1PC Semi White Paper

In our ever changing and uncertain market conditions, it is extremely important to ensure capital investments are successful and economically viable. As we exploit new fields in difficult and harsh environments, the Paired Column Semisubmersible (PC Semi) brings a number of advantages to deepwater developments.

Thoroughly engineered, model tested, qualified, and approved in principle by DNV GL, the PC Semi is a uniquely innovative alternative to the conventional semi. The PC Semi utilizes column pairs that allow for exceptional steel catenary riser (SCR) performance in a wet tree configuration, and permits drilling and the support of surface trees in a dry tree configuration.

By splitting the larger single corner column of a conventional semi into two smaller columns, platform motions are improved and thus the functionality of the platform is also enhanced. The PC Semi increases safety, lowers cost, improves project delivery time and is riser friendly.

When compared to a conventional four-column semisubermsible, Atkins’ PC Semi can cost 17% less. This comparison is based on a 10,000 mt topside payload in the central Gulf of Mexico in a water depth of 5,000 ft, with eight risers and an oil production rate of 80,000 bpd. This CAPEX saving includes the engineering, fabrication, installation and pre-commissioning of the hull, mooring, risers and topsides.

New Semisubmersible Design Increases Safety, Lower Costs, Improves Project Delivery Time and is Riser Friendly

Fabrication of hull - PC Semi cost is 18% less than the conventional semi

Dry transport of hull - PC Semi cost is 18% less than the conventional semi

Topsides deck fabrication - PC Semi cost is 7% less than the conventional semi

$ M

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Semi Hull CAPEX Differentials

PC Semi in Central

GoM

Conventional Semi in

Central GoM

748

39

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Pile

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713

Mo

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Hull and topsides integration - PC Semi cost is 20% less than the conventional semi

Mooring and foundation procurement, fabrication, transport and installation - PC Semi cost is 23% less than the conventional semi (PC Semi needs only eight lines while a conventional semi requires 12 lines)

Riser procurement fabrication, transport and installation - PC Semi cost is 30% less than the conventional semi (PC Semi uses standard SCRs while a conventional semi requires SLWRs in this example)

The key cost differentials between the PC Semi and the conventional semi are:

900

880

860

840

820

800

780

760

740

720

700

2PC Semi White Paper

Distribution by Execution Element

PC Semi (MM USD)

Conventional Semi (MM USD)

Hook Up and Commissioning, $19.4, 3%

Transportation, Installation and Logistics, $128.1, 17%

Project Management $22.5, 3%

Engineering $74.5, 10%

Procurement $232.4, 31%

Construction and Fabrication $271.1, 36%

$748.0 $876.5

Transportation, Installation and Logistics, $171.9, 20%

Hook Up and Commissioning, $19.4, 2%

Project Management $22.6, 3%

Engineering $73.6, 8%

Procurement $264.6, 30%

Construction and Fabrication $324.3, 37%

PC Semi Topsides, $296.3, 40%

Conventional Semi Topsides, $303.4, 35%

PC Semi Hull, $249.6, 33%

Conventional Semi Hull, $289.0, 33%

PC Semi Foundations, $23.2, 3%

PC Semi Mooring, $42.3, 6%

PC Semi SCRs, $136.7, 18%

Conventional Semi Foundations, $29.9, 3%

Conventional Semi SLWRs, $199.0, 23%

Conventional Semi Mooring $55.5, 6%

PC Semi (MM USD)

Conventional Semi (MM USD)

Distribution by Functional Element

$748.0 $876.5

The following graphs summarize the total cost comparison between the PC Semi and a conventional semi. This does not include integrated owner team costs, global project contingency, EPCI fees, or intangibles such as risk associated with additional offshore exposure during installation of a conventional semi, and the technical risk associated with lazy wave riser and its installation.

3PC Semi White Paper

A conventional semi has four large vertical columns arranged in a square pattern and four horizontal pontoons connecting the lower portions of the columns. These columns form a foundation to support the topsides and provide buoyancy and stability in both pre-service and inplace conditions. Unfortunately, the key design characteristics of a conventional semi are strongly coupled, making it difficult to achieve a “balanced” solution. This difficulty is described below:

• To achieve suitable motion characteristics for steel catenary risers (SCRs), platform draft is increased which leads to an increase in column spacing or column size in order to meet stability requirements.

• Increasing column spacing will lead to an increase in deck steel weight which will offset any gains in stability.

• Increasing column size will lead to more diffraction and wave crest enhancement under the deck resulting in an increase in column freeboard to meet minimum airgap requirements. Stability will be adversely affected due to the resulting increase in vertical center of gravity.

