MODU mobilization

Before you can put a mobile offshore drilling unit (MODU) to work, you have to get it to the well site. This can be a complex and expensive operation depending on the type of unit, distance, and conditions.

Introduction

Mobilizing a MODU usually falls into three categories:

• Field moves

• These are short and require no special preparation other than standard marine items.

• They are usually defined as < 500 miles in the same environment and geographical area, with the availability of safe haven if required by weather conditions.
• In large bodies of water, such as the U.S. Gulf Coast, mobilizations within the entire area are classified as field moves.

• Area moves

• If a MODU is moved from the U.S. Gulf Coast to Mexico, for example, it would be considered an area move.

• Long/international moves

• Any moves across the Atlantic or Pacific Oceans, of significant distance in Southeast Asia, from Europe to West Africa, etc., would be considered long/international moves.

Through their more favorable marine design, ships and semis have less metocean restrictions on moves than jackups and submersibles. Depending on the drilling contractor’s arrangement with the insurance underwriters and third-party surveyor, a surveyor may or may not be required to be present during the move. The surveyor and underwriter are keenly interested in the seaworthiness of the MODU. The degree of preparation is controlled by the category of move. The long/international move, which is the most restrictive, requires the most preparation. Usually, there is a long list of conditions, including:

• Mooring gear requirements
• Water tightness of openings
• Crew training and licensing
• Radio and communication gear
• Tug hookup and emergency lines
• Weather forecasting
• Class and regulatory compliance
• Routing of the tow
• Post-tow inspection
• General overall condition of the MODU

Methods of mobilization

A unit may be moved in two basic ways, by wet tow with a tug or a dry tow with a heavy lift ship or barge.

Wet Tows

Dry Tows

Tugs for wet tows come in all sizes and capabilities. Small 600- to 900-hp tugs are often used to move submersibles in shallow water near shore. For field moves in the open waters of the GOM, 4,600- to 9,000-hp units are often used, usually two to three units at a time, depending on the size of the MODU, length of tow, and type of MODU. For ocean-going wet tows, tugs with > 20,000 hp are not uncommon. In the past 15 to 20 years, a new type of tug has become popular for semis that can pull/run anchors, act as a supply boat, and tow. Some of these vessels are very large with horsepower ratings > 20,000 hp.

The second mode of transport is the use of a heavy lift ship or barge (Fig. 1). This is the most expensive transport but usually travels at > 10 knots, which, depending on the MODU, is two to three times faster than a wet tow. If collecting the MODU contract day rate is an issue for the drilling contractor, the heavy lift ship is cheaper overall because it gets to location much faster. The insurance rate is also a third to a quarter of that for a wet tow. The use of heavy lift ships has become more popular for many reasons, mainly safety and speed. There are also non-propelled submersible barges that load the same way as the heavy lift ships.

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  Fig. 1—Heavy lift ship performing a dry-tow transport of a semi (Atwood Eagle) from Angola to Australia in 2004. Courtesy Atwood Oceanics.

Factors that impact mobilization of units

A key issue with any MODU is the site condition. This usually centers on soil characteristics, especially for jackups and submersibles , which sit on bottom, and less so for semis and drillships, which are concerned only with the anchor-holding power of the soil. The issue of punch through of a leg by an independent leg jackup is of major concern; thus, soil borings are usually required for these locations. With information on soil conditions from soil borings, punch-through conditions can usually be determined. A punch-through condition usually is associated with a hard, thin sand layer with weak soil underneath it. When the jackup preloads by filling its preload tanks with seawater and thus increases its weight, the load may become so great that the soil fails and a punch through occurs with usually just one leg-spud can. Should this occur, the other two legs will probably be quickly overstressed. If the punch-through is deep enough, the legs usually bend, and the jackup must go to the shipyard for extensive leg and hull repairs.

For submersibles, the issue is usually uneven settling or scouring under the hull. If the hull should settle unevenly because of scouring resulting from ocean currents, the hull will most likely be overstressed, resulting in possible structural damage. Although this event is very uncommon, “hogging” or bending the keel of the submersible is a very serious situation. To prevent this, some submersibles have scouring skirts around the edge of the hull. Mat jackups also may experience scouring, especially if in shallow water with high currents; therefore, a 2-ft-deep knife edge is placed all around the mat’s perimeter to help prevent scouring. Cement-filled sandbags have been used to prevent scouring under submersible and jackup mats. Uneven settling is a more severe condition for a mat jackup in that, with misalignment over 1 to 1½ degrees vertical tilt, the cylinder legs will become wedged in the jack house, and the rig cannot jack because of friction between the leg and jack house.

Pipelines and underwater structures (e.g., natural reefs and old shipwrecks), protected underwater creatures (e.g., tube worms), and even old wells must be mapped and acknowledged. For semis, running anchor lines across and resting the anchor chains on some of the latter objects is not allowed or considered good practice. In this case, anchor patterns are altered or special mooring-line configurations are considered. Options include the use of:

• Spring buoys that lift the mooring line off the object
• Special vertical load anchors that do not require the anchor-line scope of a dynamically installed drag anchor
• Special composite mooring-line makeup. Sandbags full of ready-mix cement have often been laid on pipelines in shallow water to keep mooring lines from cutting or lying on them

Drilling next to shipping lanes and/or fairways requires planning and extra precaution. Ships sometimes stray out of their designated lanes and have, on occasion, collided with MODUs. Floaters may have anchors and/or anchor lines in the fairway, so coordination with the proper authorities is mandatory so that vessel traffic will not hit the MODU’s mooring lines. Usually, the mooring line must be at a depth under full tension that will not threaten vessel traffic. For the GOM and other areas, notice must be given to the proper authority that a “navigation hazard” has moved into an area so that vessel traffic will be aware of the MODU and not collide with it.

Using a MODU requires the operator to plan ahead, determine conditions, and make arrangements for unusual conditions.

See also

MODU types

MODU selection

PEH:Offshore_Drilling_Units