FABRICATION/ASSEMBLY METHODS: Non-conventional deck lifts for deepwater Spars, TLPs

Managing loads and vessel dynamics

Nov 1st, 2000
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A comparative workability assessment for a deck installation on a jacket structure off West Africa.
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Deepwater developments remain the current "crest of the wave" and many operators continue to make aggressive moves to increase their production from deepwater leases around the world. The stakes are high for the entire industry, and with oil prices riding historical highs, segments of the world economy are also watching closely.

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Typical development costs, including drilling, can now routinely exceed the $1 billion mark - making such investments highly visible at the corporate level. This, coupled with the longer development cycle times and more complex environments (encompassing commercial, economic, technical, operational and political dimensions), adds further pressure to obtain maximum returns while managing the associated risks and uncertainties.

One possible solution to help maximize development performance is the installation of large, pre-commissioned production facilities. A brief review of those deepwater developments in the planning phase shows production rates in the range of 50,000-100,000 boe to be the norm, with operating weights in the range of 15,000-20,000 tons. There are also the "super-deepwater" developments under review, with decks of 30,000 plus tons.

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Based on recent experience, offshore hookup and commissioning of such typical deepwater facilities, installed offshore on a modular basis can take six months and have hookup and commissioning costs of $25-50 million.

Given anticipated production rates, the appeal of reducing time to first production as well as dramatically lowering offshore costs is significant. In order to realize this opportunity, however, operators and developers may need to consider non-traditional deck installation methods.

Large decks

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The installation of a large integrated deck requires, first of all, construction and loadout. Historically, this has not been a routine operation for decks with loadout weights in excess of 10,000 tons. Large decks, however, have been constructed and loaded out by US Gulf coast fabricators, with a number of yards having this capability. The Shell Auger TLP deck (25,000 tons at loadout) was loaded out as a single unit onto the I-650 at McDermott's Amelia facility.

Similarly, other fabricators with piled skidways and reinforced bulkheads have similar capabilities. Conventional skidded loadouts for large decks present the following challenges:

  • Requirement for high capacity piled skidways and bulkheads in construction yards
  • Accommodation of high local loads under deck legs during the transition from dry land to the floating deck transportation barge. This transition can add additional structural steel to the deck and require the fabrication of customized skid shoes.
  • Need for large deck transportation barges to be able to access the fabrication facility.

For construction in the US Gulf of Mexico, the issue of transportation barge selection is potentially further complicated by the need to comply with all the requirements of the Jones Act.

Deepwater

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After loading out and transporting the deck offshore with a suitable barge, the process of installation commences. Consider two classes of typical deepwater development systems, tension leg platforms and spars (of all varieties). There are others such as floating production systems (FPS) and floating production, storage, and offloading (FPSO) systems, but these do not typically require offshore installation of their facilities. These structures may be categorized as compliant: tension leg platforms (TLP) are laterally compliant and vertically stiff; Spars are both laterally and vertically compliant.

Compliance implies motion, and relative motions (between installation systems and the Spar or TLP) imply potential loads. Hence, the deepwater installation methods must be able to either remove all relative motions, or be able to predict such motions and offer methods of accepting the associated loads.

There are several options for the removal of relative lateral motions. The simplest operationally is to link the installation system to the compliant substructure by short, stiff, lines. Other systems will require the use of separate mooring systems, or dynamic positioning. For these cases, however, the potential for relative lateral motions may still remain.

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The removal of relative vertical motions is not possible. Both TLPs and Spars exhibit very little heave in typical installation sea-states - TLPs, due to the pretensioning of the vertical mooring system, and spars due to their hull shape and long heave natural periods. Conversely, all barge and ship based installation vessels will exhibit some degree of heave response, giving rise to wave frequency and relative vertical motions. In addition, for Spar type substructures, the act of load transfer to the hull will also result in a vertical offset, countered by de-ballasting of the Spar. This combination of vertical motions and responses poses a complex numerical problem, requiring a combination of linear and nonlinear analysis tools and techniques.

The problem of system motions is further compounded by the nature of the marine environments at many deepwater platform locations. Offshore West Africa and Brazil are dominated by long period swells (10-17 sec periods), and the Gulf is subjected to periodic, but intense, loop currents.

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As noted earlier, technical issues form only one dimension of the overall project environment. The majority of the alternative deck installation tools are owned and operated by companies other than those with large derrick barges. Consequently, project developers will also have to address the commercial integration of these alternative installation methods into the total installation process. This can prove as challenging as all of the technical issues mentioned above.

A lift system

The Versatruss Heavy Lift System is one alternative deck installation method suitable for large, integrated topsides. It consists of a pair of flat-topped barges in a catamaran type layout, supporting an array of booms, blocks, and winches that provide the overall mechanical lifting system. Available equipment is capable of lifting decks up to 11,000 tons in weight, depending upon specific geometries. Project teams and joint industry evaluations are reviewing the use of Versatruss for deck lifts (installations and removals) up to 40,000 tons in weight, and for a number of environmental conditions.

Versatruss is similar to a number of other alternative tools in that it utilizes a catamaran barge configuration. However, the proprietary arrangement of booms, blocks and winches provide a number of unique solutions for the installation of integrated production decks onto deepwater development systems.

