otc-7576-ms dewatering and commissioning the cats

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OTC 7576

Dewatering and Commissioning the UK Central Area Transmission

System (CATS) Pipeline

G.L. Langford, T,E. Krisa, and M.L. McCrary, Amoco Production Co.

Copyright 1994, Offshore Technology Conference

This paper was presented at the 26th Annual OTC in Houston, Texss, USA., 2-5 May 1994.

Thie peper was ealec ted for presentat ion by the OTC Program Committ ee fol lowing review of Information conta ined In an abstract submiftad by the author(a ). Contents o f the pap+3r,

as presented, have not been reviewed by tha Offshore Technology Conference end are subject to C=Jrrectlonby the author(e) . The mater ial, as presanted, does not necasssrily ref lect

anv rmelt ion of the Offshore Technolow Conference or ita off icers. Permission toCOPYIsrestricted to an abstract of not more than S00 words. I lluatrat lone may not be copied. The abatract

shi ti ld centa in consp lcuoua acknowledgment o f where and by whom tha paper Is“prasented.

ABSTRACT

A rnor?oethyler?eglycol swabbing technique was used to

successfully cfewafer the 36” diameter, 410 kilometer,

Central Area Transmission System CATS gas pipeline in

the United Kingdom, and condition it for hydrate inhibition.

The chronicle and analysis of the operation provides

insightful information for design of future dewatering and

commissioning operations. The process worked so

effectively that the pipeline was not only conditioned, but

was dried such that target product water dew points were

met shortly after start-up. The method was flexible and

integrated well with other associated project construction

work. The duration of the actual dewatering/conditioning

operation was significantly less than that for the commonly

used vacuum drying method. The keys to the success

were the excellent sweep efficiency of the pig/glycol train

and the aggressive and flexible execution philosophy.

INTRODUCTION

The Central Area Transmission System consists of an

offshore riser platform in the Central Graben Area of the

North Sea, a 410 kilometer long 36” diameter high-

-pressure subsea gas pipeline, and an onshore gas

receiving and metering terminal at Teesside on the

northeast coast of England. The Central Graben

Development, comprising the installation of the North

Everest and Lomond production platforms, and the CATS

Figures at end of paper

581

project, were executed concurrently. Please refer to ‘

Figure 1 for a schematic diagram of the production and

pipeline facilities and their relative locations. The Riser

Platform, located adjacent (and bridge connected) to North

Everest, receives dry gas from the North Everest and

Lomond platforms and transfers the production do the

CATS pipeline. The CATS pipeline has capacity to

transport additional gas production from future platfortns in

the central North Sea.

*

Referring again to Figure 1, the CATS pipeline runs from a

pig launcher on the Riser Platform to landfall at Teesside,

402 kilometers away. From there, the onshore portion of

the pipeline traverses a beach valve station, runs under the

River Tees in a flooded tunnel, and terminates at a ,pig

receiver at the gas terminal. The onshore pipeline is

approximately eight kilometers long.

The North Everest and Lomond platform and CATS

pipeline projects were executed with a “fast-track”

approach, in order to deliver gas by April 1, 1993, as

required by contractual agreements. Detailed engineering

kicked-off in April 1990. The pipeline was completed and

ready for service on schedule.

To meet the schedule, glycol swabbing, rather than

vacuum drying, was used to condition the pipeline. The

glycol swabbing technique used to dewater the CATS

pipeline was a major contributor to project success. The

dewatering and commissioning objectives were to

dewatet’, condition to avoid hydrates, and fill the pipeline

— — ——— ————



2

DEWATERING AND COMMISSIONING THE UKCENTRAL

OTC 007576

AREA TRANSMISSION SYSTEM (CATS) PIPELINE

with sales quality gas from shore, prior to platform

completion, start-up, and gas production. Despite

numerous equipment problems and delays due to both

conflicting operations and bad weather, this method

allowed these objectives to be achieved.

Pipeline dewatering was accomplished using an eight-pig

train with monoethylene glycol (MEG) for water absorption

and hydrate inhibition. The train was propelled from the

onshore terminal to the offshore Riser Platform using dry

air. Valve damage resulting from the pigging operation

necessitated a variation to the original commissioning

program to allow repair. Rather than displace the remaining

air from the line after valve repair by pushing a buffer pig

train with gas from the onshore UK gas grid, the air was

displaced by low pressure nitrogen. The nitrogen was

followed by gas without involving the use of additional

pigs.

