Deep Water SAL - An Offloading Station for Large Water Depths
Authors: Aanesland Vidar , Bech Arild 
Published: October 2011

The use of FPSOs in the development of oil and gas fields in deep waters requires accessibility to safe and reliable off-take solutions. The Deep Water SAL (DW SAL) design moves the well proven Single Anchor Loading (SAL) concept to deep water applications. The SAL system has been used for the last ten years in the North Sea at eight different locations.

The DW SAL system consists of a submerged buoy, the SAL Base, moored to the seabed by typically 6 mooring legs. The Oil Offloading Lines (OOLs) are connected at one end to the FPSO and at the other end to the centre of the SAL Base. The distance between the FPSO and the DW SAL is typically in the range of 1500m to 2500m. The SAL Base is equipped with a mooring swivel and a fluid swivel, allowing the moored export tanker to freely weather vane and serves as the connection point between the export tanker and the geo-stationary mooring system.

The DW SAL can be applied to a wide range of water depths, as the system is virtually made water depth independent by placing the mooring swivel on the SAL Base. The target water depth of the SAL Base is about 45m, at which, the wave induced motions are reduced significantly compared to a surface buoy. The loads on the OOLs are consequently much lower and the fatigue life of the system components much higher. At the same time as the SAL Base will be at a safe distance below any service vessel or export tanker when idle and disconnected, it may still be reached by divers. However, the operational philosophy is based on emptying specially designed ballast tanks to blow the SAL Base to the sea surface so that maintenance and other work easily can be performed.

The extreme tension in the mooring hawser is also much lower for a DW SAL system compared to a standard calm buoy. The main reason is the combination of slow drift motion of the export tanker combined with the first order motion of the calm buoy / SAL Base. Typically the slowly varying offset of the tanker creates the mean tension level; while the wave induced motion of the buoy creates the peak loads.

The development of the DW SAL has been using cite specific data both from West of Africa as well as Santos Basin offshore Brazil.

Introduction

Various offloading concepts are available and in use today and the selection depends on location, weather conditions and infrastructure. APL has over the years developed systems like BLS (Bow Loading System), SDS (Stern Discharge System), STL (Submerged Turret Loading), SAL (Single Anchor Loading) and BTL (Buoy Turret Loading). Except for two BTLs, which essentially are specially designed calm buoys for deep water, these systems have been installed in moderate water depths, up to 350m, but designed to operate in the harsh North Sea environment. The SAL system, Figure 1, where a tanker is connected by a single mooring hawser in the bow of the vessel, has in particular shown to perform very well. More than 2000 shuttle loads have been lifted since the introduction of the first system on the Siri Field in 1999. Key elements are the mooring and fluid swivels, the mooring hawser and the flexible hose for fluid transfer. The anchor, which also serves as PLEM (Pipe Line End Manifold) is either suction anchor, pile anchor or gravity anchor. While disconnected, the mooring line and loading hose are lowered to the seabed until next hookup. Contrary to a Calm buoy, all equipment remains subsea on the seabed during storms and extreme weather. The maximum design sea state for the SAL systems in idle condition have been significant wave heights up to 13m.