Some design notes for semi-submersible and TLP

Semi-Submersible

In the design of a semi-submersible, and its configuration in particular, a clear idea of the functions it must perform should be in hand. These will strongly influence the configurational choices. Besides function like drilling, other functions include production, heavy lift, accommodations and operational support (surface, subsea). Apart from the mission and support functions, stated simply, there are two essential functions of a semi-submersible, i.e. to stably support a payload above the highest waves, to minimally respond to waves.

These are the principal factors that establish size. It is, however, the mission functions and associated support functions that most significantly contribute to configuration.

The four main configurational components are:   

Pontoons

Stability columns

Deck

Space frame bracing

Virtually, all semi-submersibles have at least two floatation states: semi-submerged (afloat on the columns) and afloat on the pontoons. The pontoons are the sole source of floatation of the semi when not semi-submerged. The stability columns are the principal elements of floatation and floatation stability while semi-submerged.  

Column and Pontoon 

The number and arrangements of pontoons and columns distinguish many configurational variants employed in the evolution of the semi. This has included as few as three to as many as a dozen or more columns. It has likewise included a simple two parallel pontoon arrangement, up to six, and even a grillage of orthogonally intersecting pontoons. 

The twin pontoon preference is principally because of its mobility. A preference for the 6- and 8-columns relates primarily to the twin pontoon option, and is influenced by the use of bracing systems. 

The function of the columns is to provide stability and the critical point of stability is when a semi is submerging,  the flotation undergoes transitions from being afloat on the pontoons to being afloat on the columns. This operation is restricted to mild conditions and requires only that there be “positive GM”. It is common to flare the columns at the pontoons to enhance stability through the critical range of drafts. 

The main problem in semi-submersible design is to adopt the right configuration for the specific functions required. Rigorous hydrostatic, stability, hydrodynamic and structural analyses should be performed once the appropriate shape and size is determined for the initial design.

In sizing of a semi, it is informative to re-examine the most fundamental functions of the type:

- To stably support a payload above the highest waves

- To minimally respond to waves 

The second basic function, “minimum response to waves” relates to the size, shape, and submergence of the pontoons relative to the column waterplane area, and the spacing of the pontoons and columns.

Generally, the fewer the columns, the lower is the cost of the structure, even if the lesser number of columns must be more robust. For the twin pontoon configuration, each column pair at least requires a transverse brace between the columns to resist the squeeze-pry forces. These are usually associated with diagonal bracing. The issue of bracing and deck configuration can be avoided in the initial, parametric stages of design, but must be addressed.

Column and Pontoon 

The number and type of arrangements for the pontoons and columns distinguish many variants employed in the design of the semi. This has included as few as three to as many as a dozen or more columns. It has likewise included a simple two parallel pontoon arrangement, up to six, and even a grillage of orthogonally intersecting pontoons.  Only the 4-, 6-, and 8-column configurations continue to be preferred in new generation semi. Similarly only the twin pontoon and the closed array pontoon arrangements are currently used. A 3-column.

closed array pontoon (triangular) arrangement has been proposed for both FPS semi-

submersible and TLP applications, and offers a steel reduction opportunity  but these designs have not been successful, perhaps because of the more complex deck arrangements.

The twin pontoon preference is principally because of its mobility. A preference for the 6- and 8-columns relates primarily to the twin pontoon option, and is influenced by the use of bracing systems. 

The pontoon-to-column connection is especially important, particularly with regard to structural connectivity. For reasons noted before, the column may be flared at the pontoon, typically rectangularly. If rectangular, aligning the internal bulkheads within the pontoons as continuations of the column sides can significantly reduce stress. Generally there are at least two or four pump rooms in the pontoons under the corner columns depending on the rig whether it is DP2 or DP3 or moored only type. The mooring equipment arrangement is also a significant aspect of the column design, most notably chain lockers (or wire storage), hawsepipes, external fairleaders, and windlasses (or winches) at the top. For DP semi, should thrusters be installed for the dynamic positioning purpose, space and special arrangements must be provided for the thrusters and their internal support systems, not to mention the fuel tanks. The importance of this is the overall space and size, particularly in that this is in or near the column base. 

The height of the deck and columns matter most to weight estimating and meeting stability requirements. As noted previously, the columns should be sufficiently tall to support the deck with sufficient wave clearance. With single deck semi-submersibles, the column tops are flush with the deck. With hull-type decks, particularly if the column is integrally connected, the column tops may be in level with the upper deck.  

Bracing configurations vary considerably and include a transverse bracing, low on the columns, to resist squeeze/pry forces and, with these, a transverse diagonal bracing.  The diagonal bracing is both to support the deck weight and, together with the horizontal transverse, provide the lateral racking strength. Often, a system of the horizontal diagonals is used to provide racking strength against quartering seas.

