187

1 INTRODUCTION

There are a number of different terminals located

offshore, outside port limits or in sheltered anchorage

areas where larger ships which cannaot approach

ports or terminals ashore are berthed and the cargo

transfer is carried out offshore [1], [3], [7], [9], [10].

In this paper we are going to describe the main

characteristics of and comparisons between the

typical Conventional Multi-Buoy Mooring

(CBM/MBM), Single Point Mooring (SPM) and

conventional Single Anchor Loading (SAL) systems

(all designated for connection to amidships cargo

manifold of a standard trading tanker) taking into

account the hydro-meteorological boundary

conditions (weather limitations) enabling ships to

carry out safe cargo and manoeuvring operation

offshore. The permanently moored vessels FSO/FPSO

and DP shuttle tankers with Bow Loading Systems

(BLS) are to be excluded from this scope.

All items described in this paper are based on the

common requirements covered by the specified oil

field offshore operation manuals (e.g. [2], [6]), ship’s

operator manuals, which include Teekay Shipping

company experience factor [10] collected from the

tanker fleet since 1979 and the author’s own

experience (e.g. [3], [11]) in ship’s cargo and

manoeuvring operations offshore collected at sea

since 1997.

2 CONVENTIONAL MULTI-BUOY MOORING

A Conventional Buoy Mooring (CBM) known also as

Multiple Buoy Mooring (MBM) typically consists of

the following main components (see Figure 1):

mooring system with buoys, mooring legs and anchor

points, pipeline end manifold (PLEM), pipeline to

shore, subsea control system and hose string with

pick-up arrangement [3], [7], [9]. The multiple buoys

are fixed to the seabed by means of mooring lines and

marine anchors in a rectangular pattern that allows

safe mooring of a vessel which is positioned between

the buoys with tug assistance.

A Comparison Between Conventional Buoy Mooring

CBM, Single Point Mooring SPM and Single Anchor

Loading SAL Systems Considering the Hydro-

meteorological Condition Limits for Safe Ship’s

Operation Offshore

G. Rutkowski

aster Mariner Association, Gdynia, Poland

Teekay Learning and Development Centre, Teekay Shipping Norway

ABSTRACT: The purpose and scope of this paper is to describe the characteristics and make comparisons

between: Conventional Buoy Mooring (CBM), also referred to as Multi-Buoy Moorings (MBM), Single Point

Mooring (SPM) and conventional Single Anchor Loading (SAL) systems considering the hydro meteorological

condition limits enabling safe ship’s cargo and manoeuvring operation offshore. These systems (CBM, SPM and

SAL) are typically used for short term mooring applications offshore associated with the offloading and loading

of bulk liquid fuel tankers transporting refined and unrefined products of crude oil. The permanently moored

vessels FSO/FPSO are excluded from this scope.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 13

Number 1

March 2019

DOI: 10.12716/1001.13.01.19

188

Figure 1. Conventional Buoy Mooring CBM known also as Multiple Buoy Mooring MBM system. Source: Author’s own

researches based on [7] and [11].

In this method the bow of the ship is secured by

using both her anchors or forward mooring buoys

and the stern is secured to buoy around it by using

stern and quarter mooring system. It holds the vessel

in a fixed position and does not allow it to

weathervane. The buoys provide the strong points to

which the vessel’s on-board mooring lines can be

attached. As soon as the tanker has approached into

position with the aid of tugs, a launch crew takes the

tanker lines, one at a time, and tows them to the

various mooring buoys.

The mooring system comprises of mooring buoys

and mooring legs, where the buoys are generally

moored to the seabed with chain legs and high

capacity anchors, piles or clump weights depending

on soil characteristics. The number of mooring lines

and/or mooring chains is a function of the vessel size,

hydro-meteorological conditions and navigational

constraints.

When berthed, the tanker remains in a fixed

position without tugs, and depending on the site-

specific conditions & sometimes without vessel

anchors. The ships’ mooring ropes are connected from

either side of the vessel from the bow and the stern to

quick release hooks on the conventional buoys. The

buoys are manufactured either from steel or polymer

materials. The buoys act as a mooring only structure.

They contain no product transfer paths. The size of

the buoys is a function of the counter buoyancy

needed to hold the anchor chain for each buoy in

position. Some mooring buoys are off-the-shelf

products, while others have been specially designed

to include features like quick disconnection couplings.

The mooring system and layout of the buoys are

always specifically designed to match the vessel’s

requirements and local environmental conditions.

A typical CBM system of buoys have mooring

assemblies through the centre of the units,

terminating in a mooring eye on the bottom and pad

eyes on top for the fitting of quick release hooks. The

mooring legs for each buoy only consist of one

anchor, unlike in the case of a CALM system where

the buoy has between six and eight anchors. This is

because the anchor chains of a CBM system only

needs to work in one direction.