EXECUTION

-Pi~elineDewatering

Equipment was mobilized at Teesside to facilitate the

launching of eight hi-directional polyurethane pigs using

alternating slugs of MEG and dry air. The trailer and skid

mounted equipment spread included seventeen air

compressors, six air dryers, two glycol pumps, and four

glycol tanks. The design called for a nominal flow of

17,000 m3/hr of 1800 kPag dry air to propel the pig train at

an average speed of 0.5 mh. Equipment mobilization at

the Riser Platform was minimal, as the overboard water

dumpline had been previously fabricated and installed by

the offshore hook-up contractor. A manifold allowed

pipeline water to be routed overboard upstream of the pig

launcher for maximum flow during the dewatering process,

or through the launcher, as required, to recover the pigs.

Refer to Figure 2, Riser Platform Pig Launcher Schematic.

The pig train was launched using the CATS Terminal

pipeline receiver as a launcher. This presented no problem

as the lines to the closed drain had been valved such that

they could be used as the “kicker” line. Refer to Figure 3

for a schematic of the CATS Terminal pig receiver. The first

pig was launched with compressed dry air. A buffer of

1500 linear meters of dry air at pipeline temperature and

pressure was injected prior to launching the second pig

with glycol. A 260 m3 slug of glycol then preceded the

launch of the third pig with dry air. This procedure was

repeated with alternating slugs of compressed air and MEG

separating the remaining five pigs. Figure 4 depicts the pig

train. An inhibitor was injected into the third glycol slug to

provide corrosion protection to the pipeline wall.

Launching the pig train took place as planned without

incident. Pig train location was determined by calculating

582

the linear distance traveled based on the metered volume

of air injected at Teesside, and by the volume of water

discharged at the Riser Platform.

With the actual pig train speed of 0.34-0.37 m/s, the

estimated time for the first pig to arrive at the Riser Platform

was approximately two weeks. However, within a week of

launching the pig train it became apparent that a conflict

with the installation of the bridge-linked North Everest

Platform would necessitate a slow-down of the pig train.

The Riser Platform would be left unattended during this

operation. No hazard was identified with continuing the

overboard dumping of water from the pipeline, but

manning would be required to recover the pigs. Thus a

slowdown was attempted to delay the arrival of the pig train

at the Riser Platform. The potential schedule conflict with

the weather-dependent platform installation was

recognized when the pig train was launched. The

likelihood of the conflict was low and the risk was accepted

in accordance with an aggressive project schedule

execution philosophy.

Although the attempt was made to just slow the train

speed, continuing weather delays extended the North

Everest installation period until a complete and continued

stoppage became inevitable. Access to the Riser Platform

was unavailable for twenty-three days, during which time

the pig train stopped twice. Meanwhile, the batteries for

the ultrasonic flow meter measuring the overboard volume

ran down, leaving metered air injection at Teesside as the

only method of determining pig train location. The first pig

arrived at the Riser four days after restating the pig train.

The pigs were recovered on the Riser Platform using the

pipeline pig launcher as a receiver. This proved difficult

because of its vertical orientation. An internal basket was

fabricated by the launcher manufacturer to catch and retain

the pigs for removal two at a time. However, the first pig

jammed in the bottom of the basket with a piece of a

welding bladder remaining in the pipeline from a previous

operation. The pig was jammed so tightly that it could not

be removed from the basket. The wear on the pigs was

minimal, therefore much of the oversized outside diameter

of the pig discs remained upon arrival at the Riser. Without

excessive disc wear, the pigs exerted sufficient

circumferential pressure on the pig launcher internal walls

to support their weight, and the decision was made to

abandon the use of the basket and recover the pigs

individually.

The second problem encountered in recovering the pigs

was a loss of sealing capability in the pipeline riser valves.

The first two pigs were recovered using the main valve

below the pig launcher for isolation, as customary.

_. —



OTC 007576

G. L. LANGFORD, T. E. KRISA AND M. L. MCCRARY

3

However, the valve started passing at a rate that prevented

the launcher from being vented down upon attempting to

remove the third pig. The emergency shutdown (ESD)

maintenance valve was then used to isolate the pipeline for

pig removal. This method was successful for recovering

the second two pigs before it, too, began passing to the

extent that the launcher could not be vented. The

remaining pigs were then successfully recovered by

closing both the pig trap isolation and the ESD

maintenance valves and leaving the overboard dump line

open to atmosphere.