A bracing system commonly found on many of the earlier generation drilling semis is shown here, where transverse bracing  in heavy dark lines and horizontal diagonals are shown in heavy dashed lines. Where continuous, strong longitudinal pontoons are employed, the longitudinal diagonals are not particularly useful and are rarely used in contemporary designs. As a structural system, the strength of the space frame truss system is typically developed in parallel series of planes between columns, following civil engineering practice, called “bents.” Each bent is a full truss, including the deck as a top chord and the horizontal, transverse brace as the bottom chord, all spanning between a pair of stability columns. Some use an “inverted-V” form of diagonals and some use an “inverted-W.” Except for a deck girder, the members consist of large diameter, thin walled cylinders. 

The Classification rules and the API Codes may require that there be 5ft (1.5m) clearance (“airgap”) between the highest wave crest and the deck. The highest wave, or the crest level above still water, is usually specified with the design seastate data. 

Excessive airgap raises the centre of gravity and thereby impairs the payload performance. Determination of the effective airgap should consider the relative motions of the vessel. For large, long period waves, a semi will tend to rise and fall synchronously with the waves, possibly as much as 20% of the wave height (single amplitude). To recognise this, in initial design, it can be conservatively assumed that the semi rises 10% of the wave height. 

For drilling semi units the, shorter columns are preferred for a lower centre of gravity for large deck loads. Drilling semi achieve deep submergence by ballasting to a deeper draft for drilling, but otherwise deballast to a desirable airgap for severe storms. It is also desirable to minimise the ballasting time and the amount of ballast water to be handled. Consequently, mobile semi-submersibles are no taller than need be, with operating drafts no more than necessary. 

For drilling rig. the maximum drafts could well be in the 70-80ft range, with a relatively small air gap. For the severe storm condition (“survival”), drafts in the 50-60 ft range would be used and a more generous air gap. Example, with a typical hurricane survival condition with Hs, = 40 ft, the extreme crest elevation would be about 45 ft above stillwater. Allowing a 5 ft of rig heave at the crest, and 5 ft crest clearance, a 45 ft calm water airgap should be sufficient. It is also considered undesirable that the pontoon tops be exposed in the trough of extreme waves. Under the same hurricane survival condition, the pontoon tops should be at least 40 ft below still water. With pontoon depth 25-30 ft deep, this would correspond to a 60-70ft survival draft and 85ft of column between the pontoon top and the deck. Correspondingly, the operating draft would be 80-90 ft with 25 ft stillwater airgap. The initial deign of a drilling semi-submersible would be based upon achieving the best drilling performance, and be based upon the shallower operating draft. 

A big issue in semi-submersible design criteria is the rig lifespan, inspectability, future class survey and repair. For permanently sited semi-submersibles, there are site-specific extreme environments and the fatigue requirements and the difficulties in structural maintenance, repair and inspections.

Conversely, mobile units can be dry-docked and can also be inspected and repaired afloat on the pontoons. However, the MODU classification rules do represent unlimited, world class service and this is actually quite severe. Also, most semi-submersibles give 30 years or more in service life. Quite often, the extreme design loads for mobile units are more severe than those of the permanently sited units. The opposite is true for the mooring systems, whereby the permanent structure mooring is usually subject to more severe requirements than the mobile units. 

Local strength is the consideration of whether the structure is sufficiently strong to resist the expected distributed load, particularly the hydrostatic pressures. This applies to the plating, the stiffening, and the framing of all watertight surfaces. It also applies to distributed deck loading as well as a variety of functional concentrated loads. In this connection it is significant that 80-85% of all hull steel is a consequence of local loading. For the column and pontoon shell, and the internal subdivision surfaces, a variety of hydrostatic heads are to be considered as potential controlling design pressures. At a minimum, the shell plating must be designed to resist the static loading for the most extreme operating draft without consideration of internal pressure. However, the water-tight shell must be designed for no less than a 20 ft head and this is where the watertight doors have to be able to take such pressure as well.

Global strength addresses the overall strength of the structure as a space frame, and of the main elements forming it. For a semi-submersible, the elements that form the space frame are the pontoons. columns, and deck and may include bracing. Global strength relates primarily to two types of loading systems: the gravitylbuoyancy load and the environmental loading. The direct loading of waves and the inertial load from consequent response are the principal environmental loads. What is unique to the global strength of the semi-submersibles is the controlling load patterns. 

An idealised distribution of deck load and concentrations of buoyancy forces at the pontoon and column lines is shown in the section view. An additional gravity load is included in the pontoons and the columns. A distribution of gravity loading on the superstructure must be supported by buoyancy concentrated at the extremities, causing a tendency to sag. This causes very large tensions in the horizontal brace to resist the sag. Additionally, the interior parts of the deck weight will transmit directly through the diagonals into the column. This exhibits one important function of the main bracing as primary structure. Particularly important are the end connections at the column, especially the efficiency of the load flow from the diagonal to the transverse. 

