Impacting a subsea development with tubulars inadvertently released from a drilling vessel would have disastrous consequences. In addition to the environmental impact, potential lost production and recovery costs could easily run to hundreds of millions of dollars.

Background
The GlobalSantaFe Development Driller Class deepwater semisubmersibles are dual activity vessels equipped with both a main and auxiliary load path. The auxiliary load path is fully

Figure 1. The EAU is inspected after a test. (Images courtesy of GlobalSantaFe)
equipped with a 500-ton top drive, 4,500 hp draw works and a 49.5-in. rotary table. When not deploying subsea equipment or performing offline drilling or casing running operations, the auxiliary load path is used to assemble the vast majority of tubulars required for drilling and completion operations. A full array of tubulars including all drill pipe, drill collars, tubing and casing sizes up to 135¼8 in. are assembled (in quadruples of range two or trebles of range three) on the auxiliary load path and racked in the 2-million-lb capacity tubular setback area. Because these rigs are designed for complex deepwater development work a means of protecting the subsea architecture on the seabed below during the offline stand-building process was an essential requirement. It was determined that a protection system composed of a closed end steel cylinder containing an integral shock absorbing system installed beneath the auxiliary rotary table would provide the most workable and cost effective solution.

Design requirements
The basic design of the rotary protective sock had to meet the following requirements:
• Contain a stand of 91¼2-in. outer diameter (OD) by 3-in. internal diameter (ID) drill collars dropped from a height of 3 ft (.914 m) above the rigfloor;
• Provide a useable working height below the rigfloor of 94 ft (28.7 m);
• Allow for efficient assembly, ease of removal and provide the means to rack the assembly in the tubular setback area;
• Interface with the existing rotary support structure;
• Provide an internal diameter of 183¼4 in. to match that of the subsea blowout preventer;
• Provide environmental containment;
• Allow sufficient clearance between the top of the rotary sock and the rigfloor to install automated power slips; and
• Provide sufficient lateral support in the moonpool to survive a 10-year Gulf of Mexico storm.
The major design challenge faced was to provide an energy absorption system capable of successfully stopping and retaining a 27,000-lb length of tubulars traveling at almost 80 ft (24.4 m) per second.

Engineering
National Oilwell Varco (NOV) had previously developed an Energy Absorption Unit (EAU) for
Figure 2. The rotary sock is shown in the moonpool.
use in similar but less demanding applications. GlobalSantaFe worked with NOV to adapt this technology and further develop the EAU for use on the Development Drillers. The EAU is composed of a series of corrugated steel plates fabricated into individual elements which in turn are stacked to a height determined by the required load capacity. The elements are designed to partially collapse when impacted and thus decelerate a dropped tubular. The controlled deceleration limits the impact load generated and transferred to the rig structure via the sock body.

Testing
Following an iterative design process whereby all of the rig requirements were integrated into the sock design and the EAU technology was expanded to cope with the much higher impact load, a full scale test of the system was conducted at a Weatherford test facility in Houston using a full size land rig and a 30-in. test well. The basic sock design has four modular sections — a top housing to interface with the auxiliary rotary beams, two 45-ft (13.7 m) intermediate sections and a lower section containing the EAU. The main sock body (20-in. OD by 183¼4-in. ID) was fabricated using grade X56 steel.

In the event that the test was unsuccessful, a secondary closed end string of 24-in. casing was first installed before installing the rotary sock complete with the EAU. In the event of an unsuccessful test, the stand of 91¼2-in. drill collars would have been contained within this secondary string of casing. In order to accurately measure the axial load generated as a result of the impact, Mohr Stress Engineering installed eight electrical resistive strain gauges at 45° intervals around the top of the rotary sock.

The actual test was very straightforward. A 125-ft (38.1-m) stand of drill collars weighing 27,000 lb was purposely dropped from a height of 97 ft (29.6 m) above the top of the EAU. The test was successful in that the stand of drillcollars was retained within the rotary sock and no obvious signs of distortion or cracked welds were observed. (MPI inspections later confirmed this is to the case.) During the test, the EAU deformed 60 in. from a starting length of 141.6 in. (42% deformation). The peak average axial load generated as a result of the impact was 934,000 lb with recoil lasting less than 1¼3 of a second. The design loading for the auxiliary rotary beams is 1 million lb. The resultant impact load and element deformation were deemed to be within acceptable limits.

Environmental loading
The required length of the sock to accommodate the EAU in addition to providing the required useable working height resulted in the bottom 20 ft (6 m) of the sock (depending on vessel draft) being through the splash zone. Analysis of the sock assembly when deployed in 10-year Gulf of Mexico winter storm conditions returned a maximum horizontal loading of just under 5 metric tons. The resultant initial design of the lateral support mechanism encompassed four rigid adjustable arms spanning between the sock body and dedicated padeyes installed in the lower moonpool. Due to difficulties during initial installations of the sock, this design was later revised to incorporate a simple sling and turnbuckle system.

Final design
The final sock design (as presently installed) consists of four modular sections. The top section or housing is sized to interface with an auxiliary rotary beam adapter. This adapter was a required addition to the rotary beams and is designed to support the top housing and transmit the load more uniformly to the rotary beams. This new configuration helps minimize the bending moments produced in the event of a dropped tubular. The top housing has recessed handling padeyes to allow the installation of the automated slips above the sock.

The housing also has integral drainage ports to ensure any run off from the rigfloor is contained within the sock. The bottom section containing the EAU has a recessed drainage valve that allows controlled drainage of the sock while providing a solid foundation for racking the system in the derrick. There are three flanged connections joining the various components.

To enable initial assembly of the rotary sock on the rig, a C-plate was included in the package. The C-plate interfaces with the master bushing profile and fits securely beneath each flanged connection allowing the sock to be suspended in the rotary table during assembly. In order to rack the sock assembly it is necessary to remove the 49-in. OD top housing. A 91¼2-in. OD running tool designed to interface with 65¼8-in. elevators and the automated pipe handling system can then be installed and the sock racked in the setback area using the automated pipe racking systems.

Summary
The rotary sock has been an extremely valuable addition to the Development Drillers. It is capable of safely containing at surface any combination of tubulars weighing up to 27,000 lb
and dropped from a height of up to 3 ft above the rig floor. The design is such that the impact load generated will be safely distributed to the rotary beams with no damage other than to the expendable energy absorption unit. The use of this innovative equipment has been expanded to include staging logging tool strings assembled offline and safely containing subsea test trees during offline assembly and testing.

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