MAJOR ASIAN SUBSEA PIPELINE TO START UP THIS YEAR

May 1, 1995
J.S. Wilburn ARCO China Inc. Shenzhen, PRC Peter M. Roberts Oman Oil Co. Ltd. Houston Yacheng 13-1 Pipeline: Major Events Gas supplies for Hong Kong power generation will start flowing later this year through a 440 mile, 28-in. subsea pipeline from the Yacheng 13-1 gas development in the South China Sea. Commissioning is set for third quarter, possibly September (see accompanying box). The line will be the region's first major (and the world's second longest) subsea pipeline, after
J.S. Wilburn
ARCO China Inc.
Shenzhen, PRC

Peter M. Roberts
Oman Oil Co. Ltd.
Houston

Yacheng 13-1 Pipeline: Major Events

Gas supplies for Hong Kong power generation will start flowing later this year through a 440 mile, 28-in. subsea pipeline from the Yacheng 13-1 gas development in the South China Sea.

Commissioning is set for third quarter, possibly September (see accompanying box).

The line will be the region's first major (and the world's second longest) subsea pipeline, after Zeepipe in the North Sea (OGJ, May 3, 1993, p. 42).

The Yacheng 13-1 gas project is under development by ARCO China Inc. acting as operator with its partners China National Offshore Oil Co. (Cnooc) and Kufpec (China) Inc. The field lies around 90 km south of Hainan Island, People's Republic of China (PRC) and nearly 800 km from Hong Kong.

Condensate-rich gas, containing some CO2 and H2S, is to be produced and processed through two offshore platforms in water approximately 90 m deep.

Field development is to take dry gas to a power station in Hong Kong and condensate in two-phase flow with gas to Hainan Island (Fig. 1).

Natural gas will be transported via a 28-in. OD pipeline to shore facilities in Hong Kong at the Black Point power station. Natural gas and condensate will also be transported via a 14-in. OD pipeline from the Yacheng 13-1 gas field to shore facilities on Hainan Island. Design life for each pipeline is 40 years.

Table 1 gives pipeline characteristics.

All installation work has been completed, and the pipelines have been flooded prior to hydrotest and precommissioning.

SCHEDULING CONSTRAINTS

Gas must be available at the new Black Point power station later this year. Contracts were let for detailed design, procurement, and project support in spring 1992, after agreements on gas sales and feasibility studies.

At that time, however, no route survey had been made and the only wave, wind, and current data available were inferred from historical records of regional weather data, rather than Project-specific measurements.

Thus, the first characteristic of the overall schedule was the need to accommodate within the detail design the processes of preliminary engineering whereby such parameters as route survey, diameters, wall thicknesses, and coatings are established for a cost-effective balanced design.

The second schedule challenge was to resolve within the given time constraints the appropriate pipeline installation technique needed to minimize cost at acceptable schedule risk levels. Options included the following:

  • Use a high lay-rate, third-generation lay barge. Expect a high mobilization cost and higher day rate for fewer days and less susceptibility to weather downtime. Wait until the end of the 1993 typhoon season to start laying. Let the coating yard build up a sufficient inventory of coated pipe to support the high lay rate once started.
  • Use lower lay-rate second-generation lay barges. Expect to avoid high mobilization costs and obtain a modest day rate for more days. Begin sufficiently early before the 1993 typhoon season to achieve the scheduled end date. And expect less pressure on the coating yard and less need for coated line-pipe inventory.

Prefeasibility work, going back over the previous decade, had tended to favor local second-generation barge utilization.

At the beginning of detailed design in April 1992, the objective was to award construction contracts by November 1992 so that the option of using second-generation barges could be preserved, leaving both 1993 and 1994 available for pipe-lay.

This objective was addressed in two ways:

1. Bids for line-pipe purchase and coating were solicited against a range of pipe sizes and grades, before design was completed. Full environmental design data were not available until spring 1993.

2. The main length of offshore trunkline was put out to bid ahead of the more complex but shorter Hainan Island and Pearl River estuary sections, which were allocated to separate contracts.

The trunkline inquiry was built around specially summarized scope-of-work route documents because full data were not at that time available.

To preserve possible economics of scale or other synergies associated with one contractor taking several contracts, however, the Pearl River estuary and Hainan Island contracts were scheduled for parallel evaluation and award later in the winter of 1993-94.

Fig. 2 summarizes the resulting schedule.

The marketplace favored the larger, higher lay-rate barges. A very wide range of semisubmersible vessels was offered by the pipe-lay industry, and the initial project pressure to get out to early bid provided space in the schedule to consider competing bids.

The flexibility which lay behind this interactive approach to the overall schedule and the holding of optional positions well into detailed design were deemed to have been successful.