The product transfer system for CBMs consists of

offloading hoses (connects the tanker to the subsea

PLEM), marine breakaway coupling (which acts as a

weak link and prevents rupture of hoses/hawser and

subsequent oil spills). The CBM system is especially

valuable when no quay sites are available. It can also

be combined with a fluid transfer system that enables

connection of (subsea) pipelines to the amidships

manifold of a conventional tanker. The end of the

hose is provided with a pick-up line and a marker

buoy. The hose string is picked up by a small support

vessel that also assists the tanker with mooring to the

buoys. After mooring the tanker to the buoys, pickup

of the submerged hose strings and connecting the

hoses to the amidships manifold, the loading or

offloading operation may start. CBMs can be operated

with up to four separate product lines. When no

tanker is moored, the submersible hose or hoses are

stored on the seabed away from the influence of the

waves. For cryogenic fluids, the aerial hose is

suspended from a tower to the amidships manifold of

the liquefied gas carrier.

Typically, these systems are designed for

nearshore applications with water depths starting

from six metres up to approximately 30 m water

depth, in beginning environmental conditions or

conditions with a dominant directional character. The

minimum depth is a constraint of the design vessel

under keel clearance (UKC). As no weathervane mode

is possible, it is often applied on projects where

smaller tankers are employed. However, the system

can be designed to berth all sizes of tankers, with the

largest CBM system in the world designed to

accommodate vessels up to 225 000 DWT (e.g. Puma

multi-directional loading facility, Angola, 2015).

The tanker usually needs assistance in mooring

and disconnecting from the mooring and navigating

away from the CBM system. Ship usually can moor

with winds up to 30 knots and head waves of 1.0 m.

Vessel has to leave berth with winds of 60 knots and

waves higher than 2.0 m to 3.0 m. Mooring typically

takes 2 hrs (once vessel has arrived at CBM).

Disconnecting from mooring typically takes 1.0 hrs.

Hose connection and disconnection typically takes 1.0

hrs. Product unloading with winds up to 40 knots and

waves higher than 2.0 m to 2.5 m. The

offloading/loading operations can be undertaken via a

number of hoses (one to four hoses) and can handle

multiple products if required [5], [7].

189

3 SINGLE POINT MOORING

A Single Point Mooring (SPM), also known as Single

Buoy Mooring or SBM is a loading buoy anchored

offshore, that serves as a mooring point as well as an

interconnection for tankers loading or offloading gas

or liquid products. The basic principle of the buoy is

to keep the position of the vessel with respect to the

buoy steady and at the same time allowing vessels to

swing to wind and sea [3], [9]. Often a tug is provided

at the aft to keep the ship at a fixed angle and distance

from the buoy. The buoy is fixed by positioning it in

the centre of four to eight anchors connected to it. The

ship is made fast to the buoy with the help of a single

chain with hawser (or two) which is secured on board

to the bow stopper. The vessel always takes the most

favourable position in relation to the combination of

wind, current and wave and is free to align itself with

the prevailing environmental forces at the time. As

the vessel in its stationary state is always positioned

head-on into the winds/current’s direction, the total

force is less than would be experienced by a vessel on

a fixed mooring which is not always head-on into the

prevailing conditions.

The vessel will approach the buoy with its bow

into the dominant environment, thus maximising

control while minimising the need for tug assistance.

This solution is also good for DP dynamic positioning

vessel. For conventional tankers a tug is usually

required at all times during mooring and offloading

to maintain the nominal amount of tension on the

mooring hawsers to prevent collision of the tanker

with the buoy and assist with the weathervane

movements of the vessel.

The SPM system consists of four main

components: the body of the buoy, mooring and

anchoring elements, product transfer system and

ancillary components [9]. The buoy body may be

supported on static legs attached to the seabed, with a

rotating part above water level connected to the

offloading or loading tanker. These two sections are

linked by a roller bearing, referred to as the ‘main

bearing’. Alternatively, the buoy body may be held in

place by multiple radiating anchor chains. The

moored tanker can freely weathervane around the

buoy and find a stable position due to this

arrangement. The type of bearing used and the split

between the rotating and geostatic parts determines

the concept of the buoy. The size of the buoy is a

function of the counter buoyancy needed to hold the

anchor chains in position, and the chains are a

function of the environmental conditions and the

vessel size. Moorings fix the buoy to the sea bed.

Buoy design must account for the behaviour of the

buoy given applicable wind, wave and current

conditions and tanker sizes. This determines the

optimum mooring arrangement and size of the

various mooring leg components. Anchoring points

are greatly dependent on local soil condition and may

consist of ships, rig, piles or gravity anchors, sinker or

anchor chain joint to connect the buoy (SPM), anchor

chain and chain stoppers to connect the chains to the

buoy.

A tanker is moored to a buoy by means of a

hawser arrangement setup according to Oil

Companies International Marine Forum (OCIMF)

standards. The hawser arrangement usually consists

of nylon rope, which is shackled to an integrated

mooring joint on the buoy deck. At the tanker end of

the hawser, a chafe chain is connected to prevent

damage from the tanker fairlead. A load pin can be

applied to the mooring joint on the buoy deck to

measure hawser loads. Hawser systems use either one

or two ropes depending on the largest size of vessel

which would be moored to the buoy. The ropes

would either be single-leg or grommet leg type ropes.

