General Equipment, Communication, and Personnel Related Problems

Most drilling problems result from unseen forces in the subsurface. The 

major causes of these problems are related to equipment, gap in proper 

communication, and issues related to human errors (personnel-related). 

However, there are drilling problems that are directly related to formation, 

operational hazard, and geology. This section discusses the equipment, 

communication, and personnel-related problems.

2.1.3.1 Equipment

The integrity of drilling equipment and its maintenance are major factors 

in minimizing drilling problems. However, the equipment involved can 

also be a source of problems in addition to communication and personnel￾related issues. Drilling problems can significantly be reduced by proper rig 

hydraulics (i.e., pump power) for efficient mud circulation, proper hoisting 

power for efficient tripping out, proper derrick design loads and drilling 

line tension load to allow safe overpull in case of a stuck pipe problem, and 

well-control systems (ram preventers, annular preventers, internal preven￾ters) that allow kick control under any kick situation. Specific mud prop￾erties and required horsepower are needed for bottom hole and annular 

space cleaning, proper gel strength to hold the cuttings. Proper monitoring 

and recording systems are necessary to monitor trend changes in all drill￾ing parameters and can retrieve drilling data at a later date. Proper tubular 

hardware is specifically required to accommodate all anticipated drill￾ing conditions. Effective mud-handling and maintenance equipment will 

ensure the mud properties that are designed for their intended functions. 

The following drilling equipment may create potential drilling problems 

while drilling: i) rig pumps, ii) solids control equipment, iii) the rotary 

system, iv) the swivel, v) the well control system, and vi) offshore drilling. 

In the majority of cases, equipment failure may happen due to corrosion in 

addition to bending, fatigue, and buckling.

The integrity of drilling equipment and its maintenance are major fac￾tors in minimizing drilling problems. The following are all necessary for 

reducing drilling problems:

Proper rig hydraulics (pump power) for efficient bottom and 

annular hole cleaning

Proper hoisting power for efficient tripping out

Proper derrick design loads and drilling line tension load to 

allow safe overpull in case of a sticking problem

Well-control systems that allow kick control under any kick 

situation (i.e., proper maintenance of ram preventers, annu￾lar preventers, and internal preventers)

Proper monitoring and recording systems that monitor 

trend changes in all drilling parameters and can retrieve 

drilling data at a later date

Proper tubular hardware specifically suited to accommodate 

all anticipated drilling conditions

Effective mud-handling and maintenance equipment (i.e., it 

will ensure that the mud properties are designed for their 

intended functions)

2.1.3.1.1 Case Study

Inspection of the below-grade wellhead equipment has shown corrosion 

damage to the buried landing base, casing spools and surface casing, espe￾cially in water injection and supply wells in onshore fields in the Middle 

East. Occurrence of corrosion damage has been a concern in the buried 

wellhead equipment and surface casing immediately below the landing 

base in the onshore fields. Initial random inspections of the below-grade 

wellhead equipment in the mid-eighties showed corrosion damage to the 

buried landing base, casing spools, and surface casing. Typical landing base 

and surface casing equipment for onshore wells is depicted in Figure 2.2. 

The 13-3/8 casing is either welded or screwed on to the 13-3/8 l3-5/8

landing base. The 18-5/8 conductor pipe is cemented at a distance ranging 

from a few inches to 2–3 feet below the landing base.

                          Procedure and Data: A typical landing base inspection operation involves 

excavating the cellar to below the landing base to expose three to six feet 

of the surface casing or until hard cement is encountered below the land￾ing base, whichever is earlier. The exposed section is sand blasted and 

then inspected for evidence of corrosion. The data from such inspections 

for the last six years (1991 through 1996) is presented in Table 2.1, while 

Figures 2.3–2.5 illustrate some cases of severe corrosion damage on the 

landing base and surface casing on oil as well as water wells.

