20 YEARS DECOMMISSIONING EXPERIENCE - EVOLUTION OF INNOVATIVE SOLUTIONS

Decommissioning is an increasing market sector which has been gradually gathering momentum particularly in the North Sea. The forecast expenditure for removing existing platforms increases with time. This comes together with the increasing complexity of the decommissioning operation. Experience and expertise in this field are key for success. Saipem have been contracted to perform a number of ‘removals’ ranging from subsea templates, subsea pipelines, seabed debris clearance, jacket structures and topside modules. This paper provides an account of our experience gained over the last 20 years performing decommissioning activities. The paper presents the evolution of the techniques developed and focuses in particular on the Lift and Tow method developed after 2004 for a number of subsea applications. Problems always materialise post contract award due to inadequate data. The paper gives a detailed description of the Lift and Tow method along with various innovative techniques developed for this method, ranging from lifting operations supported by motion forecasting through to personnel access onto the structures.

An Experimental Study of Damage Detection on Typical Joints of Jackets Platform Based on Electro-Mechanical Impedance Technique

Many methods have been used in the past two decades to detect crack damage in steel joints of the offshore structures, but the electromechanical impedance (EMI) method is a comparatively recent non-destructive method that can be used for quality monitoring of the weld in structural steel joints. The EMI method ensures the direct assessment, analysis and particularly the recognition of structural dynamics by acquiring its EM admittance signatures. This research paper first briefly introduces the theoretical background of the EMI method, followed by carrying out the experimental work in which damage in the form of a crack is simulated by using an impedance analyser at different distances. The EMI technique is used to identify the existence of damage in the welded steel joints of offshore steel jacket structures, and Q345B steel was chosen as the material for test in the present study. Sub-millimetre cracks were found in four typical welded steel joints on the jacket platform under circulating loads, and root average variance was used to assess the extent of the crack damage.

Floating Wind Turbine Model Test to Verify a MoorDyn Modification for Nonlinear Elastic Materials

As the Floating Offshore Wind industry matures it has become increasingly important for researchers to determine the next generation materials and processes that will allow platforms to be deployed in intermediate (50-85 m) water depths which challenge the efficiency of traditional catenary chain mooring systems and fixed-bottom jacket structures. One such technology, synthetic ropes, have in recent years come to the forefront of this effort. The challenge of designing synthetic rope moorings is the complex nonlinear tension-strain response inherent of some rope material choices. Currently, many numerical tools for modeling the dynamic behavior of FOWTs are limited to mooring materials that have a linear tension- strain response. In this paper an open source FOWT design and analysis program, OpenFAST, was modified to capture the more complex tension-strain responses of synthetic ropes. Simulations from the modified OpenFAST tool were then compared with 1:52-scale test data for a 6MW FOWT Semi- submersible platform in 55m of water subjected to representative design load cases. A strong correlation between the simulations and test data was observed.

An Efficient Solution to Mudmat Design for Jacket Structures in Soft Seabed Soil

Conventional jacket structures are normally equipped with mat foundations for support during offshore installation when the jacket sits on the seabed before piling. An efficient mudmat design is required to support the jacket since the weight of the mudmat contributes about 20% to the overall structural weight. It is challenging to analyze and to find an exact solution when calculating the bearing capacity of the soil beneath the mudmat because the seabed conditions vary from hard to very soft soil: this is especially true for a relatively slender jacket on very soft soil. The paper presents an efficient method for conducting such design.

Life-Extension Project Applies Assessment of Reinforced Concrete to Nonjacket Structures

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 31250, “Wandoo B: Application of Advanced Reinforced Concrete Assessment for Life Extension for Non-Jacket Structures,” by Robert Sheppard, Spire Engineering; Colin O’Brien, Vermilion Oil and Gas; and Yashar Moslehy, Spire Engineering, et al., prepared for the 2021 Offshore Technology Conference, originally scheduled to be held in Houston, 4–7 May. The paper has not been peer reviewed. Copyright 2021 Offshore Technology Conference. Reproduced by permission. Wandoo B is a concrete gravity-based structure (GBS) and is the main production facility for the Wandoo field offshore northwest Australia. It was installed in 1997 with a design life of 20 years. The structural assessments discussed in this paper are part of a comprehensive life-extension project encompassing wells, subsea systems, marine and safety systems, and topsides facilities and structures to demonstrate fitness for service through the end of field life. Background The GBS serves as the support structure for the Wandoo B facility and provides oil storage for the Wandoo field. The structure has four shafts approximately 11 m in diameter that support the top-sides facilities and a base structure with permanent ballast and oil storage cells (Fig. 1). It was originally developed as an ExxonMobil-led project and now is owned and operated wholly by Vermilion Oil and Gas Australia. The reinforced concrete (RC) shafts and the base top slab are pretensioned. In the shafts, tendons are enclosed in 20 ducts distributed around the circumference. The top of the shafts provides a mating point with the steel topsides structure with the connection formed by embedded anchor bolts in a bulge in the shaft cross section. The topsides structure is a three-level braced steel frame system supporting production operations for 12 well conductors contained within the northeast shaft and three outboard well conductors. Life-Extension Project The facility was designed with a target life of 20 years. The life-extension project was intended not only to satisfy the operator’s responsibility to continue safe operations and adhere to their safety case but also to meet the expectations of the regulator. The structural aspects of the project included four phases, the first two of which are detailed in this synopsis: - Design assessments per latest standards and modifications where required - Ultimate capacity assessments with retrofit modifications where required - Risk studies and workshops to demonstrate that risk is as low as reasonably practicable (ALARP) - Integrity-management manual and inspection plan The first two phases were addressed using the latest condition-assessment, weight, and environmental data available. The phased approach allowed the assessment team to use basic linear approaches to demonstrate code compliance and only use the more-advanced analysis techniques to evaluate the critical components that did not satisfy code or were needed to provide input to the ALARP assessment and establish target reliability for the facility.