ENVIRONMENTAL CONDITIONS

Beginning in April 1992, ARCO China had prepared for survey and environmental data collection prior to detailed design. The survey vessels worked May through September 1992, and final data were received during fall-spring 1992-93.

Two concerns in particular were evident at the outset.

The area had no previous oil industry infrastructure and no "go-by" environmental data. The area is also subject to May-December typhoons.

Data available at the outset were from ship-borne wind measurements and were generated excluding the influence of typhoons: ships avoid typhoons and therefore don't record their effects.

Marine pipeline design has evolved in mature provinces where the environmental data are highly developed. Design techniques which can be used substantially to reduce capital costs require data much more sophisticated than regional historical records.

For example, an entire series of presumptions on seabed-soil quality and current directionality were in effect for the Yacheng trunkline.

Application of the American Gas Association's methodology to the analysis of the pipeline's seabed stability resulted in an unexpected opportunity to remove all pipeline trenching from the scope-of-work of the main trunkline contract.

This substantial saving was only as real as the hypothetical data on which it was based. Field data commissioned by ARCO China, when subsequently received, validated the presumed figures.

To address the requirement for acquiring and evaluating environmental data for the 28-in. Yacheng trunkline, the 14-in. Hainan Island pipeline and the platform site, ARCO China let two contracts.

The first scope included collection of oceanographic and meteorological data such as currents, waves, and water levels at locations along the pipeline route from January 1992 to April 1993.

The second was to develop a model to predict the environmental conditions along the pipelines routes. Once the model was developed, the data collected were compared with model predictions to calibrate and refine the model.

The focus on typhoons shouldn't blur the importance of regular, mon-soon-driven winter weather. This is sufficiently severe significantly to interfere with the progress of an older second-generation barge, for example. In fact, even the third-generation lay-barge experienced in the order of 5% weather downtime.

Also in the Pearl River estuary, the stability of a trenched but not artificially backfilled pipeline was found to be controlled by winter monsoon rather than summer typhoon weather.

The major parameter in this unexpected result was the current force from freshwater runoff down the estuary after winter rains.

ROUTING

The importance of picking the right route for a major subsea trunkline is evident when it is remembered that a meandering route is longer and more costly.

Of much more significance, however, is avoiding mud slides, boulder fields or sharp changes of gradient, and most of all excessive pipeline freespans over seabed corrugations. This is because only subsequent, costly subsea intervention can rectify excessive spans.

Fig. 3 shows a profile of the trunkline. Cross-reference to Fig. 1 shows an area of seabed corrugation to the southeast of Hainan Island. This had been indicated in earlier data but not surveyed in detail.

The water is too deep for these features to be sandwaves; they are now presumed to be old river beds from an earlier geological age. The route selected can be seen to deflect to the south of the straight line route to miss the worst of these ridges.

The interface between the trunkline and Pearl River estuary work was set early in the project at a water depth of 16 m. The 28-in. trunkline has no discontinuity, however, at this point.

The estuary work was segregated because of the complexity and duration of the expected engineering work, to permit an early barge commitment to be obtained on the 707 km of trunkline installation while design was proceeding on the 71 km of line through the Pearl River estuary and into Black Point in Hong Kong.

The 16-m contour was chosen as an approximate minimum working draft for the larger lay vessels which it was surmised would be bidding the deeper water trunkline. All pipeline trenching below this depth was also subsequently eliminated by design. The estuary route eventually selected is shown in Fig. 4.

A reasonable amount of environmental and shipping data was initially available in the public domain, and the first decision necessary was to select from two optional route strategies:

  • Run up the west side of the estuary in very shallow water to obtain protection from shipping interaction; or,
  • Cross to the east side of the estuary in deeper water to reduce the amount of trenching needed.

WESTERN ROUTE

The selection process for the estuary pipeline route is too involved adequately to be discussed here. But in summary, the selected route, the western option, responds to several factors:

  • Along much of the route, the water depth is just sufficient to give access to a shallow draft lay barge. Only smaller craft use these shallow waters. The hazard to the pipeline presented by the size of vessels able to ply these water depths is negligible.
  • Significant volumes of oceangoing shipping sail north-south into the Pearl River to and from Guangzhou. Also, there is significant shipping between Lan Tau Island and the mainland using the Urmston Roads between Hong Kong and Guangzhou.
  • It is appropriate to protect with rock armor a pipeline which crosses such shipping channels. Crossing them at right angles and at their narrowest points is therefore both safer and more cost effective. The chosen route gives an effective crossing of the Zhoujia, Linding, and Urmston Roads channels.
  • There are several vessel anchoring and turning zones off Black Point. The chosen route picks its way through these.

There are several environmentally sensitive areas in the estuary. These include breeding grounds for the rare white dolphin and oyster beds. The route avoids these.

The selected route is not without its difficulties.