Causes: The damage was occurring in spite of an apparently success￾ful cathodic protection program that has reduced the number of casing 

leaks due to external corrosion damage. The possible causes of the cor￾rosion damage were: leakage of water from surface piping and wellhead 

valves during various operations on water related wells, presence of highly 

saline and corrosive water close to surface in the area, and impediments to 

effective cathodic protection at shallow depths.

Preliminary Solution: In view of the safety and environmental hazards 

associated with possible shallow leaks from corroded casing or failure of 

wellhead equipment, a number of steps have been taken to control the dam￾age. These include regular inspection and repairs at regular intervals, pro￾tection with field-applied corrosion resistant coatings, and a requirement 

to coat all new wells immediately after the rig release.

Lessons learned: Corrosion damage could jeopardize well safety to the 

below-grade wellhead equipment and the upper few feet of the surface 

casing. This was recognized as a potential problem and it could result in 

flow of well fluids outside the wellbore. An effective protection program 

                            

has been implemented that includes regular inspection, standardized

repair procedures, and initial protection of all new wells with protective

coatings as well as sacrificial anodes.

2.1.3.2 Communication

There are no better issues in drilling process safety than communication.

Communication in drilling begins before the first foot is drilled. It begins

in the pre-planning and pre-spud meeting. Communication does not

stop at the pre-spud meeting, rather continues throughout all the various

meetings that are held. At the operator/contractor meeting, which should

be private, operators need to review their respective responsibilities, the

multimedia messaging service (MMS) requirements, the IADC report, the

BOP drills (e.g., reaction and trip drills), land covenants and the BOP clos￾ing-in procedure. The pre-cement meeting is more of a people plan. The

responsibilities of the company supervisor, drilling engineer, tool pusher,

driller, feed pump operator, chief cementing engineer and mud engineer

are all spelled out and delegated.

In addition to good communication at the various meetings, there also

needs to be good communication between the crew and the home office. The 

crew on-site needs to be very thoughtful and detailed in their reports of any

problems. Their communication needs to include the trends and related facts,

their operational plan to correct the problem and their recommendations.

Besides communication between the various parties, there is another

type of communication which is extremely important in a drilling opera￾tion. The driller must learn to communicate with the bottom of the hole. He

can do this through monitoring trends. The various trends tell the driller

exactly what is happening down below and gives him the information that

everyone needs to make critical decisions on a daily basis. In order to see

these trends, they must be written down. Some of these trends that he must

monitor include: i) pressure and stroke trends, ii) torque trends, iii) drag

trends, iv) rate of penetration trends, v) mud trends, and vi) pit trends.

The trends, daily reports, appraisals and other records are all effective

tools in communication. The logging records help the geologists pick their

sites and make better plans. The bit records help the drilling team in their

future bit selection. The reports and records help the engineer do his post￾appraisal of the well. It helps him to determine whether the program was

followed or the deviations were necessary and how future programs can be

improved during planning. Good communication helps management to

properly supervise and optimize their operations.

Good drilling training programs do not merely give out information,

they help drillers, engineers, rig foremen, and service companies learn to

communicate with each other, optimize their drilling operations, and prop￾erly supervise the well. When Bill Murchison started Murchison Drilling

Schools in 1977, he set out five objectives for his Operations Drilling

Technology and Advanced Well Control Course. They were: i) how to

supervise a drilling operation, ii) how to preplan field operations, iii) how

to analyze and solve drilling problems, iv) how to prevent unscheduled

events, and v) how to communicate on the rig. Twenty-four years later,

the same five objectives are helping companies around the world to super￾vise, optimize, and communicate better on the rig. The training has proved

so valuable that many oil companies, contractors, and service companies

have made it standard policy to put all their new service men through

the Murchison Drilling Schools Operations Drilling Technology and

Advanced Well Control Course. It has become part of their overall training

that they receive before going out into the field.

In order for effective communication to take place in that meeting, many

issues must be considered. Here are just a few of those considerations:

1. The meeting must be well planned by the engineer (e.g., he

must meet with a number of people before he even makes

his plans).

