The fast growing offshore industry has come up with advanced technology that makes it possible to produce oil and gas on deeper offshore sites. One of the interests now is to look for a structure and systems that are reliable and cost effective for ultra deep water operation. Tension leg platform (TLP) concept is currently been viewed for this ultra deepwater operation. TLP differ from other type of floating platform in a sense that the hull buoyancy exceed its weight, the hull is vertically moored at a draft below the displacement thus creating constant tension in the mooring line. The problem for the development of TLP beyond 1500 meters water depth is the material for the tendon. Conventional tendon made by steel is too heavy and proven to be unpractical in ultra deep water. New material is needed, that is lightweight and stiffer, and this material needs to meet the structural performance characteristic for practical development. This research is intended to demonstrate the performance of reduced weight tendons in ultra-deep water. The effect of weight reduction on motion response of the TLPs in regular and irregular waves and the performance differences between conventional tendons and reduced weight tendons in term of motion response also will be investigated. Based on the review it is known and widely accepted that numerical analysis is the most practical and economical way to analyze the structure, but because of the simplification and assumption of the numerical calculation a validation is needed. The numerical analysis will be done using industrial accepted software, ANSYS AQWA. For the purpose of validation, model test is proposed to be done in the test basin, but due to very limited water depth in the test basin, truncation method will be used instead of common traditional test. This method also known as hybrid method that uses numerical calculation to predict the cut off portion of the tether lines and using physical model test in the upper portion.
The motion of a structure floating or submerged in the sea and exposed to various environmental loads have long became an interest of scientific and industrial communities all over the world. Understanding and predicting this motion is very essential for the design, development and safe operation in the sea especially in remote areas. The idea of using a compliant offshore platform such as TLP was not entirely a new idea, works have been done to develop this kind of system start already in sixties. This chapter of literature review will discuss a great deal of works that have been done considering various aspects that affect the design and safe operation of the TLP. These include the environmental loads that the structure will endure, the tendon integrity itself and the various type of analytical approach. Numerous studies also been reviewed that address various response of the TLP when exposed to the random loading. Finally the literature review will discuss the type of model test that have been done to address the ultra-deep water condition and an overview of rules and regulation for these compliant structure.
1.2 Environmental loads
The most important part that needed to be considered in any of the design, construction, installation, and operation of any offshore structure is the environment that the structure will be placed in. This environmental factor or forces that act on the structure involved waves, wind, tide, and current. Other extreme cases such as earthquake, hurricane and storm also sometimes seem realistic to be considered. Failure to understand or predict all these various forces spell catastrophe.
The basic major forces that act on the fixed offshore structure are wind, waves and current. Various studies until this day focus on these forces. Li and Kareem (1990) conducted a study of the dynamic behavior of the TLP under coincident loading of wave fields and random wind. To address and accurately evaluate the diffraction and radiation forces, a computer code that based on the boundary element method (BEM) is developed.
In the analysis they found an excellent agreement between the frequency domain and time domain analyses with the response of a typical TLP in six degrees of freedom. This agreement is manifested using power spectral density of the surge response of the structure.
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Figure: Power Spectral Density (PSD) of TLP Surge Response (Li, 1990)
The effect of regular waves on TLP is presented by Jain (1995). The nonlinear coupled response of the TLP is explained in detail base on the dynamic analysis. He noted that because of the natural periods in surge, sway and yaw is evidently more than heave, roll and pitch the resonance that correspond to surge, sway and yaw is not likely to take place. Jain also found that the resonance effect with heave frequency could trigger a large fluctuation in the tension of the TLP tendon. This happen primarily due to the fact that the heave period is regularly close to wave periods that frequently occur. With all of this good result especially in the TLP response with respect of surge, sway, yaw, heave, roll and pitch, it seems that more need to be done to understand the behavior of the TLP response, especially in regard of irregular wave which reflect a closer reality which was happening in the sea.
The fast growing industry also triggers the study of effect of the incoming waves and their effect to the TLP in greater detail. S. Chandrasekaran et al. (2006) presented a technical note discussing the response of TLP from varying waves angle. The Stokes fifth-order nonlinear wave theory is used to model the hydrodynamic loading. They concluded that the change in the angle of wave approach have a direct influence to the dynamic response of the triangular TLP. This is true to all degree of freedom except for the heave response. They also observed that the response of the TLP from pitch and roll point of view are highly stochastic in nature. This shown that these motions indicate high degree of randomness. These finding maybe true for TLP with triangular geometrical design, more need to be done for other type of TLP geometry model to further confirm all these responses.
Extreme wave effect on the TLPs is also an essential part that need be understood for the development of TLPs in deepwater. T.B. Johannessen et al. (2006) presented a study on extreme wave effects based on Snorre A model test. The study focus on accidental wave event and how to analyze them. It also touched on the measure need to be taken for the design. This study however may not applicable to the deepwater or ultra deepwater. The Snorre A TLP installed in the shallow water in the North Sea, with only 308 meters water depth.