In semi design, there must be sufficient buoyancy to balance the weight of the rig and the external forces. The required buoyancy determines the underwater volume, or “displacement.” This comprises the volume of the pontoons, the columns, and, sometimes, the bracing. Displacement is a primary determinant of size and proportions. Consequently, much of the initial design work is devoted to determining all the components of weight.

Although payload and its height above the most extreme of waves were specifically identified as the salient factors in design, payload ( or also call VDL ) is only a part of the total weight. It and all other weights as well as its centre of gravity is needed to proceed with a design, at least a first estimate. This estimate should be continuously refined throughout the designing process.  Weight is made up of two components, “Lightship” (W,) and “Variable Load”. The former comprises all the steel, equipment, and outfitting provided at completion and is usually defined and verified according to regulation. The latter comprises all weight beyond the light ship to be carried by the semi: i.e. variables like the ballast, the consumable liquids, the bulk items, the personnel and effects, 3rd party equipment, fuel, etc. and, as the name implies, varies according to the operating state of the vessel. In addition, there are a variety of external loads to consider (e.g. mooring tensions. riser tension, hook load, etc.).

As for the term "payload", this comprises all of the mission-related equipment, variable load, and external load. The necessary support system weight that is needed, regardless of the mission function (e.g. mooring equipment and other “marine systems”), is not considered to be a part of the payload. But some rig owners may like to consider mooring anchor and its wire as VDL. Payload exclusive of deck structural steel, is referred to as net payload. However, if the deck structural steel is included, it is referred to as gross payload. The net and gross distinction is needed, particularly in comparing designs, because some mission functions can have a high impact on the amount of structural steel and is not an inherent property of the semi design. Such distinctions are particularly important when the same design is used for varied applications and also when conversion and upgrades are to be considered. This distinction is also needed in the evaluation of designs in as much as many designers are not consistent. 

Response Amplitude Operator (RAO)

A wave scatter diagram provides a long-term wave description for only one specific site. Determining the stress Frequency Response Function (FRF) or Response Amplitude Operator (RAO), H (0; an,&) is one of the major efforts in the strength assessment, because it allows the transfer of the exciting waves into the response of structures. This concept of linear dynamic theory is applicable to any type of oscillatory "load" (wave, wind-gust, mechanical excitation, etc.) and any type of "response" (motion, tension, bending moment, stress, strain

etc.).

For a linear system the response function at a wave frequency can be written as

Response(t) = RAO.q(t)

where V(t) denotes the wave profile as a function of time t. The RAO could be determined using theoretical computation or experimental measurement (Bhattacharyya, 1978). Almost all of the theoretical computation has neglected viscosity and used potential flow.

The structure may be envisaged in a general terms as a ''black box", see Figure 3.7. The input to the box is time-history of loads and the output from a structural analysis is time-history of the response. The basic assumption behind the RAO concept is linearity, that allows superimpose the output based on superimpose of the input. In these situations, the response to regular oscillatory loading of any waveform can be obtained by expressing the load as a Fourier series, and then estimate the corresponding Fourier series of the response for each component. A typical RAO is shown in Figure 3.8, that is a roll RAO of a barge in beam seas.

The RAO is given in degrees (or meters/A) of motion amplitude, per meter (or A) of wave amplitude and expressed as a function of wave period (second). The RAO may be calculated using the first order wave theory as wave fkequency response.

Another application of the RAO is to calculate loads in irregular waves. It is suggested that the total response of a vessel in an irregular seaway is the linear superposition of the response to the individual components that may be determined using RAO.

In the calculation of H (o,a&), a suitable range of wave frequency, number of frequency points, and wave headings should be used. The commonly used parameters for an FPSO analysis are:

Frequency increment: 0.05 rad/s

Wave heading: 0" to 360" with 15" increment

If a finite element method is used, the pressure distribution needs to be mapped from a hydrodynamic model onto a finite element model with NAXNFXNH loading cases, where:

Frequency range: 0.20 I o I 1.80 rad/s

TLP- Tension Leg Platform

Tension Leg Platforms have been used exclusively as production and drilling platforms,with the exception of the “East Spar” platform, which is a control buoy. Figure  shows the latest platform P61 recently completed in Brazil

Like semi-submersible, the TLPs consist of columns and pontoons. The unique feature is the mooring system, which consists of vertical tendons (sometimes called “tethers”), which restrain the heave motion.

As with the semi-submersible, the main problem in TLP design is to address the specific functions and rational sizing. Even more so than semi-submersibles, the construction program consideration is a salient design issue. Here too the approach is to initiate the

design with a straight forward process for sizing, and to discourage multiple trial-and-error analyses with inappropriately rigorous detail and methods.