The purpose of the meeting needs to be very clearly spelled

out. Here are five purposes for that pre-spud meeting: i) to

open all doors of communication, ii) to reduce unscheduled

events, iii) to review the well plans, iv) to review the geologi￾cal considerations, and finally v) to coordinate the responsi￾bilities between the contractors, service companies and the

operators. The meeting must have an agenda which helps

accomplish these purposes.

3. The meeting needs to have the presence of the right people.

The operator’s superintendent and the contractor’s superinten￾dent both need to be there. The tool pushers and drillers, the

foremen, the engineers, the geologist, the offshore installation

manager and the representatives from the service companies

all need to be at this meeting. Unless all these key individu￾als are at the meeting to both communicate their concerns

with others and come to a mutual understanding of how the

program is to be implemented, the efficiency, profitability and

success of the entire drilling operation is jeopardized.

2.1.3.3 Personnel

Given equal conditions during drilling/completion operations, person￾nel are the key to the success or failure of those operations. Overall well

costs as a result of any drilling/completion problem can be extremely high.

Therefore, continuing education and training for personnel directly or

indirectly involved is essential to successful drilling/completion practices.

For example, four of every five major offshore accidents are caused by

human errors. This highlights the need to make safety, which is the back￾bone of any offshore company’s corporate culture. Over recent years, there

has been a growing recognition of the importance of human factors in the

management of safety-critical industries. Many of the concepts are new to

the oil and gas industry with much of the seminal work and development

of techniques having arisen from the nuclear and aviation domains. These

have set the standard for human factors practice.

Human factors has identified the aetiology of most major incidents as

being linked to human failure. The findings have been that, although most

will have multiple causes, over 80% will have a cause which is related to

human performance. Human factors is a relatively new science. It is con￾cerned with adapting technology and the environment to the capacities

and limitations of humans. The challenge for human factors is to act in a

prescriptive way to make systems and working practices safer and more 

efficient. Many drilling incidents have been found relating to human fac￾tors. However, currently there is not yet a special approach by which drill￾ing safety professionals may rationally evaluate the actual human factor

risk lever and accordingly select appropriate risk control measures for a

given drilling process.

It has been found that more than 80% of incidents are related to human

factors in the global drilling industry. After studying the human error fea￾tures in 59 serious drilling blowout cases from 1970 to 2006 in China, it

shows that the percentage of the human factor as direct cause of a blow￾out incident can reach 93.53%. It includes the individual violation and

management deficiency which reveals the human factor.

Up to now, there is no special approach by which drilling safety profes￾sionals may rationally evaluate the actual human factor risk control and

accordingly select appropriate risk control measures for a given drilling

process. Therefore, it is necessary to create a special method for quanti￾ficational evaluation of the drilling human factor risks, so that strategi￾cally measures can be taken to control the risks associated with drilling

activities.

Many accident investigation techniques and other methods used by the

petroleum industry today list a set of underlying human-related causes and

subsequent improvement suggestions. Norsok (2001) defines an accident

as “an acute unwanted and unplanned event or chain of events resulting in

loss of lives or injury to health, environment or financial values.” Another

way of putting it is energy gone astray (Hovden et al., 2012). What differen￾tiates two accidents is primarily the type and amount of energy astray. The

knowledge of accidents is important in order to operate with efficient risk

management and preventative work. In order to increase the knowledge of

accidents, they must be investigated. Accident investigation models aim to

simplify complex events to something tangible and understandable.

2.1.4 Stacked Tools

Stacked tool is defined as “if a tool is lost or the drillstring breaks, the

obstruction in the well is called junk or fish.” It cannot be drilled through if

there is stacked tool. The preventive measure is to educate the crew. Special

grabbing tools are used to retrieve the junk in a process called fishing.

In extreme cases, explosives can be used to blow up the junk and then the

pieces can be retrieved with a magnet.

Wellbore debris is responsible for many of the problems and much of

the extra costs associated with producing wells, especially in extreme water

depths and highly deviated holes. Even a small piece of debris at the right 

place at the wrong time can jeopardize well production. For this reason,

debris management has become a major concern for oil and gas producers.