These are usually connected to an OCIMF chafe chain

on the export tanker side (either type A or B

depending on the maximum size of the tanker and the

mooring loads) [9]. This chafe chain would then be

held in the chain stopper on board the export tanker.

A basic hawser system would consist of the following

(working from the buoy outwards): buoy-side shackle

and bridle assembly for connection to the pad eye on

the buoy, mooring hawser shackle, mooring hawser,

chafe chain assembly, support buoy, pick-up

messenger lines, marker buoy for retrieval from the

water.

The heart of each buoy is the product transfer

system. This system transfers products between the

tanker and a pipeline end manifold (PLEM) located

on the seabed. The basic product transfer system

components are: flexible subsea hoses, generally

referred to as ‘risers’, floating hose strings, marine

breakaway coupling, product swivel, valves and

piping. The risers are flexible hoses that connect the

subsea piping to the buoy. The configuration varies

on depth, sea state, buoy motions, tidal depth

variation, lateral displacement due to mooring loads

etc. In all cases the hose curvature changes to

accommodate lateral and vertical movement of the

buoy, and the hoses are supported at near neutral

buoyancy by floats along the length. These are:

Chinese lantern, in which two to four mirror

symmetrical hoses connect the PLEM with the buoy,

with the convexity of the curve facing radially

outwards. Lazy-S, in which the riser hose leaves the

PLEM at a steep angle, then flattens out before

gradually curving upwards to meet the buoy almost

vertically, in a flattened S-curve. Steep-S, in which the

hose first rises roughly vertically to a submerged

float, before making a sharp bend downwards

followed by a slow curve through horizontal to a

vertical attachment to the buoy.

Floating hose string, connects the buoy to the

offloading tanker. The hose string can be equipped

with a breakaway coupling to prevent rupture of

hoses/hawser and subsequent oil spills. The product

swivel is the connection between the geostatic and the

rotating parts of the buoy. The swivel enables an

offloading tanker to rotate with respect to the

mooring buoy. Product swivels range in size

depending on the size of attached piping and risers.

Product swivels can provide one or several

independent paths for fluids, gases, electrical signals

or power. Swivels are equipped with a multiple seal

arrangement to minimize the possibility of leakage of

product into the environment [3], [8].

190

Figure 2. Catenary Anchor Leg Mooring (CALM) as example of Single Point Mooring (SPM)/Single Buoy Mooring (SBM)

system with amidships cargo manifold side connection for operation offshore. Source: Author’s own researches based on [9]

and [11].

Other possible components of SPMs are: a boat

landing, providing access to the buoy deck, fenders to

protect the buoy, lifting and handling equipment to

aid materials handling, navigational aids for maritime

visibility, and fog horn to keep moving vessel alert,

electrical subsystem power navigation aids or other

equipment (provided either by batteries which are

recharged on a regular basis or solar power systems

to maintain the charge in the battery packs) and

optionally the hydraulic system to allow the remote

operation of the PLEM valves if required [9].

SPMs are the link between geostatic subsea

manifold connections and weather-vanning tankers.

They are capable of handling any size ship, even very

large crude carriers (VLCC) where no alternative

facility is available. In shallow water SPMs are used to

load and unload crude oil and refined products from

inshore and offshore oilfields or refineries, usually

through some form of storage system. These buoys

are usually suitable for use by all types of oil tanker.

In deep water oil fields, SPMs are usually used to load

crude oil direct from the production platforms, where

there are economic reasons not to run a pipeline to the

shore. These moorings usually supply to dedicated

tankers (DP class shuttle tankers) which can moor

without assistance.

Nowadays there are several types of Single Point

Mooring (SPM) systems in use. Most of the SPM

buoys are typically used for short term mooring

applications offshore. This system is preferential

when deep waters are not available close to shore.

The minimum depth is a constraint for the design

vessel Under Keel Clearance (UKC). A commonly

used configuration for SPM is the Catenary Anchor

Leg Mooring (CALM) and Single Anchor Leg

Mooring (SALM).

CALMs are usually located in water depths

between 20 to 100 meters and connected to a shore

storage facility (tank farm) or to offshore production

platforms by means of a submarine pipeline. Since

early 2000, the CALM design has been also used and

adapted to deep-water conditions, greater than 1,000

meters [3], [9], [11]. For this application, the CALM is

used as an offloading system for a deep-water

Floating Production Storage and Offloading unit

(FPSO) which are not covered by the scope of this

paper.

CALM holds the buoy in place by an anchor chain

that extend in catenaries to anchor points some

distance from the buoy. The SALM system is similar,

except that the SALM is anchored by a single anchor

leg [5], [7], [9]. The SALM system prevents collision

damage to the swivels by placing them underwater

and below the keel level of the tanker. Any damage

should then only affect the simple surface buoy and

be relatively cheap to repair. The underwater swivels

do however have maintenance disadvantages. To

prevent the flexible loading pipe clashing with the

mooring chains the catenary is replaced by a single,

nominally vertical, tensioned chain mooring leg. In

shallow water the fluid swivels are on the base. In

deep water the fluid swivels could be attached part

way up the mooring leg. This would ease

maintenance of the swivels and the flexible pipes

from the swivels to the tanker. In such case the

primary benefit of a CALM Buoy over a SALM Buoy

is ease of maintenance. The mechanical U-Joints of a

SALM are removed, and the fluid swivel is located

above the water surface.