The lift system fits to the required deck configuration and does not drive overall geometry. The boom and tension chord connection details provide an efficient load transfer into the deck during lifting, and transportation, if transported in a catamaran configuration. This provides the opportunity to optimize deck structural weight and cost. The load distribution between boom pairs can be precisely controlled. This can help manage decks with large center of gravity offsets, as well as further optimizing the load transfer into the deck.

The use of multiple sets of booms, winches, and blocks provides a redundant lift system. If the use of a Versatruss system is considered during the design of the deck, redundancy can be made to reduce installation risks. The mechanical lifting process offers considerable flexibility in terms of controlling the load transfer during set-down of a deck onto a Spar, as well as providing an immediately reversible lift process. For deck installations on Spars, this allows for the rapid initial transfer of about 20-30% of deck weight to the Spar, followed by a combination of de-ballasting and mechanical set-down for the remainder of the load. This fast initial set-down (typically 3-5 min) helps provide a fixed vertical connection between the deck and spar, preventing separation and potential impact loads during installation.

As with the alternative barge-based installation methods, the top-level deck can be fully outfitted with equipment, as access to padeyes and sling laydown areas are not necessary. This also offers the opportunity to set and pre-commission the drill rig packages, prior to installation of the deck.

For decks with installation weights up to 16,000 tons, the system uses flat-topped barges available in the Gulf of Mexico.

Operational experience with the system has shown the mooring forces between the lift system and jacket substructures to be low (10-20 tons). Analysis shows that similar mooring loads can be expected for installations on floating systems, thereby providing a simple solution for the removal of lateral relative motions.

The lift system offers a loadout solution for large decks. The system can be used to place a completed deck onto a transportation barge without a conventional skidding process. Deck design, barge selection and skid shoe requirements can be greatly impacted through the use of this method.

Finally, the catamaran barge configuration can offer improved workability characteristics, particularly for swell dominated environments.

An accompanying figure presents a comparative workability assessment for a deck installation on a jacket structure off West Africa. The comparison is between a single barge floatover and a lift system installation for an 8,000-ton deck. For this example, the workability limit was set as a maximum RMS heave response of three feet for the deck legs. The curves presented in the figure describe the significant wave height for a given predominant wave period, in which the installation system can operate. Finally, the wave persistence data is overlaid with the workability curves. The catamaran lift system offers a workability advantage for predominant wave periods in the range of 6-15 sec (has a greater limiting Hs value).

Numerical tools

Major offshore installation involves the understanding of design risks and challenges for the selected installation methods or system. In support of this, a suite of comprehensive numerical tools is essential for predicting and evaluating the hydrodynamic behavior of the installation vessel or vessels.

The most common numerical tools used for the analysis of a typical offshore deck installation involve using either frequency domain or time domain approaches. The frequency domain approach assumes the coupled effects between bodies to be linear relationships. This linear approximation works well for many aspects of offshore marine operations, and is numerically very efficient allowing the efficient assessment of many different conditions. However, in the event of some form of non-linear or transient behavior, the time domain approach is required to provide a more accurate representation of the system behavior.

Frequency domain commercial programs based on 3D diffraction theory such as MORA and WAMIT can model either a single-body or multiple-body hydrodynamic characteristics effectively. However, in order to calculate the relative motion responses between the multiple vessels, a pre-assembled stiffness matrix is required as input to these programs. The SACS/WAMIT program can facilitate the generation of such a coupled stiffness matrix by modeling the complete physical system consisting of marine vessels, lift equipment, and the deck, including all connecting elements or springs.

An accompanying figure shows an example of a coupled system model for a a lift system supporting a 14,000-ton North Sea deck. Typical results from these types of analyses are statistical extremes of individual body motions, or extreme relative motions between bodies as well as the extreme reaction loads between the various system components.

Time domain solution methods are generally used during final detailed design stages and for checks on frequency domain solutions. Their primary advantage is in allowing changing boundary conditions and non-linear forcing and stiffness functions to be explicitly modeled. The time domain analysis procedure consists of a numerical solution of the rigid-body equations of motion for the offshore system subject to external forces due to wind, waves, current, and mooring systems.

Time domain analysis methods for floating bodies have been proposed by a number of authors since 1962. A direct numerical integration of the equations of motion can include nonlinear connecting spring elements, and fender or positioning systems. The transient responses including vertical impact loads during initial steel to steel contact between two bodies can be evaluated at each time step.

ABB has developed an in-house time domain analysis tool - Wampost. This tool initially converts the frequency dependent hydrodynamic force coefficients derived from WAMIT for individual bodies into time domain forcing functions. It then assembles the coupled system mass, stiffness and damping matrices. In assembling the system stiffness matrix combinations of both linear and non-linear gap/contact springs can be used to represent the interfaces or boundaries between the system components, i.e. underside of the deck legs and the support cans on a Spar hull.

Using the already calculated hydrodynamic forcing functions, the program finally solves the equations of motion for the entire multiple body systems simultaneously. The extreme responses for the system and its components can then be estimated by using a Weibull distribution fitted to the calculated responses.

Complementary tools

The installation of large, pre-commissioned production facilities can contribute significantly towards increasingly profitable deepwater developments. Such alternative installation applications, however, represent a step-change from the way in which offshore installations have traditionally taken place, although existing tools and systems with proven track records are available.

The case for change lies in the prize of reduced development costs and accelerated production, and in support of the change, the necessary analysis tools are now available. The remaining challenge is to understand how these tools can be integrated into overall deepwater development plans, viewing them as complementary to existing methods, and allowing operators to maximize the value of their deepwater portfolios.

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