Considering rig rates and completion equipment costs, debris removal is

moving into the realm of risk management.

A clean wellbore is not only a prerequisite for trouble-free well testing

and completion. It also helps ensure optimum production for the life of the

well. Debris left in the wellbore can ruin a complex, multimillion-dollar

completion. It can prevent a completion from reaching total depth. It will

never reach an optimum production level. These issues are pushing the

industry to create reliable, efficient systems for quickly ridding of wellbores

harmful debris and larger pieces of junk.

If a tool is lost or the drillstring breaks, the obstruction in the well is

called junk or fish. It cannot be drilled through. Special gripping tools are

used to retrieve the junk through a process called fishing. In extreme cases,

explosives can be used to blow up the junk and then the pieces can be

retrieved with a magnet.

During the stacked tool problem, the remedial measures are run the

junk basket, run basket with collapsible teeth (e.g., “Poor Boy” Basket),

and run magnet.

2.1.4.1 Objects Dropped into the Well

Despite utmost care, wrenches, nut-bolts, rocks or any objects (i.e., rather

than fishing objects) are inadvertently dropped into the borehole while

drilling. In addition, the LS-100 (The LS-100 is a small, portable mud rotary

drilling machine manufactured by Lone Star Bit Company in Houston,

Texas) is often operated near its design limits with a high degree of struc￾tural stress on the drill stems and tools. This will encounter unexpected

layers of very soft sand or filter or hard rock. As a result, it can cause caving

or tool breakage. Sometimes, the entire drillpipe can be lost in the hole.

If objects are dropped into the borehole after the final depth has been

reached, it may be possible to leave them there and still complete the well.

If this is not the case, it may be possible to make a “fishing” tool to set-up

on the lost gear. For example, if a length of well screen falls down the bore￾hole, it may be possible to send other sections down with a pointed tip on

the end and “catch” the lost casing by cramming the pointed end forcefully

into it. These types of “fishing” exercises require innovation and resource￾fulness suitable to the circumstances. While this task may appear to be

routine, there is no single “right” way of conducting this operation. If sedi￾ments have caved in on top of the drill bit or other tools, circulation should

be resumed in the hole and the fishing tool placed over the lost equipment. 

If the lost tools/bits/drillpipes are not critical, it is best to avoid retrieval

efforts, instead, resorting to just changing the location somewhat and start￾ing to drill a new hole. Even if the equipment is important, it is still best to

start drilling at a new location while others try to retrieve it since consid￾erable time can be spent on retrieval and there is a low likelihood of suc￾cess. The decision to retrieve can be set aside while continuing the drilling

operation.

2.1.4.1.1 Case Study

An employee was operating a workbasket inside the substructure while

doing various tasks in preparation to nipple down the annular. He had used

a 5-pound (2.3 kg) shop hammer several minutes prior to the incident in

order to break out the annular hydraulic lines. After he completed the task,

he dropped the hammer to the bottom of the man-basket. While he was

moving throughout the basket to arrange the BOP handler (chain hoist),

the 5-pound (2.3 kg) hammer was accidentally “kicked” out of the basket.

It was “launched” approximately 10 feet (3 meters) down to the driller’s

side of the substructure where it struck another employee on the hard hat.

The impact of the hammer created a pinch point between the hard hat and

his safety glasses, thus resulting in a laceration below his left eyebrow.

Cause of the incident: i) The hammer was not secured or tethered after use,

ii) employees were standing in the “line of fire” watching the employee in

the work basket complete his work, iii) the employee operating the work

basket did not call a “stop task” to move other employees out of the “danger

area”, and iv) poor housekeeping procedures in the work basket (i.e., the

hammer and other items were not placed or secured properly).

Corrective Actions: To address this incident, this company did the follow￾ing: i) employees were reminded of the importance of tethering / securing

any tools when working overhead, even when working in a work basket, ii)

personnel were instructed to discuss “line of fire” for any work, especially

when the potential for a dropped or “launched” object existed, iii) person￾nel were instructed to discuss application of Stop Work Authority (obliga￾tion) and were reminded that SWA includes stopping and asking other

personnel / bystanders to move from a “danger area”, and iv) the JSA /

Work Plan for operating in a work basket must be reviewed/revised to

include the importance of keeping the lift basket orderly (i.e., housekeep￾ing must be maintained).