The recovered glycol met the established acceptance

criteria and the dewatering was complete. Piping re-

instatement, valve investigation and repairs, and nitrogen

leak testing of the Riser topside gas process piping were

then carried out. The pipeline had to be vented to perform

the valve repairs. A gauging pig run (discussed in a later

section) also became necessary prior to commissioning.

N-Moue

n/Hydrocarbon Gas Pura

ina and Line Packinq

The pipeline was purged with atmospheric pressure

nitrogen using a nitrogen pump/vaporizer at Teesside and

opening the pipeline to flare on the Riser Platform. The

target flowrate was 3,000 m3/h, with a minimum flow of

1,200 m3/h. These flowrates were required to prevent

excessive nitrogen-air interface mixing, likely when

velocities fall below 0.5 m/s. A total of 1-1/2 pipeline

volumes of nitrogen were injected to purge the line.

Following the introduction of the first line volume, sampling

of the purge gas commenced on the Riser Platform. Once

the oxygen COtWfIt

measured

SO/.

or

k?SS,

pUrging Of the

deadleg piping on the Riser topsides was carried out.

Purging continued until the entire volume of nitrogen had

been injected at Teesside.

Hydrocarbon gas-up of the pipeline commenced

immediately after nitrogen purging. Natural gas from the UK

gas grid was used for this purpose. The gas purge was to

be at atmospheric pressure, similar to the nitrogen purge,

though the gas supply pressure was 6000 kPag. This

necessitated the use of a water bath heater to warm the

gas to at least -5°C to offset the cooling effect of the

pressure reducing valve, and thereby avoid hydrates. After

injecting )7Y0 Of a tine VOkH?N? Of natUral gas intO the

pipeline, sampling commenced on the purge gas at the

Riser Platform. Once the hydrocarbon content of the gas at

the Riser reached 950/., the pipeline was shut-in and gas

injection at Teesside continued in order to pack the line,

completing the commissioning process. The topside gas

piping at the Riser was left with a nitrogen blanket,

500 kPag above the final pipeline pressure, to prevent

hydrocarbon gas from entering the process piping prior to

platform commissioning and start-up.

The purging and commissioning operations were

successful with no problems encountered. The interfaces

between air and nitrogen, and between nitrogen and

hydrocarbon gas, were distinct, with only a fifteen-to-

twenty minute time duration between detection of the

purge gas and attainment of acceptance criteria.

DESIGN OF DEWATERING PROCESS

The objective of the dewatering/commissioning process

for the CATS pipeline was to bring the pipeline into

operation as quickly as possible, while meeting the delivety

gas dew point target as early as possible in its operating

life. The alternatives considered for the drying operation

were: (1) bulk water removal by pigging, followed by

vacuum drying, (2) swabbing with a methanol train, and

(3) swabbing with a glycol train. Vacuum drying, which

would have given the highest likelihood of achieving the

delivery gas dew point early in operating life, was not used

because of the long time period for the drying process.

Even without layout limitations at the Riser Platform end,

the duration of the vacuum drying process was

unacceptable considering project schedule constraints.

Glycol swabbing was chosen in preference to methanol,

because the low volatility of glycol results in glycol

remaining with any residual water in the pipeline after the

operation to inhibit hydrates. Glycol also poses a lower fire

risk than methanol.

To achieve the glycol swabbing, the glycol train was

designed to travel from the onshore terminal to the

offshore Riser Platform. The prime factors for flowing in this

direction were the available space onshore to arrange the

large air compression system, and the availability of a large

supply of gas from the national grid to provide an initial fill

for the pipeline.

The key elements of the pig train design were as follows:

9

583

Two 260 m3 slugs of glycol were expected to be

adequate based on experience. A third slug was

added as a safety margin to reduce the possibility of

failure which would result in the requirement to repeat

the operation.

Air was used as the propelling agent for the pig train to

enable the running of a second train if the glycol

dryness criteria was not met.

The buffer zones between glycol slugs provided the

ability to sweep bypassed ~quids and allowed liquid

accumulations in the subsea tees to drain into the

pipeline

for removalby the following pigs

— — ——— ————

— ——.



4

DEWATERINGAND COMMISSIONING THE UKCENTRAL

OTC 007576


Though the pipeline was internally coated to facilitate

gel cleaning prior to dewatering, the design of the

dewatering pigs did not consider reduced roughness

of the pipe. This was because the reduced disc wear

afforded by the coating could not be quantified.

