Architectural requirements for a fifth generation drillship

Performance standards required
significant changes in conventional design

Joaquin Lopez-Cortijo Garcia
Astano

Richard Paul Michel
Transocean Offshore

• Finite element models of critical areas. [5,368 bytes]
• Wind tunnel results - current force and wind force. [19,356 bytes]
• One year storm - mean thruster utilization. [47,091 bytes]
• Moonpool wave rise - model test results. [115,026 bytes]
• Environmental conditions for DP test program [26,644 bytes]
• Comparison of Discoverer Enterprise to Discover 534 and Transocean Richardson [64,810 bytes]

A contract was signed between Astilleros Y Talleres De Noroeste (Astano) and Transocean Offshore for the construction of a fifth generation drillship in July 1996. The vessel named Discoverer Enterprise combines a unique dual-drilling activity with a modern hull form to produce a drilling unit capable of worldwide operations in water depths up to 3000 meters.

The naval architectural design of the vessel hull presented many challenges due to the geometry requirements, essential performance characteristics, and mode of construction.

To address these issues, Astano and Transocean worked together to develop and verify the hull design to satisfy the operational requirements of the vessel. Traditional naval architectural design methods were utilized as well as extensive 3D finite element model (FEM) analysis programs and a wide range of model tests. The details of the work performed are described below.

The main dimensions of the vessel were selected taking into account the following:

• Deck area required for drilling operation
• Large accommodation module required
• Size of the power plant located aft over the main deck
• Breadth (chosen sufficiently large to ensure adequate intact and damage stability to match the MODU requirements. Large breadth was critical because of the large variable deck load (VDL), high vessel center of gravity (VCG), and the big windage area. However, it had to be sufficiently small to minimize rolling of the unit, critical for the drilling operation as well as for the design of the huge derrick needed for the dual activity.
• Large length-to-beam ratio, which has the double effect of minimizing the ship resistance, and at the same time, reducing the heave and pitch motions due to its larger length.

These considerations led to the following vessel dimensions: a length overall of 254.4 meters, a bpp length of 240 meters, a moulded breadth of 38 meters, a moulded depth of 19 meters, and a design draft of 12 meters.

Hull form

A big effort was made in the definition of the hull forms to achieve an optimum hydrodynamic behavior capable of matching the target transit speed of 15 knots. It was also necessary to take care of the transverse moment of inertia of the waterplane at the design draft, in order to provide adequate stability performance.

Finally, the need to accommodate six huge thrusters (5,000 kW each), three aft and three forward, was also of paramount importance in the hull shape definition, by avoiding large trunks protruding from the bottom, which would increase the ship resistance. Also, the maintenance of the thrusters was facilitated as much as possible.

The Maritime Research Institute (Wageningen, Holland) was committed to perform an assessment on the hydrodynamic performance of the vessel speeds for different drafts, based on their experience in multi-thruster propulsion. In order to avoid the resistance increase due to standing waves within the moonpool, different solutions were tested.

The prevention of slamming either forward or aft was considered a main concern, and the hull forms were adapted accordingly. Also, the greenwater occurrence was considered in the design. This is important when the vessel is facing the waves sternward due to the dual activity of the drilling operation. Bilge keels of suitable width and length were fitted to reduce rolling which is critical for the reasons mentioned earlier.

In the forward area of the vessel, below the main deck, the forepeak and the forward machinery space are located. One of the three thrusters is segregated from the other two. Above the main deck, the accommodation block is arranged with capacity for 200 men.

A helideck suitable for a Chinook-type helicopter is supported over the forecastle deck, forward of the accommodation block.

Arranged in the vessel mid-body under the maindeck are the slop and auxiliary tanks, two cargo tanks, the moonpool, the drill water tanks and one hold where the mud tanks (liquid and bulk) are located. All these spaces are surrounded by wing and double-bottom ballast tanks.

The moonpool (located amidships) has a rectangular shape and dimensions of 25 meters in length by 10 meters in width. Cofferdams or void spaces surround it in accordance with the regulations.

