How Do Semisubmersibles Work?

Originally conceived as a bottom-supported drilling unit, semisubmersibles eventually found their true calling. Now, semisubs are the most stable of any floating rig, many times chosen for harsh conditions because of their ability to withstand rough waters.


Deepwater Horizon

A semisubmersible is a MODU designed with a platform-type deck that contains drilling equipment and other machinery supported by pontoon-type columns that are submerged into the water. Another type of drilling rig that can drill in ultra-deepwaters, drillships are capable of holding more equipment; but semisubmersibles are chosen for their stability. The design concept of partially submerging the rig lessens both rolling and pitching on semisubs.

While in transit, semisubs are not lowered into the water. Only during drilling operations are semisubs partially submerged. Because semisubs can float on the top of the water, transporting these rigs from location to location is made easier. Some semis are transported via outside vessels, such as tugs or barges, and some have their own propulsion method for transport.

Types Of Semisubs

Based on the way the rig is submerged in the water, there are two main types of semisubmersibles: bottle-type semisubs and column-stabilized semisubs.


Noble Clyde Boudreaux

Bottle-type semisubs consist of bottle-shaped hulls below the drilling deck that can be submerged by filling the hulls with water. The first incarnation of this type of drilling rig, bottle-type semisubs originally were conceived as submersible rigs. As a submersible, the bottles below the rig were completely submerged, resting on the ocean floor.

But, as time progressed, naval architects realized that the rig would maintain its stabilization if the bottles were only partially submerged, but be able to drill in deeper waters. Mooring lines are then used to keep the semisub in place, and these anchors are the only connection the rig has with the sea floor. Eventually, these bottle-type rigs were designed to only serve as semisubs.

As a semisub, the rig offered exceptional stability for drilling operations, and rolling and pitching from waves and wind was great diminished. In addition to occasional weather threats, such as storms, cyclones or hurricanes, some drilling locations are always harsh with constant rough waters. Being able to drill in deeper and rougher waters, semisubs opened up a new avenue for exploration and development operations.


DSS 38 Semisubmersible

A more popular design for semisubmersible rigs is the column-stabilized semisub. Here, two horizontal hulls are connected via cylindrical or rectangular columns to the drilling deck above the water. Smaller diagonal columns are used to support the structure.

Submerging this type of semisub is achieved by partially filling the horizontal hulls with water until the rig has submerged to the desired depth. Mooring lines anchor the rig above the well, and dynamic positioning can help to keep the semisub on location, as well.

Mooring Patterns

Semisubmersibles and other mobile offshore facilities are moored in systematic ways, but there are many different designs for various situations. Mooring is similar to multiple anchors, and a number of spread mooring patterns are used to keep the floating rig in place, including symmetric six-line, symmetric eight-line, symmetric twelve-line and 45i-90i nine-line, among others. These mooring spreads are chosen depending on the shape of the vessel being moored and the sea conditions in which it will be moored.

Because the wellbore is extremely precise, it is very important that the semisub is kept in position, despite the waves and the winds working to move it about. Furthermore, working in ultra-deepwaters, the drilling riser pipe may span thousands of feet from the bottom of the semisub to the stationary subsea well equipment located on the ocean floor.

The drilling equipment is somewhat flexible to overcome slight movements caused by the wind and waves, but the drilling risers must not be bent beyond what it can manage, or they will break.

Additionally, dynamic positioning can be used as well as mooring lines to keep the rig in place. Dynamic positioning uses different motors or propulsion units on the vessel to counteract against the motions of the water. Many times, the dynamic positioning system is guided by telemetry signals from beacons on the ocean floor, satellite information and the angular movements of a cable.

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Floating Accommodation Units (FAU) – also known as Units for Maintenance and Support (UMS)

These units enable opportunities for companies that operate offshore to carry out maintenance campaigns offshore. “Our units can carry up to 500 passengers and offer a wide range of service and maintenance options for offshore companies,” says Strategic Developer at Teekay, Kjell Ove Breivik.


“Our unique cylindrical Sevan-based designs provide highly competitive advantages such as high uptime, excellent motion characteristics, more deck space, better stability, and more storage space than other FPSO designs,” Breivik states. “The technology is well-proven; when the 2007-builtPiranema Spirit came onstream, it revolutionised the world of FPSOs.”

Until 2007, either drill or semi-submersible ships dominated the options that offshore corporations could choose from when deciding on FPSO-solutions.

In July this year, Teekay Offshore Partners acquired Logitel Offshore Holdings – a company that was established in 2013 between Sevan Marine ASA and CeFront Technology AS. Logitel Offshore exclusively has the right to use Sevan Marine’s unique cylindrical hull design for accommodation units. “This piece of advanced engineering signifies a major change in offshore oil and FPSO solutions. The operational leadership expertise that Teekay has developed through almost 30 years of experience with dynamic positioning will enable us to grow alongside this acquisition,” Breivik says.


Over the course of the next five years, expectations are that there will be a strong demand for FAUs in markets such as Brazil, Mexico and the North Sea. “The Brazilian oil production is expected to double before 2020, hence the need for accommodation units in this market is expected to be strong,” Breivik states. “Indications suggest that oil production in the Gulf of Mexico will also increase in the years to come.” In the North Sea, low quality, lack of vessels that can handle harsh weather conditions, extension of field life and decommissioning of offshore platforms indicate rise in demand.