It is noted that this case study was taken from AIDC website for study

purposes.

Fishing Operations

Fishing is the process of removing equipment that has become stuck or

lost in the wellbore. Its name derives from a period in which a hook, simi￾lar to fishing hooks, used to be attached to a line before lowering into the

borehole in order to extract the lost item. From that period onward, any

object dropped or stuck in a well that interferes with its normal operations

is called a fish and is targeted for removal from the well. The operation of

removing these objects is called a “fishing job”. Typically, in drilling vocab￾ulary, a fishing job is simply called ‘fishing’. The fish, or lost object, is classi￾fied as tubular (e.g., drillpipe, drill collars, tubing, casing, logging tools, test

tools, and tubing) or miscellaneous (e.g., bit cones, small tools, wire line,

chain, hand tools, tong parts, slip segments, and junk). Industry-wide, 25%

of drilling costs may be attributed to fishing. Fishing jobs are classified into

three categories: i) open hole fishing: when there is no casing in the area

of fishing, ii) cased hole fishing: when the fish is inside the casing, and iii)

thru-tubing: when it is necessary to fish through the restriction of a smaller

pipe size (tubing). Figure 2.6 shows the basic fishing tools including the

spear and socket, each with milled edges. Using nails and wax, an impres￾sion block helps determine what is stuck downhole. Anything that goes in

the hole can be left there and anything with an outside diameter less than

that of the hole can be dropped in it. After a fishing job begins, any and/or 

all fishing tools in the hole may themselves have to be removed by fishing.

So precautions should be taken.

The most causes of fishing jobs are i) twist-off, ii) sticking of the drill￾string, iii) bit and tool failures, and iv) foreign objects such as hand tools,

logging instruments, and broken wire line or cable lost in the hole. When

the preparation for a fishing job becomes necessary in an uncased hole,

one has to find out as much as possible about the situation before taking

action. In addition, the questions that need to be answered before fishing

operation are: i) what is to be fished out of the hole?, ii) is the fish stuck, or

is it resting freely?, iii) if stuck, what is causing it to be stuck?, iv) what is

the condition in the hole?, v) what are the size and condition of the fish?,

vi) could fishing tools be run inside or outside the fish?, vii) could other

tools be run through the fishing assembly that is to be used?, viii) are there

at least two ways to get loose from the fish if it cannot be freed?

Any fishing operation in an open or cased hole involves the usage and

operation of the following tools and accessories: i) spears and overshoots,

ii) internal and external cutters, iii) milling tools, iv) taps and die collars,

v) washover pipe – a) washover pipe overshot (releasable), b) washover

pipe back-off connector, and c) washover pipe drill collar spear, vi) acces￾sories – a) bumper jar, b) mechanical jar, c) hydraulic jar, d) jar accelera￾tor, e) hydraulic pull tool, and f) reversing tool, vii) safety joints, viii) junk

retrievers, ix) impression blocks. In a fishing job involving the drillstring,

the operator can often ascertain whether or not the lost drillpipe is stuck

in the hole by determining what happened just before it was lost. The stuck

pipe problems will be discussed in Chapter 7.

History of the Fishing: During early years of petroleum well drilling,

spring-pole cable tools were used instead of rotary drilling. Drillers used a

hook connected by hemp rope to the pole in order to recover drilling tools

inadvertently left in the wellbore. The physical and operational similarity

to the angler’s art resulted in the process of lost tool recovery being named

“fishing” (Moore 1955). The Prud’homme family plantation near Bermuda,

Louisiana, displays in its museum a set of rotary drilling equipment, includ￾ing fishing tools, used to dig three water wells in 1823. A French engineer

designed this equipment, and an African technician built it (Brantly, 1961).