A SALM can be employed as an unmanned tanker

loading or discharge terminal with multiple fluid

transfer circuits. The configuration of a SALM is

highly elastic over a very wide range of water depths.

This inherent elasticity enables cargo transfer

operations to continue under adverse weather and

sea-state conditions.

The vast majority of Marine Terminals installed

since the mid-1990s have been CALM Buoys because

of these design improvements. This configuration

uses six or eight heavy anchor chains placed radially

around the buoy, of a size to suit the designed load

and attached to an anchor or pile to provide the

required holding power. The anchor chains are pre-

tensioned to ensure that the buoy is held in position

above the PLEM. As the load from the tanker is

applied, the heavy chains on the far side straighten

and lift off the seabed to apply the balancing load.

Under full design load there is still some shackles of

chain lying on the bottom. Figure 2 illustrates a

Turntable type SPM CALM system during

installation.

Less commonly used SPM configurations include

Vertical Anchor Leg Mooring (VALM), which is

seldom used (e.g. offshore Brazil), two types of single

point mooring tower (Jacket type, which has a jacket

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piled to the seabed with a turntable on top which

carries the mooring gear and pipework or Spring pile

type, which has steel pipe risers in the structure),

Exposed Location Single Buoy Mooring (ELSBM)

which stores the mooring line and cargo hose on

drums when not in use (configuration suitable for use

in rough sea conditions), Articulated Loading

Platform (ALP) which is also suited for rough sea

conditions.

There are no vessel constraints for an SPM system.

The SPM CALM buoy configuration can

accommodate the largest vessels, including VLCCs. In

general, while approaching single point or single

buoy moorings weather is a major criterion in

determining whether to berth the vessel or not. Calm

seas with low swell and wind force below 15 knots

are considered favourable to make an approach.

Presence of strong tidal current limits the interval for

berthing and un-berthing. The headway approach has

to be slow often less than while at the same time

approaching at a smaller angle to the buoy and then

gradually hauling in the buoy messenger rope and

pulling the vessel slowly towards the buoy using

engine kicks at short intervals to control and maintain

headway along with mooring winches to haul in the

vessel when she nears about 150-200 meters from the

buoy. For un-berthing the chain is released from the

bow stopper and a short kick on the engines going

astern swings the bow to starboard for right handed

propellers thus clearing the vessel of the buoy. Tug’s

assistance can also be used to pull the vessel astern

and clear it of the buoy.

Ships usually can moor with winds up to 30 knots

and head waves of 2.0 m to 2.5 m. Vessel has to leave

berth when winds exceed 60 knots and waves are

higher than 3.5 m to 5.0 m. Approach to the SPM can

be from any direction (from a mooring perspective).

Mooring typically takes 15 min (once vessel has

arrived at SPM buoy). Disconnecting from mooring

does not necessarily need tug assistance, although

tugs may be required depending on the local site

condition and availability of space for vessel

manoeuvring. Disconnecting from mooring typically

takes 15 min.

Hose connection and/or disconnection on

amidships manifold typically takes 1 hour. Product

unloading possible with winds up to 40 knots and

head waves of 3.0 m to 4.5 m. SPM system can be also

designated for DP dynamic positioning vessels with

BLS (Bow Loading System), where offloading hose

can be connected directly to BLS system within about

10 to 15 minutes.

For conventional tankers the fulltime tug

assistance can be required to assist with weather-

vanning and prevent collision between the vessel and

the SPM. Typically, vessels are only able to offload a

single product at a time, however SPMs with

amidships manifold connection can be designed to

handle multiple products if required.

4 SINGLE ANCHOR LOADING

Single Anchor Loading (SAL) is offshore loading

system introduced in the late 1990’s, that serves as a

single mooring point via hawser to the underwater

strong base anchor as well as an interconnection for

tankers loading or offloading gas or liquid products

[1], [2]. SAL was developed as low-cost alternative to

the Submerged Turret Loading (STL) system for use

in the situation where traditional CALM buoys cannot

be used. As the SAL is subsea the risk of collision

between the tanker and traditional CALM or SPM

buoy is eliminated. On SAL system the vessel always

takes the most favourable position in relation to the

combination of wind, current and wave and is free to

align itself with the prevailing environmental forces at

the time.

The central elements of the SAL system are a

mooring and a fluid swivel with a single mooring line

(hawser) and a flexible riser for fluid transfer

attached, anchored at the sea bed by the use of a

single anchor. The anchor also functions as the

pipeline end manifold (PLEM) for the seabed export

flow line. A tanker is hooked up to the system by

pulling the mooring line and the riser together from

the seabed and up to the bow of the vessel. Here the

mooring line is secured and the riser is connected to

the vessel. Moored to the SAL system, a tanker can

freely weathervane. Disconnection is performed by

lowering the mooring line and the riser down to the

seabed.