Above the main deck, the drilling facilities are arranged. In the aft area, the power plant is located in a deckhouse above the main deck, as well as the offloading equipment. Two diesel oil tanks, the aft machinery space, and the aft peak are located below deck. Three thrusters are placed within the machinery space, again, one segregated from the other two thrusters.

The vessel is equipped with four cranes of 75 tons capacity each for handling of the drilling risers, BOPs, removal of the thruster motors and other machinery equipment. Also, two racks run along both sides of the vessel supporting piping and cable trays, and are used as well for escape routes.

Structural design

The vessel is designed in accordance with DNV Rules (MODU) for a 20-years fatigue life in the specified areas of operation (Gulf of Mexico, West of Africa, and summer North Sea). Therefore, the vessel structure complies with the loading imposed by the 50-years return period storm (hurricane conditions in the Gulf of Mexico). Furthermore, the vessel's main structure as well as the substructure and derrick have been verified to comply with a ULS condition associated with a 50-years North Sea winter storm by the beam.

The midship section was designed so that a proper continuity is achieved at the moonpool area, thus minimizing the stress concentration in the vicinity of the moonpool corners. Finite element (FE) models covering the cargo area, as well as the moonpool area, were built to assess the structural behavior of the most critical elements. For the fatigue assessment of the moonpool corners, fine mesh models were used covering the deck and bottom areas.

The experience gained by Astano in the design of FPSOs for the North Sea was applied to the structural details, which were expected to be of special concern. Apart from other structural areas, which required FE calculations, extensive analysis was done at the interface derrick substructure/vessel main deck to accommodate the unusually huge loads (almost 5,000 tons at the most loaded leg in the ULS condition), imposed by the nearly 100-meter high derrick.

To provide an optimum fixity of the supporting legs, and at the same time to avoid fatigue of the welds subjected to tension-compression loads, all the substructure legs (10), were passed through the vessel main deck instead of being welded directly to a Z-plate on the main deck. Fine mesh FE models of each leg connection were performed to assess the structural response and the necessary underdeck reinforcement.

Vessel capacities

The vessel has the following approximate storage capacities: crude oil - 120,000 bbl; ballast - 62,000 cu meters; diesel oil - 5,800 cu meters; fresh water - 1,100 cu meters; and drill water - 2,000 cu meters.

The large ballast capacity available provides a high flexibility in the operation of the unit by selecting the most suitable draft at any loading condition (constant draft mode of operation). Also, the diesel oil capacity enables the vessel to operate 24 days continuously without refueling, with the diesel engines running at a rated 50% maximum power.

The ship-shaped hull, large deck area, and ample stability characteristics of the vessel allow the Discoverer Enterprise to carry a variable deckload (VDL) of 20,000 metric tons. The VDL is distributed between bulk and liquid mud storage tanks in the vessel hold, active mud tanks in the topside modules, drilling and completion riser in topside riser racks, drill pipe, casing, and collars in topsides pipe racks, drillfloor setback areas, and various drilling live loads.

The VDL breaks down as follows: bulk mud and cement - 1,400 tons; sack mud - 750 tons; liquid mud in hold - 2,900 tons; active mud in pits - 2,350 tons; drill riser joints - 2,700 tons; completion riser - 750 tons; drill pipe and collars - 1,450 tons; casing - 1,150 tons; BOP and subsea gear - 1,000 tons; pipe in setback - 1,100 tons; casing in setback - 450 tons; drilling live load - 1,000 tons; warehouse/stores - 1,000 tons; third party equipment - 1,000, and potable water, crew, and miscellaneous - 1,000 tons.

Dual activity center

The main feature of the drilling facilities is the "dual activity drilling system," estimated to improve drilling efficiency up to 40%. This system features two drilling areas set 40 ft apart on the 80 ft by 80 ft drill floor. Each drilling area will be equipped with a top drive, a 4,000 hp drawworks, and crown-mounted motion compensators. Automated pipe-handling equipment will allow pipe to move from either rotary to the large setback area.

