Braced caisson minimal structure for water depths up to 260 ft

Greg Ruhl
Petro-Marine Engineering

• The original Guardian is a braced caisson concept for water depths to 100 ft. [18,038 bytes]
• The Guardian II is ideal for water depths of 100-180 ft. [19,022 bytes]
• The Guardian III is used in water depths of 180-260 ft. [17,160 bytes]

In 1986, Petro-Marine Engineering developed the original Guardian concept in response to industry demands for a low-cost minimal structure that can support up to 4 wells. Fifty-five Guardians have been installed in the Gulf of Mexico and overseas since 1986, thus demonstrating the technical and commercial feasibility of the concept.

In recent years, however, operators have demanded minimal structures for deeper water depths. Petro-Marine has developed the Guardian II and Guardian III concepts, which expand the Guardian principles up to a water depth of 260 ft.

The original Guardian is a braced caisson concept for water depths up to 100 ft, in which a 48-in. caisson is braced against lateral wave forces with two 36-in. battered piles. A Guardian sleeve, which has guides for the two battered piles, is initially stabbed over a preinstalled caisson and welded to the caisson above the water surface. Battered piles are then stabbed into the guides, lowered to the mudline, driven to grade and welded to the Guardian sleeve. There are five primary advantages of the original Guardian concept:

• For a 100-ft Guardian, 6% of the structural steel, excluding the deck, is fabricated structural steel. The remaining steel is in the caisson and piling, which is typically half the cost of fabricated steel. This reduces the total fabrication cost of the structure considerably.
• All installation activities for a Guardian structure are made above the water surface; therefore, neither divers nor underwater equipment is required.
• The connection between the braces and the caisson is made with a conventional jacket-to-pile crown shim concept. Clamps, or a grout spread, are not needed for the Guardian concept.
• The lifted structural components for a Guardian are relatively small. The Guardian can be installed with either a small derrick barge or a jackup lift boat.
• The caisson can be designed to be free standing, in which case the caisson can be installed by a jackup rig if the well is commercial. This eliminates the need for mudline suspension and the cost of a rig remobilization to tie back the well.

Guardian II

The Guardian II concept is applicable for water depths between 100 ft and 180 ft. The fabricated Guardian sleeve is expanded to provide lateral support to the two battered piles approximately halfway to the mudline. The fabricated Guardian II sleeve is lifted and stabbed over the caisson in the same manner as a Guardian. As with the Guardian, all structural connections are made above the water surface where conventional jacket-to-pile connections are made. There are differences between the Guardian II and original Guardian concepts:

• The Guardian II sleeve is larger than the sleeve for a Guardian; therefore, the fabricated steel costs are slightly higher. The fabricated sleeve for a Guardian II only weighs approximately 23% of the total structural steel, excluding the deck, resulting in very low structural fabrication costs.
• The lifted structural components for a Guardian II are still relatively small. However, the water depth may preclude the use of lift boats for installation. For water depths greater than 120 ft, a small derrick barge will probably be required.
• The caisson for the Guardian II can still be designed to be free standing for water depths approaching 160 ft, however, the diameters required will be larger than the conventional 48 in. with costs increasing accordingly. Nevertheless, the high costs of a mudline suspension system and an extra drill rig mobilization to tie back the well are usually far greater than the increased cost of the larger caisson. The ability to use free standing caissons is certainly an important and economic application for Guardian II platforms.

Guardian III

The Guardian III concept is applicable in water depths between 180 ft and 260 ft. The fabricated Guardian sleeve is expanded even more to provide two points of lateral support to the battered piles approximately one-third and two-thirds the distance to the mudline. The fabricated Guardian sleeve is lifted and stabbed over the caisson in the same manner as a Guardian II. As with all Guardian concepts, the Guardian III structural connection is made above the water surface where conventional jacket-to-pile connections are made. The differences between the Guardian III and the Guardian II concept are as follows:

• The Guardian III sleeve is larger than the sleeve for a Guardian II; therefore, the fabricated steel costs are again slightly higher. However, the fabricated sleeve for a Guardian III still only weighs about 26% of the total structural steel, excluding the deck, resulting in lower structural fabrication costs.
• The Guardian III sleeve will require a mid-sized derrick barge for installation.
• Since a free standing caisson in water depths greater than 180 ft is impractical, any pre-drilled wells must incorporate a mudline suspension system. Subsequent wells can be drilled conventionally with a jackup rig.

The deck size and equipment weight for all Guardian concepts are primarily limited to what can be supported by a single column. Experience has proven that decks can be as large as 50 ft by 50 ft with a helideck mounted above. The weight of the deck alone is not a significant consideration for the Guardian design because it is resisted directly by the vertical caisson, which can easily support considerable loads.

Fabrication weights

The actual weight of the Guardian sleeve and piles is dependent on the soil properties and the number of conductors. Generally, a Guardian structure with 3 conductors in a soft clay soil will result in the larger weight within the range.

The data provided in Figure 4 is applicable to the Gulf of Mexico environment. The weights of Guardians designed for other locations around the world will tend to be proportional to the height of the design wave, but fatigue considerations in some environments may affect the design. Areas where lighter Guardian structures will result, relative to Gulf of Mexico weights, include Southeast Asia and West Africa.

Installation costs

The installation of the three Guardian structure concepts is simplified by the fact that all work is done above the water surface. A few factors which influence the installation time are as follows:

• Although the cost of a jackup lift boat is considerably less than that of a derrick barge, this is partially offset by the fact that the time required to install the structure is longer because of crane limitations.
• The water depth and soil properties will dictate the number of pile splices required, which will affect the installation time.
• The installation time for the Guardian can be significantly reduced by the installation of a free standing caisson by the drilling rig prior to demobilization.

Taking into consideration these factors, the installation time for a Guardian can range between 4 and 10 days, not including the effect of weather downtime.

With the recent decline in oil and gas prices, it is reasonable to expect the demand for minimal structures to increase. As marginal fields are discovered in deeper water, the Guardian II and Guardian III concepts should continue to be attractive minimal structure concepts for future development.

Author

Greg Ruhl is Structural Engineering Manager for Petro-Marine Engineering of Texas, Inc., a Houston and New Orleans-based engineering consulting firm serving the domestic and international offshore oil and gas industry.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.