Nowadays there are a several types of SAL

systems in use. The SAL flexible riser (offloading

hose) can either be designated for connection to a

standard shuttle tanker bow loading system BLS or to

the typical amidships manifold of a standard trading

tanker.

The SAL can be equipped with clump weight

system (e.g. Hanze SAL installed in 2001, Solan SAL

installed in 2015) or underwater buoyancy systems

(e.g. Banff SAL or South Arne SAL installed in 1998).

Typical SAL system is usually equipped with strong

mooring hawser with chafing chain designated for

tanker with chain stopper located forward for single

point mooring SPM system on board the tanker vessel

(e.g. installation of South Arne SAL with mooring

hawser capability limited to 450 tons on weak link).

However, there are also SAL installation without

upper mooring system, which are in configuration

typical for traditional offshore loading system OLS,

which are designated only for full DP shuttle tankers

equipped with bow loading system.

Nowadays most of the SAL systems are typically

used offshore for short term mooring applications

recommended for DP shuttle tankers equipped with

BLS. However, there is also the possibility to use the

SAL systems for conventional tankers with amidships

cargo manifold side offloading as presented on Figure

3. In general approach the typical SAL terminal is

quite easy to install. It has been noted that new SAL

terminal can be installed on oil field offshore just

within few weeks’ period. It has been also noted that

the old SAL systems can be easy reused and/or

upgraded (e.g. Ardmore twin SAL systems have been

re-used for Kittywake SAL installation in 2005 and

Don SAL installation in 2008, Siri SAL buoyancy

system has been upgraded from buoyancy SAL

system to clump weight SAL system in 2009).

192

Figure 3. Configuration of Single Anchor Loading (SAL) system for standard trading tankers with hose connected to

amidships cargo manifold. Source: Author’s own researches based on [2] and [11].

Figure 4. South Arne SAL buoyancy system with operation parameters for shuttle tankers based on HESS Oil Export and

SAL System Operating Procedure DOM-SA-P-001B. Source: [6].

SAL system with clump weight comprises the

following main sub-systems: anchor assembly,

mooring system and riser (offloading) system. Anchor

assembly consists of the following components:

anchor pile, anchor base with piping and valves,

turret assembly including turret table, bearing and

swivel with piping. Mooring system contains lower

polyester mooring segment, mid line clump weight

with integrated steel spool piece and upper mooring

(hawser) with chafing chain and pick-up assembly.

The SAL riser (offloading system) consists of the

following components: lower riser segment

(offloading hose) connected at in-line swivel at SAL

pipeline end manifold (PLEM), mid line clump

weight (MLCW), polyester hawser, upper riser

segment (offloading hose), hose end valve (HEV) and

pick-up assembly. A polyester hawser, as an

integrated part of the lower riser system, is installed

between the clump weight and the SAL anchor. This

polyester segment is introduced to protect the lower

riser segment in case of a drift-off of the tanker, and

also to improve lay-down tolerances for the MLCW

and HEV and simplify disconnection operations. The

riser’s lower segment is connected to the anchor and

the MLCW with a bending stiffener in each end and is

equipped with buoyancy elements to make it

buoyant. The upper riser segment is connected to

MLCW and the in-line swivel in the hose end valve

assembly, also with a bending stiffener in each end.

The offshore loading house in upper riser segment

can be floating (option only for conventional tankers

with side connection) or laid down at sea bed in an

optimal heading prior to pick-up and connection by

the shuttle tankers with BLS.

The basic principle of SAL system is to keep the

position of the vessel with respect to the SAL base

steady and at the same time allowing vessels to swing

to wind and sea. This operation is typical for shuttle

tanker on DP weather vane mode. For conventional

tanker often, a tug is provided at the aft to keep the

vessel at a fixed angle and distance from the SAL

terminal.

In buoyancy SAL systems (e.g. South Arne SAL -

see Figure 4) the mooring system is made up of the

following components: lower wire segment, mooring

line buoyancy element, lower rope segment,

additional buoyancy element, upper rope segment,

chafing chain and pick-up assembly. The lower wire

segment connects the mooring line buoyancy element

to the SAL anchor top structure. The purpose of the

mooring line buoyancy element is to serve as a spring

function in the SAL mooring system, serve as support

for the loading hose and assemble mooring lines and

loading hose at a common point. The mooring line

buoyancy element is situated between the lower wire

rope segment and the lower rope segment, and

consists of a cylindrical steel structure, which is

connected to a connecting rod. The rod is an

integrated part of the mooring line carrying the

193

mooring line tension that is no mooring forces are

transferred through the hull of the buoyancy element.

In light of SAL mooring system handling

requirements during operation, fibre ropes were

chosen instead of wire ropes. Two fibre rope

segments, a lower fibre rope segment and an upper

fibre rope segment connect the mooring line

buoyancy element and the chafing chain, via the

additional buoyancy element. Both sections of rope

are manufactured from Polyester. The additional

buoyancy element is designed to avoid contact

between the fibre rope segments and the seabed, and

to reduce the stiffness of the mooring system in

disconnected condition. However, this SAL system is

designated only for DP shuttle tanker with BLS

system.

The SAL installation is fixed by SAL anchor. The

ship is made fast to the SAL with the help of a single

chain with hawser which is secured on board to the

bow stopper. The vessel always takes the most

favourable position in relation to the combination of

wind, current and wave and is free to align itself with

the prevailing environmental forces at the time. As

the vessel in its stationary state is always positioned

head-on into the winds/currents direction, the total

force is less than would be experienced by a vessel on

a fixed mooring which is not always head-on into the

prevailing conditions. The vessel will approach the

SAL with its bow into the dominant environment,

thus maximising control while minimising the need

for tug assistance. This solution is good for DP

dynamic positioning vessel. For conventional tankers

a tug is usually required at all times during mooring

and offloading operation to maintain the nominal

amount of tension on the mooring hawsers and assist

with the weathervane movements of the vessel.

The SAL Anchor Assembly consists of the

following main components: anchor pile, anchor base

structure, turret assembly, including turret adapter,

turret table, bearings and bearing locking

arrangement, crude swivel, piping from crude swivel

to flexible riser, riser connector and support, piping

from sub-sea pipeline to crude swivel, acoustic

control system (optional), crude oil valve for branch

piping (to crude swivel) and crude oil valve for main

piping. The anchor base is fabricated on top of a

conical structure that fits to the pre-installed anchor

pile. During the final installation campaign, the

conical structure is grouted to the anchor pile to

achieve the required permanent structural capacity.

The Anchor Base structure transfers the forces from

the riser system, via the bearing arrangement in the

turret into the anchor pile. The center of the anchor

base consists of the removable turret assembly. The

connection to the removable module consists of a

robust vertical pipe with a flange connection and

guiding for the turret adapter module. The interface

between the anchor base structure and the anchor pile

is a grouted connection. The soil inside the top of the

anchor pile is dredged out after installation and a

mating insert below the SAL base structure penetrates

into the anchor pile. The void space between the

anchor pile and the mating insert on the anchor base

is filled with grouting to form a rigid structural

connection.

The turret assembly is the main component that

consists of the turret adapter, turret table bearings

and bearing locking arrangement, crude swivel, riser

connector and support and lower polyester segment

connection point. The turret assembly is connected to

the anchor base structure by a bolted flange. The

turret assembly is designed to perform without any

maintenance throughout the design life of the system

(30 years). However, if found required, the turret

assembly can be unbolted and retrieved to surface for

maintenance or repair, or it can be replaced with an

interchangeable new turret assembly. The

arrangement that transfers the global loads from the

riser system consists of a turret shaft, turret table and

a bearing arrangement. It is designed with basis in the

same turret components as the field proven STL/STP

systems. The system is normally designed to transfer

mooring loads from a passive vessel. The upper

bearing ring supporting the upper axial bearing is

locked to the central turret shaft by means of a

segment locking system fitted into a circular groove of

the centre shaft. The bearings are fixed to the turret

table. The turret table is equipped with a support

structure for the riser. The support structure ensures

that all global loads from the riser (tension, shear and

moment) are transferred into the turret table, which

provides the moment arm for turning the turret table

and the crude oil swivel.

The crude swivel is located on top of the anchor.

The top of the swivel is rigidly connected to a frame

on the top of the turret table to prevent loads from the

riser to be transferred into the crude swivel. The

rotation of the swivel will be driven by the rotating

motion of the turret table via the piping clamped to

the anchor base. The rotating motion originates

mainly from the tension loads in the riser. The inline

fluid swivel is a robust and compact unit with few

sensitive parts. It is capable of handling all relevant

external forces from the attaching piping with a good

margin. It is not sensitive to moisture and water

ingress, as it holds no roller bearing elements. The

fluid swivel comprises a set of double axials thrust

bearings and double radial bearings to take up

external loads while allowing rotation. All bearings

are self- lubricating, i.e. bronze with solid lubricant

depots. The process seals are arranged as rod seal

type (axial seals). There are double process seals, with

leak collection in-between. Additionally, to prevent

water and dust ingress into the bearing cavity there is

a scraper and a seal on the outside of the bearing

cavity.

SAL systems are usually located in water depths

between 30 to 100 meters and are connected to a shore

storage facility (tank farm) or to offshore production

platforms by means of a submarine pipeline.

Nowadays most of the SAL systems are designated

for DP shuttle tankers with BLS system. SAL

configuration can accommodate the largest vessels,

including VLCCs.