Besides the dual activity drill floor, the Discoverer Enterprise will feature other enhancements to the drilling system. Four mud pumps working from two separate mud systems will permit the ability to change from a water-base top hole mud system to a synthetic system without lost time. A total mud storage volume of 15,000 bbl permits two or three complete mud systems to be stored on board.

In conjunction with the dual-activity capabilities other features, such as improved subsea tree handling and the ability to store more than 100,000 bbl of crude on board, increase the unit's ability to perform exploratory drilling, development drilling, well completion, and extended well testing.

Power and controls

The power generation is located in two engine rooms at the stern over the main deck, except the emergency generator, which is arranged forward. The plant consists of six diesel generators providing a total power of almost 40 mW. The number of generators running at any time is controlled by the scheduling algorithm within the power management system. This is equipped with a load shedding facility to ensure that there is no progressive generator trip when a failure on a running set occurs or when a sudden load is introduced.

The generators are connected to their respective generator sections in the aft 11 kV switchboard. The 11 kV voltage is selected to reduce the short circuit levels and to economize in cable sizing. The main high voltage users are the drilling and thruster motors, and the transformer feeds to the low voltage switchboards. The 11 kV system is divided into port and starboard switchboards located in segregated rooms. The switchboard is provided with a bus tie-breaker.

An emergency generator set and associated switchboards is arranged forward within the accommodation block, within a separated fire-rated enclosed space complying with the applicable Rules of Regulations.

An integrated automation system (IAS) is used to provide control and monitoring of the vessel's marine and safety systems. The bridge/central control room is the primary control center, manned and located within the accommodation module, and will house the main IAS operator interface. The electronic equipment room (EER) within accommodation will house the main electronic racks allowing easy cable access from the main deck. The engine control room (ECR) located aft will contain mainly the power management system operator control station.

The IAS will be distributed through the vessel by means of dual-redundant data-highway routed along the main deck between the aft engine control room and forward bridge/central control room. System application and logic will be resident in dedicated intelligent process stations strategically located within the vessel in order to minimize the use of hardwire cables. In critical systems where a higher degree of system reliability is a requirement, consideration is given to having redundant central process units (CPU) within the process station.

The system can be subdivided into the following:

• Vessel management system
• Power management system
• Thruster control system and dynamic positioning
• Extended well test control system
• Drilling automation system
• Fire and gas and emergency shutdown system.

DP and safety

The unit is assigned the DNV Class Notation "Dynpos Autr." In addition, one of the thrusters in the machinery spaces forward and aft is segregated from the other two thrusters, in order to provide an enhanced integrity against fire or flooding scenarios.

The number and size of thrusters installed proved to be sufficient for operation of the vessel under the specified maximum environmental conditions. This was verified during the model test.

The thruster system consists of six variable-speed azimuthing thrusters (three forward and three aft) of 5,000 kW capacity each. The units can be dismounted in-site for repair or maintenance, without the need of external means. Adequate control systems, reference systems and environmental sensors are provided in accordance with the rules and regulations.

The safety system consists of the following:

• Fire seawater system: A pressurized ring main with four fire pumps (two forward and two aft) supplies the two by 100% of the estimated maximum water demand for the vessel.
• Foam system: This is used for the main deck, drilling area and helideck area.
• Firefighting monitors: Self-oscillating water/ foam fire monitors are located strategically to cover the helideck, topsides and accommodation.
• Deluge skid: A deluge system is provided for the heli tote tanks, drillfloor and testing area.
• Fixed fire extinguishing system: A CO2 system is provided in the engine rooms and control stations.
• Fire and gas detection/emergency shutdown system: These systems are provided with adequate redundancy to allow necessary actions to avoid the escalation of a fire or the effect of a gas cloud.
• Escape routes: An open escape route at one side of the vessel is provided to connect the forward and aft areas. In the vicinity of the derrick, a hood wall protects the personnel from a fire scenario in this area. At the other side of the vessel, an alternative open escape way is arranged. In each machinery space and accommodation, at least one trunk A-60 protected is provided, as well as other non- classified escape routes.
• Temporary refuge and evacuation area: This is located in the accommodation block. Lifeboats, 2 by 100% capacity, are provided at each side.

