal currents, poor visibility and low water temperatures makes offshore operations hazardous and costly. Although weather at its worst in the North Sea is not so violent as the worst weather in the Gulf of Mexico, the bad weather of the North Sea is constant. Throughout the year the area has experienced fewer that 2 percent calms. Winds in excess of 17 knots are encountered approximately 30 percent of the year, and 20 percent of the year waves reach heights of 8 ft or more. Tides with strong currents restrict diving operations which, even during slack tides, are further restricted by surface water conditions.

This article introduces a new method for severing pilings, employing explosive technology. Described is the first use of linear-shaped charges in the North Sea to cut 36-in. OD, 1 1/4-in. pilings for fixed platform removal. Use of new devices placed by divers resulted in a safe, fast and economical method of platform removal. The high cost of barge operations, support equipment and personnel can be reduced considerably by using these explosive tools that can be employed independent of other barge operations. This article covers potential uses of explosive devices currently under development and nearing completion.

Introduction

Offshore operations in the North Sea present a tremendous challenge due to the hostile environment. Winds of gale force are the rule, not the exception, throughout the year. Violent storms are a constant threat in the fall and winter months. Their rapid buildup and mobility seldom give operators more than 12 hours' warning. The barges depend on tugs to pull anchors and get under tow. Often it is necessary to ride out the storms since tugs have tow capabilities of only a few knots and travel time from location to port is long.

In addition to being restricted by surface water conditions, diving operations also are limited by tidal currents of 2 to 4 knots. These tides, which sweep in and out every 6 hours, restrict diving operations to approximately 2 hours during slack tide.

Removal of platforms in offshore operations, even under favorable environmental conditions, presents great operational problems and involves considerable investment of time, manpower and money. Since costs of barge operations, involving use of many support vessels and a number of highly skilled personnel, run as high as $20,000 per day, new tools and techniques to reduce operational time are needed urgently.

Technological advances in the use of explosives have resulted in a safe, time-saving and economical method of conducting underwater cutting operations. Effective use of explosive devices to abandon plugged wells and damaged producing wells, to remove platforms for re-use and to salvage damaged platforms, has been reported.

Platform removal in the North Sea previously was accomplished by the hazardous and time-consuming method of lowering divers 100 ft or more inside pilings that were to be cut with torches. With the new technique described here, shaped-charge cutters that function by the well known shaped-charge principle were used to sever 36-in. OD, 1 1/4-in. wall pilings of a 10-leg platform. Less than 24 hours of barge time was charged against this operation as compared with 14 days (336 hours) charged against the previous torch-cutting procedure. At an operational cost of $20,000 per day for barge and support equipment, the use of shaped-charge cutters resulted in a savings of $280,000 in barge time alone.