Peter Evensen, CEO and President of Teekay, commented “We continue to see strong growth fundamentals in the Brazil and North Sea offshore markets, which are our core markets. We believe that the combination of our strong operational platform and access to capital, with Logitel’s innovative accommodation rig design using Sevan’s cylindrical hull platform, will enable us to provide our customers with an attractive and reliable alternative in this growing segment.”


Sevan Cylindrical Hulls to Be Completed as Accommodation Units

DJI00084-654x534Press Release – Sevan Marine ASA has entered into an agreement to sell the semi-completed Sevan hulls number 4 and 5  on an “as-is, where-is” basis to Logitel Offshore.

The total purchase price for the Hulls is USD 41 million, to be rendered as a seller’s credit. Sevan will grant an additional loan of USD 10 million to be applied by Logitel Offshore towards its first milestone payment regarding hull number 4 to the yard which will complete the Hulls as high-end accommodation units (“FAUs”). The total USD 51 million credit is structured as a bullet loan, with a 3 per cent coupon, repayable within 24 – 36 months. The loan can be converted by Sevan into shares in Logitel Offshore. Upon sale of the Hulls, Sevan will be released from substantially all accrued, contingent and future liabilities related to the Hulls. The transaction remains subject to certain customary closing conditions.

Logitel Offshore is a stand-alone company established in cooperation with Sevan for the purpose of promoting floating accommodation units based on the Sevan design. Logitel Offshore has entered into agreements with a reputable yard for the construction of one plus one FAUs based on the Hulls, with options for two additional units. These agreements are non-recourse to Sevan. Work on hull number 4 is due to commence shortly, while work on hull number 5 is expected to commence before year end 2013, subject to certain initial milestone payments being made. While focusing on the core FPSO/FSO segment, Sevan has also been seeking to develop new markets for the Sevan design.  As part hereof, Arne Smedal, co-founder of Sevan and vice chairperson of its Board of Directors, has established CeFront Technology AS in cooperation with Sevan. Sevan and CeFront have entered into a cooperation agreement regarding, among other things, the development of new applications and projects based on the Sevan design. Thus, Sevan has secured continued and preferential access to know-how and experience in addition to its own in-house resources, while retaining all patents and other intellectual property rights pertaining to the Sevan technology.

As a result of these two transactions, Sevan estimates that the operating expenses will be reduced with approximately USD 4.5 million per year. In connection with the organizational changes, a restructuring charge (including relevant severance benefits as per existing contracts) of approximately USD 3 million will be incurred in 2013.

Sale of the Hulls

The purchase price, USD 41 million in total, equals the book value of the Hulls as of 31 December 2012. An estimated total amount of USD 11 million in liabilities will be reversed through this transaction, leading to an estimated positive effect of USD 11 million in Sevan’s Q2 2013 numbers. In addition, the USD 20 million potential VAT and customs exposure in China stated in the Q4 2012 report in connection with the write-downs of the book value of the Hulls, is eliminated, together with potential liabilities going forward related to continuing maintenance, storage and insurance, including the costs and risks associated with moving the Hulls to a different storage area. The transaction is expected to be completed during Q2 2013.

Sevan will earn a license fee of USD 5 million for each of the Hulls, payable 6 months after commencement of any charter for the respective units. A license fee of USD 10 million has been agreed for any future FAU based on the Sevan design. Logitel Offshore will hold exclusive rights for the use of the Sevan design within the accommodation and logistics market for an initial period, with options to extend under certain conditions. Logitel Offshore will also enter into a service agreement with Sevan in connection with the construction of the FAUs to procure the necessary support from Sevan.

If Sevan elects to convert the USD 51 million loan into shares in Logitel Offshore, Sevan’s ownership interest following the conversion will be virtually 100% pre any equity issues. The final ownership share will depend on the amount of required new equity to be raised, subscription price, company development and access to long term debt. Logitel Offshore is currently reviewing various financing alternatives.

The New Company CeFront

CeFront will be a company focusing on developing technology, design and projects for Sevan and other customers. Key owners and employees in CeFront are Arne Smedal and Kåre Syvertsen, who bring with them a wealth of experience and expertise as co-founders of inter alia Sevan, MCG and APL. In addition to an annual, declining retainer to CeFront, the cooperation agreement provides for incentives for CeFront to enhance development opportunities for the Sevan technology outside of Sevan’s core business. This can, potentially, bring future service income and license fees to Sevan from new markets and applications outside of Sevan’s core FPSO/FSO segment which may be developed by CeFront, as well as the option for Sevan to co-invest in such new projects.

The New Company Logitel Offshore

Logitel Offshore, a Singapore based company and 100% owned by CeFront, has entered into agreements for the construction of two FAUs based on the Hulls and Sevan design. The FAUs have been developed in close cooperation with Sevan and the yard, and will be North Sea compliant and equipped with DP3 and mooring systems.