The Hong Kong construction industry has a voracious appetite for sand. This is dredged from a layer which underlines the muddy seabed in several locations around the outer harbor. Typically, a 20-m layer of sand lies 10-m below seabed.

Generally, a square is excavated that leaves a 30-m deep pit in the sea bed, which is then refilled with mud dredged from above the alluvium in the next pit or with other spoil.

The permitting of these activities is carefully managed by a committee representing various agencies of the Hong Kong government, including those concerned with environmental monitoring and control. The PRC government is responsible for the regulation of traffic and marine traffic safety in this part of the Urmston Roads.

Both the PRC and Hong Kong governments reviewed the design for this area. The 28-in. trunkline passes through one such pit in the Urmston Roads, excavated in the 1980s and currently being refilled with mud from the new airport workings (Fig. 5). The crossing of the Urmston Roads in this area was achieved by a combination of mud dilution and dredging.

Installation of this part of the line has been completed. It should be noted that several issues relating to this extremely complex piece of pipeline engineering were addressed.

These issues included detailed agreement with the authorities on the management of the interface between construction vessels and commercial shipping, the permitting of dredging and spoil dumping activities, and the monitoring and control of water quality during construction activities.

LANDFALLS

Pipeline landfalls in areas such as Hainan Island, which experience cyclonic weather conditions, are among the more difficult aspects of marine pipeline design. Beach face erosion can expose pipelines in the landfall area.

Core samples in the beach area revealed the presence of a coarse sandy material; the inference is of a high wave-energy environment at the beach from which fines have been leached.

Diffraction analysis of the littoral environment disclosed a potential for beach face erosion and sediment transport at approximately a 5-year return period in tropical-storm conditions. The analysis technique was developed by the U.S. Corps of Engineers and published in 1984. (Users of the technique are cautioned that substantial upgrading and correction have since been made but remain under preparation for publication.)

The mechanism of erosion is one in which, in tropical storms, steep, short-period waves move beach material out into the near shore area, forming a sand spit or bar. Under normal conditions, the material tends to come back to the beach over a period of time.

Survey data revealed a near shore lens of fine sand, suggesting that the as-surveyed beach and near shore seabed was part-way through this cycle.

A pipeline trench depth of 2 m to top of pipe below natural nearshore seabed was selected to give clearance below the lens of fine sand, thus giving confidence of nonexposure throughout the sediment transport cycle.

Analysis of littoral sediment migrations also provided insight into the suitability of the landfall. The location is actually considerably sheltered in the lee of a hill, which takes energy from the predominant direction of storm waves.

It is also to be expected that the exposed boulders in the shallows either side of the landfall will absorb further wave energy, although no way is known of quantifying this benefit in a practical way.

The Hong Kong pipeline landfall is into a large reclaimed area of the harbor on which gas reception facilities and the Black Point power station are being built. A "gateway" through the sea wall was left for a conventional pipeline pull.

The schedule pressure on survey data and lack of good "go-by" data necessitated real-time route selection on the survey vessel.

One on-the-spot rerouting decision is noteworthy: the need to take the trunkline south of the seabed corrugations, already mentioned, which lie southeast of Hainan Island.

Having a pipeline engineering capability aboard the survey vessel with computerized span-assessment techniques reduces the chances of having to remobilize the vessel to seek alternative route diversions if spanning problems subsequently become apparent.

The Pearl River estuary route selection was much more complex, as already noted. Having real time route-assessment capabilities aboard the survey vessel was unnecessary because several alternate routes were surveyed.

HYDRAULIC ANALYSIS

The concept of a dedicated pipeline feeding a power station is familiar. What may be less commonly understood is the effect of this on pipeline design.

The pipeline had to be capable of 12-hr on/12-hr off operation. It was designed, in effect, as a storage reservoir, supplying gas by day and repacking by night. Fig. 6 shows a typical pressure characteristic for inlet and outlet pressures of the 28-in. trunkline.

A major consideration was the duration of platform downtime which could be tolerated without delivery pressure falling below specification. It depends on which time of day the supply interruption occurs.

Hydraulic analysis was undertaken to investigate several supply-interruption scenarios at different times in the demand cycle and with differing line sizes. The confirmation of 28-in. diameter was influenced as much by consideration of supply integrity as by conventional flow-rate calculation.

Using internal fusion-bonded epoxy coating to increase delivery was considered but ruled against, however, because wall friction played a relatviely minimal role in achieving design requirements.

The effect of pipeline inventory was, conversely, significant.

To maximize economy of line-pipe purchase, extensive cooperation among operator, pipeline designers, and platform designers kept maximum operating pressure and design pressure as close to each other as possible.

A 17.12-mm W.T. was selected for the 28-in. line, Grade X 65. This non-API wall thickness was selected to give the lowest possible cost by minimizing steel quantities within code allowables.

