1.0 INTRODUCTION

Mid Water Arch (MWA) is one of the buoyant subsea structures that tethered on the mean sea level by certain buoyancy forces. MWA is commonly used to support an array of flexible riser and umbilical to perform a designed riser configuration. Thus, the cumulative riser tension can be reduced at the both end of the riser and umbilical. This is able to protect the integrity of the flexible riser and umbilical. Apart from that, MWA is designed to maintain the allowable riser curvature and avoid the clashing or entanglement of riser with not affecting the touch down point of the riser.

1.1 Scope

The purpose of this document is to collate the design calculations that have been performed to access the structural integrity of Mid Water Arch System. This document outlines the ASME calculations and FEA calculations that have been performed for the design of MWA tank and MWA structure. In this report, the design basis of buoyancy tank will be discussed in detail and the effect of loading on MWA and buoyancy tank will be discussed in detail as well.

1.2 Abbreviations

1.3 References

1.4 Design Tools

Table 1 listed the design tools that have been used to perform the ASME calculations and FEA calculations for the design and structural integrity check for MWA tank and MWA structure

File Name SoftwareTask

MathcadDesign Calculations and structural integrity check of MWA according to ASME

Abaqus 6.143D Modelling and Structural integrity check of MWA tank and MWA structure

Table 1: Design Tools

2.0 MWA SYSTEM DESCRIPTION AND DEFINITIONS

2.1 MWA Riser ConfigurationsThe following figures below are the riser configurations that are commonly been used along with MWA as the supported buoyant.

Figure 1: Lazy-S configuration Figure 2: Lazy-wave configuration Figure 3: Plaint S configurationa. Lazy-S configurationLazy-S is made up with a tethered buoy between the lower J-catenaries and the upper U catenaries. This type of configuration is well suited in a wide range of water depths where the risers do not need to be tied back beneath the floating production vessel.b. Lazy-wave configurationLazy-wave over bended with multiple small buoys usually is set fairly deep to minimize riser movement from current and wave action. As depth increases beyond 1,000 m, setting lazy-wave riser buoyancy at more than 50% of total depth leaves significant weight applied to turret.

c. Plaint S configurationPlaint-S configuration is a lazy-S with the addition of a tether restraining the touchdown point.This configuration includes a lower tether and clamp attached to the riser just above the touch down. The lower tether reduces the dynamic loads in the riser at touch down and with the combination of bend restrictors, prevents over bending. This configuration is normally applied on shallow water where large vessel offsets are difficult to accommodate.All the riser configurations above are commonly used in conjunction with Floating Production Storage and Offloading (FPSO) facilities.

*Note: There are more than three types of riser configurations available nowadays. Based on lazy-wave and lazy-s riser configurations, the risers are supported by a buoy. Mid water arch (MWA) is commonly used as buoy to support the risers in order to form the risers configuration (Lazy-wave and Lazy-S configuration)

2.2 General Arrangement of MWA Structure

A MWA structure consists of buoyancy tanks, gutters and a gravity base are shown in Figure 2.

Figure 2: Components of Mid Water Arch Structure

2.2 MWA DESCRIPTIONMWA consists of two buoyancy tanks and is designed to support the risers and umbilical at certain weight with the gutters. The following describe the each MWA components in detail.

2.2.1 Buoyancy TanksThe tanks are designed to provide a sufficient net buoyancy to support the riser and umbilical at the designed water depth. Apart from that, sufficient net buoyancy is required for the tanks in order to prevent any slack in the tethers and the mooring system.

2.2.2 Horizontal GuttersThe horizontal gutters are provided at the chosen curvature to ensure that all the risers or umbilical lay at the correct operational moment bending radius (MBR).

2.2.3 Vertical GuttersThe vertical gutters are designed with certain number of channels and spacing in order to enhance the installation aid of the riser and umbilical. Apart from that, the gutters are designed to be curved in order to ensure that the operational moment bending radius (MBR) is always greater than the minimum allowable MBR.

2.2.4 Gravity Base

3.0 DESIGN DATAThere are several design data such as environment factors, marine growth weight and operating water depth have to be considered while designing the MWA system. The following are the summary of all the related and important design data that need to be concerned.

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