Semi designed for conversion to drilling/production unit

John Vecchio, Denis Graham
Diamond Offshore Drilling Inc.
Houston

The technical demands of deepwater drilling and production require that floating facilities be designed with more operational flexibility, more available deck loading, and more complex station keeping capabilities than those offered by the industry's existing fleet of semisubmersibles.

Field economics strictly define the limits of expenditure that can be applied to these kinds of facilities. For this reason, many operators and contractors investigate the feasibility of converting second generation semis into fifth generation units that will provide the ability to perform simultaneous drilling and production operations.

One such conversion design is for the Ocean Legend, which uses the Ocean Victory class semisubmersible as a building block for developing a floating platform capable of carrying more than 12,000 long tons of variable deck load and operating in up to 10,000 ft of water.

The design concept is to convert the existing cruciform shape of the Ocean Victory class semi into a squared-off shape (Fig. 1 [40761 bytes]). The squared-off shape yields main deck dimensions of 268 ft longitudinally by 242 ft transversely. This increase in deck area allows for expansion of drilling and ship services that will enable the vessel to work in ultradeep water and that will provide an additional 32,000 sq ft for two levels of production equipment.

This design was developed to handle deepwater drilling and development individually, in specific stages, or in a phased development program. The phased development could have several stages, such as the following scenario:

• Exploratory drilling after the rig conversion

• Extended well tests, after minor modifications

• Development drilling while the modular process facilities are fabricated

• Process equipment installation and hookup for production.

This staged development plan keeps the construction downtime to a minimum while optimizing the possible production module layout for a particular development. If the exploration drilling or well tests do not indicate an economical field, the unit can quickly be moved to another prospect.

The Victory class semisubmersible lends itself to this magnitude of a conversion in that no other semisubmersible can be upgraded by this cruciform shape with 12 supporting columns. Four new corner columns are added to the unit connecting to lower hull extensions. The added columns and hull extensions will house the new mooring system and chain lockers. They will also yield a considerable increase in variable deck load while meeting the latest regulatory requirements with respect to damage control and stability.

In deepwater applications, the fundamental task is to provide ample clear deck space for the support of simultaneous drilling and production operations. However, the designer is also challenged with the responsibility of providing a floating vessel that can maintain position in a much more hostile environment than would be required if drilling were the only function of the unit. Drilling allows the operator to suspend operations and disconnect all equipment tied to the well in the event of a severe storm, but production does not.

Semisubmersibles designed only for drilling often rely on draft changes to meet survival condition stability requirements, and because disconnecting the riser from the well is the designed procedure for this type of unit, mooring excursions are not critical. If the vessel is simultaneously producing wells while drilling occurs, it is not feasible to consider disconnecting the producing risers and their associated equipment during a storm. This requirement to keep production risers connected in severe weather necessitates that draft changes be eliminated and that vessel excursions be limited to the extent that the semisubmersible, riser, and subsea wells are not damaged.

Simultaneous deepwater drilling and production design requirements define the necessity to increase the size of the semisubmersible and its mooring equipment beyond the limits of any existing unit in operation.

Because the criteria of simultaneous drilling and production exceed the design parameters including variable deck load, mooring, and deck space of the newest and largest fourth generation units, a new generation with improved characteristics is required.

Design basis

The concept of a second generation semisubmersible conversion offers the operator an alternative to the costly solutions presently being used in deepwater production. These are a 20 well drilling and production model in 4,000 ft of water with an expected production rate of 100,000 bo/d and an 800 gas/oil ratio (GOR). An Ocean Legend-type conversion is estimated to be less than one third the cost of a tension leg platform (TLP) operating under the same conditions.

A first generation semi is generally defined as a unit that can drill in 600 ft of water. A second generation semi can drill in up to 2,000 ft of water and has a variable deck load of about 2,000 tons.

A third generation semi can drill in up to 3,000 ft of water and has a variable deck load of 3,500 tons. A fourth generation semi can drill in 5,000 ft of water and has a variable deck load of about 5,000 tons.

A fifth generation unit will be able to operate in conditions beyond those.

The major trade-off in selecting a catenary moored semisubmersible over a TLP is that the wells must be completed using subsea wellheads instead of surface or platform-mounted equipment.

To achieve economic and technical success for deepwater drilling and production conversions of second generation units, care and planning based on solid engineering are required as these units vary widely regarding variable deck load, deck area, configuration, global strength, fatigue life, operating draft, etc. Even units of the same class may not be equal because of operational history, maintenance records, and detailed design items such as profile grinding. These parameters will affect a unit's suitability and conversion cost for a major enhancement to achieve the desired design criteria.

After an extensive fleet evaluation, Diamond Offshore Drilling Inc. concluded the Ocean Legend would be built using the Ocean Victory class semisubmersible as its internal foundation.

