DYNAMIC POSITIONING OF SEMI COST EFFECTIVE FOR SHALLOW WATER EXPLORATION

John P. Edmondson
BP Exploration Operating Co. Ltd.
Aberdeen

The use of a dynamic positioning (DP) system for station keeping a semisubmersible in water depths as shallow as 101 m (330 ft) has allowed substantial cost savings for short duration exploration wells in the North Sea..

BP Exploration Operating Co. Ltd.'s semisubmersible drilling unit Ocean Alliance is capable of drilling exploration wells in anchored mode in water depths to about 550 m (1,800 ft) and in dynamic positioning mode in water depths to approximately 1,370 m (4,500 ft).

The design of the DP system offers total redundancy of every component. This redundancy, coupled with BP's experiences with drilling several deepwater wells in DP mode, made the use of DP in shallow water feasible.

The dynamic positioning system maintains vessel position and heading through automatic control of the direction and amount of thrust from the vessel's engines to counteract the environmental forces. To carry out this function, the DP system must have the following basic subsystems:

A method of measuring external forces
A method of measuring the vessel's position
A method of calculating required counter forces
A method of providing counter forces
A method of providing a man/machine interface (Fig. 1).

Wind sensors on the rig measure external forces (wind speed and direction). To optimize position keeping, these forces must be counteracted before they move the vessel from its position, rather than to correct position after movements have occurred. To implement this compensation, a technique known as feed forward is used to predict vessel response (Fig. 2).

The rig uses no direct measurement input of wave and current forces. These forces are derived from a calculation of the differences between known wind and thruster output forces.

Several methods are used to measure the vessel's position and heading. Each system is independent such that no single failure mode is common to all, thereby giving the system optimum reliability. The Ocean Alliance is equipped with four types of position reference:

Hydro-acoustic transponders
Taut wire
Flex joint angle inclinometers
Riser angle acoustic beacons.

The heading is measured by gyro compasses.

The vessel movement has to be compensated to minimize errors in the position reference sensors. A vertical reference unit (VRU) measures heave, sway, and surge to calculate the compensation.

Computers are used to calculate the required response to the sensor information. The computers use a mathematical model of the vessel combined with special filtering techniques to perform the calculations. The vessel has an uninterruptible power supply to ensure that the computers receive a stabilized continuous power supply. This system also supplies power to some of the other subsystems (VRU, gyro, etc.).

Thruster units mounted under the lower hulls provide counter thrust. The Ocean Alliance is fitted with eight azimuthing thrusters which can be operated in fixed or free azimuthing conditions combined with variable pitch settings.

DP EQUIPMENT

The foremost fundamental principle of the Ocean Alliance dynamic positioning (DP) system is redundancy.

The Ocean Alliance is fitted with four wind sensors: two positioned on diagonally opposite corners of the rig and two on the top of the derrick. The wind sensors measure both wind speed and direction and produce voltage signals that are fed into the computers.

VRU

The DP operator must exercise care and judgment in selecting which sensor to activate. Changing sensors may lead to sudden thruster activity if, for instance, the one which had been in use was shielded from the wind and not giving a true reading, and the new choice gives a true reading. Also, the DP operator should be aware of the effects of helicopter downdraft on the sensors and should normally deactivate affected sensors.

Pitch and roll movements of the vessel can lead to inaccuracies in the measurements of the vessel's position and heading. Therefore, vertical reference unit, (VRUs) are included in the DP system configuration. These movements are monitored and used by the system computers to correct the measurements from the position reference systems.

The VRU measures the angular difference between the vessel's reference plane and the true horizontal plane along the port/starboard (pitch) and the fore/aft (roll) axes of the unit.

GYRO COMPASS

The gyro compass provides the DP system computers with a heading reference with respect to due north for the vessel. This information enables determination of actual heading, desired heading, and yaw.

The compass is based on a gyro mounted in a two-gimbal platform which serves as a sensor to an electronic control circuit. The circuit actuates the gimbal motors to stabilize and align the compass card. The design of the system gives a short alignment time and minimal errors under dynamic conditions.

The Ocean Alliance is equipped with three gyro compasses.

RISER ANGLE

Inclinometer devices measure the angle of the riser at the flex joint above the blowout preventer (BOP). These inclinometers measure the riser and BOP displacement from the vertical in degrees fore/aft and port/starboard. One sensor is placed below and one sensor above the flex joint. The algebraic difference of the displacements gives the flex joint angle.

The flex joint angle must be kept at or near zero to avoid "key seating" of the drillstring on the riser. The rig maneuvers to maintain this position and avoid damage to the riser.

Two sets of inclinometers are fitted (four units in total) designated blue and yellow to correspond to the BOP controls. The angle information is transmitted via the BOP multiplex transmission system to the computers.

HYDRO-ACOUSTICS

The rig uses hydro-acoustic reference systems which are based on the transmission of sound in water. A signal is generated within the hull and sent via an external transmitter to a beacon on the seafloor. Upon receiving the signal, the beacon responds. A receiver on the hull picks up the return signal, and the range and direction of the beacon are then calculated.

