Japan Drilling Company’s semisubmersible drilling rig Hakuryu-5 was recently upgraded with Exmar Offshore’s Deepdish pontoon sponson, an innovative combination of pontoon sponsons and integrated cross bracing designed to increase the capabilities and operating life of ageing rigs. Exmar’s Joel Wetmore and Ed Nagel discuss the design process that led to the successful application of the Deepdish concept.
Existing mobile offshore drilling units (MODUs) are under continuous pressure to maintain their contract rates and compete with new units entering the market. This often requires a rig to withstand greater drilling loads and operate in deeper waters than those anticipated by its original designer. Conversely, a drilling contractor is usually reluctant to remove a revenue earning asset from service for the amount of time typically required for a comprehensive upgrade.
Operators looking to improve their rig’s capabilities and performance are typically required to add displacement and improve stability via the attachment of pontoon sponsons, column blisters, and/or additional stability columns to the existing hull. While it may be relatively easy to define the basic objectives and work scope of such an upgrade, integration problems may not be discovered until after the rig has arrived at the shipyard, causing schedules to slip and costs to escalate.
Mitsubishi Heavy Industries (MHI) built the Japan Drilling Company (JDC) Panama/UMW Drilling semisubmersible Naga 1 (formerly Hakuryu-3) in 1974. This rig is limited to a maximum design water depth of 1640ft at a time when there is increasing demand for MODUs capable of working in water depths greater than 3000ft. JDC has maintained the rig well throughout its lifetime. It was therefore possible for JDC to consider favorably the cost-benefit relationship of a life enhancement upgrade and the technical feasibility of a 20-year service life extension.
JDC, however, has been required by its classification society to maintain an annual inspection programme due to fatigue issues with the existing brace end connections.
This obligation was difficult to accommodate and so, when JDC began to evaluate options for a life enhancement and capability upgrade, it desired an engineering solution that would:
Inspired by the cross pontoon and diamond brace arrangement possessed by some fifth generation semisubmersibles, JDC and its technical consultants developed an upgrade configuration that included cross pontoons, outboard pontoon side sponsons, and column sponsons. Also included was the removal of the existing horizontal and inboard vertical braces to eliminate fatigue-prone areas. In 2003, JDC requested Exmar Offshore to evaluate their configuration and propose alternatives.
Although readers may be familiar with a general association between sponsons and upgraded capability, each of the before mentioned sponson types are designed to improve the performance of a specific operating mode and its corresponding draft. To briefly summarize:
Cross pontoons: A sea state will subject a semisubmersible’s braces to cyclic pry-squeeze, pitch connecting, and racking loads. The Naga 1’s braces and end connections are graceful in their complexity but are, unfortunately, particularly sensitive to fatigue. Cross pontoons are designed to connect the two existing pontoons and to be stiff enough to cause a load transfer from the existing braces to the cross pontoons.
Pontoon sponsons: Like many other semisubmersibles, the Naga 1 has a reserve of stability when at the transit draft and its transit draft payload is limited by displacement. Pontoon sponsons, together with the cross pontoons, increase transit draft displacement so that the greater VDL can be carried with the minimum possible increase in hydrodynamic resistance. The latter consideration is often of particular concern to operators which anticipate wet tow voyages.
Column sponsons: The operating and survival drafts of the Naga 1 are stability limited. Column sponsons are attached to columns to increase a draft’s metacentric height with the smallest possible increase in waterplane area. The optimization of column sponson geometry is therefore an exercise in providing the required amount of stability without imposing an adverse effect on motions response.
Pencil columns: Pencil columns serve in a manner similar to that of column sponsons and are, in some cases, easier to integrate with the existing hull structure. It is, however, not always possible to design a pencil column that contributes enough additional waterplane area to the stability of the transition draft (the draft at which the pontoons become immersed), thus resulting in an instantaneous reduction in metacentric height.
Exmar developed preliminary upgrade configurations that consisted of various combinations of all four sponson types described above. Exmar, together with JDC, evaluated all configurations with respect to cost, constructability, and performance. The upgrade configuration eventually recommended by Exmar and selected by JDC was a solution that used pontoon bottom sponsons combined with integral and co-planar cross pontoons and diamond braces. Dubbed the ‘Deepdish’ by Exmar, the 2.5m-deep assemblage was designed to be pre-fabricated in its entirety on a drydock floor such that the existing rig could be floated over, aligned, and lowered onto the Deepdish for integration.