Carl Lieungh, Sevan’s CEO, stated: “We are pleased with these transactions, which will provide Sevan with a new line of license business going forward, while at the same time removing liabilities associated with the hulls. This helps strengthening our balance sheet. Also, the convertible loan gives us an attractive option to participate in a tightening accommodation market, with an increasing demand side illustrated by recent contracts. We have actively marketed the hulls for FPSO projects in recent years, but for various reasons it has proven difficult to convert keen customer interest into firm contracts. The Logitel transaction reduces down side risk in Sevan while increasing the upside potential and free cash flow, and brings with it the option to become a Logitel shareholder. We look forward to working with CeFront, while at the same time focusing our resources on the opportunities in the FPSO/FSO segment.”

Arne Smedal, CEO of CeFront and Logitel Offshore, stated: “The new structure will release resources to focus on developing new opportunities in both Sevan and CeFront. CeFront gives us the opportunity to work on miscellaneous projects for a wider group of customers, while at the same time maintaining the best of relations with Sevan. Through Logitel Offshore, we look forward to building a company with a new fleet within the accommodation and offshore logistics market based on Sevan’s design. We are also already looking at other potential applications and markets for the Sevan design, which is a commercially and technically competitive solution to semi-submersible platforms in various offshore markets.”


Defects in Lifting-Eyes: Repair and Root Cause Analysis

padeyeCase Study

Lifting-eyes, known as pad eyes, were welded into the top corners of a module for lifting the module on to an offshore platform. Each pad eye consisted basically of a 50mm thick vertical plate measuring approximately 2m x 1m which contained a hole reinforced by rings welded to each side of the plate. This plate was welded to a 35mm thick base plate 0.5m wide and also to horizontal and vertical stiffeners.

Four pad eyes were inserted into the top corners of the module and welded to the main girders. Welding of the pad eyes, during sub-assembly and on the module, was carried out with basic electrodes baked at 400°C and stored at150°C and the joint regions were preheated to 100°C in accordance with the recommendations of BS 5235.

The steel was to DIN St-52-3M (analysis 0.18%C, 1.49%Mn, 0.43%Si, 0.017%S, 0.027%P). The steel had been aluminium treated during production. Magnetic particle inspection and ultrasonic examination of the pad eyes before they were inserted in the module showed that the welds met the requirements of ASME VIII and the pad eyes were then stress-relieved in the furnace.

Final inspection was carried out after all welding on the module had been completed and some toe cracks were found in the cruciform joints in two of the pad eyes. In addition to this there were indications of lamellar tearing in the base plates of each pad eye under the full penetration T butt welds.

Fortunately the toe cracks were shallow and could be ground out and repaired even though the access for the welder was somewhat restricted. However the only access for the repair of the lamellar tears was from the inside of the module. The base plate in each case had to be air carbon arc gouged and ground in the overhead position to reveal the defective areas which were then checked by dye penetrant examination. Welding repairs were then carried out with the preheat of the joint regions increased to 150°C to compensate for the lower energy input when welding in the overhead position.

Inspection of the repairs was carried out four days after completion of welding and the success of the operation was largely due to the skill of the welders working in the overhead position with restricted access and preheat.

Causes of Cracking

Because of the restricted access as well as production deadlines, it was not possible to carry out metallurgical examination to confirm causes of cracking. The toe cracks reported were most likely to have been caused by hydrogen and may have been due to some relaxation of the welding procedure in respect of preheat, energy input or drying of electrodes. It seems likely that for the delay that occurred, in fact seven days, before HAZ cracking occurred some additional stress would have to have been applied to the welded joints.

Pad eyes of similar design have been fabricated previously from BS 4360G 50D steel without any indications of lamellar tearing. The St-52-3M steel used in the present case was known to have been aluminium treated, in which case the principal non-metallic inclusions would be manganese sulphide with a smaller amount of alumina inclusions. The MnS inclusion content of a plate is directly related to the sulphide content and a knowledge of the sulphur level can provide an estimate of the short transverse (ST) ductility of the plate which is one of the factors that governs the risk of lamellar tearing. The other factor is the degree of restraint of the welded joint.

The ST ductility of the steel involved was estimated to be 5-15% in terms of reduction of area. Correlations between %STRA and the risk of lamellar tearing that have been built at TWI indicate that the St-52-3M steel would be likely to suffer from lamellar tearing in highly restrained joints. It is possible that additional stresses could have been applied to the pad eyes some time after welding because of the application of heat to other parts of the structure. This could have been caused by preheating or welding at positions remote from the pad eyes which could have set up reaction stresses which would have added to any residual welding stresses.

Stresses of this nature can cause lamellar tearing or they may cause the faces of very tight pre-existing flaws to open up so that they can be detected by ultrasonic examination. It is also known that such defects can be made more readily detectable by stress relief heat treatment, which can cause some separation of the crack faces. For this particular highly restrained item, it was recommended by TWI that the final NDT inspection should in future be carried out not less than one week after welding or other heating operations on the module have been completed.

To avoid any possibility of lamellar tearing, plates for pad eyes should be ordered with guaranteed adequate through-thickness ductility. In highly restrained fabrications, the recommendations given in BS 5135 to avoid hydrogen cracking should be implemented with rigorous control and supervision.


