In recent years due to use of drilling risers with larger and heavier BOP/LMRP stacks, fatigue loading on subsea wellheads has increased, which poses potential restrictions on the duration of drilling operations. In order to track wellhead and conductor fatigue capacity consumption to support safe drilling operations a range of methods have been applied: • Analytical riser model and measured environmental data; • BOP motion measurement and transfer functions; • Strain gauge data. Strain gauge monitoring is considered the most accurate method for measuring fatigue capacity consumption. To compare the three approaches and establish recommendations for an optimal approach and method to establish fatigue accumulation of the wellhead, a monitoring data set is obtained on a well offshore West of Shetland. This paper presents an analysis of measured strain, motions and analytical predictions with the objective of better understanding the accuracy, limitations, or conservatism in each of the three methods defined above. Of the various parameters that affect the accuracy of the fatigue damage estimates, the paper identifies that the selection of analytical conductor-soil model is critical to narrowing the gap between fatigue life predictions from the different approaches. The work presented here presents the influence of alternative approaches to model conductor-soil interaction than the traditionally used API soil model. Overall, the paper presents the monitoring equipment and analytical methodology to advance the accuracy of wellhead fatigue damage measurements.

Subsea rigid jumpers are designed to meet numerous criteria including thermal and pressure effects, environmental and riser / pipeline interaction loads, slugging, and other field specific requirements. Jumper VIV can be a concern in fields with strong bottom currents. Without the benefit of detailed VIV fatigue analysis, designers must rely on experience and engineering judgement on placement of strakes if VIV is identified as a concern. VIV mitigation is even more challenging because the jumpers can contain numerous long and short design options to accommodate tolerances for subsea well locations and installation tolerances of subsea PLETS and manifolds. This paper will discuss a case study on optimization of 12 M-shaped jumpers designed for a sour service application in Gulf of Mexico. VIV fatigue assessment of the preliminary jumper design and the methodology adopted to optimize the jumper design and placement of VIV suppression will be discussed. Challenges in meeting high target fatigue life due to sour service application will be discussed. The key challenges whilst optimizing an acceptable VIV suppression solution are the assumed effectiveness of strakes, cost / available inventory of strakes, and physical limitations for placement of strakes. This paper will highlight the trade-offs that are required to strike a balance between strength and fatigue design requirements when using straked buoyancy modules vs. regular strakes. The paper will also highlight the current limitations in design code that relies on standards developed for pipeline application. An alternative method / modification to the DNV F105 approach used to calculate the cross-flow induced in-line VIV fatigue damage is also discussed.

In the current low oil price market, innovative low cost solutions are necessary for development of new fields and life extension of existing fields. For deep water applications, Steel Catenary Risers (SCRs) and Steel Lazy Wave Risers (SLWRs) provide low cost alternatives to flexible risers and offer flexibility during design and life extension for Floating Production Systems (FPS) in deepwater. The majority of deepwater fields in offshore West Africa and the North Sea have traditionally been developed using flexible risers. SLWRs, a variation of the steel catenary riser (SCR) with added buoyancy near the touchdown point at the seabed, have recently been deployed in the GoM and Brazil for deepwater applications. Due to simplicity of design, good track record and qualified suppliers, fabrication and installation methods for SCRs, SLWRs have become a logical extension of the SCR. Buoyancy installed on the SLWR helps reduce top tension and decouple vessel motions from the riser touchdown zone, which ensures that the required strength and fatigue performance can be achieved. SLWR and SCR configurations are developed for mild environment, deepwater applications representive of offshore West Africa and in severe environment moderate water depths, representive of the North Sea. Riser configurations, hang-off loads, strength and fatigue responses are compared for the configurations. The advantages and disadvantages of SCR and SLWRs are discussed and compared to other riser types. Costs for flexibles, SCR’s and SLWR’s are also compared.