• Larger column spacing or column size will result in larger environmental loads due to wind, waves and current resulting in larger mooring line sizes and/or number of mooring lines to meet offset limit requirements.

On the other hand, with its ability to efficiently support the topsides deck with the inboard columns and optimize stability with the outboard columns, the PC Semi addresses the above issues in the following creative manner:

• Topsides deck support is decoupled from stability to promote a more efficient and compact design.

• SCR performance is greatly improved because of low motions and pontoons arranged closer to the center of the platform.

• Reduced column size means the PC Semi is much more transparent to waves and current which reduces wave exciting loads and improves airgap.

• The PC Semi is extremely well-suited for either wet tree or dry tree developments in the most severe metocean environments.

• Conventional off-the-shelf riser tensioning equipment is viable due to reduced vertical motions and low riser stroke.

In addition to the above, the PC Semi has better damage tolerance, flexibility on topsides deck/hull design, and is expandable and easily scalable. The PC Semi does not require the use of high maintenance components such as hinges, suspended hydrodynamic plates, or any other moveable parts.

PC Semi Explained

How the PC Semi Works…

• Larger column of a conventional semi is split into two smaller columns

• Inboard columns support the deck at an optimum structural/

weight spacing

• Outboard columns are spaced to optimize stability

• Column pairs decouple deck support from stability

• Column pairs more transparent to waves, i.e., less load, improved

motions and airgap

• Permits use of top tensioned risers for dry tree developments

• Excellent SCR riser performance eliminating need for lazy wave

steel risers

• Safer and more robust

Conventional Semi

PC Semi

4PC Semi White Paper

The idea for the PC Semi was first introduced in 2008 (Zou, 2008), and it was ultimately selected by Research Partnership to Secure Energy for America (RPSEA) as the winning concept for a dry tree deep water development program. Computational studies were confirmed by wind tunnel and wave basin model tests (RPSEA, 2009 and 2010). A series of subsequent engineering efforts were performed on such contemporary issues as vortex induced motion (VIM) response (Zou et al., 2013) (Zou et al., 2014) and characteristic structural responses including shear forces and pry/squeeze loads (Das and Zou, 2015).

A brief summary of the PC Semi development history is as follows:

A significant effort spanning nearly eight years has resulted in the PC Semi being declared “Major Capital Project Ready” for the Gulf of Mexico. Within this time period, PC Semi technology has also been applied to offshore Western Australia (Zou and Chianis, 2011) (Zou, 2012) and offshore Brazil (Zou, 2012). In addition to its more traditional applications, the PC Semi has also been designed for small floating liquefied natural gas (FLNG) production offshore Western Australia (Zou, 2017).

PC Semi Development History

Introduced by Houston Offshore Engineering (HOE), now an Atkins company2008

Selected by RPSEA in Industry-wide competition2009

Wind tunnel tests completed at Texas A&M University2009

Technology Review with the Bureau of Ocean Energy Management and the US Coast Guard

2009

2010Wave basin model tests performed at Offshore Technology Research Center

Technology Readiness Review by RPSEA2010

2011

2013

2014

DNV GL Technology Assessment and Workshop

VIM model tests completed at Maritime Research Institute Netherlands

Approval in Principle awarded by DNV GL PC Semi Wave Basin Test

5PC Semi White Paper

The key benefits of a PC Semi are measurable, and taken as a whole, form a very strong value proposition when compared to a conventional semi. Safety, low cost, efficient delivery, and its effectiveness as a riser host are discussed.

Safety

• The PC Semi is fully capable of quayside integration. This simplifies offshore installation operations, reduces offshore working hours, and minimizes installation risk. The PC Semi is robust and has improved damage tolerance:

• Outer columns provide protection to the inner columns for accidents such as boat collision.

• An eight-column hull has less impact on stability due to flooded compartment scenarios.

• Paired-column arrangement offers the best protection to all top tensioned risers (TTRs) in the wellbay for the dry tree configuration.

• Paired-column arrangement offers the best opportunity to route all hard piping on the hull away from potential impact locations.

Benefits of the PC Semi

Wet Tow of a Dry Tree PC Semi with Topsides

Whatever Happened to Dry Trees in the Gulf of Mexico?

Of the 42 floating platforms in the U.S. Gulf of Mexico, a great majority accommodate dry trees. Why has this been so? Presumably, the Gulf of Mexico producers utilized dry trees because this type of development increased the value of their investment. Lately, however, there’s been a noticeable shift to wet tree developments. This shift can mostly be attributed to hub-type developments where multiple fields are produced through a single floater or to situations where a single reservoir is too spread-out to accommodate one drilling center. But it also seems true that producers are sometimes opting for wet trees, even if the reservoir enables a single drill center. There are obvious advantages of dry trees, a few of which are itemized below:

• Subsea kit is very expensive, especially for not-yet-approved 20K psi systems.

