An Integrated Approach To The Safety of Surface Well Testing On Mobile Offshore Rigs
An Integrated Approach To The Safety of Surface Well Testing On Mobile Offshore Rigs
This paper was prepered for presentation at the First International Conference on Health, Safety and Environment held in The Hague, The Netherlands, 10-14 November 1991.
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  ABSTRACT
                                                                                                         The risk inherent to any production operation is the
  Offshore well testing during the exploration and appraisal                                             uncontrolled release of hydrocarbons into the atmosphere
  phases requires the production of hydrocarbons, often at high                                          which may lead to explosion, fife, toxic effects and pollution.
  pressure, through a temporary system set up on a mobile
  offshore rig. These operations require the operator, the drilling                                      This study uses the production systems and safety standards of
  company and the testing company to reconcile the three key                                             fixed offshore production platforms as a reference to analyze
  issues of keeping risk as low as reasonably achievable,                                                the testing systems and procedures used on mobile rigs. The
  collecting as much information as possible and limiting cost                                           analysis focuses on coping with logistical constraints while
                                                                                                         maintaining standard drilling safety practices as well as the
  The process equipment designed and manufactured for                                                    safety standards associated with the production operation. The
  permanent installations is compared to those used on mobile                                            second part of the study reviews the interaction required
  offshore rigs. The associated risks are analyzed. Standards of                                         between companies to plan and perform the test. Conclusions
  professional training and experience are discussed.                                                    are then drawn concerning the risk level of well testing on
                                                                                                         mobile offshore rigs, the importance of professional input and
 The interaction required for test acceptance, job planning and                                          the need to reinforce the formalization of companies
 job execution is described. This interaction involves the                                               interaction and safety integration.
 operator, the testing company, the drilling company,
 regulatory bodies and certifying authorities. Potential
 improvements are proposed.                                                                              BACKGROUND:
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 2     AN INTEGRATED APPROACH TO TIlE SAFETY OF SURFACE'WELLTESTING ON MOBIlE OFFSHORE RIGS                                    SPE 23250
blowout preventer located on the seabed, as shown in Figure               The safety system should provide two levels of protection
2. This development not only improved the safety aspects of               which should be independent of and in addition to the control
the test, it also allowed for the safe operation of wireline              devices used in normal operation.
conveyed devices to monitor downhole well test data in real               Note: "overpressure" is an example of an undesirable event
time and for the use of coiled tubing for stimulation
operations.                                                               Safety analysis techniques will determine, using a Safety
                                                                          Analysis Table (SAn, the minimum safety requirement for a
There are two kinds of production test systems used on                    component as an independent unit of the system. When
mobile offshore rigs. One of these is the conventional testing            process components are fully protected and combined into a
system used to test exploration and appraisal wells. The                  production system, no additional threats are created. Therefore,
systems are in operation approximately 500 times per year on              if all process component safety devices are logically integrated
mobile offshsore rigs outside North America and represent the             into a safety system, the entire facility will be protected. This
bulk of mobile offshore testing systems. A typical offshore               is controlled through Safety Checklists (SAC) and a Safety
setup of a conventional testing system is shown in Figure 1.              Analysis Function Evaluation (SAFE) whose completion
The other type system is called an Extended Well Test System              provides a means of verifying the design logic of the basic
(EWn or Early Production Facility (EPF). These systems                    safety system.
may have some components of conventional testing
equipment and some semi-permanent components. There have                  Emergency Support Systems (ESS) minimize the effect of
been approximately 30 extended well tests/early production                escaped hydrocarbons on production facilities. They include
facilities installed since the technique was developed in the             the Emergency Shut Down System (ESD) to provide a
mid-seventies. The production capacity of these systems                   method to manually initiate production system shutdown by
ranges from 10,000 BOPD to 30,000 BOPD. Some of them                      personnel observing abnormal conditions or undesirable
have been in operation for weeks while others have been, and              events. The Subsurface Safety Valves (SSSV) which may be
still are, in operation after several years. These extended tests         self-actuated or activated by the ESD system or a fire detection
or early production facilities allow the operator to gather               system. The containment system to collect escaped liquid
information about the reservoir (i.e. boundaries,                         hydrocarbons and initiate shutdown. Combustible gas
heterogeneities, drainage systems) while producing the                    detection and fire detection devices to trigger alarms and/or
reservoir. These systems are used while the reservoir is being            shutdown.
analyzed, prior to making a final commitment on field
development or while permanent production facilities are
being designed and constructed. Such systems have to be                   MOBILE RIG INSTALLATIONS COMPARED TO FIXED
engineered and mobilized much quicker than permanent                      PLATFORM SYSTEMS. AS A REFERENCE
facilities (15 to 20 weeks against 15 to 20 months). They are
considered as being as safe as permanent systems. Since these             The production system, from reservoir up to hydrocarbon
systems represent a middle-of-the-road approach, they will not            processing, storage and disposal, is made of similar basic
be discussed further.                                                     components whether it is designed for commercial production
                                                                          or for testing the discovery well.