Success for the dewatering operation was established

as a 95”A glycol concentration in the final slug of glycol

arriving at the Riser Platform. This was selected as a

level which would ensure that any residual water in the

line was well-inhibited for hydrates during start-up

operations.

A few additional problems were anticipated during the

design stage of the pipeline. The subsea safety valve at

the Riser end of the pipeline was a swing check valve. This

valve had to be capable of being held open to allow pigs to

traverse the check in either direction. Any materials carried

by the pig train had the potential to damage the ESD valve

at the platform. The risks associated with this potential

problem were accepted, because the valve was designed

so that it could be repaired in place, and the pipeline would

be gel cleaned after construction. The final glycol slug

contained a corrosion inhibitor to protect the pipeline from

corrosion induced by residual water, until the drying

process was completed and the pipeline was in normal

operation.

PIG DESIGN AND TRIALS

uLQ.wm

A number of design characteristics were identified as being

necessary for a successful pig design. Foremost, the pig

was to be a hi-directional design to offer maximum flexibility

in case pig sticking problems were encountered. The

sweep efficiency of the pigs needed to be high to minimize

the residual water on the pipeline wall. Thus, the pig

sealing discs were oversized to 1070/. of the pipeline

internal diameter to allow for wear as the train traveled the

410 km length of the pipeline. The discs were constructed

of a proprietary “high performance” polyurethane for

additional strength and wear resistance.

The pig design was further complicated because the pigs

were required to navigate the short radius ells of the tunnel

under the River Tees, and had to pass through the check

valve at the base of the Riser Platform in the reverse

direction. The pigs needed to be long enough to “bridge”

the drop-off in the check valve, but not so long that they

would have difficulty passing through the short radius ells.

It was also desirable to keep the weight of the pigs to a

minimum, again to help prevent the nose sections from

dropping down and hanging in the check valve, and also to

reduce the energy required to propel the pig train. Finally,

the front and rear sections of the pigs featured nose cones

to aid the pigs in traversing the check valve. See Figure 5

for a general arrangement drawing of the final pig design.

-

To ensure that the pigs would be able to navigate the ells

in the pipeline and pass through the check valve in the

reverse direction, pigging trials were conducted. Sections

of 24” pipe were joined with ells and a mock-up of the

check valve. A 24” pig scaled down from the dimensions

for a 36” pig was used in conducting the trials. Tests

included propelling the pig with both compressed air and

water, and with varying degrees of simulated disc wear.

The test pig passed through the ells and mock check valve

in all cases, significantly increasing confidence in the pig

design.

The pig trials also confirmed the pressure drop necessary

to propel the pigs, thus verifying the validity of the

assumptions used for determining the compression

requirements. The trials were beneficial and are highly

recommended for future applications where the design

effectiveness is uncertain.

PROCESS RESULTS

From a process and operating standpoint, the success of

the dewatering far exceeded design expectations. The

first glycol slug met the acceptance criteria of at least

95’ glycol for the swabbing operation and the latter two

slugs exceeded the criteria substantially. When production

operations began in May of 1993, the gas in the pipeline

met the dew point target for delivery gas shortly after stait-

up. Dew point data for the gas arriving at the onshore

terminal for the first two weeks of operation is shown

below. (The data shown is a spot reading at the same time

each day for the first two weeks of operation. Flow rates for

the first few days were very low and represent a very small

volume of gas. Additionally, there were typical start-up

problems with the dew point analyzers and, particularly in

the early days, calibration was in question).

Dew ?olnt

C) Dew Point ~ Q

1 6.7

8

-22.1

2

5.8 9

-24.2

3

-14.4

10

-28.7

4

-18.5

11 -29.4

5

-21.9

12

-28.9

6

-23.9 13

-26.5

7

-22.7 14

-27.1

No liquids were collected at the onshore terminal. Overall,

the dewatering process was a complete success, resulting

584



OTC 007576

in an essentially dry pipeline,

inhibited condition expected.

SCHEDULE

G. L. LANGFORD, T. E.KRISA AND M. L. MCCRARY

5

rather than the wet, but

The dewatering and commissioning operations took place

over a five month period, due to the schedule conflict with

the North Everest production platform installation, valve

testing and repairs, a tanker grounding incident, and

weather delays. Excluding mobilization, demobilization,

and uncontrollable stoppages and delays, however, the

principal dewatering and commissioning activities were

accomplished in seventeen days and eleven days,

respectively. The following table lists key dates during the

dewatering and commissioning operations:

Nov. 2

Nov. 11

Nov. 13

NOV.