Skills Required for Proposal Preparation

proposal-management-pageA number of different skills are needed in the preparation of the proposal. Whether these skills are possessed by one person or by an organization, it is necessary to arrange for their availability and application to the proposal preparation process during the appropriate time phase of the proposal activity. The quality of the mix of skills types and the skill levels used to develop proposals has a great bearing on the overall credibility, accuracy, and completeness of the resulting proposal. The key generic skills used in proposal preparation follow, along with a description of the function they perform and an indication of the skill levels required for credibility and quality in proposal preparation.

Business and Finance Skills

Business and finance skills are an essential part of the proposal preparation process, particularly in the preparation of the cost volume. A knowledge of accounting procedures and techniques and an awareness of changing economics and business policies are needed. For example, a person with this knowledge will have a full appreciation of many of the “hidden costs” that must be covered by the cost proposal such as :

  • Direct charges that are added to basic direct costs by “factoring”,
  • Overhead cost,
  • General and administrative costs,
  • Profit or fee.

Since the purposes of bidding on a proposal and winning the contract are to make a profit, the cost volume must be constructed in a way that will do more than merely recover the costs of labor and materials. Business and finance skills are mandatory to understand these facets, so that all costs of the work output will be included in the cost volume, with sufficient allowance for profit.

Engineering and Technical Skills, and Skills of Functional Managers

Engineering and technical skills, acquired by actual on-the-job experience, are the basis for a sound, competitive, credible, and realistic proposal. A completed proposal must be based on practical knowledge of the work activity as well as on the theory of design of the work activity or work output. Although educational background and knowledge of theory are important, this theoritical knowledge must be suplemented by actual hands-on prior experience in producing a similar or identical work output; therefore, experts in the technical field required by the request for proposal must be available to the proposal team.

In addition to the technical experts, functional line managers should be made available to the proposal team on at least a part-time basis. These are the people who will be supervising the technical aspect of the work and who will be able to contribute realism and credibility to the technical approach as well as to the estimates of resources required to do the job.

Manufacturing and Assembly Skills

For work  activities or work outputs that involve manufacturing and assembly operations, detailed knowledge of each manufacturing, assembly, test, and/or inspection function is essential. This detailed knowledge requires people who have had experience in manufacturing and assembly operations. The most valuable attribute of these individuals is their ability to originate and organize the manufacturing and assembly plan for the proposed work output and to plan the effort to eliminate gaps, overlaps, and duplications. Should the proposal involve production line operations, these skills are even more important. The most common fault in manufacturing plans is the omission of essential steps in the process. Simple steps such as receiving and unpacking raw materials or parts, inspection of incoming parts, in-process inspection, attaching labels and markings, and packaging and shipping of the final product are often inadvertently omitted. Team members skilled in manufacturing and assembly will assure proposal accuracy and credibility in these areas.

Management Skills

Part of any proposal team’s expertise must consist of abilities in the area of project management. A skilled and experienced project manager will be able to be correlate the need for workers, material, equipment, and systems with the proposed work output or work activity. The manager will be able to envision and plan the management tools, resources, and expertise required to effectively carry out the proposed job and will be able to effectively communicate the management control, schedule control, and cost control aspects of the job to the customer.

Mathematical, Statistical, and Data-Processing Skills

Higher mathematics, the application of statistics, and data-processing skills are not always required in the development of credible and supportable proposals, but in high-technology and multidisciplinary work activities and work outputs, these skills have become essential. Often, a design will not be fully developed and various matemathical or statistical techniques will be necessary to develop data for the technical and cost volumes. When new products are designed and new services are envisioned, it is always best to verify the performance and cost projections by use of mathematical and statistical techniques or computer simulations. Data-processing skills are also required for the creation of the proposal itself.

Production-Planning and Industrial Engineering Skills

Production-planning and industrial-engineering skills are closely related to the manufacturing and assembly skills mentioned earlier, but these skills are usually learned and applied at a higher organizational level. Where the manufacturing and assembly skills used in proposal preparation are derived from hands-on experience by workers or their immediate supervisors, production-planning and industrial engineering skills are acquired from an overall knowledge of the workload and work flow in an office, factory, or processing plant. Production-planning and industrial engineering skills are particularly important for work activities or work outputs that involves high rates or larges quantities of production. Knowledge of automation and labor-saving techniques in the shop, factory, or office become important in these applications.

Writing and Publishing Skills

Since the proposal is primarily a sales document, it must present the best possible picture of the proposing company. Writing style, contents, quality of graphic reproduction, even the choice of cover or binding may have an effect on the evaluating team. Individuals capable of writing and editing material while working under pressure are essential. It is necessary for the proposal team to have available a knowledge of the mechanics of the writing and publishing process, including expertise ini storyboarding, proposal layout and design, desktop publishing, reproduction, and binding.

In soliciting the skilled personnel required to work on proposals, the recruiter should remain the participants that the proposal preparation process is often regarded as an essential step in developing the careers of future project managers, business managers, and corporate management. Because an in-depth knowledge of the company and one or more of its products or service is developed during preparation of a proposal, proposal team participation has historically been a vital asset in the career path of future managers. Management usually puts its best people on proposals and therefore expects these best people to grow into positions of higher responsibility and authority.

(Stewart, Rodney D., and Ann L. Stewart, “Proposal Preparation”. New York: John Wiley & Sons, 1992.)

