Page 67: of Offshore Engineer Magazine (Feb/Mar 2015)

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East Africa

Dry gas reservoirs characterized by a very narrow conti- may simply move the problem from one increase the probability of slug ? ow and nental shelf. To the south, it is less than Although oil has been discovered location to another further downstream.

high pressure drops. Pipeline routes will 5km wide. However, to the north, it can offshore Kenya, the major ? nds to date For short tie-back distances this may therefore need to be selected that minimize extend out to about 50km. Water depths have been dry gas reservoirs offshore be manageable, but the optimum solution these undulations in order to reduce the on the shelf are up to 400m. Mozambique and Tanzania. One of the would be to reduce the inventories of impact of terrain-induced slugging and

This is followed by a steep drop to the development options being considered is water and solids. This would have the overall system pressure drops.

ocean ? oor and water depths of more ‘subsea to beach’ full wellstream transfer High pressure drops are also undesir- added advantage of debottlenecking pro- than 2000m. Slope gradients are typically with no ? oating production systems. able if sand or solids enter the production cessing facilities later in ? eld lives, when up to 10%, although these can exceed However, dry gas reservoirs are de? ned system. The increasing gas velocity may water handling facilities may become 15% in places. The seabed then slopes as having no hydrocarbon liquids pres- eventually exceed the erosional velocity capacity-constrained. more gently downwards to water depths ent. They will contain formation water. and the particles may damage pipework. In order to achieve this, three-phase in excess of 4000m. As these reservoirs are pro- subsea separation and pumping 0

The ? oor itself has a large number of duced, pressures and temperatures with produced water and sand -200-200 -400-400 ridges and sea mounts that are hundreds within the production systems reinjection is required. While this -600-600 of meters in height. There is also a series will fall. Water, initially present is a mature technology in water -800-800 of deep underwater canyons running out as vapor, will begin to condense. depths less than 1000m, further -1000-1000 perpendicular to the shoreline that are hun-

These systems will therefore need development and quali? cation -1200-1200 dreds of meters wide and kilometers long.

to operate, at least in part, in the would be required for deeper -1400-1400

Water depth (m)

All of these obstacles, the very undu- multiphase region. water applications.

-1600-1600 -1800

In addition, since water will be lating terrain and steep slopes will pose 0 50,000100,000150,000200,000

Pre-investment present, it is likely that liquid cor- signi? cant challenges for identifying

Route length (m)

Offshore East Africa, pipelines rosion and hydrate inhibitors will suitable routes and landfalls for future Typical seabed pro? le and ? owlines are likely to require need to be injected for transporta- ? owlines and pipelines.

N more complex seabed routings in tion over relatively long distances.

In the Indian Ocean to the north of

S order to avoid major obstacles and

This will increase the volume of

Madagascar, the surface current circula-

S to minimize the impact of pressure liquids in the production systems tion is normally clockwise, although drop. Subsea boosting facilities and push them further into the there is an equatorial counter-current

Plan (aerial) view

S may then be retro? tted where and multiphase region. in winter. However, to the south of 500 when required in order to maintain

Madagascar, the current changes direc- 00 50 100 150 200 250 300

Multiphase ? ow production and increase reservoir tion and circulates counter-clockwise. -500

As pressures and temperatures recovery. However, pre-investment

Between Madagascar and the mainland, -1000 S

S fall, the gas velocity increases. should be made in spool pieces, tie- the Mozambique current ? ows from -1500

S

Although a production system with in points and spare umbilical hoses north to south and is magni? ed by the

Elevation low liquid loading might start in to simplify subsequent installation.

channeling effect between the two land

Possible pipeline route with tie-in points for subsea the strati? ed ? ow regime, the effect This will require a truly multi- masses. pressure boosting.

of the increasing gas velocity will disciplinary project approach to

There are also two monsoons. The be to move it into strati? ed wavy ? ow and, develop designs that are suitable for instal- north-east monsoon or kaskazi brings In addition, in low ? ow or no ? ow condi- eventually, into slug ? ow. Each of these lation and operation, both initially as well dry air in from the Persian Gulf from tions, gravity becomes dominant. With ? ow regimes has a greater pressure drop as during possible retro? t modi? cations

November to April and the warm, moist insuf? cient gas velocity to carry liquids per unit length, so pressure drops will later in ? eld life. kusi monsoon blows in from the south- and solids, they will drain downhill and increase as the ? uid moves closer to shore.

east from April to October. The slightly back into the production system. This article is based on a presentation

With the pressures and temperatures cooler kusi brings the heavier rains, from In the worst cases, the system may made in October 2014 at the DNV GL decreasing, more liquids will condense. late March to early June. There is then become liquid locked or blocked and the Pipeline Day London 2014.

The presence of more liquids increases the a second rainy season in November and reservoir pressure available on restart may likelihood of a change in ? ow regime, lead-

December. These may therefore place not be large enough to clear these accu- Alex Hunt is the ing to increasing pressure drops. These in limitations on installation activities. mulations, leading to loss of production. founder of Woodview turn cause more liquids to condense. The

Because this is a frontier area, the oil Overall, although the shortest distance Technology Ltd., a situation therefore escalates.

and gas industry currently has limited between two points may be a straight consultancy special- operational experience and knowledge. When the ? uids reach the base of the line, the most ef? cient route to minimize izing in the identi? -

The seabed bathymetry is now being continental shelf, the production systems pressure drop and liquid or solid build-up cation, development surveyed and mapped. There is maritime will need suf? cient pressure for the may not be the shortest or straightest. and implementation information available on surface currents, ? uids to be able to climb the slope and of emerging and new

Managing liquids wind speeds and directions. reach the shore. The back-pressure in the technologies for the oil and gas industry.

However, more data is needed on system will limit natural ? ow and reduce In order to avoid production systems With over 30 years’ experience, he has the strengths and directions of currents reservoir recovery unless some form of becoming liquid-locked and to offset previously held positions with Texaco, through the water column. Long-term data pressure boosting is provided. the effects of pressure drop as reservoir Total and BG Group, developing technol-

Slugging has two main causes. pressures decline, one option is to retro? t ogy strategies and establishing and gathering campaigns using acoustic dop-

Hydrodynamic slugging is governed by pler current pro? lers (ADCPs) will provide some form of subsea pressure boosting. managing portfolios of research and gas to liquid ratios and ? ow velocities. the information required for the design of

Two-phase gas-liquid separation and development projects. He also lectures on

However, terrain-induced slugging is risers and moorings for ? oating production multiphase pumping or compression ? ow assurance, subsea issues, deep water related to the seabed topography. The major offer one alternative. However, as the technologies, emerging trends and systems and will also support the planning undulations of the East African seabed will volumes of produced water increase, this technology needs.

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