Safety In Production Systems:
Protection Conce.Pts - Safety Analysis                                    Having briefly reviewed the objectives, history and evolution
Emergency SUQPOrt Systems                                                 of offshore testing, having as well stated the protection
                                                                          concepts and safety analysis applying to any production
Most threats to safety and environment protection from a                  system, we now analyze its main components:
production system involve the release of hydrocarbons. Thus,
the analysis and design of a production system should focus               -   Wellheads and upstream safety barriers.
on preventing such releases, stopping the flow of                         -   Piping systems.
hydrocarbons if it occurs, and minimizing the effect of                   -   Process equipment.
hydrocarbons that are released.                                           -   Storage - disposal.
                                                                          -   Emergency support systems.
The basic protection concept used in the safety system
analysis consists in identifying each undesirable event that              Production facilities on fixed platforms represent the state-of-
could affect a component of the system. Undesirable events                the-art in terms of safety standards and environmental
are analyzed according to:                                                constraints. Their production systems are built for decades, to
                                                                          operate round the clock in any weather condition.
- Their cause.                                                            Recommended practice for their design and installation is
                                                                          outlined in API 14 E. Recommended practice for analysis,
- Their effect and detectable abnormal conditions.                        design, installation of basic surface safety systems is outlined
                                                                          in API 14 C. It will be our reference.
- The kind of primary and secondary protection which should
  prevent or react to their occurrence.                                   The production system used for testing a discovery or an
                                                                          appraisal well is made of components which are rigged up
                                                                          only for the purpose of this test, as shown in Figure 1. The
                                                                    448
  SPE 23250                                         J.P.OLLIER B. IMRIE' R. TALBaIT                                                    3
setup will be subsequently dismantled when the well is                   Downhole testing techniques like drillstem testing are adding
plugged and abandoned.                                                   one more safety barrier close to the hydrocarbon reservoir.
                                                                         Consequently, there are more safety barriers in temporary
Wellheads And UPstream Safety Barriers                                   production strings upstream of the wellhead than on fixed
                                                                         production platforms. Some of these protections are integrated
Christmas trees on permanent installations and flowheads on              to the drilling safety systems, inside the BOP stack which
temporary setups provid.e the same wellhead function. They               provides the additional annulus protection above the downhole
allow a shutdown of the production string at surface. Both are           packer.
designed and manufactured as per the specifications of API 6
A as a minimum requirement Both have at least one remotely               Piping Systems
controlled valve connected to an emergency support system.
Their differences are the result of different logistical                 On production platforms, piping systems are designed to
constraints.                                                             handle the maximum flowrate expected during the lifetime of
                                                                         the facility. It is also advisable to add a surge factor of 20 to
Christmas trees are flanged on top of the tubing hanger and              50 per cent unless surge expectations have been more
only have to support their own weight. Flowheads, on the                 precisely determined. Both flow velocity and pressure drop
other hand, are screwed on top of the last tubing or drillpipe.          should be considered for liquid lines, single phase gas lines,
During drillstem testing, they have to support the weight of             and gas/liquid two phase lines, respectively. Design and
the whole drilling string and allow rotation of that string              installation is as per API 14 E recommended practice as a
under pressure. Because of its position above the rig floor and          minimun requirement
its screwed bottom connection, a flowhead has weight and
compactness constraints. In this respect, the choke controlling          Setting up temporary piping systems on mobile offshore rigs
the flowrate, which is normally part of a Christmas tree, is             creates several challenges. The rig may have no permanent
not mounted on the flow head. Consequently, wellhead                     production flowlines at all. Some areas of the deck may be
pressure has to be brought from the flowhead to the choke                out of reach for rig cranes and derrick catlines. Deck space,
manifold on the rig floor through a flowline.                            particularly on small jack-up rigs, is scarce which means that
                                                                         process equipment such as choke manifolds, heat exchangers,
Offshore, producing wells are equipped with a subsurface                 separators and gauge tanks may very well be scattered around.
safety valve located below the sea bed. It closes the well in a          Such an environment implies the following constraints for
fail-safe mode should the production string be damaged or                the piping system:
destroyed above the mud line. It is also surface controlled and
integrated to the emergency support system. Fire detectors,              The layout can very seldom be accurately determined and or
high-low pressure sensors and emergency shutdown systems                 pre-assembled onshore. Each piece of pipe is to be hand-
will trigger the closing mechanism.                                      carried and assembled by no more than two or three people.