To evaluate the tether loading accurately due to harsh environmental forces T.B. Johannessen et al. conducted hydrodynamic model test with a scale 1:62.5 in the wave basin. This test is expected to clarify some of the major concern such as offset and slowly varying motions, drift forces and direct response to waves. Although the test is a great help in clarifying these issue some constraint also presented. These test exclude the effect of inconsistent and changes in mass distribution in the topside and it is also cannot be employed to closely understand the riser dynamics in the TLP overall system.
Other extreme cases including the effect of vertical seismic excitation on the TLP motion response also become the centre of many studies. Kawanishi et al. (1987) analyzed TLP motion with earthquake excitation. Base on the analysis, they recommend increasing the number spring constant to reduce the maximum relative displacement. They also found that between maximum absolute acceleration and the change of the ratio basically have no clear and consistent relationships. A good note that interesting to address in this paper is that this finding are all base on theory with many assumptions, a good experiment need to be done to test this theory.
The effect of this seismic excitation is further studied by Chandrasekaran and Gaurav (2008). They tested a triangular TLP under high waves and done the earthquake motion analysis. They found in the result that based on Kanai-Tajimi ground acceleration spectrum, tether tension showing a nonlinear variation. This variation is analytically higher than the tether tension set by the rules and regulation and call for examination for seismic safety purpose. They further verify using the numerical method that a much less response in shown by the TLP built on deeper water to the wave and earthquake forces. Their focus on triangular TLP, suggested that further studies need to be done to understand the response of other various geometrical type of TLP.
Extreme weather case such as the occurrence of Hurricane Lili force an thorough assessment for deep water structure especially in regard with the structural integrity. Morandi et al. (2004) provide a statistical analysis of the TLP tendon tension in the course of hurricane Lili. These data, that been taken based on Shell Brutus TLP. They found that the TLP behave well and the measured responses were less than the design limit. This favorable result is reached although hurricane Lili environmental condition approached 100-year value. There is no notable loss of tendon tension during the extreme environmental event, this suggest that the design can be improved economically.
Hurricane Katrina that struck in 2005 call for modification of future development of TLP in Gulf of Mexico. Murray et al. (2008) presented a new design procedure for TLP in the region of Gulf of Mexico, this new design must consider the new metocean criteria after the hurricane happened. To measure the response of TLP for this rare extreme condition a model test is set up on the scale of 1:92 in the basin. A worth notable result from the model test show poor agreement using uncoupled model, these lack of agreement including setdown in the extreme offsets, riser and tendons prediction, and also extreme response. In contrast the fully coupled simulation shows good consistencies with the model test data measured. A lot of work still needs to be done to improve the TLP responses in the extreme weather cases.
1.3 TLP tether system
Although many research focuses on the environmental loads acting on the TLP, all these environmental loads are natural, some of them are proven to be random, cannot be changed and must be understand and accepted as a challenge for the design of the TLP offshore. But one always can improve the structure and design of the TLP. It is obvious the most important structural part of the TLP is the tendon or the tether system. Over recent years many researchers work on the tether part of the TLP. They mainly focused on fatigue, reliability, stability, length, cost effective and hydrodynamic part of the TLP’s tether.
One of many reasons a great effort been put to further increase the tether reliability is the fact that the high cost of in-service inspection. Wirsching et al. (1995) based on Heidrun TLP that been installed in the Norwegian sea done a maintainability analysis. They also included the fatigue and fracture reliability analysis for the TLP tether system that constantly exposed to the tensile load that is oscillating during the operation. This analysis claimed to quantify the performance of the structure and to check the structural integrity. They noticed that compared to fracture in course of extreme loading, fatigue remain as a dominant mode. The estimated reliability also enhanced notably when the assumption is made regarding the stress endurance limit.
Also based on the Heidrun TLP, Dalane (1998) done a fatigue reliability analysis. The study discussed the comparison of the numerical calculations and the forces acting on Heidrun tethers during the operational phase. He noted that to increase the fatigue reliability, the most effective way is to measure the fatigue loading. He further stated in the result that the cause of TLP resonant response for heave, roll and pitch periods is also because of first-order response that occur and not primarily caused by the second-order of loading. To better design the TLP, further studies need to be done to address this high frequency tether response.
Siddiqui and Suhail (2001) come up with the assessment of TLP tethers in term of reliability. This assessment used mainly fatigue reliability and fracture mechanics approaches. As the random loading from waves and wind constantly acting, the tether is exposed to fatigue failure. To estimate this damage Palmgren-Miner’s rule is used while the estimation of reliability is done using Monte Carlo simulation and first order reliability method (FORM).
It is marked in the conclusion that the increase of water depth with a constant tether length shown a decreasing reliability index. They also noted that single tether joint is more reliable than a number of tether joints. While for the overall system the reliability index is marked at about 30%. These finding is very important especially for the designer in the early stage of TLP development, but as the industry grows and the exploration began to go deeper there is a need to further increase the reliability.