Six Basic Steps in Proposal Preparation


There are six basic steps that are taken in proposal preparation, not necessarily in the order list. They are: (1) marketing; (2) analyzing and making a bid decision; (3) planning; (4) designing; (5) estimating and (6) publishing the proposal. Although these steps generally are taken in the order listed, the marketing, analysis, and planning steps continue throughtout the process.


The principal step required to start out in a business venture is to find or identify a need and fill it. As a subset to this step, one can identify and generate new needs that are waiting to be filled by someone who makes available a work activity or work output on which the need is dependent.(…) Successful businessmen realize that the idea of aneed must be planted in the minds of their customers long before the proposal cycle is initiated. Throughout the proposal preparation process, the marketing function plays an important role in shaping and directing the policies, ground rules, and procedures used. As will be mentioned later, care must be taken to counterbalance the marketing departement’s optimism and desire to win new work for the company with realistic independent planning, scheduling, and estimating of the project by the performing elements of the organization.


It is through an analysis of the customer’s needs and of the proposing company’s capabilities of performing useful work that identifiable products and potential proposals are generated. This analysis will take many factors into account, but the criteria for selection of an identified need for further pursuit in the form of a specific proposal will most frequently be the profitability of the venture.


(…) Good planning and definition will help avoid dead-end projects, partially completed projects, and unnecessary duplication or overlap of work activities. A better job of proposal preparation, backed on conceptual or preliminary design and testing effort, means less wasted effort, more projects completed on time and within cost, and fewer dead end projects. Businesses are developing an awareness of the need for more systematic planning and financial analysis before initiating a venture. Recognition of the significant effect of economic factors and of the need for good planning has expanded the scope and content of the proposal from a simple document into a comprehensive technical, organizational business plan for accomplishing o work activity or producing a work output.

Planning, an essential step in proposal preparation, coupled with its close cousin, scheduling, is required to provide the realism and credibility needed in the proposal. In planning a work activity or work output, it is necessary to concentrate on as few alternatives as possible. One must beware of the professional planner who wants to be in continous planning mode with two or more alternatives being simultaneously analyzed in depth. Comparison of alternatives early in the need-identification phase is always helpful; but an early choise of a single alternative is usually very beneficial. The choise of a single alternative forces the resolution of key questions and assumes that the work can and will be performed in a selected manner. Planning includes all of the technical, organizational, and management aspects of the project and considers all marketing inputs, such as projected quantities, cost targets, and capture potential for the work.


Design work that is done as part of proposal preparation is usually conceptual design or preliminary design, although a final design of the work activity or work output will occasionally be required. When proposal is to be submitted for performing final design work, conceptual or preliminary designs are all that is needed. Design work for proposal preparation includes preliminary sketches, plant layout, flow diagrams, scale models, mockups, and prototypes. The degree of completion of design work, as evidenced by the number and types of drawings, models, mockups, components, or prototypes, is often a source evaluation factor. The design step of the proposal preparation process usually culminates in the preparation of the technical volume or technical section of the proposal.


Estimating is one of the most important steps in the proposal preparation process. Estimating includes predicting or forecasting the amounts of materials, number of labor hours, and costs required to accomplish the job. Credible estimating cannot be done without adequate planning and preliminary design of the work activity or work output being proposed. Estimating requires unique skills, usually multidisciplinary in nature, that must either be acquired by experience or by training in a special mixture of technical and business disciplines. This unique mix is most nearly approached by the industrial engineering profession, but includes business skills that analyze and optimize profitability, which have not traditionally been a part of the industrial engineering discipline. The estimating step of the proposal preparation process culminates in the cost volume or cost section of a proposal.


The publication step of proposal preparation includes the organization, writing, editing, art work, printing, and binding of the proposal document or documents. The publication capability and publication team should be an integral, responsive part of the proposing company’s organization. Opportunities for using high technology in the proposal publication process must be taken if one is to submit a competitive proposal. The appearance and accuracy of a proposal, although not usually numerically scored by the evaluator or by evaluation team, are important factors in the general impression made by proposal. They can engender confidence and could be a basis for initial acceptance or rejection. Fancy or elaborate formats or displays are unnecessary and in many instances are even undesirable as they give an impression of lack of cost consciousness. A neat, accurate, easily read, easily referenced proposal is an aid to evaluators and is an indication to the costumer of the type of work that he or she can expect to receive in reporting and documentation during the performance of the work.

(Stewart, Rodney D., and Ann L. Stewart, “Proposal Preparation”. New York: John Wiley & Sons, 1992.)

Novel Hull Concepts Address Ultradeep Gulf of Mexico Production

Source :


Jelena Vidic-Perunovic

Doris Inc.

Bill Head 
Research Partnership to Secure Energy for America
Sugar Land, Tex.

By 2020, according to industry forecasts, deepwater oil production will constitute a third of the world’s total oil supply. A strong upswing in deepwater Gulf of Mexico oil exploration is likely (OGJ, May 6, 2013, p. 81), as well as an increase in oil production in Outer Continental Shelf blocks in the gulf’s Lower Tertiary. Exploitation of the gulf’s Paleogene reservoirs, however, faces several environmental, technical, economic, and geophysical challenges.

