West of Africa CALM Buoy Offloading Systems

1 West of Africa calm buoy Offloading Systems Predicting the coupled response of calm Buoys for Offloading system design Michael O SullivanMCS InternationalIntroduction This article discusses calm (Catenary Anchor Leg Mooring) buoy -based Offloading Systems with emphasis on the prediction of calm buoy motions and the critical design and analysis requirements for Offloading lines. Recent developments in first and second order coupled response analysis are presented. FPSO-based hydrocarbon production in remote, deepwater fields presents challenges to traditional Offloading methods.

2 This has consisted of Offloading via a shuttle tanker adjacent to an FPSO or to a pipeline system to shore. Safety concerns in deepwater and the size of export tankers (VLCCs) precludes the former. Because of the large distances from these fields to shore, the latter option is not cost effective. As a result of these difficulties the use of calm and TALM (Taut Anchor Leg Mooring) buoy Systems are considered particularly suited for remote deepwater FPSO Offloading Systems in areas such as West of Africa (WoA). Fig.

3 1 shows a typical deepwater field layout incorporating a deep draft floater, FPSO, Offloading lines and calm buoy . Typically a distance of up to 2km could separate the FPSO and calm buoy . It is not practical, in deepwater, to run the Offloading lines along the seabed. Consequently the Offloading lines typically take up a wave-type configuration suspended below the wave zone as shown in Fig. 2. Because the Offloading lines have to accommodate large volumes, they tend to be of large diameter (> 16 inches). For this reason the lines tend to be heavy (steel or flexible pipe); this, coupled with the desirability of a wave configuration necessitates the use of distributed buoyancy.

4 The resulting wave configuration also provides flexibility in the system to accommodate the relative motions of the FPSO and calm buoy . Due to the separation distances, diameter requirements and cost issues it is preferable to use steel pipes for the Offloading lines. However the wave configuration has proven to be sensitive to excessive fatigue damage, in particular due to large surge motions experienced by the calm buoy . Accurate prediction of these motions is therefore critical for the fatigue analysis of the Offloading lines.

5 The Offloading system is a coupled dynamic system in which the attached mooring and Offloading lines, the FPSO and shuttle tanker influence the motions of the calm buoy . The response of each component will be influenced by coupling of first and second order wave loading on the system as a whole. Furthermore, hydrodynamic and mechanical coupling also exists between the various components of the system . Coupling occurs between the following components: i) Offloading lines and mooring lines ii) First order response of the calm buoy iii) Second order response of the FPSO and/or shuttle tanker iv) First order response of the calm buoy , Offloading lines and mooring lines De-coupled vs.

6 Coupled Approaches Traditionally, the prediction of the motions of floating bodies, such as calm buoys, and the prediction of loading and stresses in attached components such as mooring and Offloading lines is carried out in a de-coupled approach, using a two-step procedure. This is shown schematically in Fig. 3. First, the mean, low-frequency and wave-frequency motion response of the calm buoy is determined by de-coupling it from the mooring and riser Systems , in a radiation/diffraction analysis. The stiffness characteristics of the riser and mooring system may or may not be included.

7 Second, the vessel motions calculated in the first step are applied as prescribed boundary conditions in the analysis of the Offloading system [1]. With increasing water depth the de-coupled approach described above no longer adequately captures the interaction between the calm buoy and the attached lines (mooring and Offloading ). This is principally due to the effects of dynamics of the mooring and Offloading lines and the magnitude of the mooring and Offloading line inertia being of similar order to that of the calm buoy itself. All relevant interactions between the calm buoy and the mooring and Offloading Systems can be taken into account by using a fully coupled finite element analysis procedure in the time domain (Fig.)

8 4). In the coupled analysis of the Offloading lines, the floating vessel is regarded as a nodal component in the finite element model at ExxonMobil 1 Deepwater Field Layout Fig. 2 Offloading Line Wave Configuration which vessel forces, including wind, wave, current and hydrodynamic, are applied. The inertia, buoyancy stiffness and damping effects of the calm buoy , Offloading lines and moorings are modelled. Mean loading forcing functions due to wind and current on the calm and subsurface structures are considered together with first and second order wave frequency dependent, inertia, damping and forcing functions on the floater.

9 Specifically, for the coupling of first and second order inertia, damping and forcing functions, a Fast Fourier Transform (FFT) algorithm is used to calculate the spectrum for each ensemble. The actual output spectrum is calculated as an average of all individual ensemble spectra. The general equation of motion for a floating structure in six degrees of freedom may be stated as follows [2]: ()()6 ..,,,ktFxCdx)-(tRx)m(MkjjktkjjjkJjk2161= =+++ &&&where: xj = displacement in the j-th DOF Fk(t) = dynamic external force in the k-th DOF M = inertia matrix m = added inertia matrix R = matrix of retardation functions C = matrix of buoyancy stiffnesses Once the system of coupled differential equations is obtained, arbitrarily time-varying loads such as wave induced loads, current forces, non-potential fluid reaction forces and non-linear mooring forces are incorporated as external force contributions.

10 MCS International has developed a version of its time domain program, Flexcom-3D, with a generalised floating body coupled first and second order analysis capability. This version of Flexcom-3D is used to analyse the response of a calm buoy in a WoA environment in the next section. West of Africa calm buoy Coupled Analysis A coupled analysis of an Offloading system similar to that shown in Fig. 4 has been carried out using the coupled version of Flexcom-3D. The Offloading system comprises an FPSO, two oil Offloading lines and calm buoy .