Both rotation and reciprocation were powered by a fifteen-man prime

mover. Most fishing tools were designed for cable-tool drilling and for

production operations, then adapted for rotary drilling. Fishing tools have

been necessary ever since commercial drilling operations were started.

It is generally accepted that fishing operations account for 25% of drilling

costs worldwide (Short, 1995). Since fishing is a non-routine procedure, 

all personnel connected with a given operation are more likely to commit

operational error. Study on fishing art is needed which can be beneficial for

engineering, geological, operational, and accounting staff.

Conventional Fishing: In oil-field operations, fishing is the technique of

removing lost or stuck objects from the wellbore. The term fishing is taken

from the early days of cable-tool drilling. At that time, when a wireline

would break, a crew member put a hook on a line and attempted to catch

the wireline to retrieve, or “fish for,” the tool. Necessity and ingenuity led

these oil-field fishers to develop new attraction. The trial-and-error meth￾ods of industry’s early days built the foundation for many of the catch tools

used currently. A fish can be any number of things, including: (i) stuck pipe,

(ii) broken pipe, (iii) drill collars, (iv) bits, (v) bit cones, (vi) dropped hand

tools, (vii) sanded-up or mud-stuck pipe, (viii) stuck packers, or (ix) other

junk in the hole. There are some conventional fishing jobs such as: (i) wash

overs, (ii) overshot runs, (iii) spear runs, (iv) wireline fishing, (v) stripping

jobs, and (vi) jar runs which are among many fishing techniques developed

to deal with the different varieties of fish.

Some care should be taken when an object is pulled out of the hole with

most tools and fishing so it does not create a swelling action. Care should

also be taken to prevent pulling into a tight place such as a key seat so you

cannot go back down. Fishing jobs are very much a part of the planning

process in drilling and workover operations. Drill operators will often bud￾get for fishing with the increasing cost of rig time and depth, and due to

more complicated wells. When a fishing operation is planned for a work￾over, the operator will work closely with a fishing-tool company to design a

procedure and develop a cost estimate. Taking into account the probability

of success, the cost of a fishing job has to be less than the cost of redrilling

or sidetracking the well for it to make economic sense.

Figure 2.7 shows the bit components such as bit cones, nozzles, and

other pieces of junk which are typically small enough to be retrieved by a

magnet (Figure 2.8) or junk basket (Figure 2.9). The most common fish is

bit cones. Cones are run off for several reasons: i) poor solids control, ii)

poor hydraulics, iii) improper bit choice, iv) operator error such as drop￾ping or pinching, v) manufacturing defects, vi) excessive time on bottom,

vii) inordinately abrasive lithology, and viii) unsuspected junk on bot￾tom. Magnet is used to retrieve small pieces of ferrous material from the

hole. Some junk magnets have circulating ports that enable cuttings to be

washed away from the junk. In general, circulation of drilling fluid lifts

the junk off-bottom. Beneath the tool joint, mud velocity decreases as the

annulus grows wider. This decrease in mud velocity allows the junk to 

use a wedding band (collar clamp). Make-up torque failures can be avoided

by the use of a gauge. Wear can be found by inspection, collar clamp loss

stopped by adequate supervision, and harmonic stress can be minimized

by proper rotary speed. Poor shopping is a matter that is harder to deal

with, and it seems to be on the increase. As shown in Figure 2.10, excessive

torque can cause a drillstring to part downhole. Here (left), the drillpipe

has twisted off beneath the tool joint. Even thick-walled drill collars may

be subjected to wear and fatigue.

Figure 2.11 shows the overshot assembly, which is divided into three

segments. The top sub connects the overshot to workstring. The bowl has 

a tapered helical design to accommodate a grapple, which holds the fish in

place. The guide helps position the overshot onto the fish.

Fishing for Bit Cones, Tong Dies, and Small Tools: When the bit is on the

bank and the small junk is in the hole, several choices present themselves.