In this context, different concepts for ultradeep gulf production and storage have been assessed. Non-ship-shaped, round floating production, storage, and offloading (FPSO) hulls currently appear to be the most appropriate production solution for the ultradeep gulf, capable of complying with physical criteria, safety regulations, and operators’ requirements.

This article describes concepts being developed in production technologies through the Research Partnership to Secure Energy for America’s (RPSEA) ultradeepwater technologies development program. RPSEA manages a federal research program for the US Department of Energy (DOE), under guidance from the National Energy Technology Laboratory (NETL).

The program was created by the 2005 Energy Policy Act, Section 999, Subtitle J. Section 999 funding by Congress specifically directs research aimed at increasing production of federal reserves in a safe and environmentally sound manner in the ultradeepwater Gulf of Mexico.

Existing technologies

Traditional platforms and towers, applied in shallower waters, rapidly become uneconomical as depth increases. Commonly used concepts for operation in deep water are FPSOs, floating storage and offloading units (FSOs), semisubmersibles, spars, tension-leg platforms (TLPs), and emerging hybrid designs. Functional requirements, depth, subsequent weight of risers, environmental conditions, and the size of topsides usually determine the appropriate concept to be applied for a discovery.


Table-1 outlines existing technology. Concepts are represented by the exemplary production units, covering main subtypes, geographical and operation requirements (storage and production), and the level of technological readiness.

With reservoir discoveries in deepwater Gulf of Mexico, offshore operations move towards water deeper than 3,000 m (more than 9,800 ft) with distances of about 300 km (186 miles) from shore.


These ultradeep and remote locations are distant from existing pipeline infrastructure (Fig. 1). Pipeline extensions into ultradeepwater are complicated due to difficult pipeline design, installation, inspection, maintenance, and repair. In addition, characteristics of marginal field reservoirs are uncertain with limited life span.

Therefore, operators look towards local oil storage, offloaded via shuttle tankers, in order to facilitate exploitation of Lower Tertiary gulf reservoirs. It should be noted that large storage capacity has the advantage of minimizing offloading frequency and affords safe reserve storage in case of shut-in due to hurricanes. The size of shuttle tankers for the gulf is limited by ports’ water depths (Galveston’s port draft limit, for example: 45 ft at present)1 and tanker availability due to effects of the Jones Act.

Future use of deeper draft vessels will increase after gulf ports have been deepened in response to the Panama Canal expansion (OGJ, Mar. 15, 2010, p. 39). It is reasonable to assume storage capacity of at least 1 million bbl of oil will be sufficient, corresponding to topside production rate of 60,000 bo/d. Thus, oil produced in 16 days can be accommodated allowing a 2-week shuttle cycle.

Further, technology readiness level (TRL) is used as a measure of the level of development of one technology (0-7 scale, according to API 17N or DNV-RP-A203 (July 2011)). Although application of innovative concepts may reduce costs, a higher level of concept maturity is desired by operators because it will shorten the concept’s time to market.

Assessment in Table 1 indicates the following hull types with local storage comprise standalone solutions:

  • Ship-shaped FPSO.
  • Non-ship-shaped FPSO (represented by Sevan 1000, SSP Base, SSP 320 Plus; Figs. 2a-c).2-6
  • Deep-draft semisubmersible FPSO (represented by Octabuoy, 1 million bbl; Fig. 2d).7

It should be noted that any ultradeepwater oil production concept can be considered in combination with an additional FSO system. The need for a low-cost solution, however, makes a system consisting of more than one unit unlikely to be cost competitive.


These hull types are applicable as standalone solutions: satellite services platform (SSP) by SSP Offshore, SSP Base model (a); SSP Plus model (b); Sevan cylindrical hull by Sevan Marine ASA (c); and Octabuoy hull by Moss Maritime (d). (Fig. 2; sources: References 2-7).

Concept assessment

The essential elements for hull applicability in the ultradeep gulf include several aspects of environmental and human safety, operational efficiency, and perceived cost for any floating production and storage solution option.

To withstand wave and storm stresses that damage structures, offshore structural design criteria must include responses to the latest hurricane standards associated with (normal) environmental conditions (recommended design waves by API,8 re-evaluated after hurricanes Ivan (2004), Katrina (2005), and Rita (2006); Fig. 3).


Studying storm tracks of recent hurricanes, such as Ivan (2004), Katrina (2005), and Rita (2006), helps in formulating offshore structural design criteria (Fig. 3; source: Reference 26).

The three selected concepts—ship-shaped, non-ship-shaped round hull, and semisubmersible—summarized in Table 2 are discussed here for their main advantages and possible shortcomings.



The US Coast Guard (USCG) is the lead regulatory authority over floating OCS structures, as outlined in the Memorandum of Agreement between the Minerals Management Service (MMS) and the USCG (MMS/USCG MOA: OCS-04, Subject: Floating Offshore Facilities).9

From reorganization of MMS in 2010 emerged the Bureau of Safety and Environmental Enforcement (BSEE), which became responsible for enforcing safety and environmental regulations related to oil and gas. Classifications recognized by the USCG apply “intact” and “damage stability” standards for different types of offshore installation hulls. For operations in the gulf, stability of a floating unit is reviewed by both the USCG and BSEE.9

Round floaters (Figs. 2a-c), featuring a deeper cross section than a ship-shaped unit, will have a better restoring capability (center of buoyancy, center of gravity) from all wave directions than a traditional ship shape, once the vessel heels from the equilibrium position.