The initial design should include the best working weight estimate, hull displacement, and hydrostatics available.

The key analytic areas for preliminavy analyses of the initial design for a TLP include the following:

Weights and CG’s

Wind Forces

Current forces

Global Performance Analyses

-Motions

-Drift force

-Tendon tensions

Global Strength

With a well executed initial design, a model can be quickly established for the above analyses to rigorously proceed in parallel without the need for additional major design iterations. Results for the preliminary analyses, based upon the initial design, can be used for a reasonably conclusive revision, this being the preliminary design. The initial design is also adequate for the beginning of specialised subsystem (topsides, tendons, riser, installation, etc.) design and analyses to proceed concurrently. The following is about how to develop such a model.

Unlike semi-submersibles, hydrostatics and stability are not salient design issues for a TLP, although they are important considerations for addressing transport and installation. The TLPs to date have been exclusively used for permanently sited production systems, most with drilling or workover functions. They have fewer functions to consider and therefore limited configurational variants.

Although it is implicit in any design, that the construction, transport and installation scenario is a particularly important aspect of TLP design with considerable impact on certain design choices. Usually, the hull and deck are separately fabricated, with either an inshore hull-deck mating or an offshore heavy lift. In either case transit, ballast to the operating draft, stability, and installation of the tendons are key issues in the design of TLP.

Besides the mission and support functions, the essential function of a TLP is to the support a payload above the highest waves. More specifically, the hull is to provide buoyancy, both for the support of weight and to provide tendon tension. It should also be tall enough to give the deck wave clearance in all modes of operation. Tendon tension has as much influence on hull size as the payload.

While the heave motion of a semi-submersible is a salient design issue. vertical motion of a TLP is far less of an issue and entirely different. While the TLP does not heave, it will undergo set-down with offset. Like a semi-submersible, the TLP is laterally compliant and will surge, sway and yaw. In both platform types, there is relatively little design-wise that can be done to affect lateral motions, although steady offset can be minimised by increased tendon tension.

The three main configurational components are:

Pontoons

Stability columns

Deck

Decks of TLPs (and some production semi-submersibles) are unique. Virtually all TLP decks are separately

built from the hull (often on a different side of the world) and joined later, either at dockside, offshore, or in a separate, sheltered location. An underlying issue with this uniqueness is more administrative than technical. With TLP hull installation benefitting from minimisation of topside weight, usually some parts of the deck system are placed after installation of the TLP. The deck-to-column interface is of considerable importance, with many options for load transfer and securing, and many being proprietary.

A semi-submersible is a true, free floating structure, restrained with compliant spread moorings and/or

dynamic positioning, a TLP is kept in place through lateral forces developed by the tendons when the TLP is moved off from centre. The lateral force is dependent upon the tendon tensions. Consequently, a major portion of the TLP buoyancy is devoted to development of tendon tension.

A TLP is highly compliant to lateral forces and. at the same time, is highly resistant to vertical forces. Offset from lateral forces is not altogether different from surge,’sway response of any compliantly restrained floater. However, what is truly different for a TLP is set-down.  Nevertheless, it is noted that tendon response periods higher than say 3seconds should be avoided in the initial design. To achieve this, tendons need sufficient stiffness relative to the TLP mass. This precludes certain material for tendons and tends to pose limits on TLP water depth.

Generally the internal outfitting within the TLP hulls is considerably less than found on semi-submersibles. Typically, there is no mooring equipment. In as much as there is very little active ballast, piping systems are much different. In service, most internal spaces are considered to be voids and may be piped differently than in a semi-submersible. The ballast is used for installation (and removal). but this may be through a temporary system deactivated after installation. Otherwise, any “operable” system must be maintained in working order. Often a less robust system is employed for damage control dewatering, but this depends upon the damage mediation strategy.  The TLP is essentially a fixed-draft, constant buoyancy system and, once installed, does not rely on floatation stability. It was also noted that a large part of the buoyancy is provided to develop tendon tension. While small changes in the sea level (e.g. tide) and set-down do occur. these result in small changes in tendon tension.

Variable load on TLPs is mostly on the deck and includes operating liquids, supplies, drilling and other consumables as well as personnel and effects. Liquid consumables (e.g. fuel, drill water, etc.) may be on the deck or within the hull. Additionally, and significantly, is the tension of top tensioned risers. Hull variable load will include ballast and risers, export in particular, but sometimes subsea well riser systems. Hull variable load on a TLP is usually quite small. While there may be very little difference between the normal operating loading condition and that for a severe storm, there can be significant differences related to the state of production riser deployment. There may, however, be special ballast distributions related to tendon tensions. There also may be special ballast distributions related to tendon failure or internal flooding. Without further detail, transport and installation will address a number of loading conditions, most expedient to the objectives.