The system installed on the Ocean Alliance is based on the ultrashort baseline principle-the hull transmitter and receiver are mounted together in a compact transducer.

The system can measure the range and position of several beacons (transponders) by use of a different interrogation and reply frequency for each. A screen on the main console displays each transponder position.

In addition to these transponders, the rig is equipped with special inclination transponders which, in addition to range and bearing, give displacement from the vertical in the fore/aft and port/starboard axes. These transponders can give riser inclinations and can therefore be used as an alternative reference system.

The Ocean Alliance is equipped with four transducers, two of which are fixed head and two of which are tracking head. One of each type is located in each lower hull. The main console is fitted with two display screens such that one screen and one of each type of transducer makeup one system.

TAUT WIRE

The taut wire is an over-the-side position reference system. This reference system has a depressor weight which is suspended from an arm and lowered to the seabed on a wire kept under constant tension. A sensor located at the end of the lowering arm has two potentiometers mounted in a gimbal which follows the direction of the wire. The direction of the wire from the vertical is derived in x and y (fore/aft and port/starboard) coordinates. An incremental encoder measures the length of wire out which is translated into depth.

The Ocean Alliance has one taut wire installed at the forward end between the lifeboats.

COMPUTER SYSTEM

It is necessary to provide the DP computers and essential equipment with a stable, uninterruptible power supply. The vessel's power supply is interconnected through electronic circuits in such a manner that should one supply fail, there is a bumpless transfer to an alternate supply.

The semisubmersible is fitted with a double uninterruptible power supply which incorporates two main supplies and two battery backup supplies.

The computer in the DP system is the main controlling mechanism (Fig. 3). By assimilating all the information provided by the sensors and the software model, the computer controls the thruster output. The Ocean Alliance has two independent computer systems for this task.

The main system which activates the automatic dynamic positioning (ADP) 503 console consists of three computers (A, B, and C) located side by side in the computer room behind the pilot house. The A computer is normally in command, and the B computer, identical to A, runs in passive parallel on the same data as the A computer. The C computer continuously controls the other two computers and automatically takes care of switching to the backup B computer if A fails.

A separate computer, also running in passive parallel, is located in the ADP 311 console. This computer has its own separate sensor inputs.

The console provides the man/machine interface. The DP operator can select the mode of operation, position reference, and other sensors. Color screens graphically display position and other information as various "pages" of information selected by the operator.

The ADP 503 is the main DP control console and is equipped with dual screens and full mode and sensor choice. The console also houses the two hydro-acoustic reference display screens.

The ADP 311 backup console is the main transit control position and the backup DP control position in event of total failure of the ADP 503. This console has a single screen and a choice of sensor inputs.

DP PERFORMANCE

The incremental fuel burn between anchored operations and DP operations averages 25 metric tons/day. Fig. 4 shows the net DP fuel consumption as it varied with weather conditions for October 1990.

The positioning accuracy of the DP system is 1% of water depth. In most weather conditions, the station keeping averages 3-5 m, exceeding the specifications.

The yellow alert watch circle is normally set at 3 lower flex joint angle. The red alert watch circle is normally set at 5 lower flex joint angle.

On the shallow water well drilled in DP in 101 m water depth, the yellow alert watch circle was set at 7 m, and red alert watch circle set at 10 m.

WESTERN NORTH SEA

The Ocean Alliance has drilled four wells to date on the Western United Kingdom Continental Shelf in water depths ranging from 450 m to 706 m. Cost calculations are presented for one of the wells which was drilled in DP mode in 550 m of water.

The cost calculations were based on the following:

Three anchor handlers required to run and pull anchors
Three days total required to run and pull anchors
Anchor handlers cost 10,000/boat/day ($19,000/boat/day)
DP fuel consumption of 25 metric tons/day at 130/ton ($250/ton)
Comprehensive cost, including services, helicopters, fuel, etc., for the Ocean Alliance at 110,000/day ($209,000/day)
Well duration of 40 days
DP beacons deployed and recovered while ballasting and deballasting, respectively
No tow boat required-the Ocean Alliance is self propelled
Four additional DP operators at 1,000/day ($1,900/day).

For anchored operations, the outward journey would require three anchor handling boats: 1/2 day to load up anchor handling equipment, 1 day in transit to the well location, 1 1/2 days to run anchors, and 1 day to return to Peterhead, Scotland.

The inward journey would also require three anchor handlers with the same time breakdown. Anchor handling would require a total of 24 boat days.

The estimated cost to anchor is 570,000 ($1,083,000), which includes 24 boat days (240,000) and 3 days of rig time during anchor handling (330,000).

The actual additional cost to drill dynamically totaled 175,000 ($332,500), which comprised DP fuel for 40 days (130,000) and DP operators for 45 days (45,000).

For this well, the cost saving by drilling in DP mode was 570,000 - 175,000, or 395,000 ($750,500).