The Victory class semi comprises four lower hull pontoons and 12 columns (Fig. 2 [24529 bytes]). The additional structural requirements to build the Ocean Legend include the following:

• Extending the two 25.5-ft diameter outboard hulls with four 58-ft wide by 88-ft long deep pontoon sections

• Adding four 42-ft diameter by 91.5-ft tall stability columns mounted on the new pontoon extensions

• Adding a 238-ft wide by 248.5-ft long by 17.5-ft deep upper hull on the existing box girder grid of the Victory class main deck

• Adding foundation and support for a 12-point mooring system

• Adding a new heliport and new living quarters or refurbishing existing quarters.

The Ocean Legend is being designed to operate in severe environments worldwide. Enhanced safety, low motion response, improved stability, structural strength, high performance, and operating efficiency are guidelines for this unit. Regulatory guidelines are being incorporated into the design for each specific area of operation.

The American Bureau of Shipping classifications for an "A1 Column Stabilized Drilling Unit Floating Production System" will also be incorporated. To maintain regulatory and classification requirements after construction, the design will allow for all future inspections to be conducted offshore, on location, without interruption of operations.

Dry docking requirements will be satisfied using underwater surveys. Internal tank inspections in the lower hulls will be possible at operating draft by access provided to all lower hull, column, and brace tanks and voids.

Stability

The Ocean Legend design incorporates the most severe design criteria for both intact and damaged stability. Rule requirements for intact stability specify a minimum area ratio of 1.3, 70-knot wind criteria for operating conditions, and 100-knot wind for survival conditions.

(In stability analysis, righting moment curves are plotted along with heeling moment curves. The American Bureau of Shipping mobile offshore drilling unit rules specify that the area under the righting moment curves or the second intercept or the down-flooding angle is not to be less than 1.3 times the area under the heeling moment curve to the same limiting angle.)

The Legend design is based on an area ratio greater than 1.3 and 100-knot wind criteria for both operating and survival conditions. Rule requirements for damaged stability specify 50-knot wind criteria and an allowable angle of heel to the first down-flooding point. The Legend design is based on 100-knot wind criteria and a damaged heel angle of less than 15°.

The stability analysis on this design used a model based on a light ship weight of 22,369 long tons and a total operating displacement of 51,644 long tons. Draft was considered to be 80 ft for both intact and damaged conditions. Results of the analysis indicated that a 13,671 long ton variable deck loading capability could be achieved while maintaining an allowable KG margin greater than 20 ft.

Fig. 3 [16619 bytes], Fig. 4 [16488 bytes] and Fig. 5 [14817 bytes] show calculated motion response curves for heave, roll, and pitch. These motion responses are considerably less than those encountered by the Ocean Victory class semi and are very close to the low responses found on the Ocean America class semis.

Mooring

The Ocean Legend has been targeted to meet drilling and production requirements for water depths from 1,500 to 10,000 ft. Mooring configurations will vary depending on the water depth.

A 12-point catenary combination chain and wire rope system has been established as the basis for water depths up to 6,000 ft. Beyond this depth it is envisioned that the catenary system would be deployed but that azimuthing thrusters would also be used to reduce vessel excursions.

To maintain the dual function of drilling and production, the vessel design will meet 100-year storm criteria for the area of operation. For example, in the deepwater Gulf of Mexico the 100-year design criteria could be considered 72-knot sustained winds with 100-knot, 1-min gusts and 40-ft maximum wave height with a 3.4-knot surface current.

A mooring analysis for the Legend design operating in up to 6,000 ft of water using these 100-year hurricane conditions yields the following component requirements:

• 12 mooring lines configured in a 15-45-75° pattern (Fig. 6 [33745 bytes])

• 20 metric ton high holding capacity anchors

• 5,500 ft of 4-in. stud link chain connected to each of the anchors

• 10,000 ft of 41/4-in. diameter wire rope connected to each of the anchor chains.

The mooring winches and windlasses will be installed in the four new corner columns. The mooring components can be fully contained on the vessel using chain lockers, wire rope traction winches, and wire rope storage reels (Fig. 6 [33745 bytes]).

Fatigue and global strength

Fatigue is probably the primary factor of concern with regard to structural considerations for converting existing semisubmersibles for extended operations or production service. The basic reasons for this concern include the age of the rig and the cumulative nature of fatigue damage. Another factor is that the tools to calculate fatigue and welding procedures of offshore structures were not as advanced or sophisticated during early semisubmersible construction as they are today.

The Ocean Victory class foundation used more strict design and construction techniques than other rigs of similar vintage. The combination of low stress levels and contour profile grinding during original construction of the critical connections provides long calculated fatigue life of these Victory class hulls. All Ocean Victory units built with these criteria yield a minimum fatigue life of 84 years based on the U.K. DEN 1970 criteria.