• Completions and workovers can utilize a small inexpensive rig.

• If it makes sense, full drilling is available.

• For supplemental lift, downhole pumping using proven ESP’s are less costly, more reliable with platform rig access, and utilize far less power when compared to seabed pumping.

• Simpler well maintenance leads to a greater number of barrels produced.

Perhaps a reason for this trend toward wet trees is that traditional dry tree platform types, TLP’s and spars, have at least partially fallen out of favor. The TLP has water depth restrictions. The spar has size restrictions and does not enable quayside integration. Maybe the answer is a low motion, low cost semisubmersible that could trigger a return to more economical dry tree developments. Is the PC Semi this vehicle?

6PC Semi White Paper

Low Cost

At a project level, the PC Semi has cost advantages for both CAPEX and OPEX. Low CAPEX comes from the following aspects:

• The PC Semi is capable of quayside integration.• Deck steel weight is reduced due to optimized inner

column spacing and smaller pry/squeeze loads (Das and Zou, 2015).

• Hull steel weight is reduced due to smaller platform displacement and associated reduction of global shear forces, split loads and pry/squeeze loads (Das and Zou, 2015).

• Response characteristics are improved from wind, wave, current and VIM, resulting in more efficient mooring and riser systems.

• The mooring system can be reduced by smaller line sizes and/or the number of mooring lines.

• The potential need for lazy wave steel risers is eliminated as conventional and lower cost SCRs can be utilized.

• The paired-column hull permits flexibility during fabrication and enables a fast track execution plan. Late increases in topside payload can be handled efficiently.

Due to more favorable motion and VIM response characteristics, OPEX is also low:

• It is not necessary to perform mooring pay-in/pay-out to shift the touch down points of risers in order to mitigate potential strength and fatigue issues.

• There is also no need to adjust chain links at fairleads to mitigate potential fatigue due to out-of-plane and/or in-plane bending.

Besides the specific CAPEX and OPEX cost savings, enabling the use of dry trees on a semi can lead to further savings, such as:

• Drilling from the platform• Completions and workovers utilizing a smaller rig• Downhole pumping using proven ESP’s with far less

power consumption when compared to seabed pumping• Simpler well maintenance which leads to a greater

number of barrels produced

Fast Track Delivery

The spacing between inner columns depends solely on the optimized support location for deck structure and can be determined and frozen at an early stage of the project. The requirements on stability and buoyancy can be met by adjusting either the distance between the inner and outer columns, the outer column size or a combination of both. This can be done at a late stage of the project without much penalty. Thus, the paired-column arrangement permits maximum flexibility from the start of the project throughout its entire project lifecycle.

Benefits of the PC Semi Continued

PC Semi for Dry Tree Application

7PC Semi White Paper

Riser Friendly

Extensive wave basin model tests have been performed to validate the suitability of the PC Semi with a conventional off-the-shelf riser tensioner for dry tree applications. The test results, and associated post analysis (Poll et al., 2013), decisively confirm riser stroke is within an acceptable range, even in the Central Gulf of Mexico region for 1,000 yr hurricanes. Additional studies comparing PC Semi and Truss Spar were also performed (Kumar and Zou, 2016).

VIM for floating deepwater structures plays a significant role on both the strength and fatigue of mooring elements and SCRs. The PC Semi underwent extensive VIM model testing for mooring and SCR design and analysis. The number of columns and column slenderness all contributed to the PC Semi demonstrating highly desirable VIM results. The PC Semi is a proven riser friendly host for both TTRs and SCRs.

Benefits of the PC Semi Continued

VIM Model Testing for the PC Semi: Model Setup (left), Horizontal Trajectory (upper right), and VIM Response Spectra (lower right)

Wet Tree PC Semi Hull with Moorings and Risers

8PC Semi White Paper

A conventional semi for the production of oil and gas has one large column per corner. Creation of a column-pair is a simple geometry change that results in two very important benefits:

1. The inboard columns support the deck, topsides equipment, and all associated environmental loads at an optimum structural spacing. The outboard columns are spaced independently in order to optimize hydrostatic stability.

2. The paired-columns lead to improved motions which enable improved platform functionality.

The decoupling of deck support from stability promotes a more compact and efficient design, and allows topsides and hull designers to work independently. The smaller column pairs are much more transparent to waves and current, thereby reducing wave exciting loads and improving airgap.