Model testing

A model testing program was performed on the Discoverer Enterprise to analyze the seakeeping motions, resistance and propulsion characteristics of the vessel. The results of these model tests was used to verify the hull form, determine the thruster size and main power plant generating capacity, and to calibrate the computational-model hull-form damping characteristics.

Additional measurements taken during the model tests were used to study a possible crude off-loading scenario, and to measure wave impact and slamming forces. The results of the resistance and propulsion tests lead to a later series of tests to measure wave rise behavior in the moonpool.

Wind tunnel tests were performed to determine wind and current force coefficients for use in the seakeeping tests. The tests were performed at TNO Institute of Environmental Sciences in Apeldoorn, Netherlands, and later repeated at Danish Maritime Institute (DMI) in Lyngby, Denmark.

Tests were performed on the above water and underwater portions of the vessel at the minimum transit (8 meters) and maximum operating draft (13 meters). The tests were performed at a 1:150 scale, and at a full-scale wind speed of 100 knots. Measurements of wind force and moment about the three axes were taken at fifteen degree (15!) increments throughout 360!. During the above-water portion tests, measurements were made with and without setback in the derrick in order to measure the effect of full setback on overall wind load.

Test results

The results are typical in that they are generally symmetrical about the longitudinal and transverse axes. The peak force value in the longitudinal direction occurs at a vessel heading angle 10-15! off the bow, and the peak force value in the transverse direction occurs at a vessel heading angle 10-15! off the beam. This trend was consistent for both above-water and underwater portions of the model. The measured values of wind force were greater than the force values calculated using conventional programs.

The effect of the large setback area (50 ft long by 20 ft wide by 135 ft tall) was found to add approximately 10% to the wind force. This percentage increase varied only slightly throughout the range of wind directions tested.

As the wind force measurements taken in the tests performed at TNO were higher than the force values calculated by computer program, the tests were repeated on the same model at DMI. When the tests were repeated, the force measurements closely matched those originally measured at TNO.

When the model force measurements were modified to full-scale values, however, the full-scale force values resultant from the tests at DMI were closer to the calculated values. This was due mainly to the blockage correction factor applied.

The full-scale length of the Discoverer Enterprise is 240 meters. With a model scale of 1:150, the model length was approximately 1.6 meters. As the above-water portion of the model was rotated during the testing program, the blockage in the wind tunnels varied from approximately 2% in head wind to 7% in beam wind. By applying different industry accepted blockage correction formulas the full-scale wind forces can vary by up to 10% between different wind tunnel facilities in the beam wind condition.

For the seakeeping tests, the full-scale wind forces used were the ones derived from the tests performed at TNO. Further, the results from the tests performed with full setback were used in all seakeeping tests. These test results produced the highest values of wind force in all directions.

Seakeeping tests

The seakeeping tests were performed on a 1:70 scale model at the Maritime Research Institute (Marin). The test program consisted of the following components:

• Tests in soft springs
• Roll decay test
• Full dynamic positioning (DP) tests
• Stationary tests
• Turning tests
• Squall test
• Side-by-side off-loading test

The seakeeping tests were performed in the Wave and Current Basin at Marin, which corresponds to a full-scale water depth of 140 meters. Wind forces were generated using a series of wind fans. Current was generated by means of water pumps. Waves were generated by flap-type wave generators.

The thruster system consisted of six model thrusters matching the full-scale characteristics of thrust, azimuthing speed, and RPM control speed. The thrusters were controlled in three main groups. Group one contained the three aft thrusters, group two contained the two lateral forward thrusters, and group three consisted of only the forward-most centerline thruster. While each group was controlled independently, all thrusters within each group were given the same commands with regard to rpm and heading.

The control of the thruster groups was via an automatic DP system. The system uses as input the vessel position, heading, wind feed forwarding, and thruster control parameters. Two of the components of the control loop of the DP control system were an extended Kalman filter algorithm and a PID feedback controller. The control system also contained certain thruster allocation logic with regard to forbidden sectors, heading control, surge and sway control, and others.