If the hole is mudded up and a fishing magnet is immediately available, go

directly back to bottom and try to catch the fish. If not mudded up, or if a

magnet is not on location, run a used bit below two junk subs and attempt to

bust and wash by the junk. If no hole can be made, mud up and call for a junk

basket. When it arrives or mud up is complete, round trip placing the junk

basket on bottom. Cut hole equivalent to the length of the junk basket and

withdraw from the hole. The junk basket is similar to a core barrel and will

retain the fish and core by means of retainer springs. If the fish is recovered,

drill ahead. If not, run a used bit and attempt to drill and wash by them. If no

hole can be made, mill the junk with a concave mill. The concavity will center

the cones or tools and bust them up. The two junk subs should remain in the

string until the iron has been accounted for. Especial care should be taken to

remove all metal junk from the hole before a diamond or P.D.C. bit is run.

Milling: Besides the dressing of fish tops, mills are used to grind up junk

(Figure 2.12). They are also used to cut casing windows, to ream out

casing, to cut fishing necks, and to mill up tubulars that cannot be fished

(e.g., drillpipe cemented in the hole). Clustered tungsten carbide such as

Klustrite is used to face mills. Larger particles are used for milling larger

objects. Too much weight will knock the larger particles off the mill face.

High speed and high weight certainly do not invariably yield high rate of

penetration. One or two magnets should be used in the possum belly and

cleaned continuously while milling. Cuttings are known to build up in the

stack, which should be inspected and cleaned as needed.

Fishing Equation: The decision to fish or not must be weighed

against a need to preserve the wellbore, recover costly equipment or 

  

comply with regulations. Each choice is fraught with its own costs, risks

and repercussions. Before committing to a specific course of action, the

operator must consider a number of factors: i) well parameters: proposed

total depth, current depth, depth to top of the fish and daily rig operating

costs, ii) Lost-in-hole costs: the value of the fish minus the cost of any

components covered by tool insurance, iii) fishing costs: daily fee for

fishing expertise and daily rental charges for fishing tools and jars, and

iv) fishing timetable: time spent mobilizing fishing tools and personnel,

estimated duration of the fishing job and the probability of success.

Earlier research has derived equations to determine how long fish￾ing operations can be economically justified. These were based on Gulf

of Mexico wells. The work presented here investigates the economics of

fishing in the North Sea. The effort was justified by an early BP task force

review, which showed that the Gulf of Mexico and the North Sea have sig￾nificantly different sticking problems. In the Gulf of Mexico, most stuck

pipe is due to differential sticking. Spotting a diesel based pill is considered

to be the most successful remedy. In the North Sea, mechanical sticking is

the major problem and the best remedial action is less obvious. Spotting a

pill is only one option among a number of possible options. In 1984, Keller

et al. (1984) introduced the concept of Economic Fishing Time (EFT).

They developed an equation to calculate the time at which the cost of fish￾ing becomes equal to the cost of an immediate side-track. They considered

that probability of successful fishing can be estimated as:

The fishing times an EFT were characterized using the Weibull distribution

which has the following probability density function (PDF)

Economic Considerations: There is an important trade-off that must be

considered during any fishing operation. Although the actual cost of a

fishing operation is normally small compared to the cost of the drilling

rig and other investments in support of the overall drilling operation, if

a fish or junk cannot be removed from the borehole in a timely fashion,

it may be necessary to sidetrack (i.e., directionally drill around the

obstruction) or drill another borehole. Thus, the economics of the fishing 

operation and the other incurred costs at the well site must be carefully

and continuously assessed while the fishing operation is underway. It is

very important to know when to terminate the fishing operation and get

on with the primary objective of drilling a well. In such case, Eq. (2.1) can

be rewritten in terms of number of days (Df

) that should be allowed for

fishing operation as:

Optimum Fishing Time (OFT): OFT is an economically attractive alter￾native to EFT because it attempts to minimize total costs. When fishing

operations are started, there are only two possible outcomes: getting free or

failing to free. The costs associated with these outcomes are:

OFT is the point at which the gradient becomes zero which can be

derived from Eq. (2.11) as: