Seismic Design Challenges of High Pressure Riser Systems on Gravity Based Structures

Abstract

This paper presents two case studies of the seismic analysis of high pressure riser and conductor systems used on shallow water fixed platforms (approximately 120m water depth) offshore Newfoundland and Labrador, Canada. The case studies presented consider the Hebron and White Rose extension Husky gravity-based concrete platforms. A methodology is presented to assess the dynamic and resonant response of each conductor and riser system to seismic loading using a fully integrated conductor and platform interaction model. Seismic loading is an application of an earthquake-generated agitation to a structure. This occurs at contact surfaces either with the ground, or with adjacent structures, or with gravity waves. Nonlinear time history analyses of the riser system subjected to various ground motion records are performed to simulate the seismic load. Design considerations that drive the HP riser design are discussed. The paper addresses the intricacies of the gravity base concrete platform-riser system interaction, initial configuration and dynamic seismic response. The riser and conductor system response is used to determine HP riser connector and centralization requirements. The learnings taken from the detailed modeling method are presented along with the advantages of this methodology. NOMENCLATURE HP – High Pressure GBP – Gravity Based Platform CCG – Continuous Conductor Guide BOP – Blowout Preventer BHGE – Baker Hughes, a GE Company C-NLOPB – Canada-Newfoundland and Labrador Offshore Petroleum Board ID – Internal Diameter OD – Outer Diameter VBR – Variable Bore Ram DES – Drilling Equipment Set UPM – Utilities/Process Module SME – Subject Matter Expert INTRODUCTION Gravity based concrete platforms (GBP) are unique offshore structures that facilitate oil and gas production and wind power generation in locations that offer extreme operational challenges. Traditionally these structures were deemed essential for developments that require oil or condensate storage and applications that involved heavy topsides, [1]. Off the coast of Newfoundland, Canada, the gravity-based structures enable production operations in the harsh ocean conditions of the North Atlantic. These conditions include extreme ocean waves, sea ice and icebergs. The fully enclosed conical (shown in FIGURE 2) or stepped caisson designs provide a safe enclosure for the platform well conductors and offer protection against direct impact loading from both floating sea ice and large-scale icebergs. The average weight for a Grand Banks-area iceberg is 100,000-200,000 tonnes about the size of a cubic 15-story building. However, the stiff steel reinforced concrete structure of the GBP can pose different design challenges for the well conductors and the top side high pressure risers which are more Learn more at www.2hoffshore.com 2 Copyright © 2019 by ASME compliant structures. These design challenges are exacerbated in areas of high seismic activity. TYPICAL STRUCTURES On a gravity-based concrete platform the structural conductor and casings of each well come all the way up to the surface wellhead. The conductor casing is designed to support the weight of the subsequent casing and tubing strings and is cemented back to the CGS (concrete gravity substructure) base. The HP (high pressure) riser is a pressure containing structural member that must withstand all the operational loads and accidental loads induced by motions of the platform. The conductor structure on which the HP riser is installed can have different types of support structures, boundary conditions and loading mechanisms that can affect the compliance and rigidity of the entire conductor-wellhead-high pressure riser assembly. The conductor below the HP riser itself can have multiple configurations. For example: • A continuous conductor guide (CCG) is sometimes used to protect the conductor. • The conductor or CCG may be centralized at one or more platform guides. • The conductors and casings may be fully cemented or uncemented. • The conductors may support varying numbers of concentric internal well casings. The basis of this paper are 2 platforms, the Hebron and White Rose platforms for which BHGE has supplied the HP riser assembly. While the results presented are mostly based on analysis conducted for Hebron, the system on the White Rose platform is used to describe the design philosophy and to highlight the variations in platform conductor and HP riser designs. On the Hebron platform, the conductor is installed in a continuous conductor guide (CCG). The CCG is laterally supported by the GBP decks at several locations along the length. The conductor is installed in the CCG without any centralizers, thus the loads transferred to the conductor are dependent on the initial preset configuration of the conductor pipe inside the CCG. For the Husky platform the conductor is directly supported by lateral restraints at various elevations without a