The PC Semi has been model tested for critical current conditions and was found to have excellent VIM characteristics. The platform’s low motions and favorable VIM characteristics make the PC Semi an exceptional riser host. Should the development call for a dry tree floater, the PC Semi can also support drilling and dry trees. TTR stroke was found to be within off-the-shelf tensioner design limits. The PC Semi is able to handle harsh deepwater environments in the Gulf of Mexico, Western Australia, North Sea, and offshore Brazil.

Thoroughly engineered, model tested, qualified, and approved in principle by DNV GL, the PC Semi is the previously missing piece from a complete portfolio of deepwater platform concepts: a low motion semi which, 1) allows exceptional SCR riser performance in a wet tree configuration, and 2) permits drilling and the support of surface trees in a dry tree configuration. All critical maturation works have been extensively developed, carefully examined and thoroughly validated. The PC Semi is ready for market.

With over 40 years’ experience in the oil and gas sector, Atkins has been delivering industry defining innovation throughout the lifecycle of onshore and offshore assets on some of the world’s most challenging projects. For more information, visit www.atkinsglobal.com/oilandgas  

Conclusion

Roll Motion RAOs

Pitch Motion RAOs

Heave Motion RAOs

Roll Motion RAOs

Pitch Motion RAOs

Conventional Semi

Equivalent PC Semi

100-year Hurricane

1,000-year Hurricane

1,000-year Hurricane

1,000-year Hurricane

Equivalent PC Semi100-year Hurricane

Equivalent PC Semi

100-year Hurricane

Conventional Semi

Conventional Semi

Wave Period (s)

Wave Period (s)

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9PC Semi White Paper

Zou, J., “Dynamic Responses of a Dry Tree Semisubmersible Platform with Ram Style Tensioners in the Post-Katrina Irregular Seas.” International Society of Offshore and Polar Engineering Conference 2008, Vancouver, Canada.

Research Partnership to Secure Energy for America; RPSEA (2009-2010). “Ultra Deepwater Dry Tree System for Drilling and Production”, RPSEA UDW 1402, Stage 1 Final Report and Stage 2 Summary Report, H08130-G-RPT-GN-15002 & H08130-G-RPT-GN-15003.

Zou, J., Poll, P., Roddier, D., Tom, N., and Peiffer, A., “VIM Testing of a Paired Column Semisubmersible”, Proceedings of International Conference on Ocean, Offshore and Arctic Engineering, June 9 - 14, 2013, Nantes, France. OMAE2013-10001.

Zou, J., Poll, P., Antony, A., Das, S., Padmanabhan, R., Vinayan, V., and Parambath, A., “VIM Model Testing and VIM Induced Mooring Fatigue of a Dry Tree Paired-Column Semisubmersible Platform”, Offshore Technology Conference 2014, Houston, Texas, USA. OTC 25427.

Das, S and Zou, J., “Characteristic Responses of a Dry Tree Paired-Column and Deep Draft Semisubmersible in Central Gulf of Mexico”, SNAME2015, Feb. 2015.

Zou, J., and Chianis, J., “Paired-Column Semisubmersible Platform for Wet Tree with Steel Catenary Riser Application in Offshore Western Australia.” 1st International Conference on Ocean System Engineering, 2011, Seoul, Korea.

Zou, J., “Semisubmersible platforms with Steel Catenary Risers for Western Australia and Gulf of Mexico”, International Journal on Ocean System Engineering, June, 2012, vol. (2), pp 99-113.

Zou, J., “Dry Tree Paired-Column Semisubmersible Platform for Ultra-Deepwater Offshore Brazil.” 17th SNAME Texas Section Offshore Symposium 2012, Houston, Texas, USA.

Zou, J., “Conceptual Study of a Paired-Column SemiSubmersible Platform for a 1.5 MTPA FLNG”, SNAME2017, Feb. 2017.

Poll, P., Zou, J., Shi, S., and Kurup, N., “An Evaluation of Strength, Fatigue and Operational Performance of Dry Tree Semisubmersible Riser Tensioning Equipment”, Offshore Technology Conference 2013, Houston, Texas, USA. OTC 23926.

Kumar, B and Zou, J., “Comparative Study of a Paired-Column Semisubmersible and a Truss Spar with Direct Vertical Access Feature in Ultra-Deep Water of West Gulf of Mexico,” SNAME2016, Feb. 2016.

References

© Atkins Ltd except where stated otherwise

www.atkinsglobal.com/oilandgas

Robert (Bob) Harrell, Director281.436.6203

robert.harrell@atkinsglobal.com

Chris Sherertz, Director of Business Development713.576.8550

christopher.sherertz@atkinsglobal.com