16

Nov. 20

Nov. 21

NOV.

25

NOV.

27

NOV. 28

NOV.

29

Dec. 10

Dec. 14

Dec. 17

Dec. 23

Jan. 19

Feb. 22

Mar. 2

Mar. 6

Mar. 13

Mar. 14

Mar. 16

Mar. 17

Mar. 18

Mar. 25

Mar. 26

Commence personnel and equipment

mobilization

Launch first dewatering pig (No. 1)

Launch last dewatering pig (No. 8)

De-man Riser Platform in anticipation of North

Everest Platform installation

Compressor spread shut down 8 hours to slow

pig train

Pig train travel ceases

Pig train commences movement at low speed

Pig train travel ceases once more. Operation put

on hold until personnel able to re-board Riser

North Everest installation complete

Temporary access to Riser allows closing of

dewatering discharge valves

Regain access to Riser. Compressor spread re-

started, pig train commences moving

Pig No. 1 recovered

Pig No. 8 recovered

Commence Riser mainline valve seat testing

Testing complete

Repairs to 36 ESD valve complete

ESD Valve and Riser topside piping leak testing

complete. Tanker drags anchor across pipeline

route during storm

Commence gauging pig run

Gauging run complete

Commence nitrogen purging

Receive consent to operate pipeline from UK

authorities (required to introduce hydrocarbon

gas)

Nitrogen purge complete, commence

hydrocarbon purge

Hydrocarbon purge complete, pipeline shut-in,

continue line packing

Pipeline at 4600 kPag, line packing complete

Receive Certificate of Fitness for Riser Platform

{required to start operation)

Mar. 27 Demobilize pipeline commissioning equipment

Much of

and personnel

the delay during the valve testing and repair

period was due to extended off-station time for the flotel

because of weather. A key to success is starting activities

as soon as possible to allow for unavoidable delays. Even

with the problems and delays encountered, the CATS

pipeline was ready for operation before the April 1, 1993

target date.

The glycol swabbing technique afforded substantial

schedule savings compared to vacuum drying. The actual

vacuum drying process could not have started until the

bulk of the water had been removed by a pigging

operation. Therefore, the several months required to

vacuum dry could not have started until essentially the end

of the glycol swabbing operation.

EXECUTION ISSUES

@ioment ~out and @ace R~emen

Space requirements for launching the dewatering pig train

were significant, with the compressor spread required to

generate 17,000 m3/hr of 2000 kPag compressed air, and

the ancillaty dryers, glycol, diesel and nitrogen tanks,

stores, control cabin, pig handling equipment, etc. The

space required did not present a problem because the

train was launched from an onshore location. Equipment

layout and spacing, as well as weight limitations on the

platform, must be considered in the case of using this

method between two offshore locations. Minimal

equipment was required at the Riser Platform for receiving

the pig train and consisted of small equipment stores and a

control cabin.

The glycol required for the three 260 m3 slugs was

delivered by five road tankers making eight round trips

each, wohing on a 24-hour basis. Delivery and storage

logistics are therefore another critical consideration. In

order to ensure that adequate preparations are made and

facilities are available, the commissioning contractor should

be consulted as early as practical during the design phase.

orafy PlOework

The original design for the temporaty pipework on board

the Riser Platform called for a hard-piped dewatering

manifold with a 12“ flexible hose routed overboard. This,

however, proved to be impractical. The weight and size of

12” flexible hose of appropriate pressure rating makes it

difficult to handle, and requires large arcs to bend.

Furthermore, air slugs tend to generate large forces that

require robust piping

and supports

for strength and rigidj?y.

The final piping configuration consisted of 12” steel piping

585



6

DEWATERING ANDCOMMISSIONING THE UK CENTRAL

OTC 007576


from the manifold to just above sea level, where a

12“ flexible hose was attached to absorb the wave action

as it discharged subsea. This arrangement proved

satisfactory.

Mete nalp _

i ia Train Po

sition/Tracking

Pig train location was determined from volumetric

calculations of air injected behind the pig train and water

discharged on the Riser Platform. The air was measured

with an orifice meter and the discharge water was

measured using an ultrasonic flow meter. Originally, the

primary tracking mechanism was to be the water discharge,

with the air measurement as a

he k However, during the

extended delay, the battery pack for the ultrasonic flow

meter ran down, and cumulative water discharge readings

were lost. Though such a delay was considered during the

design, the actual delay exceeded the battery capacity.