Shuttle tankers can moor to SAL system with

significant sea waves up to 4.5 m (maximum wave

heights 8.5 m) and stay connected to SAL when

loading with significant sea waves up to 5.5 m

(maximum wave heights 9.5 m) and disconnection in

seas as high as 7.0 m [1], [2]. Conventional tankers

usually can moor with winds up to 30 knots and head

waves of 2,0 m to 2.5 m. Vessel has to leave berth

when winds exceed 60 knots and waves are higher

than 3.5 m to 5.0 m. The above values are normative

194

and must be considered in connection with current

weather conditions and whether conditions are

improving, stabilizing or worsening. Approach,

loading and discontinuation of these activities are

always subject to the Master’s final approval.

The operational limits for single anchor loading

(SAL) loading are the same as for SPM loading. In

addition, there is a limited sector (±5°) for positioning

when picking up the line from the bottom. On DP

shuttle tankers two different loading modes, rotation

and non-rotation modes may be used for loading on

SAL systems (see figure 4). Non-rotation mode (Auto

Position) is normally used when weather conditions

allow it, to reduce strain on the loading system.

Rotation mode (Weather Vane) is normally used

when weather conditions are above winds of Beaufort

force 4 and/or sea state 2 or above. Hawser tension

limits for SAL are normally higher than other systems

for offshore loading with shut down (green line

failure) on 200 t. Restricted zones are defined where

other installations are placed nearby and when there

is a risk that the vessel, in an emergency, may drift

towards other offshore installations.

Approach to the SAL can be from any direction

(from a mooring perspective). If needed the SAL

system can be rotated and laid down in an optimal

heading by supporting tug prior to pick-up and

connection by approaching tanker. Mooring typically

takes 30 min to 1 hour (once vessel has arrived at SAL

installation and connect mooring pick up assembly).

Disconnecting from mooring does not necessarily

need tug assistance, although tugs may be required

depending on the local site condition and availability

of space for vessel manoeuvring. For conventional

tankers the fulltime tug assistance can be required to

assist with mooring, unmooring and weather-

vanning. Disconnecting from mooring typically takes

about 30 min. Hose connection and/or disconnection

on amidships manifold typically takes 1 hour.

Product unloading possible with winds up to 40 knots

and head waves of 3.0 m to 4.5 m. Connection to BLS

usually takes about 15 minutes. Typically, vessels are

only able to offload a single product at a time,

however SPMs with amidships manifold connection

can be designed to handle multiple products if

required.

5 COMPARISON SUMMARY

Summing up the above, it can be assumed that in the

offshore sector, the expensive offshore loading

systems will be replaced by the cheaper and equally

effective solutions. The author also believes that the

traditional single anchor loading (SAL) systems with

the Hawser mooring system will be replaced by the

modified SAL systems in the OLS configuration

(without the Hawser system attached), which is

designated only for DP shuttle tankers.

Conventional buoy mooring system CBM and

single point mooring system SPM will continue to be

found in common use by conventional tankers and/or

storage units FSO on shallow waters (CBM up to 30 m

depth and SPM up to 100 m depth).

A summary comparison between conventional

buoy mooring system CBM, single point mooring

system SPM and single anchor loading system SAL

considering operational limits and the hydro-

meteorological condition limits for safe ship’s

operation offshore has been presented in the

following table:

Table 1. Comparisons between conventional buoy mooring system CBM, single point mooring system SPM and single

anchor loading system SAL considering operational limits and the hydro-meteorological condition limits for safe ship’s

operation offshore. Source: Author’s own researches.

__________________________________________________________________________________________________

No Criteria CBM SPM SAL

__________________________________________________________________________________________________

1. Vessel size Vessel sizes typically up to Vessel sizes unlimited Vessel sizes unlimited

80 000 dwt, but up to

225 000 dwt has been

installed

2. Operating water Usually up to 30 m water Up to 100 m water depth Up to 100 m water depth

depth depth

3. Mooring method Multi-buoy mooring system Single buoy mooring system Single point mooring system with

optional with ship’s anchor with one or two hawsers one hawser

4. Approach Can only approach from Can approach from any position – and therefore can choose to

limited positions approach into prevailing weather conditions

5. Mooring time Typically, about 2.0 h Typically, about 15 min Typically, about 30 min

(after arriving at

mooring position)

6. Hose connection Typically, about 1.0 h Typically, about 1.0 h Typically, about 1.0 h

time amidships

7. BLS connection N/A Typically, about 15 min Typically, about 15 min

8. Time for hose Typically, about 1.0 h Typically, about 1.0 h Typically, about 1.0 h

disconnection

9. Offloading time Function of vessel pumps capacity, line size and distance

10. Time for mooring Typically: 1.0 h Typically: 15 min. Typically: 20 min to 30 min.

disconnection

11. Net total time Typically: 2.5 h longer Typically: 2.5 h quicker Typically: 2 h quicker compared to

difference compared to SPM compared to CBM CBM

12. Mooring conditions Able to moor with winds up Able to moor with winds up to 30 knots and head waves of 2.0 m

for conventional to 30 knots and head waves to 2.5 m

tanker of 1.0 m

13. Operating limit for Product unloading with wind Product offloading possible with wind up to 40 knots and head

conventional up to 40 knots and waves waves of 3.0 m to 4.5 m

195

tanker higher than 2.0 m to 2.5 m

14. Mooring Vessel has to leave berth with Typically, vessel has to leave berth with winds of 60 knots and

disconnection winds of 60 knots and waves waves higher than 3.5 m to 5.0 m

higher than 2.0 m to 3.0 m

14. Mooring N/A, restriction similar as for Connection: significant sea waves (Hs) up to 4.5 m, max. sea waves

conditions and conventional tankers (No (HSmax) up to 8.5 m, Period (Tmax)= 15 sec. Loading: significant

operational limits DP operation) sea waves (Hs) up to 5.5 m (max. sea waves heights (HSmax) up to

for DP BLS shuttle 9.5 m). Period (Tmax)= 15 sec.