Full DP test

The DP test program environment consisted of a 1-year storm operating condition, a 50-year Gulf Of Mexico (GOM) Storm and a 50-year hurricane condition. The criteria for investigation of the thruster system was to maintain station and heading in the 1-year and 50-year storm cases, and to maintain heading only in the 50-year hurricane condition with an acceptable level of 100% thruster utilization (saturation). Further, a "one thruster down" condition was run in the 50-year storm case.

The 50-year storm tests were run in two sets "parallel" with wind, wave and current coming from the same direction, and "transverse" where there is separation between the headings of wind, wave, and current. The vessel was also tested with the stern into the environment.

The results showed that the vessel could maintain heading and station in this environment, with reasonable thruster saturation (approximately 25%), within a vessel heading angle of approximately +/- 20! off the bow. The ability to maintain heading was easier in the transverse weather condition, as the heading set-point was between the wind and wave, reducing the yaw moment on the vessel. The window for a stern heading was not tested in these tests, but it is expected to be slightly less than the bow-on case due to higher wave drift forces when the environment is stern-on.

In the "one thruster down" case, the thruster made inactive was the forward centerline thruster. This thruster was selected, as it was the most critical for heading control. Further, the DP control system logic was not modified for this thruster being out of service. These parameters were considered as a worst case scenario. Even in this case, the thruster system was able to maintain heading and station, though the allowable heading window was less than the intact case.

The 50-year hurricane condition test was performed twice: once with the nominal thrust capacity, and a second time with the thrust increased 15%. The second case was run because the thruster system has an allowable overload capacity built into it, and the effect on heading recovery time was to be studied. In both cases, the thruster system demonstrated the capability to maintain heading into the environment, and the 15% overload capability visibly decreased the heading recovery time. The 1-year storm tests are described in the stationary and turning tests section below.

Turning and squall tests

The Discoverer Enterprise is a dual activity drillship. This dual-activity feature allows well operations to be performed simultaneously from the two rotary tables located on the vessel centerline. The possibility exists, then, that the vessel may have two well strings connected to the seafloor at any given time. In this scenario, the vessel does not have the liberty of maintaining vessel heading into the environment at all times.

The possibility exits, then, for the requirement to turn the vessel 180! to realign the vessel bow into the weather. To test the thruster system's capability to maintain station and heading in the 1-year storm operating case, a series of tests were performed.

These tests consisted of maintaining station and heading for 30 minutes (full-scale), then rotating the vessel 30! using the thruster system, holding the vessel for 30 minutes, turning 30! more, etc., until the vessel had rotated 180!. The thruster system was able to maintain station and heading throughout the series of tests, with the beam-on case resulting in the highest percentage of thruster saturation.

The ability of the thruster system to respond to a sudden squall on the beam and rotate the vessel 90! into the wind was investigated. The vessel was heading into a 1-year storm wave and current when wind fans were turned on from the beam, and increased to a speed of 38.5 meters/second in a full scale time of five minutes.

The heading set-point for the thruster system was changed in 15! steps, until the vessel was rotated bow-on into the wind. The tests showed that the thruster system could successfully turn the vessel 90! into the wind while maintaining station. Even though high thruster saturation was observed, the test was considered acceptable due to the severity of the environment and the thruster control strategy utilized.

The side-by-side off-loading tests demonstrated that off-loading to a 30,000 DWT shuttle tanker is feasible in 1-year storm. Thruster performance characteristics are improved and mooring hawser loads are reduced if the shuttle tanker is on the lee side of the Discoverer Enterprise.

Throughout the tests greenwater impact was measured on the forward and aft faces of the superstructure, and bow flare impact was also measured. Virtually no wave impact loads were measured on the bow flare, and the greenwater impact loads measured, though the number of occurrences was low, during the 50-year GOM storm and 50-year hurricane were used to design the superstructure of the vessel.

Resistance, propulsion tests

The resistance and propulsion tests were performed on a 1:41 scale model in the Towing Basin at Marin. All tests were calm-water tests. The tests were performed at two drafts, eight meters corresponding to a light-load draft, and 11.25 meters corresponding to the fully-loaded transit draft.