The metered air values had compared favorably with the

water discharge values prior to losing the ultrasonic meter,

so confidence remained in the ability to use the metered air

values to track pig position. Large air slugs in front of the

pig train as it approached the Riser Platform further

reduced the effectiveness of the ultrasonic meter. The

ultrasonic meter was, however, helpful in determining the

pig train velocity. The discharge flow measurement device

was designed early in the project, prior to involvement by

the commissioning contractor. It would have been

beneficial to have had the pipeline commissioning

contractor involved in its design to ensure correct

operation and functionality, calling on experience with

similar operations.

The first and last pigs were equipped with electronic

“pingers” that could be used to locate the pig train

accurately in the case of it becoming stuck. A more

complex system that would have allowed precise tracking

during the operation could have been utilized, but the

additional cost was not justified. The pipeline had been

gauge pigged and gel cleaned prior to being flooded for

hydrotest, therefore the likelihood of the pig train

becoming stuck was considered low, and the pingers

provided adequate ability to locate the pig train should it

have become necessary.

Process Inter

m

An expected key to a successful dewatering operation was

maintaining a pig train velocity between 0.1 and 0.5 tis to

achieve the required pig sweep efficiency. In practice, the

minimum velocity was not maintained, and the train

stopped completely on two separate occasions, once for

four days and again for thirteen days. In spite of these

stoppages, the glycol slug concentrations were well within

the 951?40cceptance criteria. This success is believed to be

due to the combined effect of oversized pig discs to effect

a good seal, and the strength of the high-performance disc

material, which prevented the pigs from settling during the

stoppages. The internal coating of the pipeline significantly

reduced disc wear and probably helped disc sealing.

Pia Ret

rieval

The original procedure called for recovering the pigs in

batches, primarily to simplify the recovery process with

regards to the number of times the pig receiver had to be

vented, opened, and reclosed. However, it became

necessary to recover the pigs individually after the first pig

became stuck in the bottom of the retrieval basket and the

basket was removed and abandoned.

The design of the pig recovery procedure should consider

the possibility of valve seat leakage. Contingencies should

be developed to accommodate such an occurrence. Also,

additional valves may be necessary around the pig trap

pipework if pig receivers are to be used as launchers and

launchers as receivers. Early review of pig trap design and

appurtenance piping by the pipeline commissioning

contractor is beneficial. The design should be specific for

the pigs involved and consider the likelihood of debris. A

recovery basket should be avoided in the interest of

simplicity if possible.

.RiserMainline

Valve Problems

As previously mentioned, valve seat leakage became a

problem during the pig train recovery operation. The ESD

maintenance valve and the pig trap isolation valve were

both used in the pig recovery operation, and began

noticeably leaking after two to three losures The pig train

was successfully and safely recovered after modifying the

venting method as described earlier. The use of

compressed air as the propelling mechanism allowed the

acceptance of minor valve leakage during pig recove~.

Although not operated during the pig recovery operation,

the ESD valve began passing during testing that followed

the pigging process,

again showing evidence of

worsening with continued valve closures. Both the ball and

seat of the ESD valve were severely scored. The specific

cause of the damage was never determined. The quantity

of debris recovered from in front of the pigs was less than

one cubic meter, which was small considering the size of

the pipeline. One possible cause of the valve damage was

the oversized pig discs lifting the spring loaded seats off

the ball and allowing dirty water and debris into the valve

cavity. Valve operations following this may have scored the

seats and ball. Oversizing the bore of pipeline valves may

reduce the possibility of this occurring. During similar

dewatering operations on the 20” gas pipeline between

Lomond and the Riser Platform, valve damage did not

occur. For that operation, the ESD valves were removed

586

— — ——— ————

— ——.



OTC

007576

G.

L. LANGFORD, T. E. KRISAAND M. L. MCCRARY

7

and inhibited water was flushed through the cavities of the

remaining valves. It is not possible to make a direct

comparison between the two operations, however,

because of differences in pipeline size, length, and valve

manufacturers.