The line was designed to ANSI B31.8 which is considered the code most compatible with an economic design. Heavier, 25.4-mm W.T. line pipe, Grade X-65, was selected for the riser.

Buckle-propagation calculations showed a need for buckle arrestors in the deeper water sections. Cost-risk optimization dictated placement of a buckle arrestor every 600 m, or 50 pipe joints, in the vulnerable area.

One final consequence of the cyclic pressure fluctuations in the line was a concern for fatigue. The line was demonstrated to be acceptable within appropriate code provisions.

The cumulative fatigue fraction through life was, however, significant, and it is believed that a concern for fatigue might on similar projects be found to militate against the use of X-70 or higher grade steels.

For the 14-in. Hainan Island line, the range of input parameters is unusually wide for a multiphase line because gas demand will be a consequence of infrastructure development in southern Hainan Island.

PIPE PROCUREMENT, COATING

A formal prequalification exercise was made of worldwide pipe mills for the 28-in. and 14-in. line pipe. Compliant and competitive bids were received from several areas including Japan and Europe.

An order was subsequently placed early in 1993 for supply from seven Japanese mills and for delivery to the coating yard.

The 28-in. pipe in both wall thicknesses is double submerged arc welded (DSAW) formed by the UOE process. The thin-wall, 14-in. is high frequency electric resistance welded (HF-ERW), continuously produced pipe. The heavy-wall, 14-in. line pipe is seamless.

The use of HF-ERW line pipe subsea is still less common than seamless. ARCO China took advantage of successful ARCO experience in other areas of the world again to select HF-ERW.

The specification features heat treatment and non-destructive testing (NDT) measures designed to give assurance of a level of integrity appropriate to subsea use and the gas composition. ERW pipe is considerably less expensive than seamless.

A pipe-coating location at Zhanjiang where there is an existing offshore supply base was considered. The Zhanjiang site required some land reclamation and upgrading of pipe-handling facilities, plus access dredging. Zhanjiang can be seen in Fig. 1 to be roughly an equal distance from each end of the 28-in. trunkline.

A site at Basno Gang on Hainan Island was also considered.

Other existing pipe-coating facilities are located in Malaysia and Indonesia.

Competitive bids were solicited from several contractors at all the above sites. The 28-in. trunkline installation was bid with all the options for coated-pipe storage locations.

Both coal-tar enamel and fusion-bonded epoxy were bid as technically adequate corrosion-coating options. A source of epoxy was believed to be available in PRC but the additional electrical power needed in its application was a drawback.

Zhanjiang and coal-tar enamel were eventually selected.

EXPANSION; COMMISSIONING

Provision has been made for the possibility of expanded gas delivery in the future. The optimum location of an intermediate compression platform was determined and the minimum tie-in facilities consistent with supply continuity specified.

The projected tie-in facilities are shown in Fig. 7.

Should a compression platform need to be installed later, a by-pass will be installed.

The central part of the 28-in. line will then be isolated by the double-block-and-bleed valves and replaced by piggable lines to and from compression facilities.

Precommissioning of the Yacheng 13-1 gas project pipelines began Apr. 15.

The trunkline underwent the longest travel of the first pig for a subsea pipeline and is the longest line designed to meet base delivery without intermediate compression.

The precommissioning is therefore a major undertaking. The pipelines are being vacuum dried and filled with low-pressure nitrogen. The line has been successfully flooded and gauged.

The use of internal cleaning prior to pipe load-out and of gels in the flooding pig train has proven sufficient to handle residual debris and oxidation. This experience revalidated the decision to not coat the line internally.

REGULATIONS

The Peoples Republic of China has developed a suite of regulations relating to offshore work that more resemble the proscriptive European style than the North American style of self-regulation within the law.

The PRC requires offshore pipelines in its waters to be certified by a recognized agency. By contrast, this is not a requirement in either the U.S. or U.K. and more stringent in its implementation than Norwegian regulations.

PRC authorities approved the route, the design, arrangements for construction vessels crossing shipping channels in the Pearl River estuary, and they also approved environmental impact provisions. The route approval was received from the State Oceanographic Administration and the Ministry of Communications.

Certified medical fitness and survival training are required for offshore workers. There is a genuine understanding by the PRC authorities of the destructive capability of tropical storms and of the need for evasion or evacuation procedures for offshore craft.

In the case of the Yacheng trunkline, an environmental impact assessment has been needed as part of the approval procedures with PRC authorities.

During the manufacturing process, the PRC authorities have taken an active role in satisfying themselves that any material entering the PRC meets the relevant codes and standards as being fit for its intended purpose.

Teams of inspectors from the Commodities Inspection Bureau of the PRC have undertaken NDT examination of incoming line pipe and have reviewed pipe mill quality assurance/quality control facilities.

Copyright 1995 Oil & Gas Journal. All Rights Reserved.