Fatigue Strength Curve for Tubular Joints of Offshore Structures under Dynamic Loading

Offshore structures are subjected to dynamic environmental loads (wave and wind loads). A stress-life fatigue strength curve is proposed for tubular joints which are in the splash zone area of offshore jacket structures. The Det Norske Veritas (DNV) offshore structures standards given design T-curve in the air is modified with the environment-dependent parameters to obtain this fatigue strength curve. Validity of the curve is done by comparing fatigue lives given by the proposed curve with experimental fatigue lives of tubular joints tested in seawater under different loading conditions. The fatigue assessment of a case study tubular joint is performed using the proposed curve. Nominal stress ranges of the members, which are connected to the joint, are obtained by dynamic analysis of the jacket structure. Stress concentration factors are utilized with the nominal stresses to obtain the hot spot stress ranges. Fatigue lives are calculated and compared with the conventional approach. Hence the applicability and significance of the proposed fatigue strength curve are discussed.

Wandoo B: Application of Advance Reinforced Concrete Assessment for Life Extension for Non-Jacket Structures

Wandoo B is a concrete Gravity Base Structure (GBS) and is the main production facility for the Wandoo field offshore NW Australia. It was installed in 1997 with a design life of 20 years. The structural assessments discussed in this paper are part of a comprehensive life extension project encompassing wells, subsea systems, marine and safety systems, topsides facilities and structures to demonstrate fitness for service through the end of field life (EOFL). The challenge was to demonstrate compliance efficiently and effectively for a large structure with a range of materials (steel, reinforced concrete (RC)) and operations supported (oil storage, drilling, production) under increased loading criteria compared to the original design. There is comprehensive industry guidance for assessing existing steel jacket structures, but far less for a concrete GBS such as Wandoo B. Demonstrating compliance required a combination of computer model results, project-specific tools to check reinforced concrete sections, and engineering judgement to define how much damage constitutes failure. A number of global and local structural models were developed to assess the linear and nonlinear performance of the reinforced concrete and steel structure. A phased approach was employed using basic, conservative approaches in initial phases to demonstrate code compliance, and progressing to more advanced, less conservative approaches for those components under higher stress. Developing models that more accurately simulate the behavior of the different structural components and materials was a large part of the project scope, particularly for the nonlinear behavior of the reinforced concrete and the interface connections between the steel and reinforced concrete structures. It was inefficient to develop a detailed steel and reinforced concrete solid model of the large GBS shafts and base, so an equivalent shell model was developed and tested to determine the global behavior and onset of damage. This equivalent model aimed to predict behavior accurately for metocean and seismic loads under material tension and compression. Local detailed models were then developed including a constitutive model of reinforced concrete and used to define the extent of the damage and predict where failure would occur.

RMS Control of Response of Jacket Structure Using Tuned Liquid Column Ball Gas Damper Under Random Ocean Waves

Jacket structures are one of the most important offshore structures for extracting oil and gas. The fatigue life is affected due to the continuous dynamic wave force experienced by the structure. Generally, the structure is designed so that the dynamic response is small, which increases the cost. So, controlling its response is a good alternative to increase its life span. In this work, a simplified jacket structure under a random sea state is controlled for its response using a tuned liquid column ball gas damper (TLCBGD). The jacket structure in a water depth of 60m is modeled in a surge degree of freedom. The parameters of TLCBGD are optimized using a genetic algorithm for achieving better control in response quantities. For the analysis purpose, the wave is considered stochastic and presented by Pierson–Moskowitz (PM) spectrum of significant wave height 10m. In such a case, the jacket structure response can be presented using the root mean square (RMS) values obtained from the Lyapunov technique. Based on the random vibration analysis theory, the Lyapunov method can be employed to obtain the RMS of the system driven directly without solving the governing differential equation. This method requires the system to be driven by white noise. So, in this study, filters are developed to get the required narrow banded ocean spectrum. It is noticed that the response quantity is highly sensitive to the filter parameters. This is because a slight change in excitation parameters and a change in filter parameters near the system’s natural frequency affect the response significantly. Further, it is seen that the use of the genetic algorithm for tuning the TLCBGD gives very good control on the response quantity of the jacket structure.