The vessel was powered by six model thrusters; and the tests were run in speed increments of 1.0 knot between 11.0 and 17.0 knots. All tests were run with the moonpool open. One set of tests was performed at the 11.25-meter draft with the moonpool closed by means of a "door" located at the vessel baseline. This was modeled by a wood plank, effectively closing the moonpool, though not making it buoyant.

The tests produced EHP versus speed curves for the vessel with open moonpool at the 8.0 meter and 11,25 meter drafts. Also, curves were produced for the vessel with a closed moonpool at the 11.25-meter draft. From the results of the tests, the predicted speed of the vessel was 14.2 knots at 8.0-meter draft, and 13.5 knots at 11.25-meter draft with an open moonpool. Closing of the moonpool added approximately 1.0 knot to the predicted vessel speed at 11.25-meter draft.

Though the model tests satisfactorily demonstrated the speed characteristics of the hull form, and clearly showed the benefit of closure of the moonpool, historical problems with moonpool door deployment and service life led to the decision to search for an alternative to the moonpool door.

Further, due to the large size of the moonpool (25 meters long by 10 meters wide) wave action within the moonpool became significant, particularly at shallow draft and high speed. Additionally when the wave action within the moonpool was significant, the vessel was seen to visibly surge. For these reasons, a series of moonpool wave rise tests. as described below, were performed.

Moonpool wave rise

A series of towing tests were performed at DMI, using the same model as was used in the original propulsion and resistance tests. The purpose of the tests was to quantify the effect different optional configurations had upon the wave rise in the moonpool, vessel longitudinal acceleration, and on vessel resistance. Each test was run independently, that is, the modification made to the vessel in each test was removed before the following test was run.

The openings in the hull where the six thrusters were fitted in the original tests were closed up before the tests at DMI were begun. DMI also removed the port bulkhead of the moonpool and replaced it with a Plexiglas wall. A video camera was fitted outside this wall looking in.

The interior of the starboard moonpool bulkhead was painted in a 2 meter by 2 meter (model scale) checkerboard pattern, so wave rise could be easily seen. Also, streamers were fitted on the interior of the starboard moonpool bulkhead below the 8 meter waterline so the water flow could be witnessed. In all tests, the following measurements were made:

• Wave rise in the moonpool using a wave rise sensor located in the aft end of the moonpool, on centerline.
• Resistance of the vessel
• Longitudinal surge acceleration
• The wave rise in the divided moonpool (in certain tests the moonpool was effectively divided in two portions) the wave rise in the moonpool using a wave rise sensor located in the forward end of the moonpool, on centerline
• In the "Door C" and "Door W" option tests, the hydrostatic pressure on the door, 3 meters forward (full scale) of the aft moonpool bulkhead, on centerline.
• Also, tests were run at drafts of 8.0 and 11.25 meters. The purpose of these tests was to:
• Quantify the wave rise in the moonpool and vessel surge accelerations
• Test the vessel in irregular waves, to verify if the behavior witnessed at Marin was only a calm water phenomenon
• Investigate various options to reduce the wave action in the moonpool and surge acceleration without, hopefully, causing a detrimental effect on vessel resistance.

The following options were tested: open moonpool, tests in irregular waves; open moonpool, tests in regular waves; swash transverse bulkhead at moon pool midship, 3.5 meters high; swash transverse bulkhead at moon pool midship, 6.0 meters high; swash transverse bulkhead at moon pool midship, 9.0 meters high; turbulence inducing lip across the aft end of the moonpool, at baseline; turbulence inducing lip across half of the aft end of the moonpool, at baseline; moonpool door at the 8.0 meter elevation; and flap at the aft end of the moonpool.

Some of the important findings of the tests are:

• The irregular waves had no effect on the wave rise in the moon pool.
• The wave rise amplitude could not be excited by wave action. Running regular waves at the calculated and observed natural period of the moon pool had no effect on wave rise.
• The swash bulkheads were most effective when they broke the still waterline. To be effective over a range of drafts makes the bulkhead so large that the overall vessel resistance increases undesirably.
• The most effective options tested were the moon pooldoor and flap.

