Part II: Types of Offshore Platforms

Ladies, here we meet again for Part II. Ok last time I already explained about the introduction of the offshore structures and now we move to the main contents, types of offshore structures. Either fixed or float, as simple as that. Ah, layman terms. Fixed platforms is a platforms that its substructure attached / piled to the seabed while floats platforms does not. However, floating systems normally moored i.e chained or anchored.

Fixed Platforms

Steel Jacket Platform

Semangkok-A Platform

In Malaysia, especially in Peninsular Malaysia (PM) blocks, water depth is range to 50-70 metres (shallow waters). Hence, it is economically feasible to built a steel jacketed platforms. But the application of the jacket platform usually up to 300 metres. Topside of jacket platform are normally built either integrated (alltogether) or modular type (section per section e.g Living Quarters + Power Generation + Booster Compression + bla bla bla). For middle school level of understanding, JACKET PLATFORM = TOPSIDE + JACKET.

Compliant Tower

Compliant Tower Design

Petronius - Compliant Tower Platform

Obviously, the compliant tower platform are designed for water depth more than jacket platform which are normally up to 600 metres. The difference with jacket is that it has more narrowed frame support structure by a piled foundation. Compliant towers are designed to sustain significant lateral deflections and forces. Kids, read this out loud. COMPLIANT TOWER = TOPSIDE + FLEXIBLE TOWER.

Concrete Gravity Structure

Hibernia Platform

Practically, this type of structures are being used in the harsh environments such as North Sea and Scandinavian. Also known as Gravity Base Structure (GBS) platform, it is design to withstand the loads (environment + iceberg) and also for the storage of its base support. Normally GBS is applicable for water depths up to 300 metres. When completed, a GBS is towed to its intended location and sunk. Again kiddos, GBS = TOPSIDE + CONCRETE BASE.

Floating Systems

Semisubmersible

Na Kika Semisubmersible

Gumusut-Kakap Semi Floating Production System

Semisubmersible platform consist of hulls (box or cylindrical) for its substructure while atop of it is the topside. Sinple explanation always brought a simple expression. SEMISUB = TOPSIDE + HULL. What the hell is hull? Hull consist of columns and pontoons with providing sufficient buoyancy to cause the structure to float. It is used for storage and also for ballasting operations. The advantage of the floating systems that is can be moved or towed from one location to another. Moreover, some of the semisub platforms was converted from semisubmersible drilling rigs. Normally, this type of platforms is used for deepwater productions which is water depth is above 1000 metres.

Spar


Kikeh DTU Truss Spar

Spar platform consist of cylindrical steel of large diameter, vertically placed and moored to support the superstrucure of the platform (topside). The cylidrical structure that holds the topside always referred as hull (substructure of spar). In  mathematical expression, SPAR = TOPSIDE + HULL.

Spars can be classified into three types, namely Classic spar, Truss spar and Cell spar. As always, figures really really helps. The only difference for these three design is the hull parts, which have its pros and cons. For more understanding of trus spar, you can have the preview from this post.

Classic Spar Design

 

Typical Truss Spar Platform



Cell Spar Design

Tension Leg Platform

Moses TLP

Ram Powell TLP

Tension-Leg Platform also known as TLP is a platform that consist of topside and hull (same like the previous floaters). But noted that the hull's mooring system are different. TLP system used a set of tension legs or tendons attached to the platform and connected to a template or foundation on the seafloor. The tension leg mooring system allows for horizontal movement with wave disturbances, but does not permit vertical, or bobbing, movement, which makes TLPs a popular choice for stability, such as in the hurricane-prone Gulf of Mexico. Normally TLPs are applicable for depths from 400 - 1500 metres. Out loud ladies, TLP = TOPSIDE + HULL/TENDONS.

FPSO/FSO

FPSO Kikeh

FSO Abu

Floating, Prodution, Storage and Offloading (FPSO) is a unit floating vessel that consist of production process equipment on the deck or simply can be said topside. A FPSO vessel is designed to receive oil or gas produced from nearby platforms or subsea wellheads, process it, and store it until oil or gas can be offloaded onto a tanker or transported through a pipeline. On the other hand, Floating, Storage and Offloading (FSO) vessel only meant to store the produced oil while being tansported or offloaded to a tanker. These type of floating units is advantegous for lower cost development and also ideal for deepwater development fields. Both FPSOs and FSOs can be newly built or conversion from marine tanker. As a conclusion, by a mathemical equation FPSO = TOPSIDE + VESSEL + TURRET while FSO = VESSEL + TURRET.

Ah, forget to explain about turret. Turret is a structure installed on the forward position of a vessel for umbilical and risers and also serve as mooring systems for FPSOs & FSOs. Meaning, it is purposedly designed to support the transmission of crudes through pipes via turret. Turret can be either internal turret or external turret. As an example, FPSO Kikeh consist of an external turret and it is suitable for benign waters like Malaysia offshore. The internal turret contradicts the external turret thus it is popularly used at harsh sea environments.

External Turret