                                                                         That is a pipe weight constraint. Pipes are assembled using
Temporary production strings provide at least one safety                 quick connections because of time and manpower constraints.
barrier upstream from the flowhead:                                      The standardization on H2S-Proof equipment for most
                                                                         exploration tests has resulted in the elimination of chicksan-
A lubricator valve, located a few feet below the rig floor,              type articulated piping. They have been replaced by scores of
allows electrical or mechanical wireline tools to be run in the          90° elbows and straight pipes assembled by quick
production string. It can be closed from the rig floor and acts          connections. A simple 3-in. nominal 90° elbow with its quick
as a safety barrier should any leak or rupture occur at or below         connection weighs between 30 and 50 Ibm according to its
the flowhead. It can subsequently be pumped open by the rig              working pressure. A simple 3-in. nominal 6 ft straight pipe
mud pumps to circulate and control the well.                             connection weighs between 80 and 150 Ibm. A standard 2,000
                                                                         ft piping setup will amount to more than 300 different pipes
In addition, on floating rigs, a subsea test tree is positioned          and 300 connections. It requires about 16 man/days to rig up
inside the drilling BOP stack. This device has three                     and subsequently 12 man/days to pressure test In the North
simultaneous safety functions:                                           Sea, each piece of piping has to be traceable and recertified on
                                                                         a regular basis. High temperature production tests require the
a) To allow a quick disconnection of the production string               elimination of elastomeric seals in the quick connection. They
   should the rig need to move off location in an emergency              are replaced by metal to metal seals associated with different
   (less than 20 seconds).                                               types of screwed connections or clamping devices.
b) To cut any cable or coil tubing which happen to be inside             Today, the flowline connecting the flowhead outlet to the
   the production string at the time of disconnection.                   choke manifold is a flexible pipe composed of unbonded
                                                                         layers of plastic and steel. Maximum allowable working
c) To plug the production string left downhole and, through a            pressure can be up to 15,000 psi and its maximum operating
   retainer valve, to plug the upper part of the string just             temperature up to 160°C. For an internal diameter of 2.8-in.,
   above disconnecting point. This prevents dumping                      an external diameter of approximately 6-in., it weighs nearly
   hydrocarbons into the sea. Liquid would pollute the                   40 lbm/ft Since it is located on the rig floor, it is easily
   environment, gas would be a threat to the rig safety.                 moved and handled.
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 4     AN INTEGRATED APPROACH TO mE SAFETY OF SURFACE WELLTESTING ON MOBllE OFFSHORE RIGS                                    SPE 23250
An alternate solution is to have at least part of the piping             10,000 BOPD assuming a one minute retention time. While
system permanently installed on board the drilling rig. It can           in operation. normally for flowing periods not exceeding a
be designed, particularly the low pressure side of the system,           few hours, test separators are continuously manned. In this
in larger sizes, with less elbows and with proper supports in            type of environment, the field experience of specialists can
order to restrict vibrations. Piping has to be inspected after           hardly be replaced by automated equipment and pre-established
each production test in order to detect any erosional damage             computer programs.
resulting from well clean up or sand production. Such an
integrated setup is normal practice on Norwegian rigs where              Storage - Disposal
most of the process equipment like steam exchangers,
separators, burner booms remain permanently rigged up. In                On platforms. besides engine fuel. there is no storage of
other areas, some semi-submersible rigs have part of the                 hydrocarbons. Both gas and liquids are segregated and pumped
piping and burner supporting booms permanently installed. It             through monophasic pipelines to onshore storage facilities.
is seldom the case with jack-ups owing to rig space and rig              Tankers may provide some intermediate crude oil storage.
move constraints.                                                        particularly for extended well tests or early production
                                                                         facilities.
A temporary piping system is a containment system as safe
as permanent piping provided it is inspected after each job and          On mobile offshore rigs, whatever the type of test. gas is
pressure tested before each job. However, even though it is              flared. During exploration tests, storage of large quantities of
common practice that flow velocity may exceed. for short                 crude oil on the rig deck would be hazardous in terms of risk
periods, the limits set for permanent installations (provided            of explosion and fire and limited in terms of deck space and
sand is not produced), the flowing capacity of the whole                 deck load. Disposal through floating storage facilities would
production system may be restricted, and rigging up and                  be uneconomical as regards mobilization costs. eventhough
pressure testing the system are time-consuming.                          technically feasible. Such an environment implies that today
                                                                         liquid hydrocarbons are disposed of through crude oil burners.