tankers

15. Offloading steps Steam from anchorage, moor, connect hoses, pump product through, disconnect hoses,

disconnects mooring, steams away

16. Tug assistance Required during mooring and For conventional tankers tug required full time during mooring

disconnection from mooring and disconnection and to assist with weathervane movements. DP

buoys shuttle tanker normally can operate without tug assistance.

17. Under Keel Function of the vessel, offshore terminal installation and hydro-meteorological conditions

Clearance

18. Risk of collision Possible with floating buoy Possible with SPM buoy As the SAL is subsea the risk of

during mooring and during mooring, unmooring collision between the tanker and

unmooring operation (tug and weathervane (for traditional CALM / SPM buoy is

assistance is required). When conventional tanker full tug eliminated.

tanker is moored to CBM in assistance is required).

fixed position the collision

risk is minimal.

19. Weather impact More prone to adverse Less prone to adverse Less prone to adverse conditions

conditions and swell delays conditions and swell delays and swell delays than a CBM

than a SPM than a CBM

20. Weathervane N/A. Tanker is moored to The vessel always takes the most favourable position in relation to

around the buoy CBM on fixed heading the combination of wind, current and wave and is free to align

itself with the prevailing environmental forces at the time.

21. Night operations Possible limitation on night Possible limitation on night time mooring depending on local

time mooring and operational, safety and environmental procedures

disconnecting from mooring

depending on local Can disconnect from moorings 24 h a day

operational, safety and

environmental procedures

22. Track record Successfully installed around Since 1959 successfully Since introduced in the late 1990’s

the world for close to a installed around the world successfully installed around the

century world

23. Cost Approximately in the USD Approximately in USD SAL was developed as low cost

from 12 000 to 250 000 per 15 000 000 for SPM CALM alternative to the STL.

buoy alone [5] buoy alone [5] Typical cost for the SAL system will

be in the USD from 12 000 000 to

15 000 000 range [1]

24. Suppliers Non-proprietary technology Proprietary technology and limited suppliers

25. Maintenance Less complex maintenance Additional complex maintenance activities compared to a CBM –

compared to SPM swivel, bearings, mooring line tensions, etc.

Hoses damage more due to

abrasion from storage on seabed

__________________________________________________________________________________________________

REFERENCES

[1] APL Advanced Production and Loading, National

Oilwell Varco, website: www.nov.com/fps, accessible:

March-April 2018.

[2] APL Document No.: 1586-APL-O-KA-0001, National

Oilwell Varco, SAL Operation Procedure, Rev. B, 27-06-

2011, accessible: March-April 2018.

[3] Bluewater, website: http://www.bluewater.com,

accessible: March-April 2018.

[4] Operation of the dynamically positioned ships

(Eksploatacja statków dynamicznie pozycjonowanych),

Volume 8 of the series ‘Contemporary Technologies of

the Sea Transport’ (Współczesne Technologie

Transportu morskiego). Trademar Publishing House,

ISBN 978-83-62227-44-0, Gdynia 2013.

[5] EPCM consultants, website: https://epcmconsultants.co,

accessible: March-April 2018.

[6] HEES internal document No.: DOM-SA-P-001B, Oil

Pipeline and Loading Operating Procedure, Rev. 7

1.12.2015. Document accessible: March – April 2018.

[7] Marine Insight website: https://www.marineinsight.com,

accessible: 30 March 2017.

[8] Rutkowski G.: Numerical Analysis for the Dynamic

Forces and Operational Risk Accomplished Forces and

Operational Risk for Hiload DP1 Unit Docked to MT

Navion Anglia at Sea Waves. TransNav, the

International Journal on Marine Navigation and Safety

of Sea Transportation, Vol. 8, No.4, DOI

10.12716/1001.08.04.04, pp. 505-5012, 2014.

[9] SBM, website: www.sbmoffshore.com, accessible: 10

Nov.2017.

[10] Ships and off-shore technologies in outline (Statki i

technologie off-shore w zarysie), Cydejko J., Puchalski J.,

Rutkowski G., ISBN 978-83-62227-24-2, Trademar

Publishing House, Gdynia 2010/2011.

[11] Teekay internal document No.: SP0978 ver. 10, Teekay

Requirements for Offshore Loading Operations (Shuttle

Tankers), accessible: 07 April 2018..