On the Discoverer Enterprise, Transocean decided to install the moonpool flap. Though the results of the test showed results for the flap similar to the full moonpool door, the smaller size makes it easier to handle.

Test Correlation

The wind tunnel test results produced larger above-water wind forces than those estimated by calculations. The difference between the model test values and calculated values was greatest in the beam-wind condition, though the magnitude of this difference varied, depending on the blockage correction factor applied to the wind tunnel test measured results.

The underwater portion of the tests produced force values close to what was calculated. The major difference came in the determination of the lateral underwater center of resistance. The center of lateral resistance is not a directly measured value; it is extrapolated by measurements of forces and moments on the underwater portion of the vessel hull.

In calculations, this value was typically slightly greater than half the draft of the vessel. When the overturning moment of the thrusters was added, the center of lateral resistance approached the keel, particularly at light drafts. The wind tunnel test measurements predicted the center of lateral resistance below the keel of the vessel, in headings approaching beam-on.

• DP tests versus calculations: The DP model tests verified calculations, which had been performed prior to the tests. The DP calculations had predicted a slightly larger heading window (+/- 5!) through which the DP system would be able to maintain station. The reason for the tested heading window being smaller than the predicted window can be directly traced to the use of the wind tunnel model test forces being used as an input parameter to the DP tests. At angles of attack greater than 20! off the bow, the wind force component is at least 2/3 of the total force on the vessel, in the environments tested. An increased wind force component had the most significant effect on the determination of the operating limits of the vessel's DP system.
• Motion tests versus calculations: Of all the comparisons made between model test results and calculations, the vessel motion calculations showed the highest degree of correlation with the model test results. After the model test results were quantified, virtually no correction was required to the roll-damping coefficients, which were used as an input to the motions-calculation computer program. Perhaps the fact that the vessel's lines are fairly conventional in nature led to this close correlation between calculated versus tested results.
• Propulsion test versus calculations: Before the model test program was carried out, Marin was committed to perform a preliminary assessment on the vessel speed prediction for different drafts at full power. The propulsion test at Marin yielded 14.5 knots at 11.25-meter draft, and 15.2 knots at 8.0-meter draft, showing excellent agreement between the estimated and measured values.

Conclusion

When the Discoverer Enterprise is delivered in year 2000, and after final outfitting in the Gulf of Mexico, it will be the world's premier drilling unit, and the only vessel of its kind. Though the dual activity drilling system is expected to cut well costs by up to 40%, a large part of the unit's success will derive from the capabilities and features of the vessel hull and systems.

The vessel hull structure supports all activities necessary to safely store over 100,000 bbl of crude, plus carry 20,000 tons of VDL and drill in a fully DP mode from a dual-activity derrick. This is done while satisfying the stability requirements of the HSE. Yet the hull form is such that during transit operations, the vessel can cruise at an expected speed of 15 knots.

This combination of carrying capacity and transit speed was accomplished through the use of fine mesh model 3D FEM analysis, and an aggressive model testing program.

Authors

Joaquin Lopez-Cortijo Garcia is the Basic Engineering Manager for Astano.

Richard Paul Michel is with Transocean Offshore's Discoverer Enterprise Project.

References

J.C. Cole, SPE, and RP.Herrmann, SPE, and R.J. Scott, Transocean Offshore, and J.M. Shaghnessy, SPE, Amoco Corporation.

SPE/IADC 37659, 1997 SPE/IADC Drilling Conference, Amsterdam, The Netherlands, 4-6 March 1997.

!-- Predicted vessel speed at full power versus draft Mean draft T 7.00 8.00 9.65 11.25 13.00 M Power PS 30,000 30,000 30,000 30,000 30,000 KW Trail speed Vs 16.10 15.85 14.94 14.25 13.54 Knots --Editor's note: This paper was presented at the Deepwater Offshore Technology (DOT) Conference in The Hague in November 1997.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.