A Solution for Keeping Submersibles on Location
Authors: Childers Mark 
Published: December 2007

When the submersible Mr. Charlie, the world's first purpose built MODU, went to its first location in 1954, it initially slid off location. Since that time anti scouring skirts, small piles, shell pads and other efforts have helped keep, for the most part, submersibles on location; however, in severe weather with weak soil conditions the units often had recurring sliding problems. The Atwood "Richmond" has had an excellent operating and safety record since returning to the Gulf of Mexico in 1988 while successfully drilled 82 locations. Unfortunately, special bottom conditions at 8 of these locations created sliding problems and of those, the bottom condition on 2 prevented completion of the well.

In 2000 after sliding was experienced on one of the two subject locations, an extensive search of existing technology and exploration of new ideas was carried out. As a result of this extensive review, the use of the new technology of suction piles in shallow water was decided upon. A Design Basis was developed to hold the unit on location in a 10-year hurricane event with the hull sitting on 100-psf shear strength soil. To meet this, four (4) self contained and self-installable/retractable piles were located on the four outer columns. Calculations resulted in the 31-ft. long piles located in shucks being 10 ft. in diameter with a requirement to penetrate 19 ft. under the most severe conditions. Jet nozzles were placed at the foot of each of the 32 ton, 1-1/2 inch wall thickness piles thus allowing them to be jetted-in or out of the soil with the high-pressure mud pumps. Four 35 Hp variable speed AC pumps were installed allowing finite control of the evacuation (suction installation) of water from the interior of the pile. To extract the piles, the rig's saltwater service pumps were manifolded to "pump" the piles out of the soil. Suction piles had previously never been used in this application or in water this shallow.

Since installation in late summer 2000, the system has been successfully used on 9 locations with the unit maintaining location in severe weather up to collinear 45 – 50 knot sustained winds, 10 – 12 ft waves and an estimated 2-1/2 knot current. These conditions far exceeded previous combinations of environmental and soil conditions where sliding was experienced; however, the Atwood "Richmond" held location with no movement. The suction pile system (Atwood US Patent) has been an unqualified success.

Introduction and History

The submersible barge was the first open water Mobile Offshore Drilling Unit (MODU). Submersibles characteristically have a large lower hull that floats during transit and sits on bottom during drilling. Structural latticework holds up an upper deck that houses the drilling rig, accommodations, power plant and stores. The first units could only work in very shallow waters but later units worked in water depths as deep as 175 ft. in non-hurricane season.

Forerunners of the submersible were inland barges, which had been used in the swamps of Louisiana from as far back as the early 1930's. Posted barges followed and are units designed with the drilling platform elevated above the barge via latticework. Starting in the 1950s operators started working in the bays of Louisiana that were exposed to Gulf of Mexico (GOM) fetches and open water environment. The posted barge did not fare well in not sliding and staying on location even in mild environments. This was due to most locations having very soft and low shear strength soil bottoms, the hull having a very large exposed area for wave forces to act upon, and the windage of the upper deck, derrick and other structures. Operators and drilling contractors used shell pads and drove piles to help solve the sliding problem all with limited success.