The area of the unit with the minimum fatigue life is at the tapered section of the four structural columns; all other areas exceed 84 years. None of the Ocean Victory class semis which used these criteria has ever had a structural failure.

In converting the unit, the critical connections, both existing and new, will be identified, and the fatigue strength will be reanalyzed using the latest technology. In conjunction with the calculations, a complete nondestructive test (NDT) inspection will be conducted on all the critical existing connections. This inspection will include blasting all the welds and heat-affected parent metal at the critical connections to white metal and performing a magnetic particle examination of these areas.

During the construction phase, the same standards of low stress levels and contour grinding will be used. A vigorous NDT program will be maintained along with visual verification of the critical welds and fit-up throughout the conversion. All inspections will be documented.

Recent inspections of Diamond Offshore's Ocean Victory class units indicated little or no wastage or deterioration. Nonetheless, an extensive survey will be performed to establish the precise "as-is" condition of each unit. The examination will consist of a comprehensive grid of ultrasonic thickness tests and complete visual inspection of the hull. Upon completion of the inspection, the as-is condition will be used in lieu of the thickness indicated on the original construction drawings for reevaluating global strength.

A semisubmersible mobile offshore drilling unit is normally brought to sheltered water for inspection or repairs; however, a floating production unit will have to remain on location even if damage is sustained. The Ocean Legend is designed with structural redundancy to ensure that the resulting overall strength can withstand a 1-year storm even with the loss of a main load-carrying component or a buoyancy compartment. This redundancy is incorporated so that permanent repairs can be completed on location.

The combination of the inspection of the existing critical connections, fatigue and global calculations, examination of new critical connection fabrication, and the rig's documented inspection history will form a package to demonstrate the unit's ability to meet or exceed the requirements of 20 years uninterrupted field service.

Topsides and vessel services

The top deck will have clearly defined areas to allow maximum separation for functions to enhance the safe operation of the unit. The processing facilities will be located on the new deck areas at the forward part of the rig. The drill floor and cellar deck will remain in the center of the rig.

The mud processing equipment and pits will remain in the starboard side. The pipe rack and lay down area at the aft will be expanded to the starboard side to accommodate the increased amount of riser. The quarters and machinery houses will be located on the port side.

The area located forward at the main deck reserved for process equipment is approximately 62 ft long by 248 ft wide. If required, a second deck of equal or less space can easily be installed above the main deck. For additional safety, a blast wall will be installed aft of the process deck to separate operations.

The existing drilling systems of the rig will be upgraded to include the latest technology for deep water, high pressure, high temperature wells.

The quarters, which are located as far away from the process facilities as practical, will be expanded aft and another level added to accommodate a crew of 116. The expanded quarters will also house additional office space, the control room for the process facility, the ballast and control room, and enlarged recreational and dining facilities.

The accommodations will incorporate the temporary refuge design philosophy used in the North Sea. The quarters bulkheads will have A-60 and H-120 ratings as required for crew safety. The rig's safety equipment will be upgraded to suit the enlarged crew capacity.

Various components of the vessel services equipment will require upgrading or removal to suit the new hull configuration and production service. The existing Victory class units are currently outfitted with a conventional propulsion system. The propulsion system was evaluated and found undersized for the increased hull profile. Also, any benefit from the system is questionable because of long-term operation at one location.

The propulsion system's shafts are subject to periodic regulatory inspections which require the rig to be brought to sheltered water and the shafts drawn. The shaft's seals are subject to normal wear and deterioration which presents the possibility for leaks that could require repairs in sheltered waters.

Because the propulsion system yields little benefit and could present future problems for long-term operations at one location, the system will be removed and the stern tubes blanked off. The pilot house would thus also be removed.

Another major modification to the ship is the ballast system. The new corner hull extensions and columns will need to be subdivided into ballast tanks, chain lockers, and compartments. In lieu of connecting the new ballast tanks in each corner to the existing ballast piping, new pump rooms will be added to each corner in the lower hull.

The pump rooms in each corner will be subdivided into two separate compartments, each with a ballast pump so that if either is flooded, the corner retains its ability for any ballasting operation. This system design will allow the unit to deballast to transit draft with the loss of any pump room, including the unmodified existing pump rooms. All ballasting operations will be controlled from the ballast control room in the quarters.

Because the Ocean Legend significantly expands the upper deck area, the cranes will need to be modified. The quantity, capacity, and arrangement of the cranes will be designed to suit the requirements for the process decks.

The power generation system will include diesel generators for drilling requirements and gas turbines for the production facilities, if the gas supply is sufficient.

The silicon controlled rectifier (SCR) will be integrated to handle both power supplies.

The Authors