Vessel stability in unloaded condition is provided by use of ballast water. A ballast system is designed so that the capacity and location of ballast tanks ensure the vessel meets stability requirements, maximum trim/heel angle limits for the operation of process facilities, and minimum draft to avoid bottom slamming.

In general, stability of hemispherical bottom shapes is better than that of a flat cylinder. Therefore the original spherically shaped SSP320 should not need active ballasting.2 Permanent ballast has been applied in flat-bottom round vessels with bilge box.

A semisubmersible hull with storage, such as the Octabouy, has been arranged with a complicated ballast system in the octagonal pontoon and columns.7

A well-functioning ballast system and integrated ballast control are vital to this type of hull’s operation and safety to withstand changing environmental conditions that cause frequent changes in draught and trim. Such changes in ballast dynamics may happen quickly due to pronounced hull sensitivity to weight during loading or unloading.


Hull motion is an important consideration for riser selection because it affects riser fatigue life, motion-sensitive process equipment, and everyone aboard. Structures represented by the reduced waterplane area and deep draft (such as round FPSO hulls, spars, semisubmersibles, and TLPs), feature long natural periods in heave.

Natural period of angular motion around horizontal axes is influenced by the metacentric height,10 a measure of the initial static stability of a floating body; yet this parameter must comply with the stability requirements of both physics and regulators.

Motion amplitude, on the other hand, depends on the level of excitation and damping in the system. Rigid body translatory motions and angular displacements in ships are mainly excited by linear waves in the significant energy region. Thus, a ship-shaped FPSO in hurricane conditions may experience excessive motions affecting her operability.

Pitch motion influences a ship-shaped FPSO when risers are located in the fore hull, as is the case with bow turret systems. Baffling is especially important in topsides hydrocarbon separation to compensate for hull motions due to motion-caused dispersion of liquid phases at the interface between oil and water. With round FPSOs, hull motions’ frequencies are moved away from the main excitation wave energy content, simplifying the riser location selection on the vessel.

In the absence of direct wave excitation, roll motion can be parametrically excited in the system, due to oscillations in the restoring term, i.e., the metacentric height. The phenomenon is caused by heave and roll oscillations at resonant frequency condition. This is typical for column stabilized units or spars.

The column shape of the semisubmersible hull Octabuoy has been modified to eliminate parametric roll by the introduction of conical columns just below the water line (Fig. 2d). This architecture, however, might disrupt the first-order heave motion. With cylindrical FPSO hulls (for the same heave amplitude), the heave effect becomes less pronounced with increases in hull size (waterplane area).

Structural response

The still-water bending moment (“sagging,” “hogging”) in a floater occurs solely because of the variation of mass and buoyancy along the hull. Wave-bending moment is further increased by longitudinal wave lengths close to the hull’s girder length (or pontoon length on a semisubmersible). For a ship-shaped FPSO hull in the gulf, the important design wave lengths correspond to maximum energy waves (API RP 2A11).

In principle, longitudinal strength in a round floater is improved compared with a long beam because the bending stress is reduced due to hull form (i.e., increased sectional modulus). Ship-shaped FPSOs may suffer notable hull deflections as a result of vertical bending that affects deck and topside interface stool structures, topside structural supports being areas of high stress concentration.12

Hull vertical bending effect on topsides is insignificant on round floaters when the diameter-to-height ratio is close to or less than unity. With semisubmersible hulls, the largest bending moment may be induced on columns, yet the bending stresses are largest on the pontoons and deck.

To reduce dynamic stresses transferred to the Octabuoy deck structure and topsides, a structural brace system is arranged at the top of the columns.7 Column brace joints (designed with small diameter in order not to increase the waterplane area) are susceptible to fatigue damage due to concentration of local stress.

Hull configuration

Both BSEE and the USCG are involved in production and drilling in US waters and in actions related to oil discharge from a floating facility.13 A ship-shaped vessel storing oil is required by the USCG to have a double-hull structure in the tank area (Oil Pollution Act of 1990 resolution).

(The primary purpose of a double-hull structure is to prevent oil leaks in the event of low-energy collision or grounding. Stationary vessels that are not required to escape extreme weather by disconnecting the mooring are not susceptible to grounding while laden.)

A non-ship-shaped hull design also reviewed by both the USCG and BSEE has no specific requirement related to double hull (neither sides, nor bottom).14 Thus, a double-hull structure becomes a criterion under the assumption that a non-ship-shaped production and storage vessel would have to comply with the same regulatory requirements as a ship-shaped FPSO categorized as a vessel.

Water on deck

To mitigate “green water” loading (or deck wetness) effects on the deck, ship-shaped FPSOs are designed with protective breakwaters and bulwarks in front of critical structures on deck. Under fully laden conditions when freeboard is minimum, the deck area may be subjected to heavy green water (more than spray or foam); the most severe would be beam-sea due to focusing of directional wind sea and current.

The associated impact loads may threaten people, hull structure, and topside equipment.15 Owing to its volumetric properties, as compared to a tanker form, a round-shaped hull can be designed economically with significantly greater freeboard for added safety.

