Abstract

Gulf Oil Corp. recently completed an unusual type of operation in the Gulf of Mexico. This involved a task which was performed below water level by oil field personnel without the aid of diving gear. The purpose was to develop another approach to deep water operating problems.In this case a wildcat well located in 90 ft. of water, which had been drilled with a deep water mobile rig, resulted in a discovery. The well was cut off under water at a depth of approximately 60 ft and the rig was moved.A tender-type platform was constructed following proof of commercial production, and at a convenient time this platform was installed over the submerged stub by a standard derrick barge.

A 48-in. diameter steel tube - "snorkel tube" - was lowered through guides within the platform, over the stub and on to the ocean floor. The tube then was driven to refusal with a large pile driver, and water inside the tube was evacuated with a pump. Regular oilfield personnel. who would work under atmospheric pressure. were lowered into the tube to the underwater well stub.A new coupling was welded oil to the 13 3/8-in. casing at the bottom of the tube. An old coupling was washed off of the pin on one joint of 9 5/8-in. casing with the welder's torch, which left new threads exposed on top of the stub. New casing joints were lowered into the well from the platform, and these were screwed on to the threads on the casing stub. This casing extended up and out the top of of the snorkel tube. Wellhead controls were installed at the top. Thus, the well was readied for completion by the cheaper tender-type rig which would be used to drill other development wells from the same platform. A few platform installations over subsea wells had been made prior to the Gulf job. It appears that none had used the snorkel tit be in the manner or to the extent of this one. The snorkel tube technique offers another solution to three major problems. It simplifies the platform installation where the mobile rig is employed, making the mobile rig more useful in deep water. It provides a simple, positive means of completing wells drilled by floating rigs. It can be used to repair well head equipment and other parts of wells which are completed under water in the ocean floor environment. Future applications include plans to fully complete wells prior to moving the portable rig which drilled them.

Introduction

Gulf Oil Corp. recently conducted a field experiment to perfect an unusual technique for use in the Gulf of Mexico. The approach is known as the "snorkel tube technique". The principal purpose of this technique is to provide a better and cheaper means of completing certain types of new wells in deep water (90 to 600 ft). it also is designed to provide a means of repairing subsea wells. The snorkel tube technique has many desirable features. It enables men to perform common tasks on oil wells below the water at ocean floor level without diving gear. Personnel ordinarily employed in the oil industry are used. The men work comfortably and safely under atmospheric pressure. No "special" equipment is required in any phase of this technique. These features could prove to be of value in the exploration and profitable development of many deep water oil fields around the world.

Applications for the snorkel tube technique exist under the following conditions:

  1. When exploratory wildcat wells are being drilled with either deep water mobile rigs or floating drilling rigs:

  2. When wells are being drilled without the benefit of platform jackets, exceptionally large diameter caissons, or similar expensive structures which only provide lateral support for the well column;

  3. When water depths vary between 90 and 600 ft (proportionally greater benefits may be derived with increasing water depths, particularly in depths exceeding 100 ft); and

  4. When it would be desirable to build a platform only after proving the wells to be commercially productive.

In simplest analysis, the snorkel tube technique consists of cutting off a slender well column below the water near the ocean floor, normally while the mobile rig which drilled it remains on location. A conventional platform is installed over the well stub after the rig leaves. This platform may be either a multiwell tender-type platform, a single-well platform, or a self-contained platform. The principal feature of this technique, a long steel tube of sufficient internal diameter to provide working space for personnel, is lowered through guides in the jacket over the well stub and driven firmly into the ocean floor. Water is removed from the tube. Men are lowered into the tube on a winch line to the top of the well where they cut off couplings to expose new casing threads. These submarine workers then help attach new joints of casing, which are extended back to the surface, and wellhead controls are installed above water in conventional manner.

JPT

P. 1171^

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