The ESD maintenance valve and pig trap isolation valve

were of all-welded construction, and therefore not suitable

for in-situ repair. Fortunately, they were not leaking as

badly as the ESD valve and were brought into acceptable

standards by injecting sealant into the seat rings. A top

entry valve was chosen for the main ESD valve

construction, which allowed the valve to be repaired in

place. The repair consisted of replacing the ball, seats, and

all other sealing components.

Following these repairs, all three valves were rigorously

tested with high pressure nitrogen and proved to be

acceptable. A valve for injecting nitrogen into the area

between the ESD maintenance valve and the ESD valve

was added to the pipeline during this time in order to test

the downstream seat of the ESD valve without pressurizing

the entire pipeline. Although the original intent during the

design of the pipeline was to minimize the number of leak

paths below the ESD valve, a pressure injection point in

this location was necessary to properly test the sealing

integrity of the ESD valve prior to placing the pipeline in

service.

Ta

nker Groundina Damaae Assessment/Gaua na ~

Following the repair and testing of the mainline valves at

the Riser Platform, and just prior to the nitrogen purging

operation, a tanker ran aground in the immediate vicinity of

the shore approach at Teesside during a storm. As part of

the resulting investigation, a gauging pig train was run past

the point of concern to insure that the pipeline had not

been damaged by the vessel or its anchors. The train

consisted of three pigs separated by glycol slugs.

The pipeline was first pressurized with air to 600 kPag to

provide a driving force to allow recovery of the gauging

pigs at the terminal, rather than at the Riser Platform. The

train was then launched from Teesside using compressed

air. After the pig train had traveled sixteen kilometers, the

terminal side was vented, allowing the pipeline pressure to

propel the pig train back to shore. The pig train stopped in

the tunnel under the River Tees, and required additional

pressure to dislodge. Fortunately, the river crossing is

between the terminal and the onshore beach valve station.

Air compression equipment was mobilized at the beach

valve station and the line segment further pressurized until

the pigs were dislodged. Upon recovery, the gauging pigs

were examined and found to be in satisfactory condition,

and further investigation was deemed unnecessary.

587

SAFETY/ENVIRONMENTAL/START-UP ISSUES

Safet Considerations

A hazard analysis of the procedure for dewatering and

commissioning the CATS pipeline was performed to

identify safety risks that could be eliminated or reduced.

The analysis team was composed of engineering,

operations, safety, and commissioning contractor

representatives. The study was beneficial in contributing to

the safe performance of the dewatering and

commissioning operations.

All dewatering and commissioning activities were carried

out using a work permit system. This ensured that all

parties on location were aware of the operations being

performed and that appropriate safety procedures were

followed. it also helped in preventing conflicts with other

ongoing hook-up activities. Impromptu safety meetings

were heId prior to each critical activity to reinforce the safe

performance of the operations.

f20mmunicat ons

Communication between the onshore and offshore

locations was initially provided by a patched radio link

through a nearby platform, then through the permanent

platform telecommunication system when it became

available.

The flotel ship-to-shore radio was available as a

backup if required. Levels of communication were

established prior to commencement to ensure proper

transfer of information during critical activities or in case of

an emergency.

E~

A lockout of the ESD systems at the terminal and Riser

Platform was necessary during the pigging operations to

prevent spurious alarms from inadvertently closing the

ESD valves. Considerable damage to the valves and pigs

would have occurred if a valve closed while a pig was

passing through. An operator was designated to manually

trip the actuated ESD valve in the event of an emergency.

The lockouts were removed and the systems thoroughly

tested prior to the introduction of hydrocarbon gas.

9VefbOad Rum rw

oi f Inhibited Water and Glycoj

The pipeline volume of inhibited hydrotest water and the

three MEG slugs were dumped directly overboard during

the dewatering operation. The concentrations of biocide,

corrosion inhibitor, and oxygen scavenger in the hydrotest

water were low enough to allow this, and MEG is

biodegradable. No environmental damage was incurred,

and approval from the Scottish Office, Agriculture and Fish

Division Marine Laboratory was received prior to obtaining a

Gonsent to Discharge from the Department of Trade WI

Industry.



8

DEWATERING AND COMMISSIONING THE UKCENTRAL

OTC 007576


en Leak Tests

Following the removal of temporary pipework and

reinstatement of dismantled permanent pipework, the

Riser topsides gas process piping was leak tested with

high pressure nitrogen using helium as a leak tracer. The

purpose of the testing was to ensure gas containment at

normal operating pipeline pressure upon start-up. This

testing was performed at the Teesside terminal as well.