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Figure: Heidrun Tension Leg Platform (Wirsching, 1995)
The needs to better understand the reliability of TLP tendons in shown by the emerging studies on this subject. Cicilia et al. (2008) presented design criterion for TLP tendons base on reliability. To generate the partial safety factors using the storm environmental situation in deep waters from Campeche Bay offshore Mexico, a long-term reliability-based method is applied. They found that a less scattered distinction of reliability indexes when the tendons designed using Load and Resistance Factor Design (LRFD). The result also observed the change of tendons thickness when the safety factors is calibrated. These findings serve as a good basis to further understand the methodology to assess the reliability, but other factors also needed to be address in order to establish a final safety factor for a specific TLP. Among of the factors include hull size, tendons material characteristic, updated environmental data and geometrical configuration of the TLP.
Besides the reliability part of the TLP tethers, the stability of the tethers also is a major concern from the design point of view. Chandrasekaran et al. (2005) used Mathieu stability analysis in order to define the value of vibration in the tethers. The numerical calculation proven the fact that to achieve the stability with increasing hydrodynamic loading, the tether tension needed to be increased. This increasing tether tension is also is a good help to gain platform stability. The study also noted that for the application of TLP in deep water, higher initial pretension is essential to improve the stability. This understanding provide a good basis in order to understand the relationship between increasing water depth and tether pretension.
The hydrodynamic part of the TLP tendon also one of the aspect that cannot been discarded when designing the TLP tendon. Wang and Zou (2006) discussed this subject and focused on several key hydrodynamic factor that include tendon springing and ringing response, vortex induced vibration (VIV) and tendon bottom tension slacking. De Boom et al. (1983) compared computational method based on potential theory and experiment in model basin to predict the tether motion and forces. They emphasize that during the early design phase the computational method are fairly applicable. They further noted that because of small underestimation of wave drift force using the computational method some inconsistencies also found. These inconsistencies need to be investigate further.
El-gamal and Essa (2013) conducted a study to observe the effect of tether length to the response of square TLP. The numerical calculations were executed in the time domain using modified Morrison equation with Airy’s linear wave theory. The analysis however considers only the regular wave from surge direction. Based on the result they establish the relationship that is directly proportional between tether length and the natural time period. They also observe the decreasing tether stiffness when the tether tension is increased. This finding however needs to be further investigated considering all other wave propagating from other direction beside the surge direction.
1.4 TLP tendon material
The material aspect of the tendon is one of the big concerns in the development of TLP. Going deeper than 1000 meters made steel tendons unpractical, this is because of the fact that the weight is excessively high. Steel tendons also have a serious issue with elasticity that causes a resonance problem. Other factor that excludes the use of steel tendon in deep water is the cost effective issue. All these pushed the development of a new lightweight tendon.
One of the early notable and important studies on TLP tendon is made by Hanna et al. (1989), where they account the effect of several parameters including tension, weight, material and hydrostatic pressure on the TLP tendons. Lightweight materials are proposed to reduce tendon top tension, which have a lower tendon weight in air. Steel tendons can be used in deep water by increasing the ratio between diameter and wall thickness (D/t), but due to hydrostatic collapse in deep water, D/t ratio for steel tendons allowable only up to 15. For this reason new lightweight material are needed for deep water purpose, and their performance need to be clarify.
The performance of composite tendon compared to conventional steel tendon is studied by Wu (1999). This comparison shown some several superiority of composite tendon compared to traditional steel tendon, these includes significant reduction of horizontal offset, reduce pretension and maximum dynamic tension. The results of the TLP motion at center gravity are given in Table 3.
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Table 3 - Summary of TLP motion at center gravity (Wu, 1999)
As the steel tendons are viewed as unpractical and uneconomical, new materials have been developed to provide alternative solution. Botker et al. (2002) discussed the use of composite that are been developed by the Composite Alliance that consist of Kvaerner and Conoco in 1995. This composite material that consists of carbon fiber rod is set to be used for tether with 3000 meter water depth. They stated that composite tethers are better in term of stiffness to weight ratio and also strength to weight ratio. These composites have been proven to be competent through various testing including prototype testing.
The compatibility of carbon fiber composite to be used in the tether for TLP in ultra deep water is further clarified by Sparks et al. (2003). The new material that been discussed is the carbon fiber composites that have been developed by joint action of Freyssinet, Soficar, Bostik Findley, Institut Francais du Petrole and Doris Engineering. The carbon fiber tendon which consists of 19 rods is able to overcome the water pressure in deep water without damage, their also naturally buoyant and can be spool to be shipped to the field location. Other new technology in materials such as cellular tendons as proposed by Yu (2013) also provides a possible alternative for a more cost effective material in deep water. With all these structural capabilities of the tendons, a thorough analysis from economic point of view must be performed considering the material cost, fabrication and installation to establish its applicability.