CENTRAL NORTH SEA

The Ocean Alliance has drilled one well to date in DP mode in the central North Sea. This well was drilled in 101 m water depth.

The cost savings were based on the following:

Two anchor handlers required to run and pull anchors
One day total required to run and pull anchors
Anchor handler hire at 10,000/boat/day
DP fuel consumption of 25 metric tons/day at 165/ton
Total cost for the Ocean Alliance at 110,000.day
Well duration of 17.7 days
Additional time to deploy and recover beacons was 3 1/2 hr
No tow boat required
Additional four DP operators at a cost of 1,000/day.

For anchored operations, the outward and inward journeys would each require two anchor handling boats: 1/2 day in transit to well location, 1/2 day to run or pull anchors, and 1/2 day to return to Peterhead.

The estimated cost to anchor is 170,000 ($323,000), which includes 6 boat days (60,000) and 1 day of rig time during anchor handling (110,000).

The actual additional cost to drill dynamically totaled 171,000 ($324,900): 21,000 for DP operators for 21 days, 73,000 for 17.7 days of fuel for DP operation, and 16,000 for 3 1/2 hr of rig time for deployment and recovery of beacons.

The cost saving by drilling in DP mode was 170,000 - 110,000, or 60,000 ($114,000).

WEATHER DELAYS

Weather delays can substantially affect anchor handling time, thus making DP drilling more cost effective. The following cost analysis is for a well drilled in anchored mode in 118 m of water in the central North Sea.

The decision to drill this well in anchored mode was influenced by two major factors: The well was likely to test and testing in DP in shallow water presents some risk, and the reliability of a newly installed BOP multiplex system was yet unknown.

A fast-acting subsea test tree is presently under development to allow a drill stem test (DST) to be performed safely in DP mode. Until this system is fully developed and field tested, DSTs can only be performed in anchored mode. Depending upon the likelihood of a DST on a particular exploration well, a decision analysis is conducted. The decision analysis must assess the relative costs and likelihood of a DST to help determine the most appropriate course of action-either to anchor up from the start or drill DP and only run anchors if a DST is to be performed.

The cost calculation is based on the following:

Two anchor handlers required to run and pull the anchors
Time for anchor handling and waiting on weather was 2.81 rig days
Anchor handlers cost 10,000/boat/day
DP fuel consumption of 25 metric tons/day at 144/ton.
Total rig day rate was 110,000/day
Well duration of 32.3 days
Additional time to deploy and recover beacons estimated at 3 1/2 hr
Actual rig waiting-on-weather time to pull anchors was 43 1/2 hr
No tow boat required for the rig
Additional cost for four DP operators at 1,000/day.

The boat time on this well totaled 11.19 boat days, including the time the boats were in transit, running and pulling anchors, and waiting on weather.

The actual cost to anchor totaled 421,000 ($800,000), including 112,000 for 11.19 boat days and 309,000 for 2.81 rig days during anchor handling.

The cost to drill in DP mode is estimated at 168,000 ($319,200): 116,000 for fuel for DP operations for 32.3 days, 36,000 for 36 days of DP operators, and 16,000 for 3 1/2 hr of rig time for deployment and recovery of beacons.

Drilling this well in DP mode could have saved 421,000 - 168,000, or 253,000 ($480,700).

PARTNER SAVINGS

These potential cost savings by drilling in dynamic positioning (DP) mode are based upon the gross rig rate for the Ocean Alliance of about 85,000/day. The rig rate paid by BP's partners is substantially less because their costs are based on the commercial market rate. For example, the commercial rate on the central North Sea well was 25,000/day ($47,500), and BP subsidized the remaining 60,000/day ($114,000/day).

The cost savings for this central North Sea well have been reworked to illustrate the actual savings for the partners (Table 1). The partner shares ranged from 21% to 0.5%.

For DP operations, an additional four DP operators are required to supplement the rig crew. The cost of this additional personnel amounts to 1,000/day and is covered by the BP subsidized rate.

QUICK INTERVENTION

The flexibility of the DP system on the Ocean Alliance was put to good use in January 1991.

During a central North Sea operation, a marine riser was lost to the seabed in 93 m water depth. The resultant force of the riser left the wellhead and BOP stack leaning at an angle of approximately 7.

The extent of damage to the BOP stack was unknown. Therefore, recovery operations could have been extensive if subsea repair work was required on the BOP or its hydraulic control system. If subsea repair work had been required, then the appropriate course of action would have been to mobilize a dive vessel for the work.

The Ocean Alliance was effectively used in DP mode to assess the damage to the BOP stack. In between exploration wells, the rig made a short detour to the well location, set up on DP, ran a subsea control pod, and assessed that no damage had occurred to the stack. Thus, no dive vessel was required. The rig completed the damage assessment operation in just 18 hr, including the rig move.

The use of an anchored drilling unit to perform this type of quick intervention operation would have been extremely expensive (additional cost estimated at 150,000) because of the additional time and cost of anchoring operations.