Process Equipment
                                                                         Emergency Shutdown Systems
For a given process capacity. the design and manufacturing of            Pressure Relief Systems
separators is very similar, whether they are part of a fixed
platform setup or mobile separators. The reference is ASME               The operating mode of the system is as per API RP 14 C
Section VIII, division 1 and 2 and NACE MR 0175 as a                     minimun requirements:
minimum requirement. Their difference is in the degree of
automation and packaging.                                                -   Automatic monitoring and automatic protective action if
                                                                             an abnormal condition indicating an undesirable event can
On platforms. process equipment is tailored to the field                     be detected by a sensor.
characteristics. that is operating pressure, flow rates. gas oil
ratio, water oil ratio. in order to optimize the hydrocarbons            -   Automatic protective action if manually actuated by
recovery. Mobile test separators are designed to cope with 90                personnel who observe or are alerted to an unsafe condition
per cent of unknown flowing conditions. which may as well                    by an alarm.
include exposure to mud, sediments and debris. They need to
be ruggedized in order to endure transportation on supply boat           -   Continuous protection by support systems that minimize
decks and offloading on rigs in almost any weather condition.                the effects of escaping hydrocarbons.
Then. they may remain stored on the deck, unattended for days
or weeks and nevertheless they have to be ready to operate in a          Considering the relatively short flowing periods in
matter of hours. Such an environment implies the following               exploration testing. from a few hours to 48 hours as a
constraints:                                                             maximum, and the potentially unstable flowing conditions,
                                                                         safety system operating modes are essentially based on
- Equipment must be light enough to be moved by the rig                  personnel. There are only a limited number of sensors,
  cranes.                                                                basically pressure safety low pilot and pressure safety high
                                                                         pilot, located between flowhead and test separator, that will
- Equipment must be rugged enough to withstand offshore                  automatically shutdown the flow at and below the flowhead in
  transport and offshore storage. This prohibits the use of              case of flowline rupture or plugging. Any other shutdown is
  most electronic equipment in remote locations.                         manually actuated either from the emergency shutdown panel
                                                                         or from selected remote manual ESD stations. As a secondary
- Equipment must be versatile in terms of gas and liquid                 protection against overpressure. pressure safety valves located
  capacity in order to cope with unknown flowing                         on each pressure vessel can release the flow overboard through
  conditions.                                                            relief lines.
                                                                   450
 SPE 23250                                          IP. OILIER. :8'. IMRIE . R. TALBaIT                                                5
                                                                    451
6    AN INTEGRATED APPROACH TO TIlE SAFETY OF SURFACE WELLTESTING ON MOBILE OFFSHORE RIGS   SPE 23250
ACKNOWLEDGMENT
REFERENCES
                                                               452
                                                                                                       SPE   23250
TABLE 1
Companies Interaction
4 Commjssjonjog
                                                                          ·;;.
                                  o   GAS       •   WELL STREAM
                                                                          I
  2. Perforated tailpipe           • 13. E-Z TREE control unit and
  3. PosiTest packer                     glycol injection pump             . ,
                                                                           \/7   1.. .•
  4. Hydraulic jar                    14. Flow head
                                      15. Flow head safety valve
  5. Pressure recorders
  6. Test valve                       16. Wireline wellhead equipment
                                                                               -,
                                      17. Offshore wireline unit with          "1
  7. Reverse valves
                                    •     COMPUTEST system
                                                                              " -,1
 Subsea Safety Equipment              18. Data acquisition units
• 8. E-Z TREE-Safety valve            19. Emergency shutdown
     latch assembly with glycol           system
     injection system                 20. Data header
  9. Retainer valve                   21. Choke manifold                             7
 10. Deep sea hydraulic control       22. Steam exchanger                            6
     pod                              23. Three-phase separator
                                                                                     5
 11. Lubricator valve                 24. Oil manifold
                                      25. Surge tank                                 4
                                      26. Transfer pump                              3
                                      27. Air compressor
                                                                                     2
                                      28. Gas manifold
                                      29. Supporting boom                            1
                                      30. Burner
  Figure 1
  Well Testing Schematic
• Mark of Schlumberger
                                                     453
                                                                       SPE    23250
           Hydraulic Control
           Hose Reel Console Accumulator
                              Skid Unit
                                               IIW.L,m,---   Riser
       1[1;01----          Hose Protector                    Disconnected
            1----          Hydraulic
                           Control Pod
Figure 2
Subsea Test Tree Assembly
- Mark of Schlumberger
                                         454
                                                                                                                                                                       SPE   2}250
PRELIMINARY
PROPOSAL
                                                                       .--
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                                                           ;._-        _._ VENT     _._._._._._.
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TL:!=----'!:::::=:.::r----- :-----,
RISER DECK
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Figure 3
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                                      HYDROCARBON
                                      LINE:S
                                      VENT LINES
                      +
                      ~
                                   H'f't)RAULIC liNtS
                                     PILOT LINts
                       NOTt ;- WEIGHT AS GNEN A8O'v't tS "WET WEIGHT", i.e. WEICHT OF' EQUIPMENT WHE
                                    ruu.
                                    OF' fRESH WA.ltR.
455