Besides green water loads, deck structure on a low air-gap semisubmersible hull is prone to slamming wave impact loads and vortex forces. To reduce the wave loads on deck and topsides, the minimum air-gap between the wave crest and the deck, required by the recommended rules of the classification societies, equals 1.5 m.

Topside arrangement

Arrangement of topsides on the hull is directly related to the unit’s floating equilibrium, i.e., trim and stability. To balance a hull on still-water-level, the main consideration hull and topsides weight distribution, riser connection location, and storage and ballast tanks position.

Information on present and future topsides weight is important for maintaining required freeboard and trim. Deck area on a ship-shaped floater or a semisubmersible offers enough space for the safe and functional arrangement of process equipment.

The arrangement of topsides on round floaters might be slightly more challenging in case of reduced deck area, which may lead to an overhang or modules designed in several levels. However, some of the advantages of having slightly shorter distances between the modules are significantly reduced length of piping and cable runs and better crane accessibility.

Hull disconnection

When compared with other deepwater floater solutions, ship-shaped FPSO motion characteristics represent a clear disadvantage that affects riser loads and overall safety and limits the choice of risers in Gulf of Mexico applications. Thus, from the regulatory point of view, it is unlikely that BSEE will approve a permanently moored ship-shaped FPSO in the gulf.

Currently a disconnectable turret mooring-riser system is recommended in order to reduce the effect of environmental loading in severe weather such as hurricanes.16 A self-propelled vessel can sail away, after being disconnected, to avoid extreme weather.


Where will hurricane Katrina go? (Fig. 4; image from NOAA-NASA GOES 12 Satellite)

Planning an escape route, though, might be difficult for the crew in wind conditions when, as shown in Fig. 4, most of the gulf is affected because exact path predictions may come too late for a safe haven route.

The main safety hazards recognized with the turret operation are turret seizure and hydrocarbon leakage from flexible risers.17 A risk of cracking is associated with the vessel structure around the internal turret cavity, the upper bearing being the main area of load transfer to the vessel.18 Production downtime is associated with disconnection in hurricane weather.

The need to disconnect in stormy weather and shut down production has been eliminated in case of non-ship-shaped floaters and column-stabilized units due to their favorable motion characteristics.

Lightship weight

Hull steel weight constitutes a substantial part of total cost of fabrication as well as number of yards capable of building a given hull. Rough estimation in Fig. 5 is based on data provided by SSP Offshore, Sevan Marine, Moss Maritime, published characteristics of truss spar and semisubmersible without storage by Floatec,27 and data published by DNV for a concept vessel Triality.28


As seen in Fig. 5, the hull steel weight of possible concepts with 1 million bbl storage varies between ~20,000 and 30,000 tons. The exception is the 1 million bbl Octabuoy of 61,000 tons, whose hull weight is more than double the comparative round hull types of the same storage capacity, thus affecting the low-cost assumption.

It should be noted that the prediction in Fig. 5 includes uncertainties with regard to both consistency in hull design basis and the technical method of lightship weight estimation. A detailed cost analysis of each selected concept, including mooring, riser and offloading system, as well as topside facilities, must be included in each concept’s cost assessment.

BW Pioneer

It has been more than a year since BW Pioneer, the first and only ship-shaped FPSO in the gulf Cascade/Chinook field, received her first oil. During Hurricane Isaac in 2012, FPSO production was shut down and later successfully restarted, yet the mooring system did not have to be disconnected because the storm’s wind force was below mooring capability.

With production moving to deeper and more remote fields, stability, weight, motions, structural response, ability to accommodate expanded topside production, safety, and reliability become major concerns. Round hulls maximize storage capacity while meeting production capacity in a floating unit. Non-ship-shaped hulls accord safe operation of a unit encountering gulf conditions.

Moreover, because deepwater operations are very expensive, low capital expenditure and quick return on investment are desired. Future innovation should increase the technological readiness of circular hull solutions for ultradeepwater gulf application.


This article represents part of a technology status assessment study under RPSEA project No. 10121-4404-03. Authors acknowledge support by the US Department of Energy, the NETL, and RPSEA Deepwater Program 2010 and assistance of Rick Haun, Doris Inc.


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The authors


Jelena Vidic-Perunovic ( is a senior analyst in naval architecture and risers at Doris Inc., Houston. She has 15 years of experience in applied hydrodynamics and global structural analysis of ships and offshore structures, from both industry and academia. Before joining Doris Inc. Vidic-Perunovic worked at Technical University of Denmark, University of California Berkeley, National Oilwell Varco, and the American Bureau of Shipping. Vidic-Perunovic holds a PhD in maritime engineering from the Technical University of Denmark.

Bill Head ( is a project manager for RPSEA’s Ultra Deepwater program with more than 38 years of experience in the oil and gas industry. Past positions include general manager and vice-president of technology at independent oil companies; vice-president and chief operating officer at major service company; exploration supervisor and applied researcher at major oil companies, and project manager of applied research for the US Geological Survey’s Rocky Mountain Division. Head holds BA, MS, MBA, and JD degrees focusing on intellectual property and is an active member of SEG, AAPG, and SPE.