The Deepdish pontoon sponson, together with column sponsons, offers an elegant solution to all of JDC’s requirements and provides ancillary benefits that include improved motions behavior (particularly with respect to heave, roll and pitch). Pre-fabrication reduces the time that the rig is required to be in the shipyard and the associated loss of revenue. The 7x2.5m transverse cross pontoons and 3.5x2.5m diagonal braces are rigid enough to absorb most of the loads previously supported by the existing braces. This load transfer also results in a large increase in the fatigue lives of the existing brace end connections, and eliminates the need to remove or reinforce the existing braces.
In June 2005 the Naga 1 secured a long term contract with its original capabilities, thus delaying the implementation of the 3500ft upgrade. JDC, however, had similar life enhancement objectives for its Hakuryu-5, a Mitsubishi MD 501 design that MHI delivered in 1977. JDC therefore contracted with Exmar to prepare a 3500ft upgrade basic design package for the Hakuryu-5 that was based on the Naga 1 package.
In preparing the basic design package for the Hakuryu-5 upgrade, Exmar was responsible for sponson sizing, performing the various engineering analyses required to obtain class approval of the concept, and producing class-approved scantling and piping schematic drawings for JDC’s shipyard tender package.
Due to concerns over the fatigue life of brace end connections, Exmar performed a detailed fatigue analysis of both the existing and upgraded hull configurations. Global finite element models (FEMs) were subjected to spectral fatigue analyses to identify critical areas. Fine mesh FEMs of these areas were then studied in greater detail. JDC had maintained detailed records of the rig’s operational history and so Exmar was able to accurately quantify the fatigue life consumed by the existing structure prior to the upgrade, and to demonstrate that the upgraded rig would have an expected post-upgrade fatigue life of over 20 years.
The global FEM was also used to establish maximum misalignment tolerances between the Deepdish and existing structure. Although JDC provided Exmar with as-built drawings of the Hakuryu-5, it was expected that the existing hull geometry would deviate to some degree from the theoretical arrangement. It was also possible that the mating process might have introduced some amount of misalignment. Element stress data from the spectral fatigue analysis was therefore combined with stress concentration factors to determine the fatigue lives associated with a range of different misalignment values. Exmar then modified some of the interface details to reduce fatigue sensitivity and worked with ABS to establish maximum misalignment tolerance of ±50 mm for major structure.
The piping system modifications required by the Deepdish were minimal. The majority of the pontoon ballast tanks were extended into the Deepdish pontoon and, once it was determined that the existing ballast pumps could accommodate the additional 2.5m of suction head, the existing ballast lines were simply extended down into the Deepdish. The remaining Deepdish compartments were designated as void spaces. JDC elected to equip these spaces with a leak detection system and portable pumps and so, with no bilge system modifications, the remaining piping systems were limited to vent and sounding lines.
JDC has decided to carry out the life enhancement of the Hakuryu-5 in multiple phases. It contracted to have IHI Marine United perform what has been called the Deepdish Conversion, a project that involved the detailed design, fabrication, and integration work required to add the Deepdish and column sponsons to the Hakuryu-5. It should be noted that, as no modifications were made to the existing mooring, drilling or motion compensation systems, the Deepdish Conversion has only provided an increase in VDL. Exmar worked with IHI Marine United and ABS to refine construction details and to modify the design as required to suit the shipyard’s construction preferences.
The Hakuryu-5 initially entered the IHI Aichi Works drydock in October 2008 for hull cleaning and a detailed dimensional survey of the hull. Floatover and mating took place the following month and was followed by the welding programme required for Deepdish and column sponsons integration. IHI Marine United delivered the upgraded Hakuryu-5 on schedule in February 2009, having required only half of the time estimated for an equivalent conventional upgrade.
Exmar has performed extensive engineering work to optimize the Deepdish configuration, demonstrate that it will result in improved global strength, fatigue life, and payload carrying ability, and to obtain classification society approval of the concept. The cost, schedule and performance benefits of the Deepdish have been proven in practice by its application to the Hakuryu-5. The project was an excellent example of how good cooperation between the owner, designer and shipyard has resulted in the successful execution of an innovative concept.
It is expected that the Deepdish pontoon sponson can be successfully applied to other column stabilized drilling, production and accommodation units.
Exmar has also built upon the work of the Hakuryu-5 3500ft upgrade design with a basic design package for 5000ft operating water depth upgrade.
This project represents a greater level of complexity as it will require the replacement of much of the existing drilling, mooring, and power generation/distribution systems. Additional sponsons will be required to carry further increases in payload, while major modifications to the upper hull structure will also be required to accommodate improved BOP, subsea tree, and riser handling systems. It is hoped that, at some time in the future, JDC will secure a contract for the Hakuryu-5 that justifies the investment of time and cost of the 5000ft upgrade. OE
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