INTERLOCKING MATS SUPPORT DRILLING RIG ON FROZEN SWAMP

Conoco Inc.'s Anchorage, Alas., division employed a unique mat system to reduce environmental impact and to support the drilling rig on its Astosch No. 1 exploratory well at Granite Point in the Trading Bay Wildlife Refuge. The site is on the west side of Cook Inlet (Fig. 1).

April 15, 1991

4 min read

The site is on the west side of Cook Inlet (Fig. 1).

During winter, the travel time from Anchorage to the base camp near the Tyonek Indian village was 5 hr by ice road or 45 min by fixed wing aircraft. Eighteen miles of existing gravel roads were used from this base camp to the edge of the frozen muskeg swamp, and from there, Conoco constructed 7 miles of ice road to the well site.

Conoco constructed a snow and ice pad with two impermeable liners and then installed Uni-Mat International Inc.'s patented interlocking mats for the final foundation (Fig. 2). After moving in the rig, a snow berm was built around the perimeter of the location, and an impermeable liner was then draped and secured over the berm.

The drilling rig for this well is Grace Drilling Co.'s western division rig No. 158. The rig is rated to 12,000 ft with a gross nominal capacity of 700,000 lb and has complete winterized facilities.

The drilling operation was scheduled during the freezing temperatures from December 1990 to March 1991 with completion anticipated in March. Ice pad construction alone is the normal practice for the area, but Conoco wanted a location that would support the drilling operation should an early thaw occur. Additionally, the company desired minimal impact on the environmentally sensitive muskeg swamp area. The Uni-Mat system provided this environmental protection through its load-dispersing properties and double liners.

INSTALLATION

At this drilling site, 540 mats were used to build a location of 30,240 sq ft. All of the mats were installed with two forklifts and two 3-man crews, each working a 12-hr shift, in less than 1 day in -35 F. weather.

A basic mat measures 8 ft x 14 ft and weighs approximately 1,400 lb. Each mat is constructed of hardwood lumber with one solid side and one side with staggered members for interlocking.

The modular system consists of four of the mats placed solid side down, staggered side up. These four bottom mats are then locked into place by a top mat placed solid side up at the intersection of the four base mats.

The pad is then extended as wide and long as desired by adding bottom and top layers. The Uni-Mat system provides unique load dispersion because any heavy load on one top mat is spread to four times its area for a ground pressure effect of 25% of the original load. The reduction of load effect on the swamp below reduces damage to the delicate plant growth, root stock, and top soil.

The mats reduce drill site environmental impact because the hardwood lumber insulates the ice pad from the heat of the rig engines and mud tanks. Because the rig is elevated off the ice pad, air is able to pass through the mats to serve as a secondary cooling area to prevent melting of the ice pad.

During the latter part of the drilling schedule in March, temperatures fluctuate above and below freezing. With a conventional ice pad alone, continual melting and refreezing forms dangerous ice patches where men and equipment slip and slide.

The mat system, however, virtually eliminates this problem because its 6-in. height above the constructed ice pad allows for drainage and containment of rig runoff and melted snow. Thus, the location is safer for men and equipment moving around.

When the thaw does occur, the mats will be removed and transported out as quickly as they were installed. This will reduce the traffic and time on the ice pad during the thaw/breakup and, therefore, minimize the effect on the swamp. Nature should have no problem regenerating the muskeg swamp to its natural state.

This project is Uni-Mat International's entry for a North Slope environmental achievement award, sponsored by British Petroleum Exploration Co. and the Alaska Support Industry Alliance, that recognizes outstanding environmental efforts by companies and individuals involved in oil and gas exploration, development, and production on the North Slope.