.

ctlon of Hydrocarbon Gas

Safety was always a high priority during the dewatering

operations, but became even more so during the gassing-

up operations, as this was the first introduction of

hydrocarbon gas to the offshore facilities. From this point

on, hot work permits were required with appropriate

operations coordination and approval for all activities.

Though the gassing-up procedure was developed by the

commissioning contractor, all valve operations were

performed directly by, or under the guidance of, the

permanent facility operators.

After closing in the pipeline for line packing, the Riser

Platform topside process and flare system piping was

purged with nitrogen. The piping section immediately

above the ESD valve was pressurized with nitrogen to

500 kPag above the pipeline pressure to ensure that any

seat leakage of the isolation valves would be to leak

nitrogen into the pipeline and not hydrocarbon gas ,into the

topsides piping.

The final operation was to mobilize a diving vessel and crew

to lower the flap on the subsea check valve located at the

base of the Riser Platform to its normal operating position.

KEY POINTS FOR FUTURE PROJECTS

The Central Area Transmission System (CATS) pipeline

was successfully dewatered and commissioned using

glycol swabbing followed by a low pressure nitrogen purge

to achieve a low dew point gas delivery on start-up. The

following key points were identified during the process and

should be considered in future applications:

The glycol swabbing method provides a reliable means

of removing residual hydrotest water from a pipeline to

achieve a low dew point gas delivery on start-up.

The combination of the oversized, high-performance

polyurethane sealing discs and the pipeline’s internal

coating resulted in high sweep efficiency for the pigs.

Using proper methods and technique, large pipelines

can be purged of air and commissioned without the

use of separation pigs.

588

w

Pig train stoppages are not necessarily detrimental to

the success of the glyml swabbing method.

Pigging trials are beneficial in designing and proving

critical application pigs.

Open and early discussion and coordination with

vendors, contractors, engineering, and operators are

crucial to the success of the operation.

The implications of pigging through valves need to be

considered early in the design phase and options

explored to minimize or handle valve problems if they

occur.

Careful thought should be given to the design of

tempora~ pipework, especially overboard dumplines.

The pipeline commissioning contractor should be

consulted early in the facility design for issues such as

pig retrieval methods, temporary pipework and

blinding, equipment requirements and layout, and

metering applications.

Activities must commence as early as possible, and

sufficient time allowed in schedules to accommodate

unexpected delays and problems. Contingency plans

should be developed as appropriate for all potential

eventualities.

ACKNOWLEDGMENT

We thank Amoco (UK) Exploration Company and Nowsco

Pipeline Services for their cooperation in preparing this

paper.

— — ——— ————

— ——.



Figure 1

Central Graben Development Project and

Central Area Transmission System CATS Pipeline

Valve ’1’yp

r“:z’?l “0”’”

/

Beach Valve

/

Station

/

36” CATS

589



Figure 2

Riser Platform Pig Launcher Schematic

To Flare

@ I

t

13as Fmm

Pmcesa

-P

To Pipeline

Flare

t-

i

1

I

T

Pressurizing

r’ -1

Sample Point

Tempora~

Maintenance

I

. . . .

Level

[

,Irs,m

u

Ultrasonic

Flowmeter

sea

To Ov&board

F@me 3

CATS Terminal Pig Receiver Schematic

I

I To Flare

I

l---M________

t 1~ I Ill 1

36” CATS

I

Pipel ine c

I

I

l ’b

alve added to

36” CATS

Pipeline

---

I

I

I

I

1

I

I

I

I

1

enable use as

I

“Launcher”

I

-——- -——— ———— ———— —

———

4

I

Glycol/Air/Purgeas

Injection Point

+TOclOsedDrain PILOOP

4

addedfor Gss

Pig Launching

Commissioning

Injection Point

590

— — ——— ————

— ——.



Figure 4

36” Pipeline Dewatering Pig Train Schematic

PIG NO. 8

+ +p’GNO”’

PIG NO. 7

PIG NO. 5 PIG No, 4

PIG NO. 3

PIG NO. 2 PIG NO. 1

I

L MEG slug s Linear Metera (Typ,)

I

Dry Air Slug -1500 Linear Metera (Typ. )

n

Dry Air a t Work ing

Preseure

Hydrotest

Water

F@re 5

CATS 36” Dewatering Pig General Arrangement

Seali ng Disc

support D isc

Nose Cone

591

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otc-7576-ms dewatering and commissioning the cats

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