CCG as shown in FIGURE 1. Configuration 1 shows the conductor with the casings fully cemented, whereas configuration 3 shows a contingency scenario of the casings uncemented. FIGURE 1-CEMENTED AND UNCEMENTED CONFIGURATIONS FIGURE 2-TYPICAL GRAVITY BASED PLATFORM CONFIGURATION, HEBRON DESIGN CONSIDERATIONS In the initial phase of high-pressure riser design for such an application, the OEMs use pressure rating of the riser as the key Learn more at www.2hoffshore.com 3 Copyright © 2019 by ASME parameter for riser pipe design selection. In the tendering or preFEED phase riser pipe rated for higher than the anticipated shutin, test and accidental pressure surge conditions is selected. Using the same philosophy, a riser connector that is of equivalent pressure rating or higher than the riser pipe is selected. A detailed riser analysis is not feasible at this stage due to the lack of availability of interface load data at this stage in the project. The emphasis is on ensuring all functional requirements are met with the proposed design and include the following: • Drift diameters of the riser pipe and connectors; • Wall thickness specified for anticipated design loads and pressure requirements; • Material selection for the application; • Weldability of pipe and connectors; • Installation considerations; • Environmental loading; • Weight limitations of handling equipment; • Angular misalignments and its effects on functionality. Detailed global riser analysis is typically undertaken during the execution phase to ensure that the riser system can withstand all the system loads in addition to the design pressure. There is no single design code that covers the design of a HP riser for a fixed platform application. Hence, designers typically rely on engineering judgement and design codes that are partially applicable to the riser design. API RP 2RD is used for dynamic design of risers for floating production platforms; this code can be used as a guidance for structural design of the HP riser. The HP riser, however interfaces with the platform conductor. Fixed platform designs rely on guidance from API RP 2A-WSD and The Institute of Petroleum’s Guidelines for the Analysis of Jackup & Fixed Platform Well Conductor Systems. Thus, the designer must apply multiple design criteria simultaneously to ensure the system can withstand the loads expected during its life cycle. The designer must also consider regional permitting requirements. For the two projects described in this paper, operating offshore Canada, the HP riser design must comply with Canada-Newfoundland and Labrador Offshore Petroleum Board (C-NLOPB) Guidelines. Considering the different operating modes and loads anticipated throughout the lifecycle, the following riser design criteria are identified: • Global strength and stability • Internal overpressure for normal operating, extreme, accidental and system test pressure • Installation loads encountered during different stages of installation with associated configurations • Environmental loads including wind, waves and current • Seismic loads for a range of seismic events. Upon review of all the design considerations a load case matrix is developed such that all potential loading scenarios are assessed. Of all the different loading scenarios experienced by the riser, the wind loading and riser buckling checks are deemed as Quasi-static loads and can be assessed in isolation independent of the conductor. However, the dynamic loading scenarios require a system level approach such that the interaction between the HP riser and the platform conductor and the interaction between the platform conductor and the concrete platform structure are correctly captured. For the purpose of this paper, the analysis performed on the Hebron platform will be fully discussed. Design of the HP riser for global strength & stability checks under static construction loads and for internal pressure is done using code checks. For stability the riser joints must support the weight of the surface BOP. In addition, when the drill string is hung-off the variable bore ram of the BOP, the entire weight of the drill string must also be supported by the riser. These loads are experienced as compression by the riser, possibly inducing buckling in the riser pipe. Treating both these loads as column buckling loads can lead to over design of the riser joints. The BOP and drill string compression loads are inherently different in nature and can be treated as external and internal compressive loads as per the refined Baur and Stahl method approach, [3]. This method is based on modified API RP 2A WSD criteria, [2] and is commonly used in the assessment of platform conductors. Burst checks for internal operating pressure and test pressures are performed using von Mises stress formulations given in API RP 2RD, [4]. The structural response of the HP riser should be assessed for different stages of installation in addition to the final installed configuration. It is possible that during installation, the rise